Focal-plane array testing

(article under development)

Focal Plane Array testing (FPA testing) is the test engineering process of validation and verification (V&V) of operation of focal plane array imaging devices, device under test (DUT), at various levels of the development and/or production assembly process. V&V can be done internal to the DUT (detector array and readout circuit), such as with built-in self-test (BIST) or external, such as with automatic test equipment. Functional and environmental testing is part of V&V. An FPA is basically composed of a photon or phonon detector array and a readout integrated circuit. There are basically five processing steps performed by these two sub-components, including: getting the electromagnetic energy in the detector; generating a consequent charge in the detector; charge collection; charge to voltage conversion; signal transfer; and digitization. Testing at various levels of the entire process can filter out FPA's with excessive number of unit cell defects before the complete process is carried out.

Overview

FPA testing (automated or semi-automated) utilizes hardware and software to characterize the DUT by measuring parameters such as: signal transfer function; signal transfer function vs differential temperature; spatial noise power spectral density; noise equivalent temperature difference; modulation transfer function; RMS and fixed pattern noise; temporal noise; responsivity and detectivity; spectral response; crosstalk; minimum resolvable temperature difference; intensification gain; field of view; spatial resolution; dynamic range; focus adjustment; harmonization; alignment; distortion. Software analyses the DUT sensor images in real time, including array failure maps. Source temperature settings and target selection are computer-controlled. The software can integrate a complete test bench comprising collimator, source, optical table and data acquisition system.

Test & Evaluation primer

The research & development budget of focal plane arrays for U.S. Government applications (military, space, and otherwise) is overwhelming compared to commercial expenditures. It is appropriate to largely view focal plane array testing from a government test perspective.

In 1983, Congress established the Office of the Director of Operational Test and Evaluation (DOT&E) to coordinate, monitor, and evaluate operational testing of major weapon systems. As part of the Office of the Secretary of Defense, DOT&E is separate from acquisition (that also conducts developmental and operational testing) and therefore is in a position to provide the Secretary and Congress with an independent view. Congress created DOT&E in response to reports of conflicts of interest in the acquisition community's oversight of operational testing leading to inadequate testing of operational suitability and effectiveness and the fielding of new systems that performed poorly.

By law, DOT&E serves as the principal adviser on operational test and evaluation in DOD and bears several key responsibilities, including: 1) monitoring and reviewing all operational test and evaluation in DOD; 2) reporting to the Secretary of Defense and congressional committees whether the tests and evaluations of weapon systems were adequate and whether the results confirmed that the system is operationally suitable and effective for combat before a decision is made to proceed to full-rate production; and 3) submitting to the Secretary of Defense and congressional decision makers an annual report summarizing operational test and evaluation activities during the preceding fiscal year.

In 1993, DOD's advisory panel on streamlining and codifying acquisition laws concluded that DOT&E was impeding the goals of acquisition reform by: 1) promoting unnecessary oversight; 2) requiring excessive reporting detail; 3) inhibiting the services' discretion in testing; and 4) limiting participation of system contractors in operational tests where such involvement is deemed necessary by the services. The following year, DOD proposed legislative changes that would have reduced the scope and authority of DOT&E. These changes were directed at perceived rather than documented problems and if adopted would undermine a key management control over the acquisition process--independent oversight of operational test and evaluation.

Although the legislative proposals were not adopted, in 1995 the Secretary of Defense implemented several operational test and evaluation initiatives in the Department to: 1) involve operational testers earlier in the acquisition process; 2) use models and simulations effectively; 3) combine tests where possible; 4) combine tests and training; and 5) applying these initiatives to all acquisition programs. The goals of these initiatives included saving time and money by identifying and addressing testing issues earlier in the acquisition process; merging or closely coordinating historically distinct phases, such as developmental and operational testing to avoid duplication; and using existing technologies and training exercises to create realistic and affordable test conditions.

difference between “test” and “evaluation”

While the terms “test” and “evaluation” are most often found together (as in T&E) and frequently inadequately applied by defense contractors and others, these terms actually denote distinctly different functions in the research, development, test and evaluation process of DoD.

A “test” is any procedure that is designed to obtain, verify, or provide data for the evaluation of: 1) progress in accomplishing developmental objectives; 2) the performance, operational capability, and suitability of systems, subsystems, components, and equipment items; and 3) the vulnerability and lethality of systems, subsystems, components, and equipment items.

“Evaluation” denotes the process whereby data are logically assembled, analyzed, and compared to expected performance to aid in making systematic decisions. Evaluation is the process for review and analysis of qualitative or quantitative data obtained from design review, hardware inspection, modeling and simulation, testing, or operational usage of equipment.

“Test and Evaluation” is the process by which a system is exercised and results analyzed to provide performance information. The information has many uses including risk identification and risk mitigation and empirical data to validate models and simulations. T&E enables an assessment of the attainment of technical performance, specifications, and system maturity to determine whether systems are operationally effective, suitable, and reliable.

difference between “validation”, “verification”, and “accreditation”

Validation is the process of determining: 1) the manner and degree to which a model is an accurate representation of the real world from the perspective of the intended uses of the model; and 2) the confidence that should be placed on this assessment. Verification is the process of determining that a model implementation accurately represents the developer’s conceptual description and specifications. Accreditation is the official certification that a model or simulation is acceptable for use for a specific purpose.

management of test and evaluation

The Test and Evaluation process identifies levels of performance and assists the developer in correcting deficiencies. It is a significant element in the decision-making process providing data that support trade-off analysis, risk reduction, and requirements refinement. Program decisions on system performance maturity and readiness to advance to the next phase of development take into consideration demonstrated performance. The issue of paramount importance is: will it fulfill the mission? will it work? will it do what it was intended to do? The T&E process provides data that shows how the system is performing during development. The Program Manager must balance the risks of cost, schedule, and performance to keep the program on track. The responsibility of decision-making authorities centers on assessing risk tradeoffs.

development test and evaluation

Development Test and Evaluation (DT&E) is conducted to demonstrate that the engineering design and development process is complete. It is used to reduce risk, validate and qualify the design, and ensure that the product is ready. The results are evaluated to ensure that design risks have been minimized and the system will meet specifications. DT&E serves a critical purpose in reducing the risks of development by testing selected high-risk components or subsystems. DT&E is a 2nd or 3rd party tool used to confirm that the system performs as technically specified and that the system is ready for field testing.

operational test and evaluation

OT&E is conducted for major programs by an organization that is independent of the developing, procuring, and using commands. Some form of operational assessment is normally conducted in each acquisition phase. Each assessment should be keyed to a decision review in the material acquisition process. It should include typical user operators, crews, or units in realistic simulations of operational environments. The OT&E provides the decision authority with an estimate of: 1) The degree of satisfaction of the user’s requirements expressed as operational effectiveness and operational suitability of the new system; 2) The system’s desirability, considering equipment already available, and operational benefits or burdens associated with the new system; 3) The need for further development of the new system to correct performance deficiencies; 4) The adequacy of doctrine, organizations, operating techniques, tactics, and training for employment of the system; of maintenance support for the system; and of the system’s performance in the countermeasures environment.

FPA primer

A focal plane array uses the infrared detector for the first step in digital imaging (detection of light and collection of photocharge into pixels). The circuits are fabricated with the same methods that produce computer chips, but special amplifiers are required to sense the small packets of photocharge produced by weak sources. The silicon readout integrated circuit converts the charge to voltage with an amplifier in each pixel, and transfers the signal to the edge of the array. The analog-to-digital conversion can be done on the imaging array, or in the focal plane electronics. The pixels of the detector array are attached to the pixels of the readout via Indium interconnects - one Indium bump per pixel. Since the readout multiplexes the signals from each pixel to the off-chip electronics, the readout is a multiplexer (one of many functions provided by the readout).

The readout converts the individual pixel outputs into a pulse train thereby reducing the number of output wires. The frame rate is the number of times the array is read out per second. Frame rates of 30-60 Hz are typical. The frame rate must be small enough to avoid smearing the image and large enough to allow sufficient energy to be collected before read out. Readout analog-to-digital speed limits the frame rate. All pixels are exposed to the field of view for a fraction of each frame cycle. During this time the photoelectrons are collected in capacitors in the readout (called buckets). The storage capacity of the buckets limits the dynamic range and signal-to-noise ratio. An array of microlenses can be added to the FPA to increase its sensitivity and the fill factor.

detector

A focal-plane-array detector is any detector that has more than one row of detectors. The focal plane of an optical system is a point at which the image is focused. An array of detectors (FPA) is located at a point where the image is focused. Typical infrared FPA systems have an array of 256 x 256 detectors.

FPA detectors have high-resolution IR imaging capabilities. An array of detectors staring at the scene rather than a single detector being scanned across the scene means IR cameras can be much smaller, lighter, and more power efficient than a camera with elaborate scanning components.

Not all the surface of the detector is sensitive to IR energy. Around the rows and columns of individual IR detectors making up the array is an inactive region surrounding each of the detector. The inactive areas can serve as pathways for electronic signals. The ratio of active IR sensing material to inactive row and column borders is called the fill factor. An ideal detector would have a very high fill factor because it would have a large percentage of its area dedicated to collecting IR photons and a very small area dedicated to detector segregation. Typical infrared FPA systems have fill factors of 85%.

A camera with a high fill-factor detector will typically provide better sensitivity and overall image quality than one with a lower fill factor. High fill-factor detectors offer better cooling efficiency so less power is used to cool the detector to operating temperature.

There are two types of infrared FPAs: monolithic and hybrid. Monolithic FPAs have both IR-sensitive material and signal transmission paths on the same layer. Monolithic FPAs are easier and less expensive to manufacture than hybrids because fewer manufacturing steps are required. Monolithic FPAs have lower performance than hybrids because having the detector material and signal pathways on the same level results in a significantly lower fill factor.

The difference between a system with a monolithic FPA array and a hybrid array manifested in poorer image quality. This difference is noticeable when viewing low temperatures or small temperature differences. Hybrids have advanced features such as variable integration time.

A hybrid array has the IR-sensitive detector material on one layer and the signal-transmission and processing circuitry on another layer. The two layers are bonded together by small indium bumps which transmit the signal from each detector element to its respective signal path on the multiplexer below. Although this process requires more steps and can be more expensive, it results in FPAs with a significantly higher fill factor (85%-90%). The higher fill factor resulting from this geometry provides much higher sensitivity than usually found in corresponding monolithic FPAs.

The detector material is a key factor in array performance. Two primary measures are quantum efficiency and dark current. The best material is HgCdTe grown by molecular beam epitaxy. With substrate removal the HgCdTe responds to visible light and quantum efficiency can be increased for wide bandpass. This enables focal plane arrays to provide wide spectral sensitivity, ultraviolet to infrared.

In the wavelength range of 0.3um to 30um, detector materials includes: 1)Si PIN; 2) InGaAs; 3) Ge PIN; 4) SWIR HgCdTe; 5) MWIR HgCdTe (on Silicon); 6) Thinned VisSWIR HgVdTe; 7) Si:Ga; 8) PtSi; 9) MWIR HgCdTe CdZnTe; 10) VisMWIR InSb; 11) LWIR PV HgCdTe; 12) LWIR PV HgCdTe; 13) VLWIR PV HgCdTe; 14) Si:AsIBC; 15) VOx. Generally: PtSi MWIR 77K; HgCdTe LWIR 77K; InSb MWIR 77K; HgCdTe SWIR 200K; Microbolometer 1-14um roomtemp (uncooled).

For cooled detectors, detector materials are MCT (HgCdTe), InSb and GaAs-GaAlAs (QWIP) and InAs/InGaSb (super lattice structure) in MWIR band. MCT, GaAs-GaAlAs (QWIP) and InAs/InGaSb (super lattice structure) are sensor materials used in LWIR spectral band. Vanadium oxide (VOx), amorphous silicon (Si), and titanium oxide (TiO) are the materials used for uncooled detectors.

Mercury Cadmium Telluride (HgCdTe) has advantages over other materials and it is why it is the most used variable gap material in infrared focal plane arrays. A main limitation of HgCdTe is the size of the lattice-matched bulk Cadmium Zinc Telluride (CdZnTe) substrates used for epitaxial-grown HgCdTe. The standard size for production are difficult and expensive to scale in size, which does not support the increasing demand for larger FPA formats such as 2048x2048. Worst yet, only one die can be made per wafer (3-inch). Heteroepitaxial Si-based substrates offer a cost effective alternative that can be scaled to large wafer sizes (6-inch). Standard practice is to grow HgCdTe on Si(112) substrates using Molecular Beam Epitaxy (MBE) process. Future process will be to extend this to large wafers, large formats, wider range of wavebands (LWIR/MWIR), multicolor arrays, higher temperatures, using other processes such as liquid phase epitaxy (LPE) and MOCVD.

readout

After the photocharge has been collected into pixels the charge flows to the readout via indium bumps. A key aspect of the hybrid CMOS imager architecture is that the readout is fabricated with the same equipment that is used for making high performance integrated circuits. With the highly advanced tools used for IC design, the functionality of the readout is limited only by the designer and space constraints of the pixel. Space constraints can be overcome by using finer design rules. The most basic functionality of the pixel is charge-to-voltage conversion (photovoltaic) and transmission of the voltage signal off-chip. It is also possible to include signal processing within the pixel such as range detection or background subtraction. Hundreds of transistors within a pixel are possible.

There are two basic types of readout devices for taking each detector`s signal and getting it to the camera`s signal processor: a charge-coupled device (CCD) detector and a complementary metal-oxide semiconductor (CMOS) detector. The CCD detector operates in a mode in which the signal from each detector is determined by transferring its electrons from one detector to the next down the same row until it reaches the end column where it is read out. The CCD transfer process is not perfect because some of the charge is lost along the way (known as charge-coupled transfer loss phenomenon). Also, when one detector cell becomes overfilled with photons from a hot source, the photons can overflow into the adjacent detector cells (blooming). CCD detectors require significantly more power than their CMOS counterparts and thus usually require higher-power cooling devices.

CCD detectors are widely used in imaging applications because the losses encountered by the loss phenomenon and blooming are typically not relevant in non-measurement scenarios. When a CCD detector is used in a measurement infrared FPA camera the errors must be compensated.

A CMOS detector has a readout made up of a series of metal-oxide-silicon field-effect transistors that provide direct access to the signal from each detector. In a CMOS detector, the signal from each detector is read out column by column and row by row until each detector has been addressed individually and its exact value provided to the signal processor.

CMOS circuits are ideal for low-power applications. Power dissipation in a detector readout circuit is critical because it must be cooled with the detector to -200°C. Even with a highly efficient cooler, each milliwatt of power dissipated by the readout requires about 25 mW of battery power for cooling. Maximum battery life is achieved by using a CMOS multiplexer detector readout and high-efficiency rotary Stirling engine cooler. CMOS detectors provide better accuracy for measurements as a result of their direct access readout capability. A multiplexer is the device that organizes and formats the signals from each detector in a repeatable fashion. Typically, a multiplexer takes the signal from each individual detector and feeds it to a signal processor through one or more output devices.

cooler

In the temperature range of 30K to 400K, cooler technologies includes: 1) Turbo-Brayton; 2) Single Stage Stirling; 3) Hybrid; 4) Pulse Tube; 5) Joule-Thomson Cryostat; 6) Radiative; 7) Thermoelectric cooling; and 8) liquid nitrogen (LN2)/dewar.

FPA testing primer

1) Assist system development. Perform tests to verify system functionality and get key data for system hardware and software development. 2) Test piecemeal. Perform tests first to characterize performance at the subsystem level . Tests of multiple subsystems and their interfaces are based on experience at the subsystem level. Avoid testing at a high system level. Faults at this high level are difficult to isolate. 3) Isolate faults and troubleshoot. Detect and isolate faults in the system to reduce risk of failure in a costly higher-end test. Ensure the system meets its requirements. 4) Probe the system for likely vulnerabilities. Devise tests that realistically stress the system. These are tests beyond the acceptance tests performed at manufacturing level. 5) Integrate test and simulation. Simulations are important in evaluating system performance over the many scenarios that would be impractical to test. Integrating simulation development and test programs is a good practice. Simulation models are typically validated through testing as they are developed. Test data are used as inputs to define model parameters. 6) Provide rapid response and documentation. Respond quickly in investigating unexpected problems and devising solutions. Test results and analyses are typically documented either in the form of a report or a formal presentation within weeks. 7) Avoid using the system to test itself (except BIST). Using measurements from the device under test to gauge its own performance is a common practice. Unfortunately, there have been cases where these data were wrong leading to false test results and conclusions. Before such data are relied on during tests, it is standard practice to validate the data using external calibration devices. 8) Develop test facilities that emphasize flexibility, portability and expansion. Performance compromises should not be made in the test equipment to accommodate potential applications that might come later. Construct test equipment into modular building blocks that can be reconfigured, modified, or upgraded gradually. Provide flexibility for incremental testing.

(under development)...thermographic testing versus radiometric testing/include calibration distiction between the two

Technology primer

Night vision is considered two different technologies, image intensification and thermal imaging. Image intensification depends on reflected light from objects in the scene was developed earlier than thermal imaging. Thermal imaging depends on blackbody radiation from objects in the scene.

Image intensification, as it exists in the latest third-generation used in aviation goggles, may be the end of the line for intensified systems based on the vacuum intensifiers. Development is focusing on solid state intensifiers and exploitation of the 1,000–2,000-nm wavelength region.

Early terrestrial thermal imaging systems operated primarily in two spectral wavelength regions: MWIR (3–5 um) and LWIR (8–11 um). These systems originally depended on cooled detector arrays for peak sensitivity, but in the 1990s detectors were developed that required minimal or no cooling. While the uncooled arrays do not achieve the high sensitivity of cooled detector arrays, there are numerous applications that are not possible with the cooled arrays. Without the requirement for cooling engines that consume power, lightweight, affordable systems such as personal viewers and vehicle driving aids are possible. A number of civil applications are appearing in 2000 because of the lower cost.

Space systems operate in several other spectral regions, mainly in the very long wavelength IR (VLWIR) region beyond 11 μm. Ultraviolet (UV) applications also exist. These are expensive and tend to use exotic detector materials with limited production.

Early thermal-imaging systems used scanned linear arrays, and much of the operational inventory has these systems. Upgrades to second-generation staring systems are underway, and most planned systems employ staring arrays that require no mechanical scanning.

The key developing technologies in thermal imaging include the following: 1) Larger cooled staring arrays; 2) Multicolor and hyperspectral arrays; 3) Improvements in uncooled arrays; 4) Exploitation of other spectral regions such as short wave IR (SWIR) and UV; 5) Improved affordability and producibility.

image intensifiers

Parameters: 1) Low-light level (LLL); 2) 980 × 1,280 pixels with 10 μm pixels, wavelength range: 1,000–2,500 nm; 3) High-temperature operation using thermoelectric (TE) cooling. Critical Materials: Mercury cadmium telluride (HgCdTe) and indium gallium arsenide (InGaAs). Test, Production Equipment: Modulation transfer function (MTF) testers, night-vision scene simulators. Commercial Applications: vacuum intensifiers (police and industrial surveillance, rescue missions, and sporting activities). Must be low cost because the sensors will be used at the “soldier level.” Current image-intensification night-vision devices operate mainly in the visible spectrum and extend into the near infrared (NIR) by a very small amount. This response is referred to as photopic response. They are not operationally sensitive in the SWIR.

imaging sensors (1-2 um)

Parameters: 1) Solid-state imaging sensors capable of LLL sensitivity; 2) Optimize sensitivity at the wavelength of “eye safe” laser illuminators; and 3) 480 × 640 pixels. Critical Materials: group III-V and II-VI semiconductors.

uncooled infrared array

Parameters: 1) High performance: sensitivity = 0.01 °C; 2) resolution = 1 mil pixels, 1,000 × 1,000 pixels. Micro sensor: 160 × 120 pixels, 2 mil × 2 mil pixel size, no cooling, expendable, 1 oz., 10 mW with power management. Critical Materials: Microbolometer and thin-film ferroelectric materials. Commercial Applications: Vehicle driving aid, perimeter surveillance. Must be low cost because it is used at the "individual soldier" level. Objectives of this technology: 1) Smaller pixels and increased sensitivity; 2) Larger formats; 3) No mechanical chopper (as in pyroelectric detectors); 4) No temperature stabilization; 5) Lower power requirements; 6) Higher frame rates; and 7) Use of low-cost optics Payoffs: 1) Lower cost; 2) Longer autonomous life; 3) Lighter weight; 4) Smaller volume; and 5) High performance (comparable to cooled arrays).

cooled staring fpa

Parameters: 1) Mega pixel arrays (from 1,024 × 1,024 to >2,048 × 2,048); 2) higher operating temperature using thermoelectric (TE) or mechanical cooling (120–180 K) cooling; 3) smaller pixels, 18 × 18 mm multi-color. Critical Material is HgCdTe.

long/dual wavelength infrared fpa

Parameters: 1) 1,024 × 1,024 LWIR FPAs and 128 × 128 LWIR hardened for space; 2) Cutoff wavelength in the 14–25-μm range for space surveillance; 3) MWIR FPAs for threat warning. Critical Materials: HgCdTe, silicon (Si)/HgCdTe, gallium indium antimony/indium arsenide (GaInSb/InAs) superlattice and gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) quantum well materials. Low-cost detectors production.

imaging sensors (solar blind)

Parameters: Solar blind detector with a noise-equivalent power (NEP) of 10–14. Critical Materials: Gallium aluminum indium nitride (GaAlN) with a low (<107 cm–2) defect density substrate material. An objective of this technology is the development of III-V nitride materials and detector technology to demonstrate an imaging ultraviolet FPA. Current operational systems use vacuum, solar-blind photomultiplier tubes (PMTs) and are non-imaging.

3-color detector

Parameters: Highest spatial resolution detector arrays which have the ability to sense red, green, and blue wavelengths by means of layered detectors on a single-pixel-element basis as required. The device, called the buried triple P-n junction (BTJ) structure can be fabricated using conventional bipolar or bipolar complementary metal oxide semiconductor (BICMOS) processes. Spatial resolution is tripled by this triad-stacking process. This technique eliminates the need for spectral filters. The amount of charge carriers generated depends on both the wavelength of the light and the depth at which it is absorbed. The BTJ exploits this by having three buried (layered) junctions, each producing a photocurrent. Sensitivity has been shown to be comparable or to exceed current visible-array detectors. Temperature compensation sensitivity algorithms need to be developed. Mapping the device color to the visual range over a wide temperature range is needed. Commercial Applications: If the processes can be developed for mass production capability on current chips, the market is wide open. This technology will eventually replace current triad (side-by-side) color camera detector arrays. This technology should be more affordable than current technology at comparable resolution.

infrared antennas

Antenna-coupled, uncooled IR FPAs provide high-speed, polarization-resolved, wavelength resolved IR imagery using uncooled FPA technology. Goals are uncooled IR sensors of 10–100 NETD. These sensors will be in FPA format of nominal 512 × 512. These sensors will be tunable in wavelength response over the 3–5 and 8–12 μm bands, with 0.5-μm bandpass. Sensors will be tunable in polarization response for all linear polarization states as well as Left Circular and Right Circular. Tuning in response to a dc voltage of 100 mV. Production method: direct-write electron-beam lithography. Testing method: custom apparatus currently under development for assessment of tuning functionality. Inspection method: usual IC inspection techniques. Commercial Applications: Satellite IR remote sensing systems. Goals are increased information gathering capability and reduced payload weight. Affordability: Sensor costs are a declining portion of overall imaging systems because of cost reductions inherent in uncooled IR FPA technology as compared to HgCdTe cooled FPAs. Key advantage is the ability to provide wavelength-resolved and polarization-resolved imagery in a non-moving parts configuration. Reduces system weight and complexity. Present uncoooled IR FPA development programs are concentrated on realization of sensors operating at the background fluctuation limit of sensitivity, and near-equality with cooled sensor performance. This goal, which represents a fifty times increase in sensitivity relative to current uncooled IR sensors, must be achieved in order to realize the full benefits of polarization-resolved imagery.

DUT Examples

• Charge Coupled Device (CCD)
• Complementary metal–oxide–semiconductor (CMOS)
• Charge Injection Device(CID)
• Available Random Access CID arrays (RACID)
• Cryocooler FPA
• Uncooled FPA
• Microbolometers
• SWIR/MWIR/LWIR FPA
• Dual-band FPA
• Readout integrated circuits
• Multi/Hyperspectral FPA
• Polarimetric FPA
• QWIP FPA
• InGaAs FPA
• HgCdTe FPA
• Infrared camera systems
• Foward looking infrared (FLIR)

Test Equipment

Commercial automatic test equipment for FPA testing is very expensive (prober, parameter analyzer, etc). However, minimal production testing is possible on a boot-strap budget. A relatively low cost test system for testing non-uniformity and signal to noise (S/N) ratio of an uncooled focal plane array can use programmable logic devices to generate the necessary pulses for the DUT and low dropout regulator to obtain low noise bias. A proportional integral derivative (PID) thermoelectric cooler that is microprocessor or microcontroller controlled can stabilize the DUT. A PC-based data acquisition card can then be used as an analog-to-digital converter (ADC) to convert DUT output to digital input for computer analysis. The 12-bit ADC capability provides sufficient accuracy for evaluating the S/N ratio and non-uniformity of 128 x 128 pixels DUT. High level software is used to control test procedures and analyze the signals.

infrared scene simulators/projectors

(under developemnt)The state-of-the-art in laboratory testing of infrared (IR) missile seekers and target acquisition systems is centered on advancements with infrared scene projectors (IRSP). IR scene projection provides realistic and repeatable operational-type test scenarios in a controlled synthetic environment. When using resistor array scene projectors, there is an inherent variability in the micro-fabrication process for each emitter element that manifests as fixed-pattern noise or non-uniformity. An IRSP is a complete projection system instead of a set of components to be integrated by the test engineer.

target projectors

(under development)

collimators

The collimator projects an image of the target, back lit by the blackbody, to the infrared imager device under test. The collimator preserves the apparent angular size of the target features regardless of the distance from the unit under test to the target. The image produced by the collimator is an erect virtual image at infinite distance from the device under test (DUT).

targets/target wheels

Targets are solid disks of copper through which slots or holes are machined to create the target features. If the target features are very small then the target is fabricated from thin sheets of metal and the features are chemically etched through the material. The target is coated with a special high emissivity coating. Many target patterns are possible. However, there are two target types that are standard for particular tests: window or aperture targets and bar targets. Window and aperture targets are used for tests in which a line of video is stripped out of the device under test signal and signal levels from the differential blackbodies temperature areas of the image are measured. Bar targets are used in resolution and focus tests. Others: four-bar; edge; window; pinhole. The target wheel eliminates the risk of damage to coating delicate surfaces by keeping them enclosed and changing them without the need for handling. The target wheel establishes the position of the targets with great accuracy and repeatability.

thermometers

(under development)

optical benches

(under development)

blackbodies

Blackbodies are used when high temperatures and high IR flux are required. They are incorporated into test systems that are used to test bare detectors. They are also used when filtering of the IR energy will be required. These blackbodies will often be used with a chopper. Typical blackbodies have an aperture size of one inch and a temperature range of 50 to 500C.

infrared detector preamplifiers

(under development)

energy modulators

(under development)

pulse integration systems

(under development)

flare test and spectral radiometer

(under development)For analysis of counter-measure flare; radiant intensity and IR emissions measurements.

array integrators

(under development)

environmental chambers

(under development)

field tester

A portable field tester is a compact, lightweight, self-contained IR collimator. It is used in the field for checking the serviceability of thermal sight-based observation systems without disassembling and transporting to the lab. They perform minimum resolved temperature difference (MRTD) tests and enables a single operator Go/No Go testing.

test station

A thermal imaging test station is used for testing a wide range of multi-sensor systems including a NFOV FLIR, WFOV FLIR, visible sensors, laser designators, laser range finders, and more. The test software is a turnkey solution for testing multi-sensor FLIR, CCD, and laser systems in production and at all maintenance levels. The system includes: 1) Collimator; 2) Cavity black body; 3) Extended area black body; 4) He-Ne laser diode; 5) Visible source; 6) Electronic console; and 7) Optical bench.

Test Considerations

Testing focal plane arrays requires: 1) writing a concise test procedures with unambiguous system specifications; 2) identifying all appropriate test parameters; 3) differentiating between observer variability and system response during MRC and MRT testing; 4) understanding how jitter, linearity, amplitude normalization, frequency scaling, and noise affect CTF and MTF measurements; 5) discerning the difference between poor system performance, peculiarities of the system under test and measurement errors; and 6) understanding how sampling affects test results.

measurement techniques

(under development)Targets and collimators... differences between radiometry and photometry; define responsivity in radiometric, photometric, and differential temperature.

image resolution

(under development)Boundary detection...Resolution refers to a FPA’s ability to image targets and to distinguish between closely spaced objects.

responsivity

(under development)System response...differences between radiometry and photometry; define responsivity in radiometric, photometric, and differential temperature units (Radiometry/Photometry and System Responsivity) select appropriate equipment to measure responsivity identify possible causes of poor responsivity

field of view

Field of view is the angular width a FPA can see. The field of view of a single pixel of a FPA is called the instantaneous field of view. The field of view is its instantaneous field of view multiplied by the number of pixels in a row or column of the FPA (typical is 1 degree).

noise

(under development)Thermal, shot... ergodicity; random, fixed pattern, and other noise components; nonuniformity; three-dimensional noise model; 3-D noise components vs. real noise sources; measurement techniques to quantify each noise component.

uniformity

The detectors within an infrared focal plane array characteristically have responses that vary from detector to detector. It is desirable to remove this non-uniformity for improved image quality. Factory calibration is not sufficient since non-uniformity tends to drift over time. Field calibration can be performed using uniform temperature sources but requires briefly obscuring the field of view and leads to additional system size and cost. Alternative scene-based approaches are able to utilize the normal scene data when performing non-uniformity correction (NUC) and therefore do not require the field of view to be obscured. These function well under proper conditions but at times can introduce image artifacts such as “ghosting”. Ghosting results when scene conditions are not optimal for NUC.

transfer functions

(under development)Linearity, fourier, modulation... CTF and MTF; MRT/MDT and MRC/MDC-why these metrics are a measure of image quality. Metrics considered include modulation transfer function (MTF), and contrast transfer function (CTF). For thermal imaging systems, image "quality" metrics include the minimum resolvable temperature (MRT) and minimum detectable temperature (MDT). For systems operating in the visible, these metrics become the minimum resolvable contrast (MRC) and minimum detectable contrast (MDC). Since all imaging systems spatially sample the scene, sampling artifacts occur in all imagery. Sampling effects become evident when viewing most test targets (typically bar patterns). Signal Transfer function (STF) is the slope of the output voltage versus the differential temperature.

modulation transfer function

The modulation transfer function (MTF) is a property of an imaging system that describes the effect that the system has on the sharpness of an object. It is an important image quality metric. The MTF can be produced by aligning an edge perpendicular to the measurement scan line. The scan averages all the pixels along the edge (parallel to the edge) and a function of reflectance versus position (referred to as the edge spread function) can be produced. The edge spread function is then differentiated and operated on by a Fourier transform to produce the MTF.

image quality

The minimum resolvable temperature difference (MRTD) is an important parameter of thermal imaging systems that enables the estimation of probability of detection, recognition, and identification of targets. Unfortunately, MRTD is a subjective parameter that describes the ability of the system (human included) to detect low contrast details. MRTD can be described as a function of the minimum temperature difference between a 4-bar target and the background required for a human to resolve the image versus the frequency of the target. The large measurement uncertainty results in low repeatability.

ergodicity

(under development)

chromatic aberration

(under development) Chromatic aberration is a phenomenon in which different wavelengths of light are not all focused at the same time. Chromatic aberration can occur in IR systems because systems sense energy over a wide range of wavelengths at one time. Without correction, energy at 3.5 µm might be focused and energy at 5.0 µm might be fuzzy. The resulting image would not be crisp and could be subject to measurement errors. Can correct for this by developing color-corrected IR lenses.

spot size ratio

The spot size ratio (SSR) is the maximum distance a camera can be from a target of a given size and still maintain temperature measurement accuracy. A SSR of 200:1 indicates that at a distance of 200 meters the camera will accurately measure the temperature of an object of one square meter.

frame rate

The frame rate is the number of times the scene is viewed to produce a frame, i.e. several fields are integrated to produce a frame. If the frame rate equals the field rate then no integration is necessary. Frame rate is more significant when comparing IR systems. Slow frame rates produce "streaky" images even if the camera is moved slowly.

frame repetition rate

This is the time taken for the infrared imager to scan and update every pixel element in the detector, expressed as frames per second.

diffractive lenses

(under development) Diffractive lenses provide the color-correction capability of a set of multiple lenses with a single diffractive element. By doing the work of several lens elements with only a single element, the size, weight, and transmission properties of a lens can be improved. Diffractive lenses can be distinguished from standard lenses by the rings etched in the surface of the lens. These diffractive grooves cause lightwaves to be bent in a manner that corrects for chromatic aberration.

automation

(under development)Objectivity, productivity, reproducibility, ...

standardization

(under development)Role of industry organizations, SPIE...

statistical analysis

(under development)Objective vs subjective quality...

adjustable integration time

Integration time is the time that the FPA collects IR photons. An FPA runs at a maximum integration time of 16.7 ms (one complete frame). Arrays with adjustable integration time can capture photons over shorter periods of time. This reduces the amount of energy that the detector captures at any given temperature. FPAs with adjustable integration time have high-temperature imaging and measurement capabilities without needing filters. Some FPAs will operate at high temperatures by using an adjustable integration time. Adjustable integration time FPA's saves time because scenes at higher temperatures can be viewed by changing the electrical characteristics of the detector rather than installing an optical filter.

detector performance metrics

(under development)Noise equivalent power; Detector noise; FPA noise; Background limited performance; Source radiance at detector; Reverse bias current density (dark current); D-star; responsivity; noise equivalent irradiance.

blackbody radiation laws

Planck law for radiant emittance; Planck law for photon emittance; Stefan-Boltzmann law (blackbody emittance); Wein law; Wien displacement law; Total Radiation law; Emissivity/Emittance/Kirchoff law/reflection/absorption/transmission; Lambert law.

radiant and luminous quantities

radiant flux; radiant intensity; radiant emittance; radiance; irradiance; luminous flux; luminous intensity; luminance; iluminance.

FPA Acceptance Testing

The FPA Accepting Test Procedure (ATP) should be performed at the highest operating temperature in the range of temperatures given in the specification of the DUT. The highest operating temperature is used to insure that DUT is tested at the most aggressive temperature. The FPA ATP consist of the following 5 basic parts: 1) Current / Voltage read back and power dissipation; 2) Starvation and Saturation voltage measurements; 3) LN2 Background and RMS noise measurement; 4) Low background offset and RMS noise measurement;l and 5) High background offset and reset noise measurements. The ATP should be a batch test.

From the measured data, responsivity, Noise Equivalent Irradiance (NEI) and the DC output offset are calculated. An operable pixel is defined as one that meets the DC offset uniformity, responsivity and response uniformity, and NEI requirements. These radiometric measurements are performed using an extended black body illuminating the DUT through an appropriate aperture, using cold spectral filters and neutral density filter. The cold neutral density filter is necessary to achieve the low background flux needed while viewing an approximately room temperature black body source. All measurements should utilized a constant and appropriate integration time.

Response uniformity should be met with an appropriate response sigma / mean value. Use plots for test results. Examples: responsivity gray scale and histogram (number of elements vs responsivity); NEI gray scale and histogram (number of elements vs NEI); DC uniformity gray scale and histogram (number of elements vs DC volts).

Measured and required parameters should include: temperature; leak current; pixel size; NEI; power dissipation; clock frequency; integration time; dynamic range (instantaneous and total); charge capacity; and filter bandpass.

FPA Testing Using Dark Current

Dark current is the flow of charge in p-n junction under bias with no light. Since it is not possible to distinguish dark current from photocharge and since dark current has a Poisson noise distribution, very low dark current is required for sensing low illumination. Molecular beam epitaxy-made detectors have the lowest dark current of all mercad detectors. Dark current is a function of pixel area, temperature, and cutoff wavelength. Benefits of low dark current are: 1) enables the most sensitive flux measurements to be made at very cold temperatures; 2) enables operation at the highest temperature for a given level of performance, decreasing thermal load on the instrument cooling system.

With flip-chip technology a detector (focal plane array) can be hybridized to a readout integrated circuit with Indium bumps. A pre-binding test is needed on the FPA and readout, but conventional probe-testing is time comsuming and raises probe-related risks. Measuring the dark current of the photodiodes provides an alternative test technique. The off-current from the MOS switches, however, needs to be calibrated. A good approximation of the calibrated off-current (if the number of columns and rows are large) can be calculated. Use of a test socket chip to test detectors using dark current measurements with single automated probe that can cancel out calibrated off-current can be a useful alternative to conventional tests.

FPA Testing Using Parallel Pattern Scanning

The Parallel Pattern Scanning (PPS) test is composed of scanning a full row of cells in parallel for each row, and followed by a full column of cells in parallel for each column. Consequently the number of test times is reduced to just sum of columns and rows. The two steps for a PPS test are: 1) For each row, measure the total currents of one full row. All columns are selected ON, and only the interested row is selected ON. Sensors in the interested row will contribute their currents to the measurement. Therefore, if there is no faulty sensor cell in the row, the measured current will be row times of one cell’s. And if there are faulty cells, the measured current will be different from the expected value. And; 2) For each column, measure the total currents of one full column. All rows are selected ON, and only the interested column is selected ON. Sensors in the interested column will contribute their currents to the measurement. Therefore, if there is no faulty sensor cell in the column, the measured current will be column times of one cell’s. And if there are faulty cells, the measured current will possibly be different from the expected value. A full row or column of cells can be tested in parallel at the same time. From the test results of rows and columns a faulty cell can be identified. Applying a PPS test can speed test time from rowsXcolumns to rows+columns.

Camera System Testing

A camera systems includes packaged focal plane arrays with optics, cryocoolers, and board electronics with requirements from several engineering diciplines such as mechanical, optical, thermal, and electrical. Testing camera systems incorporate system-level tests which are appropriate with system engineering objectives. A camera test engineer may be responsible for development of functional test systems for visible and infrared cameras, and electronics; specifying and/or designing of test equipment, fixtures and tooling, setup and maintenance of image acquisition computers and lab instrumentation; settting up and maintenance of cryogenic and high vacuum systems required for integrated camera testing; setting up and using test equipment to perform testing and troubleshooting; locating and identifying problems at the sub-assembly level; participating in new product design to ensure Design For Test principles are incorporated, and in transfering test methods to manufacturing. Manufacturing test includes generating Test Plans, Test Procedures, Work Instructions, and general technical support to production.

Performance test considerations differ depending on the design and application of an IR camera system. Image quality is most important for scenery surveillance/tracking cameras, such as missile seekers. Accuracy is most important for cameras used in measurement of absolute temperatures, such as non-destructive testing.

(under development)...hardware-in-the-loop; distributed testing...

camera system components

(under developemnt)...fpassemblies vs fparrays

• Lens (optics, focus, aperture control, neutral density filters, lens coatings, lens materials .2-30um: Ge, ZnS, Si, ZnSe, CdTe, InSb, MgF2, Al2O3, SiO2 IR fused silica, CaF2, BaF2, KBr, NaCl)
• FPA/ROIC/grating optics
• cryocooler (LN2/dewar, peltier, stirling/He)
• electronics (video processing, a/d converter, uC/memory...)
• software/firmware: A) built-in self-test; B) image processing firmware (for Field Programmable Gates Arrays, FPGA’s) algorithms to improve image quality for uncooled microbolometer. Specifically, two algorithms: 1) Histogram Equalization; and 2) Histogram Projection.
• packaging (interfaces, ...)
• display monitor/panel & controls
• signal processing - Signal processor takes inputs from detector electronics, and processes it to form real-time images. Signal processing includes amplification, multiplexing, digitization of IR signal, non-uniformity calibration and correction for each detector element, bad pixel replacement and digital scan conversion to generate video, which is displayed on a suitable monitor. Modern detectors perform on-focal plane processing which includes amplification and multiplexing. They also have digital on-focal plane processing capability and are compact systems. Off-focal plane signal processor includes automatic gain and offset control, dynamic range compression (DRC) with plateau control and various image processing features. Design and implementation of signal processors are based on field programmable gate array (FPGA).

camera performance tests

1) Subjective image quality (MDTD/MRTD); 2) Response and noise (STF, dynamic range, saturation, SRF, ATRF, fix pattern noise, defective pixels); 3) Image resolution (IFOV, MTF, CTF, AWAR); 4) Geometric (FOV, magnification, distortion, boresight alignment); 5) Accuracy (min error, noise generated error, temperature stability); 6) Spectral (spectral sensitivity function); 7) Operational (focus).

camera/fpa calibration

(under development) atmospheric window, compensation, offset/gain/firmware normalization, NUC, radiance/a-d counts/temperature, drift compensation, ...)

FPA-related Test Standards

• ASTM E 1543-94 Standard Test Method for Noise Equivalent Temperature Difference of Thermal Imaging Systems.
• ASTM standard E 1213-2002 “Standard Test Method for Minimum Resolvable Temperature Difference for Thermal Imaging Systems”.
• ASTM standard E 1311-99 “Standard Test Method for Minimum Detectable Temperature Difference for Thermal Imaging Systems”.
• NATO STANAG 4349 and 4350, Measurement of minimum resolvable thermal difference (MRTD) of thermal cameras, 4349(6/19/1996).
• ASTM F1623-96 Standard Terminology Relating to Thermal Imaging Products.
• NIST International Temperature Scale of 1990 (ITS-90).
• ASTM F 1405 Standard test method for determining the dynamic thermal response of direct thermal imaging products - Atlantek method.
• ASTM F 1623 Standard terminology relating to thermal imaging products.

Testing Challenges

• high density (small unit cells and large format arrays)
• small electrical currents (small unit cell)
• probe point impossibilities (flip chip technology)
• pre-binding test required
• conventional probe is time consuming
• MOS off-current calibration (dark current)
• serial vs parallel testing
• built-in current sources

References and external links

• Raytheon Vision Systems [1]
• Teledyne Imaging Sensors [2]
• Redstone Technical Test Center [3]
• SPIE-The International Society for Optical Engineering [4]