Understanding Infrared Thermography Reading 7 Part 2 of 2.pdf

Infrared Thermal Testing 

Reading VII Part 1 of 2 

My ASNT Level III, 

Pre-Exam Preparatory 

Self Study Notes 

12 June 2015 

Charlie Chong/ Fion Zhang

6. Basic Elements Of An In-house Program 

The creation of an in-house program to utilize infrared thermography would 

be customized to each facility’s methods of conducting operations. The basic 

elements of each program, however, would probably be much the same. This 

section outlines a generic approach to developing and implementing a 

comprehensive infrared thermography program. A discussion of the basic 

elements is followed by a sample program. 


6.1 Basic Elements 

An in-house program can be developed by many different approaches. A 

program that is limited to the use of only qualitative thermal imaging 

instruments (as compared to radiometric/quantitative) is likely to be less 

comprehensive. Assuming that a program was created to make full use of a 

radiometric/quantitative imager and image processing software, the following 

topics would need to be addressed: 

• Introduction 

• Definitions 

•Scope 

• Responsibilities 

• Precautions 

• Prerequisites 

• Conduct of the Survey 

• Acceptance criteria 

• Reporting requirements 

• Qualification of personnel 

• Scheduling 

• Equipment matrix 

• References 


6.1.1 Introduction 

This section provides a discussion of the purpose and goal of the IR survey. 

6.1.2 Definitions 

In order to put the program in the proper context, the definitions should be at 

the front. This will allow the reader or reviewer to have an easy reference for 

the terminology that follows. 

6.1.3 Scope 

The scope of the program should be very specific as to what is covered and 

what is not. The applications for infrared thermography are very broad. 

Inspections of roofs and buildings should not be addressed in a document 

that has inspections of safety-related equipment as its main purpose. An 

addendum to the main procedure should be used to avoid confusion. 


6.1.4 Responsibilities 

This section should clearly delineate who is responsible for the various 

aspects of the program from administration through corrective action. The 

main areas of responsibility are administration, inspection (Infrared 

Thermographer), and corrective action. Most of the difficulty in applying this 

technology is in image interpretation and diagnosis. It might be necessary to 

use others in this effort and, if so, their role should be specifically identified. 

6.1.5 Precautions 

Many of the infrared inspections necessitate that panels be removed from 

energized electrical equipment. Precautions as to electrical and personnel 

safety should be included. 


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6.1.6 Prerequisites 

All of the prerequisites for conducting the survey should be identified here. 

This should include the qualification of personnel, calibration of equipment, 

approvals needed from Operations and/or Management, and the required 

resources (equipment and personnel). 

6.1.7 Conduct of the Survey 

This section could reference or include specific procedures for inspections. 

Specific techniques and a suggested sequence of inspections could also be 

included. 


6.1.8 Acceptance Criteria 

All survey results should be compared to either a baseline thermogram or 

other industry accepted standards. Problems or anomalies should then be 

reviewed for determination of which corrective action, if any, should be 

undertaken. The following acceptance criteria provide a generic example but 

would need adaptation for component-specific use. 

An alternative to the above classification is that used in Military Standard MIL- 

STD-2194 (1988). The MIL Standard uses four categories as follows: 


The main difference between the two methods of problem classification is that 

the MIL Standard references temperature rise above ambient and the guide 

classification relates to a temperature rise above a reference value. That 

reference value could be ambient or, in the case of three-phase electrical 

circuits, a temperature rise above an adjacent phase. Each facility should 

adopt criteria that provide a balance between maintenance requirements and 

operational considerations. 


6.1.9 Reporting Criteria 

A rigid process should be established when reporting the results of infrared 

inspections. This rigidity is necessary due to the ease of misinterpretation of 

the thermograms by untrained personnel. A typical quarterly survey of 

electrical equipment might result in 25 to 50 problems in 200 pieces of 

inspected equipment. The vast majority of these problems might be minor in 

nature and require corrective action on a low priority. The process that works 

best, based on industry responses, is one that keeps the report distribution 

and decision-making in the hands of the right people (operations, 

maintenance, and/or program managers). The format for the report should 

also be consistent. 


At a minimum, it should include the following: 

•Time/date 

• Equipment identification 

• Location 

• Specific problem 

• Corrective action recommended 

• Problem action criteria 

• Visible photograph 

• Infrared photograph 

• Inspector’s name and signature 


6.1.10 Qualification of Personnel 

Personnel responsible for conducting the surveys and interpreting the results 

should be trained in the use of the equipment and certified by their employer. 

The training and certification criteria, established by the American Society for 

Nondestructive Testing (ASNT), should be adapted and incorporated into the 

program. These criteria are outlined in their document SNT-TC-1A and will be 

discussed in more detail in Section 7. 

6.1.11 Scheduling 

The documentation requirements and listing of equipment to be evaluated 

during the survey should be established in advance so that trends in 

equipment operation can be translated easily into predictions of future results. 

This is the key to predictive maintenance. The program must also be flexible 

enough to accommodate emergency inspections and inspections during 

unplanned outages. Typically, the administrator of the IR program provides 

this interface. 


6.1.12 Equipment Matrix 

The equipment to be surveyed, the selection criteria, and the locations and 

frequency of inspection should be compiled in a matrix. Typically, the 

electrical equipment is grouped together, as are the other major component 

groups. An alternate approach would be to list the equipment in a route of 

survey-format, which might save time for the infrared thermographer. 

6.1.13 References 

References to any helpful information should be provided. These typically 

include training materials, textbooks on the subject, and equipment operation 

manuals. 


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Basic Elements of an In-House Program 

6.2 Sample Program 

This section incorporates the above recommendations and could serve as the basis for a program 

using infrared thermography as part of a predictive maintenance program. 

1.0 INTRODUCTION 

1.1 This program is for the administration and conduct of an infrared inspection program of 

electrical and mechanical equipment. The purpose of this program is to identify 

equipment that requires maintenance and to improve its reliability through the use of 

infrared thermography (IR). 

1.2 This document contains the recommended scope, frequency, and corrective action criteria 

for routine and unscheduled infrared surveys. 

1.3 Requests for changes to this program and questions relative to it shall be directed to the 

administrator of the IR program. 

2.0 DEFINITIONS 

2.1 Infrared – Electromagnetic radiation having wavelengths that are greater than those of 

visible light, but shorter than microwaves. As it applies to IR thermography, the 

wavelengths are between 3 to 15 micrometers. 

2.2 Infrared Survey – A comprehensive examination of components and equipment with an 

infrared imaging system. 

2.3 Emissivity – The ratio of radiance from a surface to the radiance at the same wavelength 

from a perfect blackbody at the same temperature. Functionally, this is the radiation 

efficiency of a surface in the infrared spectrum. 

2.4 Radiosity – Thermal energy of a surface as seen by the infrared detector. 

2.5 Thermogram – A recorded, displayed, or hard-copy image of the output of an infrared 

imaging system. 

2.6 Isotherm – A thermal contour on a thermogram where all of the spots along it are at the 

same apparent temperature. 

2.7 Infrared thermographer – An individual who is trained and qualified to operate infrared 

imaging equipment and to interpret the images. 

3.0 SCOPE 

3.1 The requirements of this procedure shall apply to all safety-related components. It shall 

also be applicable to non-safety-related equipment where financial benefit might be 

achieved by monitoring (that is, increased plant availability, decreased maintenance 

costs, and so on). 

3.2 This procedure includes guidelines for the following: 

• Component selection 

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• Interval selection 

• Determining component acceptability 

4.0 RESPONSIBILITIES 

4.1 Administrator of IR – It is the administrator’s responsibility to oversee the program. This 

includes making changes to the procedure. All surveys, whether they are scheduled or 

conducted on an emergency basis, shall be approved by the administrator or his/her 

designee. The administrator shall be responsible for budgeting, planning, and interfacing 

with outside organizations. 

4.2 Infrared Thermographer – The infrared thermographer is the only person trained and 

qualified to operate the infrared imaging equipment. He/she is responsible for conducting 

the surveys, interpreting the images, writing the reports, and acting as a technical 

resource to other plant departments. The infrared thermographer is responsible for the 

maintenance and calibration of the infrared imaging equipment. 

4.3 Cognizant Engineer – At the request of the infrared thermographer, a discipline-cognizant 

engineer will provide assistance in diagnosing a problem. The cognizant engineer will 

also suggest corrective action and provide coordination with other plant disciplines. 

4.4 Root Cause – Determination of root cause and the subsequent applicable action level 

shall be the responsibility of plant management. When necessary, the infrared 

thermographer shall request assistance from a cognizant systems or maintenance engineer 

in determining the root cause or the recommended corrective action. 

5.0 PRECAUTIONS 

5.1 Many of the components that are being inspected represent potential plant trip hazards; 

exercise extreme care. 

5.2 All safe work practices as outlined in the plant safety manual, shall be followed. These 

practices include exhibiting caution near energized electrical equipment, rotating 

equipment, and hot pipes. All surveys shall be conducted from a safe stable location. 

5.3 Infrared surveys within the Radiological Controls Area shall be conducted within the 

guidelines of the Health Physics Department. In areas of potential contamination, the 

infrared thermographer shall be responsible for covering the equipment with plastic as 

directed by Health Physics. 

5.4 When practical, surveys in areas of airborne contamination should be avoided. When this 

is not possible, a thin piece of polyethylene or plastic can be placed over the lens. If this 

is done, the transmittance of the covering must be taken into account. 

6.0 PREREQUISITES 

6.1 Personnel – The infrared thermographer and one craft person constitute the minimum 

personnel necessary to conduct a survey when the operating or opening of equipment is 

necessary. 

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6.2 Approvals – The required approvals to conduct a survey shall be coordinated with the IR 

administrator. The control room should be notified both prior to the start of the survey 

and at its end. If requested, the infrared thermographer will inform the control room prior 

to opening equipment that presents a possible plant trip hazard. 

6.3 Emergencies – In cases where requests for surveys are done on an emergency basis, the 

infrared thermographer shall fulfill the duties of the IR administrator and provide the 

necessary coordination. 

7.0 CONDUCT OF THE SURVEY 

7.1 The equipment survey matrix shall identify the equipment to be surveyed and the 

frequency of the survey. 

7.2 The sequence of the survey is not important unless specifically stated in the procedure or 

requested by either Maintenance or Operations. All equipment on the matrix must be 

surveyed unless it is not in operation or conditions dictate otherwise. The infrared 

thermographer shall note any exceptions in the inspection report. 

7.3 Standard practice is to videotape all surveys and to include an audio track for verbal 

identification and discussion. 

7.4 The thermal images must be of sufficient resolution to identify the components and any 

problem areas. 

7.5 When problems are identified, the thermographer shall reposition the imager and obtain 

more than one view. This is done to eliminate the possibility of apparent problems being 

caused by reflections from hot objects. The hard-copy images should be obtained from 

the position that provides the best image. 

7.6 All problems are to be photographed in the visible as well as in the infrared. This is to 

allow proper and easy identification of the problem areas, which will facilitate 

maintenance activities. 

7.7 The problems shall be customarily reported as a temperature rise. This rise can be 

calculated from ambient, thermal baseline data, or made by comparison in the cases 

where similar equipment exists. 

7.8 When absolute temperatures are requested or required, the infrared thermographer shall 

determine and use the target's effective emissivity to assure accuracy. A standard table of 

effective emissivities will be developed by measurement and will be maintained by the 

infrared thermographer. 

7.9 Important information relating to test conditions, such as load, flow, and pressure shall be 

noted by the thermographer if it is available. This information will be used in component 

trend analysis. 

7.10 The components shall be inspected with the imager aimed along a line normal 

(perpendicular) to the target surface whenever possible, to minimize the potential for 

errors due to reflections. 

6-7



7.11 During the infrared inspection, the components must also be inspected visually and any 

discolorations, questionable noise, or smell should be reported. 

7.12 In cases where precise measurements must be obtained, the instrument background 

radiation effects must be taken into account. Instrument background temperature can be 

determined by placing a good diffuse reflector (such as a piece of aluminum foil that has 

been crumpled and re-flattened) in ambient air and measuring its apparent temperature 

with the imager’s emissivity set to 1.0. 

7.13 Where external optics, such as telescopic and wide-angle lenses are used, the 

transmittance of the optics must be taken into account. The information that corrects the 

effects of these devices is supplied by the manufacturer and is entered directly into the 

imager software. 

7.14 When measurements are being made on targets, the size of the target and the distance 

must be known. The IFOVmeas (Instantaneous Field of View for measurement) of the 

instrument must fit comfortably within the required target spot at the measurement 

distance. If these criteria are not satisfied, the instrument must be moved closer to the 

target and/or a higher magnification lens must be used. (See section 3.3.4 for a more 

detailed discussion of this subject). 

7.15 The survey should be done with the imager scanned at a speed that does not cause 

blurring of the image so that acceptable thermograms can be obtained from the videotape 

on playback. 

7.16 If requested or desired, a second (backup) measure of temperature can be obtained 

through the use of contact thermocouples or spot radiometers. (Care should be used in 

evaluating the results of measurements that are not calibrated.) 

7.17 In general, equipment shall be surveyed when in a normal operational state. In cases 

where equipment is not energized or running normally, the thermographer shall note it in 

the IR inspection report. 

7.18 Equipment such as batteries shall be surveyed during both normal operation and during 

discharge tests. 

7.19 Requests for equipment operation for the sole purpose of an infrared inspection shall be 

coordinated with operations by the IR administrator. In most cases, this should be 

avoided. 

7.20 All infrared inspections, whether done by on-site personnel or outside contractors, will be 

performed under the guidance and procedures listed in this program. Special tests outside 

of the normal inspection shall be reviewed and approved in advance by the IR 

administrator. 

8.0 ACCEPTANCE CRITERIA 

8.1 Subsequent to an initial thermal baseline, the following action levels are to be used to 

classify each problem: 

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Advisory (Level 1) 

Intermediate (Level 2) 

Serious (Level 3) 

Critical (Level 4) 

1°F to 15°F rise 



in excess of 100°F rise 

8.2 When indications on components fall into levels 2, 3, 4, section 9 of the program shall be 

followed for reporting. 

8.3 To determine acceptability of the inspection, the results and final report shall be 

compared against the criteria set forth in this program. 

9.0 REPORTING REQUIREMENTS 

9.1 Every scheduled and unscheduled infrared inspection shall be documented and reported 

in accordance with the requirements of this section (see Figure 6-1). 

9.2 At a minimum, the report shall contain the following: 

• Summary of inspection and findings 

• Equipment list 

• Data sheets with IR and visible photographs of anomalies 

• Root cause analysis and corrective action 

• Comments 

9.3 The report shall be issued to the IR administrator within five working days of the 

completion of the survey. 

9.4 A verbal report shall always be given to the on-site IR administrator upon completion of 

the survey. 

9.5 The reporting of problems that fall within the four acceptance action levels are as 

follows: 

Advisory (Level 1) 

Normal cycle of corrective maintenance. 

Intermediate (Level 2) High priority during an unscheduled shutdown. 

Serious (Level 3) 

Critical (Level 4) 

Alert Operations—potential failure. Correct ASAP. 

Alert Operations, Management. Remove from service ASAP. 

9.6 Items classified as serious are to be immediately reported to the IR administrator who 

will advise Maintenance and Operations. 

9.7 Items classified as critical are to be immediately reported to Operations, Maintenance, 

and the IR administrator. 

6-9



10.0 QUALIFICATION OF PERSONNEL 

10.1 The infrared thermographer shall be qualified by examination and certified by the plant to 

conduct the survey. 

10.2 The qualifying examination and training shall meet the guidelines of ASNT SNT-TC-1A 

(current edition). 

10.3 In addition to the ASNT qualifications, the thermographer shall be knowledgeable in the 

following areas: 

• Equipment-specific operation 

• Infrared theory 

• Heat transfer modes 

• Safety practices 

10.4 Certification of the thermographer shall be made through a written and a practical 

examination. 

10.5 The plant Training Department shall administer the initial and re-qualification training. 

11.0 SCHEDULING 

11.1 The IR administrator is responsible for scheduling all routine infrared inspections. 

11.2 The Equipment Matrix (Program, section 12.0) lists the frequency of inspection for each 

component. 

11.3 Inspections on an emergency basis or for a special test shall be scheduled and coordinated 

by the IR administrator. 

12.0 EQUIPMENT MATRIX 

12.1 Component Selection Criteria 

12.1.1 The components that are to be included in the thermographic analysis program should be 

selected based on the perceived or documented benefit of thermography on the type of 

equipment and the following criteria categories: 

A. Critical: Critical equipment shall be defined as: 

• Equipment whose function is necessary and must be available at all times. 

• Equipment upon which thermography has been used to deviate from a specific 

vendor-recommended preventive maintenance activity. 

• Equipment necessary to maintain full-power generating capabilities (that is, nonredundant). 

B. Vital: Vital equipment shall be defined as those components whose function is 

necessary but that, through redundant design, do not have to be available at all times. 

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C. Vendor Recommended: Vendor-recommended equipment whose manufacturer or 

vendor recommends the periodic monitoring of the equipment with infrared 

thermography. 

D. Non-Vital: Non-Vital equipment shall be defined as: 

• Equipment whose replacement cost versus periodic monitoring cost does not differ 

greatly and does not fall into category A or B above. 

• Components that are used very infrequently and do not fall into category A or B. 

12.1.2 The IR administrator shall maintain a listing of all of the components in the 

thermographic analysis program, the category to which they belong, and their monitoring 

interval. 

12.1.3 Equipment in category D that has a failure history relating to thermography might be 

included in the program in order to determine root cause, or to prevent failure recurrences 

or significant inconveniences. Otherwise, equipment in category D should be omitted 

from the program. 

12.1.4 The above recommended component selection criteria should be applied predominantly 

to electrical equipment such as: 

• Motor control centers 

• Load centers 

• Transformers 

• Switchgear 

• Battery chargers 

• Switchyard equipment 

• Large motor termination 

12.1.5 The above criteria can also be applied to: 

• Pumps/motors 

• Steam traps 

• Valves 

12.2 Performance Intervals 

12.2.1 The selection of performance intervals should be based upon several factors, such as: 

• The impact of the component on plant operation and personnel safety if an 

unexpected failure were to occur. 

• The speed at which a component fault manifests itself into a stage of degradation, 

which affects the component’s operability. 

• Vendor/manufacturers’ recommendations. 

• The category of the component as stated in section 12.1.1 of the program. 

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12.2.2 When considering the vendor’s recommended frequency for thermography, the 

application of the equipment should be taken into consideration (that is, the run time 

experienced by the equipment in this installation versus what the vendor expects for 

typical run times). Also, if the component falls into categories A or B of 12.1.1, then the 

most limiting interval (between the vendor-recommended interval and the recommended 

interval in section 12.2.3 of the program) shall be used for the monitoring of the 

equipment. 

12.2.3 The following recommended intervals for the given categories should be used: 

A. Critical Equipment 

• Monitor quarterly for those components that are operated continuously or are optested 

at least quarterly. 

• Monitor semi-annually for those components that are operated continuously or are 

run-tested at least semi-annually. 

• At start-up, monitor when the component is placed on-line, is at a stabilized 

temperature, and has not been monitored for at least one monitoring interval. 

• Equipment less than 240 V does not require periodic monitoring. 

B. Vital Equipment 

• Monitor equipment greater than 480 V quarterly. 

• Monitor equipment greater than 240 V but less than 480 V semi-annually. 

• Equipment less than 240 V does not require periodic monitoring. 

12.2.4 Changes to monitoring intervals should be reviewed carefully prior to making changes in 

order to ensure that maximum component availability and program efficiency is 

provided. 

12.2.5 At a minimum, documentation for interval changes shall be maintained, by the IR 

administrator. 

12.2.6 Components need not be operated for the sole purpose of collecting thermography data. 

13.0 SUGGESTED PROGRAM REFERENCES 

13.1 Infrared Thermography Guide (Revision 3), (formerly NP-6973) 

13.2 Plant Administrative Procedures Manual 

13.3 Plant Safety Manual 

13.4 Plant Training Manual 

13.5 Plant Quality Assurance Procedures Manual 

13.6 Plant Systems Training Manual 

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13.7 Infrared Imager Instruction Manual 

13.8 Plant Predictive Maintenance, INPO Good Practice 89-009. 

13.9 Wolfe, W. L. and Zissis, G.J., The Infrared Handbook. Environmental Research Institute 

of Michigan (1996). 

13.10 Mil-Std-2194, Infrared Thermal Imaging Survey Procedure Electrical Equipment. 

13.11 American Society for Nondestructive Testing Standard Practice SNT-TC-1A, 

Qualifications Guidelines. 

13.12 American Society for Nondestructive Testing Infrared and Thermal Testing Handbook, 

2001. 

13.13 American Society for Nondestructive Testing Level III Study Guide: Infrared and 

Thermal Testing Method, 2001. 

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Figure 6-1 

Infrared Survey Results 

6-14

7. TRAINING AND CERTIFICATION 

This section deals solely with the efforts of the American Society of 

Nondestructive Testing (ASNT) in the training and certification of infrared 

thermographers. The purpose is to provide guidelines for training individuals 

who will be able to deliver the best level of service possible. It is important to 

understand that certification via the ASNT Certification Program, does not 

imply authorization or licensing of the certificate holder to perform infrared 

thermography tasks. It is solely the employer's responsibility to review the 

individual’s qualification records for completeness and to authorize individuals 

to perform infrared thermography tasks. 


7.1 Background 

Commercially available infrared imagers are quite easy to both use and 

misuse. Many small, independent contractors, from electricians to engineers, 

provide a wide range of services to many different industries. In the absence 

of formal training, most of these people have learned on the job while working 

with more experienced individuals. At the request of many ASNT members, a 

committee was formed in the fall of 1989 to propose modifying ASNT 

Recommended Practice No. SNT-TC-1A, the qualification guideline for 

nondestructive testing, to accept and recognize infrared thermography as a 

valid nondestructive examination method. At this writing, all of the training, 

qualification, and certification guidelines are in place and SNT-TC-1A has 

been updated (1996) to include the T/IR (Thermal Infrared) method. Two 

additional ASNT publications were released in 2001 to support training and 

certification: 

• ASNT Infrared and Thermal Testing Handbook, 2001 

• ASNT Level III Study Guide: Infrared and Thermal Testing Method, 2001 


Recommended training and certification guidelines for infrared 

thermographers are summarized in the ASNT Infrared and Thermal Testing 

Handbook on pages 15 -18, and are explained in detail in SNT-TC-1A. 

The ASNT training program is intended to supplement equipment-specific 

training that might be offered by the manufacturers. Certification is the 

responsibility of the individual employer. SNT-TC-1A states the following 

in this regard: 

“Written Practice. The employer shall establish a written practice for the 

control and administration of nondestructive personnel training, examination 

and certification. The employer’s written practice should describe the 

responsibility of each level of certification for determining the acceptability of 

materials and components in accordance with applicable codes, standards, 

specifications and procedures.” 


7.2 Levels of Qualification 

The recommended Levels of Qualification for infrared thermographers follow 

those of traditional NDE methods. These levels are as follows: 

■ Level I 

A Level I infrared thermographer shall be qualified to perform specific IR 

inspections in accordance with detailed written instructions and to record the 

results; the Level 1 infrared thermographer shall perform inspections under 

the cognizance of a Level II or Level III. The Level I shall not independently 

perform nor evaluate inspection results for acceptance or rejection when such 

inspection results are for the purpose of verifying compliance to code or 

regulatory requirements. (if the result is not for the purpose of verifying 

compliance to code or regulatory requirements; then the Level I could 

independently perform and evaluate inspection result?) 


■ Level II 

A Level II infrared thermographer shall be qualified to set up and calibrate 

equipment, conduct inspections, and to interpret inspection results in 

accordance with procedure requirements. The individual shall be familiar with 

the limitations and scope of the method employed and shall have the ability to 

apply techniques over a broad range of applications within the limits of their 

certification. The Level II shall be able to organize and report inspection 

results. A Level II must have the ability to correctly identify components and 

parts of components within the scope of the IR inspection. 

■ Level III 

A Level III infrared thermographer is capable of designating a particular 

inspection technique, establishing techniques and procedures, and 

interpreting results. The individual shall have sufficient practical background 

in his/her area of expertise to develop innovative techniques and to assist in 

establishing acceptance criteria where none are otherwise available. The 

individual shall have general familiarity with other nondestructive evaluation 

(NDE) methods and inspection technologies. The Level III individual shall be 

qualified to train and examine Level I and Level II personnel for qualification 

and certification as an infrared thermographer. 


7.3 Training Requirements 

The training requirements for each level of the infrared thermographer 

qualification parallel those for the other traditional NDE methods in that onthe-job 

training, educational background, and classroom work all count 

toward qualification. There are qualification examinations and annual requalification 

requirements at all levels. It is up to the utilities’ training 

organization and individual employers to implement the appropriate 

recommendations of the training program set forth in SNT-TC-1A. 


The experience and education recommendations for the three levels are: 

• Level I A high school diploma (or equivalent) or 6 months of experience 

• Level II A two-year college or technical degree or 18 months of experience 

• Level III A four-year technical degree from a college or university or 5 

years of experience 

The required classroom training is as follows: 

• Level I 40 hours of instruction, 50-question written examination, classroom 

experiment 

• Level II 40 hours of instruction, 75-question written examination, 

classroom experiment 

• Level III 40 hours of instruction, 75-question written examination, 

procedure preparation for classroom experiment 


The classroom training is based on the body of knowledge reviewed, adopted, 

and updated by ASNT, summarized in ASNT Recommended Practice No. 

SNT-TC-1A, and reviewed in ASNT Level III Study Guide: Infrared and 

Thermal Testing Method, 2001. The depth that is covered by these areas 

corresponds to the level of the training. This translates into more extensive 

training at Level III than Level I, even though the classroom hours are the 

same. The four areas for training and associated practical aspects are listed 

below. At the conclusion of training, the trainee will: 

A. Radiosity or Target Exitance 

• Understand the concepts of radiosity and associated parameters. 

• Be able to measure emissivity, reflectance, transmittance, background 

temperature, foreground temperature, and target temperature. 

• Be cognizant of potential errors in the measurement of the above 

parameters, caused by variation across the target surface. 


B. Spatial Resolution 

• the concept of spatial resolution. • Understand the difference between 

image resolution and measurement resolution. 

• Understand the effect on measurement of the distance between the 

instrument and the target. 

• Be able to calculate measurement spot size. 

• Be able to exploit equipment- pecific aids to determine measurement 

adequacy. 


C. Heat Transfer 

• Understand the fundamental concepts of heat transfer including 

conduction, convection, and radiation. 

• Understand the difference between steady state and transient heat flow 

and application dependence. 

• Understand the effect of the environmental conditions of sky temperature, 

view factor, wind velocity, and surface orientation. 

• Understand the potential problems if evaporation or condensation occur at 

the target surface. 


D. Equipment Operation 

• Be able to set up and operate the necessary equipment. 

• Understand dynamic range and its implication in image acquisition. 

• Demonstrate good data acquisition practices. 

• Demonstrate the use of accessories. 

• Understand how to compensate for external optics. 

• Understand the implications of system spectral response. 

The written examination is derived from a pool of 200-300 questions that are 

reviewed and approved by the ASNT T/IR committee members. During 

training, practical exams are conducted through classroom experiments and 

are focused on one particular concept, such as transient thermal heat transfer. 

The actual practical exam is determined by the trainer and is conducted 

within the guidelines for each particular level. Infrared thermography was 

adopted as a nondestructive inspection method in the fall of 1991. 


7.4 Predictive Maintenance (PdM) Level III Certification 

Program 

Recognizing that there are areas of specialization within the infrared 

thermography discipline, the ASNT T/IR committee has promoted the 

development of specialty certification. The Predictive Maintenance Level III 

Certification Program has been developed by ASNT in response to this effort. 

Developed to meet the needs of the predictive maintenance sector of the 

industry, this program incorporates the vibration analysis (VA) and 

infrared/thermal (IR) test methods. A PdM-specific body of knowledge, 

including knowledge of the Recommended Practice No. SNT-TC-1A and the 

ANSI/ASNT CP-189 standard, is used for the two-hour PdM basic 

examination. The VA and IR method tests are the same as those used in the 

ASNT NDT Level III program. A separate and distinct PdM Level III certificate 

is issued for this certification. 


The PdM basic examination is more specific than the ASNT NDT Level III 

basic examination, and thus, PdM certificate holders wishing to gain 

traditional NDT Level III certification will still be required to sit for the ASNT 

NDT Level III basic examination, as well as taking an ASNT NDT Level III 

method test. 

Certification via the ASNT PdM Level III Certification Program, as with the 

ASNT NDT Level III program, does not imply authorization or licensing of the 

PdM certificate holder to perform PdM tasks. It is solely the employer’s 

responsibility to review the individual’s qualification records for completeness 

and to authorize individuals to perform PdM. 


The Expert!

Appendix-A 

The Science Of Thermography (Practical 

Application Of Thermographic And Thermal 

Sensing Equipment) 


A.1 Introduction 

This appendix is presented as a reference guide to provide the practical 

thermographer with an understanding of the science behind the 

measurements. It is intended as an aid in performing and understanding 

non-contact thermal and thermographic measurements using infrared sensing 

equipment. The deployment and operation of infrared sensing instruments 

was, at one time, cumbersome and difficult. Thermographers were often 

required to perform on-the-spot calculations in order to reduce their 

measurement data and determine actual temperature values; this is no longer 

so. Modern instruments are light in weight, portable, and rugged. 

Menu-driven on-board software now makes it relatively simple to operate 

equipment and to gather data directly in terms of target temperature. Because 

of this very ease of operation, it is also relatively simple to misinterpret the 

results so easily and quickly obtained. 


Erroneous conclusions can have an extremely negative effect on the 

measurements program and on the credibility of the thermographer. 

A solid understanding of the basis on which thermographic measurements 

are made will go a long way toward minimizing operator error and ensuring 

the success of the thermographic program. 

The subject matter in this appendix begins with a discussion of heat transfer 

and how radiative heat transfer is the basis for infrared thermography. The 

basic physics of infrared radiation and how it applies to instrument 

performance is explained. Finally, the performance parameters of infrared 

point-sensing and imaging instruments are discussed, including how to select, 

calibrate, and evaluate the performance of the instrument that is best suited 

to your application. 


A.2 Heat Transfer and Radiation Exchange Basics for 

Thermography 

This section is to provide the reader with an understanding of how heat 

transfer phenomena affect non-contact infrared thermal sensing and 

thermographic measurements. Infrared thermography depends on measuring 

the distribution of radiant thermal energy (heat) emitted from a target surface, 

thus, the thermographer requires an understanding of heat, temperature, and 

the various types of heat transfer as an essential prerequisite in preparing to 

undertake a program of IR thermography. 


A.2.1 Heat and Temperature 

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

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

in one object is converted to heat and flows to another object. 

Heat can be defined as thermal energy in transition. It flows from one place or 

object to another as a result of temperature difference, and the flow of heat 

changes the energy levels in the objects. 

Temperature is a property of matter and not a complete (that means it need 

other input to completely quantify the internal energy) measurement of 

internal energy. It defines the direction of heat when another temperature is 

known. Heat always flows from the object that is at the higher temperature to 

the object that is at the lower temperature. As a result of heat transfer, hotter 

objects tend to become cooler and cooler objects become hotter, approaching 

thermal equilibrium. To maintain a steady-state condition, energy needs to be 

continuously supplied to the hotter object by some means of energy 

conversion so that the temperature and, hence, the heat flow remains 

constant. 


A.2.2 Converting Temperature Units 

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

absolute scales called degree 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 Kelvins or zero Rankines (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: 

T Celsius = 5/9 (T Fahrenheit - 32 ) 

T Fahrenheit = 9/5 T Celsius + 32 

T Rankine = T Fahrenheit + 459.7 

T Kelvin = T Celsius + 273.16 

Absolute zero is equal to -273.1°C and is also equal to -459.7°F. 


To convert changes in temperature or delta T between the English and Metric 

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

ΔT Fahrenheit (or Rankine) = 1.8 ΔT Celsius (or Kelvin) 

Table A-1 is a conversion table to allow for the rapid conversion of 

temperature between Fahrenheit and Celsius values. Instructions for the use 

of the table are shown at the top. 

(ΔT ≡ temperature interval) 


Table A-1 Temperature Conversion Chart Instructions for Use: 

1. Start in the Temp. column and find the temperature that you wish to convert. 

2. If the temperature to be converted is in °C, scan to the right column for the °F equivalent. 

3. If the temperature to be converted is in °F, scan to the left column for the °C equivalent. 


A.2.3 The Three Modes of Heat Transfer 

There are three modes of heat transfer: (1) conduction, (2) convection, and (3) 

radiation (and nothing else) . All heat transfer processes occur by one or 

more of these three modes. Infrared thermography is based on the 

measurement of radiative heat flow radiation (and nothing else) and is, 

therefore, most closely related to the radiation mode of heat transfer. 

A.2.4 Conduction 

Conduction is the transfer of heat in stationary media. It is the only mode of 

heat flow in solids, but can also take place in liquids and gases. It occurs as 

the result of molecular collisions (in liquids) (fluid, both liquid and gas) and 

atomic vibrations (in solids), whereby energy is moved one molecule at a time, 

from higher temperature sites to lower temperature sites. Figure A-1 is an 

illustration of conductive heat flow. The Fourier conduction law expresses the 

conductive heat flow through the slab shown in Figure A-1. 


Figure A-1 Conductive Heat Flow 


The Fourier Conduction Law: 

Q/A 

Q 

= K (T 1 -T 2 ) / L 

= K∙ΔT∙A / L 

Where: 

Q/A 

L 

T 1 

T 2 

K 

= the rate of heat transfer through the slab per unit area 

perpendicular to the flow 

= the thickness of the slab 

= the higher temperature (at the left) 

= the lower temperature (at the right) 

= the thermal conductivity of the slab material 


Thermal conductivity is analogous to electrical conductivity and is inversely 

proportional to thermal resistance, as shown in the lower portion of Figure A-1. 

The temperatures, T 1 and T 2 , are analogous to voltages V1 and V2, and the 

heat flow, Q/A, is analogous to electrical current, I, so that: if: 

R electrical = V1 - V2/ I 

then: 

R thermal 

= T1 - T2 / Q /A = L/K 

Heat flow is usually expressed in English units. K is expressed in 

BTU/hr∙ft²∙°F and thermal resistance (1/K) would then be expressed in 

°F∙hr∙ft²/BTU. 


A.2.5 Convection 

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

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

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

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

temperature difference necessary for heat transfer produces density changes 

in the fluid and the warmer fluid rises as a result of increased buoyancy. In 

convective heat flow, heat transfer takes effect by means of two mechanisms, 

(1) the direct conduction through the fluid and (2) the motion of the fluid itself. 

Figure A-2 illustrates convective heat transfer between a flat plate and a 

moving fluid. 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 velocity, V∞, of the fluid. It is greater for free convection and 

smaller for forced convection. The rate of heat flow depends on the thickness 

of the convection layer, as well as the temperature difference between Ts and 

T∞ (Ts is the surface temperature, T∞ is the free field fluid temperature 

outside of the boundary layer.) 


Newton’s cooling law defines the convective heat transfer coefficient: 

(h is expressed in BTU/hr-ft²-°F) 

rearranged: 

= ΔT∙h 

where: 

Rc = 1/h and is the resistance to convective heat flow 

Rc is also analogous to electrical resistance and is easier to use when 

determining combined conductive and convective heat transfer. 


Figure A-2 Convective Heat Flow 



A.2.6 Radiation 

Radiative heat transfer is unlike the other two modes in several respects: 

1. It can take place in a vacuum. 

2. It occurs by electromagnetic emission and absorption. 

3. It occurs at the speed of light. 

4. The energy transferred is proportional to the fourth power of the 

temperature difference between the objects (ΔT 4 or T 4 ?) . 

The electromagnetic spectrum is illustrated in Figure A-3. Radiative heat 

transfer takes place in the infrared portion of the spectrum, between 0.75 µm 

and about 100 µm (0.1mm) , although most practical measurements can be 

made out to 20 µm. (µ or µm stands for micrometers or microns. A micron is 

one-millionth of a meter and is the measurement unit for radiant energy 

wavelength.) (radiative heat only take place at the aforementioned portion of 

spectrum?) 


Figure A-3 Infrared in the Electromagnetic Spectrum 


A.2.7 Radiation Exchange at the Target Surface 

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

temperature measurement and thermal imaging (or thermography). The 

process of thermal infrared radiation leaving a surface is called exitance or 

radiosity. It can be emitted from the surface, reflected off of the surface, or 

transmitted through the surface. This is illustrated in Figure A-4. The total 

radiosity is equal to the sum of the emitted component (E), the reflected 

component (R), and the transmitted component (T). The surface temperature 

is related to E, the emitted component only. 


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

or transmitted as illustrated in Figure A-5. Kirchhoff’s law states that the sum 

of the three components is always equal to the received radiation (the 

percentage sum of the three components equals unity): 

A (absorptivity) + R (reflectivity) + T (transmissivity) = 1 

(ε + ρ + τ = 1) 

When making practical measurements, the specularity or diffusivity of a target 

surface is taken into effect by accounting for the emissivity of the surface. 

Emissivity is discussed as part of the detailed discussion of the 

characteristics of infrared thermal radiation in section A.3. 



Figure A-4 Radiative Heat Flow 


Figure A-4 Radiative Heat Flow 

W ε = σεT e 

4 

W ρ = σρT r 

4 

W τ = στT t 

4 


Figure A-5 Radiation Exchange at the Target Surface 


A.2.8 Specular and Diffuse Surfaces 

It should be noted that the roughness or structure of a surface will determine 

the type and direction of reflection of incident radiation. A smooth surface will 

reflect incident energy at an angle complementary to the angle of incidence. 

This is called a specular reflector. A rough or structured surface will scatter or 

disperse some of the incident radiation; this is a diffuse reflector. 

No perfectly specular or perfectly diffuse surface can exist in nature. All real 

surfaces have some diffusivity and some specularity. 


Specular or Diffuse Surfaces 



Diffuse 

Reflector 

Specular 

Reflector? 


http://www.hunantv.com/v/3/56616/f/750962.html?f=lb#

Specular and Diffuse Surfaces 

Diffuse 

Reflector 

Specular 

Reflector? 


Specular and Diffuse Surfaces 

Confused 

Specular 

Reflector? 




Specular reflection is the mirror-like reflection of light (or of other kinds of 

wave) from a surface, in which light from a single incoming direction (a ray) is 

reflected into a single outgoing direction. Such behavior is described by the 

law of reflection, which states that the direction of incoming light (the incident 

ray), and the direction of outgoing light reflected (the reflected ray) make the 

same angle with respect to the surface normal, thus the angle of incidence 

equals the angle of reflection θ 2 = θ 1 in the figure), and that the incident, 

normal, and reflected directions are coplanar. 


Reflections off Specular and Diffuse Surfaces 


Reflections off Specular and Diffuse Surfaces 


A.2.9 Transient Heat Exchange The discussions of the three types of heat 

exchange in sections A.2.4, A.2.5, and A.2.6 deal with steady-state heat 

exchange for reasons of simplicity and easier understanding. Two fixed 

temperatures are assumed to exist at the two points between which the heat 

flows. In many applications, however, temperatures are in transition, so that 

the values shown for energy radiated from a target surface are the 

instantaneous values from the moment that measurements are made. There 

are numerous instances where existing transient thermal conditions are 

exploited in order to use thermography to reveal material or structural 

characteristics in test articles. 


The thermogram of the outside surface of an insulated vessel carrying heated 

liquid, for example, should be relatively isothermal and somewhat warmer 

than the ambient air. Insulation voids or defects will cause warm anomalies to 

appear on the thermogram, allowing the thermographer to pinpoint areas of 

defective or damaged insulation. Here a passive approach can be taken 

because the transient heat flow (or it is a steady state heat flow?) from the 

liquid through the insulation to the outside air produces the desired 

characteristic thermal pattern on the product surface. Similarly, water 

saturated areas on flat roofs will retain solar heat well into the night; long after 

the dry sections have radiated their stored heat to the cold night sky, the 

saturated sections will continue to radiate and exhibit distinct anomalies to the 

thermographer. When there is no heat flow through the material or the test 

article to be evaluated, an active, or thermal injection, approach is used to 

generate a transient heat flow. 

Comment: In general steady state heat flow always lead to thermal 

equilibrium, for IRT, transient heat flows are exploited to reveal abnormalities. 


This approach requires the generation of a controlled flow of thermal energy 

across the laminar structure of the sample material under test, thermography 

monitoring of one of the surfaces (or sometimes both) of the sample, and a 

search for anomalies in the thermal patterns that will indicate a defect in 

accordance with established accept-reject criteria. This approach has been 

used extensively and successfully by the aerospace community in the 

evaluation of composite structures for impurities, flaws, voids, disbonds, 

delaminations, and variations in structural integrity. Most recently, time-based 

heat injection methods have been applied successfully to measure the depth 

of voids, as well as their location. This is effective because thinner sections of 

a given material will heat more rapidly than thicker sections. 


Steady-state conduction 

Steady state conduction is the form of conduction that happens when the temperature 

differences ΔT driving the conduction are constant, so that (after an equilibration time), the 

spatial distribution of temperatures (temperature field) in the conducting object does not change 

any further. Thus, all partial derivatives of temperature with respect to space may either be zero 

or have nonzero values, but all derivatives of temperature at any point with respect to time are 

uniformly zero. In steady state conduction, the amount of heat entering any region of an object is 

equal to amount of heat coming out (if this were not so, the temperature would be rising or falling, 

as thermal energy was tapped or trapped in a region). 

For example, a bar may be cold at one end and hot at the other, but after a state of steady state 

conduction is reached, the spatial gradient of temperatures along the bar does not change any 

further, as time proceeds. Instead, the temperature at any given section of the rod remains 

constant, and this temperature varies linearly in space, along the direction of heat transfer. 

In steady state conduction, all the laws of direct current electrical conduction can be applied to 

"heat currents". In such cases, it is possible to take "thermal resistances" as the analog to 

electrical resistances. In such cases, temperature plays the role of voltage, and heat transferred 

per unit time (heat power) is the analog of electrical current. Steady state systems can be 

modelled by networks of such thermal resistances in series and in parallel, in exact analogy to 

electrical networks of resistors. See purely resistive thermal circuits for an example of such a 

network. 


https://en.wikipedia.org/wiki/Thermal_conduction

Transient conduction 

In general, during any period in which temperatures change in time at any place within an object, 

the mode of thermal energy flow is termed transient conduction. Another term is "non steadystate" 

conduction, referring to time-dependence of temperature fields in an object. Non-steadystate 

situations appear after an imposed change in temperature at a boundary of an object. They 

may also occur with temperature changes inside an object, as a result of a new source or sink of 

heat suddenly introduced within an object, causing temperatures near the source or sink to 

change in time. 

When a new perturbation of temperature of this type happens, temperatures within the system 

change in time toward a new equilibrium with the new conditions, provided that these do not 

change. After equilibrium, heat flow into the system once again equals the heat flow out, and 

temperatures at each point inside the system no longer change. Once this happens, transient 

conduction is ended, although steady-state conduction may continue if heat flow continues. If 

changes in external temperatures or internal heat generation changes are too rapid for 

equilibrium of temperatures in space to take place, then the system never reaches a state of 

unchanging temperature distribution in time, and the system remains in a transient state. 

An example of a new source of heat "turning on" within an object, causing transient conduction, 

is an engine starting in an automobile. In this case the transient thermal conduction phase for the 

entire machine is over, and the steady state phase appears, as soon as the engine reaches 

steady-state operating temperature. In this state of steady-state equilibrium, temperatures vary 

greatly from the engine cylinders to other parts of the automobile, but at no point in space within 

the automobile does temperature increase or decrease. After establishing this state, the transient 

conduction phase of heat transfer is over. 


https://en.wikipedia.org/wiki/Thermal_conduction

A.3 The Basic Physics of Infrared Radiation and Sensing 

All targets radiate energy in the infrared spectrum. The hotter the target, the 

more energy is radiated (∝T 4 ). 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, at about 6000 K, appears to glow white-hot; a tungsten filament, at 

about 3000 K, has a yellowish glow, and an electric stove element, at 800 K, 

glows red. As the stove element cools, it loses its visible glow but it continues 

to radiate. We can feel it with a hand placed near the surface but we can’t see 

the glow because the energy has shifted from red to infrared. Infrared 

detectors can sense infrared radiant energy and produce useful electrical 

signals proportional to the temperature of target surfaces. Instruments that 

use 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 0.1°C, and with response times as fast as 

microseconds. Instruments that combine this measurement capability with 

capabilities for scanning the target surface are called infrared thermal imagers. 


They can produce thermal maps or thermograms where the brightness 

intensity or color hue of any spot on the map is representative of the 

temperature of the surface at that point. In most cases, thermal imagers can 

be considered as extensions of radiation thermometers or as a radiation 

thermometer with scanning capability. The performance parameters of 

thermal imagers are extensions of the performance parameters of radiation 

thermometers. 


A.3.1 Some Historical Background 

The color of a glowing metal is a fair indication of its temperature (the higher 

the temperature, the whiter the color). The ancient sword-maker and 

blacksmith knew from the color of a heated part when it was time to quench 

and temper. This technique is still in use today; precision optical matching 

pyrometers are used to match the brightness in color of a product with that of 

a glowing filament. The brightness of the filament is controlled by adjusting a 

knob that is calibrated in temperature. The next logical step is to substitute a 

photomultiplier for the operator’s eye and, thus, calibrate the measurement. 

Finally, a differential measurement is made between what the brightness of 

the product is and what it should be (the set point), and the differential signal 

is injected into the process and used to drive the product temperature to the 

set point. With the advent of modern infrared detectors, the precision 

measurement of thermal energy radiating from surfaces that do not glow 

became possible. Measurements of cool surfaces, well below 0°C, are 

accomplished routinely with even the least expensive of infrared sensors. 


A.3.2 Non-Contact Thermal Measurements 

Infrared non-contact thermal sensing instruments are classified as infrared 

radiation thermometers by the American Society of Testing and Materials 

(ASTM), even though they don’t always read out in temperatures. The laws of 

physics allow for the conversion of infrared radiation measurements to 

temperature measurements. This is done by first measuring the self-emitted 

radiation in the infrared portion of the electromagnetic spectrum of target 

surfaces, and then converting these measurements to electrical signals. In 

making these measurements, three sets of characteristics need to be 

considered: 

• The target surface 

• The transmitting medium between the target and the instrument 

• The measuring instrument 


A.3.3 The Target Surface 

The chart of the electromagnetic spectrum (Figure A-3) indicates that the 

infrared portion of the spectrum lies adjacent to the visible. Every target 

surface above absolute zero (0 Kelvins or -273° Centigrade) radiates energy 

in the infrared. The hotter the target, the more radiant energy is emitted. 

When targets are hot enough, they radiate or glow in the visible part of the 

spectrum as well ( and beyond that, again becoming invisible again, example 

UV & ɣ ray) . As they cool, the eye becomes no longer able to see the emitted 

radiation and the targets appear to not glow at all. Infrared sensors are 

employed here to measure the radiation in the infrared, which can be related 

to target surface temperature. The visible spectrum extends from energy 

wavelengths of 0.4 µm for violet light to about 0.75 µm for red light. (µ or µm 

stands for micrometers or microns. A micron is one-millionth of a meter and is 

the measurement unit for radiant energy wavelength.) For practical purposes 

of temperature measurement, the infrared spectrum extends from 0.75 µm to 

about 20 µm. 


The visible spectrum extends from 

energy wavelengths of 

0.4 µm for violet light to about 0.75 

µm for red light. For practical purposes of 

temperature measurement, the infrared spectrum 

extends from 0.75 µm to about 20 µm. 

for my ASNT Exam 


Figure A-6 shows the distribution of emitted energy over the electromagnetic 

spectrum of targets at various temperatures. The sun, at 6000 K, appears 

white hot because its emitted energy is centered over the visible spectrum 

with a peak at 0.5 µm. Other targets, such as a tungsten filament at 3000 K, a 

red-hot surface at 800 K, and the ambient earth at 300 K (about 30°C), are 

also shown in this illustration. It becomes apparent that, as surfaces cool, not 

only do they emit less energy, but the wavelength distribution shifts to longer 

infrared wavelengths. Even though the eye becomes no longer capable of 

sensing this energy, infrared sensors can detect these invisible longer 

wavelengths. They enable us to measure the self-emitted radiant energy from 

even very cold targets and, thereby, determine the temperatures of target 

surfaces remotely and without contact. 

Keypoints: 

The visible spectrum extends from energy wavelengths of 0.4 µm for violet 

light to about 0.75 µm for red light. 

For practical purposes of temperature measurement, the infrared spectrum 

extends from 0.75 µm to about 20 µm. 


Figure A-6 

Blackbody Curves 

at Various 

Temperatures 



λ m = b/T = (2897/T μm) 


http://www.nasa.gov/centers/goddard/news/topstory/2004/0107filament.html

Two physical laws define the radiant behavior illustrated in Figure A-6: 

The Stephan-Boltzmann Law (1): 

W = εδT 4 

and Wien’s Displacement Law (2): 

λ m = b/T = (2897/T μm) 

Where: 

W = Radiant flux emitted per unit are a (watts/cm²) 

ε = Emissivity (unity for a blackbody target) 

δ = Stephan-Boltzmann constant = 5.673 x10 -12 watts cm -2 

T = Absolute temperature of target (K) 

λ m = Wavelength of maximum radiation (µm) 

b = Wien’s displacement constant = 2897 (µm∙K) 


According to (1), the radiant energy emitted from the target surface (W) 

equals two constants multiplied by the fourth power of the absolute 

temperature (T 4 ) of the target. The instrument measures W and calculates T. 

One of the two constants, δ, is a fixed number. 

Emissivity (ξ) is the other constant and is a surface characteristic that is only 

constant for a given material over a given range of temperatures. 

For point measurements, one can usually estimate the emissivity setting 

needed to dial into the instrument from available tables and charts. One can 

also learn, experimentally, the proper setting needed to make the instrument 

produce the correct temperature reading by using samples of the actual target 

material. This more practical setting value is called effective emissivity (e*). 


According to (2), the wavelength at which a target radiates its peak energy is 

defined as simply a constant (b = 2897≈ 3000) divided by the target 

temperature (T) in Kelvins. For the 300 K ambient earth, for example, the 

peak wavelength would be (λ max = 2897/300) or ≈ 10 µm. This quick 

calculation is important in selecting the proper instrument for a measurement 

task, as will be discussed in section A.4. 

Target surfaces can be classified in three categories: (1) black bodies, (2) 

gray bodies, and (3) non-gray bodies. 

The targets shown in Figure A-6 are all blackbody radiators (or black bodies). 

A blackbody radiator is a theoretical surface having unity emissivity at all 

wavelengths and absorbing all of the energy available at its surface. This 

would be an ideal target to measure because the temperature calculation 

within the instrument would be simply mechanized and always constant. 

Fortunately, although blackbody radiators do not exist in practice, the 

surfaces of most solids are gray bodies, that is, surfaces whose emissivities 

are high and fairly constant with wavelength. 


Figure A-7 shows the comparative spectral distribution of energy emitted by a 

blackbody, a gray body, and a non-gray body (also called a spectral body), all 

at the same temperature. For gray body measurements, a simple emissivity 

correction can usually be dialed in when absolute measurements are required. 

For non-gray bodies, the solutions are more difficult. To understand the 

reason for this, it is necessary to see what an instrument sees when it is 

aimed at a non-gray target surface. 

Keywords: 

non-gray body (also called a spectral body) 


Figure A-7 Spectral Distribution of a Blackbody, a Gray Body, and a Non- 

Gray Body 


Figure A-7 Spectral Distribution of a Blackbody, a Gray Body, and a Non- 

Gray Body 


Figure A-8 shows that the instrument sees three components of energy: first, 

emitted energy (ε); second, reflected energy from the environment (ρ); and 

third, energy transmitted through the target from sources behind the target (τ). 

The percentage sum of these components is always unity (1). The instrument 

sees only ε, the emitted energy, when aimed at a blackbody target because a 

blackbody reflects and transmits nothing. For a gray body, the instrument 

sees ε and ρ, the emitted and reflected energy. The instrument sees all three 

components when aimed at a nongray body because a non-gray body is 

partially transparent. 

Keywords: 

because a non-gray body is partially transparent.(?) 


Figure A-8 Components of Energy Reaching the Measuring Instrument 


If the emissivity of a gray body is very low, as in the case of polished metal 

surfaces, the reflectance becomes high (reflectance = 1 - emissivity) and can 

generate erroneous readings if not properly handled. Reflected energy from a 

specific source can generally be redirected by proper orientation of the 

instrument with respect to the target surface, as shown in Figure A-9. This 

illustrates the proper and improper orientation that is necessary to avoid 

reflected energy from a specific source. 


Figure A-9 Aiming the Instrument to Avoid Point Source Reflections 


Under certain conditions, an error in temperature indication can occur as the 

result of a high temperature background, such as a boiler wall (behind the 

instrument), reflecting off of a reflective target surface and contributing to the 

apparent temperature of the target. Most instrument manufacturers provide a 

background temperature correction to compensate for this condition. Often, in 

practice, the troublesome component is T, the energy transmitted through a 

non-gray target from sources behind the target. A discussion of solutions to 

this type of problem is included in section A.4. 

Non-Gray body – An object whose emissivity varies with wavelength over the 

wavelength interval of interest. A radiating object that does not have a 

spectral radiation distribution similar to a blackbody; also called a “colored 

body” or “realbody”. Glass and plastic films are examples of non-graybodies. 

An object can be a graybody over one wavelength interval and a non-gray 

body over another. http://www.infraredtraininginstitute.com/thermography-terms-definitions/ 


Blackbody, Graybody & Non-graybody (colored body or real body) 


http://www.moistureview.com/resources/infrarods-blog/page/4

EXAM score! 

Non-graybody 

(colored body or real body) 

for my ASNT exam 



A.3.4 The Transmitting Medium 

The transmission characteristics of the medium in the measurement path 

between the target and the instrument need to be considered in making nonontact 

thermal measurements. No loss of energy is encountered when 

measuring through a vacuum. For short path lengths, a few feet for example, 

most gases including the atmosphere, absorb very little energy and can be 

ignored (except where measurements of precision temperature values are 

required). As the path length increases to hundreds of feet, or as the air 

becomes heavy with water vapor, the absorption might become a factor. It is 

then necessary to consider the infrared transmission characteristics of the 

atmosphere. 


Figure A-10 illustrates the spectral transmission characteristics of 0.3 km of 

ground level atmosphere (what is the object to detector distance in tabulating 

the chart? or this is not a factor as the transmittance is given as a ratio (%) 

with respect to transmittance in vacuum (Transmittance in vacuum=100%)). 

Two spectral intervals can be seen to have very high transmission. These are 

known as the 1.5 µm and the 8.14 µm atmospheric windows, and almost all 

infrared sensing and scanning instruments are designed to operate in one or 

the other of these windows. (unless) Usually, the difficulties encountered with 

transmitting media occur when the target is viewed by the instrument through 

another solid object such as a glass or quartz viewing port in a process. 

Keywords: 

These are known as the 1.5 µm and the 8.14 µm atmospheric windows. 


Figure A-10 Infrared Transmission of 0.3 km of Sea Level Atmosphere 


Figure A-10 Infrared Transmission of 0.3 km of Sea Level Atmosphere 


Figure A-11 shows transmission curves for various samples of glass and 

quartz. Upon seeing these, our first impression is that glass is opaque at 10 

µm where ambient (30°C) surfaces radiate their peak energy. This impression 

is correct and, although in theory, infrared measurements can be made of 

30°C targets through glass, it is hardly practical. The first approach to the 

problem is to attempt to eliminate the glass, or at least a portion of it, through 

which the instrument can be aimed at the target. If, for reasons of hazard, 

vacuum, or product safety, a window must be present; a material that 

transmits in the longer wavelengths might be substituted. 


Figure A-11 Infrared Spectral Transmission of Glass 


Figure A-11 Infrared Spectral Transmission of Glass 


Figure A-11 shows transmission curves for various samples of glass and 

quartz. Upon seeing these, our first impression is that glass is opaque at 10 

µm where ambient (30°C) surfaces radiate their peak energy (?). This 

impression is correct and, although in theory, infrared measurements can be 

made of 30°C targets through glass, it is hardly practical. The first approach 

to the problem is to attempt to eliminate the glass, or at least a portion of it, 

through which the instrument can be aimed at the target. If, for reasons of 

hazard, vacuum, or product safety, a window must be present; a material that 

transmits in the longer wavelengths might be substituted. 



http://www.technicalglass.com/fused_quartz_transmission.html

EXAM score! 

Glass is opaque 

λ > 5µm at 30ºC? 



Figure A-12 shows the spectral transmission characteristics of several of 

these materials, many of which transmit energy past 10 µm. These materials 

are often used as lenses and optical elements in low-temperature infrared 

sensors. 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. 


Figure A-12 Characteristics of IR Transmitting Materials 



The characteristics of the window material will always have some effect on 

the temperature measurement, but the attenuation can always be corrected 

by pre-calibrating the instrument with a sample window placed between the 

instrument and a target of known temperature. In closing the discussion of the 

transmitting medium, it is important to note that infrared sensors can only 

work when all of the following spectral ranges coincide or overlap: 

1. The spectral range over which the target emits 

2. The spectral range over which the medium transmits 

3. The spectral range over which the instrument operates 

3 

2 

1 


IR Lenses – Sapphire Lens 


http://www.ecvv.com/product/3411419.html

IR Lenses – LWIR Len 


http://eom.umicore.com/en/infrared-optics/product-range/25-mm-f-1.2/

IR Lenses – Fresnel Len 


http://www.glolab.com/pirparts/pirparts.html

IR Lenses – Fresnel Len 


http://www.glolab.com/pirparts/pirparts.html

A.3.5 The Measuring Instrument 

Figure A-13 shows the necessary components of an infrared radiation 

thermometer. Collecting optics (an infrared lens, for example) is necessary in 

order to focus the energy emitted by the target onto the sensitive surface of 

an infrared detector, which, in turn, converts this energy into an electrical 

signal. 


Figure A-13 Components of an Infrared Radiation Thermometer 

Thermal or photon 

detector, single 

element or FPA. 


When an infrared radiation thermometer (point-sensing instrument) is aimed 

at a target, it collects energy within a collecting beam, the shape of which is 

determined by the configuration of the optics and the detector. 

The cross- section of this collecting beam is called the field of view of the 

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

surface that is measured by the instrument. 

On thermal imaging instruments, this is called the instantaneous field of view 

(IFOV) and becomes one picture element on the thermogram. 

Comment: 

for single element detector; FOV = IFOV 

for FPA multi element detector; IFOV (D) = θ(rad) x d 

Where, d= focal to object distance 


Infrared optics are available in two general configurations, refractive and 

reflective; 

■ 

Refractive optics (lenses), which are at least partly transparent to the 

wavelengths of interest, are used most often for high- temperature 

applications where their throughput losses can be ignored. 

■ 

Reflective optics (mirrors), which are more efficient but somewhat 

complicate the optical path, are used more often for low-temperature 

applications, where the energy levels cannot warrant throughput energy 

losses. 

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

spectral region or band of the energy reaching the detector. The reasons for 

spectral selectivity will be discussed later in this section. 


The processing electronics unit amplifies and conditions the signal from the 

infrared detector and introduces corrections for such factors as (1) detector 

ambient temperature drift and (2) target surface emissivity. Generally, a meter 

indicates the target temperature and an analog output is provided. The analog 

signal is used to record, display, alarm, control, correct, or any combination of 

these. 

Figure A-14 illustrates the configuration of a typical instrument employing all 

of the elements outlined. The germanium lens collects the energy from a spot 

on the target surface and focuses it on the surface of the radiation thermopile 

detector. The 8.14 µm filter limits the spectral band of the energy reaching the 

detector so that it falls within the atmospheric window. The detector generates 

a dc emf proportional to the energy emitted by the target surface. The autozero 

amplifier senses ambient temperature changes and prevents ambient 

drift errors. The output electronics unit conditions the signal and computes the 

target surface temperature based on a manual emissivity setting. The analog 

output terminals accept a 15 - 30 VDC loop supply and generate a 4 - 20 

milliampere signal, proportional to target surface temperature. 


All infrared detector-transducers exhibit some electrical change in response 

to the radiant energy impinging on their sensitive surfaces. Depending on the 

type of detector this can be (1) an impedance change, (2) a capacitance 

change, (3) the generation of an emf (voltage), or (4) the release of photons. 

Detectors are available with response times as fast as nanoseconds or as 

slow as fractions of seconds. Depending on the requirement, either a 

broadband detector or a spectrally limited detector can be selected. 

Keywords: 

Depending on the type of detector this can be 

(1) an impedance change, (Z) (thermal detector?) 

(2) a capacitance change, (C) (thermal detector?) 

(3) the generation of an emf (voltage), (Emf) (thermal detector?) 

(4) the release of photons. (E=hѵ) (photon detector?) 


Sequences of Events: 

1. The germanium lens collects the energy from a spot on the target surface 

2. focuses it on the surface of the radiation thermopile detector. 

3. The 8.14 µm filter (pass) limits the spectral band of the energy reaching 

the detector so that it falls within the atmospheric window. 

4. The detector generates a dc emf proportional to the energy emitted by the 

target surface. (thermal detector) 

5. The auto-zero amplifier senses ambient temperature changes and 

prevents ambient drift errors. (electronic) 

6. The output electronics unit conditions the signal and computes the target 

surface temperature based on a manual emissivity setting. (W = εσT 4 ) 

7. The analog output terminals accept a 15 - 30 Volt, DC loop supply and 

generate a 4 - 20 milliampere signal, proportional to target surface 

temperature. 


Figure A-14 Typical Infrared Radiation Thermometer Schematic 


Germanium Len 


Germanium Len 


Germanium Len 


Thermopile Detector 






http://wanda.fiu.edu/teaching/courses/Modern_lab_manual/stefan_boltzmann.html



https://www.adafruit.com/products/2023










The Grid-EYE 64-thermopile infrared array sensor from Panasonic adds state-of-the-art sensing technology to Avnet Abacus' 

passives portfolio. Based on Panasonic’s advanced MEMS technology, the 8x8 grid format infrared array sensor combines a builtin 

thermistor and an integrated circuit for temperature sensing in a small SMT package measuring only 11.6x4.3x8.0mm. Grid- 

EYE enables contactless temperature detection over the entire specified area. It can use passive infrared detection to determine 

temperature differentiation allowing it to detect multiple objects simultaneously. It is able to measure actual temperature and 

temperature gradients, providing thermal images and identifying the direction of movement of people or objects. The device’s 64 

pixel range yields accurate temperature sensing, within the range of -20°C to 100°C, over a viewing angle of 60° provided by a 

silicon lens. It uses an external I²C communication interface, enabling temperature measurement at speeds of 1 or 10 frames/s. 

An interrupt function is also available. The operating voltage of the device is 3.3 or 5.0V. 


http://www.electronics-eetimes.com/en/64-thermopile-infrared-array-sensor-available-fromavnet-abacus.html?cmp_id=7&news_id=222915463



http://www.electronics-eetimes.com/en/64-thermopile-infrared-array-sensor-available-fromavnet-abacus.html?cmp_id=7&news_id=222915463

Thermopile Detector - DR46 Thermopile Detector 

Features- A two-channel or a one-channel compensated thin-film thermopile in a TO-8 package. Each active area is 4mm x 

0.6mm. Offers high output with excellent signal-to-noise ratio. An internal aperture minimizes channel-to-channel crosstalk 

increasing sensitivity. Applications: Gas analysis, non-contact temperature measurement, fire detection / suppression. 


http://www.dexterresearch.com/?module=Page&sID=dr46

The IR Detectors 

Infrared detectors fall into two broad categories: 

■ 

■ 

thermal detectors, which have broad, uniform spectral responses, 

somewhat lower sensitivities, and slower response times (on the order of 

milliseconds), and 

photodetectors, (or photon detectors), which have limited spectral 

responses, higher peak sensitivities, and faster response times (on the 

order of microseconds). 

Thermal detectors will generally operate at or near room temperature, while 

photodetectors are generally cooled to optimize performance. The mercury- 

Cadmium-telluride (HgCdTe) detector, for example, is a photodetector cooled 

to 77 K for 8.14 µm operation and to 195 K for 3.5 µm operation. Because of 

its fast response, this detector is used extensively in high-speed scanning 

and imaging applications. 


The radiation thermopile, on the other hand, is a broadband thermal detector 

operating uncooled. It is used extensively for spot measurements of cool 

targets. It generates a dc emf proportional to the radiant energy reaching its 

surface and is ideal for use in portable, battery powered instruments. Figure 

A-15 illustrates the spectral responses of various infrared detectors. 


Figure A-15 Spectral Sensitivity of Various Infrared Detectors 


Thermal Detectors & Photon Detectors 

Photon 

Detector 

Thermal 

Detector 


The Mercury- Cadmium-telluride (Hgcdte) Detector, 


Discussion 

Subject: Why there are many curves for HgCdTe. 


http://irassociates.com/index.php?page=hgcdte

The Mercury- Cadmium-Telluride (HgCdTe) Detector – FPA 

WISE Mercury Cadmium Telluride Focal Plane Mount Assembly (HgCdTe FPMA). This picture 

shows one of the four WISE detectors. The sensitive area shows as green and contains 1 million 

pixel elements. 


http://wise.ssl.berkeley.edu/gallery_detector.html



http://spie.org/x91246.xml


October 24, 2011 - All Eyes on Oldest Recorded Supernova 

This image combines data from four different space telescopes to create a multi-wavelength view of all that remains of the oldest documented example of a 

supernova, called RCW 86. The Chinese witnessed the event in 185 A.D., documenting a mysterious "guest star" that remained in the sky for eight months. 

X-ray images from the European Space Agency's XMM-Newton Observatory and NASA's Chandra X-ray Observatory are combined to form the blue and 

green colors in the image. The X-rays show the interstellar gas that has been heated to millions of degrees by the passage of the shock wave from the 

supernova. 



Discussion 

Subject: Why it wasn’t pixel-like correspond to the spatial resolution of 10 6 ? 




Sept 29, 2011 - Portrait of Two Asteroids in Different Light - This animation illustrates the benefits of observing asteroids in infrared light. It begins by 

showing two artistic interpretations of asteroids up close. They are about the same size but the one on the right is darker. The animation zooms away to 

show how a visible-light telescope would see these two space rocks, located at the same distance millions of miles away from Earth, against a background 

of more distant stars. The one on the left would be much easier to see because it reflects more visible light from the sun. The animation then transitions to 

an infrared view of the same two objects. Both asteroids are equally as bright because the telescope is picking up infrared light coming from the bodies 

themselves, as a result of being heated by the sun. The measurements are not strongly affected by how light or dark an asteroid is, a property called albedo. 

Instead, the brightness is more directly related to an asteroid's size. Therefore, infrared telescopes like WISE are better at both finding the small, dark 

asteroids and determining asteroid sizes. 



Sept 29, 2011 - Portrait of Two Asteroids in Different Light - This animation 

illustrates the benefits of observing asteroids in infrared light. 

■ 

http://wise.ssl.berkeley.edu/video/quicktime/V2-TwoAsteroids-HD.mov 



Point-sensing instruments for measuring very hot targets, usually operate in 

shorter wavelengths (0.9 - 1.1 µm, for example), and instruments for 

measuring cooler targets usually operate in longer wavelengths (3.5 µm or 

8.14 µm, for example). Most infrared thermal imagers operate in either the 3.5 

µm or 8.14 µm spectral region. 

The spectral Selectivity: 

■ Very hot - 0.9 - 1.1 µm 

■ Hot - 3.5 µm 

■ Cool - 8.14 µm 


A.3.6 Introduction to Thermal Scanning and Imaging Instruments 

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 

(field of view) of the instrument relative to the target). This can be 

accomplished by: 

(1) inserting a movable optical element into the collecting beam, or 

(2) by employing a multi-detector array or mosaic, and scanning the array 

electronically. (line scanner & FPA) 

A brief overview of scanning and imaging instruments follows. A more 

detailed overview can be found in section 2. 


A.3.6.1 Line Scanning 

The purpose of spatial scanning is to derive information concerning the 

distribution of radiant energy over a target scene. Quite often, a single straight 

line scanned on the target is all that is necessary to locate a critical thermal 

anomaly. The instantaneous position of the scanning element (or the position 

of the element in the linear array) is controlled or sensed, so that the 

radiometric output signal can be accompanied by a position signal output and 

be displayed on a chart recorder, an oscilloscope, or some other recording 

device. 

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 (as illustrated in Figure A- 

16). The output signal information is in real-time computer compatible format 

and can be used to monitor, control or predict the behavior of the target. 


Figure A-16 Scanning Configuration of an Infrared Line Scanner 

The line scanner 

could be a single 

element or linear 

array detector. 


A.3.6.2 Two-Dimensional Scanning 

The purpose of spatial scanning is to derive information concerning the 

distribution of infrared radiant energy over a target scene. Scanning can be 

accomplished either opto-mechanically or electronically. 

Opto-mechanical scanning can be done by moving the target with the 

instrument fixed, or by moving (translating or panning) the instrument, but it is 

more practically accomplished by inserting movable optical elements into the 

collected beam. Although an almost infinite variety of scanning patterns can 

be generated using two moving elements, the most common pattern is 

rectilinear, and this is most often accomplished by two elements, each 

scanning a line normal to the other. A typical rectilinear scanner employs two 

rotating prisms behind the primary lens system (refractive scanning). An 

alternate configuration uses two oscillating mirrors behind the primary lens 

(reflective scanning). This is also commonly used in commercial scanners, as 

are combinations of reflective and refractive scanning elements. 


Now, electronically scanned thermal imaging is accomplished by means of an 

infrared focal plane array (IRFPA), whereby a two-dimensional staring array 

of detectors collects radiant energy from the target and is digitally scanned to 

produce the thermogram. In the case of the line scanner (Figure A-16), the 

opto-mechanical scanning approach is gradually being superceded by 

replacement of the single-element detector with an electronically scanned 

linear focal plane array (a line of detectors), thus eliminating the scanning 

mechanism entirely. At the time of this writing, focal plane array imagers have 

all but completely replaced optomechanically scanned imagers in 

manufacturers’ inventory and product literature. Because many optomechanically 

scanned line scanners and imagers are still in use throughout 

the predictive maintenance community, the following discussion is included in 

this appendix. 


Opto-mechanical Scanner 

A typical commercial rectilinear opto-mechanical scanner is shown 

schematically in Figure A-17. It employs two oscillating mirrors (reflective 

scanning) behind the primary lens and is commonly used in commercially 

available scanners. This approach has the advantage of a broad spectral 

response limited only by the spectral characteristics of the detector and the 

primary lens system. The main disadvantage is that the elements and their 

associated drive mechanisms must be arranged so that there is no optical or 

mechanical interference. This makes compact design more difficult. An 

alternate approach to scanning employs two rotating prisms behind the 

primary lens system. This instrument, using refractive scanning elements, has 

the advantage of compact design, because all of the scanning elements can 

be arranged in a line. It has the disadvantage of spectral limitation in that 

each element must transmit the entire portion of the infrared spectrum for 

which the instrument was designed. Some energy is absorbed by each 

refractive element, reducing the throughput somewhat, and the rather high 

cost of infrared transmitting materials add to the instrument cost. It should be 

pointed out that opto-mechanical scanners can employ refractive or reflective 

scanning elements or even combinations of both elements. 


Figure A-17 Schematic of a Typical Opto-Mechanically Scanned Imager 


Electronic scanning 

Electronic scanning involves no mechanical scanning elements.the surface is 

scanned electronically. The earliest type of electronically scanned thermal 

imager is the pyrovidicon. 

Pyrovidicon thermal imagers 

Pyrovidicon thermal imagers (pyroelectric vidicons) or thermal video 

systems are devices in which charge proportional to target temperature is 

collected on a single pyroelectric detector surface within an electronic 

picture tube, and scanning is accomplished by an electronic scanning 

beam. The pyrovidicon is a video camera tube that operates in the 

infrared (2.14 µm) region instead of in the visible spectrum. Electronically 

scanned thermal imaging systems based on pyrovidicons and operating 

in the 8.14 µm atmospheric window are in common use today. They 

provide qualitative thermal images and are classified as thermal viewers. 


Focal plane array (FPA) imagers 

Focal plane array (FPA) imagers have, over the last decade, become the 

imagers of choice over opto-mechanically scanned imagers, replacing 

them in virtually all commercial applications. Manufacturers of FPA 

imagers offer a wide choice of both cooled and uncooled detector arrays, 

with a wide selection of spectral ranges for both measuring (quantitative) 

and non-measuring (qualitative) applications. A more detailed discussion 

of focal plane array imagers can be found in Section 2. 

Published performance characteristics of currently available infrared 

commercial thermal imaging systems, including detailed discussions of 

diagnostic software and image recording methods, can also be found in 

Section 2, Table 2-1. 

Figure A-18 is a schematic of a typical focal plane array based thermal 

imager. 


Figure A-18 Schematic of a Typical (Staring) FPA-Based Thermal 

Imager 


Staring Array 

A staring array, staring-plane array, focal-plane array (FPA), or focal-plane is 

an image sensing device consisting of an array (typically rectangular) of lightsensing 

pixels at the focal plane of a lens. FPAs are used most commonly for 

imaging purposes (e.g. taking pictures or video imagery), but can also be 

used for non-imaging purposes such as spectrometry, LIDAR, and wave-front 

sensing. 

In radio astronomy the term "FPA" refers to an array at the focus of a radiotelescope 

(see full article on Focal Plane Arrays). At optical and infrared 

wavelengths it can refer to a variety of imaging device types, but in common 

usage it refers to two-dimensional devices that are sensitive in the infrared 

spectrum. Devices sensitive in other spectra are usually referred to by other 

terms, such as CCD (charge-coupled device) and CMOS image sensor in the 

visible spectrum. FPAs operate by detecting photons at particular 

wavelengths and then generating an electrical charge, voltage, or resistance 

in relation to the number of photons detected at each pixel. This charge, 

voltage, or resistance is then measured, digitized, and used to construct an 

image of the object, scene, or phenomenon that emitted the photons. 


http://military.wikia.com/wiki/Staring_arrayc

Applications for infrared FPAs include missile or related weapons guidance 

sensors, infrared astronomy, manufacturing inspection, thermal imaging for 

firefighting, medical imaging, and infrared phenomenology (such as observing 

combustion, weapon impact, rocket motor ignition and other events that are 

interesting in the infrared spectrum). 

Comparison To Scanning Array 

Staring arrays are distinct from scanning array and TDI (time-domain 

integration) imagers in that they image the desired field of view without 

scanning. Scanning arrays are constructed from linear arrays (or very narrow 

2-D arrays) that are rastered across the desired field of view using a rotating 

or oscillating mirror to construct a 2-D image over time. A TDI imager 

operates in similar fashion to a scanning array except that it images 

perpendicularly to the motion of the camera. A staring array is analogous to 

the film in a typical camera; it directly captures a 2-D image projected by the 

lens at the image plane. 



A scanning array is analogous to piecing together a 2D image with photos 

taken through a narrow slit. A TDI imager is analogous to looking through a 

vertical slit out the side window of a moving car, and building a long, 

continuous image as the car passes the landscape. 

Scanning arrays were developed and used because of historical difficulties in 

fabricating 2-D arrays of sufficient size and quality for direct 2-D imaging. 

Modern FPAs are available with up to 2048 x 2048 pixels, and larger sizes 

are in development by multiple manufacturers. 320 x 256 and 640 x 480 

arrays are available and affordable even for non-military, non-scientific 

applications. 



Staring 


A.4 Performance Parameters of Thermal-Sensing 

Instruments 

To select an instrument suitable to a particular application, the thermographer 

needs to understand how to determine and specify its required performance. 

This section provides information regarding the performance parameters of (1) 

point-sensing instruments and (2) scanning & imaging instruments. 


A.4.1 Point-Sensing Instruments 

For point-sensing instruments (infrared radiation thermometers), the following 

performance parameters should be considered: 

• Temperature range: The high and low limits over which the target 

emperature can vary 

• Absolute accuracy: As related to the National Institute of Standards and 

Technology (NIST) standard 

• Repeatability: How faithfully a reading is repeated for the same target 

• Temperature sensitivity: The smallest target temperature change that the 

instrument needs to detect 

• Speed of response: How fast the instrument responds to a temperature 

change at the target surface 

• Target spot size and working distance: The size of the spot on the target to 

be measured, and its distance from the instrument (FOV/IFOV) 

• Output requirements: How the output signal is to be used 

• Spectral range: The portion of the infrared spectrum over which the 

instrument will operate 

• Sensor environment: The ambient conditions under which the instrument 

will operate 


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 with an absolute accuracy ± 2°C over the 

entire range. This could alternately be specified as ± 1% absolute accuracy 

over full scale. On the other hand, we might require the best accuracy at 

some specific temperature, say 100°C. In this case, the manufacturer should 

be so informed. The instrument can then be calibrated to exactly match the 

manufacturer’s laboratory calibration standard at that temperature. 

It is difficult for a manufacturer to comply with a tight specification for absolute 

accuracy because absolute accuracy is based on traceability to the National 

Institute of Standards and Technology (NIST) standard. An absolute accuracy 

of ±0.5°C ± 1% of full scale is about as tight as can be reasonably specified. 

Repeatability, on the other hand, can be more easily assured by the 

manufacturer, and is usually more important to the user. 


Temperature sensitivity is also called thermal resolution (≠ spatial resolution) 

or noise equivalent temperature difference. It is the smallest temperature 

change at the target surface that must 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. We should specify temperature sensitivity, 

therefore, at a particular target temperature, and this should be near the low 

end of the range of interest. We might, for example, specify temperature 

sensitivity to be 0.25°C at a target temperature of 25°C, and be confident that 

the sensitivity of the instrument will be at least that for targets hotter than 

25°C. 

Keywords 

Temperature sensitivity is also called thermal resolution or noise equivalent 

temperature difference (NETD). 


EXAM score! 

Temperature sensitivity is also 

called thermal resolution or 

noise equivalent temperature 

difference (NETD). 



EXAM score! 

thermal resolution 

(≠ spatial resolution) 



NETD - Noise Equivalent Temperature Difference 

Noise Equivalent Temperature Difference is used to measure the 

performance of a infrared cameras ability discern the minimum level of 

thermal sensitivity and is very similar to the MRTD with the exception that the 

test is based on the output of the detector only, without taking into 

consideration the performance of the infrared cameras image as it would be 

displayed to a thermographer. The results are usually expressed as the 

NETD. A common specification for an IR cameras NETD is 0.02 deg. C at 30 

deg. C. 

MRTD - Minimum Resolvable Temperature Difference 

Minimum Resolvable Temperature Difference is a test developed by the 

Department of Defense (ASTM Standard E1213) and used to measure the 

performance of a infrared cameras ability discern the minimum level of 

thermal sensitivity that a operator of the camera can see. The test involves 

selecting the smallest test pattern (4 bars with a 7:1 length to width aspect 

ratio) that can be clearly distinguished by the operator as viewed on a display. 


http://www.prothermographer.com/training/IRBasics/qualitative_thermography/mrtd 

_minimum_resolvable_temperature_difference.htm

NETD - Noise Equivalent Temperature Difference 

NETD is used to measure the performance of a infrared cameras ability 

discern the minimum level of thermal sensitivity and is very similar to the 

MRTD with the exception that the test is based on the output of the detector 

only, without taking into consideration the performance of the infrared 

cameras image as it would be displayed to a thermographer. The results are 

usually expressed as the NETD. A common specification for an IR cameras 

NETD is 0.02 deg. C at 30 deg. C. 

MRTD - Minimum Resolvable Temperature Difference 

METD is a test developed by the Department of Defense (ASTM Standard 

E1213) and used to measure the performance of a infrared cameras ability 

discern the minimum level of thermal sensitivity that a operator of the camera 

can see. The test involves selecting the smallest test pattern (4 bars with a 

7:1 length to width aspect ratio) that can be clearly distinguished by the 

operator as viewed on a display. 


http://www.prothermographer.com/training/IRBasics/qualitative_thermography/mrtd 

_minimum_resolvable_temperature_difference.htm

Speed of response is generally defined as the time it takes the instrument 

output to respond to 95% of a step change at the target surface. 

Figure A-19 shows this graphically. Note that the sensor time constant is 

defined by convention to be the time required to reach 63% of a step change 

at the target surface. Instrument speed of response is about 5 time constants, 

and is generally limited by the detector used. As previously discussed, this 

limit is on the order of microseconds for photodetectors and milliseconds for 

thermal detectors. There is, however, a tradeoff between speed of response 

and temperature sensitivity. As in all instrumentation systems, as the speed of 

response becomes faster (wider information bandwidth), the sensitivity 

becomes poorer (lower signal-to-noise ratio). We learn from this that the 

speed of response should not be over-specified. 

Keywords: 

■ 63% 

■ 95% 


Figure A-19 Instrument Speed of Response and Time Constant 


Target spot size (also called spatial resolution) and working distance can be 

specified as just that (1 cm at 1 meter, for example), or we can put it in more 

general terms such as field of view angle (10 milliradians, 1 degree, 2 

degrees) or a field of view (spot size-to -working distance) ratio (D/15, D/30, 

D/75). 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, for 

example). 

Figure A-20 illustrates the fields of view for several instruments and how an 

instrument can be selected based on the spot size and working distance 

required. An examination of the collecting beams of the instruments shown 

also shows that, at very close working distances, this simple ratio does not 

always apply. If close-up information is not clearly provided in the product 

literature, the instrument manufacturer should be consulted. For quick 

reference, a method of approximating spot size based on manufacturerrovided 

information is illustrated in Appendix C, Plate 2. 


Figure A-20 Fields of View of Infrared Radiation Thermometers 







An examination of the collecting 

beams of the instruments shown also 

shows that, at very close working 

distances, this simple ratio does not 

always apply. If close-up information 

is not clearly provided in the product 

literature, the instrument 

manufacturer should be consulted. 


The output requirements are totally dependent on the user’s needs. If a 

readout indicator is required, a wide selection is usually offered. An analog 

output suitable for recording, monitoring, and control is commonly provided. In 

addition, most manufacturers offer a broad selection of output functions 

including digital (BCD coded) outputs, high, low, and proportional set-points, 

signal peak or valley sensors, sample and hold circuits, and even closed-loop 

controls for specific applications. Many currently available instruments, even 

portable hand-held units, include microprocessors that provide many of the 

above functions on standard models. 


As previously noted, the operating spectral range of the instrument is often 

critical to its performance. For cooler targets, up to about 500°C, most 

manufacturers offer instruments operating in the 8.14 µm atmospheric 

window. For 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 

effective emissivity incorrectly will result in smaller temperature errors when 

measurements are made at shorter wavelengths. A good general rule to 

follow, particularly when dealing with targets of low or uncertain effective 

emissivities, is to work at the shortest wavelengths possible without 

compromising sensitivity or risking susceptibility to reflections from visible 

energy sources. 


Spectrally selective instruments employ interference 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 highly selective to a specific material whose 

temperature is to be measured in the presence of an intervening medium or 

an interfering background. For example, for measuring the temperature of 

objects from 200°C to 1000°C inside a heating chamber with a glass port, or 

inside a glass bell jar, an instrument operating in the 1.5 to 2.5 µm band will 

see through the glass and make the measurement easily. A very important 

generic example of the need for spectral selectivity is in the measurement of 

plastics in the process of being formed into films and other configurations. 

Keywords: 

interference filters 


Thin films of many plastics are virtually transparent to most infrared 

wavelengths but do emit at certain wavelengths. Polyethylene, polypropylene, 

and other related materials, for example, have a very strong, though narrow, 

absorption band at 3.45 µm. Polyethylene film is formed at about 200°C in the 

presence of heaters that are at about 700°C. 

Figure A-21 shows the transmission spectra of 1.5- mil thick polyethylene film 

and the narrow absorption band at 3.45 µm. The instrument selected for 

measuring the surface of the film has a broadband thermal detector and a 

3.45 µm spike band pass filter. The filter makes the instrument blind to all 

energy outside of 3.45 µm, and enables it to measure the temperature of the 

surface of the plastic film without seeing through the film to the heaters. 


Figure A-21 Spectral Filtering for Polyethylene Temperature 

Measurement 


The object is opaque to 3.45 µm radiation, by using 3.45 µm pass filter, only 

the object’s 3.45 µm is monitored, all other bandwidth from the object or 

transmitted from the process hot roller are filtered off. 

3.45 µm pass filter 


Figure A-22 shows a similar solution for 0.5-mil thick polyester (Mylar) film 

under about the same temperature conditions. Here, the strong polyester 

absorption band, from 7.7 to 8.2 µm, dictates the use of a 7.9 µm spike filter 

placed in front of the same broadband detector. 


Figure A-22 Spectral Filtering for Polyester Temperature Measurement 


A.4.2 Scanners and Imagers.Qualitative and Quantitative 

The parameters used for assessing the performance of infrared thermal 

imaging scanners are complex and the methods used for testing performance 

have generated some controversy among manufacturers and users of these 

instruments. A thermal image is made up of a great number of discrete point 

measurements, however, many of the performance parameters of infrared 

thermal imagers are the same as those of radiation thermometers (pointsensing 

infrared radiometers that read out in temperature). Others derive from, 

or are extensions of, radiation thermometer performance parameters. 

Qualitative (non-measuring) thermal imagers, also called thermal viewers, 

differ from quantitative (measuring) thermal imagers, also called imaging 

radiometers, in that thermal viewers do not provide temperature or thermal 

energy measurements. 

It should be noted, therefore, that for users requiring qualitative rather than 

quantitative thermal images, many of the parameters discussed herein are of 

no importance. 


A.4.3 Performance Parameters of Imaging Radiometers 

The Environmental Research Institute, Michigan (ERIM) Infrared Handbook 

[13] provides an extensive table of terms and definitions (section 19.1.2) and 

a list of specimen specifications (section 19.4.1). The section of the 

Handbook covering infrared imaging systems does not, however, deal with 

the imager as a quantitative measurement instrument, and so the 

performance parameters related with temperature measurement need to be 

added. Some simplifications can be made, which result in some acceptable 

approximations. Bearing these qualifications in mind, the following definitions 

of the key performance parameters of infrared thermal scanners are offered: 


• Total field of view (TFOV): the image size, in terms of scanning angle. 

(example: TFOV = 20°V x 30°H) 

• Instantaneous field of view (IFOV): the angular projection of the detector 

element at the target plane; imaging spatial resolution. (example: IFOV= 2 

milliradians ) 

• Measurement spatial resolution (IFOVmeas): the spatial resolution 

describing the minimum target spot size on which an accurate temperature 

measurement can be made. (example: IFOVmeas = 5 milliradians) 

• Frame repetition rate: The number of times every point on the target is 

scanned in one second. (example: Frame rate = 30 /second) 

• Minimum resolvable temperature (MRT) (NETD? / MRDT?) : The smallest 

blackbody equivalent target temperature difference that can be observed; 

temperature sensitivity (example: MRT=0.1°C @ 30°C target temperature) 


Minimum resolvable temperature MRT 

MRT and the terms relating to spatial resolution are interrelated and cannot 

be considered independently. (unlike the point sensing: IR thermometer) 

Other parameters, such as spectral ranges, target temperature ranges, 

accuracy and repeatability, and focusing distances, are essentially the same 

as those defined previously for infrared radiation thermometers, although they 

can be expressed differently. Dynamic range and reference level range, for 

example, are the terms that define the target temperature ranges for thermal 

imagers. While the operating spectral range of a radiation thermometer is 

often critical to its performance, the spectral range of operation of a thermal 

imager is not usually as critical to the user, except for a few specialized 

applications. Most commercial thermal imagers operate in either the 2.5 µm 

or the 8.12 µm atmospheric window, depending on the manufacturer’s choice 

of detector. Filter wheels or slides are usually available to enable users to 

insert special interference filters and perform spectrally selective 

measurements when necessary. 


Despite some manufacturers’ claims to the contrary, there is usually little 

difference in overall performance between an imager operating in the 2.5 µm 

band and an imager operating in the 8.12 µm band, all other parameters 

being equal. 

For a specific application, however, there might be a clear choice. One 

example of this would be selecting an imager operating in the 2 .5 µm band to 

observe a target through a quartz window. There would be no alternative 

because quartz is virtually opaque in the 8.12 µm region. Another example 

would be selecting an imager operating in the 8.12 µm band to observe a cool 

target through a long atmospheric path. The choice would be obvious 

because long-path atmospheric absorption is substantially greater in the 2.5 

µm window than in the 8.12 µm window. 


For qualitative (non measuring) thermal viewers, parameters relating to 

temperature range are only applicable in the broadest sense. Absolute 

accuracy and stability parameters are not applicable. MRT is applicable only 

as an approximation because stability cannot be assured. IFOV meas is not 

applicable. 

Secondary features, such as field uniformity and spatial distortion, are design 

parameters and are assumed to be handled by responsible manufacturers. A 

discussion of the significant performance parameters (figures of merit) follows. 


A.4.3.1 Temperature Sensitivity, Minimum Resolvable Temperature 

Difference 

(MRTD) or Minimum Resolvable Temperature (MRT) Temperature sensitivity, 

also called thermal resolution or noise equivalent temperature difference 

(NETD) for a radiation thermometer, is the smallest temperature change at 

the target surface and can be clearly sensed at the output of the instrument. 

For an imaging system, the MRT or MRTD defines temperature sensitivity but 

also implies spatial resolution (IFOV). MRTD is expressed as a function of 

angular spatial frequency. Testing for MRTD is usually accomplished by 

means of a subjective procedure developed by the Department of Defense 

community. 

Keywords: 

■ the MRT or MRTD defines temperature sensitivity but also implies spatial 

resolution (IFOV) (for 2D thermography; both thermal viewer & thermal 

radiometric imaging) . 

■ MRTD is expressed as a function of angular spatial frequency. (?) 


This involves selecting the smallest (highest frequency) standard periodic test 

pattern (four bars, 7:1 length-to-width aspect ratio) that can be distinguished 

as a 4 bar contrast target by the observer, and recording the smallest 

detectable element-to-element temperature difference between two 

blackbody elements on this pattern. Unlimited viewing time and optimization 

of controls is allowed and the target is oriented with the bars normal to the 

horizontal scan line. 

Figure A-23 illustrates the setup using an ambient pattern and a heated 

background. The MRTD curve shown is a function of spatial frequency 

(cycles/mRad). Additional points on the curve are achieved by changing the 

pattern size or the distance to the scanner. 


Figure A-23 Test Setup for MRTD Measurement, MRTD Curve 

heated background 



A.4.3.2 Spot Size (FOV) , Instantaneous Field of View (IFOV), Imaging 

Spatial Resolution (?) , Measurement Spatial Resolution (IFOVmeas) 

For thermal imagers, the instantaneous field of view (IFOV) expresses spatial 

resolution for imaging purposes but not for measurement purposes. 

Measurement instantaneous field of view (IFOVmeas) expresses spatial 

resolution for measurement purposes. 

The modulation transfer function (MTF) is a measure of imaging spatial 

resolution. Modulation is a measure of radiance contrast and is expressed: 

Modulation = (L max -L min ) / (L max + L min ) 

L = luminosity? 

Modulation transfer is the ratio of the modulation in the observed image to 

that in the actual object. 


For any system, MTF will vary with scan angle and background, and will often 

be different when measured along the horizontal than it is when measured 

along the vertical. 

For this reason, a methodology was established and accepted by 

manufacturers and users alike to measure the MTF of an imager and, thereby, 

to verify the spatial resolution for imaging (night vision) purposes. A sample 

procedure follows for a system where IFOV is specified at 2.0 milliradians. 

This is shown in Figure A-24 and uses the same setup as illustrated in Figure 

A-23: 


■ 

A standard 4 bar (slit) resolution target (7:1 aspect ratio) with a 2-mm slit 

width is placed in front of a heated blackbody reference surface at a 

distance of 1 meter from the primary optic of the instrument. The ratio of 

the 2-mm slit width to the 1-meter working distance is 2 milliradians). The 

target is centered in the scanned field (oriented so that the horizontal axis 

is normal to the slit), and a single line scan output signal is monitored. 

The analog signal value of the 4 peaks (Vmax), as the slits are scanned, 

and the analog signal value of the 3 valleys (Vmin), are recorded using 

the bar target surface ambient temperature as a base reference. The 

MTF is expressed as a ratio equal to (Vmax -Vmin) / (Vmax + Vmin). If 

this ratio is at least 0.35, the 2 milliradian IFOV is verified. 

There are some disagreements among users and manufacturers regarding 

the acceptable minimum value of MTF to verify imaging spatial resolution, 

with values varying between 0.35 and 0.5, depending on the manufacturer 

and the purpose of the instrument. For most users, a tested value of MTF, 

equal to or greater than 0.35 for a slit width representing a specified spatial 

resolution is generally considered sufficient to demonstrate that spatial 

resolution for imaging purposes. 


Figure A-24 Modulation Transfer Function, Imager Spatial Resolution 


Both MRTD and MTF are functions of spatial frequency for any given system. 

This is illustrated in Figure A-25, reprinted from J.M. Lloyd, Thermal Imaging 

Systems [14], for a typical system rated by the manufacturer to be 1 

milliradian. The cut-off frequency is where the IFOV equals 1 cycle (one bar 

and one slit) so that the intersection of the two curves at the half-cut-off 

frequency represents the actual performance of the system for an MRTD of 

1°C. MTF is seen to be about 0.22 for this system. 


Figure A-25 MRTD and MTF for a System Rated at 1.0 Milliradian 



For measurement purposes, of course the slit width should, ideally, be 

increased until the modulation reaches unity. For this reason the MTF method 

was found to be unsatisfactory for commercial thermal imagers where 

quantitative temperature measurement and control are often necessary. 

Another procedure, called the Slit Response Function (SRF), was developed 

for this purpose and is generally accepted for measuring IFOVmeas. In this 

method, illustrated in Figure A-26, a single variable slit is placed in front of a 

blackbody source and the slit width is varied until the resultant single-line- can 

signal approaches the signal of the blackbody reference. Because there are 

other errors in the optics, the 100% level of SRF is approached rather slowly, 

as shown in the curve of Figure A-26. The slit width at which the SRF 

reaches 0.9, divided by the distance to the slit (W/d), is usually accepted 

as the IFOVmeas of the instrument under test. Figures A-23 and A-26 are 

adapted from the Ohman paper, .Measurement Versus Imaging in 

Thermography. [15], which provides a detailed description of the Slit 

Response Method, setup diagrams, and a discussion of imaging and 

measurement spatial resolution figures of merit. The step-by-step procedure 

for measuring SRF is described in detail in Appendix C, Plate 6. 


IFOVmeas: The slit width at which the SRF reaches 0.9, divided by the 

distance to the slit (W/d), is usually accepted as the IFOVmeas of the 

instrument under test. 

for IFOV or IFOV geometric : 

D = σ∙d, σ (IFOV) = D / d 

σ 

D 

d 

for IFOV meas = D meas /d 

σ 

D meas 

d 


Figure A-26 Setup and Curves for Slit Response Function Test 



Note: Because FPA imagers have all but replaced opto-mechanically 

scanned imagers, many experienced thermographers suggest that the SRF 

measurement procedure be performed in both the horizontal and vertical 

scan-line direction. The larger of the two results is then accepted as the 

IFOV meas of the imager under test. 


FPA 


http://spie.org/x34358.xml

FPA 


FPA 


FPA 


A.4.3.3 Speed of Response and Frame Repetition Rate 

Speed of response of a radiation thermometer is generally defined as the time 

it takes the instrument output to respond to 95% of a step change at the 

target surface (about 5 time constants). This parameter is not applicable to 

thermal imagers. Frame repetition rate is the measure of the data update of a 

thermal imager. This is not the same as field repetition rate. (Manufacturers 

might use fast field rates with not all of the picture elements included in any 

one scan, and then interlace the fields so that it takes multiple fields to 

complete a full frame. This might produce a more flicker-free image and be 

more pleasing to the eye than scanning full data frames at a slower rate. 

Frame repetition rate is the number of times per second every picture element 

is scanned. 


Figure A-19 Instrument Speed of Response and Time Constant 


A.4.4 Thermal Imaging Software 

In order to optimize the effectiveness of thermography measurement 

programs, the thermographer needs a basic understanding of thermal image 

processing techniques. The following is a broad discussion of thermal image 

processing and diagnostics. A detailed description of thermal imaging and 

diagnostic software currently available from manufacturers is provided in 

section 2. 

Thermal imaging software can be categorized into the following groupings: 

• Quantitative thermal measurements of targets 

• Detailed processing and image diagnostics 

• Image recording, storage, and recovery 

• Image comparison 

• Archiving and database* 

*Although data and image database development is not an exclusive 

characteristic of thermal imaging software, it should be considered an 

important part of the thermographer’s tool kit. 


With the introduction of computer-assisted thermal image storage and 

processing, thermography has become a far more exact science, and the 

ability to perform image analysis and trend analysis has greatly expanded its 

reach. Innovative software has been tailored specifically for detailed image 

and thermal data analysis, and has been rapidly updated and expanded. 

Most software packages for thermography image analysis and diagnostics 

offer a number of standard features. These include spot temperature readout, 

multiple X and Y analog traces, monochrome and multiple-color scale 

selection, image shift, rotation and magnification, area analysis with 

histogram display, image averaging and filtering, and permanent disk storage 

and retrieval. Some of these capabilities are offered as part of the basic 

instrument and some are found in a diagnostics package offered separately. 


The newest field-portable instruments allow the thermographer to store 

images to disc (or data card) during field measurements, and perform detailed 

image analysis upon return to home base (see Section 2 for details). The 

ability to perform differential thermography is a most powerful feature of 

thermographic software routines. This is the capability for archiving thermal 

images of acceptable operating components, and assemblies and 

mechanisms, and using these stored images as models for comparison to 

subsequently inspected items. Subtractive routines produce differential 

images, illustrating the deviation of each pixel (picture element) from its 

corresponding model. 

Another powerful routine that was recently introduced is an emissivity 

determination and correction program, which produces true surfacetemperature 

thermograms of microelectronic devices and other very small 

targets. 

Keywords: 

Subtractive routines 


To perform this function, the unpowered device is heated sequentially to two 

known low-level temperatures, and the stored thermal images are used to 

allow the computer to calculate emissivity of each pixel. The device is then 

powered and the image produced is corrected, point by point, for the 

emissivities previously computed. There is great interest in applying this 

spatial emissivity correction to larger targets such as circuit cards. The 

difficulty in developing a reliable emissivity matrix lies in achieving tight 

control over the temperature and temperature uniformity while heating a 

target of this size. 

For the professional thermographer, the maintenance of an historical 

database is most critical, and thermography software allows this to be done 

systematically. The historical data included with stored images (time, date, 

location, ambient conditions, distance to target, estimated effective emissivity, 

scanner serial number, and additional stored comments) serve as important 

inputs and subsequent backup for the written report. New software to aid the 

thermographer in the efficient and rapid preparation of professional looking 

reports is also available from most manufacturers of thermal imagers (see 

Section 2). 


Appendix B 

Measuring Emissivity, Reflectance & Transmittance 


B.1 Introduction 

An infrared radiometer measures the sum of the emitted (We), reflected (Wr), 

and transmitted (Wt) energies coming from the target of interest. Figure B-1 

(repeated from Appendix A, Figure A-8) demonstrates this graphically. The 

sum of We + Wr + Wt is called Exitance or Radiosity. To determine the 

temperature of the target, the emitted energy must first be subtracted from the 

reflected and transmitted energies. This value must then be corrected to 

account for the emissivity of the target and to obtain a blackbody equivalent 

value. The blackbody equivalent value is then converted to temperature by 

referencing a calibration curve. All of the techniques discussed below for 

measuring emissivity, reflectance, and transmittance assume that the user 

has a thermal imager. Also note that the values for emissivity, reflectance, 

and transmittance are valid only for the spectral range of that instrument. 


Figure B-1 Target Radiosity 


B.2 Measuring Emissivity 

There are several common techniques for the measurement of emissivity 

using a single band radiometer, two of which are illustrated below. 

■ The first technique, known as the reference emitter technique, is 

accomplished by direct comparison with a known emitter at the same 

temperature. 

■ The second technique, known as the reflective emissivity technique, is 

accomplished by calculating emissivity indirectly using measured values of 

reflectance (and transmittance if applicable). 


The reference emitter technique works well when the target is at a different 

temperature than the background, such as in the case of a steam inlet valve 

whose body is at system operating temperature, while the applied emissivity 

reference is at the same temperature as the target. 

The reflective emissivity technique works well for smooth surfaces such as an 

electrical connection. The reflective emissivity technique is independent of 

target temperature, although the temperature of the target must remain 

constant throughout the measurement. 

A third, field-type method for estimating the effective emissivity of a specific 

target under specific conditions, is described in Section 3.3.3 and is illustrated 

in Appendix C, Plate 5. 


B.2.1 Reference Emitter Technique 

The reference emitter technique assumes both that the transmittance through 

the target is zero, and that a constant temperature difference between the 

target and the background is maintained. Ideally, this temperature difference, 

either hotter or colder, should be in the range of at least 15°F to 25°F. If the 

target is colder than the background, it should be above the dew point so that 

condensation on the surface of the target cannot occur. 

The reference emitter technique will only work if a reference emitter is applied 

to the surface of the target. 

Good reference emitters (E) are foot-powder, dye check developer, or black 

electrician’s tape, as previously discussed in sections 4.1.2 through 4.1.4. 

The procedure for determining the effective emissivity of a target using the 

reference emitter is as follows (refer to Figure B-2): 


1. Apply the reference emitter (E) to a portion of the target (an area of at least 

one square inch is normally adequate). 

2. Set the imager to measure isotherm units. 

3. Measure the background thermal level (B) adjacent to the target. Do this 

by placing a piece of cardboard to which is applied a crumpled, flattened 

piece of aluminum foil. Take this measurement over a large area of the foil. 

(An area of at least one square foot is normally adequate.) 

4. Measure the target thermal level (T). 

5. Measure the reference emitter level (R). The reference emitter must be in 

thermal equilibrium with the target. This thermal equilibrium condition will 

be apparent when the reference emitter thermal level is not changing. (In 

the case of dye check developer, its application cools the surface as the 

propellant evaporates. Wait at least 15 minutes after application unless the 

target is very warm.) 

6. Calculate the emissivity by using the equation: Emissivity=(T-B)/(R-B) 

7. Measure the emissivity several times. Determine the final value by taking 

an average of all measured emissivity values. 


Figure B-2 Using the Reference Emitter Technique 


B.2.2 Reflective Emissivity Technique 

The reflective emissivity technique involves measuring the reflectance of the 

target and subtracting it from 1.0 (emissivity = 1 minus target reflectance). 

ε= 1-ρ 

The procedure for determining emissivity using the reflective emissivity 

technique works best when dealing with highly reflected or mirrored surfaces, 

such as mirror insulation, and when dealing with pipes or electrical contacts. 

Some of these surfaces naturally have a low emissivity. In this technique, the 

target should not be coated with a reference emitter and must be kept at a 

constant temperature. Also, once a range is chosen for measuring 

temperature, both measurements must be made on that range. This 

technique is temperature independent. The emissivity, using the reflective 

emissivity technique, is calculated from the ratio of the thermal level 

differences. The procedure for determining the reflective emissivity technique 

follows (refer to Figure B-3). Note: The temperatures of the two sources must 

be constant and with a substantial spread between them (15°F to 25°F). 


1. Establish that the two sources are at different temperatures and are 

thermally stable. This can be adequately accomplished with a hand-held 

contact pyrometer. The exact temperature of each surface does not need 

to be known, only the ΔT. The ΔT, however, is limited by the temperature 

range of the imager. 

2. Aim the imager at each source and measure the direct isotherm levels (S a 

and S b ). 

3. Reposition the imager so that the sources are reflected off the target. 

Measure the reflected isotherm levels (T a and T b ). In most situations, this 

requires reflecting one source at a time (the exception is when they are 

reflected off a large uniform surface). 

4. Calculate the target reflectance: Reflectance = (T a -T b ) /(S a -S b ) To ensure 

that the data is reliable, take the average of several of these 

measurements over several parts of the surface, particularly if the surface 

is non-uniform in appearance. The exception to this is when an imager, 

either directly or through software, allows an area to be defined and 

averaged. 


Figure B-3 Using the Reflective Emissivity Technique 

Reflectance = (T a -T b ) /(S a -S b ) 


B.2.3 Transmittance Measurement 

The transmittance of non-opaque targets is measured similar to the 

reflectance measurement technique. As shown in Figure B-4, two sources are 

again used. In this case, the target is placed directly in front of the two 

sources rather than reflected off of it. To calculate transmittance, substitute 

the reflected levels in the equation cited previously for reflectance (Section 

B.2.2) with the transmitted thermal levels. 


Figure B-4 Using the Transmittance Technique (Measuring 

Transmittance) 

Transmittance = (T a -T b ) /(S a -S b ) 


B.2.4 Generic Emissivity Values 

Table B-1 lists broadband, generic normal emissivity values for several 

common materials (repeated from Section 4, Table 4-1. These values should 

only be used as references until the user can compile a library of values 

based on actual measurements. 

Table B-1 Normal Emissivity Values of Common Materials 


Appendix C 

Quick Reference Charts And Plates 


Calculating Instantaneous Field Of View, Quick Calculation 


MTF Determination 

Using An Ir Imager 


Minimum Resolvable 

Temperature Difference 

(MRTD) Estimate Using An Ir 

Imager 


Measuring And Setting Effective Emissivity Using An Imager Or A Point 

Sensor 


MEASURING IFOV meas OF AN IMAGER 

USING THE SLIT RESPONSE FUNCTION (SRF) 


Classification Of Faults (Guidelines) Relating To 50% Of Maximum Load 

Joule.s Law: P = I 2 R. Use this to proportion the temperature rise to 50% of the 

load. 

For example: 

At 20% of load, an 8°C rise is seen. To proportion it to 50% of load, multiply 

by the square of the load ratio as follows: 

(50/20)² = 6.25; 6.25 x 8°C = 50°C equivalent temperature rise 




End Of Reading Three 


Reading: Four 

Emissivity: Understand the difference 

between apparent and actual IR 

temperatures 


http://reliableplant.com/Read/14134/emissivity-underst-difference-between-apparent,-actual-ir-temps

Emissivity: Understand the difference between 

apparent and actual IR temperatures 

Taking infrared temperature measurements is certainly a lot easier than it 

used to be. The tricky part is understanding when an infrared reading is 

accurate as is and when you need to account for certain properties of the 

materials you’re measuring, or for other things like heat transfer. 

The most common use of infrared temperature measurement is for the 

inspection of electrical power distribution equipment. Let’s look at a typical 

three-phase fused power disconnect (Figure 1) and the corresponding 

infrared image (Figure IR1) below. 



Figure 1 shows a typical three-phase fused power disconnect. The 

corresponding infrared image, figure IR1, was taken with the emissivity 

setting at 1 on our thermal imager. The temperature span and color scale for 

the infrared image is set to 95.5 ºF referring to black, with warmer 

temperatures indicated progressively by blue (105 ºF), green (115 ºF), red 

(125 ºF) and white (133 ºF and hotter). We also measured the load in phase 

A, B and C (from left to right), at approximately 34 amps each. 

A simple analysis of the thermal image indicates that Phase A is significantly 

hotter than phases B and C. The fuse clip at the top of Phase A indicates 

133.4 F, while the end of the fuse, specifically the metal cap of the top of the 

fuse, appears much cooler with a temperature of 103.6 ºF and the fuse body 

just below the cap appears to be 121.9 ºF. 



Figure 1: Fused power disconnect. 



Figure IR1: Corresponding infrared image. 



Can this be true? Is the metal cap only 103 ºF? No. You are seeing an 

example of the apparent temperature and the effect of emissivity. The fuse 

end cap is a highly reflective metal, in this case copper. Notice that the body 

of the fuse also appears hotter than the metal cap. The temperature of the 

cap is actually as hot as the fuse body it’s in contact with. 

To explain why the apparent temperature seen through a thermal imager can 

be significantly different than the actual temperature, let’s review our 

knowledge of physics. 



Thermal radiation and properties of materials 

All objects emit infrared (thermal) radiation. The intensity of the radiation 

depends on the temperature and nature of the material’s surface. At lower 

temperatures, the majority of this thermal radiation is at longer wavelengths. 

As the object becomes hotter, the radiation intensity rapidly increases and the 

peak of the radiation shifts towards shorter wavelengths. The relationship 

between total radiation intensity (all wavelengths) and temperature is defined 

by the Stefan-Boltzmann Law: (broad band) 

Q = eσT 4 

where: 

Q = radiation intensity 

e = emissivity of material 

σ = Stefan-Boltzmann constant 

T = absolute temperature 



At a given temperature, the maximum radiation is achieved when the object 

has an emissivity of 1. This is referred to as blackbody radiation, because 

with an emissivity of 1, the object is a perfect radiator. However in our real 

world, there are no true blackbodies – that is, no perfect radiators. Since real 

materials are less than perfect radiators, the relevant issue is “how much less 

than perfect are they?” Emissivity is defined as the measure of how much 

less than perfectly a material radiates when compared to a blackbody. But, 

emissivity is only one of three factors that cause an object to be less than a 

perfect radiator. 



The thermal nature of materials. 

Materials (objects in everyday life, whether they be solids, liquids or gases) 

are constantly affected by their surroundings. Thermally, all objects attempt to 

exchange energy with other objects in their natural drive toward thermal 

equilibrium with their surroundings. In this search for thermal equilibrium, heat 

is exchanged between objects via three mechanisms: conduction, convection 

and radiation. 

Conduction is defined as heat transfer between two solid bodies that are in 

physical contact with each other. Convection is heat transfer usually between 

a solid material and a liquid or gas. Conduction and convection are 

dependent on physical contact between materials. Radiation is a process of 

heat transfer, characteristic of all matter (at temperatures above absolute 

zero). Radiation passes through a vacuum and can also pass through gasses, 

liquids and even solids. 



When radiative power is incident on an object, a fraction of the power will be 

reflected (ρ), another portion will be absorbed (α), and the final portion will be 

transmitted through the object (τ). The transmitted fraction is τ. All of this is 

described by the Total Power Law: 

ρ + α + τ = 1 

where: 

ρ = fraction reflected 

α = fraction absorbed 

τ = fraction transmitted 

The ability of an object to absorb radiation is also related to its ability to emit 

radiation. This is defined by Kirchhoff's Law 

α = ε 

where 

α = absorbance coefficient 

ε = emissive coefficient 



So in plain English, when the thermal imager observes the thermal radiation 

from real objects, part of what the thermal imager sees is reflected from the 

surface of the object, part is emitted by the object, and part may be 

transmitted through the object. In our example of a steel part, the 

transmission is zero (opaque, τ = 0), but to the degree that the part is 

reflective, it is less emissive and therefore real objects will usually appear 

cooler than they actually are. Except when there is something hotter in the 

vicinity; since with opaque materials, the lower the emissivity, the higher the 

reflectivity. The result in this case is materials appear to be hotter than they 

actually are! Let’s examine some real objects to illustrate these effects. 



Applying emissivity to real objects 

In the figure IR1 example, not only is the fuse end cap temperature actually 

much hotter than the 103.6 ºF that it appears, the hot spot above it is most 

assuredly hotter than the 133.4 ºF that it appears. 

So, how much hotter might it be? This fused power disconnect is electrically 

energized, so let’s conduct a simple experiment with a metal part that is not 

electrically energized. Note: While this experiment may not be shocking, it 

can still burn you. 

Picture a round stainless steel block sitting at ambient temperature. 



Figure 2: Stainless steel block. (at ambient temperature) 



Observed with our thermal imager (with emissivity set to 1), the metal 

appears to vary in temperature from about 74 ºF to 87 ºF. This seems to 

make sense, since the block could have picked up a little heat from our hands 

during handling. Actually, the metal block is very uniform in temperature. The 

apparent hot spot is a reflection of my face on the surface of the metal. Can 

you see my eye glasses in the image? (Figure IR2) 

Figure IR2: Thermal image of stainless steel block. 



Observed with our thermal imager (with emissivity set to 1), the metal 

appears to vary in temperature from about 74 ºF to 87 ºF. This seems to 

make sense, since the block could have picked up a little heat from our hands 

during handling. Actually, the metal block is very uniform in temperature. The 

apparent hot spot is a reflection of my face on the surface of the metal. Can 

you see my eye glasses in the image? (Figure IR2) 

Figure IR2: Thermal image of stainless steel block. 



Can you see my eye glasses in the image? (Figure IR2) 


Can you see my eye glasses in the image? 


https://en.wikipedia.org/wiki/Douglas_MacArthur

The block appears to vary in temperature from about 92 ºF to 110 ºF – and 

you can see the image of my face in the warm metal surface even more 

clearly than before. Using a thermocouple, we measure the surface 

temperature and find that it’s actually 169 ºF (see Figure 2a). 

Figure 2a: DMM with thermocouple, measuring surface temperature of the 

steel block. 



How can the thermal imager’s readings appear reasonable when the metal 

part is at room temperature and be so wrong (still producing a mirror image of 

my face on the hot surface) when the part is 169 ºF? 

At room temperature, the block appears to be room temperature because the 

block is primarily reflecting the thermal radiation from everything around it. 

Since the ambient temperature in the room is in the 70s, the reflection from 

the surface of the block appears also to be similar. When the same part is 

heated in the oven, the part becomes much hotter than the surroundings, so 

the thermal imager is able to see an increase in radiant energy, albeit 尽然 

much lower in apparent temperature because of the low emissivity value of 

the surface. 

Let’s modify our experiment to better demonstrate what the thermal imager 

sees. We take another stainless steel block and paint half of it with a flat black 

paint (flat black paint has an emissivity of 1 or 0.98 to be a little precise) and 

bake it (in a slightly warmer oven) another three hours (Figure 3, Figure IR3). 



Figure 3: Steel block, left side painted black. 

Figure IR3: Corresponding thermal image of steel block. 



When we remove the block from the oven this time, the unpainted side 

appears to be 92 ºF, but the thermal imager now indicates the painted sided 

to be 198 ºF. We can make a very good estimation of the actual emissivity of 

this material by observing the unpainted surface with our IR camera and 

adjusting the emissivity value on the thermal imager until the reading matches 

the temperature observed on the painted side. In this case, the emissivity is 

found to be approximately 0.12. 

Assumed 

ε = 0.98 

Adjusted 

ε = 0.12 



Emissivity is a cantankerous 脾气坏且抱怨不休的 variable 

As we’ve seen, emissivity varies by surface condition, but also by viewing 

angle, and even by temperature and by spectral wavelength. A table of 

common emissivity values is published in the operating manual for your 

thermal imager. The table should be considered only a rough guide in 

estimating an emissivity value to use with any particular material. If actual 

temperature values are required, it is best to perform experiments as 

described here, to properly characterize the emissivity for the material and its 

application. 

The two most common techniques for providing a higher emissivity reference 

surface are the application of a flat black high emissivity paint to the surface 

(as discussed in the previous section), or application of common black 

electrical tape to the material’s surface. Both black electrical tape and flat 

black tape have an emissivity of approximately 0.96. Another option is to use 

an infrared thermometer with adjustable emissivity, and a contact probe, 

adjusting the emissivity until the contact probe and infrared temperature 

displays equilibrate. 



In this experiment we see that the difference between the apparent 

temperature on the unpainted side and actual temperature is an error of 106 

ºF. If we were to conduct a similar experiment with a high-temperature 

infrared sensor, and examine steel at 2,000 ºF, the error between the actual 

and apparent temperatures could be more than 400 ºF. Of course, neither 

black paint or tape could survive 2,000 ºF. It’s often useful to use a narrow 

spectral band similar to the wavelength of the object’s radiant energy. 

Wien’s displacement law helps us determine the peak wavelength of the 

object’s peak radiant energy for an object at a certain temperature. 

λ max = b / T 

where: 

λ max = peak wavelength of radiant energy 

b = 2897 μm/ °K 

T = temperature (Kelvin) 



Wien’s Displacement Law 


http://www.sun.org/encyclopedia/electromagnetic-spectrum



When you are working with high-temperature materials, you can greatly 

reduce the errors due to uncertainty in emissivity by selecting infrared 

detectors that operate at narrow wavelength bands at shorter wavelengths. 

The math and physics necessary to prove this is beyond the scope of this 

application note. However, calculations demonstrate that by choosing an 

infrared sensor with a wavelength band close to 1 μm (rather than the 8 μm 

to 14 μm spectral band used by most thermal imagers), the maximum 

difference between the 2,000 ºF actual and apparent temperatures would be 

closer to 50 degrees (without knowing the precise emissivity of the material 

with better certainty). 

(the reflected low temperature spectrum is filter out leaving the high energy 

narrow wavelength representing the high temperature λ max ) 

To summarize: Temperature measurement without knowledge in this case 

would result in an error of more than 400 ºF. Making the same measurement 

with knowledge would reduce the error to 50 ºF, with no better determination 

of the material’s emissivity. 



Using a narrow band filter 

to measure this 

temperature range 

The ambient reflection 

ρ is filter out 



Discussion 

Subject: Are the following statement true? 

• Is the absolute emittance (≠ emissivity, ε) of an object constant with 

disregard of reflectance ρ? 

• Is the 1= ε + ρ + τ, a weighted ratio of three contributing factor meant for 

IR thermographic measurement purpose and not a physical property of the 

object? 

• What ever the reflectance be (by surface conditioning, texture, shielding, 

by raising the T amb to T obj etc.) , the emittance from the object is always the 

same as long the T obj remain the same? 

• Could be say that will reflectance, transmittance coupled with the object 

emissivity, the actual power radiating from the object is higher than the 

black body? 


Emissivity, the variable’s variable! 

Back to our steel block example, let’s discuss another very significant 

phenomena. We will take our unpainted metal block and drill three holes part 

way into the body. All three holes are one-eighth of an inch in diameter. The 

first is one-eighth-inch deep, the second is one-fourth-inch deep, and the third 

is three-eighths-inch deep. 

Figure IR4: Thermal image of steel block with three holes. 



Bake the block for another three hours, then remove the block and observe it 

again with the camera. Interestingly, the hot block surface appears to be 

about 84 F, and now appears to have three hot spots. 

■ 

■ 

■ 

The one-eighth-inch deep hole appears to be 106 ºF. 

The one-fourth-inch deep hole appears to be 112 ºF; and 

the three-eighth-inch deep hole appears to be 125 ºF. 

We know that the metal block is actually about 175 ºF (measured by a 

thermocouple) and the surface finish is uniform and has an emissivity of 

approximately 0.12. The reason the temperature appears to be higher in the 

holes is that a hole in a body enhances the emissivity. The greater the 

depth/diameter ratio of the hole, the greater the emissivity enhancement. By 

adjusting the emissivity on the thermal imager to match the actual 

temperature at each hole, we find that the emissivity appears to be 0.25 for 

the one-eighth-inch deep hole. The emissivity of the one-fourth-inch deep 

hole appears to be 0.35 and the three-eighth-inch deep hole appears to have 

an emissivity of 0.45. This is an extremely important effect. Let’s look at 

another piece of electrical equipment to see why. 



Emissivity and electrical equipment 

In Figures 5 and IR5, you see another power disconnect with the conductors 

bolted in place using Allen head bolts. The corresponding infrared image 

shows a hot connection on the middle phase. 

Figure 5: 3-phase power disconnect. 



Figure IR5: Corresponding thermal image. 

Notice the apparent hot spot in the hot Allen socket head. The well of the bolt 

head appears hotter primarily because the well illustrates the blackbody effect 

of a hole. 



In manufacturing processes, steel or aluminum rolls are often used to heat or 

cool a material such as in paper or plastic film processing. These rolls are 

usually polished metal surfaces, and it’s important to understand the thermal 

profile since the manufacturing process depends on thermal uniformity across 

the rolls. The temperature of these rolls can be difficult to measure with a 

thermal imager because they have very low emissivities. However, there are 

often points where the material passes between two rolls. The tangent point 

between two rolls also tends to simulate the blackbody effect, allowing for 

effective temperature measurement in an otherwise difficult situation. 

This effect is illustrated in common electrical equipment as well. Look at 

Figure 6. 



Figure 6: Power 

disconnect with knife 

blade connectors. 

Figure IR6: 

Corresponding thermal 

image. 



In this case, we have another power disconnect with knife blade switches. 

This type of switch utilizes shiny metal blades, and the proximity of the blades 

with narrow gaps simulates the blackbody effect for greatly improved effective 

emissivity. The important message here is to develop your understanding of 

apparent and actual temperature measurement. Actual temperature 

measurement requires an intimate understanding of physics, heat transfer 

and characteristics of materials. 

Aimed here 



Qualitative vs. quantitative infrared thermography 

Emissivity difficulties are not a barrier to effectively using infrared 

thermography for predictive maintenance (PdM). ASTM standards exist to 

guide thermographic PdM inspections. These standards describe the use of 

thermal imagers for qualitative and quantitative infrared inspections. 

Quantitative infrared inspections require determining the emissivity of each 

component, to make accurate temperature measurements possible. This 

practice may not always be necessary for routine inspections, unless the 

exact temperature value is needed for long term tracing. Qualitative methods, 

in contrast, allow you to leave the emissivity at 1.0 and evaluate the 

equipment on a relative basis: Has it changed, or is it different? The basis for 

qualitative evaluation is comparing similar equipment under similar loads. 

Looking back at Figure 1 and IR1, you can see that there is little value to be 

gained in spending time estimating or debating the emissivity of the various 

parts in the power disconnect. The value is in understanding that Phase A is 

hotter than phase B and C. In addition to realizing that a phase is hotter, it is 

essential to measure the load of the three phases. 



Figure IR1: Corresponding infrared image. 

there is little value to be gained in 

spending time estimating or debating the 

emissivity of the various parts in the 

power disconnect. The value is in 

understanding that Phase A is hotter than 

phase B and C. 

A B C 



Greater electrical load inherently means more heat is present 

W = I 2 R 

where 

W = power in watts (heat) 

I = current in amps 

R = resistance in ohms 

The first rule of thermography in predictive maintenance PDM is to compare 

comparable equipment under comparable loads. In electrical power 

distribution, comparable equipment is usually the easy part since each 

electrical phase is usually similar in materials to the phase next to it. Load is a 

very different matter. Figure 7 illustrates an electrician measuring the 

electrical load. 



Figure 7: Measuring the loads on a power disconnect. 



So, just observing that there is a hot spot does not indicate a problem. 

Electrical components can be appropriately hot for the electrical load and 

conditions. If you measure the loads, you can determine if the presence of a 

thermal anomaly indicates a problem. Thermal imagers do not identify 

thermal problems – trained, knowledgeable, qualified people make educated 

assessments of equipment. This leads to real value in preventive 

maintenance and reduced frequency of equipment breakdowns. 



Summary 

Predictive maintenance PDM with a thermal imager can be effectively 

performed by utilizing qualitative analysis of equipment. 

Qualitative techniques allow the emissivity setting on the thermal imager to be 

kept at 1.0 and apparent temperatures used for comparisons between similar 

equipment under similar load. With basic training, most technicians can 

reliably perform qualitative analysis. 

Quantitative infrared analysis requires a deeper understanding of thermal 

theory and application to be truly effective. It refers to the attempt to measure 

actual temperatures of materials using infrared thermography. Actual 

temperature measurement involves more than simply adjusting for emissivity. 

Total incident radiance requires dealing with the effect of reflection and 

transmission in addition to emissivity. 

Today’s thermal imagers are becoming increasingly affordable and easy to 

use. But, what does easy mean? The practice of infrared thermography looks 

straight forward and simple; but it has its tricks. It is much like most 

endeavors in life: the more you learn, the more you discover that there is 

more to learn. 



It is much like most endeavors in life: the more you learn, the more you 

discover that there is more to learn. 


It is much like most endeavors in life: the more you learn, the more you 

discover that there is more to learn. 


Good Luck 


Good Luck 


Good Luck 


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

Chong/ Fion Zhang

Understanding Infrared Thermography Reading 7 Part 2 of 2.pdf

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