Understanding infrared thermography reading 7 part 2 of 2

Infrared Thermal Testing Reading VII Part 2 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.

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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

• Reporting requirements

• Definitions

• Qualification of personnel

• Scope

• Scheduling

• Responsibilities

• Equipment matrix

• Precautions

• References

• Prerequisites • Conduct of the Survey • Acceptance criteria Charlie Chong/ Fion Zhang

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.

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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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IR Viewing Window

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IR Viewing Window – Opaque Polymer Grill

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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.

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

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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.

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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.

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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

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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.

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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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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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• •

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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.

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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)

1°F to 15°F rise

Intermediate (Level 2)

16°F to 50°F rise

Serious (Level 3)

51°F to 100°F rise

Critical (Level 4)

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:

9.3

•

Summary of inspection and findings

•

Equipment list

•

Data sheets with IR and visible photographs of anomalies

•

Root cause analysis and corrective action

• Comments 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)

Alert Operations—potential failure. Correct ASAP.

Critical (Level 4)

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.

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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: • • • 12.2

Pumps/motors Steam traps Valves

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.

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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

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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.

Charlie Chong/ Fion Zhang

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

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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.�

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7.2 Levels of Qualification The recommended Levels of Qualification for infrared thermographers follow those of traditional NDE methods. These levels are as follows: â– 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?)

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â– 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. â– 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.

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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.

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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

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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 ď Ž Understand the concepts of radiosity and associated parameters. ď Ž Be able to measure emissivity, reflectance, transmittance, background temperature, foreground temperature, and target temperature. ď Ž Be cognizant of potential errors in the measurement of the above parameters, caused by variation across the target surface.

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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.

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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.

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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.

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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.

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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.

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The Expert!

Appendix-A The Science Of Thermography (Practical Application Of Thermographic And Thermal Sensing Equipment)

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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.

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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.

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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.

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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.

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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.

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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)

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Table A-1 Temperature Conversion Chart Instructions for Use: 1.

2.

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

If the temperature to be converted is in 째C, scan to the right column for the 째F equivalent.

3.

If the temperature to be converted is in 째F, scan to the left column for the 째C equivalent.

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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.

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Figure A-1 Conductive Heat Flow

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The Fourier Conduction Law:

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

= K∙ΔT∙A / L

Where: Q/A = the rate of heat transfer through the slab per unit area perpendicular to the flow L = the thickness of the slab = the higher temperature (at the left) T1 = the lower temperature (at the right) T2 K = the thermal conductivity of the slab material

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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, T1 and T2, 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.

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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.)

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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.

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Figure A-2 Convective Heat Flow

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A.2.6 Radiation Radiative heat transfer is unlike the other two modes in several respects: 1.

2.

3.

4.

It can take place in a vacuum.

It occurs by electromagnetic emission and absorption.

It occurs at the speed of light.

The energy transferred is proportional to the fourth power of the temperature difference between the objects (ΔT4 or T4?) .

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?)

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Figure A-3 Infrared in the Electromagnetic Spectrum

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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.

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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.

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Figure A-4 Radiative Heat Flow

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Figure A-4 Radiative Heat Flow

Wε = σεTe4 Wρ = σρTr4 Wτ = στTt4

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Figure A-5 Radiation Exchange at the Target Surface

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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.

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Specular or Diffuse Surfaces

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Specular or Diffuse Surfaces

Diffuse Reflector

Specular Reflector?

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

Specular and Diffuse Surfaces Diffuse Reflector

Specular Reflector?

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Specular and Diffuse Surfaces

Confused Specular Reflector?

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Specular or Diffuse Surfaces

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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 θ2 = θ1 in the figure), and that the incident, normal, and reflected directions are coplanar.

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Reflections off Specular and Diffuse Surfaces

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Reflections off Specular and Diffuse Surfaces

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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.

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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.

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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.

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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.

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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.

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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 (�T4). 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.

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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.

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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.

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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

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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.

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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

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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.

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Figure A-6 Blackbody Curves at Various Temperatures

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位m = b/T = (2897/T 渭m)

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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 = εδT4 and Wien’s Displacement Law (2):

λm = 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) λm = Wavelength of maximum radiation (µm) b = Wien’s displacement constant = 2897 (µm∙K)

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According to (1), the radiant energy emitted from the target surface (W) equals two constants multiplied by the fourth power of the absolute temperature (T4) 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*).

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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 (λmax = 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.

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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)

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

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

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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.(?)

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Figure A-8 Components of Energy Reaching the Measuring Instrument

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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.

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

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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/

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

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

EXAM score!

Non-graybody (colored body or real body)

for for my my ASNT ASNT exam exam

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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.

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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.

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

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

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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 ¾m 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.

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Figure A-11 Infrared Spectral Transmission of Glass

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Figure A-11 Infrared Spectral Transmission of Glass

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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 ¾m 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.

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

EXAM score!

Glass is opaque λ > 5µm at 30ºC?

for my ASNT exam

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Figure A-12 shows the spectral transmission characteristics of several of these materials, many of which transmit energy past 10 Âľm. 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.

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Figure A-12 Characteristics of IR Transmitting Materials

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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

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IR Lenses – Sapphire Lens

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

IR Lenses – LWIR Len

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

IR Lenses – Fresnel Len

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

IR Lenses – Fresnel Len

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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.

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Figure A-13 Components of an Infrared Radiation Thermometer

Thermal or photon detector, single element or FPA.

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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

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Infrared optics are available in two general configurations, refractive and reflective; â– 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. â– 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.

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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 Âľm 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.

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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=hัต) (photon detector?)

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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 ﾂｵm 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 = ﾎｵﾏサ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.

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Figure A-14 Typical Infrared Radiation Thermometer Schematic

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Germanium Len

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Germanium Len

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Germanium Len

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Thermopile Detector

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Thermopile Detector

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Thermopile Detector Charlie Chong/ Fion Zhang

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

Thermopile Detector

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

Thermopile Detector

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Thermopile Detector

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

Thermopile Detector

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

Thermopile Detector 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. GridEYE 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.

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

Thermopile Detector

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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.

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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 mercuryCadmium-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.

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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.

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Figure A-15 Spectral Sensitivity of Various Infrared Detectors

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Thermal Detectors & Photon Detectors

Photon Detector

Thermal Detector

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The Mercury- Cadmium-telluride (Hgcdte) Detector,

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Discussion Subject: Why there are many curves for HgCdTe.

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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.

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

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

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http://spie.org/x91246.xml

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

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

Discussion Subject: Why it wasn’t pixel-like correspond to the spatial resolution of 106?

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

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

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

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

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

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

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

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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.

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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 A16). 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.

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Figure A-16 Scanning Configuration of an Infrared Line Scanner

The line scanner could be a single element or linear array detector.

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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.

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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.

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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.

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

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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 Âľm) region instead of in the visible spectrum. Electronically scanned thermal imaging systems based on pyrovidicons and operating in the 8.14 Âľm atmospheric window are in common use today. They provide qualitative thermal images and are classified as thermal viewers.

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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.

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

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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.

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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.

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

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.

Charlie Chong/ Fion Zhang

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

Staring Charlie Chong/ Fion Zhang

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.

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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 Charlie Chong/ Fion Zhang

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.

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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).

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EXAM score!

Temperature sensitivity is also called thermal resolution or noise equivalent temperature difference (NETD).

for my ASNT exam

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EXAM score!

thermal resolution (≠spatial resolution) for my ASNT exam

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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.

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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.

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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: â– 63% â– 95%

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Figure A-19 Instrument Speed of Response and Time Constant

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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.

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Figure A-20 Fields of View of Infrared Radiation Thermometers

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Figure A-20 Fields of View of Infrared Radiation Thermometers

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Figure A-20 Fields of View of Infrared Radiation Thermometers

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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.

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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.

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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 ¾m atmospheric window. For hotter targets, shorter operating wavelengths are selected, usually shorter than 3 ¾m.

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.

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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 ¾m 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

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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.

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Figure A-21 Spectral Filtering for Polyethylene Temperature Measurement

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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

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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 Âľm, dictates the use of a 7.9 Âľm spike filter placed in front of the same broadband detector.

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Figure A-22 Spectral Filtering for Polyester Temperature Measurement

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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.

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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:

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 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)

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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 ¾m or the 8.12 ¾m 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.

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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.

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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. IFOVmeas 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.

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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: â– the MRT or MRTD defines temperature sensitivity but also implies spatial resolution (IFOV) (for 2D thermography; both thermal viewer & thermal radiometric imaging) . â– MRTD is expressed as a function of angular spatial frequency. (?)

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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.

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Figure A-23 Test Setup for MRTD Measurement, MRTD Curve

heated background

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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 = (Lmax- Lmin) / (Lmax + Lmin) L = luminosity?

Modulation transfer is the ratio of the modulation in the observed image to that in the actual object.

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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:

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â–

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.

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Figure A-24 Modulation Transfer Function, Imager Spatial Resolution

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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 1째C. MTF is seen to be about 0.22 for this system.

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Figure A-25 MRTD and MTF for a System Rated at 1.0 Milliradian

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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.

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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 IFOVgeometric : D = σ∙d, σ (IFOV) = D / d

D

σ d for IFOVmeas = Dmeas /d

Dmeas

σ d

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Figure A-26 Setup and Curves for Slit Response Function Test

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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 IFOVmeas of the imager under test.

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FPA

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http://spie.org/x34358.xml

FPA

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FPA

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FPA

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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.

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Figure A-19 Instrument Speed of Response and Time Constant

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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.

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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.

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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

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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).

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Appendix B Measuring Emissivity, Reflectance & Transmittance

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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.

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Figure B-1 Target Radiosity

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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. â– The first technique, known as the reference emitter technique, is accomplished by direct comparison with a known emitter at the same temperature. â– The second technique, known as the reflective emissivity technique, is accomplished by calculating emissivity indirectly using measured values of reflectance (and transmittance if applicable).

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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.

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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):

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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.

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Figure B-2 Using the Reference Emitter Technique

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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).

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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 (Sa and Sb).

3. Reposition the imager so that the sources are reflected off the target.

Measure the reflected isotherm levels (Ta and Tb). 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 = (Ta-Tb) /(Sa-Sb) 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.

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Figure B-3 Using the Reflective Emissivity Technique Reflectance = (Ta-Tb) /(Sa-Sb)

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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.

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Figure B-4 Using the Transmittance Technique (Measuring Transmittance)

Transmittance = (Ta-Tb) /(Sa-Sb)

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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 Charlie Chong/ Fion Zhang

Appendix C Quick Reference Charts And Plates

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Calculating Instantaneous Field Of View, Quick Calculation

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MTF Determination Using An Ir Imager

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Minimum Resolvable Temperature Difference (MRTD) Estimate Using An Ir Imager

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Measuring And Setting Effective Emissivity Using An Imager Or A Point Sensor

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MEASURING IFOVmeas OF AN IMAGER USING THE SLIT RESPONSE FUNCTION (SRF)

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Classification Of Faults (Guidelines) Relating To 50% Of Maximum Load

Joule.s Law: P = I2R. 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 Charlie Chong/ Fion Zhang

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End Of Reading Three

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Reading: Four Emissivity: Understand the difference between apparent and actual IR temperatures

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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.

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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.

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Figure 1: Fused power disconnect.

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Figure IR1: Corresponding infrared image.

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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.

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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 = eĎƒT4 where: Q = radiation intensity e = emissivity of material Ďƒ = Stefan-Boltzmann constant T = absolute temperature

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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.

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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.

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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

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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.

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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 ÂşF that it appears, the hot spot above it is most assuredly hotter than the 133.4 ÂşF 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.

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Figure 2: Stainless steel block. (at ambient temperature)

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Observed with our thermal imager (with emissivity set to 1), the metal appears to vary in temperature from about 74 ยบF to 87 ยบF. 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.

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Observed with our thermal imager (with emissivity set to 1), the metal appears to vary in temperature from about 74 ยบF to 87 ยบF. 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.

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Can you see my eye glasses in the image? (Figure IR2)

Can you see my eye glasses in the image?

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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.

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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).

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Figure 3: Steel block, left side painted black.

Figure IR3: Corresponding thermal image of steel block.

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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

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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.

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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. λmax = b / T where: λmax = peak wavelength of radiant energy b = 2897 μm/ °K T = temperature (Kelvin)

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Wien’s Displacement Law

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http://www.sun.org/encyclopedia/electromagnetic-spectrum

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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 λmax) 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.

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Using a narrow band filter to measure this temperature range The ambient reflection Ď is filter out

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http://www.sun.org/encyclopedia/electromagnetic-spectrum

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 Tamb to Tobj etc.) , the emittance from the object is always the same as long the Tobj 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?

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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.

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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.

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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.

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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.

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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.

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Figure 6: Power disconnect with knife blade connectors.

Figure IR6: Corresponding thermal image.

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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

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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.

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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

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B

C

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Greater electrical load inherently means more heat is present W = I2 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.

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Figure 7: Measuring the loads on a power disconnect.

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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.

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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.

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It is much like most endeavors in life: the more you learn, the more you discover that there is more to learn.

Charlie Chong/ Fion Zhang

It is much like most endeavors in life: the more you learn, the more you discover that there is more to learn.

Charlie Chong/ Fion Zhang

Good Luck

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Good Luck

Good Luck

Charlie Chong/ Fion Zhang

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