Product Code Database
A staring array, also known as staring-plane array or focal-plane array ( FPA), is an image sensor consisting of an array (typically rectangular) of light-sensing pixels at the focal plane of a lens. FPAs are used most commonly for imaging purposes (e.g. taking pictures or video imagery), but can also be used for non-imaging purposes such as spectroscopy, lidar, and wave-front sensing.
In radio astronomy, the FPA is at the focus of a radio telescope. At optical and infrared wavelengths, it can refer to a variety of imaging device types, but in common usage it refers to two-dimensional devices that are sensitive in the infrared spectrum. Devices sensitive in other spectra are usually referred to by other terms, such as CCD (charge-coupled device) and CMOS image sensor in the visible spectrum. FPAs operate by detecting photons at particular wavelengths and then generating an electrical charge, voltage, or resistance in relation to the number of photons detected at each pixel. This charge, voltage, or resistance is then measured, digitized, and used to construct an image of the object, scene, or phenomenon that emitted the photons.
Applications for infrared FPAs include guided missile or related weapons guidance sensors, infrared astronomy, manufacturing inspection, thermal imaging for firefighting, medical imaging, and infrared phenomenology (such as observing combustion, weapon impact, rocket motor ignition and other events that are interesting in the infrared spectrum).
Scanning arrays were developed and used because of historical difficulties in fabricating 2-D arrays of sufficient size and quality for direct 2-D imaging. Modern FPAs are available with up to 2048 x 2048 pixels, and larger sizes are in development by multiple manufacturers. 320 x 256 and 640 x 480 arrays are available and affordable even for non-military, non-scientific applications.
Some materials (and the FPAs fabricated from them) operate only at cryogenic temperatures, and others (such as resistive amorphous silicon (a-Si) and VOx ) 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 Air-to-Air Weapons - Royal Air Force
Crosstalk can inhibit the illumination of pixels.
Another method was to add a flat thinned substrate membrane (approximately 800 angstroms thick) to the FPA. This was reported to eliminate pixel-to-pixel cross talk in FPA imaging applications. In another an avalanche photodiode FPA study, the etching of trenches in between neighboring pixels reduced cross talk.
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