Attachment_6_-_TOP_6_3_040_dtd_20_Jul_2009.pdf

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1. REPORT DATE (DD-MM-YYYY)

20-07-2009

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Final

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4. TITLE AND SUBTITLE

Test Operations Procedure (TOP) 6-3-040 Thermal Imaging Systems

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6. AUTHORS

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

TEDT-RT-TT

US Army Redstone Technical Test Center Redstone Arsenal, Alabama 35898-8052

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REPORT NUMBER

TOP 6-3-040

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Test Business Management Division (TEDT-TMB) US Army Developmental Test Command 314 Longs Corner Road Aberdeen Proving Ground, MD 21005-5055

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Same as item 8

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Approved for public release; distribution unlimited.

13. SUPPLEMENTARY NOTES

Defense Technical Information Center (DTIC), AD No.:

This TOP supersedes ITOP 6-3-040, dated 14 November 2000

14. ABSTRACT

This TOP describes the instrumentation and procedures for measuring the optical and electro-optical characteristics of military thermal imaging systems.

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a. REPORT B. ABSTRACT C. THIS PAGE Unclassified Unclassified Unclassified 19b. TELEPHONE NUMBER (include area code)

US ARMY DEVELOPMENTAL TEST COMMAND

TEST OPERATIONS PROCEDURE

DTIC AD No.:

Test Operations Procedure (TOP) 6-3-040 20 July 2009

THERMAL IMAGING SYSTEMS

Page

PARAGRAPH 1. SCOPE

1.1 Purpose

1.2 Limitations and Assumptions

2. FACILITIES AND INSTRUMENTATION

3. REQUIRED TEST CONDITIONS

3.1 Preliminary Remarks

3.2 Pre-Test Planning

3.3 Preparation

3.4 Test Item

4. TEST PROCEDURES

4.1 Minimum Resolvable Temperature Difference (MRTD)

4.2 Minimum Detectable Temperature Difference (MDTD)

4.3 Signal Transfer Function (SiTF)

4.4 Noise Equivalent Temperature Difference (NETD)

4.5 Operational Readiness Time (ORT)

4.6 Field-of-view (FOV)

4.7 Coincidence of Fields-of-view

4.8 Linearity

4.9 Modulation Transfer Function (MTF)

5. DATA REQUIRED

5.1 General

5.2 Uncertainty Analysis

6. PRESENTATION OF DATA

APPENDIX A. DEFINITIONS .................................................................................... A-1 B. ADDITIONAL PROCEDURES ......................................................... B-1 C. REFERENCES .................................................................................... C-1

* This TOP supersedes ITOP 6-3-040, dated 14 November 2000.

Approved for public release; distribution unlimited.

TOP 6-3-040

20 July 2009

1. SCOPE.

1.1 Purpose.

This TOP describes the instrumentation and procedures for measuring the optical and electro-optical characteristics of military thermal imaging systems.

1.2 Limitations and Assumptions.

This TOP comprises optical and electro-optical procedures that are predominantly for laboratory evaluation. However, some field test procedures are also included. The test procedures included in this TOP should be applied to the performance parameters specified for the test item. Not all of the procedures will be required for every test item. It is assumed that the operator has a working knowledge of laboratory practices and the principles of optics and thermal imaging. It is further assumed that the test operator is familiar with the test item’s characteristics. It is essential that appropriate guidelines and standard operating procedures for safety are followed.

This TOP is intended to cover both scanning and staring focal plane array military thermal imagers operating in the mid-wave infrared (3-5µm waveband) and long-wave infrared (8-14µm waveband) spectral regions. Applications of this TOP are subject to test equipment variations dependent upon the specific type of thermal imager. Collimator efficiencies are usually given or measured as radiometric quantities, but are typically implemented as a multiplying factor of the source temperature. This discrepancy is a common source of error which can be minimized by maintaining a constant ambient temperature. The field test procedures should be used when the thermal imaging sensor/subsystem cannot be separated from the host vehicle; however, the accuracy obtainable in field testing may not reach that obtainable in the laboratory. The references contained in Appendix C are for information purposes only and to aid understanding of the test methods to be applied.

2. FACILITIES AND INSTRUMENTATION.

Due to the significant number of procedures described in this TOP, the specific facilities and instrumentation guidance are covered within each associated test procedure.

3. REQUIRED TEST CONDITIONS.

3.1 Preliminary Remarks.

Although this section provides a general overview of the required test conditions, specific guidance for this subject is covered within each associated test procedure.

3.2 Pre-Test Planning.

a. Test Plan: Prior to initiating the test program a test plan must be prepared. The test plan shall:

(1) Describe each test to be performed in sufficient detail to be understood by potential test personnel and approving authorities.

(2) Describe the measurements to be made, the test criteria, verification method, data required, data reduction and analysis techniques to be used.

(3) Describe the test set-up for each test and the instrumentation to be used.

(4) Identify the required environmental conditions, profiles and limits for each test, as well as the number and location of environmental condition sensors.

b. The pre-test planning shall also include:

(1) Adequate safety procedures for test personnel.

(2) A brief to test personnel on all aspects of the test program, including the purpose of each test, the measurement requirements, and the preparation and operation of all test instrumentation.

(3) Sufficient copies of all test item and instrumentation operating instructions and safety procedures.

3.3 Preparation.

3.3.1 Facilities.

Preparation of test facilities shall include:

a. Adequate lead time to ensure the facility will be available for the duration of the test program.

b. Assurance that facility environmental equipment is in proper operating condition.

c. In terms of laboratory requirements, conformance to optical darkroom standards is essential, as is good floor stability, although extreme levels of vibration damping are not normally required. However, care must be taken to ensure that the levels of vibration, air turbulence, temperature variation etc, do not interfere with resolution measurements (e.g.

MRTD, MDTD, and MTF). The working area should provide easy access to all parts of the system.

d. If the laboratory area is clean and dry, clean room conditions (temperature, dust and humidity control) are not normally required. Unless otherwise specified, an ambient temperature of 20 °C to 25 °C and a relative humidity of about 50% are generally satisfactory. For some measurements, control of environmental conditions may be required to meet test plan requirements.

3.3.2 Instrumentation.

a. Prior to initiating the test program:

(1) Ensure that all test instrumentation is in proper operating condition.

(2) Ensure that test personnel are adequately trained to operate the test instrumentation. For measurement of MRTD a trained observer should be used.

(3) Ensure all test instrumentation is available for the test program.

(4) Ensure that the calibration of all test instrumentation is current and will extend through the test period.

(5) Ensure that the accuracy of all test instrumentation will meet the test requirements. This is referred to as Test Accuracy Ratio (TAR) of the measurement parameter.

b. The use of automated test instrumentation should be considered during the test planning process.

Specific test instrumentation requirements are listed in the individual test procedures.

3.4 Test Item.

a. Prior to initiating the test program:

(1) Inspect the test item for damage, completeness, deterioration, or manufacturing defects and record any deficiencies.

(2) Ensure that the test item is in proper operating condition.

(3) Ensure that operating manuals for the test item are available.

b. Provide a description of the test item(s). If available, the description should include, but not be limited to, the following:

(1) A description of the intended use of the test item.

(2) A description of each operating mode of the test item.

(3) Photographs of the test item mounted in the host vehicle and, if possible, outside the host vehicle.

(4) The test item specifications for each attribute to be measured.

(5) The physical characteristics (i.e. weight and dimensions) of the test item.

(6) A schematic diagram of the test item, its components and its interface to other sub-systems in the host vehicle.

(7) A description of all control features of the test item.

c. Test personnel shall be adequately trained to operate the test item and be familiar with all operating modes and controls.

4. TEST PROCEDURES.

Note: for any measurements where optical alignment or image resolution or quality is involved, the test item must be mounted in such a fashion that vibrations are dampened to normal field operational levels. In some cases, when sensors are hard mounted in lab environment, these vibrations are more pronounced than that found in operational environments and are of such a magnitude that they adversely impact the measurement values. This is especially true when dealing with sensors that have scanning mirrors or other vibration sources.

4.1 Minimum Resolvable Temperature Difference (MRTD).

4.1.1 Scope.

The purpose of this test is to measure the thermal resolution of a thermal imaging device as a function of spatial frequency for each test item field-of-view, in a laboratory environment (reference STANAG 43491**

). Note that this reference is limited to analog or properly sampled thermal imaging sensors.

4.1.2 Facilities and Instrumentation.

a. Facilities.

Item Requirement Optical Laboratory Standard Equipment

MRTD Thermal Targets A differential blackbody source with a thermal range of -10 °C to + 30 °C from ambient and several 7:1 (height to width) aspect ratio 4-bar patterns bounding the published specification for MRTD when used with the below described collimator.

** Superscript numbers/letters correspond to those in Appendix C, References.

Collimator(s) Diffraction limited, capable of covering the span of spatial frequencies specified for the test item, using the MRTD thermal targets. The waveband of operation must accommodate the waveband of the test item. The average radiometric efficiency (collimator transmission/reflection and atmospheric transmission) in the waveband of the test item must be known. The collimator aperture must be sufficient to overfill the entrance pupil of the test item at the collimator/test item separation (collimator working distance).

b. Instrumentation.

Device Measurement Accuracy Collimated MRTD Thermal Targets

Spatial frequency: ± 5%, Temperature: better than 10% of the thermal resolution specified for the test item, or 0.01 °C, whichever is larger.

4.1.3 Test Conditions.

a. The laboratory ambient temperature should be set at the recommended ambient background temperature for the test item and maintained to control residual drift. Steps must be taken, by shielding or by other means, to avoid significant local background temperature variations or transient fluctuations.

b. Mount the test item in a fixture so that an observer can view the image of the target.

The test item is positioned so that it views the target through a collimator as shown in Figure 1.

Ensure that the test item is within the collimator working distance. The target plate is placed in front of a blackbody source and positioned at the focal point of the collimator. The collimator shown in Figure 1 is an off-axis paraboloid (OAP). The temperature difference between the blackbody source (target) and the target plate (background) provides the thermal contrast. The temperature difference between the blackbody seen through the apertures in the target plate and the solid parts of the target plate is the target-to-background temperature difference. The fundamental spatial frequency of the bar target is defined by the period of the 4-bar target and collimator focal length.

Thermal Imager

Test Item

Black body Target plate

Targets are a series of four bar, 7:1 aspect ratio apertures.

Observer

Display

OAP

Figure 7. Typical MRTD Test Configuration.

c. The MRTD is the minimum target-to-background temperature difference at which the 4-bar target can just be resolved by a trained observer. A Forward MRTD is measured with the blackbody temperature above the background. A Reverse MRTD is measured with the blackbody temperature below the background.

d. The Forward and Reverse MRTD values are measured at different spatial frequencies by using a selection of standard 4-bar targets. The standard 4-bar target patterns range in size to represent spatial frequencies typically ranging from 0.1FN to just beyond FN, where FN = Nyquist frequency (cy/mr).

e. The temperature difference between the background and the blackbody is varied between -10 °C and 30 °C for the Forward and Reverse MRTD. The target plate is placed at the focus of a collimator so that the correct spatial frequency is presented. Forward and reverse measurements are required in order to null the effects of the emissivity of the background/target.

Emissivity will be cancelled out if the measurements are averaged. Care must be taken to maintain the correct positive and negative values in all calculations.

f. The horizontal MRTD is measured with the vertical bar pattern placed in the on-axis position (OA) and, if required, in one or more off-axis FOV positions. These are defined in Figure 2, where OA = on-axis and UR = upper right, etc. The horizontal separation between positions UL and UR or LL and LR and the vertical separation between positions UL and LL or UR and LR will typically subtend 80% of the field-of-view.

g. If required, this measurement can also be made with horizontal bar patterns for Vertical MRTD.

Figure 1. Typical MRTD Test Configuration.

UL

LL LR

UR

OAHorizontal MRTD Target Orientation

Vertical MRTD Target Orientation

Figure 8. Horizontal and Vertical MRTD Target Orientations and MRTD FOV Positions.

4.1.4 Test Procedures.

a. Use an observer who is considered / recognized as trained in the area of resolving 4-bar targets to determine the MRTD. The observer’s visual acuity shall be at least 3/6 or equivalent. While viewing the target on the display in a dark-adapted environment, permit the observer to adjust all control settings and background illumination conditions for optimum image quality. Once set, the white-hot/black-hot polarity setting should be recorded and retained for the duration of the test. The target plate (background) temperature shall be homogeneous and close to the laboratory ambient temperature. Record the laboratory ambient temperature and the target plate temperature (if different from ambient).

b. The criterion to be used for the MRTD measurements is that it should be possible to just resolve 75% to 100% of the area of the bars and spaces between the bars (not just some modulation) on the display, 50% of the time. It is not necessary that the whole of each bar be visible at the same time.

c. MRTD is calculated to give apparent temperature difference values and is given by MRTD = η∆T; where η is the radiometric efficiency and ∆T is the actual target-to-background temperature difference (°C).

d. For each combination of target spatial frequency, orientation and field-of-view position (see Figure 2), perform the following operations to obtain values for Forward and Reverse MRTD.

(1) Adjust the temperature of the blackbody to make the target bar pattern temperature much higher than the target plate (background) temperature so that the bars are clearly distinguishable.

Figure 2. Horizontal and Vertical MRTD Target Orientations and MRTD FOV Positions.

(2) Optimize the test item display controls to obtain the best image. The test item position may be slightly adjusted in pitch and/or yaw to obtain the maximum visibility of the image. Any aliasing effects (e.g. changes in bar width or spacing) should be noted and, if possible, photographed.

(3) Change the blackbody temperature towards the ambient temperature until the bars are clearly unresolvable.

(4) Slowly change the temperature of the blackbody away from the ambient temperature.

(5) Slowly increase the temperature until the bar pattern is just discernable and the top and bottom of the pattern can be identified and the relative width of the spaces and bars are approximately equal. This is the point where the measurement should be taken.

(6) Calculate and record the minimum apparent temperature difference between blackbody and background for which the target bar pattern is just distinguishable (Forward

MRTD).

(7) Adjust the temperature of the blackbody to make the target bar pattern temperature much lower than the background and clearly distinguishable.

(8) Repeat steps 2 through 5, except record as the Reverse MRTD.

(9) Calculate the MRTD by taking half the difference of the Forward and Reverse MRTDs.

(10) Where multiple observations are made by a single observer, calculate and record the arithmetic mean MRTD of the observations.

e. If more than one observer is required, repeat the test as described in paragraph 4.1.4.d for each observer.

f. Calculate the geometric mean of the MRTD for the observers and record as the overall geometric mean MRTD at each combination of target spatial frequency, orientation and field-of-view position. (The geometric mean is used because the observer-to-observer variability typically follows a log-normal distribution.) Typical observer-to-observer variations in MRTD are shown in Figure 3.

Spatial Frequency (cy/mr)

Figure 9. MRTD Observer Variation.

- Observer A

- Observer B

- Observer C

M

RT

D (l og te m pe ra tu re

4.1.5 Data Required.

a. Number of observers.

b. Radiometric efficiency.

c. Test item field-of-view.

d. Target plate temperature (if different from ambient).

e. Laboratory ambient temperature.

f. Target orientation.

g. Target spatial frequencies used.

h. Calculated geometric mean of MRTD values for each combination (arithmetic mean if one observer).

4.1.6 Presentation of Data.

a. For each target orientation, provide a table of the calculated MRTD values as illustrated in Table 1.

Figure 3. MRTD Observer Variation.

Table 1. MRTD Measurements.

MRTD MEASUREMENTS

Number of Observers: Target Plate Temperature:

Target FOV Position: Test Item FOV:

Lab Ambient Temperature: Radiometric Efficiency:

Spatial Freq (cy/mr) Overall Mean MRTD at FOV Position

Horizontal MRTD Vertical MRTD 0.1

b. For each target field-of-view position, provide curves of the horizontal and vertical MRTD as a function of target spatial frequency, as illustrated in Figure 4. The curves are fitted to the overall mean values from Table 1.

Figure 10. Typical MRTD Curves.

Spatial Frequency (cy/mr)

MRTD

(Vertical)

MRTD

(Horizontal)

M

RT

D (l og te m pe ra tu re

4.2 Minimum Detectable Temperature Difference (MDTD).

4.2.1 Scope.

The purpose of this test is to measure the thermal resolution of a thermal imaging device as a function of spatial frequency, for each test item field-of-view, in a laboratory environment.

Figure 4. Typical MRTD Curves.

4.2.2 Facilities and Instrumentation.

MDTD Thermal Targets A differential blackbody source with a thermal range of -10 °C to + 30 °C from ambient and several square, circular, or equilateral triangular targets.

Collimator(s) Diffraction limited, capable of generating targets varying in size from 0.1 to 10 times the test item’s detector angular subtense, using the MDTD thermal targets. The waveband of operation must accommodate the waveband of the test item. The average collimator efficiency (transmission/reflection) in the waveband of the test item must be known. The collimator aperture must be sufficient to overfill the entrance pupil of the test item at the collimator/test item separation (collimator working distance).

Collimated MDTD Thermal targets

Angular subtense: ± 5%, Temperature: better than

0.1 of the thermal resolution specified for the test item, or 0.01 °C, whichever is larger.

4.2.3 Test Conditions.

a. The laboratory ambient temperature should be set at the recommended ambient background temperature for the test item and maintained to control residual drift. Steps must be taken, by shielding or by other means, to avoid significant local background temperature variations or transient fluctuations.

b. Mount the test item in a fixture so that an observer can view the image of the target.

The test item is positioned so that it views the target through a collimator as shown in Figure 1.

Ensure that the test item is within the collimator working distance. The target plate is placed in front of a blackbody source and positioned at the focal point of the collimator. The collimator shown in Figure 1 is an off-axis paraboloid (OAP). The temperature difference between the target plate and the blackbody source provides the thermal contrast. The temperature difference between the blackbody seen through the aperture in the target plate and the solid part of the target plate is the target-to-background temperature difference. The angular subtense of the target is defined by the width (square), diameter (circle), or base-to-apex distance (triangle) of the target and the collimator effective focal length.

c. The temperature difference between the target plate (background) and the target blackbody is varied between -10 °C and 30 °C.

4.2.4 Test Procedures.

a. Use an observer who is considered / recognized as trained in the area of resolving targets to determine the MDTD. The observer’s visual acuity shall be at least 3/6 or equivalent.

While viewing the target on the display in a dark-adapted environment, permit the observer to adjust all control settings and background illumination conditions for optimum image quality.

The background temperature of the MDTD target shall be homogeneous and close to the laboratory ambient temperature. Record the laboratory ambient temperature and the target plate temperature (if different from ambient).

b. The target extent should be varied between 0.1 and 10.0 times the Detector Angular Subtense (DAS).

c. Adjust the temperature of the blackbody to make the target temperature much higher than the background, so that the target is clearly distinguishable.

d. Optimize the test item display controls to obtain the best image. The test item position may be slightly adjusted in pitch and/or yaw to obtain the maximum visibility of the image.

e. Lower the blackbody temperature towards the ambient temperature until the target is clearly undetectable.

f. Increase the blackbody temperature above the ambient temperature at a rate which is easily controlled. Record the target temperature above background (Forward MDTD) at which the observer can just detect the target. Repeat the test for each target size.

g. If the forward MDTD for any target size is less than 1°C, then decrease the target temperature below the ambient, at a rate that is easily controlled. Record the target temperature below ambient (Reverse MDTD) at which the observer can just detect the target. Calculate the MDTD for the observer for the target size by taking half the difference of the Forward and Reverse MDTDs. Where multiple observations are made by a single observer, calculate and record the arithmetic mean MDTD of the observations.

h. If more than one observer is used, repeat the above test for each combination in paragraphs 4.2.4c to 4.2.4h.

i. Calculate and record the geometric mean MDTD of the observations. (The geometric mean is used because observer-to-observer variability typically follows a log-normal distribution.)

4.2.5 Data Required.

a. Number of observers.

b. Radiometric efficiency.

c. Test item field-of-view.

d. Laboratory ambient temperature.

e. Target Plate temperature (if different from ambient).

f. Target description (square, circular or triangular).

g. Target angular subtense used.

h. Calculated geometric mean of MDTD values (arithmetic mean if one observer).

4.2.6 Presentation of Data.

a. MDTD data for each target size shall be presented in tabular form as shown in Table 2.

Table 2. MDTD Measurements.

MDTD MEASUREMENTS

Number of Observers: Radiometric Efficiency:

Target Plate Temperature:

Lab Ambient Temperature: Test Item FOV:

Reciprocal Target Subtense (1/mr)

Target Description Mean MDTD(°C)

b. The MDTD data will be graphically displayed as a function of target size as shown in Figure 5. The curve is fitted to the overall mean values from Table 2.

Figure 5. Typical MDTD Curve.

4.3 Signal Transfer Function (SiTF).

4.3.1 Scope.

The purpose of this test is to measure the relationship of the temperature difference between the target and the background, to either the test items analog or digital output video signal or display luminance.

4.3.2 Facilities and Instrumentation.

Collimated thermal target Size: approximately 0.1 of the test item field-of-view. Temperature: -10°C to +30°C relative to background.

Digital Data Acquisition Device (digital video output frame grabber)

Dynamic range > test item range.

Bandwidth > test item bandwidth.

Digital Oscilloscope (for analog video signals)

Dynamic range > test item range.

Bandwidth > 5 times test item bandwidth.

Photometer (for illuminance measurements)

Dynamic range > test item display range.

4.3.3 Test Conditions.

a. Mount the test item in a fixture so that an observer can view the target.

b. The thermal target will be heated to a sufficient temperature difference so that all observers can readily discern the target pattern.

c. Where SiTF is measured as part of NETD (Section 4.4), this must be within one half hour before the NETD measurement.

4.3.4 Test Procedures.

a. Present the collimated thermal target to the test item with the target image near the center of the raster. Measure and record the output video or luminance corresponding to the displayed target with a suitable data acquisition device or a photometer.

b. Adjust the target temperature such that there are at least 5 data points over the linear unsaturated part of the SiTF curve. For each data point, permit the temperature to stabilize and record the peak output video signal.

c. For cases where contrast and brightness controls are used to vary the video output, repeat for at least six combinations of contrast and brightness settings; three with variable brightness (min, mid, max) and fixed mid-point contrast, and three with variable contrast (min, mid, max) and fixed mid-point brightness. These combinations are illustrated in Figure 6.

4.3.5 Data Required.

a. Test item brightness and contrast settings.

b. Background temperature.

c. Target temperatures relative to background temperature.

d. Measured values for output video signal or the luminance of the display at each target temperature setting.

Figure 6. Variations in Video Output for SiTF.

4.3.6 Presentation of Data.

a. Tabular presentation of required data for each combination as illustrated in Table 3.

Note that Table 3a is for output video signal measurements and Table 3b is for luminance measurements.

Table 3a. SiTF Analog or Digital Video Measurements.

SiTF MEASUREMENTS

Background temperature (°C):

Brightness Setting: Contrast Setting:

Trial Target temperature relative to background temperature (°C)

Signal Voltage (V) or Digital Counts

N

Table 3b. SiTF Luminance Measurements.

SiTF MEASUREMENTS

Background temperature (°C):

Brightness Setting: Contrast Setting:

Trial Target temperature relative to background temperature (°C)

Luminance (cd/m2)

b. For each combination, provide a graphical representation of the output video signal or the display's luminance as a function of the target’s temperature difference relative to the background temperature. An example is shown for the voltage measurement in Figure 7.

4.4 Noise Equivalent Temperature Difference (NETD).

4.4.1 Scope.

The purpose of this test is to measure the high frequency component of the random temporal noise associated with the thermal imager output, relative to the SiTF. The SiTF value (the slope of the SiTF curve in the linear region of response as shown in Figure 7) obtained from section

4.3 is required. Depending on the thermal imager, the noise measurement can be made in at least one of the following three ways:

a. Using the RMS voltage noise from the analog video output,

b. Using the RMS digital count noise from the digital video output, or

c. Using the RMS illuminance noise of the thermal imager’s display.

The method chosen must coincide with the method chosen for the SiTF measurement. It should be noted that voltage or digital count measurements are preferred to display photometric measurements for RMS noise. The procedure is not suited to field testing.

4.4.2 Facilities and Instrumentation.

Figure 7. SiTF Curve.

Felt or Foam Cover Must be uniform in temperature and emissivity and large enough to cover the imager’s entrance aperture.

Digital Data Acquisition Device (digital video output frame grabber)

Dynamic range > test item range.

Bandwidth > test item bandwidth.

Digital Oscilloscope (for analog video signals)

Dynamic range > test item range.

Bandwidth > 5 times test item bandwidth.

Photometer (for illuminance measurements)

Dynamic range > test item display range.

4.4.3 Test Conditions.

a. Mount the test item in a fixture so that a cover can be placed over the entrance aperture of the imaging system.

b. The SiTF must have already been measured (within the last one-half hour) and the SiTF value is readily available for the NETD calculation (see section 4.3). When calculating the NETD, the SiTF value is taken as the slope of the SiTF curve in the linear region of response (near the small signal temperature differential).

c. The region of interest within the test item FOV is specified or assumed for the NETD calculation processing (e.g. the center 1/3 x 1/3 of the field-of-view).

Note: Historically, NETD was defined at the output (analog) of the post amplifier. For this classical measurement, a simple filter was added with a 3-dB break frequency equal to the reciprocal of twice the detector dwell time. For system measurements as described in this TOP, NETD is defined at the analog video output to the monitor or the digital video output, where this classical external filter is not used.

4.4.4 Test Procedures.

a. Place the felt or foam cover over the entrance aperture of the test item to present a uniform out-of-focus source, so that the imager’s output is a constant signal with maximum gain.

b. For voltage measurements, measure the RMS noise of the analog output video signal (e.g. RS-170, CCIR) of the test item using a digital oscilloscope.

(1) Set up the digital oscilloscope to capture the first video line in the region of interest.

(2) Set up the digital oscilloscope to capture only that portion of the video line corresponding to the region of interest for processing (the sync, reference, front porch, back porch, etc. signals must not be included).

(3) Collect (capture or calculate) a time-averaged waveform of at least 256 frames of the video line region of interest and save this as the time-averaged waveform.

(4) Subtract the time-averaged waveform from 4 consecutive waveforms, resulting in 4 pedestal removed waveforms.

(5) Fit a 2nd order polynomial curve to each of the four pedestal removed waveforms and subtract it from each of the respective waveforms to remove the low frequency “trend” component of the video.

(6) Calculate the standard deviation of the samples for each of the 4 waveforms that have been processed for pedestal and trend removal.

(7) Calculate the average value of the 4 standard deviations and record this as the RMS voltage noise for the selected video line.

(8) Repeat steps 2 through 7 for each video line in the region of interest.

(9) Calculate the system RMS voltage noise by taking the average of the RMS voltage noise of all the video lines; recording this as Vrms_noise[Volts].

(10) Calculate and record the NETD using the RMS voltage noise and the corresponding SiTF value from paragraph 4.4.3b, using the equation:

[ ]C Deg VoltsSiTF

VoltsV NETD rms_noise= Celsius

c. For digital count measurements, the RMS noise is measured on the digital output imagery of the test item.

(1) Capture a minimum of one second of continuous digital frames from the region of interest and store in a three dimensional data set whose size is T x V x H. This data set is denoted UTVH. T is the number of frames captured, V is the number of vertical pixels and H is the number of horizontal pixels. V and H are determined by the specified region of interest, where V begins and ends with the first and last row of the region of interest respectively; and H begins and ends with the first and last column of the region of interest respectively.

(2) Calculate a two dimensional average in the T dimension (i.e. a frame average of the data set UTVH). Subtract the two dimensional average from each two dimensional VxH ‘frame’ of UTVH resulting in a new three dimensional data set with the temporal average removed. The size of the new data set is still T x V x H, and is denoted UtTVH.

(3) Calculate a two dimensional average in the V dimension on UtTVH. This two dimensional average shall be subtracted from each two dimensional TxH ‘frame’ of UtTVH resulting in a new three dimensional data set with the temporal and vertical averages removed.

The size of the new data set is still T x V x H, and is denoted UtvTVH.

(4) Calculate a two dimensional average in the H dimension on UtvTVH. This two dimensional average shall be subtracted from each two dimensional TxV ‘frame’ of UtvTVH resulting in a new three dimensional data set with the temporal, vertical and horizontal averages removed. The size of the new data set is still T x V x H, and is denoted UtvhTVH.

(5) If necessary, remove the low frequency portion of UtvhTVH, which is sometimes referred to as trends. It is suggested that this be accomplished by subtracting UtvhTVH by a 2nd order least squares three dimensional fit to UtvhTVH.

(6) After removal of the low frequency trends, calculate the standard deviation of the data set UtvhTVH and record the value as σrms[Counts].

(7) Calculate and record the NETD using the RMS digital counts noise and the

][ CDeg sCountSiTF sCountσ NETD rms= Celsius

d. For luminance measurements, measure the RMS luminance of the display using a photometer that has a measurement spot of, at most, one-tenth of the impulse response width of the imager system displayed on the monitor.

(1) Record the RMS luminance as Lrms_noise[cd/m2]

(2) Calculate and record the NETD using the RMS luminance noise and the

][ CDeg cd/mSiTF cd/mL

NETD

rms_noise= Celsius

e. When calculating the Noise Equivalent Temperature Difference, the SiTF value (see paragraph 4.4.3b) is taken as the slope of the SiTF curve in the linear region of response (near the small signal temperature differential), for normal brightness and contrast settings usually specified or assumed. The specified combination of brightness and contrast settings should be recorded, if available.

4.4.5 Data Required.

a. Test item settings.

b. Region of interest within the FOV.

c. Ambient temperature.

d. System NETD.

4.4.6 Presentation of Data.

NETD is presented in tabular form as illustrated in Table 4.

Table 4. Noise Equivalent Temperature Difference (NETD).

NETD MEASUREMENTS

Test Item Settings: Region of Interest within the FOV:

Ambient Temperature (degrees C): NETD (degrees C):

4.5 Operational Readiness Time (Warm-up Time).

4.5.1 Scope.

The purpose of this test is to measure the time interval between switch on and readiness for use for a thermal imaging system, in a laboratory environment.

4.5.2 Facilities and Instrumentation.

Optical laboratory Standard Equipment

MRTD thermal targets A differential blackbody source with a thermal range of + 30 °C from ambient and several 7:1 (height to width) aspect ratio 4-bar patterns.

Collimator Diffraction limited, capable of covering the span of spatial frequencies specified for the test item, using the MRTD thermal targets. The waveband of operation must accommodate the waveband of the test item. The collimator aperture must be sufficient to overfill the entrance pupil of the test item.

Collimated MRTD thermal targets

Spatial frequency ± 5%.

Timer (stopwatch) ± 0.1s over timed interval.

4.5.3 Test Conditions.

a. Ensure that the MRTD target source is adequately shielded to avoid local background temperature variations.

b. Mount the test item in a fixture so that the observer can view the image of the target through the collimator. Note that for image comparison purposes, one observer should be used throughout the test.

c. Ensure that the collimator aperture overfills the entrance pupil of the test item.

4.5.4 Test Procedures.

a. Switch on the test item and allow it to stabilize. Permit the observer to adjust all control settings to obtain an image.

b. Set the target condition to the specified spatial frequency and temperature difference.

When there is no specification for these parameters, select a target pattern which is close to, but distinctly within the thermal resolution of the test item.

c. Present the collimated thermal target to the test item. Position the target at the center of the field-of-view. Adjust the test item control settings to achieve good display contrast without saturation.

d. With the image quality and position established as described, switch off the thermal target and the test item. Leave the test item in the off condition until the detector has reached ambient temperature (this is highly dependent on the test item and can range from several hours to overnight). Do not change the test item and laboratory conditions during this period.

e. Switch on the thermal target at the previous settings and allow the temperature to stabilize.

f. Verify that the ambient temperature is the same as at first switch on. Switch on the test item and use the timer to record the interval between switch on and the point at which the image quality reaches the level previously established at step c (the operator may adjust control settings if needed). The recorded period is the operational readiness time (maximum).

Note: If the test item has a standby mode of operation, the procedure may be repeated by timing the interval between this condition and the attainment of established image quality. The recorded period is the operational readiness time (standby).

4.5.5 Data Required.

a. The spatial frequency of the selected thermal target pattern.

b. The differential target temperature and the ambient temperature.

c. The operational readiness time (maximum and/or standby).

4.5.6 Presentation of Data.

A tabular listing of the required data as shown in Table 5.

Table 5. Operational Readiness Time.

OPERATIONAL READINESS TIME (ORT)

Ambient Temperature:

Target Spatial Frequency (cy/mr)

Differential Target Temperature (K)

ORT Maximum/Standby* (seconds)

* Delete as appropriate

4.6 Field-of-View (FOV).

4.6.1 Scope.

The purpose of this test is to measure the field(s)-of-view of a thermal imaging device. The test will be conducted in a laboratory environment when the test item is removable from the host vehicle. When the test item cannot be removed from the host vehicle, a procedure is outlined for field testing.

4.6.2 Facilities and Instrumentation.

and/or

Test Range ≥500m line-of-sight

Collimated thermal point, edge or line source

Collimated beam > entrance pupil diameter of the test item.

Distinguishable from background

Rotary Table (pitch and yaw)*

Sufficient size to support the test item or collimator Rotation ≥ 110% of test item FOV Resolution ≤ 2 arc min

Accuracy ≤ 1 arc min

Distance measuring equipment Target range ±0.5 m

*Note: It is possible to use a single axis rotary table that can be reconfigured to accommodate both pitch and yaw angles.

4.6.3 Test Conditions.

a. Laboratory Tests.

(1) Securely mount the test item or collimator on the rotary table. Position the test item and collimator such that the axis of rotation of the rotary table is perpendicular to their optical axes and is in the plane of the entrance pupil of the test item, as shown in Figure 8.

(2) Position the collimator system such that the collimated beam overfills the entrance pupil of the test item under all rotation conditions.

(1) The point or line source must have a sufficient temperature difference so that the observer can readily discern the target from the background.

(3) The optics of the test item must be focused with the image at the center of one edge of the field-of-view.

b. Field Tests.

(1) If the test item cannot be removed from the host vehicle, then a test range is required where the observer can view a point source target at a range of 500 meters or greater.

(2) Position the test item using the host vehicle systems so that an observer can view the distant target.

(3) There should be a clear line of sight over the entire field-of-view of the test item.

(4) The optics of the test item must be focused with the image at the center of one edge of the field-of-view.

Figure 8. FOV Test Configuration.

4.6.4 Test Procedures.

a. Laboratory Tests.

(1) Present a collimated thermal target to the test item. Either the collimator must be rotatable in pitch and yaw or the test item must be rotatable about its entrance pupil (see Figure 14).

(2) Rotate the collimator/test item to produce an image at the center of the left-hand edge of the test item field-of-view. Record the collimator/test item position (angles) as indicated in Figure 9.

Figure 9. Image Positions for Horizontal FOV.

(3) Rotate the collimator/test item to produce an image at the center of the right-hand edge of the test item field-of-view. Record the collimator/test item position (angle) as indicated in Figure 9. The difference in the two recorded collimator/test item positions is the test item horizontal field-of-view (yaw angle).

Figure 10. Image Positions for Vertical FOV.

(4) Repeat the procedures of b and c but with the image at the center of the top and bottom edges of the test item field-of-view, as shown in Figure 10. The difference in the two recorded collimator/test item positions is the test item vertical field-of-view (pitch angle).

(5) Repeat the procedure of b to d for each required field-of-view of the test item.

b. Field Tests.

(1) In-Vehicle Method

(a) Position a thermal target at a distance of ≥ 500m from the test item with the test item installed on the host platform. The host platform must be horizontal.

NOTE: This method assumes there is test item line-of-sight pitch and yaw control on the host platform/vehicle. It is further assumed that relative line-of-sight angle positions can be monitored.

(b) Rotate the test item to produce an image at the center of the left-hand edge of the test item field-of-view. Record the test item position (angle) as indicated in Figure 9.

(c) Rotate the test item to produce an image at the center of the right-hand edge of the test item field-of-view. Record the test item position (angle) as indicated in Figure 9. The difference in the two recorded test item positions is the test item horizontal field-of-view (yaw angle).

(d) Repeat the procedures of (2) and (3) but with the image at the center of the top and bottom edges of the test item field-of-view, as shown in Figure 10. The difference in the two recorded positions is the test item vertical field-of-view (pitch angle).

(e) Repeat the procedures of (2) to (4) for each required field-of-view of the test item.

(2) Remote Marker Method.

(a) Focus the test item on an object at a range between 1000 and 1500 meters. An observer, using the thermal imaging system under test, shall direct an assistant to place a reference marker at a point corresponding to the center of the system reticle.

(b) Establish a line perpendicular to the test item line-of-sight. At the direction of the observer, the assistant shall move to the right along the established line until he/she is at the edge of the field-of-view. The assistant shall mark this position. Record the distance from this position to the reference mark. Calculate the field-of-view right half-angle as shown in Figure 11.

(c) Determine the distance to the left edge of the field-of-view in the manner described above. Calculate the left half-angle field-of-view.

(d) Record the system field-of-view as the sum of the right and left-half angles.

(e) Repeat the procedure of (2) to (4) for each required field-of-view of the test item.

Figure 11. Remote Marker Method for FOV.

4.6.5 Data Required.

a. Ambient temperature.

b. Calculated values for each required horizontal and vertical field-of-view for each field-of-view of the test item.

4.6.6 Presentation of Data.

The field-of-view data shall be presented in tabular form as illustrated in Table 6.

Table 6. Field-of-view.

FIELD-OF-VIEW MEASUREMENTS

Ambient Temperature:

Axis Field-of-View

Horizontal

Vertical

4.7 Coincidence of Fields-of-View.

4.7.1 Scope.

The purpose of this test is to measure the angular coincidence between the different fields-of-view of a thermal imaging system. Alternative methods are provided for this test. The test should be conducted in a laboratory environment when the test item is not installed in the host vehicle. When the test item is installed in the host vehicle, the alternative test method can be applied in the field environment. In any case, the most appropriate test method should be chosen.

4.7.2 Facilities and Instrumentation.

Optical laboratory Standard equipment and/or

Test range with remote thermal target

500m - 1000m line-of-sight

Collimated thermal point or crosshair target

Collimated beam > entrance pupil diameter of the test item, temperature high enough to make target easily distinguishable from background but not high enough to cause detectable image spreading in the test item.

Target subtense at test item to be just larger than central reticle or aiming mark, i.e. diameter of dot or width of crosshair.

Rotary table

(pitch and yaw)*

Sufficient size to support test item.

Rotation = ± 15 deg

Resolution ≤ 30 arc sec

Accuracy ≤ 15 arc sec

*Note: It is possible to use a single axis rotary table that can be reconfigured to accommodate both pitch and yaw angles.

4.7.3 Test Conditions.

a. Laboratory Test.

(1) Laboratory ambient temperature should remain constant within ±1 °C, during the conduct of this test unless otherwise required by the test plan.

(2) Securely mount the test item on the rotary table. Position the test item so that the axes of rotation of the rotary table are perpendicular to the optical axis of the test item.

(3) Position the collimator system so that the collimated beam overfills the entrance pupil of the test item for all rotation conditions.

(4) If the test range target can be viewed from the optical laboratory and is to be used for the test, position the rotary table so that there is a clear and unobstructed line of sight between the test item and the target location.

(5) The observer must be able to easily discern the point or crosshair target source from the background in each field-of-view.

(6) For conditions where the test item reticle pattern can be repositioned, the central reticle marks for each required field-of-view must be symmetrically superimposed prior to the test.

b. Field Test.

(1) The test plan should identify the test item temperature range, profile and limits. It should indicate the number and the location of the temperature sensors used for the conduct of this test.

(2) Position the test item using the host vehicle systems so that an observer can view the distant target.

(3) The point or crosshair target source must have a sufficient temperature difference so that the observer can easily discern the target from the background in each field-of-view.

(4) For conditions where the test item reticle pattern can be repositioned, the central reticle marks for each required field-of-view must be symmetrically superimposed prior to the test.

(5) Exercise care to avoid movement of the host platform/vehicle during the test, e.g.

rocking on its suspension.

4.7.4 Test Procedures.

a. Laboratory Test.

(1) Present the collimated thermal target or the test range target (> 500m from the test item) to the test item, as appropriate.

(2) Switch on the test item and permit the observer to adjust all control settings for optimum image quality. Verify that the central reticle marks of all required fields-of-view are aligned to the same target.

(3) Set the test item to its narrowest optical field-of-view. Using the rotary table, position the central reticle mark to be symmetrically superimposed on the target. If the test item is equipped with electronic zoom, then this feature may be used to optimize positioning. Note the readings (pitch and yaw) of the rotary table at this position.

(4) Without disturbing the set-up, change the test item setting to a wider field-of-view and observe the position of the central reticle mark relative to the target.

(5) If the center of the reticle mark in the wider field-of-view is displaced from the center of the target, use the rotary table to restore symmetrical…