A staring array, also known as staring-plane array or focal-plane array (FPA), is an
image sensor An image sensor or imager is a sensor that detects and conveys information used to form an image. It does so by converting the variable attenuation of light waves (as they refraction, pass through or reflection (physics), reflect off objects) into s ...
consisting of an array (typically rectangular) of light-sensing pixels at the
focal plane of a
lens
A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...
. FPAs are used most commonly for imaging purposes (e.g. taking pictures or video imagery), but can also be used for non-imaging purposes such as
spectrometry,
LIDAR
Lidar (, also LIDAR, an acronym of "light detection and ranging" or "laser imaging, detection, and ranging") is a method for determining ranging, ranges by targeting an object or a surface with a laser and measuring the time for the reflected li ...
, and
wave-front sensing.
In
radio astronomy
Radio astronomy is a subfield of astronomy that studies Astronomical object, celestial objects using radio waves. It started in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming from the Milky Way. Subsequent observat ...
, the
FPA is at the
focus of a
radio telescope
A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...
. 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
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
spectrum. Devices sensitive in other spectra are usually referred to by other terms, such as CCD (
charge-coupled device
A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...
) 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
missile
A missile is an airborne ranged weapon capable of self-propelled flight aided usually by a propellant, jet engine or rocket motor.
Historically, 'missile' referred to any projectile that is thrown, shot or propelled towards a target; this ...
or related weapons guidance sensors, infrared astronomy, manufacturing inspection, thermal imaging for firefighting, medical imaging, and infrared phenomenology (such as observing combustion, weapon impact, rocket motor ignition and other events that are interesting in the infrared spectrum).
Comparison to scanning array
Staring arrays are distinct from
scanning array and
TDI imagers in that they image the desired field of view without scanning. Scanning arrays are constructed from linear arrays (or very narrow 2-D arrays) that are rastered across the desired field of view using a rotating or oscillating mirror to construct a 2-D image over time. A TDI imager operates in similar fashion to a scanning array except that it images perpendicularly to the motion of the camera. A staring array is analogous to the film in a typical camera; it directly captures a 2-D image projected by the lens at the image plane. A scanning array is analogous to piecing together a 2D image with photos taken through a narrow slit. A TDI imager is analogous to looking through a vertical slit out the side window of a moving car, and building a long, continuous image as the car passes the landscape.
Scanning arrays were developed and used because of historical difficulties in fabricating 2-D arrays of sufficient size and quality for direct 2-D imaging. Modern FPAs are available with up to 2048 x 2048 pixels, and larger sizes are in development by multiple manufacturers. 320 x 256 and 640 x 480 arrays are available and affordable even for non-military, non-scientific applications.
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(V) 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
In electronics, a multiplexer (or mux; spelled sometimes as multiplexor), also known as a data selector, is a device that selects between several Analog signal, analog or Digital signal (electronics), digital input signals and forwards the sel ...
, 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 resistive amorphous silicon (a-Si) and VOx
microbolometer
A microbolometer is a specific type of bolometer used as a detector in a thermal camera. Infrared radiation with wavelengths between 7.5–14 μm strikes the detector material, heating it, and thus changing its electrical resistance. This resista ...
s) can operate at uncooled temperatures. Some devices are only practical to operate cryogenically as otherwise the
thermal noise
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
would swamp the detected signal. Devices can be cooled evaporatively, typically by
liquid nitrogen
Liquid nitrogen (LN2) is nitrogen in a liquid state at cryogenics, low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, mobile liquid whose vis ...
(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 missile
An air-to-air missile (AAM) is a missile fired from an aircraft for the purpose of destroying another aircraft (including unmanned aircraft such as cruise missiles). AAMs are typically powered by one or more rocket motors, usually solid-fuel roc ...
s and
anti-tank missile
An anti-tank guided missile (ATGM), anti-tank missile, anti-tank guided weapon (ATGW) or anti-armor guided weapon is a missile guidance, guided missile primarily designed to hit and destroy armoured fighting vehicle, heavily armored military v ...
s such as the
AIM-9X Sidewinder,
ASRAAM
The Advanced Short Range Air-to-Air Missile (ASRAAM), also known by its United States designation AIM-132, is an imaging infrared homing air-to-air missile, produced by MBDA UK, that is designed for close-range combat. It is in service in the ...
Cross talk can inhibit the illumination of pixels.
[
]
Applications
3D LIDAR Imaging
Focal plane arrays (FPAs) have been reported to be used for 3D LIDAR
Lidar (, also LIDAR, an acronym of "light detection and ranging" or "laser imaging, detection, and ranging") is a method for determining ranging, ranges by targeting an object or a surface with a laser and measuring the time for the reflected li ...
imaging.[Goldberg, A.; Stann, B.; Gupta, N. (July 2003). "Multispectral, Hyperspectral, and Three-Dimensional Imaging Research at the U.S. Army Research Laboratory" (PDF). ''Proceedings of the International Conference on International Fusion th'. 1: 499–506.]
Improvements
In 2003, a 32 x 32 pixel breadboard was reported with capabilities to repress cross talk between FPAs. Researchers at the U.S. Army Research Laboratory used a collimator to collect and direct the breadboard’s laser beam onto individual pixels. Since low levels of voltage were still observed in pixels that did not illuminate, indicating that illumination was prevented by crosstalk. This cross talk was attributed to capacitive coupling
Capacitive coupling (electronics), coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) node (circuits) , nodes, induced by the electric field. This coup ...
between the microstrip lines and between the FPA’s internal conductors. By replacing the receiver in the breadboard for one with a shorter focal length, the focus of the collimator was reduced and the system’s threshold for signal recognition was increased. This facilitated a better image by cancelling cross talk.[
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.][{{Cite journal, last1=Itzler, first1=Mark A., last2=Entwistle, first2=Mark, last3=Owens, first3=Mark, last4=Patel, first4=Ketan, last5=Jiang, first5=Xudong, last6=Slomkowski, first6=Krystyna, last7=Rangwala, first7=Sabbir, last8=Zalud, first8=Peter F., last9=Senko, first9=Tom, editor1-first=Eustace L, editor1-last=Dereniak, editor2-first=John P, editor2-last=Hartke, editor3-first=Paul D, editor3-last=Levan, editor4-first=Ashok K, editor4-last=Sood, editor5-first=Randolph E, editor5-last=Longshore, editor6-first=Manijeh, editor6-last=Razeghi, date=2010-08-19, title=Design and performance of single photon APD focal plane arrays for 3-D LADAR imaging, url=https://spie.org/Publications/Proceedings/Paper/10.1117/12.864465, journal=Detectors and Imaging Devices: Infrared, Focal Plane, Single Photon, volume=7780, pages=77801M, publisher=SPIE, doi=10.1117/12.864465, bibcode=2010SPIE.7780E..1MI, s2cid=120955542, url-access=subscription]
See also
* Focal-plane array (radio astronomy) Focal-plane arrays (FPAs) are widely used in radio astronomy. FPAs are arrays of Receiver (radio), receivers placed at the Focus (geometry), focus of the optical system in a radio-telescope. The optical system may be a Reflecting telescope, reflecto ...
References
Infrared imaging
Image sensors