Understanding infrared thermography reading 3 part 1 of 2

Infrared Thermal Testing 

Reading III- SGuide-IRT Part 1 of 2 

My ASNT Level III Pre-Exam Preparatory 

Self Study Notes 29th April 2015 

Charlie Chong/ Fion Zhang

Infrared Thermography 






DEADLY French Military Mistral Anti Aircraft Missile System 

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

Charlie Chong/ Fion Zhang 

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

See Through & Fun Thermal Camera Experiments 

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


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

LEAKED Body Scan Images From The TSA! 

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


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

How to see through clothing 2 

■ 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 

■ 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/ 




http://greekhouseoffonts.com/

Greek letter 


IVONA TTS Capable. 


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 


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 



■ 

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 


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 

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


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

Ɛ 

ρ 

τ 


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 

Kirchhoff's law 


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 






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. 


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 


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 


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 


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 

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 


Figure 1.12: Transmission curves of various infrared transmitting material 


Figure 1.12: Transmission curves of various infrared transmitting material 


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 


Partial Table 2.1 


Partial Table 2.2 



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) 


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 


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 


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 


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

Thermistor 


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 



Brightness Pyrometers –Wien’s Law 



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 


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 


Discussion 

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


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 


Figure 3.2: Response Curves of Various Infrared Detectors 

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. 

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 


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 


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 


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 


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 


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 


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. 

Figure 3.4: Line scanner scanning configuration 


Figure 3.10: Total field of view (TFOV) determination for an infrared imager 


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! 

Temperature sensitivity is 

also called: thermal resolution 

or 

noise equivalent temperature 

difference (NETD). 



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) 


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 

IFOV ratio = d/D or 1/σ 

(when calculation IFOV ratio 

care on unit used!) 



FOV - Animation 


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 


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 






https://www.eutech-scientific.de/products-services/power-generation/euflame.html



https://www.eutech-scientific.de/products-services/power-generation/euflame.html


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 

E 2 



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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Platinum Silicide IrFPA 


http://www.bealecorner.com/trv900/thermal/therm.html

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. 


End Of Reading 


Good Luck 


Good Luck 


Charlie https://www.yumpu.com/en/browse/user/charliechong 

Chong/ Fion Zhang

Understanding infrared thermography reading 3 part 1 of 2

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