Glossary


A brief Glossary of Terms used in Infrared Imaging

 

1st Gen Infrared – The first generation of infrared imaging, generally characterized by use of a linear array of detectors opto-mechanically scanned to cover the field of view. Typically these were linear mercury cadmium telluride detectors with 60 to 180 elements. (Also called Common Module).

 

2nd Gen Infrared – scanned 480 x 4 or 240 x 4 pixel photovoltaic HgCdTe focal plane arrays operated in a TDI (Time Delay and Integration) mode in which the same scene is scanned over different detector elements to reduce noise. Staring arrays are also considered to be 2nd Gen although many manufacturers refer to them as 3rd Gen.

 

3rd Gen Infrared – a large staring focal plane array with dual-band (MWIR/LWIR) spectral response at each pixel.

 

3D Imaging – An imaging technology that includes range information usually acquired by the addition of a pulsed laser. A 3D FPA provides both a two-dimensional image, as well as detailed range information for each portion of the object being viewed.

 

a-Si (Amorphous Silicon) – A non-crystalline form of the element silicon that is used as the active material in some microbolometers.

 

APD (Avalanche Photodiode) – An optical or infrared detector in which the electrical bias is sufficiently high that the electrical charges generated by the absorption of radiation multiply, resulting in high gain and a short time constant.

 

Afocal – a lens or lens system in which the object and image are at infinity.

 

B-Kit – A thermal imaging sensor that can be installed on a variety of military platforms. For 2nd Gen systems, the B-kit consists of a SADA II, 1 watt linear Stirling cooler, an imager (scanner), afocal optics and an electronics unit. The electrical/mechanical interface to specific platforms is called an “A-kit.”

 

Blackbody – A surface that perfectly absorbs all radiation incident upon it. Such a surface emits radiation having spectral distribution and intensity described by a predictable formula that depends only on the surface temperature.

 

Bolometer – An infrared sensor that operates on the principle that the resistance of certain materials varies with temperature.

 

BST (Barium-Strontium Titanate) – A ferroelectric material used as the active element in many first-generation uncooled thermal imaging sensors, which required the use of a continuously-moving synchronous chopper.

 

CCD (Charge-Coupled Device) – An integrated circuit that facilitates the processing of signals by moving charge packets along a rectilinear array of sites in the circuit. Often, the charge packets are generated by optical or infrared detectors, and sometimes the packet sites are themselves the detectors. “CCD” is also sometimes used to refer to a type of visible imager.

 

CMOS (Complementary Metal-Oxide Semiconductor) – An integrated circuit technology that facilitates the fabrication of both n-channel and p-channel field-effect transistors (FETs) on the same substrate. The technology is used to fabricate optical imagers.

 

Core – An assembly containing as a minimum, a focal plane array and the electronics necessary to drive it. The core may also include a lens and additional signal/image processing.

 

Crosstalk – An undesirable effect wherein an optical signal incident on one detector pixel produces an extraneous signal on an adjacent pixel.

 

Cryocooler (Cryogenic Cooler) – A miniature refrigerating device used to cool some infrared detectors.

 

Current Mode – A mode of detector operation in which an input signal modulation produces an output current modulation. Current mode is sometimes referred to as constant-voltage mode, because a constant-voltage bias causes the detector to act as a current source.

 

D* (Specific Detectivity) – A figure of merit for comparing infrared detector pixels of different areas and measured with different electrical bandwidths. It is defined as the square-root of the pixel optical area multiplied by the square-root of the electrical bandwidth, divided by the noise-equivalent power (NEP). It is a useful metric primarily when the detector NEP is proportional to the square-roots of both the pixel area and the electrical bandwidth.

 

Dark Current – A current that flows in a detector pixel even in the absence of optical (or infrared) signal, and is indistinguishable from signal current. Low dark current is a critical feature of high-performance photodetectors.

 

Diode – An electrical component that allows current to flow in only one direction.

 

Dual Band – Describes a sensor that detects radiation from two different spectral bands.

 

Dual Color – Describes a sensor that detects radiation from two different sub-bands within a single spectral band.

 

DVE (Driver’s Vision Enhancer) – An infrared imaging device used in a motorized vehicle to improve the driver’s vision at night or under other low-visibility conditions.

 

DVE (Degraded Visual Environment) - A poor visibility environment (due to dust, smoke, etc.), especially such as faced by helicopters when landing or hovering near the ground. DVE is also sometimes referred to as a Pilotage System that can see through this environment. It may include Infrared, Lidar, and/or Millimeter wave technologies.

 

EBAPS (Electron Bombarded Active Pixel Sensor) – A low-light-level imager that consists of a photocathode and a micro-channel plate of an image intensifier coupled to an addressable solid-state sensor (which absorbs electrons emitted by the micro-channel plate).

 

EFVS (Enhanced Flight Vision System) - an EVS system that is coupled with a HUD (Head Up Display) and is approved by the FAA for Landing Credits to descend below the usually accepted decision height as seen by the naked eye.

 

EMCCD (Electron Multiplying Charge-Coupled Device) – A CCD imager having for each charge packet an additional series of transfers at sufficiently high voltage to cause electron multiplication, thereby increasing both the signal and the noise so as to minimize the effect of extraneous noise sources.

 

ENVG (Enhanced Night Vision Goggle) – A night vision goggle enhanced by the addition of a thermal imaging sensor, the images being registered and combined either optically (ENVG-O) or digitally (ENVG-D).

 

EO/IR (Electro-Optical/Infrared) – Used as an all-inclusive term to group together electronic imaging of the optical (i.e., the visible) and infrared spectral regions.

 

EVS (Enhanced Vision System) – A thermal imaging system mounted in an aircraft to provide real-time video for improved visibility at night, in fog and other low-visibility conditions. Also see EFVS.

 

Emissivity – The radiation emitted by a surface, expressed as a fraction of that emitted by an ideal blackbody at the same temperature.

 

Epitaxy – the growth of high-quality crystalline films on a lattice-matched substrate.

 

F# (Pronounced “eff-number”) – Usually written f/#, the ratio of the effective focal length of an optical system to its aperture diameter. Image brightness for an ideal optical system is inversely proportional to the square of the f/#.

 

Ferroelectric – A material that becomes electrically polarized after being temporarily subjected to an electric field, and whose polarization reverses after being subjected to a reversed field.

 

FET (Field-Effect Transistor) – The basis of all modern semiconductor circuits, it is a transistor whose conductance is governed by the voltage applied to the gate, which is a conductive plate separated from the transistor substrate by an insulating layer. The term is in contrast to a junction or bipolar transistor.

 

FLIR (Forward-Looking Infrared) – another name for a thermal imager. The term originated to distinguish stand-alone thermal imaging devices from the very earliest, which were downward-looking devices mounted in an aircraft and having a linear array of detectors aligned orthogonally to the direction of aircraft motion, the aircraft motion being use to “sweep” the array and form a two-dimensional image of the ground below.

 

FMS (Foreign Military Sales) – Sales of military hardware to foreign governments arranged through the U.S. Government. (In contrast to “Direct Military Sales”, which occur directly from a supplier to a foreign government).

 

FOM (Figure of Merit) – A number or group of numbers that characterizes the relative value of a particular device such as an infrared detector or an imaging system.

 

FOV (Field of View) – The angle, usually measured in degrees, of the region that is visible to an imaging device. A circular sensor such as an image intensifier has a single FOV, whereas a rectangular sensor such as an infrared FPA has a horizontal FOV (HFOV) and vertical FOV (VFOV).

 

FPA (Focal Plane Array) – A two-dimensional array of detector pixels, as distinguished from a linear array.

 

FWS (Family of Weapon Sights) - a new family of weapon sights being developed by the U.S. Army for individual, crew-served or sniper use.

 

GaSb (Gallium Antimonide) – A compound semiconductor that absorbs radiation of wavelengths shorter than about 1.7µm, making it a candidate material for detectors in the NIR spectral band. It is also used as a substrate for T2SL detectors.

 

GPS (Global Positioning Satellite) – One of an collection of Earth orbiting satellites that emit signals that permit a specially-designed receiver to determine its precise location on Earth.

 

Gyrostabilized – Stabilized by means of gyroscopes.

 

HgCdTe (Mercury-Cadmium Telluride) – A semiconductor compound often used as the active element in infrared photodetectors in the LWIR, MWIR and SWIR spectral bands, owing to the tunability of its spectral response by adjusting the mercury-to-cadmium ratio. Also commonly referred to as MCT (and, in the U.K., sometimes referred to as CdHgTe and CMT).

 

Hyperspectral – A sensor that is capable of simultaneously measuring the intensity of radiation in each of many spectral sub-bands.

 

Hz (Hertz) – the standard unit of frequency, equal to one cycle per second.

 

II-VI – Pronounced “2 – 6”, denotes elements in columns II (e.g., Hg, Cd) and VI (e.g., Se, Te) of the periodic table of the elements.

 

III-V – Pronounced “3 – 5”, denotes elements in columns III (e.g., In, Ga) and V (e.g., As, Sb) of the periodic table of the element.

 

I2 (Image Intensifier, also I2) – A device that amplifies available light and forms a visible image at very low ambient light levels.

 

IDCA (Integrated Detector-Cooler Assembly) – A subsystem that combines an infrared detector array with the cryocooler necessary for its operation.

 

IDIQ (Indefinite Delivery Indefinite Quantity) – refers to a government contract for which neither the number of items nor the delivery schedule is initially specified.

 

IFOV (Instantaneous Field of View) – the angle in object space viewed by a single pixel.

 

InAs/GaSb (Indium Arsenide/Gallium Antimonide) – A material combination used in Type 2 superlattices (T2SL) for infrared detectors.

 

InGaAs (Indium Gallium Arsenide) – A compound semiconductor used to make Vis/NIR/SWIR detectors with a cutoff wavelength of 1.68µm and, with special processing, out to 2.6µm.

 

InSb (Indium Antimonide) – A compound semiconductor used to make very high performance MWIR detectors that require cooling to about 80K (-193°C).

 

Ion Implantation – A method for implanting desired impurities in semiconductors by using electrically accelerated ions.

 

IP (Internet Protocol) – The principal communications protocol for relaying packets of information across network boundaries, thereby enabling the internet.

 

IR (Infrared) – Refers to that part of the electro-magnetic spectrum having wavelengths longer than visible light (~0.75µm) and shorter than microwaves (~300µm).

 

IRST (Infrared Search and Track) – An infrared sensor that provides broad-area (usually 360°) coverage and provides means to automatically detect and alert against incoming threats such as missiles.

 

LRF (Laser Range Finder) – A device that determines the distance to a remote object by measuring the time for laser radiation to reflect from the object and return to the device.

 

LWIR (Long-Wave Infrared) – The region of the infrared spectrum just above the MWIR and just below the VLWIR, with wavelengths ranging from about 7µm to about 15µm; more narrowly, referring to the atmospheric transmission window contained within that band, often referred to as the 7-to-14 band, the 8-to-12 band, or a similar descriptor.

 

MBE (Molecular Beam Epitaxy) – A highly-controlled, but slow method of growing high-quality films on a lattice-matched substrate in an ultra-high vacuum system.

 

MCT (Mercury-Cadmium Telluride) – See HgCdTe.

 

MEMS (Micro-Electro-Mechanical Systems) – Miniaturized mechanical and electrical-mechanical devices fabricated using processes developed for standard silicon integrated circuits. MEMS devices are used as support structures for microbolometers and some other thermal detectors.

 

MRTD (Minimum Resolvable Temperature Difference, often written simply MRT) – The smallest temperature difference at which a standard four-bar target is completely visible against a uniform background. MRT is usually specified as a curve, being a function of the size of the four-bar target.

 

Microbolometer – An array of pixels, each of which is a bolometer fabricated using MEMS processing techniques.

 

Micro-Channel Plate – The gain element in an image intensifier tube, it is a thin wafer of hexagonal, closely-packed hollow fibers, each of which is coated on its inner wall with a material that emits multiple electron when bombarded with a single high-energy electron.

 

Micron – A millionth of a meter. Abbreviated as “µm”.

 

mK (milli-Kelvin) – A thousandth of a Kelvin; a thousandth of a degree Celcius.

 

MOVPE (Metal-Organic Vapor Phase Epitaxy) – A method of growing an epitaxial film using two or more complex organic gases, each containing a different metal, the gases reacting to release the metals to react with one another and form the desired film.

 

MTF (Modulation Transfer Function) – A measure of the crispness of the image created by an optical system, MTF is the modulation (i.e., the difference between the maximum and the minimum) of the image of a target whose intensity varies sinusoidally in one dimension, compared to that of an ideal optical system. MTF is generally expresses as a curve, being a function of the spatial frequency of the target.

 

MWIR (Mid-Wave Infrared) – The region of the infrared spectrum just above the SWIR and just below the LWIR, with wavelengths ranging from about 3 µm to about 7 µm; more narrowly, referring to the atmospheric transmission window contained within that band, often referred to as the 3-to-5 band.

 

Nanotube – A long, thin, hollow structure formed by a single layer of atoms or molecules chemically bonded to one another in a regular pattern, and often having unusual mechanical, electrical and/or thermal properties.

 

nBn – An infrared or optical photodetector formed by two n-type semiconductor layers (usually antimonides) separated by a thin barrier layer that blocks majority carriers while allowing minority carriers to flow, thereby reducing dark current and permitting operation at a higher temperature. See also pBp and XBn.

 

NEP (Noise-Equivalent Power) – For a detector pixel, the incident radiant power necessary to produce a signal equal to the rms noise.

 

NETD (Noise-Equivalent Temperature Difference) – For a thermal imaging system, the scene temperature difference necessary to produce a signal difference equal to the rms noise.

 

NIR (Near Infrared) – The region of the infrared spectrum just above the visible and just below the SWIR, with wavelengths ranging from about 0.75µm to about 1.5µm.

 

NVG (Night Vision Goggle) – a low-light-level imager typically operating with image intensifier tubes.

 

pBp – An infrared or optical photodetector formed by two p-type semiconductor layers separated by a thin barrier layer that blocks majority carriers while allowing minority carriers to flow, thereby reducing dark current and permitting operation at a higher temperature. See also nBn.

 

PbS (Lead Sulfide) – A compound semiconductor used in infrared photodetectors (1 – 3 µm).

 

PbSe (Lead Selenide) – A compound semiconductor used in MWIR photodetectors.

 

Photocathode – The light-sensitive surface of an image intensifier tube.

 

Photodetector – A detector in which the absorption of a photon directly modifies an electrical property of the sensitive material, and for which the output signal is proportional to the number of photons absorbed. See also Thermal Detector.

 

Photodiode – A diode that generates a current proportional to the rate of photon absorption.

 

Photovoltaic – A mode of detector operation in which the output voltage is proportional to the rate of photon absorption. More loosely, the mode of operation of a photodiode, sometimes even when operating in the current mode.

 

Pixel – Originally a contraction of “picture element”, it is one of a multitude of “dots” that together form an image. When referring to a detector array, it relates to a single detector element.

 

PIR (Passive Infrared) – Refers in an infrared sensor that uses no illumination source and detects only radiation emitted by the scene. Typically, this term is used to refer to single element motion detectors used for security.

 

PtSi (Platinum Silicide) – A MWIR photodetector formed by the interface (a Schottky barrier) between a silicon wafer and platinum contacts. PtSi arrays were initially promising because of excellent uniformity, but soon fell out of favor because of low quantum efficiency.

 

Pyroelectric – The property of some polar materials, such as ferroelectrics, wherein the polarization changes with temperature.

 

Quantum Efficiency – For a photodetector, the fraction of incident photons converted into electrical charges.

 

QWIP (Quantum Well Infrared Photodetector) – An infrared detector whose spectral response is determined by an engineered quantum state resulting from a potential well created by a very thin layer (a few monolayers) of semiconductor material between two layers of wide bandgap materials, effectively restricting electrons to a two-dimensional region.

 

Radiometric – Quantitative, as opposed to relative, measurement of absolute radiation intensity. A radiometric imager provides quantitative values of either the temperature or the power being emitted by the various portions of the scene.

 

Range-Gated Imaging – A mode of active imaging wherein a laser pulse is emitted, and the detector is turned on thereafter only for a brief time, resulting in an image being formed of only objects within a relatively narrow range of distances.

 

ROIC (Read-Out Integrated Circuit) – An integrated circuit that is coupled to an imaging array to facilitate the multiplexed output of the array with minimal losses and minimal extraneous noise.

 

SADA I (Standard Advanced Dewar Assemble I) – The standard detector, cooler and dewar for special (airborne) U.S. military 2nd generation thermal imaging systems. The detector array is 480 x 4 LWIR MCT in which 4 rows are operated in TDI. The detector temperature of 67 K is maintained by a 1.5 watt linear Stirling cryocooler.

 

SADA II (Standard Advanced Dewar Assemble II) – The standard detector, cooler and dewar for U.S. military 2nd generation thermal imaging systems. The detector array is 480 x 4 LWIR MCT in which 4 rows are operated in TDI.. The detector temperature of 77 K is maintained by a 1 watt linear Stirling cryocooler.

 

SLS (Strained-Layer Superlattice) – A superlattice in which the alternating layers have different lattice constants (spaces between atoms), resulting in distortion of the lattice and consequent modification of the optical and electrical properties of the structure.

 

SOI (Silicon-on-Insulator) – A semiconductor substrate in which devices are fabricated in a thin silicon layer separated from the bulk silicon wafer by an insulator, the purpose being to reduce parasitic capacitance and increase circuit speed. SOI is also used for a type of uncooled infrared sensor.

 

Shutter – An opaque flag that is periodically interposed between an FPA and the scene to provide a uniform reference source for updating non-uniformity correction.

 

Snapshot Mode – An imaging mode in which the signals from all pixels are generated simultaneously as opposed to, for example, row-by-row.

 

Spectral Band – One of the conventionally-defined spectral regions commonly designated as Vis, NIR, SWIR, MWIR, LWIR or VLWIR.

 

Stirling Cryocooler – A closed-cycle cryocooler based on an approximation to the thermodynamic Stirling cycle.

 

Superlattice – A structure fabricated by alternating thin epitaxial layers of two different materials, resulting in unique optical and electronic properties. Typically, these are made of III-V materials, such as InAs/GaSb. Please also see T2SL.

 

SWIR (Short-Wave Infrared) – The region of the infrared spectrum just above the NIR and just below the MWIR, with wavelengths ranging from about 1.5µm to about 3µm; more narrowly, referring to the atmospheric transmission window contained within that band.

 

TDI (Time Delay and Integrate) – A function used in some scanned thermal imagers to accumulate multiple exposures and thus effectively increase the integration time. Typically, several redundant rows of pixels are used to accumulate the signal and thus improve the signal-to-noise ratio.

 

TEC (Thermoelectric Cooler) – A solid-state device used for cooling infrared detectors and other electronic devices/equipment, using the Peltier effect. TEC designs can range from multi-stage devices capable of cooling small heat loads to as low as 170K (-103°C), or they can be single-stage devices capable of removing large amounts of heat without reducing the temperature.

 

TEC-less – Operating without a TEC. This typically refers to uncooled FPAs that need no temperature stabilization.

 

T2SL (Type 2 Superlattice) – A superlattice detector designed so that electrons and holes are confined in different layers, reducing the recombination rate and therefore the dark current, enabling efficient operation at higher temperatures.

 

TWS (Thermal Weapon Sight) – A family of military thermal imaging systems for fire control of small arms ranging from individual weapons such as rifles to crew-served weapons such as machine guns and grenade launchers. The three members of the TWS family are Light (LTWS), Medium (MTWS) and Heavy (HTWS).

 

Thermal Detector – A detector in which the absorption of a photon changes the temperature of the sensitive material, which possesses some temperature-dependent electrical property, and for which the output signal is proportional to the power in the absorbed photons. See also Photodetector.

 

Thermal Imager – A device that images radiation emitted from a scene by virtue of the temperature distribution of objects in the scene.

 

Thermocouple – A device comprising lengths of two dissimilar materials in contact with one another at one end, contacting opposite poles of an electrical measuring device at the other, and used to measure the temperature difference between the connected ends and the input of the measuring device.

 

Thermography – Radiometry in which the quantitative output is temperature.

 

Thermopile – Two or more thermocouples connected electrically in series and thermally in parallel. A type of infrared sensor.

 

UAS (Unmanned Aerial System) – A UAV and its associated payload and launch and retrieval system.

 

UAV (Unmanned Aerial Vehicle) – An aircraft piloted by remote control.

 

Vis (Visible) – The spectral band of visible light, nominally from 0.39µm to 0.75µm.

 

VLWIR (Very Long Wave Infrared) – The region of the infrared spectrum above the LWIR, with wavelengths ranging from about 15µm to about 300µm.

 

Voltage Mode – A mode of detector operation in which an input signal modulation produces an output voltage modulation. Voltage mode is sometimes referred to as constant-current mode, because a constant-current bias causes the detector to act as a voltage source.

 

VOx (Vanadium Oxide) – A compound semiconductor of various specific compositions used as the active element in some microbolometers and some opto-electronic devices.

 

XBn – An infrared or optical photodetector formed by two semiconductor layers (usually antimonides), one of which (X) can be n-type or p-type separated by a thin barrier layer that blocks majority carriers while allowing minority carriers to flow, thereby reducing dark current and permitting operation at a higher temperature. See also pBp and nBn.