Construction and Materials
The difficulty in constructing high-quality, high-resolution FPAs derives from the materials used. Whereas visible imagers such as CCD and CMOS image sensors are fabricated from silicon, using mature and well-understood processes, IR sensors must be fabricated from other, more exotic materials because silicon is sensitive only in the visible and near-IR spectra. Infrared-sensitive materials commonly used in IR detector arrays include mercury-cadmium-telluride (HgCdTe, "MerCad", or "MerCadTel"), Indium Antimonide (InSb, pronounced "Inns-Bee"), Indium Gallium Arsenide (InGaAs, pronounced "Inn-Gas"), and Vanadium Oxide (VOx, pronounced "Vox"). A variety of lead salts can also be used, but are less common today. None of these materials can be grown into crystals anywhere near the size of modern silicon crystals, nor do the resulting wafers have nearly the uniformity of silicon. Furthermore, the materials used to construct arrays of IR-sensitive pixels cannot be used to construct the electronics needed to transport the resulting charge, voltage, or resistance of each pixel to the measurement circuitry. This set of functions is implemented on a chip called the multiplexer, or readout integrated circuits (ROIC), and is typically fabricated in silicon using standard CMOS processes. The detector array is then hybridized or bonded to the ROIC, typically using indium bump-bonding, and the resulting assembly is called an FPA.
Some materials (and the FPAs fabricated from them) operate only at cryogenic temperatures, and others (such as VOx microbolometers) can operate at uncooled temperatures. Some devices are only practical to operate cryogenically as otherwise the thermal noise would swamp the detected signal. Devices can be cooled evaporatively, typically by liquid nitrogen (LN2) or liquid helium, or by using a thermo-electric cooler.
A peculiar aspect of nearly all IR FPAs is that the electrical responses of the pixels on a given device tend to be non-uniform. In a perfect device every pixel would output the same electrical signal when given the same number of photons of appropriate wavelength. In practice nearly all FPAs have both significant pixel-to-pixel offset and pixel-to-pixel photo-response non-uniformity (PRNU). When un-illuminated, each pixel has a different "zero-signal" level, and when illuminated the delta in signal is also different. This non-uniformity makes the resulting images impractical for use until they have been processed to normalize the photo-response. This correction process requires a set of known characterization data, collected from the particular device under controlled conditions. The data correction can be done in software, in a DSP or FPGA in the camera electronics, or even on the ROIC in the most modern of devices.
The low volumes, rarer materials, and complex processes involved in fabricating and using IR FPAs makes them far more expensive than visible imagers of comparable size and resolution.
Staring plane arrays are used in modern air to air missiles and anti-tank missiles such as the AIM-9X Sidewinder, ASRAAM
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