Understanding Infrared Thermography Reading 3

Infrared Thermal Testing 

Reading III- SGuide-IRT 

My ASNT Level III Pre-Exam Preparatory 

Self Study Notes 29th April 2015 

Charlie Chong/ Fion Zhang

Aerial thermography 

Charlie Chong/ Fion Zhang 

http://horuslab.eu/

Infrared Thermography 

~-- 

... ... .. 






DEADLY French Military Mistral Anti Aircraft Missile System 

I' 

■ https://www.youtube.com/embed/_3c0NpYapM0 


https://www.youtube.com/watch?v=_3c0NpYapM0

See Through & Fun Thermal Camera Experiments 

I' 

■ https://www.youtube.com/embed/pXAzZoWLzSo 


https://www.youtube.com/watch?v=pXAzZoWLzSo

LEAKED Body Scan Images From The TSA! 

I' 

■ https://www.youtube.com/embed/QRkWmRVs-nk 


https://www.youtube.com/watch?v=QRkWmRVs-nk

How to see through clothing 2 

I' 

■ https://www.youtube.com/embed/0wQlyCNPw8M 


https://www.youtube.com/watch?v=0wQlyCNPw8M

Bf4 little bird ah-6j night vision infrared real combat footage helmet cam 

montage funker tactical. – 金头盔 

I' 

■ https://www.youtube.com/embed/dRra63kOwWE 


https://www.youtube.com/watch?v=XfXShaTzAhI&list=PL7D451B08CD9A119B

Apache IR Thermal Weaponry 

■ 

https://www.youtube.com/embed/XfXShaTzAhI?list=PL7D451B08CD9A119B 


https://www.youtube.com/watch?v=XfXShaTzAhI&list=PL7D451B08CD9A119B

Infrared Electrical Testing 

■ 

https://www.youtube.com/embed/DgXsmvv7Q9o 


https://www.youtube.com/watch?v=DgXsmvv7Q9o


Fion Zhang at Shanghai 

29th May 2015 

http://meilishouxihu.blog.163.com/ 


Greek alphabet 

Letter Name 

Sound 

Ancient 141 Modern 151 Letter Name 

Sound 

Ancient 141 Modern 151 

A a alpha [a] [a:] [a] Nv nu [n] [n] 

813 beta (b] (v] - ~ Xi (ks] (ks] 

rv gamma (g] (y] - (j] Oo omicron (o] (o] 

[j.(j delta (d) [OJ nn pi (p] (p] 

EE epsilon (e) (e) Pp rno (r] (r] 

Z< zeta [zdt [z] L al~[lJ sigma (s] (s] 

Hr) eta (e:] [i] TT tau [I] [I] 

08 theta [t•] [6] Yu upsilon (y] (y 1 [i] 

I I iota [i] [i:] [i]

A Alpha r Gamma NNu T Tau 

(al-fah) (gam-ah) (new) (taw) 

B Beta 

(bay-tah) 

HEta 

(ay-tah) 

Q Omicron 

( om-e-cron) 

y 

X Chi 

I Iota II Pi Q Omega 

Upsilon 

(up-si-lon) 

(kie) (eye-a-tah) (pie) ( oh-may-gah) 

q 

~Delta K Kappa 8 Theta ~ Xi 

E Epsilon A Lambda p Rho 'I' Psi 

(del-ta) (cap-pah) (thay-tah) 

p 

h 

d 

(zie) 

(ep-si-lon) (lamb-da h) (roe) (sigh) 

Phi MMu L Sigma Z Zeta 

(fie) (mew) (sig-ma) (zay-tah) 


http://greekhouseoffonts.com/

Greek letter 

l 

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u 

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IVONA TTS Capable. 

e Text-To-Speech 

1uona 

Text-To-Speech 


http://www.naturalreaders.com/

SGuide-IRT 

Content 

Part 1 of 2 

■ Chapter 1 - Introduction to Principles & Theory 

■ Chapter 2 - Materials and Their Properties 

■ Chapter 3 – Thermal Instrumentation 

Part 2 of 2 

■ Chapter 4 – Operating Equipment and Understanding Results 

■ Chapter 5 – Applications 

■ Appendices A, B, C 


Chapter 1 

Principles & Theory 


1.1 Introduction to Principles & Theory 

Infrared/thermal testing involves the use of (1) temperature and (2) heat flow 

measurement as a means to predict or diagnose failure. 

This may involve the use of contacting or noncontacting devices, or a 

combination of both. A fundamental knowledge of heat flow and the thermal 

behavior of materials is necessary to understand the significance of temperature 

and temperature changes on a test sample. 

Contacting devices include thermometers of various types, thermocouples, 

thermopiles and thermochromic coatings. 

Noncontacting devices include convection (heat flux) devices, optical pyrometers, 

infrared radiation thermometers, infrared Line scanners and infrared thermal 

imaging (thermographic) equipment. 

Infrared thermography is the nondestructive, non-intrusive. noncontact mapping 

of thermal patterns on the surface of objects. It is usually used to diagnose 

thermal behavior and, thereby, to assess the performance of equipment and the 

integrity of materials, products and processes. 


Keywords: 

Principles: 

• temperature and 

• heat flow measurement as a means to predict or diagnose failure. 

Techniques: 

• contacting or 

• noncontacting devices, 

• or a combination of both. 

Contacting devices include: 

• thermometers of various types, 

• thermocouples, 

• thermopiles and 

• thermochromic coatings. 

Noncontacting devices include: 

• convection (heat flux) devices, 

• optical pyrometers, 

• infrared radiation thermometers, 

• infrared Line scanners and 

• infrared thermal imaging (thermographic) equipment. 


The infrared thermal imaging equipment used in infrared thermography is 

available in numerous configurations and with varying degrees of complexity. 

The thermal maps produced by infrared thermal imaging instruments are 

called thermograms. To understand and interpret thermograms, the 

thermograpber must be familiar with the fundamentals of temperature and 

heat transfer, infrared radiative heat flow and the performance of infrared 

thermal imaging instruments and other thermal instruments. 

An understanding of the equipment, materials and processes being observed 

is also important to effectively assess the full significance of infrared/thermal 

measurements. A more detailed discussion of the performance parameters of 

infrared thermal imaging instruments is provided in Chapter 3. 

Keywords: 

■ infrared thermography - The thermal maps produced by infrared thermal 

imaging instruments are called thermograms. 


1.2 Fundamentals of Temperature and Heat 

Transfer 

Heat is a transient form of energy in which thermal energy is transient. What 

is often referred to as a heat source (such as an oil furnace or an electric 

heater) is really one form or another of energy conversion – the energy stored 

in one object being converted to heat and nowing to another object. 

Heat flow is thermal energy in transit and heat always flows from warmer 

objects to cooler objects. (transient 

Temperature is a measure of the thermal energy contained in an object - the 

degree of hotness or coldness of an object that is measurable by any of a 

number of relative scales. 

Comments: 

“HBNDEv C9 -Transfer of heat energy can be described as either steady-state or transient 暂态 . 

In the steady-state condition, heat transfer is constant and in the same direction over time.” – 

However, In this PPT, both steady state and transient are both transient form of energy. 


The three modes of heat transfer are: 

■ conductive, 

■ convective and 

■ radiative. 

All heat is transferred by one of these three modes. In most situations, beat is 

transferred by a combination of two or all three modes. Of these three modes 

of heat transfer, infrared thermography is most closely associated with the 

radiative process, but it is essential to study all three to understand the 

meaning of thermograms and to pursue a successful program of 

thermography. As a result of heat transfer, objects tend to increase or 

decrease their temperature until they come to thermal equilibrium with their 

surroundings. To maintain a steadystate heat flow condition, energy must be 

continuously supplied by some means of energy conversion so that the 

temperature differential, and hence the heat flow remains constant. 


The three modes of heat transfer are: 

■ conductive, 

■ convective and 

■ radiative. 


http://www.chem.purdue.edu/gchelp/liquids/character.html

The three modes of heat transfer are: Water in 3 phases. 

http://dli.taftcollege.edu/streams/Geography/Animations/WaterPhases.swf 

H + 

0 

H + H 

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Water molecule 

(polarity) 

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bond 

Gas 

(Water vapor) 

u 

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Latent heat and phase changes 

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20 11lD 200 400 6Dil 

calories 

BOO 

Solid 

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Solid ====+ Liquid ==~ Gas ===to Solid ====to Gas ===to Liquid ~ Solid 

(Ice) (Water) (Water vapor) (Ice) (Water vapor) (Water) (Ice) 


http://dli.taftcollege.edu/streams/Geography/Animations/WaterPhases.html

Temperature and Temperature Scales 

Temperature is expressed in either absolute or relative terms. There are two 

absolute scales called Rankine (English system) and Kelvin (metric system). 

There are two corresponding relative scales called Fahrenheit (English 

system) and Celsius or centigrade (metric system). Absolute zero is the 

temperature at which no molecular action takes place. This is expressed as 

zero Kelvin or zero degrees Rankin (0 K or 0° R). Relative temperature is 

expressed as degrees Celsius or degrees Fahrenheit (°C or °F). The 

numerical relations among the four scales are as follows: 

converting ºC to ºF, (9/5 x ºC +32) ºF 

converting ºF to ºC, (5/9 x ºF -32) ºC 

T Rankine = T Fahrenheit+ 459.7 

T Kelvin = T Celsius + 273.16 

Exercise: Temperature (not temperature interval) 

0 ºC = 32 ºF 

thus -273.16 ºC = (-273.16 x 9/5 + 32) ºF = 459.7 ºF 



Quick Celsius (°C) I Fahrenheit (°F) Conversion: 

Conversion Tool 

Just t ype a va lue ill eitlle r box: 

oc: l -273. 15 1 

° F: I -459. 6'6999999999996 I 

Or use the I nteractive Thermometer , 

Or this m1ethod : 

° F to ° C Deduct 32, then m ultiply by 5 1 

then divide by 9 

()C to ° F Multiply by 9 1 

then divide by 5, then add 32 

( Explanation Below .. . ) 

0'C 

OF 

220 

210 

200 

90 

190 

80 180 

170 

70 160 

150 

60 140 

130 

50 120 

110 

40 

100 

30 90 

80 

20 70 

60 

10 50 

40 

0 30 

20 

10 

0 

-10 

-20 

-30 

-40 - -40 

■ 

http://www.mathsisfun.com/temperature-conversion.html 



REMEMBER 

0ºC = 32ºF 

for my ASNT exam 

converting ºC to ºF, (9/5 x ºC +32) ºF 


Boston Tea Party – New governances not the Old Fahrenheit & ⅝”. 


Boston Tea Party – New governances not the Old Fahrenheit & ⅝”. 

/ 


The Mighty Fahrenheit & ⅝”, 

English System. 


The Mighty Fahrenheit & ⅝”, English System. 


Absolute zero is equal to - 273.16 °C and also equal to approximately - 459.7 

°F. To conveIt, a change in temperature or delta T (∆T) between the English 

and metric systems, the simple 9/5 (1.8 to 1) relationship is used: 

∆T Fahrenheit (or º Rankine) = 9/5 x ∆T Celsius (or Kelvin) 

or simply; 

∆T Fahrenheit (or º Rankine) = 1.8 x ∆T Celsius (or Kelvin) 

Table 1.1 (pages 12 to 14) is a conversion table that will assist in the rapid 

conversion of temperature between fabrenheit and celsius values. 

Instructions for the use of the table are shown at the top of the table. (not in 

this PPT) 


Conductive Heat Transfer 

Conductive beat transfer is probably the simplest form to understand. lt is the 

transfer of beat in stationary media. It is the only mode of heat flow in solids, 

but it can also take place in liquids and gases. 

Conductive heat transfer occurs as the result of atomic vibrations (in solids) 

and molecular collisions (in liquids) whereby energy is moved, one molecule 

at a time, from higher temperature sites to lower temperature sites. An 

example of conductive heat transfer is when one end of a section of metal 

pipe warms up after a flame is applied to the other end. There are physical 

laws that allow the amount of conductive heat flow to be calculated, and they 

are presented here to show the factors on which conductive heat flow 

depends. 

Keywords: 

■ atomic vibrations 

■ molecular collisions (atomic collisions in inert gas) 


The Fourier conduction Law expresses the conductive heat flow, Q per unit 

area A, through a slab of solid material of thickness L as illustrated in Figure 

1.1. Thermal resistance R t is defined as: 

Thermal conductivity is defined as: 

Heat flow per unit area is defined as: 


Where: 

• Q/A = the rate of heat transfer through the slab per unit area (BTU/h∙ft 2 ) or 

(W/m 2 ) perpendicular to the flow, 

• L = the thickness of the slab (ft or m), 

• T 1 =(°F) or (ºC) is the higher temperature (at the left), 

• T 2 = the lower temperature (at the right) 

• k = the thermal conductivity of the slab material (BTU/h∙ft∙ºF) or (W/m∙K) 

• R t = the thermal resistance of the slab material (°F∙h∙ft 2 fBTU) or (m 2 ∙K/W) 


The Fourier conduction Law ( One dimension heat flow) 

The mathematical relationship that describes heat transfer as a function of the 

material that heat is conducting through is known as Fourier's law and is 

given below. 

Fourier’s Law: q = k ∙ A ∙ (T H -T C ) ∙ L -1 

Where: 

q = heat transfer per unit time (W) 

A = heat transfer area (m 2 ) 

k = thermal conductivity of material (W/m∙K) 

L = material thickness (m) 


Thermal conductivity is highest for metals such as aluminum and lower for 

porous materials such as brick. It is inversely proportional to thermal 

resistance. 

K= 1/R t 

Comment: 

k α 1/R, R= thermal resistivity and the thermal resistance R t = L∙R 

Thermal conductivity is highest for metals such as aluminum and lower for 

porous materials such as brick. It is inversely proportional to thermal 

resistance. In real terms, the Fourier expression means that the rate of heat 

flow increases with increasing temperature difference. increases with 

increasing thermal conductivity and decreases with increasing slab thickness. 

Heat flow may be expressed in English units or metric units. 


Convective Heat Transfer 

Convective heat transfer takes place in a moving medium and is almost 

always associated with heat transfer between a solid and a moving fluid (such 

as air). Forced convection takes place when an external driving force, such as 

a wind or an air pump, moves the fluid. Free convection takes place when 

there is no external driving force - the temperature differences necessary for 

heat transfer produce density changes in the fluid. The warmer fluid rises as a 

result of increased buoyancy. In convective heat flow, heat transfer takes 

effect by direct conduction through the fluid and the mixing motion of the fluid 

itself. Figure 1.2 illustrates convective heat transfer between a flat plate and a 

moving fluid. 


Figure 1.2: Convective heat flow 

Too = Free Fluid Temperature 

_______ _ ________ ______ .Tbe.rmaL __ ____ _ 

Boundary Layer 

Plate 

Tsurface 


Figure 1.2: Convective heat flow 

T ∞ 

Distance from 

boundary layer 

Thermal Boundary layer 

T surface 

fluid velocity 


The presence of the plate causes the velocity of the fluid to decrease to zero 

at the surface and influences its velocity throughout the thickness of a 

boundary layer. The thickness of the boundary layer depends on the free fluid 

velocity V ∞ - the higher the free fluid velocity, the thinner the boundary layer. 

It is greatest for free convection where V ∞ = 0. The rate of heat flow depends, 

in turn, on the thickness of the boundary layer as well as the temperature 

difference between T s and T ∞ , T s being the surface temperature and T ∞ 

being the free field fluid temperature outside the boundary layer. 


Newton's cooling law defines the convective heat transfer coefficient as: 

where: h = (BTU/b-ft2-°F) or (W/m2-K) 

This is rearranged to obtain an expression for convective heat flow per unit 

area: 

If R c = 1/h is the resistance to convective heat flow, then: 


R c is easier to use than h when determining combined conductive and 

convective heat transfer because then they are additive terms. 

In real terms, this expression means that the rate of convective heat flow 

increases with increasing temperature difference, increases with higher 

convective heat flow coefficient and decreases with increasing convective 

thermal resistance. 

Conductive and convective heat transfer are very similar. In both, the heat 

transfer is directly proportional to the temperature difference and the speed at 

which th is energy is transferred (rate of heat flow) depends on the transfer 

coefficient of the media or material through which the heat energy flows. By 

comparison, radiative heat transfer takes place in accordance with a different 

set of rules. 


Radiative Heat Transfer 

Radiative heat transfer is unlike the other two modes because: 

1. it occurs by electromagnetic emission and absorption in a manner similar 

to light; 

2. it propagates at the speed of light; 

3. like light, it requires a direct line of sight; 

4. the heat energy transferred is proportional to the fourth power T 4 of the 

temperature of the objects; and 

5. it can take place across a vacuum – in fact, a vacuum is the most efficient 

medium for radiative heat transfer. 

The electromagnetic spectrum is illustrated in Figure 1.3 and shows that X- 

rays. radio waves. light waves (ultraviolet and visible) and infrared radiation 

are all related. Radioactive heat transfer takes place in the infrared portion of 

the spectrum, from 0.75μm to about 100μm, although most practical 

measurements can be calculated to about 20μm . The symbols μm (μm is 

preferred) stand for micrometers or microns. A micron is one-millionth of a 

meter and the measurement unit for radiant energy wavelength. Wavelength 

is inversely related to frequency (longer wavelengths have lower frequencies). 


Figure 1.3: Infrared in the electromagnetic spectrum 

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0.4 

0.75 1.0 

1.5 2.0 3.0 5.0 10 20 

Wavelength (Jtm) 

30 

Practical Infrared Thermography λ; 2μm to 6μm and 8μm to 14μm 


Figure 1.4: Infrared radiation leaving a target surface (ρετσ) 

T, 

Reflected Radiation (W,) 

Radiation (We) 

Ɛ 

smitted Radiation (W~ 

ρ 

τ 

%We+ % W,+ % Wt= 100% 

..----- Target Surface 

We=

1.3 Fundamentals of Radiative Heat Flow 

Radiation Exchange at the Target Surface 

The measurement of infrared/thermal radiation is the basis for non-contact 

temperature measurement and infrared thermography. The surface to be 

evaluated is called the target surface. Thermal infrared radiation leaving a 

surface is called exitance or radiosity. It can be emitted from the surface, 

reflected by the surface, or transmitted through the surface. This is illustrated 

in Figure 1.4. 

The total radiosity is equal to the sum of the emitted component (W e ), the 

reflected component (W r ) and the transmitted component (W t ). 

It is important to note that the surface temperature T e is related to the emitted 

component W e only. 

Keywords: 

■ Exitance 

■ Radiosity 


Thermal infrared radiation impinging on a surface can be absorbed, reflected, 

or transmitted as illustrated in Figure 1.5. Kirchhoff's law states that the sum 

of the three components is always equal to the total received radiation, E t The 

fractional sum of the three components equals unity or 100 percent: 

E t = E α + E ρ + E τ , (for blackbody E Ɛ = E α ) 

where: 

E t = total energy 

Likewise, the sum of the three material properties, transmissivity, reflectivity 

and emissivity, also always equals unity: 

τ + ρ + Ɛ =1 


Figure 1.5: Infrared radiation impinging on a target surface 

Radiant Energy 

Source 

Material Properties 

a. = absorptivity } 

p = reflectivity a + p + -r = 1 

-r = transmissivity 

Et 

Total 

Incoming 

Energy 

E 

Abso

Reflections off Specular and Diffuse Surfaces 

A perfectly smooth surface will reflect incident energy at an angle 

complementary to the angle of incidence as shown in Figure 1.5. This is 

called a specular reflector. A completely rough or structured surface will 

scatter or disperse all of the incident radiation. This is called a diffuse reflector. 

No perfectly specular or perfectly diffuse surface can exist in nature, and all 

real surfaces have some diffusivity and some specularity. These surface 

characteristics will determine the type and direction of the reflected 

component of incident radiation. When making practical measurements, the 

specularity or diffusivity of a target surface are taken into account by 

compensating for the effective emissivity (Ɛ*) of the surface. The 

thermographer's use of effective emissivity is reviewed as part of the detailed 

discussion of equipment operation in Chapter 5. 

Keywords: 

■ Specular reflector 

■ Diffuse reflector 



Specular Reflection 



d i ffu s ~e 

reflection 


Transient Heat Exchange 

The previous discussions of the three types of heat transfer deal with steady 

state heat exchange for reasons of simplicity and comprehension. Heat 

transfer is assumed to take place between two points, each of which is at a 

fixed temperature. However, in many applications, temperatures are in 

transition so that the values shown for energy radiated from a target surface 

are the instantaneous values at the moment measurements are made. In 

many instances, existing transient thermal conditions are exploited to use 

thermography to reveal material or structural characteristics in test articles. In 

infrared nondestructive testing of materials, thermal injection or active 

thermography techniques are used to generate controlled thermal transient 

flow based on the fact that uniform structural continuity results in predictable 

thermal continuity. These techniques will be discussed in greater detail in 

Chapter 5. 


Radiant Energy Related to Target Surface Temperature 

All target surfaces warmer than absolute zero radiate energy in the infrared 

spectrum. Figure 1.6 shows the spectral distribution of energy radiating from 

various idealized target surfaces as a function of surface temperature (T) and 

wavelength (A.). Very hot targets radiate in the visible as well, and our eyes 

can see this because they are sensitive to light. The sun, for example, is at a 

temperature of about 6000 K and appears to glow white bot. The heating 

element of an electric stove at 800 K glows a cherry red and, as it cools, it 

loses its visible glow but continues to radiate. This radiant energy can be felt 

with a hand placed near the surface even though the glow is invisible. The 

idealized curves shown in Figure 1.6 are for perfect radiators known as 

blackbodies. Blackbodies are defined and discussed in greater detail later in 

this chapter. Figure 1.6 also shows two key physical laws regarding infrared 

energy emitted from surfaces. 


Radiant Energy Related to Target Surface Temperature 

All target surfaces warmer than absolute zero radiate energy in the infrared 

spectrum. 

Copyright© gnurf • http :1/Vecto . rs/25 


The Stefan-Boltzmann law: W= σƐT 4 

Where: 

W = radiant flux emitted per unit area (W/m 2 ) 

Ɛ = emissivity (unity for a blackbody target) 

σ = Stefan-Boltzmann constant= 5.673 x I0 -8 W/m -2 ∙K -4 

T = absolute temperature of target (K) 

(Comments: for blackbody Ɛ=1, α=Ɛ.) 

illustrates that W, the total radiant flux emitted per unit area of surface, (the 

area under the curve) is proportional to the fourth power of the absolute 

surface temperature T 4 . It is also proportional to a numerical constant σ, and 

the emissivity of the surface, Ɛ. 


Figure 1.6: Typical blackbody distribution 

curves and basic radiation laws 

Stefan-Boltzmann Law 

Radiant Flux per Unit Area In W/cm 2 

W= σƐT 4 

Ɛ = emissivity (unity for a blackbody target) 

σ = Stefan-Boltzmann constant 

= 5.673 x I0 -8 W/m -2 ∙K -4 

T = absolute temperature of target (K) 

Wien's Displacement Law 

λ max = b/T 

where: λ max = peak wavelength (μm) 

b = Wien's displacement constant 

(2897 or 3000 approximately) 


Figure 1.6: Typical blackbody distribution curves and basic radiation laws 

Am ax 

10 8 

10 7 3,000K 

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10- 1 0.1 0.4 0 .8 1.0 10 100 1,000 

Wavelength in Micrometers ( J.Lm) 


Wien's displacement law: 

λ max = b/T 

Where: 

λ max wavelength of maximum radiation (μm) 

b Wien's displacement constant or 2897 (μm∙K) 

illustrates that the peak wavelength, λ max (μm) at which a surface radiates, is 

easily determined by dividing a constant b (approximately 3000) by the 

absolute temperature T (Kelvin) of the surface. 


1.4 Practical Infrared Measurements 

ln practical measurement applications, the radiant energy leaves a target 

surface, passes through some transmitting medium. usually an atmospheric 

path, and reaches a measuring instrument. 

Therefore, when making measurements or producing a thermogram, three 

sets of characteristics must be considered: 

1. characteristics of the target surface, 

2. characteristics of the transmitting medium and 

3. characteristics of the measuring instrument. 

This is illustrated in Figure 1.7. 


Figure 1.7: Three sets of characteristics of the infrared measurement 

problem 

Ɛ obj 

ρ amb 

τ assumed = 0 

Ɛ atm 

τ atm 


Characteristics of the Target Surface 

Target surfaces are separated into three categories; blackbodies, graybodies 

and nongraybodies (also called real bodies, selective radiators or spectral 

bodies). 

The target surfaces shown in Figure 1.6 are all perfect radiators (or 

blackbodies). A blackbody radiator is defined as a theoretical surface having 

unity emissivity at all wavelengths and absorbing all of the radiant energy 

impinging upon it. 

Emissivity, in turn, is defined as the ratio of the radiant energy emitted from a 

surface to the energy emitted from a blackbody surface at the same 

temperature. Blackbody radiators are theoretical and do not exist in practice. 

The surface of most solids are graybodies, that is, surfaces with high 

emissivities that are fairly constant with wavelength. Figure 1.8 shows the 

comparative spectral distribution of energy emitted by a blackbody, a 

graybody and a nongraybody, all at the same temperature (300 K). 


Figure 1.8: Spectral distribution of a blackbody, graybody and nongraybody 

100 

Blackbody at 300 K 

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H- 

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1 5 10 15 20 

Wavelength (llm) 


Referring back to Figure 1.5, the total exitance available to the measuring 

instrument has three components: 

• emitted energy (We), 

• reflected energy (Wr) from the environment and other reflecting sources, 

and 

• for nonopaque targets, energy transmitted through the target (Wt) from 

sources behind the target. 

Because a theoretical blackbody has an emissivity Ɛ of 1.00, it will reflect and 

transmit no energy ρ = 0, τ = 0. 

Real targets, however, are not blackbodies. and figure 1.9 shows the three 

components that comprise Wx, the total exitance that an instrument sees 

when aimed at a real Ufe target surface. Because only the emitted 

component, We, is related to the temperature of the target surface, it 

becomes apparent that a significant part of the measurement problem is 

eliminating or compensating for the other two components. This is discussed 

in greater detail in Chapter 4. 


Figure 1.9: Components of energy reaching the measuring instrument 

Target Surface Properties 

e = emissivity 

p :::: reflectivity 

} 

e + p + t = 1 

1: =transmissivity 

(Target Exitance or Radiosity) 

Wx= We+ W,+ Wt 


Characteristics of the Transmitting Medium 

Because lhe infrared radiation from the target passes through some 

transmitting medium on its way to the target, the transmission and emission 

characteristics of the medium in the measurement path must be considered 

when making non contact thermal measurement. No loss of energy or self 

emission (Ɛ atm ) is encountered when measuring through a vacuum. However. 

most measurements are made through air. For short path length (a few 

meters, for example), most gases (including the atmosphere) absorb and emit 

very little energy and can be ignored. However. when highly accurate 

temperature measurements are required, the effects of atmospheric 

absorption must be taken into account. (τ atm , Ɛ atm ). 


As the path length increases to more than a few meters, or as the air 

becomes heavy with water vapor, atmospheric absorption may become a 

significant factor. Therefore, it is necessary to understand the infrared 

transmission characteristics of the atmosphere. Figure 1.10 illustrates the 

spectral transmission characteristics of a 10 m (33 ft) path of ground level 

atmosphere at a temperature of 25 °C (77 °F) and 50 percent humidity. 

It is immediately apparent that the atmosphere is not as transparent in the 

infrared ponion of the spectrum as it is in the visible ponion. Two spectral 

intervals have very high transmission. These are known as the 3 to 5 μm and 

the 8 to 14μm atmospheric windows, and almost all infrared sensing and 

imaging instruments are designed to operate in one of these two windows. 

The absorption segments shown in Figure 1.10 were formed by carbon 

dioxide and water vapor, which are two of the major constituents in air. For 

measurements through gaseous media other than atmosphere, it is 

necessary to investigate the transmission spectra of the medium before 

validating the measurements, which is explained in greater detail in Chapter 2. 


Figure 1.10; Transmission of 10m (33ft) of ground level atmosphere at 50 

percent humidity and 25 °C (77ºF) 

Percentage Transmission 

80 

70 

=o 

0 

0 

0 

0 

D 

Wave Length μm 


When there is a solid material, such as a glass or quartz viewing port, 

between the target and the instrument, the spectral characteristics of the solid 

media must be known and considered. Figure 1.11 shows transmission 

curves for various samples of glass. Most significant is the fact that glass 

does not transmit infrared energy at 10μm where ambient (30 °C, 86 °F) 

surfaces radiate their peak energy. In practice, infrared thermal 

measurements of ambient targets can never be made through glass. One 

practical approach to this problem is to eliminate the glass, or at least a 

portion through which the instrument can be aimed at the target. If a window 

must be present for personal safety, vacuum, or product safety, a material 

might be substituted that transmits in the longer wavelengths. Figure 1.12 

shows the spectral transmission characteristics of several infrared 

transmitting materials, many of which transmit energy past 10μm. In addition 

to being used as transmitting windows, these materials are often used as 

lenses and optical elements in infrared sensors and imagers. Of course, as 

targets become hotter, and the emitted energy shifts to the shorter 

wavelengths, glass and quartz windows pose less of a problem and are even 

used as elements and lenses in high temperature sensing instruments. 

Characteristics of the measuring instrument are addressed in Chapter 4. 


Figure 1.11: Transmission, absorption and reflectance characteristics of 

glass 

C1) 

u 

c: 

C'O 

Q,)- 

rn(,) 

c~ 

ocu 

100 

90 

D. a: 80 

rn, 

Cl.)c 70 

a:C'O 

-('OCI.) 

60 

~u 

-c 

UC'O 

Q,)t: 

Q.,_ 

50 

cne 40 

Cl.)tn 

;::C'O 

>C: 30 

~-= 20 Q,)- 

a:c: 

C1) 

u 

~ 

10 

Transmission of 

Glass Envelopes 

Transmission 

0.2mm 

Absorpt~on 

Reflectance of 1.3 mm 

Thick Glass Sample 

cf 

1 

2 3 4 

5 6 7 8 

Wavelength (11m) 

9 10 11 


Figure 1.12: Transmission curves of various infrared transmitting material 

c: 100 

0 90 

~ 80 

- 70 

~ 60 

c: 50 

tV 

t!: 40 

- 30 

c: 

Q) 

20 

e 10 

0.. 1 

Germanium (ar-coated at 10 J..Lm) 

Transmission 

Q) ~--------------~----~------ 

c: 100 

0 90 

·- (/) 80 

!! 70 

E 60 

(/) 

c: 50 

tV 

j!: 40 

- 30 

c: 20 

Q) 

5 10 15 20 

Wavel ength (~m) 

Zinc Selenide (ZnSe) 

Transmission 

~ 10 L-~------~------~------~----- 

~ 1 5 10 15 20 

Wavelength (11m) 

c: 100 

0 90 

·- (/) 80 

!! 70 

E so 

(/) 

c: 50 

CIS 

j!: 40 

... 30 

c: 20 

Q) 

Transmission 

KR5-5 

5 10 15 20 

Wavelength (~m) 

Sapphire 

~ 10 L-----------------------~------ 

Q) 

Q. 1 5 10 15 20 

Wavelength (~m) 


Figure 1.12: Transmission curves of various infrared transmitting material 

c 100 

0 90 

- Cll 80 

Cll 

·- 70 

E 60 

Cll 

c 50 

~ 40 

_ 30 ransc 

20 mission 

Fused Quartz (Si0 2 ) 

G.l 

~ 10 

G.l ~~----~----------------~--- 

0.. 1 5 10 15 20 

Wavelength (Jlm) 

50 

40 

30 

20 

10 

1 

Barium Fluoride (BaFJ 

5 10 15 20 

Wavelength (Jlm) 


Convective Heat Transfer 

Convective heat transfer, often referred to simply as convection, is the transfer of heat from one place to 

another by the movement of fluids. Convection is usually the dominant form of heat transfer in liquids and 

gases. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the 

combined processes of conduction (heat diffusion) and advection (heat transfer by bulk fluid flow). The term 

convection can sometimes refer to transfer of heat with any fluid movement, but advection is the more precise 

term for the transfer due only to bulk fluid flow. The process of transfer of heat from a solid to a fluid, or the 

reverse, is not only transfer of heat by bulk motion of the fluid, but diffusion and conduction of heat through the 

still boundary layer next to the solid. Thus, this process without a moving fluid requires both diffusion and 

advection of heat, a process that is usually referred to as convection. Convection that occurs in the earth's 

mantle causes tectonic plates to move. Convection can be "forced" by movement of a fluid by means other than 

buoyancy forces (for example, a water pump in an automobile engine). Thermal expansion of fluids may also 

force convection. In other cases, natural buoyancy forces alone are entirely responsible for fluid motion when 

the fluid is heated, and this process is called "natural convection". An example is the draft in a chimney or 

around any fire. In natural convection, an increase in temperature produces a reduction in density, which in turn 

causes fluid motion due to pressures and forces when fluids of different densities are affected by gravity (or any 

g-force). For example, when water is heated on a stove, hot water from the bottom of the pan rises, displacing 

the colder denser liquid, which falls. After heating has stopped, mixing and conduction from this natural 

convection eventually result in a nearly homogeneous density, and even temperature. Without the presence of 

gravity (or conditions that cause a g-force of any type), natural convection does not occur, and only forcedconvection 

modes operate. The convection heat transfer mode comprises one mechanism. In addition to 

energy transfer due to specific molecular motion (diffusion), energy is transferred by bulk, or macroscopic, 

motion of the fluid. This motion is associated with the fact that, at any instant, large numbers of molecules are 

moving collectively or as aggregates. Such motion, in the presence of a temperature gradient, contributes to 

heat transfer. Because the molecules in aggregate retain their random motion, the total heat transfer is then 

due to the superposition of energy transport by random motion of the molecules and by the bulk motion of the 

fluid. It is customary to use the term convection when referring to this cumulative transport and the term 

advection when referring to the transport due to bulk fluid motion. 


http://en.wikipedia.org/wiki/Convective_heat_transfer

Chapter 1 

Review Questions 

Q&A 

1. b 

2. d 

3. c 

4. a 

5. c 

6. d 

7. b 

8. b 

9. d 

10. d 

11. a 

12. a 

13. d 

14. e 

I5. d 

16. e 

17. b 

18. d 

19. a 

20. d 

21. b 

22. e 


Q1. At a temperature of absolute zero: 

a. hydrogen becomes a liquid. 

b. all molecular motion ceases. 

c. salt water is part solid and part liquid. 

d. fahrenheit and celsius readings are the same. 

Q2. Conductive heat transfer cannot take place: 

a. within organic materials such as wood. 

b. between two solid materials in contact. 

c. between dissimilar metals. 

d. across a vacuum. 

Q3. The only three modes of heat transfer are: 

a. resistive, capacitive and inductive. 

b. steady state, transient and reversible. 

c. conduction, convection and radiation. 

d. conduction. convection and absorption. 


Q4. Heat can only flow in the direction from: 

a. hotter objects to colder objects. 

b. colder objects to houer objects. 

c. more dense objects to less dense objects. 

d. larger object to smaller objects. 

Q5. Thermal resistance is: 

a. analogous to electrical current. 

b. proportional to the fourth power of emissivity. 

c. inversely proportional to the rate of heat flow by conduction. 

d. a measure of material stiffness. 

Q6. The radiation of thermal infrared energy from a target surface: 

a. occurs most efficiently in a vacuum. 

b. is proportional to the fourth power of the absolute surface temperature. 

c. is directly proportional to surface emissivity. 

d. is all of the above. 


Q7. The mode of heat transfer most closely associated with infrared 

thermography is: 

a. induction. 

b. radiation. 

c. convection. 

d. conduction. 

Q8. To convert a fahrenheit reading to celsius: 

a. divide by 1.8. 

b. subtract 32 and divide by 1.8. 

c. multiply by 1.8 and add 32. 

d. add 273. 

Q9. Thermal radiation reaching the surface of an object can be: 

a. absorbed only in the presence of atmosphere. 

b. reflection and absorbed only in a vacuum. 

c. transmitted only if the surface is organic. 

d. absorbed, reflected and transmitted. 


Q10. The follow ing spectral band is included in the infrared spectrum: 

a. 0.1 to 5.5 μm. 

b. 0.3 to 10.6 μm. 

c. 0.4 to 20.0 μm. 

d. 0.75 to 100 μm. 

Q11. Mosl instruments used in infrared thermography operate somewhere 

within the; 

a. 2 to 14 μm spectral region. 

b. 5 to 10 μm spectral region. 

c. 10 to 20 μm spectral region. 

d. 20 to 100 J μm spectral region. 

Q12. As a surface cools, the peak of its radiated infrared energy: 

a. shifts to longer wavelengths. 

b. shifts to shorter wavelengths. 

c. remains constant if emissivity remains constant. 

d. remains constant even if emissivity varies. 


Q13. The peak emitting wavelength of a 300 °C (572 ° F) blackbody is 

approximately: 

a. 1.5 μm. 

b. 3 μm. 

λ max = b/T( in K) = 2897/573 μm 

0. 10 μm. 

d. 5 μm. 

Q14. An opaque surface with an emissivity of 0.04 would be: 

a. transparent to infrared radiation. 

b. a fairly good emitter. 

c. almost a perfect reflector. (τ=0, Ɛ=0.04, ρ = 0.96) 

d. almost a perfect emitter. 

Q15. If a surface has an emissivity of 0.35 and a reflectivity of 0.45. its 

transmissivity would be: 

a. impossible to detennine without additional information. 

b. 0.80. 

c. 0.10. 

d. 0.20. [1-(0.35+0.45)] 


Q16. In forced convection, the boundary layer: 

a. increases as the fluid velocity increases. 

b. remains the same as the fluid velocity increases. 

c. decreases as the fluid velocity increases. 

d. increases in proportion to the fourth power of the fluid velocity. 

Q17. When heating one end of a car key to thaw a frozen automobile door 

lock, heat transfer from the key to the lock is an example of: 

a. forced convection. 

b. conductive heat transfer. 

c. free convection. 

d. radiative heat transfer. 

Q18. The infrared atmospheric window that transmits infrared radiation best is 

the: 

a. 2.0 to 3.0 μm region. 

b. 3.0 to 6.0 μm region. 

c. 6.0 to 9.0 μm region. 

d. 9.0 to 11.0 μm region. 


Q19. The spectral band in which glass transmits infrared radiation best is the: 


b. 3.0 to 6.0 μm region. 



Q20. Reflectance of infrared radiation by a glass surface is greatest in the: 


h. 3.0 to 6.0 μm region. 



Q21. A diffuse reflecting surface is: 

a. a polished surface that reflects incoming energy at a complementary angle. 

b. a surface that scatters reflected energy in many directions. 

c. also called a specular reflecting surface. 

d. usually transparent to infrared radiation. 


Q22. In the 8 to 14 μm spectral region: 

a. the atmosphere absorbs infrared radiant energy almost completely. 

b. the atmosphere reflects infrared radiant energy almost completely. 

c. the atmosphere transmits infrared energy very efficiently. 

d. infrared instruments do not operate very accurately. 


Chapter 2 

Materials and Their Properties 


2.1 Materials Characteristics 

A knowledge of the characteristics of materials is important to the 

thermographer for numerous reasons, but the two most important arc the 

need to know how a particular target surface e mits. transmits and refl ects 

infrared radiant energy. and the need 10 know how heat flows within a 

particular material. 

2.2 Surface Properties of Materials 

The surface properties of materials include emissivity. reflectivity and 

transmissivity. 


Emissivity Ɛ 

When using infrared thermography to measure surface temperature of a 

target. it is essential to know the effective emissivity (Ɛ*) of the surface 

material. This is the value that must be set into the instrument's menu under 

the specific conditions of measurement for the instrument to display an 

accurate surface temperature value. When attempting to make temperature 

measurements on a target of unknown emissivity. an estimate of emissivity 

may be the only available alternative. There are numerous reference tables 

available that list generic values of emissivity for common materials and these 

can be used as guides. Table 2.2 is an example of a reference table. As 

previously noted. emissivity depends on the material and the surface texture. 

It may also vary with surface temperature and with the spectral interval over 

which the measurement is made. These variations, though usually small , 

cannot always be ignored. 


For an emissivity reference table to be useful. conditions of target 

temperature and spectral interval (wavelength) must also be presented. If the 

temperature and wavelength listed do not correspond to the actual 

measurement conditions. the emissivity listed must be considered to be a 

rough estimate. Even if there is an exact match to the measurement 

conditions, the lookup method is not the best approach for accurate 

temperature measurement. Ideally. the way to determine effective 

emissivity is to measure it with one of the several established protocols. using 

a sample of the actual target surface material and the actual instrument to be 

used for the measurement mission. The protocols for measuring effective 

emissivity of material samples are discussed in Chapter 4. 


Reflectivity ρ 

Reflectivity of a surface generally increases as emissivity decreases. For 

opaque graybody surfaces τ=0. the sum of emissivity and reflectivity is unity 

(1.0). Therefore. an opaque graybody surface with a low effective cmissivity 

will be highly reflective, which can result in erroneous temperature readings 

even if the correct emissivity is set into the instrument. These errors can be 

the result of either point source reflections, background reflections or both 

entering the instrument . There are two components of reflected energy the 

diffuse componenl and the specular component. If the surface is relatively 

specular (smooth). most of the reflected energy is specular, that is. it reflects 

off the surface at an angle complementary to the angle of incidenct. If the 

surface is relatively diffuse (textured) most of the renected energy is scattered 

uniformly (haphazardly?) in all directions regardless of the angle of incidence. 

Keywords: 

Therefore. an opaque graybody surface with a low effective cmissivity will be 

highly reflective 


Errors because of point source reflections are usually larger when the target 

surfaces are specular, and errors because of background reflections are not 

affected by the specularity or diffusivity of the target surface. Both types of 

reflective errors are more serious when the target surface is cool compared to 

the temperature of the point source or the background because the point 

source makes a greater contribution to the total radiant exitance than the 

target does. In practice, the thermographer can learn to recognize and avoid 

errors due to point source reflections. The thermographer also can learn to 

measure and compensate for errors due to background reflection. This is 

discussed in Chapter 4. 


Transmissivity τ 

When the target surface is a non-graybody, the target material may be partly 

transparent to infrared radiation. This means the target material has a 

transmissivity greater than 0. Due to this transparency. radiant thermal energy 

may be transmitted through the target from sources behind the target. This 

energy may enter the instrument and cause temperature measurement errors 

even if the correct emissivity is set into the instrument and reflective errors 

are eliminated. Although errors due to transmission are the least common in 

practice. errors due to energy transmiued through the target usually require 

the most sophisticated procedures to correct them. In most cases, spectral 

filtering is the best solution. Methods for correcting these errors are discussed 

in Chapters 4 and 5. 

Keywords: 

■ spectral filtering 

■ non-graybody (could be any others like black body, selective emitter, could 

be a body with τ > 0) 


View Angle 

The angle between the instrument's line of sight and the surface material will 

have a minimal effect on the material properties described above, providing 

this angle is kepi as close as possible to normal (perpendicul ar) and no 

greater than ±30 degrees from normal (for many nonmetallic surfaces this 

may be increased 10 as large as ±60 degrees from normal. if unavoidable). 

If it is not possible to view a target at an angle within this range, the effective 

emissivity may Change. particularly if it is low to begin with. This will most 

likely compromise the accuracy of temperature measurements. Note that the 

emissivities listed in Table 2.2 are normal emissivities and are not valid at 

acute viewing angles. On curved (nonflat) surfaces. view angle can be even 

more critical and measurements should be made cautiously. 

Note: 

An acute angle is an angle whose degree measure is greater than 0 but less 

than 90. 


2.3 Heat Conducting Properties of Materials 

The use of infrared themlography for nondestructive material testing is 

generally based on the assumption that uniform structural continuity provides 

uniform thermal continuity. Both unstimulated and stimulated approaches to 

thermographic material testing depend on this assumption. as will be 

discussed in greater detail in Chapters 4 and 5. It is necessary. therefore, that 

the thermographer have a clear basic understanding of the manner in which 

heat flows within a material and the material properties that affect this flow. 

Keywords: 

The use of infrared themlography for nondestructive material testing is 

generally based on the assumption that uniform structural continuity provides 

uniform thermal continuity. 


Thermal Conductivity 

Thermal conductivity k is the relative one dimensional capability of a material 

to transfer heat. It affects the speed (thus time, t) that a given quantity of heat 

applied to one point in a slab of material will travel a given distance within that 

material to another point cooler than the first. Thermal conductivity is high for 

metals and low for porous materials. It is logical. therefore. that heat will be 

conducted more rapidly in metals than in more porous materials. Although 

thermal conductivity varies slightly with temperature in solids and liquids and 

with temperature and pressure in gases, for practical purposes it can be 

considered a constant for a particular material. Table 2.1 is a list of thermal 

properties for several conunon materials. 


Heat Capacity 

The heat capacity of a malerial or a structure describes its ability to store heat. 

It is the product of the specific thermal energy C p and the density ρ of the 

material. When thermal energy is stored in a structure and then the structure 

is placed in a cooler environment, the sections of the structure that have low 

heat capacity will change temperature more rapidly because less thermal 

energy is stored in them. Consequently, these sections will reach thermal 

equilibrium with their surroundings sooner than those sections with higher 

heat capacity, The term thermal capacitance is used to describe heat capacity 

in terms of an electrical analog. where loss of heat is analogous to loss of 

charge on a capacitor. Structures with low thermal capacitance reach 

equilibrium sooner when placed in a cooler environmcnt than those with high 

thermal capacitance. This phenomenon is exploited when performing 

unstimulated nondestructive testing of structures, specifically when locating 

water saturated sections on flat roofs. This is discussed in greater detail in 

Chapter 5, 


Thermal Diffusivity 

As in emissivity Ɛ. the heat conducting properties of materials may vary from sample 

to sample. depending on variables in the fabrication process and other factors. 

Thermal diffusivity α is the 3D expansion of thermal conductivity in any given material 

sample. Diffusivily relates more to transient heat flow, whereas conductivity relates to 

steady state heat flow. It takes into account the thermal conductivity k of the sample, 

its specific heat C p 

, and its density ρ. Its equation is 

α = k/ρ C p cm 2 s -1 . 

Because thermal diffusivity of a sample can be measured directly using infrared 

thermography, it is used extensively by the materials flaw evaluation community as an 

assessment of a test sample's ability to carry heat away, in all directions, from a heat 

injection site. Table 2.1 lists thermal diffusivities for several common materials in 

increasing order of thermal diffusivity. Several protocols for measuring the thermal 

diffusivity of a test sample are described by Maldague. 



Diffusivily relates more to transient heat 

flow, whereas conductivity relates to 

steady state heat flow. 


Partial 2.1 

Table 2.1: 

d i'ffusivity) 

Thermal properties of common materials (in order of Increasing thermal 

i 

Thermal Thermal Specific 

Diffusivity Conductivity Heat 

Density 

Material K c 

(g/cm 3 ) 

(cm 2Js) (calls-em- C) (cal/g- C) 

Polyisopreoe 

7.709 10 4 3.202 10 4 0.455 0.913 

Pine (parallel to 2.06 10 3 6..2l 10 4 0.669 0.45 

grain) 

Water 1.45 lO 3 1.443 [ 0 3 0.998 0.997 

I 

Glass 

3.43 10 } 1.86 10 3 0.201 2.7 


Partial Table 2.1 

Table 2.1: 

d i'ffusivity) 

Thermal properties of common materials (in order of Increasing thermal 

i 

Thermal Thermal Specific 

Diffusivity Conductivity Heat 

Density 

Material K c 

(g/cm 3 ) 

(cm 2Js) (calls-em- C) (cal/g- C) 

Polyisopreoe 

7.709 10 4 3.202 10 4 0.455 0.913 

Pine (parallel to 2.06 10 3 6..2l 10 4 0.669 0.45 

grain) 

Water 1.45 lO 3 1.443 [ 0 3 0.998 0.997 

I 

Glass 

3.43 10 } 1.86 10 3 0.201 2.7 


Partial Table 2.2 

Table 2.2: 

Normal spectr.al emlsslvliHes of common materials 

ateriaJ Temperature a elength Emissivity 

J.lm E 

I 

oc 

Alumina. brirek 17 2-5 0.68 

AJunrinuULpolished 0 8-14 0.05 

Aluminum, rough urface 0 8-14 0.07 

Aluminum, stJ:iongly oxidized 0 8-14 0.25 

Alun1inum foil" bright 17 2-5 0.09 

Asbestos board 0 8-14 0.96 

As be . tos fabric 0 8-14 0.78 

Asbestos paper 0 8-14 0.94 

Asbestos late 0 8-14 0.96 



As in emissivity Ɛ. the heat conducting properties of materials may vary from sample to sample. depending on 

variables in the fabrication process and other factors. Thermal diffusivity α is the 3D expansion of thermal 

conductivity in any given material sample. Diffusivily relates more to transient heat flow, whereas conductivity 

relates to steady state heat flow. It takes into account the thermal conductivity k of the sample, its specific heat 

Cp, and its density ρ. Its equation is 

α = k/ρ ∙ C p cm 2 s -1 . 



Chapter 2 


Q&A 

1. c 

2. b 

3. a 

4. d 

5. a 

6. b 

7. a 

8. b 

9. b 

10. b 


1. The best way to determine the effective emissivity of a target surface is: 

a. to look it up in a table. 

b. to calcu late it. 

c. to measure the effective emissivity of the material itself or a similar 

sample. 

d. all of the above. 

2. For an opaque graybody target surface, emissivity equals: 

a. 1/refleclivity. 

b. 1-reflectivity. 

c. 1.0. 

d. reflectivity to the fourth power. 

3. The effective emissivity of a surface is always affected by: 

a. the material, its surface texture and the viewing angle. 

b. the material, its thermal conductivity and humidity. 

c. the material, its surface texture and its thermal diffusivity. 

d. the material, its visible color and its thermal conductivity. 


4. When measuring the temperature of a nongraybody target: 

a, the viewing angle is not critical. 

b. always assume an emissivity of 1.0. 

c. reflections off the near surface may be ignored. 

d. errors may be caused by hot sources behind the target. 

5. The effective emissivity of a target surface: 

a, can vary at different wavelengths. 

b. is the same for all wavelengths if the viewing angle is kept constant. 

c. is always higher at longer wavelengths. 

d. is always lower at longer wavelengths. 

6. Unfinished, unoxidized metal surfaces usually have: 

a. high and uniform emissivities. 

b. low and uniform emissivities. 

c. non-graybody characteristics. 

d. low specular reflectivity. 


7. Thermal diffusivity is: 

a. high for metals and low for porous materials. 

b. the same for all metals. 

c, low for metals and high for porous materials. 

d. the same for all porous materials. 

8. Thermal diffusivity is: 

a, the same as diffuse reflectivity. 

b. related more to transient heat flow than to steady Slale heat flow. 

c. related more 10 steady stale heat flow than to transient heat flow. 

d. the same as spectral transmittance. 

9. Thermal capacitance: 

a. describes the heating of a condenser. 

b. expresses the heat capacity of a material in a form analogous to 

electrical capacitance. 

c. is zero for a blackbody radiator. 

d. describes the maximum temperature rating of a capacitor. 


10. A highly textured surface is said to be diffuse. A smooth surface is said to 

be: 

a. opaque. 

b. specular. 

c. convex. 

d. transparent. 


Chapter 3 

Thermal Instrumentation 


3.1 Thermal Instrumentation Overview 

Equipment for temperature measurement and thermography includes 

contacting as well as noncontacting devices. Contacting devices for 

temperature measurement include thermopiles. thermocouples, liquid 

thermometers, gas expansion devices (bourdon gas thermometers), liquid 

crystals (cholesterol crystals ?), heat flux indicators and fiber optic sensors. 

Aside from some specialized instruments, the vast majority of noncontacting 

temperature measurement devices are infrared sensing instruments and 

systems. Infrared sensing instruments and systems are divided into (1) point 

sensors (radiation thermometers), (2) line scanners and (3) thermal imagers. 

This chapter begins with a review of contacting thermal measurement 

instruments and a discussion of the basic configurations of infrared sensing 

and imaging instruments. This is followed by a discussion of performance 

parameters and, finally, descriptions of commercial thermal sensing and 

imaging equipment, thermographic image processing software and image 

hard copy recording accessories. 


What is Thermopile 

A thermopile is an electronic device that converts thermal energy into electrical energy. 

It is composed of several thermocouples connected usually in series or, less 

commonly, in parallel. Thermopiles do not respond to absolute temperature, but 

generate an output voltage proportional to a local temperature difference or 

temperature gradient. 

Thermopiles are used to provide an output in response to temperature as part of a 

temperature measuring device, such as the infrared thermometers widely used by 

medical professionals to measure body temperature. They are also used widely in 

heat flux sensors (such as the Moll thermopile and Eppley pyrheliometer) and gas 

burner safety controls. The output of a thermopile is usually in the range of tens or 

hundreds of millivolts. As well as increasing the signal level, the device may be used 

to provide spatial temperature averaging. Thermopiles are also used to generate 

electrical energy from, for instance, heat from electrical components, solar wind, 

radioactive materials, or combustion. The process is also an example of the Peltier 

Effect (electric current transferring heat energy) as the process transfers heat from the 

hot to the cold junctions. 


http://en.wikipedia.org/wiki/Thermopile

Thermopile- Thermoelectric Seebeck module 


http://en.wikipedia.org/wiki/Thermopile

The Working Principle: Thermopile, composed of multiple thermocouples in 

series. If both the right and left junctions are the same temperature, voltages 

cancel out to zero. However if one side is heated and other side cooled, 

resulting total output voltage is equal to the sum of junction voltage 

differentials. 


What is a IR Thermopile? (non-contact) 

A thermopile is a serially-interconnected array of thermocouples, each of 

which consists of two dissimilar materials with a large thermoelectric power 

and opposite polarities. The thermocouples are placed across the hot and 

cold regions of a structure and the hot junctions are thermally isolated from 

the cold junctions. The cold junctions are typically placed on the silicon 

substrate to provide effective heat sink. In the hot regions, there is a black 

body for absorbing the infrared, which raises the temperature according to the 

intensity of the incident infrared. These thermopiles employ two different 

thermoelectric materials which are placed on a thin diaphragm having a low 

thermal conductance and capacitance. 


http://www.ge-mcs.com/download/temperature/930-164A-LR.PDF

IR Thermopiles Sensor (non-contact) 

object 

Ob)I!Ct 

Ob) I!Ct 

ob) I!Cl 

ob) CCl 

Z60.ZS 

Z60. 39 

t~ : Z60.'9 

l~ : Z60. 71 

tet'lp: Z60.85 

aMbient t~ : 030.67 

OMb\ent t~ : 030.69 

onb\ent t~ : 030.71 

aMbient t~ : 030. 71 

aMbient t~ : 030.73 


IR Thermopile Quad Sensor (non-contact) 


Thermocouple 

General description: Thomas Seebeck discovered in 1821 that when two wires composed of 

dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous 

current which flows in the thermoelectric circuit. (Seebeck effect). The junctions can be exposed, 

grounded or ungrounded. The thermocouple is normally directly connected to a standard 

temperature controller. Thermocouples are among the easiest temperature sensors used in 

science and industry and very cost effective. (usually less than $50.00) 

thermocouple embedded in 

Dalton cartridge heater 


http://www.deltat.com/thermocouple.html

Thermocouple 

A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots, where a temperature differential is experienced by the 

different conductors (or semiconductors). It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Thermocouples are a 

widely used type of temperature sensor for measurement and control, and can also convert a temperature gradient into electricity. Commercial thermocouples are inexpensive, interchangeable, 

are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self powered and 

require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius (°C) can be difficult to achieve. 

Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a 

predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be 

important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are 

less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic 

methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the 

thermocouple, and so improve the precision and accuracy of measurements. Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, 

gas turbine exhaust, diesel engines, and other industrial processes. Thermocouples are also used in homes, offices and businesses as the temperature sensors in thermostats, and also as 

flame sensors in safety devices for gas-powered major appliances. 


http://en.wikipedia.org/wiki/Thermocouple

Liquid or Gas Expansion Devices 

Many physical properties change with temperature, such as the volume of a liquid, the length of a metal rod, 

the electrical resistance of a wire, the pressure of a gas kept at constant volume, and the volume of a gas kept 

at constant pressure. Filled-system thermometers use the phenomenon of thermal expansion of matter to 

measure temperature change. 

The filled thermal device consists of a primary element that takes the form of a reservoir or bulb, a flexible 

capillary tube, and a hollow Bourdon tube that actuates a signal-transmitting device and/or a local indicating 

temperature dial. A typical filled-system thermometer is shown in Figure 7-1. In this system, the filling fluid, 

either liquid or gas, expands as temperature increases. This causes the Bourdon tube to uncoil and indicate the 

temperature on a calibrated dial. 


Bourdon Gas Thermometers 

5 

f\G.3 0 

30 


Liquid Crystal Thermometer 

A liquid crystal thermometer or plastic strip thermometer is a type of thermometer that contains heat-sensitive 

(thermochromic) liquid crystals in a plastic strip that change color to indicate different temperatures. Liquid 

crystals possess the mechanical properties of a liquid, but have the optical properties of a single crystal. 

Temperature changes can affect the color of a liquid crystal, which makes them useful for temperature 

measurement. The resolution of liquid crystal sensors is in the 0.1°C range. Disposable liquid crystal 

thermometers have been developed for home and medical use. For example if the thermometer is black and it 

is put onto someone's forehead it will change colour depending on the temperature of the person. 

There are two stages in the liquid crystals: 1. the hot nematic stage is the closest to the liquid phase where the 

molecules are freely moving around and only partly ordered. 2. the cold smectic stage is closest to a solid 

phase where the molecules align themselves into tightly wound chiral matrixes. 

Liquid crystal thermometers portray temperatures as colors and can be used to follow temperature changes 

caused by heat flow. They can be used to observe that heat flows by conduction, convection, and radiation. In 

medical applications, liquid crystal thermometers may be used to read body temperature by placing against the 

forehead. These are safer than a mercury-in-glass thermometer, and may be advantageous in some patients, 

but do not always give an exact result, except the analytic liquid crystal thermometer which show the exact 

temperature between 35.5 to 40.5° Celsius. 


http://en.wikipedia.org/wiki/Liquid_crystal_thermometer

Liquid Crystal Thermometer 

A liquid crystal thermometer or plastic strip thermometer is a type of 

thermometer that contains heat-sensitive (thermochromic) liquid crystals in a 

plastic strip that change color to indicate different temperatures. Liquid 

crystals possess the mechanical properties of a liquid, but have the optical 

properties of a single crystal. 


Thermocouple 

Thermocouple grade wires 

Stainless steel sheath 

Flexible SS sheath 

Adjustable nut 

Wire junction 


http://www.omega.com/temperature/z/pdf/z021-032.pdf

Bimetallic Thermometers 

Dial 

Spiral \Vonnd 

E],ement 

t 

fixed End 

Free End Atl~cl!l.ed 

to Pointer Sh1'1!ft 

.. Thermowel l 

(with bimetal 

stem inserted) 

Structure of bimetallic thetmometer 


http://www.omega.com/temperature/z/pdf/z021-032.pdf

Resistance Thermometers - Resistance thermometers, also called resistance 

temperature detectors (RTDs), are sensors used to measure temperature by correlating the 

resistance of the RTD element with temperature. Most RTD elements consist of a length of fine 

coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is 

often placed inside a sheathed probe to protect it. The RTD element is made from a pure 

material, typically platinum, nickel or copper. The material has a predictable change in resistance 

as the temperature changes and it is this predictable change that is used to determine 

temperature. They are slowly replacing the use of thermocouples in many industrial applications 

below 600 °C, due to higher accuracy and repeatability. 

http://www.npl.co.uk/content/ConMediaFile/113 


http://en.wikipedia.org/wiki/Resistance_thermometer

In RTD devices; Copper, Nickel and Platinum 

are widely used metals. These three metals are 

having different resistance variations with 

respective to the temperature variations. That is 

called resistance-temperature characteristics. 

Platinum has the temperature range of 650°C, 

and then the Copper and Nickel have 120°C 

and 300°C respectively. The figure-1 shows the 

resistance-temperature characteristics curve of 

the three different metals. For Platinum, its 

resistance changes by approximately 0.4 ohms 

per degree Celsius of temperature. 

The purity of the platinum is checked by 

measuring R100 / R0. Because, whatever the 

materials actually we are using for making the 

RTD that should be pure. If it will not pure, it will 

deviate from the conventional resistancetemperature 

graph. So, α and β values will 

change depending upon the metals. 


http://en.wikipedia.org/wiki/Resistance_thermometer

Platinum Resistance Thermometer 

http://www.aoip.com/product/670-standard-platinum-resistance-thermometers/ 


Platinum Resistance Thermometer 


Resistance Temperature Detector (RTD) - Principle of Operation, 

Materials, Configuration and Benefits by Innovative Sensor Technology 

Overview 

Innovative Sensor Technology is a world-class manufacturer of thin-film RTD 

temperature sensors, capacitive humidity sensors, and mass flow sensors at the 

component level. With our state-of-the-art manufacturing technology, Innovative 

Sensor Technology offers both standard and custom sensors to satisfy unique 

applications. Additionally, our highly qualified staff is now offering R&D consulting 

services for industrial applications. Our sensors have applications in the automotive, 

HVAC, appliance, controls, and test & measurement industries. 

Resistance Temperature Detector (RTD) - Principle of Operation 

An RTD (resistance temperature detector) is a temperature sensor that operates on 

the measurement principle that a material’s electrical resistance changes with 

temperature. The relationship between an RTD resistance and the surrounding 

temperature is highly predictable, allowing for accurate and consistent temperature 

measurement. By supplying an RTD with a constant current and measuring the 

resulting voltage drop across the resistor, the RTD resistance can be calculated, and 

the temperature can be determined. 


http://www.azom.com/article.aspx?ArticleID=5573

RTD Materials 

Different materials used in the construction of RTD offer a different relationship 

between resistance and temperature. Temperature sensitive materials used in the 

construction of RTD include platinum, nickel, and copper; platinum being the most 

commonly used. Important characteristics of an RTD include the temperature 

coefficient of resistance (TCR), the nominal resistance at 0 degrees Celsius, and the 

tolerance classes. The TCR determines the relationship between the resistance and 

the temperature. There are no limits to the TCR that is achievable, but the most 

common industry standard is the platinum 3850 ppm/K. This means that the 

resistance of the sensor will increase 0.385 ohms per one degree Celsius increase in 

temperature. The nominal resistance of the sensor is the resistance that the sensor 

will have at 0 degrees Celsius. Although almost any value can be achieved for 

nominal resistance, the most common is the platinum 100 ohm (pt100). Finally, the 

tolerance class determines the accuracy of the sensor, usually specified at the 

nominal point of 0 degrees Celsius. There are different industry standards that have 

been set for accuracy including the ASTM and the European DIN. Using the values of 

TCR, nominal resistance, and tolerance, the functional characteristics of the sensor 

are defined. 



RTD Configurations 

In addition to different materials, RTD are also offered in two major configurations: 

wire wound and thin film. Wire wound configurations feature either an inner coil RTD 

or an outer wound RTD. An inner coil construction consists of a resistive coil running 

through a hole in a ceramic insulator, whereas the outer wound construction involves 

the winding of the resistive material around a ceramic or glass cylinder, which is then 

insulated. 

The thin film RTD construction features a thin layer of resistive material deposited onto 

a ceramic substrate through a process called deposition. A resistive meander is then 

etched onto the sensor, and laser trimming is used to achieve the appropriate nominal 

values of the sensor. The resistive material is then protected with a thin layer of glass, 

and lead wires are welded to pads on the sensor and covered with a glass dollop. 

Thin film RTD have advantages over the wire wound configurations. The main 

advantages include that they are less expensive, are more rugged and vibration 

resistant, and have smaller dimensions that lead to better response times and 

packaging capabilities. For a long time wire wound sensors featured much better 

accuracy. Thanks to recent developments, however, there is now thin film technology 

capable of achieving the same level of accuracy. 



Operations of RTD 

An RTD takes a measurement when a small DC current is supplied to the sensor. The 

current experiences the impedance of the resistor, and a voltage drop is experienced 

over the resistor. Depending on the nominal resistance of the RTD, different supply 

currents can be used. To reduce self-heating on the sensor the supply current should 

be kept low. In general, around 1mA or less of current is used. An RTD can be 

connected in a two, three, or four-wire configuration. The two-wire configuration is the 

simplest and also the most error prone. In this setup, the RTD is connected by two 

wires to a Wheatstone bridge circuit and the output voltage is measured. The 

disadvantage of this circuit is that the two connecting lead wire resistances add 

directly two the RTD resistance and an error is incurred. 

2-Wire Configuration 



The four-wire configuration consists of two current leads and two potential leads that 

measure the voltage drop across the RTD. The two potential leads are high resistance 

to negate the effect of the voltage drop due to current flowing during the measurement. 

This configuration is ideal for canceling the lead wire resistances in the circuit as well 

as eliminating the effects of different lead resistances, which was a possible problem 

with the three-wire configuration. The four-wire configuration is commonly used when 

a highly accurate measurement is required for the application. 

4-Wire Configuration 



Benefits of Thin Film RTD 

There are many options when considering contact temperature measurement, 

including thermocouples, thermistors, and RTD (wire wound and thin film). 

While thermocouples can handle very high temperatures and thermistors are 

inexpensive, there are many advantages of RTD. Some of these advantages 

include their accuracy, precision, long-term stability, and good hysteresis 

characteristics. Even beyond these, there are advantages of thin film RTD 

over wire wound, including smaller dimensions, better response times, 

vibration resistance, and relative inexpensiveness. New advancements has 

even made thin film technology just as accurate as wire wound at higher 

temperatures ranges. 



Thermistor 

A thermistor is a type of resistor whose resistance varies significantly with temperature, 

more so than in standard resistors. The word is a portmanteau of thermal and resistor. 

Thermistors are widely used as inrush current limiter, temperature sensors (NTC type 

typically), self-resetting overcurrent protectors, and self-regulating heating elements. 

Thermistors differ from resistance temperature detectors (RTDs) in that the material 

used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. 

The temperature response is also different; RTDs are useful over larger temperature 

ranges, while thermistors typically achieve a higher precision within a limited 

temperature range, typically −90 °C to 130 °C 


http://en.wikipedia.org/wiki/Thermistor

Thermistor 

+V 

r===~ 

Op A p 

V to A 

In 

er lstor 

- 


http://swordrock.wordpress.com/category/robotic-2/

Thermistor 

tJ o .15 Dumet wire 

Sintered electrode 

Thermistor Chi 

Glass 

(Unit: mm) 


http://en.wikipedia.org/wiki/Thermistor

3.2 Contacting Thermal Measuring Devices 

The most commonly used contacting devices include bimetallic thermometers, 

thermochromic liquid crystals, thermocouples, resistance thermometer, 

thermistors and heat flux indicators. These devices are discussed briefly here. 

For more detailed information, refer to ASNT Nondestructive Testing 

Handbook. third edition: Volume 3. Infrared and Thermal Testing. 

■ Bimetallic Thermometers 

Bimetallic thermometers are sensors constructed of dissimilar metallic strips 

bonded together. Typically. different iron nickel alloys are used. The strips 

differ in temperature coefficient of expansion such that temperature changes 

result in predictable bending of the assembly. Arranged in a spiral or helical 

configuration. one end of the bimetallic element is fixed and the other end is 

attached to a pointer. Properly calibrated, the angular position of the pointer 

can be made to indicate temperature on a scale. 


■ Thermochromic Liquid Crystals 

Thermochromic liquid crystals (also called cholesterol crystals) change color 

with temperature. Coatings made of liquid crystals are commonly used as 

temperature threshold indicators. Depending on the mixture. a coating 

applied to a surface will change color predictably when the surface exceeds a 

threshold temperature. The color change may be reversible or irreversible. 

and the sensing range for most mixtures is limited to a narrow temperature 

span. Typically. a set of liquid crystal markers provides a selection of 

transition temperatures. This allows the user to select the appropriate marker 

for the desired temperature. 

Keywords: 

Threshold temperature 


■ Thermocouple 

Thermocouples are contact temperature sensors based on the thermoelectric 

effect. or Seebeck effect. Thomas Seebeck discovered that, when two 

dissimilar metals arc joined at both ends and these ends are at different 

temperatures, a predictable direct current will flow through the circuit. The 

thermoelectric coefficient determines the relationship between this current 

and the temperature difference between the two junctions. This coefficient is 

known for each type of thermocouple. To configure a thermometer. the circuit 

is broken and the open-circuit voltage is measured by a volt meter. One of the 

two junctions is then held al a reference temperature. such as an ice bath, 

and the voltage is calibrated to indicate the temperature of the other junction. 

which then becomes the temperature sensing junction. Thermopiles arc 

banks of thermocouples connected in parallel or in series to increase output 

gradient. The reference temperature is important because of the 

thermocouples' non linear response. 

Keywords: 

thermoelectric coefficient 


■ Resistance Thermometers 

Resistance temperature detector (RTDs) arc contact sensors thaI measure 

tcmpcralUrc by a predictable change in resistance as a function of 

temperature. Platinum is the most popular resistance temperature detector 

material because of its excellent stability and its linear response to 

temperature change. Other materials used include nickel. copper. tungsten 

and iridium. In operation. the resistance temperature detector may be placed 

in a bridge circuit such that the bridge output voltage is a measure of the 

resistance and hence the temperature at the resistance temperature detector. 

A more accurate measurement may be achieved by using a constant current 

source and a digital volt meter (DVM). such that the digital volt meter reading 

is proportional to the resistance temperature detector resistance and hence 

the temperature at the resistance temperature detector. 


■ Thermistors 

Thermistors arc also sensors that measure temperature by a predictable 

change in resistance as a fun ction of temperature. Thermistors are made of 

semiconductor materials. Whereas resistance temperature detectors are low 

impedance devices. thennistors are high impedance devices. Thermistors 

typically are more sensitive to temperature changes than resistance 

temperature detectors but thermistors are not as stable. 

Keywords: 

Thermistors typically are more sensitive to temperature changes than 

resistance temperature detectors 


■ Heat Flux Indicators 

Heat flux indicators are heat flow meters and are used to measure rates in 

conduction, convection, radiation and phase change systems such as 

building walls, boiler tubes and air conditioning ducts. A typical heat flux 

indicator consists of a sensitive thermopile, composed of many fine gage 

thermocouples connected in series on opposite sides of a nat core wilh 

known and stable thermal resistance. The entire assembly is covered with 

protective material. 

The voltage generated across the thermopile is calibrated to be a measure of 

the steady state heat flux through the device. Transient heat flux can be 

related to the transient thermopile output and the geometry of the device. 


3.3 Optical Pyrometers 

Optical pyrometers include brightness pyrometers and infrared pyrometers. 

Infrared pyrometers are also called infrared radiation themlometers. Various 

types are discussed in the next section. Brightness pyrometers are also called 

matching pyrometers. They incorporate a calibrated light source (lamp) 

powered by a calibrated current supply. Looking through a viewer. the 

operator matches the brightness of the target to be measured with the 

brightness of the calibrated lamp. The adjustment control is cal ibrated in 

temperature units. such that when the brightnesses arc matched, the control 

indicates the temperature of the target to be measured. 


Pyrometer 

A pyrometer is a device that is used for the temperature measurement of an object. 

The device actually tracks and measures the amount of heat that is radiated from an 

object. The thermal heat radiates from the object to the optical system present inside 

the pyrometer. The optical system makes the thermal radiation into a better focus and 

passes it to the detector. The output of the detector will be related to the input thermal 

radiation. The biggest advantage of this device is that, unlike a Resistance 

Temperature Detector (RTD) and Thermocouple, there is no direct contact between 

the pyrometer and the object whose temperature is to be found out. 

Optical (brightness) Pyrometer 

In an optical pyrometer, a brightness comparison is made to measure the temperature. 

As a measure of the reference temperature, a color change with the growth in 

temperature is taken. The device compares the brightness produced by the radiation 

of the object whose temperature is to be measured, with that of a reference 

temperature. The reference temperature is produced by a lamp whose brightness can 

be adjusted till its intensity becomes equal to the brightness of the source object. For 

an object, its light intensity always depends on the temperature of the object, whatever 

may be its wavelength. After adjusting the temperature, the current passing through it 

is measured using a multimeter, as its value will be proportional to the temperature of 

the source when calibrated. The working of an optical pyrometer is shown in the figure 

below. 


http://www.instrumentationtoday.com/optical-pyrometer/2011/08/

Pyrometer 

A pyrometer is a type of remote sensing thermometer used to measure temperature. Various 

forms of pyrometers have historically existed. In the modern usage, it is a non-contacting device 

that intercepts and measures thermal radiation, a process known as pyrometry and sometimes 

radiometry. The thermal radiation can be used to determine the temperature of an object's 

surface. 

The word pyrometer comes from the Greek word for fire, "πυρ" (pyro), and meter, meaning to 

measure. The word pyrometer was originally coined to denote a device capable of measuring the 

temperature of an object by its incandescence, or the light that is emitted by the body as caused 

by its high temperature. Modern pyrometers are capable of interpreting temperatures of room 

temperature objects by measuring radiation flux in the infrared spectrum. 

A modern pyrometer has an optical system and a detector. The optical system focuses the 

thermal radiation onto the detector. The output signal of the detector (temperature T) is related to 

the thermal radiation or irradiance j* of the target object through the Stefan–Boltzmann law, the 

constant of proportionality σ, called the Stefan-Boltzmann constant and the emissivity ε of the 

object. 

J* = εσT 4 

This output is used to infer the object's temperature. Thus, there is no need for direct contact 

between the pyrometer and the object, as there is with thermocouples and resistance 

temperature detectors (RTDs). 


http://en.wikipedia.org/wiki/Pyrometer

Brightness Pyrometers 

OIPtical Pyrometer 

Red flllter 

Absorption sclieen 

Reference 

tern pe at!Uire 

I amp 

" 

Tempera ti!Jre 

Source 

Eye-p iece 

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Objeclril ve lens 

Battery - .-- 

Rheostat 

I ' M toE- ~-- Mu l ti me ter 

www. I nst rum e rntationToola y.com 



Brightness Pyrometers –Wien’s Law 

.8E+11 - 

I 

5500K 

hot object 

Intensity 

of 

light 

0 

500 1000 

1500 2000 

blue 

red 

Wav~e length I nm 



3.4 Basic Configurations of Infrared Radiation 

Sensing and Imaging Instruments 

In terms of configuration and operation. most thermal imagers are considered 

to be extensions of radiation thermometers or radiation thermometers plus 

scanning optics. The performance parameters of thermal imagers are 

extensions of the performance parameters of radiation thermometers. To aid 

comprehension. the basic measurement problem is discussed in this chapter 

in terms of the measurement of a single point. It is then expanded to cover 

thermal scanning and imaging. Figure 3.1 illustrates the basic configuration of 

an infrared sensing instrument (infrared radiation thermometer), showing the 

components necessary to make measurements. Collecting optics (an infrared 

lens, for example) arc necessary for gathering the energy emitted by the 

target spot and focusing this energy onto the sensitive surface of an infrared 

detector. 


The processing electronics unit amplifies and conditions the signal from the 

infrared detector and introduces corrections for such factors as detector 

ambient temperature drift and target effective surface emissivity. Generally. a 

readout. such as a meter. indicates the target temperature and an analog 

output is provided. The output signal is used to record, display. alarm, control, 

correct or any combination of these. 


Figure 3.1: Basic configuration of an infrared radiation thermometer 

Optics 

Lens Filter 

(collects (passes 

energy) selected 

spectral 

band) 

Detector 

(converts 

infrared energy 

to an 

electrical 

signal) 

Electronics 

(amplifies and 

conditions the 

signal) 

Detect 

Q) 

N 

·- C/) 

.... 

Q) I 

C) 

I \ 

I 

... I I 

{:. 

\ I 

~, 

FOV 

Output 

Measu 

Monitor 

Working Distance 

Control 


Infrared Detector 

An infrared detector is at the heart of every infrared sensing and imaging 

instrument. whatever its configuration. Infrared detectors can sense infrared 

radiant energy and produce useful electrical signals proportional to the 

temperature of target surfaces. Instruments using infrared detectors and 

optics to gather and focus energy from the targets onto these detectors are 

capable of measuring target surface temperatures with sensitivities better 

than 0.10 °C (0.18 ºF). and with response limes in the microsecond (μs) 

range. An instrument that measures the temperature of a spot on a target in 

this manner is called an infra red radiation thermometer. An instrument that 

combines this measurement capability with a means or mechanism for 

scanning the target surface is called an infrared thermal imager. It can 

produce thermal maps, or thermograms, where the brightness intensity or 

color hue of any spot on the map represents the apparent temperature of the 

surface at that point. 


Figure 3.2 illustrates the spectral responses of various infrared radiation 

detectors. Radiant energy impinging on their sensitive surfaces causes all 

infrared detectors to respond with some kind of electrical change. This may 

be an impedance change. a capacitance change, the generation of an 

electromotive force (emf) known as Voltage, or the release of photons, 

depending on the type of detector. 

Infrared detectors are divided into (1) thermal detectors and (2) photon 

detectors. Thermal detectors have broad uniform spectral responses, 

somewhat lower sensitivities and slower response times (measured in 

millisecond): photon detectors (also called photo detectors) have limited 

spectral responses. higher peak sensitivities and faster response times 

(measured in microsecond). Thermal detectors usually operate at or near 

room temperature. whereas photon detectors are usually cooled to optimize 

performance. 

Keywords: 

■ Thermal Detector- broad uniform spectral responses/ slower 

■ Photon Detector- limited spectral responses/ faster 


Figure 3.2: Response Curves of Various Infrared Detectors 

0 t2 

·- InS!;), 77K) 

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Discussion 

Subject: Why (or How) there are 2 MCT; MCT(215K), MCT(77K)? 

s. (299 ) 

.-~ 

0 5 1 2 3· 4 8 9 10 11 

g '1 · 1m1) 


The mercury cadmium telluride (HgCdTe) detectors shown in Figure 3.2 are 

photon detectors cooled to 77 K (-321° F) for operation from 8 to 12 μm and 

to 195 K (-109 ° F) for operation from 3 to 5 μm. Because of their fast 

response, these detectors are used extensively in high speed scanning and 

imaging applications. In contrast to the mercury cadmium telluride detector, 

the radiation thermopile shown in Figure 3.2, is a broad band thermal detector 

operating uncooled. It is used extensively for spot measurements. Because it 

generates a direct current electromotive force proportional to the radiant 

energy reaching its surface. it is ideal for use in portable, battery powered 

instruments. The lead sulfide (PbS) detector is typical of those used in 

radiation thermometers that measure and control the temperature of very hot 

targets. Its peak sensitivity at 3μm matches the peak energy emitted by a 

1000K (727 °C = 1340 ° F) graybody. 

Because of the atmospheric absorption considerations previously discussed. 

most infrared thermal imagers operate in either the 3 to 5 μm or the 8 to 12 

μm spectral region. 

Note: 195K = [(-273+195) x 9/5] + 32 = -108 ° F 



Indium Antimony 

Photon Detectors 


Infrared Optics - Lenses, Mirrors and Filters 

There are two types of infrared optics; (1) refractive (lenses. filters, windows) 

and (3) reflective (mirrors). Refractive optics transmit infrared wavelengths of 

interest. When used for higher temperature applications. their throughput 

losses can usually be ignored. When used in low temperature measurement 

instruments and imagers, absorption is often substantial and must be 

considered when making accurate measurements. 

Reflective optics. which are more efficient are not spectrally selective and 

somewhat complicate the optical path. Reflective optics are used more often 

for low temperature applications. where the energy levels cannot warrant 

throughput energy losses. When an infrared radiation thermometer is aimed 

at a target, energy is collected by the optics in the shape of a solid angle 

determined by the configuration of the optics and the detector. 


The cross section of this collecting beam is called the field of view (FOV) of 

the instrument and it detennines the size of the area (spot size) on the target 

surface that is measured by the instrument at any given working distance. On 

scanning and imaging instruments this is called the instantaneous field of 

view (lFOV) and becomes one picture element on the thermogram. An 

infrared interference filter is often placed in front of the detector to limit the 

spectral range of the energy reaching the detector. The reasons for spectral 

selectivity will be discussed later in this chapter. 

Processing Electronics 

The processing electronics unit amplifies and conditions the signal from the 

infrared detector and introduces corrections for factors such as detector 

ambient temperature drift and effective target surface emissivity. 

In radiation thermometers, a meter is usually provided to indicate the target’s 

apparent temperature. An analog or digital output signal is provided to record, 

display, alarm, control, correct or any combination of these. 


Field of View (FOV) 

A field of view (FOV) is a specification that defines the size of what is seen in 

the thermal image. The lens has the greatest influence on what the FOV will 

be, regardless of the size of the array. Large arrays, however, provide greater 

detail, regardless of the lens used, compared to narrow arrays. For some 

applications, such as work in outdoor substations or inside a building, a large 

FOV is useful. While smaller arrays may provide sufficient detail in a building, 

more detail is important in substation work. See Figure 4-7. 


Figure 4-7. The field of view 

(FOV) is a specification that 

defines the area that is seen in 

the thermal image when using a 

specific lens. 


What is IFOV? 

A measure of the spatial resolution of a remote sensing imaging system. 

Defined as the angle subtended by a single detector element on the axis of 

the optical system. IFOV has the following attributes: 

■ 

■ 

Solid angle through which a detector is sensitive to radiation. 

The IFOV and the distance from the target determines the spatial 

resolution. 

A low altitude imaging instrument will have a higher spatial resolution than a 

higher altitude instrument with the same IFOV 


http://www.ssec.wisc.edu/sose/tutor/ifov/define.html

What is IFOV? 

IFOV (instantaneous field of view) – smallest object detectable 

The IFOV (instantaneous field of view), also known as IFOV geo (geometric 

resolution), is the measure of the ability of the detector to resolve detail in 

conjunction with the objective. Geometric resolution is represented by mrad 

and defines the smallest object that can be represented in the image of the 

display, depending on the measuring distance. The thermography, the size of 

this object corresponds to a pixel. The value represented by mrad 

corresponds to the size of the visible point [mm] a pixel at a distance of 1 m. 


http://www.academiatesto.com.ar/cms/?q=ifov

Instantaneous Field of View (IFOV) 

An instantaneous field of view (IFOV) is a specification used to describe the 

capability of a thermal imager to resolve spatial detail (spatial resolution). The 

IFOV is typically specified as an angle in milliradians (mRad). When projected 

from the detector through the lens, the IFOV gives the size of an object that 

can be seen at a given distance. An IFOV measurement is the measurement 

resolution of a thermal imager that describes the smallest size object that can 

be measured at a given distance. See Figure 4-8. It is specified as an angle 

(in mRad) but is typically larger by a factor of three than the IFOV. This is due 

to the fact that the imager requires more information about the radiation of a 

target to measure it than it does to detect it. It is vital to understand and work 

within the spatial and measurement resolution specific to each system. 

Failure to do so can lead to inaccurate data or overlooked findings. 

IFOV, θ in milli-radian 

H 

H in mm = D∙ θ 

D in meter 


Figure 4-8. An IFOV measurement is the measurement resolution of a thermal imager that describes the 

smallest size object that can be measured at a given distance. IFOV is similar to seeing a sign in the distance 

while IFOV measurement is similar to reading the sign, either because it is closer or larger. 

README 

Instantaneous field of view (spatial resolution)/ IFOV measurement (measurement of resolution) 


3.5 Scanning and Imaging 

When problems in temperature monitoring and control cannot be solved by 

the measurement of one or several discrete points on a target surface. it 

becomes necessary to spatially scan - that is to move the collecting beam or 

the instrument's field of view relative to the target. This is usually done by 

inserting a movable optical element into the collecting beam as illustrated in 

Figure 3.3. 


Figure 3.3: Adding the scanning element(s) for imaging 

Target Surface 

(emits infrared 

energy) 

' 

I 

\ 

Lens 

(collects 

energy) 

Optics 

Filter 

(passes 

selected 

spectral 

band) 

\ 

Detector 

(converts 

infrared energy 

to an 

electrical 

signal) 

I 

Electronics 

(amplifies and 

conditions the 

signal) 

Detect 

Measure 

I 

I 

\ I 

FOV 

Monitor 

Workin Distance 

Scanning Element(s) 

for Scanners or Imagers 

Control 


Line Scanning 

When the measurement of a single spot on a target surface is not sufficient. 

infrared line scanners can be used to assemble infonnalion concerning the 

distribution of radiant energy along a single straight line. Quite often, this is all 

that is necessary to locate a critical thermal anomaly. The instantaneous 

position of the scanning element is usually controlled or sensed by an 

encoder or potentiometer so that the radiometric output signal can be 

accompanied by a position signal output and be displayed on a recording 

device and/or fed out to a computer based process control system. A typical 

high speed commercial line scanner develops a high resolution thermal map 

by scanning normal to the motion of a moving target such as a paper web or 

a strip steel process. The resulting output is a thermal strip map of the 

process as it moves normal to the scan line. The scanning configuration is 

illustrated in Figure 3.4. The output signal information is in a real time 

computer compatible format and can be used to monitor, control or predict the 

behavior of the target. 


Figure 3.4: Line scanner scanning configuration 


Two-dimensional Scanning - Thermal Imaging 

The three common imaging configurations that produce infrared thermograms 

are (1) optomechanical scanning, (2) electronic scanning and (3) focal plane 

array imaging. 

Of the three, optomechanical scanning was the most common until the mid- 

I990s. Focal plane array imagers have replaced scanning imagers in most 

applications. 


Optomechanical Scanning 

To scan optomechanically in two dimensions generally requires two scanning 

elements. Although an almost infinite variety of scanning patterns can be 

generated using two moving elements. the most common pattern is rectilinear. 

This scanning pattern is most often accomplished by two elements, each 

scanning a line normal to the other. A representative rectilinear scanner is 

illustrated in the schematic of Figure 3.5. Its scanning mechanism comprises 

two oscillating mirrors behind the primary lens, a high speed horizontal 

scanning mirror and a slower speed vertical scanning mirror. One 

performance limitation of single-detector optomechanical scanners is a trade 

off between speed of response and signal-to-noise ratio of the detector. 

These instruments require high speed cooled photodetectors that are pushed 

to their performance limits as the desired real time scanning rate is increased. 

Multidetector scanners reduce the constraints on detector performance by 

adding detector elements that share the temporal spatial burden, allowing for 

faster frame rales with no reduction in signal-to-noise ratio or improving the 

signal-to-noise ratio with no decrease in frame rate. 


Figure 3.5: Optomechanlcally scanned infrared imager 


Electronic Scanning – Pyroelectric Vidicon Thermal Imagers 

Electronically scanned thermal imaging systems based on pyrovidicons and 

operating primarily in the 8 to 14 μm atmospheric window are commonly used. 

They provide qualitative thermal images and are classified as thermal viewers. 

A pyroelectric vidicon or pyrovidicon is configured the same as a conventional 

video camera tube except that it operates in the infrared (2 to 20 μm) region 

instead of the visible spectrum. Image scanning is accomplished 

electronically in the same manner as in a video camera tube. 


Pyroelectric Vidicon Thermal Imagers 

lLIGIMI:MT 

COIL 

F OCU!IIIICi 

CIIITIUIL 

, 5 s s:s SJ.:S:S \ s s

Focal Plane Array Imaging 

First introduced to the commercial market in 1987. cooled infrared focal plane 

array (IRFPA) imagers have evolved into compact, qualitative and 

quantitative thermal imagers without scanning optics. These devices have 

been replacing optomechanically scanned imagers for many applications. 

The first uncooled infrared focal plane array imagers have been used by the 

military for several years and became available to thermographers in 1997. 

Figure 3.6 is a schematic of a typical. uncooled infrared focal plane array 

imager. Microbolometer arrays are also available. 


Figure 3.6: Typical uncooled infrared focal plane array imager 

Iris 

Array 

Bias 

Array 

-=- Address RS 170 

- Generator Video 

= Signal 

= .. __.., .. ___, . __.., .. • 

Optics 

Infrared Pre- AID Digital 

Focal amplifiers Convertors Processor 

Plane 

Ar.ray 

Package 


IRFPA - Large IR mosaic prototype array with 35 H2RG arrays. The array has 

a total of nearly 147 million pixels. Each of the H2RG arrays has 2,048×2,048 

pixels. 


http://www.osa-opn.org/home/articles/volume_19/issue_6/features/high-performance_infrared_focal_plane_arrays_for_s/

IRFPA 


http://ececavusoglu.girlshopes.com/cmoslineararraysirsensor/

Infrared sensors with 3D ROIC for cooled dual-band IR arrays 


http://www.militaryaerospace.com/articles/2013/07/army-irfpa-roic.html

3.6 Performance Parameters of Infrared 

Sensing and Imaging Instruments 

To select an appropriate instrument for an application, or to determine 

whether an available instrument will perform adequately. it is necessary for 

the thermographer to understand its performance parameters. The 

performance parameters for point sensing instruments (infrared radiation 

thermometers) are temperature range, absolute accuracy, repeatability, 

temperature sensitivity, speed of response, target spot size and working 

distance (field-of-view-spatial resolution), output requirements. sensor 

environment and spectral range. 

For scanners and imagers the performance parameters include temperature 

range. absolute accuracy, repeatability, temperature sensitivity, total field of 

view (TFOV), instantaneous field of view (lFOV), measurement spatial 

resolution (IFOVmeas), frame repetition rate, minimum resolvable 

temperature (MRT), temperature sensitivity, image processing software, 

sensor environment and spectral range. 


Qualitative Versus Quantitative Thermography 

For scanners and imagers. one distinction based on instrument performance 

limitations is that between qualitative and quantitative thermography. 

A qualitative thermogram displays the distribution of infrared radiance over 

the target surface, uncorrected for target, instrument and media 

characteristics. 

A quantitative thermogram displays the distribution of infrared radiosity over 

the surface of the target. corrected for target, instrument and media 

charactcristics so as to approach a graphic representation of true surface 

temperature distribution. 


Performance parameters of qualitative thermographic instruments. therefore, 

do not include temperature accuracy, temperature repeatability and 

measurement spatial resolution. 

Generally, instruments that include the capability to produce quantitative 

thermograms are more costly than qualitative instruments and require 

periodic recalibration. Many applications can be solved without the time and 

expense of quantitative thermography, but others require true temperature 

mapping. A discussion of the most appropriate applications for quantitative 

and qualitative thermal imagers is included in Chapter 5. 

Keywords: 

Performance parameters of qualitative thermographic instruments. therefore, 

do not include temperature accuracy, temperature repeatability and 



Performance Characteristics of Point Sensing Instruments (Radiation 

Thermometers) 

The American Society for Testing and Materials defines infrared point sensing 

instruments as infrared radiation thermometers even though they do not 

always read out in temperature units. Some read out directly in apparent 

radiant power units such as W·m -2· s -1 (or BTU· ft -2 ∙ h -1 ), some provide a 

closure or alarm signal at a selectable temperature and some others provide 

only difference indications on a light emitting diode display. 


Temperature Range 

Temperature range is a statement of the high and low limits over which the 

target temperature can be measured by the instrument. A typical specification 

would be. for example. "temperature range 0 to 1000 °C (32 to 1832 ºF).“ 

Absolute Accuracy 

Absolute accuracy, as defined by the National Lnstitute of Standards and 

Technology (NIST) standard, entails the maximum error. over the full range, 

that the measurement will have when compared to this standard blackbody 

reference. A typical specification would be, for example. "absolute accuracy 

±0.5 °C (±0.9 ºF) ± 1 percent of full scale.“ 


Repeatability 

Repeatability describes how faithfully a reading is repeated for the same 

target over the short and long term. A typical specification would be, for 

example, "repeatability (short and long term) of ±0.25 °C (±0.45ºF) “. 

Temperature range and absolute accuracy will always be interrelated; for 

example, the instrument might be expected to measure a range of 

temperatures from 0 to 200 °C (32 to 392 OF) with an absolute accuracy ±2 

°C (±3.6ºF) over the entire range. This could alternately be specified as ±1 

percent absolute accuracy over full scale. On the other hand, the best 

accuracy might be required at some specific temperature, say 100 °C 

(212 ° F). In this case, the manufacturer should be informed and the 

instrument could be calibrated to exactly match the manufacturer's laboratory 

calibration standard at that temperature. Because absolute accuracy is based 

on traceability to the NIST standard. it is difficult for a manufacturer to comply 

with a tight specification for absolute accuracy. An absolute accuracy of ±0.5 

°C (±0.9 ° F) or ±1 percent of full scale is about as tight as can be 

reasonably specified. Repeatability, on the other hand, can be more easily 

ensured by the manufacturer and is usually more important to the user. 


Temperature Sensitivity 

Temperature sensitivity defines the smallest target temperature change the 

instrument will dctect. Temperature sensitivity is also called thermal resolution 

or noise equivalent temperature difference (NETD). It is the smallest 

temperature change at the target surface that can be clearly sensed at the 

output of the instrument. This is almost always closely associated with the 

cost of the instrument. so unnecessarily fine temperature sensitivity should 

not be specified. An important rule to remember is that. for any given 

instrument. target sensitivity will improve for hotter targets where there is 

more energy available for the instrument to measure. Temperature sensitivity 

should be specified, therefore, at a particular target temperature near the low 

end of the range of interest. A typical specification for temperature sensitivity 

would be, for example, “temperature sensitivity of 0.25 °C (0.45 ºF) at a target 

temperature of 25 °C (77 ºF)." In this case, the sensitivity of the instrument 

would improve for targets hotter than 2 °C (36 °F). 

Keywords: 

Temperature sensitivity is also called thermal resolution or noise equivalent 

temperature difference (NETD). 


Temperature sensitivity is 

also called: thermal resolution 

or 

noise equivalent temperature 

difference (NETD). 



Speed of Response 

Speed of response is how long it takes for an instrument to update a 

measurement. It is defined as the time it takes the instrument output to 

respond to a step change in temperature at the target surface. 

Figure 3.7 shows this graphically. The sensor time constant is defined by 

convention to be the time required for the output signal to reach 63 percent of 

a step change in temperature at the target surface. Instrument speed of 

response is usually specified in terms of a large percentage of the full reading, 

such as 95 percent. As illustrated in Figure 3.7, this takes about five time 

constants, and is generally limited by the detector used (on the order of 

microseconds for photodctcetors and milliseconds for thermal detectors). 


A typical speed of response specification would be, for example. "speed of 

response (to 95 percent) = 0.05 s.“ It should be understood that there is 

always a tradeoff between speed of response and temperature sensitivity. 

As in all instrumentation systems, as the speed of response for a particular 

device becomes faster (instrumentation engineers call this a wider 

information bandwidth) the sensitivity becomes poorer (lower signal- to-noise 

ratio). If the speed of response is specified to be faster than is necessary for 

the application, the instrument may not have as good a temperature 

sensitivity as might be possible otherwise. 


Figure 3.7: Instrument speed to response and time constant 

100 

90 

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~ 

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Time (seconds) 


Target Spot Size and Working Distance 

Targct spot size D and working distance d define the spalial resolution of the 

instrument. In a radiation thermometer, spot size is the projcction of the 

sensitive area of the detector at the target plane. It may be specified directly, 

“1 cm at I m (0.4 in. at 3 ft)," for example, but it is usually expressed in more 

general terms such as a field of view solid angle ( 10 mrad, 1 degree, 2 

degree) or a field-of-view ratio (ratio of spot size to working distance - for 

example, d/15, d/30, d/75. 

A milliradian (mrad) is an angle with a tangent of 0.001. A d/15 ratio means 

that the instrument measures the emitted energy of a spot one-fifteenth the 

size of the working distance: 3 cm at 45 cm (1.2 in. at 18 in .) f 

or example. Figure 3.8 illustrates these relationships and also shows how 

spot size can be approximated quickly based on working distance and fieldof-view 

information furnished by the manufacturer. A typical specification for 

spot size would be. for example. "target spot size = 2 degrees from 1 m (39 

in.) to ∞.“ 


Figure 3.8: Instrument field-of-view determination 

Target 

. ' -,..-"" 

.,.,. ' > • • ..,-..... ~ 

·< s~ns~~L: 

·::"w~._..}.."'·-x~-.... 

'.:~ 

·

This would take into account the shortest working distance at which the 

instrument could be focused (1 m or 39 in.). For some instruments designed 

for very close workiing distances, the simple d∙D -1 ratio does not always apply. 

If closeup information is not clearly provided in the product literature, the 

instrument manufacturer should be consulted. For most applications and for 

middle and long working distance (greater than 1m or 3 ft), the following 

simple calculation (illustrated in Figure 3.8) will closely approximate target 

spot size: where: 

D ≡ αd 

D = spot size (approximate), 

α = field-or-view plane angle in radians, 

d = distance to the target. 

A 17.5 mrad (1 degree) field of view means a d∙D -1 ratio of 60 to1 and a 35 

mrad (2 degree) field of view means a d∙D -1 ratio of 30 to 1. (?) 


D ≡ αd 

D = spot size (approximate), 

α = field-or-view plane angle in radians, 

d = distance to the target. 

A 17.5 mrad (1 degree) field of view means a d∙D -1 ratio of 60 to1 and a 35 

mrad (2 degree) field of view means a d∙D -1 ratio of 30 to 1. (?) 

for D ≡ α∙d 

given that α = 17.5mrad, D=17.5mm if d=1000mm, thus 

d/D = 1000/17.5 = 57.296 ≈ 60 

This is to say the IFOV measurement ration = 1000 ∙ 1/α where α in mRad. 


EXAM score! 

D=σ∙d 

IFOV ratio = d/D or 1/σ 

(care on unit used!) 



Output Requirements 

Output requirements for radiation thermometers can vary widely - from a 

simple digital indicator and an analog signal to a broad selection of output 

functions, including digital output (binary coded decimal); high, low and 

proportional set points; signal peak or valley sensors; sample and hold 

circuits; and even closed loop controls for specific applications. On board 

microprocessors provide many of the above functions on even inexpensive 

standard portable models of radiation thermometers. 


Sensor Environment 

Sensor environment includes the ambient extremes under which the 

instrument will perform within specifications and the extremes under which it 

can be stored without damage when not in operation. For a portable radiation 

thermometer. a typical specifi cation for sensor environment would be as 

followas. 

1. Operating temperature is 0 to 37°C (32 to 100 °F) 

2. Humidity is at 20 to 80 percent relative (not condensing). 

3. Atmospheric pressure is at -610 m to +2440 m (-2000 to +8000 ft) above 

sea level. 

4. Storage temperature (nonoperating) ranges from -15 to +60 °C (5 to 140 

°F). 

Frequently in process control applications, the sensor must be permanently 

installed in a somewhat more extreme environment involving smoke, soot. 

high temperature and even radioactivity. For these applications, 

manufacturers provide a wide range of enclosures that offer special protective 

featu res such as air cooling, water cooling, pressurization, purge gases and 

shielding. 


Spectral Range 

Spectral range denotes the portion of the infrared spectrum over which the 

instrument will operate. The operating spectral range of the instrument is 

often critical to its performance and, in many applications. can be exploited to 

solve difficult measurement problems. The spectral range is determined by 

the detector and the instrument optics. as shown in Figure 3.9. Here, the fiat 

spectral response of a radiation thermopile detector is combined with that of a 

germanium lens and an 8 to 14 μm band pass filter. The instrument 

characterized is suitable for general purpose temperature measurement of 

cool targets through atmosphere. The transmission spectrum of a 0.3 km (0. 

19 mil) atmospheric ground level is also shown. An infrared interference filter 

is often placed in front of the detector to limit the spectral range of the energy 

reaching the detector. 


the following three classes of filters are common: 

1. High pass ti lters pass energy only at wavelengths longer than a 

designated wavelength. 

2. Low pass filters pass energy only at wavelengths shorter than a 

designated wavelength. 

3. Band pass filters similar to the one shown in Figure 3.9. pass radiation 

within a designated spectral band (8 to 14 μm. for example). 


Spectrall y selective instrumems use band pass filters to allow only a very 

specific broad or narrow band of wavelengths to reach the detector. (A 

combination of a spectrally selective detector and a filter can also be used.) 

This can make the instrument highl y selective to a specific material whose 

temperature is to be measured in the presence of an intervening medium or 

an interfering background. Solving measurement problems by means of 

spectrally selective instruments is discussed in greater detail in Chapter 4. 

For general purpose use and for measuring cooler targets cooler than about 

500 °C (932 °F). most manufacturers of radiation thermometers offer 

instruments operating in the 8 to 14 μm atmospheric window. For dedicated 

use on hotter targets. shorter operating wavelengths are selected. usually 

shorter than 3 μm. One reason for choosing shorter wavelengths is that this 

enables manufacturers to use commonly available and less expensive quartz 

and glass optics, which have the added benefit of being visibly transparent for 

more convenient aiming and sighting. Another reason is that estimating 

emissivity incorrectly will result in smaller temperature errors when 

measurements are made at shorter wavelengths. 


Thermographers have learned that a good general rule to follow, particularly 

when dealing with targets of low or uncertain emissivities, is to work at the 

shortest wavelengths possible without compromising sensitivity or risking 

susceptibility to reflections from visible energy sources. 


Figure 3.9: Spectral response of an instrument determined by detector and 

optics spectra 

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90 

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Wavelength /... (J..Lm) 


MWIR OR LWIR? 

For general purpose use and for measuring cooler targets cooler than about 

500 °C (932 °F). most manufacturers of radiation thermometers offer 

instruments operating in the 8 to 14 μm atmospheric window. For dedicated 

use on hotter targets. shorter operating wavelengths are selected. usually 

shorter than 3 μm. One reason for choosing shorter wavelengths is that this 

enables manufacturers to use commonly available and less expensive quartz 

and glass optics, which have the added benefit of being visibly transparent for 

more convenient aiming and sighting. Another reason is that estimating 

emissivity incorrectly will result in smaller temperature errors when 

measurements are made at shorter wavelengths. 

Thermographers have learned that a good general rule to follow, particularly 

when dealing with targets of low or uncertain emissivities, is to work at the 

shortest wavelengths possible without compromising sensitivity or risking 

susceptibility to reflections from visible energy sources. 


3.7 Performance Characteristics of Scanners and 

Imagers 

Because an infrared thermogram consists of a matrix of discrete point 

measurements, many of fhe performance parameters of infrared thermal 

imager are the same as those of radiation thermometers. The output of an 

infrared line scanner can be considered as one line of discrete point 

measurements. The parameters of temperature range, absolute accuracy. 

repeatability, sensor environment and spectral range are esscntially the same 

for radiation thermometers, line scanners and imagers. Others are derived 

from or are extensions of radiation thermometer performance parameters. 

Qualitative thermal imagers (also called thermal viewers) differ from 

quantitative thermal imagers (also called imaging radiometers) in that thermal 

viewers do not provide temperature or thermal energy measurements. For 

thermographers requiring qualitative rather than quantitative thermal images, 

therefore, some performance parameters are unimportant. 


Total Field of View (FOV total ) 

For scanners and imagers. total field of view denotes the image size in terms 

of total scanning angles for any given lens. An example of a typical total field 

of view specifi cation would be "TFOV = 20 degrees vertical x 30 degrees 

horizontal" (with standard Ix lens) and would define the thermogram total 

target size by a simple trigonometric relationship: 

tan θ/2 = V/2∙d -1 

V = 2 ∙ tan (y/2) ∙ d, for θ = y 

d = working distance, 

H = total horizontal image size, 

V = total vertical image size, 

x = horizontal scanning angle, 

y = vertical scanning angle. 

θ = y or x 

This is illustrated in Figure 3. 10. 


The total field of view for a line scanner consists of one scan line as shown in 

Figure 3.4 and Figure 3.10. The horizontal image size H is equal to the scan 

sector. The vertical image size V is equal to the instantaneous field of view. 

All other parameters are the same as for an imager. 

I 0 (mR 

Figure 3.4: Line scanner scanning configuration 


Figure 3.10: Total field of view (TFOV) determination for an infrared imager 

Target Size (TFOV) 

TFOV = total field of view (target size) = V x H 

at d 

IFOV = instantaneous field of view IFOV at d 1 ~ H I 

H = total horizontal Image size = 

d[2 tan (x/2)] 

V = total vertical image size = 

d[2 tan (y/2)] 

where: 

d =mean distance to the target (em, ft) 

x = image horizontal angular subtense 

(degrees) 

y = image vertical angular subtense 

(degrees) 

Imager 

l v 

~_____,. l 


Instantaneous Field of View IFOV 

Instantaneous field of view in an imager is very similar to that for a point 

sensing instrument: it is the angular projection of the detector element at the 

target plane. (resolution?) 

In an imager, however, it is also called imaging spatial resolution and 

represents the size of the smallest picture element that ean be imaged. An 

example of a typical instantaneous field of view specification would be "IFOV 

= 1.7 mRad at 0.35 MTF." The 0.35 MTF refers to 35 percent of the 

modulation transfer function test used to check imaging spatial resolution. 

This is described in detail in Chapter 4. The simple expression. D = αd, can 

be used to estimate imaging spot size at the target plane from manufacturer's 

published data by substituting the published instantaneous field of view for α. 

Keywords: 

IFOV, image spatial resolution, 

MTF-modulated transfer function 


EXAM score! 

IFOV 

is also called; 

image spatial resolution 



Recalling! 



or 





Measurement Spatial Resolution 

Measurement spatial resolution (IFOVmeas) is the spatial resolution of the 

minimum target spot size on which an accurate measurement can be made in 

lenns of its distance from the instrument. An example of a typical 

measurement spatial resolution specification would be "IFOVmeas = 3.5 mrad 

at 0.95 SRF.“ The 0.95 SRF refers to 95 percent slit response function test 

used to check measurement spatial resolution. This is described in detail in 

Chapter 4. The simple ex pression, D = αd, can again be used to estimate 

measurement spot size at the target plane from manufacturer's published 

data by substituting published measurement spatial resolution for α. 

Keywords: 

SRF refers to 95 percent slit response function test used to check 


Comments: 

IFOVmeas – IFOV measurement 


IFOV - MTF 

The 0.35 MTF refers to: 

0.35 percent of the modulation transfer 

function test used to check imaging 

spatial resolution. 



IFOV meas -SRF 

95 SRF refers to: 

95 percent slit response function test 

used to check measurement spatial 

resolution 



Fig. 2a. Slit Response Function. Camera sees slit lips of radiometric temperature T0 

(back side radiometric temperature) and The body behind the slit of radiometric 

temperature T1 (“slit “ temperature). Slit width is d and D is the distance slit-camera 

(Figure is issue from reference 4) 

B Ia ck bod y behind the slit at th·e 

rad iometric temper ature T 1 (Level 

L1) 

/ 

/~ 

Slit at the rad iometric 

tem1pe-rature T 0 (Level L 0 ) 


http://qirt.gel.ulaval.ca/archives/qirt2006/papers/025.pdf

Frame Repetition Rate 

Frame repetition rate replaces speed of response and is defined as the 

number of times every point on the target is scanned in one second. This 

should not be confused with field rate. Some imagers are designed to 

interlace consecutive fields. each consisting of alternate image lines. This 

results in images less disconcerting 令人不安的 to the human eye. The frame 

rate in this case would be one half the field rate. An example of a typical 

frame repetition rate specification for an imager would be "frame repetition 

rate = 30 frames per second." For a line scanner. the term line scan rate is 

used and it is expressed in lines per second. 

Comments: 

For interlace field rate scanning; The frame rate in this case would be one half 

the field rate. 


Minimum Resolvable Temperature Difference 

Minimum resolvable temperature (MRT) or minimum resolvable temperature 

difference (MRTD) replaces temperature sensitivity and is defined as the 

smallest blackbody equivalent larget lemperature difference Ihat can be 

observed OUI of system noise on a thermogram. As in radiation thennometry. 

this difference improves (becomes smaller) with increasing target temperature 

and is expressed in those terms. An example of a typical minimum resolvable 

temperature diffe rence speci fi cation for a line scanner or an imager would 

be "MRTD = 0.05 °C at 25 °C target temperature (0.09 OF at 77 

OF),“ Minimum resolvable temperature difference may also depend on the 

spatial frequency imposed by the test discipline. The test techniques for 

checking minimum resolvable temperature difference is described in Chapter 

4, 

Comments: Temperature sensitivity is also called: thermal resolution or 

noise equivalent temperature difference (NETD). 


Thermal Imaging Display and Diagnostic Software Overview 

Thermography applications often req uire extensive thermal imaging display 

and diagnostic software. Thermal imagers feature image processing 

capabilities that may be divided into five categories. one or more of which 

may be used in the same application. These categories are quantitativc 

thermal measurements of targets; detailed processing and image diagnostics; 

image recording. storage and recovery; image comparison (differential or 

multispectralthermography); and database and documenlalion. Applications 

using software capabilities, singly and in combination. will also be described 

in Chapter 5. 


EXAM score! 

D=σ∙d 


(when calculation IFOV ratio 

care on unit used!) 



FOV - Animation 

96,46 .. 

32,15 " 


http://www.imagerchina.com.cn/fov_calculator.html


Questions & Answers 

Subject: Answer this web queries from: http://www.thesnellgroup.com/community/ir-talk/f/9/p/1402/5433.aspx 

wonder if anyone can help me here. I am studying for my employer's Level 2 certification exam and I am using 

the ASNT supplement booklet to help. They ask a few question about IFOV and spot size calculation and I do 

not quite understand how they get the answers. basically it is not the answer I want but how they got to the 

answers. 

Question #1: A camera has an IFOV of 1.9 mRad. What is it's theoretical minimum spot size at a distance of 

100 cm? Answer is: 0.19 cm (What formula is used for this and are there any units conversion like mm to cm or 

mRad to something else?) 

Question #2: The IFOV measurement of a radiometric system is 1.2 mRad. What is the maximum size object 

this system can accurately measure at a distance of 25 m? Answer is: 3 cm (now clearly there are unit 

conversions going on here from meters to cm. So how is it done?) 

Question #3: You are looking at an electrical connection 20 m in the air. What IFOV measurement is required to 

accurately measure the temperature on the 2.54 cm (1 in.) head of a bolt? Answer is: 1.25 mRad (I know it's 

just a matter of transposing the formula, but again there is units changes and I do not know the formula to apply) 

Last question: Using an IR system with an IFOV measurement ratio of 180:1. What is the smallest size object 

you can accurately measure at a distance of 3m (3.3 ft)? Answer is: 16.6 mm or (0.65 in). 

NOW this one I kind of figured out using: 1/180 = 0.0055 & 3 m = 3000mm therefore 0.0055 x 3000 = 16.5 

Let me know if you all know how to do these problems. I think all I need is the formula and an understanding 

when and which units to convert. 


Answer: D= σ•d, IFOV ration= 1/σ = d/D 

Question #1: A camera has an IFOV of 1.9 mRad. What is it's theoretical minimum spot size at a distance of 

100 cm? Answer is: 0.19 cm (What formula is used for this and are there any units conversion like mm to cm or 

mRad to something else?) 

Calculation: D= 1.9 x 1 = 1.9mm or 0.19cm, (100cm = 1m) 

Question #2: The IFOV measurement of a radiometric system is 1.2 mRad. What is the maximum size object 

this system can accurately measure at a distance of 25 m? Answer is: 3 cm (now clearly there are unit 

conversions going on here from meters to cm. So how is it done?) 

Calculation: D= 1.2 x 25m = 30mm = 3cm 

Question #3: You are looking at an electrical connection 20 m in the air. What IFOV measurement is required to 

accurately measure the temperature on the 2.54 cm (1 in.) head of a bolt? Answer is: 1.25 mRad (I know it's 

just a matter of transposing the formula, but again there is units changes and I do not know the formula to apply) 

Calculation: 25.4 = σ x 20, σ = 1.27mRad 

Last question: Using an IR system with an IFOV measurement ratio of 180:1. What is the smallest size object 

you can accurately measure at a distance of 3m (3.3 ft)? Answer is: 16.6 mm or (0.65 in). 

Calculation: 1/ σ = d/D = 180, σ = 1/180, 

D = σ∙d, D = 1/180 x 3 = 0.01667m = 16.7mm 

(when calculating IFOV ratio, good to use the same unit for all inputs) 



Break Time 

– Kenya Coffee Picker 


http://www.kickstartcafe.com/journal/kenyan-coffee#.VWuZY52S3IU

3.8 Descriptions of Thermal Sensing and Imaging 

Equipment 

Point Sensors (Radiation Thermometers) 

Point sensors (radiation thermometers) can be further divided into 

temperature probes. portable hand held devices. online process control 

devices and specially configured devices. 

■ Temperature Probes 

Temperature probes are low priced, pocket portable, battery powered devices 

that usually feature a pencil shaped sensor connected to a small basic 

readout unit. Generally, they are optically pre-adjusted for minimum spot size 

at a short working distance. A 0.5cm (0.2 in.) spot al a 2 cm (0.8 in.) working 

distance is typical. Temperature usually ranges from about - 20 °C to 300 °C 

(- 4 ° F to 570 ° F) and a sensitivity of ±1°C (1.8 ° F) is achieved easily. 

Probes are designed for close-up measurements such as circuit board 

analysis. troubleshooting of electrical connections. inspect ion of plumbing 

systems and biological and medical studies. 


Portable Handheld Devices 


■ Portable Handheld Devices 

Portable handheld radiation thermometers are designed for middle distance 

measurements and, with few exceptions, operate in the 8 to 14 μm spectral 

region and are configured like a pistol for one-handcd operation and aiming. 

They are usually optically preadjusted for infinity focus. 

A typical 2 degree field of view resolves a 7.5 cm (3 in.) spot at a 150 cm (60 

in.) working distance and a 30 cm (1 ft) spot at a 9 m (30 ft) working distance. 

(9 x tan(2º) = 0.314m=31cm) 

Most instruments in this group incorporate microcomputers with limited 

memory and some have data logging capabilities. An open or enclosed 

aiming sight is provided and in some models a projected laser beam is used 

to facilitate aiming of the instrument as shown in Figure 3. 11. Note that the 

laser beam docs not represent the field of view. A measurement readout is 

always provided and usually the temperature is shown on a digital liquid 

crystal display. These instruments are powered with disposable batteries and 

have low power drain.Temperature ranges are typically from 0 to 1000 °C (30 

to 1800 ºF). 


Temperature sensitivity and readability are usually 1 percent of scale 1°C (2 

ºF) although sensitivities on the order of 0.1 °C (0.2 ° F) arc achievable. 

Response times are on the order of fractions of a second, usually limited by 

the response of the readout. 

Hand held radiation thermometers are used extensively in applications where 

spot checking of target temperatures is sufficient and continuous monitoring is 

not required. Handheld radiation thermometers have become an important 

part of many plant energy conservation programs. Process applications 

include monitoring mixing temperatures of food products. cosmetics and 

industrial solvents. Microcomputers enable handheld instruments to 

incorporate special features such as the ability to store sixty readings for 

future retrievals and printout. 


Figure 3.11: Hand held infrared radiation thermometer with laser aiming 


Hand Held Infrared Module 


Note that the laser beam docs not represent the field of view. 

Figure 1. Use the Fluke 66 within 5 m (15 ft.) of the intended target. 

At greater distances, the measured area will be larger (approximately 

the distance divided by 30). Field of view θ= tan -1 (1/30) = 1.91º 

D:S = 30:1 

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http://www.fluke.com/fluke/m3en/products/thermometers

Note that the laser beam docs not represent the field of view. 

Figure 2. Use the Fluke 68 within 8 m (25 ft.) of the intended target. 

At greater distances, the measured area will be larger (approximately 

the distance divided by 50). Filed of view θ= tan -1 (1/50) = 1.14º 


http://www.fluke.com/fluke/m3en/products/thermometers

■ Online Process Monitoring and Control Devices 

Online monitoring and control sensors are for dedicated use on a product or a 

process. Permanently installed where it can measure the temperature of one 

specific target. this type of instrument remains there for the life of the 

instrument or the process. With few exceptions. these instruments operate on 

line power. The measurement value can be observed on a meter. but it is 

more often used to trigger a switch or relay or to feed a simple or 

sophisticated process control loop. Most of the online monitoring and control 

sensors send signals to universal indicator control units that accept inputs 

from various types of industrial sensors. Because this instrument group is 

selected to perform a specific task, a shopping list format is provided to the 

customer by the manufacturer so that all required features can be purchased. 

including environmental features such as water cooled housings. air purge 

fittings and air curtain devices. 


Emissivity set controls, located in a prominent place on a general purpose 

instrument are more likely to be located behind a bezel 嵌槽 / 柜 on the 

sensor on these dedicated units. where they are set once and locked. The 

spectral interval over which the sensing head operates is selected to optimize 

the signal from the target, to reduce or eliminate the effect of an interfering 

energy source or to enable the instrument to measure the surface 

temperature of thin films of material that are largely transparent to infrared 

radiation. The capability for spectral selectivity has made these instruments 

important in the manufacture of glass and thin film plastics. Applications in 

these atres are discussed in Chapters 4 and 5. 


IR Sensor Module 








■ Devices with Special Configurations 

Special configurations of infrared radiation thermometers include ratio 

pyrometers (also called two color pyrometers), infrared radiometric 

microscopes, laser reflection pyrometers and fiber-optic coupled pyrometers. 

1. Two-color pyrometers or ratio pyrometers, are a special case of the online 

instrument. Ratio pyrometers are particularly useful in high temperature 

applications above 300 °C (572 ° F) and in measuring small targets of 

unknown emissivity, provided the background is cool. constant and uniform. 

The emissivity of the target need not be known if it is constant and relections 

are controlled. The target does not need to fill the field of view. provided the 

background is cool, constant and uniform. The measurement is based on the 

ratio of energy in two spectral bands. so impurities in the optical path resulting 

in broad band absorption do not affect the measurement. Ratio pyrometers 

are usually, not applicable to measurements below 300 °C (572 °F). 


Two-color Pyrometers or Ratio Pyrometers 





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Ratio Radiation - Also called two-color radiation thermometers, these devices measure the 

radiated energy of an object between two narrow wavelength bands, and calculates the ratio of 

the two energies, which is a function of the temperature of the object. Originally, these were 

called two color pyrometers, because the two wavelengths corresponded to different colors in the 

visible spectrum (for example, red and green). Many people still use the term two-color 

pyrometers today, broadening the term to include wavelengths in the infrared. 

The temperature measurement is dependent only on the ratio of the two energies measured, and 

not their absolute values as shown in Figure 3-4. 

Any parameter, such as target size, which affects the amount of energy in each band by an equal 

percentage, has no effect on the temperature indication. This makes a ratio thermometer 

inherently more accurate. (However, some accuracy is lost when you're measuring small 

differences in large signals). The ratio technique may eliminate, or reduce, errors in temperature 

measurement caused by changes in emissivity, surface finish, and energy absorbing materials, 

such as water vapor, between the thermometer and the target. These dynamic changes must be 

seen identically by the detector at the two wavelengths being used. 

Emissivity of all materials does not change equally at different wavelengths. Materials for which 

emissivity does change equally at different wavelengths are called gray bodies. Materials for 

which this is not true are called non-gray bodies. In addition, not all forms of sight path 

obstruction attenuate the ratio wavelengths equally. For example, if there are particles in the 

sight path that have the same size as one of the wavelengths, the ratio can become unbalanced. 


http://www.omega.com/literature/transactions/volume1/thermometers2.html

Figure 3-4: The “Two-Color” IR Thermometer 

E 1 

T1 

E 1 

E 2 

T2 

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Phenomena which are non-dynamic in nature, such as the non-gray bodiness 

of materials, can be dealt with by biasing the ratio of the wavelengths 

accordingly. This adjustment is called slope. The appropriate slope setting 

must be determined experimentally. Figure 3-5 shows a schematic diagram of 

a simple ratio radiation thermometer. Figure 3-6 shows a ratio thermometer 

where the wavelengths are alternately selected by a rotating filter wheel. 

Figure 3-5: Beam Splitting in the Ratio IR Thermometer 



Figure 3-6: Radio Pyometry Via a Filter wheel 

Figure 3-7: Schematic of a Multispectral IR Thermometer. 



Some ratio thermometers use more than two wavelengths. A multi-wavelength device 

is schematically represented in Figure 3-7. 

These devices employ a detailed analysis of the target's surface characteristics 

regarding emissivity with regard to wavelength, temperature, and surface chemistry. 

With such data, a computer can use complex algorithms to relate and compensate for 

emissivity changes at various conditions. The system described in Figure 3-7 makes 

parallel measurement possible in four spectral channels in the range from 1 to 25 

microns. The detector in this device consists of an optical system with a beam splitter, 

and interference filters for the spectral dispersion of the incident radiation. This 

uncooled thermometer was developed for gas analysis. Another experimental system, 

using seven different wavelengths demonstrated a resolution of +/-1°C measuring a 

blackbody source in the range from 600 to 900°C. The same system demonstrated a 

resolution of +/- 4°C measuring an object with varying emittance over the temperature 

range from 500 to 950°C 

Two color or multi-wavelength thermometers should be seriously considered for 

applications where accuracy, and not just repeatability, is critical, or if the target object 

is undergoing a physical or chemical change. Ratio thermometers cover wide 

temperature ranges. Typical commercially available ranges are 1652 to 5432° F (900 

to 3000°C) and 120 to 6692°F (50 to 3700°C). Typical accuracy is 0.5% of reading on 

narrow spans, to 2% of full scale. 



2. Infrared radiometric microscopes are configured like a conventional 

microscope and by using reflective microscope objectives and beam 

splitters, the operator can simultaneously view and measure targets down 

to 10 μm in diameter with accuracy and resolution of about 0,5 °C (1 °F). 

3. Laser reflection pyrometers use the reflected energy of an active laser to 

measure target reflectance. A built-in microcomputer calculates target 

effective emissivity and uses this figure to provide a corrected true 

temperature reading. This instrument. though expensive, is useful for 

measurement of high temperature specular target surfaces in adverse 

environments. 

4. Fiberoptic coupled pyrometers make possible the measurement of 

normally inaccessible targets by replacing the optic with a flexible or rigid 

fiberoptic bundle. This limits the spectral performance and hence the 

temperature range to the higher values, but has allowed temperature 

measurements to be made when previously none were possible. 


Infrared Radiometric Microscopes 


Fiberoptic Coupled Pyrometers 


http://www.omega.com/temperature/pdf/4121_ir.pdf

Line Scanners 

Line scanners are divided into online process control devices and special 

purpose scanners. 

■ Online Process Control Devices 

Online (monitoring and control) line scanners are high speed online 

commercial line scanners that develop high resolution thermal maps by 

scanning normal to the motion of a moving target such as paper web or a 

strip steel process. The vast majority of commercial infrared line scanners are 

in this configuration. The output signal information is in a real time computer 

compatible format and can be used to monitor, control or predict the behavior 

of the target. Like the online point sensor, these line scanners are usually 

permanently installed where they monitor the temperature profile at one site 

of the process, remaining there for the life of the instrument or the process. 

Likewise they are usually fitted with environmental housings and preset 

emissivity compensation sets. The best applications for this scanner are in 

online, real time process monitoring and control applications where they are 

integrated with the process host computer system. 


It is not unusual to find line scanners at multiple locations in a process with all 

of them linked to the host computer. In the 1990s, infrared line scanners 

based on a linear focal plane array came into use. This type of instrument 

frequently uses an un-cooled array of thermal detectors radiation thermopiles. 

This scanner has no moving parts. The linear array is oriented perpendicular 

to a process or a target moving at a uniform rate. The scanner output may be 

used to develop a thermograms or the data for each pixel can be fed directly 

to a host computer and used to monitor and control the process. Instruments 

of this type have been used to monitor moving railroad cars for overheated 

wheels and brake assemblies. 


Special Purpose Devices 

Special purpose configurations of line scanners include one type of portable 

line scanner and a number of aerial mappers that scan a line normal to the 

motion of the aircraft and develop a thermal strip map. Many of these 

mappers have been replaced by low cost forward looking infrared scanners 

(FLIRs) based on staring focal plane arrays. 


FLIR- Forward Looking Infrared 


FLIR- Forward Looking Infrared 


Imagers (Thermographic Instruments) 

Imagers (thermographic instruments) consist of both qualitative and 

quantitative imagers. 

■ Qualitative Thermal Imagers 

Qualitative thermal imagers arc also called thermal viewers. They include 

mechanically scanned, electronically scanned (pyrovidicon) and staring focal 

plane array FPA imagers. 

● Mechanically Scanned Thermal Viewers 

Mechanically scanned thermal viewers are moderately priced battery 

powered scanning instruments that produce a qualitative image of the 

radiosity over the surface of a targct. The battery packs are rechargeable and 

usually provide 2 to 3 h of continuous operation. These one-piece, lightweight 

instruments, designed to be simple to operate, feature thermoelectric detector, 

cooling provided by a battery powered cooler. Although not designed for 

absolute temperature measurements, they can demonstrably sense 

temperature differences of tenths of degrees and can be used for targets from 

below 0 °C up to 1500 °C (32 of up to 2372 °F). 


Typically, the total field of view is from 6 to 8 degrees high and from 12 to 18 

degrees wide, with spatial resolution of 2 mRad 10 mm at 2.0 m (0.4 in. at 7 

ft). Images are video recorded by means of a conventional video tape 

recorder output jack and video recorder accessories. The broad applications 

for thermal viewers are generally limited only to those in which the 

temperature measurements are not critical and recording quality does not 

need to be optimum. The combination of a thermal viewer (to locate thermal 

anomalies) and a hand held thermometer (to quantify them) can be a 

powerful and cost effective ombination. 


● Electronically Scanned Viewers (Pyrovidcon Imagers) 

Pyrovidicon imagers arc electronically scanned video cameras. The camera 

tube is sensitive to target radiation in the infrared rather than the visible 

spectrum. Aside from the tube and germanium lens, which are expensive, 

these systems use television recording accessories, in comparison with other 

infrared imaging systems, the picture quality and resolution are good, 

approaching conventional television format. 

The thermal image can be viewed or videotaped with equal convenience and 

no cooling is required. Pyrovidicon systems do not intrinsically offer 

quantitative measurement capability, but some manufacturers offer models in 

which an integrated radiation thermometer is bore sighted with the scanner 

and its measurement is superimposed on the video display along with a 

defining reticle in the center of the display thermal resolution of flicker free 

pyrovidicon instruments is between 0.2 and 0.4 °C (0.4 and 0.7 °F). 


Pyroelectric devices have no direct current response, and a basic pyrovidicon 

imager 's display will fade when the device is aimed at an unchanging thermal 

scene. Early pyrovidicon imagers needed to be panned to retain image 

definition. 

To enable fixed monitoring, crude, flag type choppers were devised to 

interrupt the image at adjustable chop rates. However, this resulted in a 

blinking image that was disconcerting to the eye. These choppers have been 

replaced by synchronous choppers that chop the image in synchronism with 

the electronic scan rate and produce flicker free images on the display. 

Pyrovidicon viewers operate well in the 8 to 14 μm atmospheric transmission 

window. Operating costs are very low because no cooler or coolant is 

required. 


● Staring Infrared Focal Plane Array Thermal Viewers 

Staring infrared focal plane array (lRFPA) thermal viewers are direct 

adaplations of devices developed for military and aerospace night vision and 

missile tracking applications. For these applications, performance emphasis 

is on picture quality rather than measurement capability. Instruments using 

cooled platinum silicide (PtSi) staring arrays with as many as 512 x 512 

elements are available. Instrument using cooled indium antimonide (LnSb) 

focal plane arrays are available in models designed to compete with top-ofthe-line 

commercial thermal imagers. Some instruments in this category have 

the size and weight of a commercial video camera that fits in the palm of the 

hand, as illustrated in Figure 3.12. 


Figure 3.12: Infrared focal plane array imager for qualitative thermography 


Infrared focal plane array imager 




Qualitative IrFPA 




■ Quantitative Thermal Imagers 

Quantitative thermal imagers include (1) mechanicatly scanned thermal 

imagers (imaging radiometers) and (2) focal plane array radiometers. 

● Mechanically Scanned Thermal Imagers 

Mechanically scanned thermal imagers (imaging radiometers) provide a 

means for measuring apparent target surface temperature with high 

resolution image quality and sometimes with extensive on-board diagnostic 

software. Mosl commercially available imaging radiometers use a single 

detector. but some manufacturers offer dual detector or multidctcctor (linear 

array) instruments. Most require detector cooling. Imaging radiometers use 

refractive reflective or hybrid scanning systems and operate in either the 3 to 

5 μm or the 8 to 14 μm atmospheric window. They generally offer 

instantaneous fields of view on the order of 1 to 2 mrad with standard optics 

and minimum resolvable temperature differences of 0.05 to 0.10 °C (0.09 to 

0.18 °F). 


On-board capabilities include isotherm graphics features, spectral filtering. 

interchangeable optics for different total field of views. color or monochrome 

(black and white) displays, flexible video recording capabilities and computer 

compatibility. Most feature compact, field portable, battery operable sensing 

heads and control/display units. A complete system including battery and 

video recorder can be handled by one person by mounting the components 

on a cart or by assembling them on a harness. 


● Focal Plane Array Radiometers 

Focal plane array radiometers are adaptations of military and aerospace 

forward looking infrared scanners. but are designed to measure the apparent 

temperature at the target surface and to produce quantitative thermograms. 

The capabilities of early infrared focal plane array imagers were slow in 

developing. The quality of measurement capabilities has improved since 1990. 

Infrared focal plane array cameras offer minimum resolvable temperature 

differences comparable to imaging radiometers (0.1 to 0.2 °C; 0.18 to 0.36 °F) 

and instantaneous field of views considerably better than imaging 

radiometers (1 mRad or better with standard optics). 

Commercially available quantitative infrared focal plane array cameras use 

detector arrays made of platinum silicide or indium antimonide, either of 

which requires cooling. Quantitative thermal imagers based on uncooled focal 

plane arrays (using bolometrie and ferroelectric detectors) have also been 

developed. With inherently faster response, no moving parts and superior 

spatial resolution infrared focal plane array cameras have been replacing 

infrared imaging radiometers for most applications. 


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Quantitative IR Image 


Quantitative IR Image 


3.9 Thermal Imaging Display and Diagnostic 

Software 

When the personal computer was introduced as part of thermal imaging 

systems, the typical imager produced raw radiometric data. whereas all of the 

diagnostic software was contained in an ancillary. separately packaged 

computer that performed all of the diagnostics back on the bench. With 

improved packaging technology in both computers and thermal imaging 

equipment, there has been a gradual trend toward providing more and more 

on board software so that more diagnostics can be performed on site. 

Depending on manufacturer and model, some software is incorporated into 

instruments and some is available only on computer driven software 

packages. Although thermographic diagnostic software packages are usually 

proprietary to a particular manufacturer, there is a trend toward universality in 

image storage. Common formats for storing electronic images include tagged 

image file format (TIFF) and other bitmapped formats. Retrieving images from 

these format is fast and easy. 


Quantitative Thermal Measurements 

Some qualitative thermograms can be converted to quantitative thermograms. 

The raw image produced by a quantitative imager may be converted to a 

quantitative thermogram; the raw image produced by a viewer may not. 

Quantitative thermal measurements provide the user with the true radiance or 

apparent temperature value of any or all points on the target surface. To 

present the thermogram in true radiance measurements, the system 

throughput attenuation must be considered as well as losses through the 

measurement medium (atmosphere, in most cases). To present the 

thermogram in true temperature values. the target effective emissivity must 

also be considered. When this capability is provided, a menu instructs the 

user to enter system calibration constants on initial setup and a system of 

prompts assures the operator that changes in aperture settings, target 

distance, inter-changeable lenses. etc., will be fed into the keyboard each 

time a change in operating conditions occurs. 


Changes in the corrections setting for target effective emissivity are also 

monitored. In addition. digital cameras are available to save visible images in 

computer compatible format for archiving with corresponding thermograms. 

For most systems. the displayed temperature readings are based on the 

assumption that the entire target surface has the same effective emissivity. 

Some systems. however. allow the assignment of several different 

emissivities to different areas of the target selected by the operator with the 

resulting temperature correction. A color scale or gray scale is provided along 

one edge of the display with temperature shown corresponding to each color 

or gray level in the selected range. The operator can place one or more spots 

or crosshairs on the image and the apparent temperature value of that pixel 

will appear in an appropriate location on the display. The isotherm feature 

allows the operator to select a temperature band or interval and all areas on 

the target within that band then appear enhanced in a predetennined gray 

shade or color hue. Detailed processing and image diagnostics relies on 

software that allows manipulation and analysis of each pixel in the 

thermogram prescnting information in a wide variety of qualitative and 

quantitative forms for the convenience of the user. Some of these capabilities 

are described in this chapler. 


In addition to the spot measurement capability discussed previously. line 

profiles may be selected. The analog trace. in X, Y. or both. of the lines on the 

image intersecting at the selected spot will then appear at the edge of the 

display. Some systems allow the operator to display as many as seven sets 

of profiles simultaneously. Profiles of skew lines can also be displayed on 

some systems. Selected areas on the thermogram in the form of circles, 

rectangles or point-to-point free forms, can be shifted, expanded. shrunk or 

rotated or used to blank out or analyze portions of the image. 

Detailed analysis of the entire image or the pixels within the area can include 

maximum, minimum and verage values. number of pixels or even a 

frequency histogram of the values within the area. Color scales can be 

created from 256 colors stored in the computer. Electronic zoom features 

allow the operator to expand a small area on the display for closer 

examination. or to expand the colors for a small measurement range. 

Autoscale features provide the optimum display settings for any image if 

selected. Three-dimensional features provide an isometric thermal contour 

map or thermal profile map of the target for enhanced recognition of thermal 

anomalies. 


Image Recording, Storage and Recovery 

Images and data can be stored in and retrieved from memory, hard disk, 

floppy diskette, video tape, optical disks (writable compact disks and 

digitalvideo disks) and Personal Computer Memory/Computer Industry 

Association (PCMCIA) cards. 

Commercial thermal imaging systems incorporate some means, such as a 

floppy disk drive or a PCMCIA card to store images in the field. Usually. about 

forty images. with all accompanying data, can be stored on a 3.5 in diskette. 

Some analysis usually can be done with on-board software; more extensive 

diagnostics usually require a separate computer. Options include IEEE or 

RS232 ports for access to additional storage and a video recorder option so 

that an entire measurement program can be recorded on video tape. Video 

tapes can be played back into the system and images can be saved to disk. 

Images can be stored from a frozen frame thermogram of a live target on 

operator command. or the operator can set up an automatie sequence and a 

preset number of images will be stored at preset time intervals. 


Stored images can be retrieved, displayed and further analyzed. Image 

comparison (differential thermography) allows the automatic comparison of 

thermograms taken at different times. This includes time based comparison of 

images taken of the same target as well as the comparison of images taken 

of different but similar targets. 

A special software program allows the operator to display two images sideby- 

ide or in sequence; and to subtract one image from another or one area 

from another; and to display a pixel-by-pixel difference thermogram. 

Comparison (subtraction) of images can be accomplished between two 

images retrieved from disk, between a live image and an image retrieved from 

disk and between a live image and an image stored in a computers random 

access memory, in this way, standard thermal images of acceptable 

components, assemblies and mechanisms can be archived and used as 

models for comparison to subsequently inspected items. It is also possible to 

subtract a live image from a previous baseline image for subsequent time 

based thermal transient measurements. 


Database and Documentation 

Records, files, data and documents can be saved in an orderly fashion. This 

capability provides thc thermographers with a filing system so that records of 

all measurement missions can be maintained on magnetic media, including 

actual thermograms, time, date, location, equipment, equipment settings, 

measurement conditions and other related observations. 

Most manufacturers of thermal imaging equipment have developed 

comprehensive report preparation software to facilitate timely and 

comprehensive reporting of the findings of infrared surveys and other 

measurement missions. These packages provide templates that allow thc 

thermographer to prepare reports in standard word processor formats into 

which tagged image file format (TIFF) images. imported from various imaging 

radiometers. can be directly incorporated. Additional diagnostic software is 

customarily provided in these packages so that analysis and trending can be 

added to reports. 


Calibration Accessories 

Infrared radiation reference sources are used by manufacturers to calibrate 

infrared sensing and imaging instruments in the laboratory before they are 

shipped. These same reference sources are used later at periodic intervals 

thereafter to ensure calibration stability. A radiation reference source is 

designed to simulate a blackbody radiator: that is. a target surface with a 

stable, adjustable known temperature and a uniform emissivity approaching 

1.0 at all appropriate wavelengths. In addition to laboratory reference sources. 

there are field portable models suitable for periodic calibration checks of 

fielded thermographic equipment and for other tasks. The setup and 

deployment of radiation reference sources is discussed in Chapter 4. 


3.10 Photorecording Accessories for Hard Copies 

Since the advent of the personal computer and its integration with thermal 

imagers, magnetic storage and archiving of data (labels. dates. conditions of 

measurement. instrument settings. etc.) as well as thermograms have 

become routine. Soft copies can be made of real time images, processed 

images enhanced images and combined images on floppy disks, analog and 

digital magnetic tape, recordable optical disks and Personal Computer 

Memory/Computer Industry Association (PCMCIA) cards. 

Report preparation software allows images to be inserted into word 

processing documents and printed by conventional laser or inkjet printers. 

Making a hard copy directly from a stored or displayed image is done in a 

variety of ways. A number of devices were introduced before magnetic media 

were available for directly photographing the display between with 

conventional or instant film. Using them generally required considerable skill 

because the ambient lighting and the screen curvature had to be considered. 

For this reason. it was difficult to achieve repeatable results. online printers 

and plotters provide reliable, good quality copies when speed is not a 

consideration. 


Online printers and plotters are relatively slow and may tie up the computer 

and related software during operation. For real time or high speed photorecording, 

portable video printers are usually selected. The video printer 

connects to the system's video output. It presents the current image on a 

remote display where it is frame grabbed and reproduced in real time under 

optimized conditions. Most video printers produce output on integral recorder 

paper. Available accessories allow a choice of direct instant hardcopies, 

negatives or slide transparencies. Although video printers are costly. they 

provide consistent quality in a reasonable time and do not require the use of 

the thermal imager or the computer during production time. 


Chapter 3 


Q&A 

1. b 

2. d 

3. a 

4. b 

5. d 

6. a 

7. c 

8. c 

9. d 

10. d 

11. b 

12. a 

13. b 

14. b 

15. a 

16. b 

17. d 

18. b 

19. e 

20. a 

21. d 

22. a 

23. a 

24. d 

25. b 


Q1. The thermal resolution of an instrument is the same as: 

a. the temperature accuracy. 

b. minimum resolvable temperature difference. 

c. temperature repeatability. 

d. the minimum spot size. 

Q2. The speed of response of an instrument is: 

a. the time constant of the detector. 

b. one half the time constant of the detector. 

c. the same as the field repetition rate. 

d. the time it takes to respond to a step change at the target surface. 

Q3. The instantaneous spot size of an instrument is related to the: 

a. instantaneous field of view and the working distance. 

b. thermal resolution. 

c. spectral bandwidth and the working distance. 

d. speed of response and the working distance. 


Q4. The performance parameters that are important for qualitative 

thermography are: 

a. absolute accuracy, repeatability and resolution. 

b. spatial resolution and thermal resolution. 

c. spatial resolution and absolute accuracy. 

d. measurement spatial resolution and thermal resolution. 

Q5. Thermal viewers do not provide: 

a. high resolution thermograms. 

b. recording capabilities. 

c. real time scan rates. 

d. quantitative thermograms. 

Q6. The thermal resolution of an instrument tends to: 

a. improve as target temperature increases. 

b. degrade as target temperature increases. 

c. remain constant regardless of target temperature. 

d. improve with increasing working distance. 


Q7. The 3 to 5 μm spectral region is ideally suited for operation of instruments: 

a. measuring subzero temperature targets. 

b. measuring targets at extremely long working distances. 

c. measuring targets warmer than 200 °C (392 ° F). 

d. operating at elevated ambient temperature. 

Q8. The total field of view of an imaging instrument determines the: 

a. imaging spatial resolution (lFOV) of the instrument. 

b. measurement spatial resolution (IFOVmeas) of the instrument. 

c. image size at the target plane for any given working distance. 

d. operating spectral range of the instrument. 

Q9. The frame repetition rate of an imager is defined as the: 

a. number of imaging pixels in a thermogram. 

b. number of frames selected for image averaging. 

c. electronic image rate of the display screen. 

d. number of times every point on the target is scanned in one second. 


Q10. The purpose of adding an infrared spectral filter to an instrument may be 

to limit the spectral band: 

a. to only wavelengths longer than a specified wavelength. 

b. to only wavelengths shorter than a specified wavelength. 

c. to only wavelengths between two specified wavelengths. 

d. any of the above. 

Q11. To quickly calculate target spot size, a useful approximation is: 

a. π =3.1416. 

b. an instantaneous field of view of 1 degree represents a 60: 1 ratio 

between working distance and spot size. 

c. there are 2π radians in 360 degrees. 

d. a 1°F temperature change is equivalent to a 1.8 °C temperature change. 

Q12. For online process control instruments, important features are: 

a. environmental housings and long term stability. 

b. ready access to emissivity compensation setting. 

c. portability and battery life. 

d. precision sighting. 


Q13. A line scanner can be used to produce a thermogram of a sheet process 

only when: 

a. emissivity is known. 

b. the sheet process is moving at a uniform rate. 

c. the process material is a non graybody. 

d. the sheet process is hotter than 200 °C (392 °F). 

Q14. Most quantitative infrared thermal imagers: 

a. are heavier than quantitative imagers and usually require line power. 

b. can store thermograms on floppy disks in the field. 

c. require frequent infusions of detector coolant in the field. 

d. use detectors that operate at room temperature. 

Q15. Infrared focal plane array imagers: 

a. have no scanning optics. 

b. cannot be used for quantitative thermography. 

c. cannot be used for very cool targets. 

d. cannot operate on rechargeable batteries. 


Q16. Most infrared focal plane array imagers: 

a. use more costly optics than scanning radiometers. 

b. offer better spatial resolution than scanning radiometers. 

c. offer better thermal resolution than scanning radiometers. 

d. offer more diagnostics features than scanning radiometers. 

Q17. The number of detector elements in an infrared focal plane array imager: 

a. affects the measurement accuracy of the imager. 

b. affects the thermal resolution of the imager. 

c. affects the spectral band of the imager. 

d. affects the spatial resolution of the imager. 

Q18. The fact that all elements in a focal plane array imager are always 

looking at the target make this kind of imager better suited than scanning 

imagers 

for observing: 

a. distant low temperature targets. 

b. targets with rapidly changing temperatures. 

c. targets with low emissivities. 

d. targets with high emissivities. 


Q19. For which of the following applications are quantitative thermograms 

most critical? 

a. Search and rescue. 

b. Nondestructive material testing. 

c. Process monitoring and control. 

d. Security and surveillance. 

Q20. Infrared thermal detectors: 

a. have a broad. flat spectral response. 

b. usually require cooling to operate properly. 

c. have much faster response times than photon detectors. 

d. have much greater sensitivity than photon detectors. 

Q21. The characteristics of infrared photodetectors 

(photon detectors) include: 

a. faster response times than thermal detectors. 

b. a requirement for cooling to operate properly. 

c. selective spectral response based on operating temperature. 



Q22. Filters, lenses and transmitting windows: 

a. are all examples of refractive optical elements. 

b. have negligible transmission loss in the infrared. 

c. are all examples of reflective optical elements. 

d. are not spectrally selective. 

Q23. Resistance temperature detectors and thermistors operate on the same 

principle. that is: 

a. a predictable change in resistance as a function of temperature. 

b. the inverse square law. 

c. the known expansion of dissimilar materials. 

d. the comparison of target brightness with a calibrated reference. 

Q24. Infrared radiation thermometers are used to measure temperature: 

a. without contacting the target. 

b. very rapidly. 

c. without causing a temperature change at the target. 



Q25. Two-color (ratio) pyrometers measure the temperature of a target by: 

a. taking into account the size and distance to the target. 

b. comparing the radiant energy from the target in two narrow spectral 

bands. 

c. incorporating tables of known emissivity. 

d. calibrating and correcting for the infrared absorption in the measurement 

path. 


SGuide-IRT 

Content 

Part 1 of 2 

■ Chapter 1 - Introduction to Principles & Theory 

■ Chapter 2 - Materials and Their Properties 

■ Chapter 3 – Thermal Instrumentation 

Part 2 of 2 

■ Chapter 4 – Operating Equipment and Understanding Results 

■ Chapter 5 – Applications 

■ Appendices A, B, C 


Chapter 4 

Operating Equipment and 

Understanding Results 


4.1 Temperature Changes 

Distinguishing real temperature changes from apparent temperature changes 

is one of the biggest challenges facingthermographcrs. Thermal imaging 

instruments register temperature changes in response to changes in radiosity 

at the target surface when in many cases, there is no change in real surface 

temperature. To complicate matters further, external mechanisms can 

exaggerate these misleading readings. To combat this situation. 

thermographers should understand the len basic causes of apparent 

temperature change - some of which are only apparent and some of which 

are the result of real temperature changes at the target surface. 

Causes of Apparent Temperature Changes 

Apparent temperature changes can be caused by difrerences in emisivity Ɛ, 

reflectivity ρ, transmissivity τ and target geometry G. 


■ Emissivity Differences ∆τ 

Emissivity differences at the target surface can change the target radiosity. 

even on an isothermal target. and may give the appearance of temperature 

variations on the thermogram. Frequently, these can be seen on painted 

metal surfaces where scratches expose bare metal that has a different 

emissivity than the paint. 

■ Reflectivity Differences ∆ρ 

Reflectivity djfferences may become apparent when heat sources external to 

the target surface reflect off low emissivity target (low emissivity ≡high 

reflectivity) surfaces into the instrument. These can be point sources or 

extended sources and they can add to or subtract from the apparent 

temperature reading as will be discussed later. 


■ Transmissivity Differences ∆τ 

Transmissivity differences can be caused by heat sources behind the target if 

the target is partly transparent in the infrared range. These will only be seen if 

the target transmissivity is high enough and the heat source is different 

enough in temperature from the target to contribute significantly to the total 

target radiosity. 

■ Target Geometry Differences ∆D 

Target geometry differences are caused by multiple reflections within 

recesses or concavities on the target surface. They are actually variations in 

effective emissivity caused by changes in surface configurations. An example 

of this is the apparent temperature gradient in the far corner of an enclosure 

that is at a uniform temperature. Geometric differences diminish as target 

surface emissivity approaches unity. (blackbody does not affected by G) 


Causes of Real Temperature Changes 

Real temperature changes may be caused by differcnces in mass transport 

(fluid flow), phase change (physical state). thermal capacitance, induced 

heating, energy conversion (friction. exothermic reactions and endothermic 

reactions), direct heat transfer by conduction, convection and radiation 

(thermal resistance) or a combination of two or more of these causes. 

■ Mass Transport Differences (Fluid Flow) 

Mass transport differences are real temperatutc changes at the target surface 

caused by various forms of fluid flow. Free and forced convection are two 

examples of mass transport differences. Cool air exiting an air conditioning 

register will cause the register to become cooler. Hot water flowing within a 

pipe will cause the inside surface of the pipe to become warmer. (This will 

result in the outside of the pipe also becoming warmer.) 


Mass Transport Differences (Fluid Flow) 


Mass Transport Differences (Fluid Flow) 


■ Phase Change Differences (Physical State) 

Phase change differenccs occur when materials change physical stale. An 

example of this is water evaporating off the surfaceof a building. As the water 

evaporates, it has a cooling effect on the entire surface. Thermal imaging 

equipment aimed at the building will register this cooling effect. 

■ Thermal Capacitance Differences ∆C p 

Thermal capacitance differences cause temperature changes in transient 

conditions when one part of a target has a greater capacity to store heat than 

another. In the thermogram of a water tank. as shown in Figure 4.1. the water 

level inside the tank is apparent because of the contrast in temperature, 

which is caused by the difference in thermal capacitance between water and 

air. This real temperature change is also evident in roof surveys as illustrated 

in Chapter 5. 


Figure 4.1: An indication of water level In a storage tank 

10.0 

9.0 

8.0 

7.0 

6.0 

5.0 


Thermal Capacitance Differences ∆Cp 


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■ Induced Heating Differences (by electromagnetic induction) 

Induced heating differences occur when ferrous metals are within a magnetic 

field. Depending on the orientation of the parts and the strength of the 

magnetic field, induced currents within the ferrous parts can cause substantial 

heating. An example of this is when an aluminum bolt in a structure is 

mistakenly replaced with a ferrous bolt. If the structure is within a magnetic 

field, the bolt may become hot. This induction effect is exploited in the 

thermographic location of steel reinforcing bars embedded in concrete 

structures. Here a magnetic field is introduced to the structure and the 

resultant warm spot on the thermogram indicate the presence of the 

reinforcing bars. 


Induced Heating Differences 

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684 

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500 

450 

400 

350 

300 

250 

200 

150 

100 

50 

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500~------~------~------~--------~----~ 

400 

300 

200 

100 



■ Energy Conversion Differences 

Energy conversion differences occur when energy is converted from one form 

to another. Friction (mechanical energy converted to thermal energy) is a 

commonly observed example of temperature changes because of energy 

conversion. Another is electrical energy converted to thermal energy, as 

illustrated in Figure 4.2, where the current carrying wire of a twisted pair 

generates heat revealing insulation discontinuities. Exothermic or 

endothermic reactions (chemical energy converted to thermal encrgy) are 

further examples. typified by the heating that accompanies the curing of 

polymers. 

■ Direct Heat Transfer Differences 

Direct heal transfer differences are also commonly observed in thermographic 

survey programs. An example of this is shown in the direct transfer of thermal 

energy through the wall of a catalytic cracker reformer vessel as illuslrated in 

Figure 4.3. The differences in heat flow illustrate the differences in thermal 

resistance between good refractory material and degraded material. 


Figure 4.2: The current carrying wire of a twisted pair generates heat that 

reveals insulation defects 

24.0 

2'2.9 

2'2.4 

22.0 

21.6 

2'! .2 

20.8 

'20." 

20.0 

2'33 

1 

i47 


Figure 4.3: Catalytic reformer vessel with insulation defects 

324 0 

::£0 •I 

2m..2 , 

278.5 

261e 

2430 

21g3 

0.0 

327 1 


Energy Conversion Differences 


Infrared Thermogram 


OF 

180 

110 

160 

150 

140 

130 

120 

110 

100 

90 

80 

10 

60 








Direct Heat Transfer Differences 


■ Combination of Heat Transfer Mechanisms 

Thermal images of operating equipment and systems will often exhibit heat 

flow by a combination of mechanisms working simultaneously. Figure 4.4 

depicts the investigation into the thermal design of a new motorcycle engine. 

The thermal signature is a combination of fluid flow (in the cooling fins), 

exothermic reactions (within the cylinders) friction (at the piston rings and 

within the bearing) and thermal resistance (in the exhaust system). 


Figure 4.3: Thermogram of a new motorcycle engine heat flow by a 

combination of mechanisms working simultaneously 


Infrared Thermogram of a running motor 


Image Interpretation 

A clearer understanding of the pitfalls possible in image interpretation helps 

the thermographer to perform the required tasks competently. As in the three 

modes of heat transfer (conduction, convection & radiation), these 

mechanisms frequently occur in combinations. Although the ability of the 

thermographer to clearly identify the causes of temperature change in a 

paticular target environment may be unnecessary when making 

measurements, it is absolutely essential for the correct and responsible 

interpretation of results. In situations where the thermographer is unfamiliar 

with the measurement environment, a knowledgeable facility representative 

should accompany the thermographer during the measurements or be 

available for consultation. By providing expert information concerning the 

processes taking place and the likely sources of temperature differences, the 

thermographer will be able to anticipate thermal behavior and better 

understand and interpret the thermographic results. 


■ Spectral Considerations in Product and Process Applications 

Many products, both simple and complex have complex spectral 

characteristics in the infrared region. Spectral filtering of the measuring 

instrument can exploit these complex spectral characteristics to measure and 

control product temperature without contact. For example, if it is necessary to 

measure the temperature of objects from 200 to 1000 °C (392 to 1832 °F) 

inside a heating chamber with a glass port , or inside a thin walled glass bell 

jar, an instrument operating in the 2 to 3 μm band will see through the glass 

and make the measurement easily. On the other hand, an instrument 

operating at wavelengths longer than 4.8 μm will measure the surface 

temperature of the glass. 


Infrared Thermogram of Glass of Water - Spectral Considerations 

4.8 μm will measure 

the surface 

temperature of the 

glass. 

2 to 3 μm band will see 

through the glass and 

make the measurement 

easily. 







..... 

" 

I 


Spectral characteristics are exploited in the monitoring of incandescent lamp 

temperatures during production as illustrated in Figures 4.5, 4.6 and 4.7. 

Figure 4.5 shows the spectral characteristics of the imaging radiometer as 

well as the transmission spectra of glass envelopes of various thicknesses. 

Using a 2.35 μm band pass filter with the instrument allows the instrument to 

see through the glass and monitor the temperature of critical internal lamp 

components. Substituting a 4.8 μm high pass filter allows the instrument to 

monitor the glass envelope temperature. 

Figures 4.6 and 4.7 are thermograms of the glass envelope and the internal 

lamp components respectively, recorded in immediate sequence. An 

important generic example of the need for spectral selectivity is in the 

measurement of plastics being formed into films and other configurations. 

Thin films of many plastics are virtually transparent to most infrared 

wavelengths, but they do emit at certain wavelengths. Polyethylene, 

polypropylene and other related materials have a very strong, though narrow, 

absorption band at 3.45 μm. Polyethylene film is formed at about 200 °C (392 

°F) in the presence of heaters that radiate at a temperature near 700 °C 

( 1292 °F). 


Figure 4.5: Spectral selectivity for measuring the surface and internal 

temperatures of incandescent lamps 

100 ,_ 

90 ...,_ 

1 

Wavelength (J.lm) 


Figure 4.6: Surface temperature thermogram of an incandescent lamp 

t 

~/TE 

LS 


Figure 4.7: temperature thermogram of an incandescent lamp 

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Incandescent Lamp 

4.8 μm high pass filter 

- bulb surface (envelope) 

temperature 

2.35 μm band pass filter 

- Incandescent 

filament temperature 


Figure 4.8 shows the transmission spectra of 40 μm ( 1.5 x 10 -3 in.) thick 

polyethylene film and the narrow absorption band at 3.45 μm. The instrument 

selected for measuring the surface of the film has a broad band thermal 

detector and a 3.45 μm spike band pass filter. The filter makes the instrument 

blind to all energy outside of 3.45 μm and enables it to measure the 

temperature of the surface of the plastic film without being influenced by the 

hot process environment. Figure 4.9 shows a similar solution for 13 μm (5 x 

104 in.) thick polyester (polyethylene terephthalate 聚对苯二甲酸乙二醇酯 ) 

film under about the same temperature conditions. Here the strong polyester 

absorption band from 7.7 to 8.2 μm dictates the placement of a 7.9 μm spike 

filter placed in front of the same broad band detector as that used in the 

polyethylene application. 


I 

I 

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Figure 4.8: Measuring temperature of polyethylene 

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Figure 4.9: Measuring temperature of polyester 

7.7 to 8.2 μm 


IR Filter 


IR Filter 


Using Line Scanners for Monitoring Continuous Processes 

Continuous processes are most often processes in constant and uniform 

motion. When this happens. an imaging system may not be required to cover 

the full process image. To monitor and control processes in motion, an 

infrared line scanner can be used, scanning normal to the process flow, to 

generate a thermal strip map of the product as it passes the measurement 

site line as illustrated in Figure 4.10. If more than one measurement site line 

is required. additional line scanners may be deployed. 


Figure 4.10: Line scanner for continuous process monitoring 

Line 

Scanner 

--- - 

Proce~s 

J 

Motion 

Normal 

to Scan Line 

, I 

Scan Line Width 

Sequential 

Scan Lines 

Generate 

Thermogram 

ObsoNed 

Une-scan Camera 

Downweb 

Direction 

Motion 


4.2 Infrared Thermographic Equipment Operation 

Because of product performance advances and meticulous ( 小心翼翼的 ) 

human engineering on the part of manufacturers, infrared thermographic 

equipment is far easier to operate in the twenty-first century than it was in the 

1990s. It is relatively simple for the novice ( 新手 ) thermographer to turn on the 

equipment. aim at a target and acquire an image. Consequently, it is also 

easier than ever to misinterpret findings. 


Preparation of Equipment for Operation 

Even when using point sensing instruments. preparation for making 

measurements requires an instrument operation check. a battery status check 

and a simple calibration check. This preparation follows a simple checklist, 

which is a critical element in the successful field operation of thermal imaging 

equ ipment. Equipment preparation is crucial in field measurements because 

of time consumption, measurement scheduling and the availability of on-sile 

personnel. 

A seemingly small oversight in equipment preparation can waste 

considerable time and money. Calibration against a known temperature 

referencc is required for all infrared measuring instrumcnts and is normally 

accomplished through radiation reference sources also known as blackbody 

simulators. These temperature controlled cavities or high emissivity surfaces 

that are designed to simulate a blackbody target at a specific temperature or 

over a specific temperature range with traceability to the National Institute for 

Standards and Technology (NIST). Factory calibration and traceability is 

provided by the manufacturer. 


Bccause most quantitative thermographic instruments measure radiant 

energy values converted to temperature readings by a computer, calibration 

information is usually stored in the computer software and is identified with a 

specific instrument serial number. If a specific instrument calibration is not 

available in the software, the computer will usually default to a generic 

calibration for that class of instrument. In addition to a blackbody calibration, 

the software is usually provided with correction functions for ambient effects 

such as atmospheric attenuation as a function of working distance and for 

emissivity correction. 

Default settings for these values are normally in effect unless the operator 

chooses to alter them. Checking calibration of a thermal imaging system in 

detail requires placing a blackbody reference source in front of the instrument 

so that ilt subtends a substantial area in the center of the displayed image 

(much greater than the instantaneous field of view). The correct measurement 

conditions must be set into the computer where applicable [example. working 

distance = 10 m (33 ft), ambient temperature = 25 °C (77 ° F), emissivity = 1, 

etc.] and the temperature reading compared to the reference source setting. 

The spot measurement software diagnostic should be used if available. The 

detailed calibration should include the widest range of temperatures possible. 


If the instrument is out of cal ibration. it may be possible to recalibrate it under 

celtain conditions. (Refer to the operator's handbook.) Otherwise, it may be 

necessary to return it to the factory for recalibration. 

A detailed calibration check should be made at least every six months. 

Periodic calibration spot checks should also be performed. Ideally, calibration 

checks should be done before and after each field measurement mission and 

can be accomplished by means of a high quality radiation thermometer and 

high emissivity sample targets. To perform a spot check. place the target in 

front of the instrument. Set emissivity the same for both instruments and 

measure the apparent temperature simultaneously with the imager and the 

radialion thermometer. Spot checks should be run at a few temperatures 

covering the range of temperatures anticipated for the specific measurement 

mission. Because the fields of view and spectral ranges of the two 

instruments may not match, exact correlation may not be possible.The errors 

should be repeatable from day to day, however, and the procedure will 

provide a high degree of confidence in the results of the measurement 

mission. 


Transfer caljbration using a radiation reference source in the field is effective 

where extremely accurate measurements are required within a narrow range 

of temperatures. Typically, instrument calibrations are performed over a broad 

range of temperatures. with certain maximum allowable errors occurring at 

temperatures within this broad range. The transfer calibration can optimize 

accuracy over a limited range. The procedure requires introducing a radiation 

reference source into the total field of view along with the target of interest 

with the reference set very close to the temperature range of interest. Using 

the diagnostic software to measure the apparent temperature differences 

between the reference and various points of the target of interest should 

provide improved accuracy. The equipment checklist used in preparation for a 

day of field measurements helps ensure that there will be no surprises on site. 

A standard checklist should be prepared to include all items in the 

thermographic equipment inventory. These should include instrument spare 

lenses, tripods, harnesses, transport cases, carts, batteries, chargers, liquid 

or gaseous cryogenic coolant, safety gear, special accessories, film, diskettes, 

spare fuses, tool kits, data sheets, operator manuals, calibration data, 

radiation reference sources, interconnecting cables, accessory cables and 

special fixtures. 


The batteries mentioned on the mission checklist should be fully charged 

batteries. It is the thermographer's responsi bility to ensure that there is a 

comfortable surplus of battery power available for each field measurement 

session. The fact that batteries become discharged more rapidly in cold 

weather also must be considered when preparing for field measurements. 


Procedures for Checking Critical Instrument Performauce Parameters 

There are established procedures for checking the critical performance 

parameters di scussed in Chapter 3. The parameters that are most important 

to most measurement programs are: 

1. Thermal resolution or minimum resolvable temperature difference (MRTD). 

2. Imaging spatial resolution or instantaneous field of view (lFOV). and 

3. Measurement spat ialresolution (IFOVmeas). 

Comments on item 1 

■ Thermal resolution or 

■ minimum resolvable temperature difference (MRTD) or 

■ noise equivalent temperature difference (NETD). 

the above describe the same phenomenon. 


Comments: 

■ 

■ 

■ 

Thermal resolution or 

minimum resolvable temperature difference (MRTD) or 

noise equivalent temperature difference (NETD). 


Recalling! 



or minimum resolvable temperature 

difference (MRTD) or 





Figure 4.11 : Test configuration for minimum resolvable temperature 

difference measurement 

Standard Test Pattern 

4 Bars, 7:1 H/WRatio 

w 

' 

, 6 T= T 2 - T 1 ' 

' , 

' , 

\ , 

\ , 

\ , 

,, 

' , 

, 

, 

, 

, 

, 

IR 

Imaging 

Sensor 


■ Thermal Resolution 

thermal resolution can be measured using a procedure developed for military 

evaluation of night vision systems. This procedure uses standard resolution 

targets as illustrated in Figure 4.11 and is described as follows: 

1. Set up the test pattern such that ΔT exceeds the manufacturer's 

specification for minimum resolvable temperature difference. 

2. Determine the spatial frequency I t of the target in cycles per milli-radian as 

follows: 

a. the number of radians equals the bar width W divided by the distance d to 

the target (example: 2 mm at 1 m = 2 mRad); and 

b. the spatial frequency, I t = 1 cycle / (1 bar + 1 space) = 1 / (W + S). 

(If W = 2 mRad and S = 2 mRad, then I t = 1/(2 + 2) = 0.25 cycles per 

milli-radian). 


3. Reduce the ΔT until the image is just lost (note ΔT H ) . Raise ΔT until the 

image is just reacquired (note ΔT C ) then: 

ΔT = ABS(ΔT H ) + ABS(ΔT C ) 

2 

4. Then change distances or use different size bar targets to plot minimum 

resolvable temperature difference for other spatial frequencies. 


Figure 4.12: Test configuration for 

modulation transfer function 

measurement 

Standard Test Pattern 

4 Bars, 3 Spaces 

7:1 HIW Ratio 

w 

- 

T1 

r 

' H 

' 

l 

Line Scan 

' 

' 

' ' ' 

Output 

-- - 

' 

I 

I 

I 

I 

I 

, 

, 

, 

I 'D. T = T2 - T1 

IR 

Imaging 

Sensor 

Vmax 

t - -- - t.. vmln 

vo 


■ Imaging Spatial Resolution 

Imaging spatial resolution of scanning imagers can be ensured using another 

procedure that stems from military night vision evaluation protocol and uses 

the same standard bar target. The procedure measures the 

modulation transfer function (MTF), a measure of imaging spatial resolution. 

Modulation is a measure of radiance contrast and is expressed: 

Modulation = 

V max –V min 

V max + V min 

where: 

V = the voltage analogue of the instantaneous radiance measured. 


Modulation transfer is the ratio of the modulation in the observed image to 

that in the actual object. For any system the modulation transfer function will 

vary with scan angle and background and will almost always be different 

when measured along the high speed scanning direction than it is when 

measured normal to it. For this reason. a methodology was established and 

accepted by manufacturers and users alike to measure the modulation 

transfer function of a scanning imager and, thereby to verify the spatial 

resolution for imaging (night vision) purposes. 


A sample setup is illustrated in Figure 4.12 for a system where the 

instantaneous field of view is specified at 2.0 mRad using the same setup as 

illustrated in Figure 4.10. The procedure is as follows: 

1. Set ΔT (where ΔT = T 2 –T 1 ) to at least 10x the manufacturer's specified 

minimum resolvable temperature difference (MRTD). 

2. Select distance to simulate the manufacturer's specified imaging spatial 

resolution. The bar width W represents one resolution element. For example. 

instantaneous field of view can be calculated where bar width W= 2 mm and 

distance d = 1 m. 

IFOV = W/d = 2mm/1m 

where: d= distances to target, W=bar width 


3. Display imager's horizontal line scan through the center of the bar target. 

4. Calculate the modulation transfer function: 

Modulation = 

V max –V min 

V max + V min 

where: 

MTF = modulation transfer function (a ratio). 

V max = maximum measured voltage V. 

V min = minimum measured voltage V. 


5. If the modulation transfer function (MTF) = 0.35* or greater. the imager 

meets the imaging spatial resolution specification. (If the signal representing 

the horizontal scan line is not accessible, consult the manufacturer for an 

alternate means by which modulation transfer function can be verified. In a 

digital image, the gray level may replace the Voltage value. Note: There are 

disagreements among users and manufacturers regarding the acceptable 

minimum value of modulation transfer function to verify imaging spatial 

resolution with values varying between 0.35 and 0.5, depending on the 

manufacturer and the purpose of the instrument.) 


■ Measurement Spatial Resolution 

Measurement spatial resolution (IFOVmeas) can be measured using a 

procedure that measures the slit response function (SRF) of the imaging 

system. This procedure was developed by instrument manufacturers and is 

generally accepted throughout the industry. In this technique, a single 

variable slit is placed in front of a blackbody source and the slit width is varied 

until the resultant signal approaches the signal of the blackbody reference. 

Because there are other errors in the optics and the 100 percent level of slit 

response function is approached rather slowly, the slit width at which the slit 

response function reaches 0.9 is usually accepted as the measurement 

spatial resolution. Again, there are some disagreements as to whether 0.9 or 

0.95 should be considered acceptable. The test can establish whether the 

imager meets the manufacturer's specifications for measurement spatial 

resolution. The test configuration for slit response function determination is 

illustrated in Figure 4.13. 


Figure 4.13: Test configuration for slit response function measurement 

Adjustable Slit Width 

~~ Extended Surface 

Blackbody 

Reference Source 

I 

I 

Infrared Imaging 

Sensor 


1. Set ΔT (where ΔT = T 2 –T 1 ) to at least 10x the manufacturer's specified 

minimum resolvable temperature difference (MRTD). 

2. Select distance and slit width to simulate the manufacturer's specified 

measurement spatial resolution. The bar width W (mm) represents one 

resolution element. For example, for a 3 mRad measurement spatial 

resolution, if d= 1m, W = (1.0 x 0.003) = 3 mm. 

3. Display imager's horizontal line scan through the center of the bar target. 


4. Open slit until V meas = V max . 

5. Close slit until V meas = 90% of V max and measure slit width (W). 

6. Compute: IFOVmeas = W·d -1 . This should be equal to or smaller than the 

manufacturer 's imaging spatial resolution specification. 

Again, if the signal representing the horizontal scan line is not accessible, 

consult the manufacturer for an alternate means by which measurement 

spatial resolution can be verified. 


Common Mistakes in Instrument Operation 

Remembering a few key cautions regarding proper equipment application can 

help the thermographer to avoid some common mistakes. The following 

guidelines should be observed. 

1. Select an instrument appropriate to the measurement application in 

accordance with the guidelines reviewed in Chapter 3. 

2. Leam and memorize the startup procedure. 

3. Leam and memorize the default values. 

4. Set or use the correct emissivi ty and be particularly cautious with 

emissivity settings below 0.5. 

5. Make sure the target to be measured is larger than the measurement 

spatial resolution of the instrument. (FOV? or IFOV?) 

6. Aim the instrument as close to normal (perpendicular) with the target 

surface as possible. 

7. Check for reflections off the target surface ρ and either avoid or 

compensate for them. 

8. Keep sensors or sensing heads as far away as possible from very hot 

targets. 


■ Learning the Startup Procedure 

Learning the startup procedure thoroughly is essential, particularly for 

thermographers who operate several different models of thermographic and 

thermal sensing equipment. Efficient startup lets the data gathering process 

begin with no unnecessary delays; it saves valuable on-site time and inspires 

confidence of facility personnel. A quick review of the operator's manual and 

a short dry run before leaving home base is usually all that is required. 


■ Memorizing the Default Values 

Memorizing the default values provided in the operator's manual is another 

important contribution to time efficiency and cost effectiveness. These include 

default values for several important variables in the measurement such as 

emissivity, ambient (background) temperature, distance from sensor to target, 

temperature scale (degrees Fahrenheit or Celsius), lens selection and relative 

humidity. It is important to remember that the instrument's data processing 

software automatically uses these values to compute target temperature 

unless the thermographer changes these values to match the actual 

measurement conditions. Typical default values are 1 m (3 ft) distance to 

target, emissivity of 1.0 and background temperature of 25 °C (77 °F). Failure 

to correct for these can result in substantially erroneous results, if, for 

example the target is known to be 10 m (33 ft) away is known to have an 

effective emissivity of approximately 0.7 and is reflecting an ambient 

background temperature of 10 °C (50 °F). By memorizing the default values, 

the thermographer will know when it is necessary to change them and when 

time can be saved by using them unchanged without referring to a menu. 


■ Setting the Correct Effective Emissivity 

Setting the correct effective emissivity is critical in making temperature 

measurements. Table 2.2 can be used as a guide when obtaining absolute 

temperature values is not critical. When measurement accuracy is important. 

it is always better to directly determine the effective emissivity of the surface 

to be measured using the actual instrument to be used in the measurement 

and under similar operating conditions. This is because emissivity may vary 

with temperature, surface characteristics and measuremenl spectral band and 

may even vary among samples of the same material. There are several 

methods that may be used to quickly estimate target effective emissivity. One 

known as the reference emitter technique can be used to detennine the 

emissivity setting needed for a particular target material. The determination 

uses the same instrument that will be used for the actual measurement. The 

procedure is illustrated in Figure 4.14 and is described as follows: 


Table 2.2: Normal spectral emissivities of common materials 

Table 2.2: 

Normal spectral ~emissivities of common mate'rials 

'laterial Temperature Wavelength Emissivity 

J.LDl E 

I 

oc 

Alumina brick 17 2-S 0.68 

AJuminum, poli hed 0 8- 14 0.05 

Aluminum, rough urface 0 8-14 0.07 

Alumi num, strongly oxidized 0 8-14 0.25 

A lu mi num foil. bright 17 2-5 0.09 

Asbestos board 0 8-14 0.96 

Asbestos fabric 0 8-14 0.78 

. 


Figure 4.14: Test configuration for the determination of effective emissivity 

using the reference emitter method. 

#5: Adjust emissivity to obtained the 

sample temperature obtained in #4 

#4; Set instrument 

emissivity using the 

known emissivity value 

and observed the 

material temperature 


Non conditioned 

Surface 

#2 This area was painted, 

taped or conditioned with 

material of known emissivity.

1. Prepare a sample of the material large enough to contain several spot 

sizes or instantaneous fields of view of the instrument. A 100 mm x 100 

mm (4.0 in. x 4.0 in.) sample may be big enough. 

2. Spray half of the target sample with flat black (light absorbing) paint, cover 

it with black masking tape or use some other substance of known high 

emissivity. 

3. Heat the sample to a uniform temperature as close as possible to the 

temperature at which actual measurements will be made. 

4. Make certain that the value for background temperature has been properly 

entered. Then set the instrument emissivity control to the known emissivity 

of the coating and measure the temperature of the coated area with the 

instrument. Record the reading. 

5. Immediately point to the uncoated area and adjust the emissivity set until 

the reading obtained in step 4 is repeated. This is the emissivity value that 

should be selected in measuring the temperature of this material with this 

instrument. 


■ Measuring and Reporting Temperature Accurately - Filling the 

Instantaneous Field of View IFOV meas 

If true temperature measurement of a spot on a target is required, the spot 

must completely fill the instrument’s measurement spatial resolution 

(IFOVmeas). lf it does not. some useful information about the target can still 

be learned. but an accurate reading of target temperature cannot be obtained. 

The simple expression. D= α∙d, can be used to compute measurement spot 

size D at the target plane from a working distance “d” where α is taken to be 

the manufacturer's published value for measurement spatial resolution. 

For example. if the target spot to be measured is 5 cm (2 in.) and the 

calculated spot size D is 10 cm (4 in .), move the instrument closer to the 

target or use a higher magnification lens if either is possible. If not, expect the 

reading to be affected by the temperature of the scene behind the target. Also. 

be sure to allow for aiming errors and instrument imperfections. An extra 30 

percent should be enough. 


■ Aiming Normal to the Target 

Aiming normal (perpendicular) to the target surface or as close as possible to 

normal is important because the effective emissivity of a target surface is 

partially dependent on the surface texture. It stands to reason, then, that if the 

surface is viewed at a skimming angle, the apparent texture will change, the 

effective emissivity will change greatly and the measurement will be affected 

by misleading reflections. These can result in cold errors as well as hot errors. 

A safe rule is to view the target at an angle within 30 degrees of normal 

(perpendicular). If the target emissivity is very high this can be increased to 

as high as a 60 degree angle if necessary. 

Note: 

where: θ angle from normal 

θ = 30º 

θ = 60º where Ɛ is high 


■ Recognizing and Avoiding Reflections from External Sources 

Recognizing and avoiding reflections from external sources is an important 

acquired skill for the thermographer. If there is a concentrated source of 

radiant energy (point source) in a position to reflect off the target surface and 

into the instrument, steps should be taken to avoid misleading results. There 

is the greatest likelihood of errors due to point source reflections when the (1) 

target emissivity is low, (2) the target is cooler than its surroundings or (3) the 

target surface is curved or irregularly shaped. 

It should be noted that, although most errors due to reflections are from 

sources hotter than the target, reflective errors from cold sources can also 

occur and should not be discounted. A common source of reflective error is 

the reflection of the cold sky off glass or other reflective surfaces. If a 

temperature anomaly is caused by a point source reflection, it can be 

identified by moving the instrument and pointing it at the target from several 

different directions. If the anomaly appears to move (changes/ varies) with the 

instrument, it is a point source reflection. Once identified. the effect can be 

eliminated by changing the viewing angle, by blocking the line of sight to the 

source or by doing both. 


Errors due to the reflection of an extended source, however, cannot be 

eliminated in this manner. The ambient instrument background (what the 

instrument sees reflected off the target surface) is the most commonly 

encountered example of an extended source reflection. 

Errors due to extended source reflections are more likely when the target 

emissivity is low or when the target is cooler than its surroundings. Most 

instrument menus include a provision for entering the ambient background 

temperature if it is different from the default setting. The system will 

automatically correct the temperature reading. This will also work if the 

ambient background is an extended source such as a large boiler. In this 

situation, substituting the boiler's surface temperature for the background 

ambient setting will correct the temperature reading. 

Keywords: 

Reflection due to Point source 

Reflection due to Extended source 


■ Measuring the Appropriate Background Temperature Using the 

Instrument 

A technique commonly used by thermographers to determine an appropriate 

setting for "ambient background temperature" requires a piece of aluminum 

foil large enough to fill the total field of view of the instrument. First. crush the 

foil into a ball and then flatten it so that it simulates a diffuse reflecting surface. 

Next, place the foil so that it fills the instrument's total field of view and reflects 

the ambient background into the instrument. Allow the foil to come to thermal 

equilibrium. With the instrument's emissivity Ɛ set to 1.00, measure the 

apparent temperature of the foil. Use this apparent temperature reading as 

the ambient background temperature setting. 


■ Avoiding Radiant Heat Damage to the Instrument 

Avoiding radiant heat damage to the instrument is always important. Unless 

an infrared sensing or imaging instrument is specifically selected or equipped 

for continuous operation in close proximity to a very hot target. it may be 

damaged by extensive thermal radiation from the target. A good rule for the 

thermographer to follow is "don't leave the instrument sensing head in a 

location where you could not keep your hand without suffering 

discomfort.“ Accessories such as heat shields and environmental enclosures 

are available from manufacturers for use when exposure to direct radiant heat 

is unavoidable. These accessories should be used to protect the instrument 

when appropriate. 


■ Temperature Differences Between Similar Materials 

Particularly in electrical applications, it is critical to measure and report 

temperature differences between similar components with similar surface 

materials. such as the fuses on different phases of the same supply. Strict 

observance of the procedures regarding the use of the correct (1) effective 

emissivity value, (2) filling the measurement spatial resolution, (3) using the 

correct background temperature, (4) setting and using the correct viewing 

angles θ will ensure that these differences are measured and reported 

correctly 


4.3 Safety and Health 

Safety and health considerations are critical to successful thermography 

programs as well as to the welfare of the thermographer and client personnel. 

Strict adherence to applicable codes is the responsibility of the 

thermographer. It is essenlial that the basics of these regulations be 

understood. 


■ Liquid and Compressed Gases 

Some instruments in the field use liquid or compressed gases for detector 

cooling. The handling of these materials can be hazardous and it is the 

thermographer's responsibility to learn safe practices and to adhere to 

them. In general, these procedures are included in the safety regulations 

for each facility. They can also be found in the operator's manuals for 

these instruments. Some instruments use liquid nitrogen LN as a detector 

coolant Liquid nitrogen is not very hazardous but some safety precautions 

should be observed. The following four guidelincs for using and storing 

liquid nitrogen are taken from the AGEMA Model 782 Operator's 

Handbook: 

1. Never store the liquid in sealed containers. Liquid nitrogen and similar 

cryogenic liquids are always stored in Dewar flasks or the equivalent 

insulated containers, with loosely fitting covers that allow the gas to vent 

without building up dangerous pressures, 

2. Never come into direct contact with liquid nitrogen. Serious frost bite injury 

(similar to a bum) can result if the liquid is allowed to splash in to the eyes 

or onto the skin. 


3. Always re place the filler cap after filling to avoid the risk of spillage and 

condensation. 

4. It is advisable to transfer some of the liquid from the storage Dewar to a 

smaller vessel (that is a vacuum jug) to effect more convenient filling and 

minimize spillage. Slowly pour a small amount into the instrument's liquid 

nitrogen chamber and wait until boiling ceases. This ensures that the 

chamber is at the same temperature as the liquid and minimizing 

splashing and spillage. Fill the chamber completely and replace the filler 

cap.“ 


Dewar Flask for LN 

Loosely fitting covers 

that allow the gas to vent 

without building up 

dangerous pressures 


■ Batteries 

Procedures for the handling of batteries and their safe disposal must also be 

followed by the thermographer. In general these procedures are included in 

the safety regulations for each facility. They can also often be found in the 

instrument operator's manuals. Generally, instructions for the safe disposal of 

batteries are provided in the literature accompanying the batteries. In the 

absence of such instructions, exhausted batteries should be considered as 

hazardous waste and handled accordingly. 


■ Electrical Safety 

Failure to recognize and observe electrical safety regulations can result in 

electrical shock and irrepairable damage to the human body. Electrical 

current flowing through the heart, even as small as a few milli-ampere can 

disrupt normal heart functions and cause severe trauma and some times 

death. In addition. body tissue can be severely and permanently damaged. 

Shock hazards are proportional to equipment operating voltage levels and 

distance from the hazard. Voltage levels as low as 60 V causing current to 

flow through the chest area with low skin resistance can be lethal. Examples 

of electric shock current thresholds and typical electrical contact resistances 

are given in Table 4.1 . 

Safety practices are important as well. One good safety rule to follow is to 

never touch electrical contacts unless qualified to do so. areing can also be 

lethal and even low voltage equipment may produce killing areas. It is 

important that only trained personnel wearing are protective gear be permitted 

to approach energized equipment. Spectators should not approach at all. 


Safety codes have been developed that specify the minimum distances to be 

maintained from live equipment and, in addition, protective clothing and 

devices (face shield, protective clothing and insulated gloves) are required in 

all facilities. Although the codes may vary from facility to facility, they all spell 

out the safety rules to which thermographers are expected to adhere. 

Examples of National Electrical Safety (NES) codes currently being observed 

in facilities in the United States and Canada that specify the minimum 

clearance zone from operating high voltage equipment in terms of voltage 

and distance are described in Table 4.2. thermographers must be aware of 

the safety regulations in force and know the recommended protective clothing. 

It is recommended that the applicable safety guidelines set forth in the 

following documents be reviewed: 

1. National Fire Protection Association NFPA 70E, Standard for Electrical 

Safety Requirements for Employee Workplace, 1995, and 

2. National Fire Protection Association NFPA 70B, Recommended Practice 

for Electrical Equipment Maintenance, 1994. 


Table 4.1: Electric shock current thresholds and skin contact resistances 

Shock Current Thresholds 

threshold of sensation 

threshoJd of pain 

mu ~; de paralysis 

stoppage of breathing 

ventricular fibrillation 

tissue burning 

household electrical circuit 

0.001 A 

0.005 A 

0.010 A 

0.030 A 

0.075 A 

SA 

l5 A 

Skin Contact Resistances 

finger touch - dry so,ooo n 

fin ger touch- wet 5,000 .Q 

holding, plit:rs - dry 5,000 .Q 

holding pliers - wet 2,000 .Q 

foot to wet ground, wet shoe 5,000 .Q 

hand in water 300 .Q 


Table 4.2: Examples of specified clearance distances from high voltage 

equipment 

United States 

Minimum Clearance Zone 

Canada 

Volts Distance phase to Volts Distance phase to 

employee 

employee 

1,000-34,000 0.6 m (2ft) 750-15,000 0.6 m (2ft) 

46,000 0.8 m (2.5 ft) 15,000-35,000 0.9 m (3ft) 

69,000 0.9 m (3ft) 35,000-50,000 1.2 m (4ft) 

138,000 1m (3.5 ft) 50,000-150,000 1.5 m (5 ft) 

230,000 1.5 m (5 ft) 150,000-350,000 2m (7ft) 

350,000-550,000 3.7 m (12ft) 


4.4 Record Keeping 

Keeping thorough and detailed records is very imponant to the thermographer, 

particularly when performing a comprehensive program of thermographic 

facility surveys. As discussed in Chapter 3, most equipment manufacturers 

sell software that provides a filing system to maintain records of all images 

and accompanying data and comprehensive report preparation software for 

timely and comprehensive reporting of the findings of infrared surveys and 

other measurement missions. Although recording the actual findings is the 

basic reason for record keeping, support records are also important. These 

records include equipment status history as well as personnel qualification 

documentation. 


Records of surveys should be documented to include: 

1. day, date, location, identification of test site and equipment or components 

inspected; 

2. thermographer's identification and qualifications; 

3. equipment used and calibration history (when last calibrated. when last 

spot check was made, etc.); 

4. what was inspected, what was not inspected and why; 

5. visual test reports of cracking, etc. with photographs if appropriate; 

6. other observations noted by the inspector. such as noise and aroma; 

7. backup video tapes of the entire measurement survey; and 

8. specific mention of any critical findings. 

All images should be maintained as files for future reference and trending. 

Reports may be tailored to include only those items considered significant 

but records should be maintained for all measurements. Maintenance and 

repair records of all equipment and accessories should also be kept. 

Easily accessible and easily understood notes and records are a measure of 

the competence and professionalism of the thermographer and lead to 

credibility in the eyes of management, whatever the industry or discipline. 


Easily accessible and easily understood notes and records are a measure of 

the competence and professionalism of the thermographer and lead to 

credibility in the eyes of management, whatever the industry or discipline. 


Chapter 4 


Q&A 

1. b 

2. d 

3. b 

4. a 

5. b 

6. a 

7. c 

8. a 

9. d 

10. d 


Q1. Apparent but not real temperature changes recorded by an infrared 

instrument can be due to: 

a. emissivity, reflectivity and mass transport differences. 

b. emissivity, reflectivity and geometric differences. 

c. thermal capacitance, reflectivity and geometric differences. 

d. thermal capacitance, mass transport and emittance differences. 

Q2. Apparent temperature changes recorded by an infrared instrument that 

are, in fact real temperature changes can be due to: 

a. emissivity, reflectivity and mass transport differences. 

b. emissivity. reflectivity and geometric differences. 

c. thermal capacitance, reflectivity and geometric differences. 

d. thermal capacitance, mass transport and energy convertion. 


Q3. Sun glints 闪耀 cause false indications of temperature changes. In this 

respect, they are similar to: 

a. solar heating. 

b. emissivity artifacts. 

c. resistive heating. 

d. mass transport. 

Q4. The lower the temperature of a target to be measured, the more 

imponant it is to: 

a. correct for ambient reflections. 

b. fill the instrument 's measurement spatial resolution with the target. 

c. use a cooled detector. 

d. keep batteries fully charged. 


Q5. The higher the temperature of a target to be measured, the less important 

it is to: 

a. fill the instrument's measurement spatial resolulion with the target. 

b. correct for ambient reflections. 

c. correct for atmospheric absorption in the measurement path. 

d. keep batteries fully charged. 

Q6. Placing a blackbody reference source next to a distant target will usually 

help correct for: 

a. the effect of atmospheric absorption in the measurement path . 

b. ambient reflections off the target surface. 

c. target surface emissivity artifacts. 

d. point source reflections. 


Q7. To make an effective infrared temperature measurement, the angle 

between the target surface and the instrument's line of sight should be: 

a. always 90 degrees (perpendicular). 

b. any angle providing the target always fills the measurement spatial 

resolution of the instrument. 

c. as close at possible to 90 degrees but not less than 60 degrees. 

d. anywhere between 30 degrees and 45 degrees. 

Q8. If a target does not fill the measurement spatial resolution of the 

measuring instrument at a convenient measurement distance, it may be 

necessary to: 

a. use a higher magnification lens or move in closer, 

b. place a blackbody reference next to the target. 

c. use the instrument 's electronic zoom feature, 

d. use more than one isothenn to make the measurement. 


Q9. Differential thermography can be very useful because it: 

a. tends to minimize the effects of surface emissivity artifacts. 

b. tends to emphasize only those areas where temperature changes occur. 

c. helps record changes for thermal trending purposes. 

d. is all of the above. 

Q10. When planning a measurement mission, it is important to remember that 

batteries: 

a. may never reach fu ll charge. 

b. are about the least reliable element at a thermographer's disposal. 

c. lose their charge more rapidly in cold weather. 

d. are all of the above. 


Chapter 5 

Applications 


5.1 Overview of Applications 

Because temperature is, by far, the most measured and recorded parameter 

in industry, it is no surprise that applications for temperature measurement 

and thermography are found in virtually every aspect of every industry. 

Because of the widespread applicability of thermal sensing and thermography, 

attempting to classify applications into formal categories meets with 

considerable overlap. Because of this ambiguity, and because the thermal 

principles of investigation involved should be well known by the qualified 

thermographer, applications are presented in this chapter by thermal 

principles of investigation categories, as set forth in the Infrared Thermal 

Testing Method, Level III Topical Outline contained in Recommended 

Practice No. SNT-TC-JA (1996) as follows: 

1. exothermic and endothermic investigations, 

2. friction investigations, 

3. fluid flow investigations, 

4. thermal resistance investigations and 

5. thermal capacitance investigations. 


5.2 Exothermic and Endothermic Investigations 

An exothermic process is one that releases heat and exhibits a temperature 

increase. An endothermic process is one that absorbs heat and exhibits a 

temperature decrease. The link between these methods is that the 

investigator does not need to apply any thermal stimulation. The relevant 

thermal pattern exists in the subject because of another process performed 

on (or within) the subject. Applications for thermograpgy in this area are 

commonly found in electrical and electronic diagnostics, chemical processes 

such as the application of foam-in-place insulation, fire detection, night vision 

and surveillance, animal studies, heating and cooling systems and other 

areas where thermal energy is released or absorbed. 

Keywords: 

the investigator does not need to apply any thermal stimulation. 


Electrical Applications 

Electrical findings represent the primary use of infrared thermography in 

facilities and utilities. They also represent the most straightforward application 

of the equipment. The most common electrical findings are caused by high 

electrical resistance, short circuits, open circuits, inductive currents and 

energized grounds. Much of the routine scanning is done qualitatively, but 

quantitative thermography is required in many instances to estimate true 

temperature rises. Specifically in electrical applications, the flow of current 

through a conductor generates heat in direct proportion to the power 

dissipated. This is directly proportional to the electrical resistance and to the 

square of the current (P = I 2 R) and is commonly called I 2 R loss. A poor 

connection or, in some cases, a defective component, will have an increased 

resistance, resulting in a temperature increase and, consequently, a 

temperature rise in the area of the discontinuity. 


Alas! 

Electrical Applications is 

NOT 

“thermal resistance investigations” 



High electrical resistance is the most common cause of thermal hot spots in 

electrical equipment and power lines. When the line current is relatively 

constant and resistance is higher than it should be, additional power is 

dissipated and a thermal anomaly occurs. This is frequently dangerous and 

always costly in terms of valuable waits lost, unwanted heat and accelerated 

aging of equipment, which results in premature replacement of equipment. 

Typical examples of resistive heating include loose connections, corroded 

connections, missing or broken conductor strands and undersized conductors. 

Figure 5.1 is an example of excessive heating caused by high resistance at a 

connection (upper center) because of deterioration of the connection. The 

connection appears to be more than 5 °C (9 °F) warmer than the adjacent 

connections. In power lines and switchyards, hot connections caused by 

deterioration are the most common findings that are associated with potential 

failures. Short circuits are another cause of electrical failure. When they occur 

in a power line, they usually are extremely brief in duration and have 

immediate and disastrous results. Within an operating component, however. 

the shorted section will cause excessive current to flow with resultant heating, 

and this frequently can be detected and diagnosed using thermographic 

equipment. 


Figure 5.1: Excessive heating due to a defective electrical connection 

The connection 

appears to be more 

than 5 °C (9 °F) 

warmer than the 

adjacent 

connections. 


Excessive heating due to a 

defective electrical 

connection 


One example of this would be shorted sections of a current transformer 

winding causing the transformer to appear hotter than normal and/or hotter 

than other similar devices. Similar problems can occur within power supplies 

and within rotating equipment such as motors and generators. Open circuits 

do not generally show up as hot spots and are often overlooked by 

inexperienced thermographers as indications of potential problems. An 

operating element running cooler than normal may indicate that the element 

is open and inoperative. A common problem with inverters, for example, is 

blown (open) capacitors that appear cool. Power supplies, resistor or 

integrated circuit chips that are open and inoperative will usually be cooler 

than normal, although the malfunction may cause excessive heating 

elsewhere in the operating clement. 


Inductive currents flowing with in ferrous components or elements that are 

within the magnetic field of large equipment (i.e., the main generator in a 

power plant) can cause excessive heating. Warm areas can appear in motor 

frames and structural elements and several examples have been documented 

where steel bolts have been inappropriately used to replace nonferrous bolts 

in framework supporting large rotating machinery. Heat caused by inductive 

heating does not always lead to failure, but should be documented by the 

conscientious thermographer. Energized grounds occur in plants and facilities, 

sometimes as the result of partial insulation breakdown in an operating 

element. These findings are, in many cases, considered life safety situations. 

Because an energized ground connection is usually extremely hot , there is 

seldom difficulty identifying it thermographically. The problem is tracing the 

cause. which may be elusive. The ground connection may also be carrying 

induced currents because of a breakdown of an element in close proximity. 

Most often the diagnosis requires considerable input from knowledgeable 

facilities personnel. 


When starting new thermography programs, it is necessary to establish 

guidelines to determine how much temperature deviation from normal 

constitutes an electrical problem. There is no simple standard because there 

are so many factors, including ambient variations, that can influence 

temperature. With this caution in mind, it is reasonable to set forth guidelines 

to assess the severity of findings based on common sense and experience as 

well as on temperature readings. Most facilities have rule-of-thumb systems 

whereby they classify the potential severity of a finding based on temperature 

rise and known load conditions. 


Moisture in Airframes 

The detection of moisture in airframe sections can be accomplished 

thermographically because of the endothermic process that takes place when 

there is moisture ingress in an airbomc structure and this water freezes. 

When thermal images are taken immediately after the aircraft lands. the skin 

above the sections where moisture has entered show up as cool spots on the 

thermogram, as seen in Figure 5.2. (does the heat capacitance qualified as 

endothermic process?) 

In chemical processes, an example of an exothermic reaction is the 

installation of foam-in-place polyurethane insulation. As the liquid chemicals 

are released into the cavity, they solidify into a foam and release heat. This 

heat is conducted into the walls of the cavity wherever the foam is produced. 

As a result. the cavity walls are uniformly heated by a successful blow. A 

thermographic investigation can evaluate the effectiveness of this process by 

mapping the uniformity of the temperature distribution on the outside walls 

cool spots would indicate sections where the foam had not migrated. 


Figure 5.2: Water ingress in an aircraft section 

29.5 

2Q.O 

2S.5 

28.0 

27.5 

27.0 I Water Ingress I 


Process Control and Product Monitoring 

For many years, infrared sensors have been used for quantitative surface 

temperature monitoring of products and processes. When measurement of 

one point in the process, or even a number of points, is not considered 

adequate to characterize the process for successful monitoring or control, 

infrared thermography can be used, The most significant aspect of this 

approach is that it is unique and unprecedented. Infrared point sensors are 

used, when appropriate, in place of conventional temperature sensors. 

Infrared scanners and imagers, however, are the only practical means to 

acquire a high resolution thermal map of an entire surface in real time (at or 

near television display rates). Full surface thermal process control was not a 

viable option until the integration of computers and image processing 

software with thermal scanners. 


Line Scanners or Imagers for Mapping of Continuous Processes 

Full image process control can be defined as using an infrared thermal image 

as a model against which to compare, and thereby control, part or all of the 

thermal surface characteristic of a product or process. If the process is 

moving at a uniform, predictable rate, a thermal image can be produccd by a 

line scanner scanning normal to the motion of the process as illustrated in 

Chapter 4. Figure 4.10. The control method is similar to thaI used in point 

sensing applications, although far broader in scope. The scanner or imager is 

first used to characterize the thermal map of the product under ideal 

conditions to produce, digitize and store a criterion image - what the ideal 

thermal distribution would be if the process resulted in perfectly acceptable 

products as designed. During the actual process, the thermal map, or any 

critical portion of the map, is constantly compared to the stored criterion 

image model by means of image subtraction and/or statistical analysis 

techniques. 


The differences produced by this comparison are used to adjust or correct the 

settings of the process mechanisms that govern the heat applied, or to alarm 

and automatically reset the process. Figure 5.3 shows the evaluation of a web 

process used on the outside of drywall construction material. 

The thermogram clearly shows excessive heat on the right edge of the 

material, a condition that can cause the paper to become brittle. The 

information derived from. The thermogram is used to correct the temperature 

distribution, thus resulting in a more acceptable product. Although the image 

is not of an automatically controlled process. it would be possible to close the 

loop to maintain ideal temperature distribution automatically. 


Spectral Considerations in Product and Process Applications 

Many products. both simple and complex, have complex spectral 

characteristics in the infrared region. Spectral filtering of the measuring 

instrument can exploit these complex spectral characteristics to measure and 

control product temperature without contact. A good example of the 

exploitation of spectral characteristics in the monitoring of incandescent lamp 

temperatures during production. An important generic example of the need for 

spectral selectivity is in the measurement of plastics being formed into films 

and other configurations. Several examples of this exploitation are illustrated 

in the detailed discussion of spectral considerations in Chapter 4. 


Night Vision, Seareh, Surveillance, Security and Fire Detection 

The level of heat given off by the human body makes it readily detectable to 

thermographic instruments. Similarly. exothermic actions of engines and 

moving vehicles make them good targets for infrared surveillance applications. 

Night vision, seareh, surveillance and security applications are, with very few 

exceptions, qualitative applications of infrared thermography. They provide 

the user with the capability to see through an atmospheric path in total 

darkness. The clarity of the image is of critical importance and temperature 

measurement is not required. Ideally, in these applications. the objective is to 

display (and sometimes to record) an image that has the very best spatial 

resolution at the longest possible range under the most adverse atmospheric 

conditions. An example of a typical surveillance image is the thermogram of a 

helicopter taken at night, shown in Figure 5.4. 


Figure 5.4: Thermogram of helicopter taken at night 


Thermogram of helicopter taken at night 


Thermogram of Jet 


Aircraft Under IR Trap 


Aircraft Under IR Trap 


Instruments used for these applications evolved from military programs based 

on the need to detect and identify tactical targets through atmosphere in the 

dark and in bad weather. For this reason, they generally operate in the 8 to 12 

μm spectral window where the atmosphere has very little absorption. 

Exceptions to this generality are infrared seeking and homing sensors that 

are sensitive to specific target emission signatures. such as rocket engine 

plumes. These instruments usually operate somewhere in the 3 to 5 μm 

region. The same qualitative instruments can be readily adapted to fire 

detection applications. From the ground or the air, they provide the capability 

of detecting incipient fires and unextinguished portions of forest fires. The 8 to 

12 μm spectral region over which they operate also provides improved 

visibility (less absorption loss) through smoke and fog. 


8 to 12 μm spectral region over which they operate also provides improved 



8 to 12 μm spectral region over which they operate also provides improved 



8 to 12 μm spectral region for Astrology 

Stellar cluster and star-forming region M 17 


Animal Studies 

Body heat allows infrared thermographic studies of animals to be made. 

Inflammation raises the temperature of infected, diseased or traumatized 

portions of the body, as illustrated in Figure 5.5. This shows the thermal 

contrast between a bruised equine foreleg (left) and a normal foreleg (right). 


Figure 5.5: Injured equine foreleg (left) appears warmer than a normal 

foreleg 


Injured equine foreleg (left) appears warmer than a normal foreleg 


http://www.thermomed.org/en/veterinarymedicine.html

5.3 Friction Investigations 

Friction generates heat as energy conversion from mechanical energy to 

thermal energy. Sliding friction is a force that acts on one body sliding over 

another. The maximum force of friction that one body is capable of exerting 

over another is directly proportional to the normal, or perpendicular force with 

which the bodies are pressed together. This proportionality is called the 

coefficient of friction and the equation for sliding friction is: 

f = μ N 

where: 

f 

μ 

N 

= the maximum force of friction. 

= the coefficient of friction. 

= the normal force with which the two bodies are pressed together. 


Work energy is expended by frictional force and converted to stored heat. 

This stored heat is then conducted. convected and radiated to the 

surroundings. which can be sensed and measured using thermal 

instruments. The heating and resultant damage from excessive friction is one 

of the most common types of mechanical failure detectable by infrared 

thermography. Many of the mechanical failures located by thermography 

occur in rotating machinery. Problems caused by friction include worn, 

contaminated or poorly lubricated bearings and couplings and misaligned 

shafts. Typical findings occur in motor bearings such as that shown in Figure 

5.6 where the temperature imbalance on a blower fan is because of uneven 

friction as seen through the end screen. The apparent temperature on the 

tower section is about 10 °C (18 °F) warmer than the upper section. Friction 

investigations applicable to thermography also include air turbulence flow 

studies in aircraft and spacecraft modeling, machine gear and belt 

temperature monitoring and effectiveness studies for the cooling of machine 

tools. 


Figure 5.6: Overhead Motor Bearing (bottom) 

27.0 

2~.4 

24.4 

23.4 

22.3 

21.3 

20.3 

19.2 

18.0 


5.4 Fluid Flow Investigations 

For successful fluid flow investigations to be performed, a temperature higher 

or lower than ambient must be induced into the fluid paths. Often, this 

condition already exists but somelimes the investigator must artificially 

introduce such a fluid. Fluid flow applications include piping, valves, heat 

exchangers, cooling towers, effluent mapping and ocean mapping. In 

predictive maintenance and plant condition monitoring, many pipe blockage 

and leakage conditions can be detected using infrared thermography. Ideally, 

the condition is simple to detect if the valve or pipe section is not covered with 

insulating material, and if the temperature of the fluid conducted by the valve 

or pipe section is sufficicnlly hotter or cooler than ambient. when conditions 

are not ideal, blockages or leakages may be difficult or impossible to detect. 

Adverse conditions include pipes or valves covered with heavy insulating 

jackets, particularly those covered with low emissivity metal cladding. Under 

most measurement conditions, a closed valve will have a distinct temperature 

gradient across it and a leaky valve will not. For example, when a hot fluid is 

blocked by a closed valve the temperature difference gradient can be 

observed thermographically. 


Steam traps are special valves that automatically cycle open and closed to 

remove condensate from sections of steam process lines. If the 

thermographer has prior knowledge of their normal operation, steam traps 

can usually be observed thermographically to determine if they are operating 

properly. Without this prior knowledge, using infrared thermography for steam 

trap diagnostics may be confusing and misleading. In the image sequence 

shown in Figure 5.7, the various operating conditions of the valve (top) result 

in clearly detectable thermal pattern changes. The thermal appearance of the 

steam trap (bottom) remains essentially the same in all three images 

Blockage of any fluid transfer line can be simple to detect thermographically if 

the fluid temperature is sufficiently hotter or cooter than ambient. If not, there 

are more sophisticated approaches that have had documented success. For 

example, the injection of uniform transient heat will often result in transient 

temperature differentials at the blockage site because of the difference in 

thermal capacity between the fluid (in liquid form) and the solid blockage. 

Heat injection techniques are discussed in greater detail in subsequent 

sections. 


Figure 1.7: Thermographs of valve (top) and steam 

trap (bottom) under three conditions of valve 

operation 


5.5 Thermal Resistance Investigations 

Thermal resistance studies are involved in any thermographic application 

where the conductive flow of thermal energy is affected by variations in 

thermal resistance that exhibits a variation in effectjve temperature at the 

target surface. Applications include building and vessel envelope studies. 

furnaces, refractory linings, hazardous heat leaks and a wide variety of 

materials testing applications. 


■ Building Insulation and Other Factors 

As previously discussed, the conductive heat flow through a laminar structure 

is related to both the temperature difference from one side of the structure to 

the other and the aggregate thermal resistance of the materials encountered. 

The higher the thermal resistance (insulating properties), the less heat will 

flow; therefore, when steady state heat flow can be established, mappingthe 

temperature on the outside of a structure and knowing the thickness and the 

inside temperature, permits the determination of the insulation properties. 

The measurement of conductive heat flow for insulation assessment is only 

one factor; however, in practical heat loss determination, other factors such 

as air infiltration and exfiltration, chimney effects. and thermal short circuits or 

bypasses can be serious enough to completely negate the benefits of good 

insulation. Thermographers have learned to consider the total structure when 

evaluating the results of thermographic surveys and to recognize and isolate 

thermal patterns typically associated with air flow as well as those caused by 

insulation deficiencies. 


Figures 5.8 and 5.9 illustrate these distinct pattern differences. Figure 5.8 

shows the distinct patterns caused by insulation deficiencies on the 

thermogram of an exterior wall of a structure heated rom within, whereas 

Figure 5.9. taken of a diffe rent structure under similar conditions. illustrates 

the effects of air exfiltration. It should be noted that most structural 

applications of thermography focus on qualitative features, such as thermal 

patterns and thermal anomalies. rather than quantitative temperature 

measurements. The only refe rence to temperature measurements was the 

stipulation in ANSUASHRAE 101- 1981 that. forthe inspection to be valid, 

"there shou ld be a minimum (difference) of 10 °C ( 18 °F) between the 

inside and outside surface tempera tures of the building for at least three 

hours prior to the survey." This stipulation was made presumably to establish 

quasi-steady state heat now thereby avoiding any misleading patterns 

because of struclUral differences in heat capacity and rendering images. 

which more rel iably represent only resistance di fferences. This standard has 

been superseded by ASTM C- I06O and ASTM C-1 155. 


Figures 5.8 and 5.9 illustrate these distinct pattern differences. Figure 5.8 

shows the distinct patterns caused by insulation deficiencies on the 

thermogram of an exterior wall of a structure heated rom within, whereas 

Figure 5.9. taken of a diffe rent structure under similar conditions. illustrates 

the effects of air exfiltration. It should be noted that most structural 

applications of thermography focus on qualitative features, such as thermal 

patterns and thermal anomalies. rather than quantitative temperature 

measurements. The only refe rence to temperature measurements was the 

stipulation in ANSUASHRAE 101- 1981 that. forthe inspection to be valid, 

"there shou ld be a minimum (difference) of 10 °C ( 18 °F) between the inside 

and outside surface tempera tures of the building for at least three hours prior 

to the survey." This stipulation was made presumably to establish quasisteady 

state heat now thereby avoiding any misleading patterns because of 

struclUral differences in heat capacity and rendering images. which more rel 

iably represent only resistance di fferences. This standard has been 

superseded by ASTM C-106O and ASTM C-1155. 


Figure 5.8: Example of missing insulation 


Figure 5.9: Example of air exfiltration 


Example of air exfiltration 


Example of air exfiltration 


■ Industrial Roof Moisture Detection 

Thermal resistance is commonly used to detect industrial roof moisture when 

there has not been adequate isolation (solar heating) to use the approach 

based on thermal capacitance. Roof moisture detection by thermal resistance 

requires that there be a minimum of 10 °C (18 °F) difference between interior 

and exterior surface temperature for at least 24 h before the survey. This 

approach is conducted at night with all surfaces clean and dry and with little 

or no wind (no greater than 15 mph). This approach is based on heat loss 

rather than solar gain. Saturated roof sections are better heat conductors 

(poorer insulators) with lower thermal resistance than dry sections, and the 

temperature difference between the interior and exterior will cause heat to be 

conducted more rapidly through wet sections than dry sections. Warmer 

areas on the exterior surface, therefore, indicate water saturation. Because 

there is a temperature differential between the interior and exterior, this 

approach is subject to artifacts caused by air flow and thermal conduction 

through the roof. For validity. the thermographer should be accompanied by 

supporting intrusive evidence such as roof core samples or by another non 

intrusive test such as electric capacitance or neutron backscatter. 


■ Refractory Systems 

Industrial structures particularly refractory structures, readily lend themselves 

to thermographic investigations. Damage or wear to a refractory structure 

invariably results in the breakdown of thermal resistance. Heat escapes 

through the worn or damaged sections and can be seen on the thermogram. 

An example of this is illustrated ill Figure 5.10 where the slight vertical crack 

in the center of the stack results in a distinct temperature increase. 


Figure 5.10: heat escaping from a worn refractory structure 


Refractory Thermogram 


Refractory Thermogram 


■ Subsurface Discontinuity Detection in Materials 

Subsurface discontinuity detection in materials is characterized by steady 

state heat flow. which may be unstimulated or stimulated. Unstimulated 

steady state heat flow uses process heat such as that produced by buildings. 

HVAC systems etc. Stimulated steady state heat flow requires the addition of 

a source of (steady) heat or cold to establish sufficient heat flow through 

material. 


■ The Unstimulated Measurement Approach to Infrared Materials Flaw 

Detection 

The unstimulated measurement approach uses the available heat flowing 

through the test sample. This occurs when products are being inspected 

during manufacture and the process being monitored produces or can be 

made to produce, the desired characteristic thermal pattern on the product 

surface. It occurs in injection molding, casting and drawing of products. An 

example of the unstimulated approach is illustrated in Figure 5. 11. On the left, 

areas of severe refractory breakdown in a boiler wall appear as the result of 

differences in heat flow because of the heat inherent in the boiler. 


Figure 5.11: An example of passive IRNDT- a refractory break down in boiler 

>112.5 11 C 

110.0 

1CXlO 

00.0 

eo.o 

70 .0 

00.0 

~.0 

40.0 

3:1.0 


■ The Stimulated Measurement Approach to Infrared Materials Flaw 

Detection 

When the desired characteristic thermal pattern on the product surface 

cannot be made to occur, or when the material samples or products are to be 

evaluated after manufacture, the stimulated, or thermal injection, approach is 

necessary. The stimulated approach can also involve thermal extraction, or 

the removal of heat from the sample, by introducing some form of cooling. 

Devices used for heat injection or extraction include the sun, air blowers, 

flood lamps, flash lamps, lasers, refrigerants, hot and cold water, chemical 

reactions, thermoelectric devices and mechanical heat sinks. In order for the 

stimulated approach to be effective, it requires the generation of a controlled 

flow of thermal energy across the Structure of the sample material under test. 

This is accompanied by thermographic monitoring of one of the surfaces (or 

sometimes both) of the sample, and the seareh for the anomalies in the 

thermal patterns so produced that will indicate a defect in accordance with 

established accept-reject criteria. 


The equipment necessary to perform infrared materials discontinuity detection 

must include thermographic scanning instrumentation and the means to 

handle the test samples and to generate and control the injection or extraction 

of thermal energy to or from the samples. These can include hot and cold air 

blowers, liquid immersion baths. heat lamps. controlled refrigerants. electric 

current, scanned lasers and induction heating. The goal is to maximize the 

normal thermal flow, minimize the lateral thermal now (along the material 

surface), cause no permanent damage to the test samples, minimize and 

carefully meter the test time and generate the most uniform thermal pattern 

possible across the surface of the test sample. Because the source of energy 

is finite in dimension, the generation of a uniform thermal pattern on the 

sample surface is often difficult to accomplish. Using a personal computer 

with appropriate diagnostic software. a thermographer has access to 

numerous image manipulation routines including keyboard controlled image 

manipulation and subtraction. This image subtraction capability can be quite 

effective in compensating for limitations in heating pattern uniformity. 


Figure 5.12 illustrates a typical infrared materials discontinuity detection 

configurat ion using the active (heat injection) method under computer control. 

When uniform heat is applied to one surface of a laminar test sample and an 

infrared scanner views the opposite surface, two types of defects are 

detectable. A metal occlusion within the structure has a higher thermal 

conductivity than the ply material and results in a warm (white) spot on the 

scanned surface. A void within the structure has a lower thermal conductivity 

than the ply material and results in a cool (dark) spot on the scanncd surface. 

The computer software can be used. when necessary, to nornlalize the 

effective temperature pattern before thermal insertion and to regulate the 

timing and intensity of the heat source. Available software also facilitates the 

precision timing and recording of test sequences so that they can be repeated 

with consistency. 


Figure 5.12: Example of active (heat injection) IRNDT for occlusion and void 

detection 

Heat Source 

Metal 

Occlusion 

Void 

Laminar 

Test 

.r IR sca~nr-e;;...__.l_ sample 

' ' ' ' 

' 

" " 

' , " 

... ,," 

, 

IR Scanner 

(Imager) 

PC 

=.\ 


Material surface characteristics, as in any other thermographic application. 

are critical to test effectiveness. Materials with high and uniform surface 

emissivity are ideally suited for evaluation by infrared materials discontinuity 

detection. When evaluating samples with low or nonuniform emissivity, the 

thermographer has several alternatives. The first is to apply a removable, thin, 

high ernissivity coating, Another is to use an image subtraction routine as 

previously discussed in Chaptcr 3. This greatly reduces emissivity artifacts 

without affecting the material. Most materials successfully evaluated by 

infrared materials discontinuity detection are composed of layers of metals. 

plastics, composites or combinations of all three. The surfaces may be metal 

or plastic and the core structure may be solid, amorphous or geometrically 

configured (i.e. a honeycomb structure). Assembled layered sections (i.e. 

aircraft lapped sections) are also tested thermographically. The surfaces of 

the test amples facing the scanner are usually uniform in appearance and 

finish, although emissivity is low and surface scratches are frequently present. 

Keywords: 

Emissivity artifact (Reflectivity) 


Typical failure modes of the material samples are(1) voids between layers, (2) 

disbonds between layers, (3) impurities or foreign material in the laminar 

interfaces and (4) significam irregularities (damage) to the geometric core 

structure. Typical defects in assembled sections are loose or damaged welds 

and rivets and erosion/corrosion between sections. often accompanied by 

material loss and thinning. 

Establishing test protocol involves determining acceptability of each part to be 

evaluated in terms of minimum size of void to be detected, minimum area of 

disbond that can be said to constitute a defect and any other void or disbond 

characteristic that is deemed significant. For this it is necessary to use known 

acceptable and known defective samples. Ideally, the defective samples 

furnished should include known defects of each classification and in the 

minimum sizes required to be detected and identified. When ideal defective 

samples are not available. it becomes necessary to synthesize flaws to 

simulate the minimum defects. 


Selecting the infrared scanning system requires matching thermographic 

equipment performance capabilities to test criteria. To be effective, 

thermographic equipment used should offer resolution, sensitivity and 

versatility somewhat beyond that envisioned to be necessary to detect and 

identify the defects, the thermographer expects to encounter. The most 

critical of the scanner performance characteristics are (1) minimum resolvable 

temperature, (2) spatial resolution and (3) scan speed. 

Figure 5.13 is an example of stimulated thermography that is ideal for the 

thermographer. Here the deicing mechanism on the wing of a DC·9 aircraft is 

evaluated. The deicing system also serves as the energizing source and the 

thermogram indicates areas that are not being heated as cool spots. The 

warm rings represent the instantaneous effect of the deicing mechanism. 


Figure 5.13: Test of aircraft deicing element showing unheated areas on the 

wing of a DC-9 


DC - 9 


DC - 9 


■ Stimulated Thermography Using Pulsed or Thermal Wave Injection 

One of the earliest applications of infrared materials discontinuity detection, 

performed as carly as 1970 was the detection of flaws in aircraft structures. 

This application continues to be an important one and most major airframe 

manufacturers have on going in-house infrared materials discontinuity 

detection programs. Innovations in heat injection techniques (i.e. the 

introduction of high intensity short-duration thermal pulses) have resulted in 

improved capability for detecting small and buried naws. These, coupled with 

the imroduclion of high speed focal plane array imagers and improvements in 

computer enhancement techniques for isolating and analyzing thermographic 

patterns and data, have had an important effect on image understanding and 

discontinuity recognition. The thermal wave technique is illustrated in Figure 

5.14. Here, high intensity xenon flash lamps are used to irradiate the target 

surface with short duration pulses (on the order of milliseconds) of thermal 

energy. In many ways, this pulsed heating is similar to using the sun's heating 

cycle for the detection of underground voids. as previously discussed. 


Figure 5.14: Conceptual sketch of thermal wave imaging 

1. Sample surface is flash heated. 2. Incident thermal wave is 

generated at the sample surface. 

Discontinuity 

5. Surface temperature is detected 

by an IR camera and sent to a 

PC-based image processor. 

4. Thermal wave "echoes" cause 

transient surface temperature 

changes. 

3. Thermal waves are scattered 

by subsurface discontinuities. 


Xenon Lamp 


In this case. however, the heat pulses and the detection intervals are 

thousands of times faster. While the surface cools, the heat is conducted into 

the material at a uniform rate until it reaches a thermal barrier or discontinuity, 

such as a flaw. At this time the temperature at the surface is lower than that at 

the discontinuity site, and a portion of the heat is conducted back to the 

surface, simulating a thermal echo. The time it takes from the generation of 

the pulse to the reheating at the surface, then, is an indication of the depth of 

the discontinuity. The behavior of the thermal energy moving through the 

material is similar in many ways to that of a wave of energy propagating 

through the material and being eflected back to the surface. For this reason 

the term thermal wave imaging has been adopted by some thermographers 

to deseribe the process. By using diagnostic software to time-gate the return 

thermal images, they can estimate the depths of flaws as well as their size 

and location, often with excellent precision. The term time resolved infrared 

radiometry is also used to describe the technique of selecting the image that 

best indicates the detected discontinuity. 


Figure 5.15 illustrates a result of thermal wave injection and computer 

enhanced image analysis. The subject is erosion/corrosion damage in an 

aircraft skin lap joint. The high speed time-gating of images is essential 

because of the extremely high thermal diffusivity of the aluminum material. 

Within the past five years, time resolved infrared thermography has been 

successful to some extent in locating wall thinning because of erosion and 

corrosion in pipes and boiler tubes in utilities. Figure 5.16 is a time-resolved 

thermogram illustrating the resu lts of flash heating of a boiler tube section. 

The high lighted areas indicate maximum thinning. 

Keywords: 

thermal wave imaging 

time resolved infrared radiometry 


Figure 5.15: Erosion/corrosion damage in a 737 aircraft lap joints elevated 

areas indicate erosion/corrosion, depressed areas are rivets 


Figure 5.16: Time-resolved thermal Image of boiler wall section showing wall 

thinning due to corrosion 


737 aircraft 


5.6 Thermal Capacitance Investigations 

Several seemingly diverse applications have in common the fact that data 

sample timing is critical to accurate detection and analysis. These 

applications are those that are investigated on the basis of (usually 

nonhomogeneous) thermal capacitance and/or thermal diffusivity. Thermal 

capacitance and thermal diffusivity are discussed in Chapter 2. 

Industrial Roof Moisture Detection 

As in most buildings and infrastructure applications. flat roof surveys are 

concerned with detection and identification of thermal patterns rather than 

quantitative measurements. These patterns are indications of subsurface 

moisture that is typically absorbed in the insulation. One approach to making 

these measurements depends on solar heating (insolation). This approach is 

conducted at night with all surface clean and dry and little or no wind (no 

greater Ihan 15 mph). 


When there has been adequate solar heating of the roof during the day 

before the survey, stored thermal energy will cause water- saturated sections, 

with their higher thermal capacitance, to store more heal. At night. the roof 

radiates thermal energy to the cold sky. At some time during the night, the dry 

sections, with less stored heat, appear cool. The saturated sections appear 

warmer and the thermographer can easily locate and identify them. This 

procedure is particularly effective even when there is no temperature 

difference between the interior and exterior of the building. Unlike the thermal 

resistance approach previously discussed, this approach, illustrated in Figure 

5.17 is subject to few thermal artifacts due to vent pipes, exhaust fans, etc. 

In 1990, ASTM C1153-90, Standard Practice for the Location of Wet 

Insulation in Roofing Systems Using Infrared Imaging was released by the 

American Society for Testing and Materials. It outlines the minimum criteria 

for an acceptable infrared roof moisture survey and clearly stipulates the 

requirement for both dry and wet core samples. It also defines the minimum 

performance specifications of thermal sensing and imaging equipment used 

to perform thermographic surveys. 


Figure 5.17: Thermogram with roof with moisture saturation. 


Liquid Level Detection 

Thermal capacitance difference also allows thermographic detection of the 

liquid levels in storage tanks and other containers. In the thermogram of a fuel 

tank at night. shown in Figure 5.18. the fuel level is clearly evident because 

the fuel has a higher thermal capacitance than the air above it. The heat 

stored through solar absorption during the day maintains a higher 

temperature on the tank walls up to the fill level. Conversely. if the entire tank 

had cooled, the liquid would warm later than the air and the wall below the fill 

level would appear cooler than above. 


Figure 5.18: Fuel level in a storage tank 


Unstimulated and Stimulated Approaches to Infrared Materials Flaw 

Detection 

Materials discontinuity detection based on thermal capacitance differences is 

similar to that based on thermal resistance differences in that a stimulated 

approach may be used when the desired characteristic thermal pattern on the 

product surface cannot be madc to occur, or when the material samples or 

products are to be evaluated after manufacture. As previously discussed, this 

can involve thermal injection in a variety of forms, but it can also involve 

thermal extraction, or the removal of heat from the sample by introducing 

some form of cooling. 


Underground Void Detection 

The detection of underground voids is based, for the most pan, on the 

difference in thermal capacitance between solid earth and the air cavities 

formed by buried tanks, eroded sewers and storm drains and improperly filled 

excavations. Typical programs to detect underground voids are performed 

using the sun as a basic source of thermal energy. During the day, the heat 

from the sun penetrates the earth and heats both the earth and the voids. The 

voids have a lower thermal capacitance and store less heat than the 

surrounding earth. On the subsequent thermographer they appear as cool 

areas. As in roof surveys, apparent findings are usually confirmed by means 

of other disciplines. Ground penetrating radar has come into use as a 

confirming discipline for thermographic underground void detection. 


Subsurface Discontinuity Detection in Materials 

Subsurface discontinuity detection in materials is characterized by nonsteady 

(varying) heat flow through the subject, which can be unstimulated or 

stimulated. Unstimulated nonsteady heat flow uses (unsteady) process heat 

or a cool down after process heating. Stimulated nonsteady heat flow 

depends on the use of an (unsteady) source of heating or cooling. 


Chapter 5 


Q&A 

1. b 

2. b 

3. c 

4. a 

5. b 

6. a 

7. b 

8. c 

9. d 

10. a 

11. d 


1. A major area of infrared nondestructive material testing is based on the fact 

that: 

a. a good structural bond normalizes emittance artifacts. 

b. uniform structural continuity provides predictable thermal continuity. 

c. a structural void is a good thermal bond. 

d. thermal imagers can be made to measure temperature with great accuracy. 

2. When analyzing a thermographic image. it is usually possible to distinguish 

between an overload condition and a loose connection because: 

a. a loose connection will appear cool compared to its surroundings. 

b. a loose connection will appear warmer than the wires on either side. 

c. an overload will cause a sharper thermal gradient. 

d. one side of a loose connection will appear much warmer than the other. 


3. The most significant advantage of thermal wave imaging over conventional 

step stimulation methods of infrared/thermal materials testing is that: 

a. it can find smaller voids. 

b. it is simpler to implement. 

c. it can providc better information regarding the depth of a 

discontinuity. 

d. it provides images with better spatial resolution. 

4. The diagnostics involved in thermography of electrical switchgear most 

frequently involves: 

a. exothermic invesligations. 

b. thermal resistance investigations. 

c. security investigations. 

d. fluid flow investigations. 


5. The diagnostics involved in detection of moisture in flat roofs most 

frequently involve: 

a. exothermic and endothermic investigations. 

b. thermal resistance and thermal capacitance investigations. 

c. friction investigations. 

d. fluid flow investigations. 

6. In time resolved thermography (thermal wave imaging) applied to materials 

nondestructive testing. the time of the return signal from a void or disband is 

most closely related to the: 

a. depth of the discontinuity. 

b. size of the discontinuity. 

c. amplitude of the heating pulse. 

d. spectral characteristics of the heating pulse. 


7. In the process monitoring of thin film plastics successful thermographic 

measurement is most closely related to: 

a. correcting the instrument for background reflections. 

b. matching the spectral characteristics of the instrument to those of the 

target material. 

c. optimizing the speed of response of the measuring instrument. 

d. optimizing the spatial resolution of the measuring instrument. 

8. The unstimulated approach to infrared nondestructive testing can usually 

be used when evaluating the condition of refractory linings of vessels 

because: 

a. refractory materials have high effective emissivities. 

b. refractory materials have high reflectivity in the infrared. 

c. a strong. uniform source of heat usually exists within the vessel. 

d. infrared focal plane imagers are available for these applications. 


9. Thermography has been successfully applied to some veterinary medicine 

applications because, in most cases: 

a. healthy animals are hotter than sick animals. 

b. the emissivity of animal hides is high. 

c. animals have higher body temperatures than humans. 

d. infection and trauma usually cause the affected portion of the body to 

become warmer. 

10. The use of thermography for the detection of moisture infiltration in 

airframes is made possible by a combination of thermal capacitance 

differences and: 

a. an endothermic effect that causes the infiltrated portions to appear 

cooler. 

b. an exothermic effect that causes the infiltrated portions to appear warmer. 

c. increased friction between the air flow and the infiltrated sections. 

d. reduced friction between the air flow and the infiltrated sections. 


11. Sulface thermal patterns can often reveal: 

a. subsurface material defects. 

b. delaminations within a structurc. 

c. impurities within a material sample. 



Appendix A 

Glossary 

The following are explanations and definitions of terms commonly encountered by the 

infrared thermographer Many of these terms have multiple definitions and the one 

provided is the one most applicable to infrared thermography. NOTE: In some cases, 

the "textbook" definition of a term is replaced by one more explicitly dealing with the 

practice o/infrared thermography. 


1. Absolute zero - The temperature that is zero on the Kelvin or º Rankine 

temperature scales. The temperature at which no molecular motion takes 

place in a material. 

2. Absorptivity, α (absorptance) - The proportion (as a fraction of 1) of the radiant 

energy impinging on a material's surface that is absorbed into the material. 

For a blackbody, this is unity (1.0). Technically, absorptivity is the internal 

absorptance per unit path length. In tthermography, the two terms are often 

used interchangeably. 

3. Accuracy (of measurement) - The maximum deviation, expressed in percent 

of scale or in degrees celsius or degrees fahrenheit. that the reading of an 

instrument will deviate from an acceptable standard reference, normally 

traceable to the National Institute for Standards and Technology (N IST). 

4. Ambient operating range - Range of ambient temperatures over which an 

instrument is designed to operate within published performance specifications. 

5. Ambient temperature - Temperature of the air in the vicinity of the target 

(target ambient) or the instrument (instrument ambient) 

6. Ambient temperature compensation - Correction built into an instrument to 

provide automatic compensation in the measurement for variations in 

instrument ambient temperature. 


Ambient temperature - Temperature of the air in the vicinity of the target (target 

ambient) or the instrument (instrument ambient) 

target ambient 

instrument 

ambient 


7. Anomaly - An irregularity, such as a thermal anomaly on an otherwise isothermal 

surface; any indication that deviates from what is expected. 

8. Apparent temperature - The target surface temperature indicated by an infrared 

point sensor, line scanner or imager. 

9. Artifact ~ A product of artificial character because of extraneous agency; an error 

caused by an uncompensated anomaly. In thermography, an emissivity artifact 

simulates a change in surface temperature but is not a real change. 

10.Atmospheric windows (infrared) ~ The spectral intervals within the infrared 

spectrum in which the atmosphere transmits radiant energy well (atmospheric 

absorption is a minimum). These are roughly defined as 2 to 5 μm and 8 to 14 μm . 

11.Background temperature, instrument - Apparent ambient temperature of the scene 

behind and surrounding the instrument as viewed from the target. The reflection of 

this background may appear in the image and affect the temperature measurement. 

Most quantitative thermal sensing and imaging instruments provide a means for 

correcting measurements for this reflection. (See Figure A- 1.) 

12.Background temperature, target - Apparent ambient temperature of the scene (1) 

behind and (2) surrounding the instrument, as viewed from the instrument. When 

the FOV of a point sensing instrument is larger than the target, the target 

background temperature will affect the instrument reading. (See Figure A-1 .) 


Figure A-1 

Apparent ambient temperature of the 

scene (1) behind and (2) surrounding 

the instrument 


Background Temperature 

Target 

Background 

Instrument 

Background 


13.Blackbody, blackbody radiator - A perfect emitter; an object that absorbs all the 

radiant energy impinging on it at all wavelengths and reflects and transmits none. 

A surface with emissivity of unity (1.0) at all wavelengths. 

14.Bolometer. infrared ~ A type of thermal infrared detector. 

15.Calibration - Checking and/or adjusting an instrument such that its readings agree 

with a standard . 

16.Calibration check - A routine check of an instrument against a reference to ensure 

that the instrument has not deviated from calibration since its last use. 

17.Calibration accuracy - The accuracy to which a calibration is performed. usually 

based on the accuracy and sensitivity of the instruments and references used in 

the calibration. 

18.Calibration source, infrared - A blackbody or other target of known temperature 

and effective emissivity used as in calibration reference. 

19.Capacitance, thermal - This term is used to describe heat capacity in terms of an 

electrical analog, where toss of heat in analogous to loss of charge on a capacitor. 

Structures with high thermal capacitance change temperature more slowly than 

those with low thermal capacitance. 

20.Capacity, heat - The heat capacity of a material or structure describes its ability to 

store heat. It is the product of the specific heat (c p 

) and the density (ρ) of the 

material. This means that denser materials generally will have higher heat 

capacities than porous materials. 


Capacity, heat (Volumetric Heat Capacity) - The heat capacity of a material or 

structure describes its ability to 

store heat. It is the product of the specific heat (c p 

) and the density (ρ) of the material. 

This means that denser materials generally will have higher heat capacities than 

porous materials. 

Heat Capacityvolumetric 

= C p x ρ 



Alas! 

Heat Capacity Volumetric 

= 

C p ∙ρ 



21.Celsius (Centigrade) - A temperature scale based on 0 °C as the freezing point of 

water and 100 °C as the boi ling point of water at standard atmospheric pressure; 

a relative scale related to the Kelvin scale [ 0 °C = 273.12 K; 1ºC (ΔT): 1 K (ΔT) ]. 

22.Color - A ternl sometimes used to deline wavelength or spectral interval, as in twocolor 

radiometry (meaning a method that measures in two spectral intervals); also 

used conventionally (visual color) as a means of displaying a thermal image, as in 

color thermogram. 

23.Colored body - See nongraybody. 

24.Conduction - The only mode of heat now in solids, but can also take place in 

liquids and gases. It occurs as the result of (1) atomic vibrations (in solids) and (2) 

molecular collisions (in liquids and gases) whereby energy is transferred from 

locations of higher temperature to locations of lower temperature. 

25.Conductivity, thermal, (k) - A material property defining the relative capability to 

carry heat by conduction in a static temperature gradient. Conductivity varies 

Slightly with temperature in solids and liquids and with temperature and pressure in 

gases. It is high for metals (copper has a k of 380 W/m∙°C) and low for porous 

materials (concrete has a k of 1.0 W/m∙°C) and gases. 

26.Convection - The form of heat transfer that takes place in a moving medium and is 

almost always associated with transfer between a solid (surface) and a moving 

fluid (such as air). whereby energy is transferred from higher temperature sites to 

lower temperature sites. 


27.Detector, infrared - A transducer element that converts incoming infrared radiant 

energy impinging on its sensitive surface to a usefu l electrical signal. 

28.Diffuse reflector - A surface that reflects a portion of the incident radiation in such a 

manner that the reflected radiation is equal in all directions. A mirror is not a diffuse 

reflector. 

29.Diffusivity, thermal, (α) - (Note: same symbol as absorptivity may be confusing.) 

The ratio of conductivity (k) to the product of density (ρ) and specilic heat (C p 

) 

[ α =k/ρ∙C p 

cm 2 s -1 ]. The ability of a material to distribute thermal energy after a 

change in heat input. A body with a high diffusivity will reach a uniform temperature 

distribution faster than a body with lower diffusivity. 

30.D* (detectivity star) - Sensitivity figure of merit of an infrared detector - detectivity 

expressed inversely so that higher D* indicate better performance; taken at specific 

test conditions of chopping frequency and information bandwidth and displayed as 

a function of spectral wavelength. 

31.Display resolution, thermal - The precision with which an instrument displays its 

assigned measurement parameter (temperature). usually expressed in degrees, 

tenths of degrees, hundredths of degrees. etc. 

32.Effective emissivity (Ɛ*) - The measured emissivity value of a particular surface 

under existing measurement conditions (rather than the generic tabulated value for 

the surface material) that can be used to correct a specific measuring instrument to 

provide a correct temperature measurement. 


33. Effusivity, thermal (e) - A measure of the resistance of a material to 

temperature change: 

e =(kρC p ) ½ cm 2 ºC -1 S 1/2 

where: 

k = thermal conductivity 

ρ = bulk density 

= specific heat 

C p 

Comments: compare diffusivity 

α =(k/ρ)∙C p cm 2 s -1 


Thermal Effusivity 

In Thermodynamics, the thermal effusivity of a material is defined as the 

square root of the product of the material's thermal conductivity and its 

volumetric heat capacity. 

e = (kρC p ) ½ cm2 ºC -1 S 1/2 

Here, k is the thermal conductivity, ρ is the density and Cp is the specific heat 

capacity. The product of ρ and Cp is known as the volumetric heat capacity. 

A material's thermal effusivity is a measure of its ability to exchange thermal 

energy with its surroundings. If two semi-infinite bodies initially at 

temperatures T 1 and T 2 are brought in perfect thermal contact, the 

temperature at the contact surface T m will be given by their relative effusivities. 

This expression is valid for all times for semi-infinite bodies in perfect thermal 

contact. It is also a good first guess for the initial contact temperature for finite 

bodies. 


34. Emissivity (Ɛ) - The ratio of a target surface's radiance to that of a blackbody at the 

same temperature, viewed from the same angle and over the same spectral interval; 

a generic lookup value for a material. Values range from 0 to 1.0. 

35. EMI/RFI noise - Disturbances to electrical signals caused by electromagnetic 

interference (EMI) or radio frequency interference (RFI). In thermography, this may 

cause noise patterns to appear on the display. 

36. Environmental rating - A rating given an operating unit (typically an electrical or 

mechanical enclosure) to indicate the limits of the environmental conditions under 

which the unit will function reliably and within published performance specifications. 

37. Exitance, radiant (also called radiosity) - Total infrared energy (radiant flux) leaving 

a target surface. This is composed of radiated. reflected and transmitted 

components. Only the radiated component is related to target surface temperature. 

38. Fahrenheit - A temperature scale based on 32 ºF as the freezing point of water and 

212 ºF as the boiling point of water at standard atmospheric pressure; a relative 

scale related to the Rankine scale [ 0 ºF = 459.67 ºR; 1 ºF (ΔT) = 1 ºR (ΔT) ]. 

39. Field of view (FOV) - The angular subtense (expressed in angular degrees or 

radians per side if rectangular, and angular degrees or radians if circular) over 

which an instrument will integrate all incoming radian energy. In a radiation 

thermometer this denotes the target spot size; in a scanner or imager this denotes 

the scan angle or picture size or total field of view. 


Alas! 

Exitance = Rodiosity 



40.Fiber optic, infrared - A flexible fiber made of a material that transmits infrared 

energy, used for making noncontact temperature measurements when there is not 

a direct line of sight between the instrument and the target. 

41.Filter, spectral - An optical element, usually transmissive, used to restrict the 

spectral band of energy received by an instrument's detector. 

42.Focal plane array (FPA) - A linear or two-dimensional matrix of detector elements, 

typically used at the focal plane of an instrument. In thermography, rectangular 

FPAs are used in staring (nonscanning) infrared imagers. These are called infrared 

focal plane array imagers. 

43.Focal point - The point at which the instrument optics image the infrared detector at 

the target plane. In a radiation thermometer, this is where the spot size is the 

smallest. In a scanner or imager, this is where the instantaneous field of view 

([FOV) is smallest. 

44.Foreground temptrature (see instrument ambient background) - Temperature of 

the scene behind and surrounding the instrument as viewed from the target. (See 

Figure A-1.) 


45.Frame repetition rate - The time it takes an infrared imager to scan (update) every 

thermogram picture element (pixel); in frames per second. 

46.Full scale - The span between the minimum value and the maximum value Ihat any 

instrument is capable of measuring. In a thermometer, this would be the span 

between the highest and lowest temperature that can be measured. 

47.Graybody - An radiating object whose emissivity is a constant value less than unity 

( 1.0), over a specific spectral range. 

48.Hertz (Hz) – A unit of measurement of signal frequency; 1 Hz = 1 cycle per second. 

Image, infrared - See Thermogram. 

49.Imager, infrared - An infrared instrument that collects the infrared radiant energy 

from a target surface and produces an image in monochrome (black and white) or 

color, where the gray shades or color hues correspond respectively to target 

exitance. 

50.Image display tone - Gray shade or color hue on a thermogram. 

51.Image processing, thermal - Analysis of thermal images, usually by computer; 

enhancing the image to prepare it for computer or visual analysis. In the case of an 

infrared image or thermogram, this could include temperature scaling, spot 

temperature measurements, thermal profiles, image manipulation, subtraction and 

storage. 


52.Imaging radiometer - An infrared thermal imager that provides quantitative thermal 

images. 

53.Indium Antimonide (InSb) - A material from which fast, sensitive photo detector 

used in infrared scanners and imagers are made. Such detectors usually requiring 

cooling while in operation. Operation is in the short wave band (2 to 5 μm). 

54.Inertia, thermal - See thermal effusivity. 

55.Infrared - The infrared spectrum is loosely defined as that portion of the 

electromagnetic continuum extending from the red visible (0.75 μm) to about 1000 

μm (1mm).Because of instrument design considerations and the infrared 

transmission characteristics of the atmosphere, however. most infrared 

measurements are made between 0.75 and 20 μm. 

56.Infrared focal plane array (lRFPA) - A linear or two-dimensional matrix of individual 

infrared detector elements, typically used as a detector in an infrared imaging 

instrument. 

57.Infrared radiation thermometer - An instrument that converts incoming infrared 

radiant energy from a spot on a target surface to a measurement value that can be 

related to the temperature of that spot. 

58.Infrared thermal imager - An instrument or system that converts incoming infrared 

radiant energy from a target surface to a thermal map or thermogram, on which 

color hues or gray shades can be related to the temperature distribution on that 

surface. 


Thermal Inertia 

Thermal inertia is a term commonly used by scientists and engineers modelling heat transfers 

and is a bulk material property related to thermal conductivity and volumetric heat capacity. For 

example, this material has a high thermal inertia, or thermal inertia plays an important role in this 

system, which means that dynamic effects are prevalent in a model, so that a steady-state 

calculation will yield inaccurate results. The term is a scientific analogy, and is not directly related 

to the mass-and-velocity term used in mechanics, where inertia is that which limits the 

acceleration of an object. In a similar way, thermal inertia is a measure of the thermal mass and 

the velocity of the thermal wave which controls the surface temperature of a material. In heat 

transfer, a higher value of the volumetric heat capacity means a longer time for the system to 

reach equilibrium. 

The thermal inertia of a material is defined as the square root of the product of the material's bulk 

thermal conductivity and volumetric heat capacity, where the latter is the product of density and 

specific heat capacity: 

e = I = √(kρC p ) See also Thermal effusivity 

k = is thermal conductivity, with unit [W m −1 K −1 ] 

ρ = is density, with unit [kg m −3 ] 

C p 

= is specific heat capacity, with unit [J kg −1 K −1 ] 

e, I = has SI units of thermal inertia of [J m −2 K −1 s −1/2 ]. 


http://en.wikipedia.org/wiki/Volumetric_heat_capacity#Thermal_inertia

59. Instantaneous field of Fiew (lFOV) - The angular subtense (expressed in angular 

degrees or radians per side if rectangular and angular degrees or radians if 

round) over which an instrument will integrate all incoming radiant energy; the 

projection of the detector at the target plane. In a radiation thermometer this 

denotes the target spot size; in a line scanner or imager it representS one 

resolution clement in a scan line or a thermogram and is a measure of spatial 

resolution. (D=α∙d) 

60. IRFPA imager or camera – An infrared imaging instrument that incorporates a 

two-dimensional infrared focal plane array and produces a thermogram without 

mechanical scanning. 

61. Isotherm - A pattern superimposed on a thermogram or on a line scan that 

includes or highlights all points that have the same apparent temperature Kelvin - 

Absolute temperature scale related to the celsius (or Centigrade) relative scale. 

The Kelvin unit is equal to 1 °C; 0 Kelvin = - 273.16 °C; the degree sign and the 

word degrees are not used in describing Kelvin temperatures. 


Instantaneous field of View 

(lFOV) 

D=σ∙d 


(care on unit used!) 



62. Laser pyrometer - An infrared radiation thermometer that projects a laser beam 

to the target, uses the reflected laser energy to compute target effective 

emissivity and automatically computcs target temperature (assuming that the 

target is a diffuse reflector) - not to be confused with laser-aided aiming devices 

on some radiation thermometer. 

63. Line scan rate - The number of target lines scanned by an infrared scanner or 

imager in one second. 

64. Line scanner, infrared - An instrument that scans an infrared field of view FOV 

along a straight line at the target plane to collect infrared radiant energy from a 

line on the target surface, usually done by incorporating one scanning element 

within the instrument. If the target (such as a sheet or web process) moves at a 

fixed rate normal to the line scan direction, the result can be displayed as a 

thermogram. 

65. Measurement spatial resolution, lFOV meas 

- The smallest target spot size on 

which an infrared imager can produce a measurcment, expressed in terms of 

angular subtense (mRad per side). The slit response function (SRF) test is used 

to measure measurement spatial resolution / IFOV meas 

. 

66. Medium, transmitting medium – The composition of the measurement path 

between a target surface and the measuring instrument through which the 

radiant energy propagates. This can be vacuum, gaseous (such as air), solid, 

liquid or any combination of these. 


Laser pyrometer - Laser pyrometer - An infrared radiation thermometer that 

projects a laser beam to the target, uses the reflected laser energy to compute 

target effective emissivity and automatically computcs target temperature 

(assuming that the target is a diffuse reflector) - not to be confused with laser-aided 

aiming devices on some radiation thermometer. 

Further reading on this subject is necessary. 


67. Mercury cadmium telluride MCT (HgCdTe) - A material used for fast, sensitive 

infrared photodetectors used in infrared sensors, scanners and imagers that 

requires cooled operation. Operation is in the long wave length region (8 to 12 

μm). 

68. Micron (micrometer) (μ or μm) - One millionth of a meter; a unit used to express 

wavelength in the infrared. 

69. Milliradian (mRad) - One thousandth of a radian (1 radian = 180/π); a unit used 

to express instrument angular field of view 

70. Minimum resolvable temperature (difference), MRT(D) - thermal resolution; 

thermal sensitivity - the smallest temperature difference that an instrument can 

clearly distinguish out of the noise, taking into account characteristics of the 

display and the subjective interpretation of the operator. 

71. Modulation - In general, the changes in one wave train caused by another; in 

thermal scanning and imaging, image luminant contrast; (Lmax - Lmin)/(Lmax + 

Lmin). 

72. Modulation Transfer Function (MTF) - A measure of the ability of an imaging 

system to reproduce the image of a target. A formalized procedure is used to 

measure modulation transfer function; It assesses the spatial resolution of a 

scanning or imaging system as a function of distance to the targe!. 



ot2 


73. Noise equivalent temperature (difference), NET(D) - The temperature difference 

that is just equal to the noise signal; a measure of thermal resolution, but not 

taking into account characteristics of the display and the subjective interpretation 

of the operator. 

74. NIST, NlST traceability - The National Institute of Standards and Technology 

(formerly NBS). Traceability to NIST is a means of ensuring that reference 

standards remain valid and their calibration remains current. 

75. Nongraybody - A radiating object that does not have a spectral radiation 

distribution similar to a blackbody and can be partly transparent to infrared 

(transmits infrared energy at certain wavelengths); also called a colored body. 

Glass and plastic films are examples of nongraybodies. The emissivity of a 

colored body has a spectral dependence. 

76. Objective lens - The primary lens of an optical system, On an infrared instrument, 

usually the interchangeable lens that denotes the total field of view. 

77. Opaque - Impervious to radiant energy. In thermography, an opaque material is 

one that does not transmit thermal infrared energy, (τ = 0). 

78. Optical element, infrared - Any element that collects, transmits restricts or 

reflects infrared energy as part of an infrared sensing or imaging instrument. 

79. Peak hold - A feature of an instrument whereby an output signal is maintained at 

the peak instantaneous measurement for a specified duration. 


Compare: 

Minimum resolvable temperature (difference), MRT(D) - thermal resolution; 

thermal sensitivity - the smallest temperature difference that an instrument can clearly 

distinguish out of the noise, taking into account characteristics of the display and the 

subjective interpretation of the operator. 

Noise equivalent temperature (difference), NET(D) - The temperature 

difference that is just equal to the noise signal; a measure of thermal resolution, but 

not taking into account characteristics of the display and the subjective interpretation 

of the operator.. 


80. Photodetector (photon detector) - A type of infrared detector that has fast 

response (on the order of microseconds), limited spectral response and usually 

requires cooled operation: photooctectors are used in infrared radiation 

thermometer. scanners and imagers, because, unlike thermal detector, direct 

photon interaction obviates 防止 external heating of the detector for the signal to 

be sensed. 

81. Pyroelectric detector - A type of thermal infrared detector that acts as a current 

source with its output proportional to the rate of change of its temperature. 

82. Pyroelectric vidicon (PEV), also called pyrovidicon - A video camera tube with its 

receiving elemen! fabricated of pyroelectric material and sensitive to wavelengths 

from about 2 to 20 μm; used in infrared thermal viewers. 

83. Pyrometer - Any instrument used for temperature measurement. (1) A radiation 

or brightness pyrometer measures visible energy and relates it to brightness or 

color temperature. (2) An infrared pyrometer measures infrared radiation and 

relates it to target surface temperature. 

84. Radian - An angle equal to 180 degrees/π or 57.29578 angular degrees. 

85. Radiation, thermal - The mode of heat flow that occurs by emission and 

absorption of electromagnetic radialion. propagating at the speed of light and 

unlike conductive and convective heal flow, capable of propagating across a 

vacuum; the form of heat transfer that allows infrared tthermography to work 

because infrared energy travels from the target to the detector by radiation. 


86.Radiation rererenee source - A blackbody or other target of known temperature 

and effective emi ssivity used as a reference 10 obtain optimum measurement 

accuracy. ideally. traceable to NIST. 

87.Radiation thermometer – See infrared radiation thermometer. 

88.Radiosity – See Exitance, thermal. 

89.Rankine - Absolute temperature scale related to the fahrenheit relative scale. The 

Rankine unit is equal to 1ºF; 0 Rankine = - 459.72 ºF ; the degree sign and the 

word degrees is not used in describing Rankine temperatures. 

90.Ratio pyrometer - An infrared thermometer that uses the ratio of incoming infrared 

radiant energy at two narrowly separated wavelengths to detennine a target's 

temperature independent of target emittance; this assumes graybody conditions 

and is normally limited to relatively hot targets (above about 149 ºC, 300 ºF). 

91.Reference junction - In a thermocouple. the junction of the dissimilar metals that is 

not the measurement junction. This is normally maintained at a constant reference 

temperature. 

92.Reflectivity, (reflectance) (ρ) - The ratio of the total energy reflected from a surface 

to total incidence on that surface; ρ = 1 - Ɛ - τ; for a perfect mirror this approaches 

1.0; for a blackbody the reflectivity ρ is 0. Technically, reflectivity is the ratio of the 

intensity of the reflected radiation to the total radiation and reflectance is the ratio 

of the reflected flux to the incident flux. In tthermography, the two terms are often 

used interchangeably. (only subtraction where is the division, ratio?) 


93. Relative humidity - The ratio (in percent) of the water vapor content in the air to 

the maximum content possible at that temperature and pressure. 

94. Repeatability - The capability of an instrument to exactly repeat a reading on an 

unvarying target over a short or long term time interval. For thermal 

measurcments, expressed in ±degrees or a percentage of full scale. 

95. Resistance, thermal (R) – A measure of a material's resistance to the flow of 

thermal energy, inversely proportional to its thermal conductivity, k. (1/R = k) 

96. Response time - The time it takes for an instrument output signal or display to 

respond to a temperature step change at the target; expressed in seconds. 

(typically, to 95 percent of the final value and approximately equal to 5 time 

constants) 

97. Resistance temperature detector (RTD) – a sensor that measures temperature 

by a change in resistance as a funct ion of temperature. 

98. Sample hold - A feature of an instrument whereby an output signal is maintained 

at an instantaneous measurement value for a specified duration after a trigger or 

until an external reset is applied. 

99. Scan angle - For a line scanner, the total angular scan possible at the target 

plane, typically 90 degrees. 

100. Scan position accuracy - For a line scanner. the precision with which 

instantaneous position along the scan line can be set or measured. 


101. Sector - For a line scanner, a portion of the total scan angle over which 

measurement is made at the target plane. 

102. Seebeck effect - The phenomenon that explains the operation of thermocouples; 

that in a closed electrical circuit made up of two junctions of dissimilar metal 

conductors, a direct current will flow as long as the two junctions are at different 

temperatures, The phenomenon is reversible: if the temperatures at the two 

junctions are reversed. the flow of current reverses. 

103. Sensitivity - See minimum resolvable temperature (difference), MRT(D). 

104. Setpoint - Any temperature setting at which an activating signal or closure can be 

preset so that. when the measured temperature reaches the setpoint, a control 

signal, pulse or relay closure is generated. 

105. Shock - A sudden application of force, for a specific time duration; also the 

temporary or permanent damage to a system as a result of a shock. 

106. Signal processing - Manipulation of temperature signal or image data for 

purposes of enhancing or controlling a process. Examples for (1) infrared 

radiation thermometer are peak hold, valley hold, sample hold and averaging. 

Examples for (2) infrared scanners and (3) infrared imagers are usually referred 

to as image processing and include isotherm enhancement. image averaging, 

alignment, image subtraction and image filtering. 

107. Slit response function - A measure of the measurement spatial resolution 

(IFOV meas 

) of an infrared scanner or imager. 


108. Spatial resolution - The spot size in terms of working distance. In an infrared 

radiation thermometer this is expressed in milliradians or as a ratio (DId) of the 

target spot size (containing 95 percent of the radiant energy, according to 

common usage) to the working distance. In scanners and imagers it is most often 

expressed in milliradians. 

109. Spectral response - The spectral wavelength interval over which an instrument or 

sensor responds to infrared radiant energy, expressed in micrometers (}lm) - also, 

the relative manner (spectral response eUlVe) in which it responds over that 

intelVal. 

110. Specular (('Occtor - A smooth refl ecting surface that reflects all incident radiant 

energy at an angle complementary (equal around the nomlal) to the angle of 

incidence, A mirror is a specular refl ector. 

111. Spot - The instantaneous size (diameter unless otherwise specified) of the area 

at the target plane that is being measured by the instrument. In infrared 

thermometry, this is specifi ed by most manufacturers to contain 95 percent of 

the radiant energy of an infin itely large target of the same temperature and 

emissivity. 

112. Storage operating range - 1be temperature extremes over which an instrument 

can be stored and. subsequently, operate within published performance 

specifications. 


113. Subtense, angular - The angular diameter of an optical system or subsystem, 

expressed in angular degrees or mRad. In thermography, the angle over which 

a sensing instrument collects radiant energy. 

114. Target - The object surface to be measured or imaged. 

115. Temperature - A measure of the thermal energy contained by an object; the 

degree of hotness or coldness of an object measurable by any of a number or 

relative scales; heat is defined as thermal energy in transit and flows from 

objects of higher temperature to objects of lower temperature. 

116. Temperature conversion – Convening from one temperature scale to another; 

the relationships are: Celsius = (Fahrenheit -32) x 5/9, Fahrenheit = 9/5 x 

Celsius + 32, 1 °C (ΔT) = 5/9 ºF (Δ.T), 0 °C = 273.12 Kelvin: 0 ºF = 459.67 

Rankine. 

117. Temperature measurement drift - A reading change (error), with time of a target 

with non-varying temperature that may be caused by a combination of (1) 

ambient changes, (2) line voltage changes and (3) instrument characteristics. 

118. Temperature resolution - See minimum resolvable temperature (difference), 

MRT(D), 


119. Thermal detector, infrared - A type of infrared detector that changes electrical 

characteristics as a function of temperature; typically. thermal detectors have 

slow response, (on the order of milliseconds) broad spectral response and 

usually operate at room temperature: thermal detectors are commonly used in 

infrared radiation thermometers and in some imagers. (See Photodetector ≡ 

photon detector) 

120. Thermal viewer - A non-measuring thermal imager that produces qualitative 

thermal images related to thermal radiant distribution over the target surface. 


121. Thermal wave imaging - A term used to describe an active technique for infrared 

nondestructive material testing in which the sample is stimulated with pulses of 

thermal energy and where the timebased returned thermal images are processed 

to determine discontinuity depth and severity; also called pulse stimulated 

imaging. 

122. Thermistor - A temperature detector. usually a semiconductor, whose electrical 

resistivity decreases predictably and nonlinearly with increasing temperature. 

123. Thermistor bolometer, infrared - A thermistor so configured as to collect radiant 

infrared energy; a type of thermal infrared detector. 


124. Thermocouple - A device for measuring temperature based on the fact that 

opposite junctions between certain dissimilar metals develop an electrical 

potential when placed at diffcrent temperatures; typical thermocouple types are; 

J iron/constantan 

K chromeValumel 

T copper/constantan 

E chromel/constantan 

R platinumlplatinum-30 percent rhodium 

S platinumlplatinum- I0 percent rhodium 

B platinum-6 percent rhodium/platinum-30 percent rhodium 

G tungsten/tungsten-26 percent rhenium 

C tungsten-5 percent rhenium/tungsten-26 percent rhenium 

D tungsten-3 percent rheniumltungsten-25 percent rhenium 


125. Thermogram - A thermal map or image of a target where the gray tones or color 

hues correspond to the distribution of infrared thermal radiant energy over the 

surface of the target (qualitative thermogram); when correctly processed and 

corrected, a graphic representation of surface temperature distribution 

(quantitative thermogram). 

126. Thermograph - Another word used to describe an infrared thermal imager. 

127. Thermometer - Any device used for measuring temperature. 

128. Thermopile - A device constructed by the arrangement of thermocouples in 

series to add the thermoelectric voltage. A radiation thermopile is a thermopile 

with junctions so arranged as to collect infrared radiant energy from a target, a 

type of thermal infrared detector. 

129. Time constant - The time it takes for any sensing element to respond to 63.2 

percent of a step change at the target being sensed. In infrared sensing and 

thermography, the time constant of a detector is a limiting factor in instrumcnt 

performance, as it relates to response time. (?) 

130. Total field of view (TFOV) - In imagers, the total solid angle scanned, usually 

rectangular in cross section. (TFOV=FOV?) 

131. Transducer - Any device that can convert energy from one form to another. In 

thermography, an infrared detector is a transducer that converts infrared radiant 

energy to some useful electrical quantity. 


132. Transfer calibration - A technique for correcting a temperature measurement or a 

thermogram for various errors by placing a radiation reference standard adjacent 

to the larget. 

133. Transfer standard - A precision radiometric measurement instrument with NTST 

traceable calibration used to calibrate radiation reference sources. 

134. Transmissivity, (transmiUance) (τ) - The proportion of infrared radiant energy 

impinging on an object's surface, for any given spectral interval thai is 

transmitted through the object. (τ = 1 – Ɛ - ρ) For a blackbody. transmissivity τ = 

O. Transmissivity is the internal transmittance per unit thickness of a nondiffusing 

material. 

135. Two-color pyrometer - See ratio pyrometer. 

136. Unity - One (1.0). 

137. Valley hold - A feature of an instrument whereby an output signal is maintained 

at the lowest inslantaneous measurement for a specified duration; opposite of 

peak hold. 

138. Working distance – The distance from the target to the instrument, usually to the 

primary optic. 

139. Zone - In line scanners. a scanned area created by the transverse linear motion 

of the product or process under a measurement sector of the scanner. 


Appendix B 

Cost Benefit Determination 


The sample worksheet at the end of this description provides a protocol for estimating 

cost benefits of any finding. Following the EPRI M&D Center Guidelines for cost 

benefit detcnnination. the benefits of detecting a failure mechanism at work on a 

system or component before failure are quantified in tcnns of probable dollars saved. 

To do this, the costs of eliminating the failure mechanism in a timely fashion are 

compared to the likely costs incurred if the failure mechanism was not corrected and 

the component or system failed. The approach used in the analysis considers three 

possible failure scenarios: 

1. worst case (catastrophic failure), 

2. possible case (moderate failure). and 

3. probable case (minor failure - the failure most likely to occur). 

The following three calculations are used to estimate failure scenarios: 

1. estimate the percentage likelihood out of 100 percent of each of the three 

scenarios occurring - with the sum of the three percentages equal to 100 percent; 

2. multiply the projected cost of each of the three scenarios by its estimated percent 

likelihood - the sum of these three products is the weighted estimated savings by 

not having to do any of them; and 

3. estimate the cost benefit by comparing the actual cost of the timely service or 

repair to thc wcighted estimated savings. 


Although the calculations are quite straightforward. the effective use of the guidelines 

is far from trivial because filling in the blanks can be a challenge. Here your historical 

database can be of substantial help. Of equal importance is a thorough knowledge of 

the criticality of the component or system to the operation of the facility to project the 

nature and extent of each of the failure scenarios. If your knowledge in this area is 

limited, rely on appropriate facility personnel for the information. 

The historical database can help you estimate the percent likelihood of each scenario. 

as well as the associated costs. When preparing cost estimates. remember to include 

man hours, transportation of parts and equipment, cost of rcplacemem parts and 

equipment and damage to adjacent equipment. Avoided maintenance may also be 

included. Another factor in cost benefit determination that is worth considering is the 

long term savings in excess power that would have been consumed by components 

and systems restored to optimum operational efficiency by timely service or repair. 

Overheated componems invariably draw more current than they should either through 

direct I 2 R loss or because of excess friction or other inefficiencies. These kilowatts of 

power lost represent lost revenue – for every every hour that the situation is not 

corrected, kilowatt hours are lost in the form of dollars that cannot be billed to 

customers. 


These dollars lost are in proportion to the square of the excess current and can be 

calculated for an electrical component if you know the excess current, I, and the 

resistance, 

1. ΔP(W) = (ΔI) 2 R 

Then divide ΔP by 1000 to convert to kilowatts and multiply by the average rate 

charged for a kilowatt hour. This will tell you how much every hour of non optimum 

operation is costing the facility, 

2. Dollars lost = (lost KWH) x (Dollars/KWH). 

In a rotating component, if you know the rated power consumption (watts) and the 

rated current, you can calculate the effective resistance and proceed as in step 1 

above. 


96 ASNT Level Ill Study Guide: Infrared and Thermal Testing Method 

COST BENEFIT WORKSHEET 

OCCURRENCEREPORTNQ FACILITY ----- -- 

LOCATION -------------------------------- DATE ----- 

ISSUE DESCRIPTION: 

PART 1. Repair/Replacement Estimate 

Worst Case Description: 

Possible Case Description: 

Probable Case Description: 

Actual Case Description: 

*Cost and Likelihood of Occurrence 

Worst Case Possible Probable Actual 

$ $ $ $ 

*it .... ,.,. .... 

% likelihood % likelihood % likelihood Real Dollars 

**Percents likelihood must add up to 100% 

PART 2. Total Cost Benefit= Total Savings - Actual Cost 

1. (Worst Case Cost) x ( __ %) = $ _ _____ plus 

2. (Possible Cost) x ( _ _ %) = $ plus 

3. (Probable Cost) x ( __ %) = $ plus 

4. (Total of 1, 2, and 3 $ projected savings 

5. Actual Cost $ (subtract from 4.) 

Estimated Cost Benefit = 

$ ______ 

NOTE: Cost estimates include man-hours, transportation of parts and equipment, cost of 

replacement parts and equipment and damage to adjacent equipment. Avoided maintenance 

may also be included. 

r:T7]EPRI 

~M&DC

Appendix C 

Commonly Used Infrared Specifications and 

Standards 


Appendix C, Commonly Used Infrared Specifications and Standards 

ANSIIASTM CJJS5 

Standard Practice for Determining Thermal 

Resistance of Br~ilding Envelope 

Components from In-Situ Data 

ANSIIASTM E220 

Stnndnrd Mnhod for Cnlibrnrion of 

Thennocouples by Comparison 

Techniques 

ANSIIASTM E230 

Standard Tempera/lire-Electromotive Force 

(EMF) Tables for Standardized 

Thermocouples 

ANSI/ASTM E344 

Tenninology Relating ro Thermometry a11d 

Hydrometry (Definitions) 


Standard Test Method for Calibration of 

Rcfractoty Metal Thermocouples Using an 

Optical Pyrometer 


Standard Practice for Preparation and Use 

of Freezing Poinr Reference Baths 

ANSIJASTM E644 

Standard Test Methods for Testing 

Industrial Resistance Them10meters 

ASTME1543 

Standard Test Method for Noise Equivalent 

Temperamre Difference of Thermal Imaging 

Systems 

IEC 548-1 

Tlu>rmJXoupiR.s- Pnrt / · Refe•enN' Tnh/e~ 

IEC 584-2 

Thennocouples - 

Part 2: Tolerances 

lEC 737 

In-Core Temperature or Prim01y Envelope 

Temperature Measuremenrs in Nuclear 

Power Reactors 

IEC 751 

lndusn·ial Platinum Resistance 

Thermometer Sensors 

ISO 1992-3 

Methods of Test, Part Ill: Temperaturt: Test 

- Commercial Refrigerated Cabinets 

ISO 4112 

Cereals and Pul.fes - Measurement of the 

Temperature of Grain Stored in Silos 

ISO 6781 

Thenna/lnsu/ation - Qualitative Detection 

of Thermal Irregularities in Building 

Envelopes, Infrared Method 




Standard Test Methods for Sheathed 

Thennocouples and Shearhed 

Thermocouple Material 


Swndard Temperature-Electromolil'e Force 

(EM F) Tables for Tungsten-Rhenium 

Thermocouples 

ANSI!ASTM E1213 

Siandard Test Method for Minimum 

Resolvable Temperature Difference for 

Thennal Imaging Systems 

ANS1/ASTM E1256 

Standard Test Methods for Radiation 

Thennomerers (Single Wa11eband Type) 

ANSIJASTM El311 

Standard Test for Minimum Detectable 

Temperature Difference for Themwl 

Imaging Systems 

ISO 6946-1 

Thenna/lnsulation- Calculation Methods 

- Part 1: Stettdy State Thermal Properties 

of Building Components 

TSO 6946.2 

Thermal Insulation - Calculation Methods 

- Part 2: Thermal/Jridges of Rectangular 

Sections in Plane 

ISO 7111 

Plastics - Thermogra vimefl)' of Polymers 

-Temperature Scanning Method 

ISO 7345 

Thermal Insulation - 

and Definitions 

Plrysical Quantities 

ISO 9251 

Thennal Insulation - Hear Transfer 

Conditions and Properties of Materials - 

Vocabulary 

ISO 9346 

Thennallnsulation - Mass 1/·an~fer 

Physical Quantities and Definitions 



ISO TR 9165 

Practical Thermal Properties of Building 

Materials and Products 

ITS-90 

Imemational Temperature Scale of 1990 

MIL-1-24698 

Infrared Thermal Imaging Systems 

MIL-HDBK-731 (VALID NOTICt) 

Nondestmctil'l! Testing Methods of 

Composite Materit1ls-Thermography 

MIL-STD-2194 

Infrared Thennallmaging Survey 

Procedure for Electrical Equipmem 

NFPA 70B 

Recommended Practice for Electrical 

Equipmem Maintenance 

NFPA 70E 

Standard for Electrical Safety Requirements 

for Employee Workplace 

USSR 18353 

Nondestructive Testing - 

Types and Methods 

USSR 23483 

Nondestructive Testing - 

-General Requirements 

Classification of 

Thermal Methods 

USSR 25314 

Thermal Nondestmctive Testing 

Terminology and Definitions 

USSR26782 

Thermal Nondestructil'e Testing - Optical 

and Thenna/ NDT Equipmem - 

Technical Requiremems 

General 


End Of Reading 


Peach – 我爱桃子 


Good Luck 


Good Luck 


Charlie https://www.yumpu.com/en/browse/user/charliechong 

Chong/ Fion Zhang

Understanding Infrared Thermography Reading 3

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