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Glossary of Terms



A graph presentation of signal versus time or signal versus space. A simplified display of the progressive temperature changes in an object over time or the thermal gradient along a cursor line.
Any undesired factor that distorts an image. Typical sources of image aberrations are noise, signal crosstalk, external signal pickup; optical effects such as chromatic aberration, vignetting, video effects such as barrel distortion, pin cushion; digitizing effects such as pixel clock crosstalk, video leveling, inadequate sampling, etc.
Absolute Temperature Scale
A temperature measurement scale based on the coldest possible temperature equal to 0. (See Rankine and Kelvin)
Absorptance (Absorptivity)
The ratio between the amount of energy absorbed and the total energy incident (irradiance), usually measured at a surface. The absorptance of a surface is equal to its emissivity.
A measure of the similarity of an instrument reading to the actual value for that reading. Instrument accuracy is affected by many things -instrument drift, environment, temperature, time, operator expertise, etc.
Active Element
The area on a detector which responds to radiation by creating a signal output. Many detectors contain both unused (inactive) elements and unused areas which are not part of the active, or responsive element area.
Active IR system
A system using an (active) IR source to illuminate a scene - much like a visible image scene is illuminated with a floodlight. An active IR system monitors the illuminated scene in much the same way as a passive system.
Usually refers to radiation in image space for a system adjusted for infinity focus. This radiation is fairly telecentric, having a very long focal length (no focal length).


Airy Disc

A measurement of the resolving capability of a lens expressing the diameter of the minimum spot size. Depends on lens, aperture and on the wavelength of passband radiation. (A.D. = 1.2 x lambda / A)
The ratio between the amount of energy reflected and the total energy incident (irradiance), usually measured at a surface. The albedo of a surface is equal to its reflectance.
Ambient temperature
The temperature of the surrounding air and environment considered the baseline measurement for heat transfers around an object.
Having an irregular structure. Amorphous crystal structures are more useful in optical assemblies since they are not as prone to fracture along a crystal boundary. Crystals can sometimes be converted from mono-crystalline to poly-crystalline to amorphous by proper annealing.
The process of heating and cooling a material at a controlled rate to improve its internal stresses and modify its internal crystalline or chemical structure. Annealing can make metals and optical materials more flexible and less brittle.
Antireflective Coating
A thin layer of material applied to the surface of an optical element to increase the transmission of the element by reducing bascattering due to surface reflections. A simple A/R coating will be comprised of a one-quarter wavelength thick coating of a material whose refractive index is between the indexes of the air and the element material.
One millimicron; a unit to measure the wavelength of high frequency electromagnetic radiation such as visible light.
A hole or opening limits the amount of radiation used by an optical system. The aperture of a system may be set by the size of the lens or window bounding the cross section of the radiation bundle.
Aperture Stop
The limiting aperture of an optical system, usually where the system energy bundle size is fully determined. Multiple aperture stops in a system with moving optical elements will usually lead to image aberrations and vignetting.
Apparent Temperature
The temperature determined for an object calculated by a remote infrared sensor based solely on the amount of radiation it emits assuming the object has Blackbody emission characteristics.
Aspect Ratio
The ratio of the vertical length to the horizontal width. For a television display the aspect ratio is 3:4.
Atmospheric Windows
Spectral radiation regions not absorbed by atmospheric gasses. These windows are transparent to radiation at those wavelengths. The most obvious window is the visible light window - if the 'smog is not too bad we can see through the atmosphere forever.
Atmospheric Attenuation Atmospheric Absorption
The amount of signal reduction occurring when IR radiation travels through the atmosphere between the target and imaging system. The amount of attenuation can be very small on clear days with short transmission paths and can be very large on foggy, smoggy, dusty days with long transmission paths. Attenuation The reduction in intensity of radiation after processing through a filter. This can be measured in dB.
Azimuth Angle
The angle measurement of horizontal dimension on an optically generated display. Normally zero azimuth is taken to be the center of the field of view and dimensions to the right and left are positive and negative azimuth angles.


Back Surface Mirror
A mirror who's reflective coating is on the opposite side of the optical material from the incident radiation. Back surface coating is used often in visible optics to help protect the mirror surface. Back surface coating is normally not used in infrared optics due to the additional attenuation caused by the passage of radiation through the optical element.
The re-reflection of thermal energy generated on the ground and reflecting off the underside of clouds or inversion layers. Also can refer to unwanted front surface reflections off of a transparent optical element.
Background Noise
The noise present in a sensor (detector) independent of the signal strength or ambient temperature. Normally caused by thermal, generant-recombinant characteristics or 1/f effects.
Background Temperature
The average temperature of the environment around an object under study. The background temperature creates the radiation level available as reflected energy.
Bandpass Filter
A filter that is transparent for a range of wavelengths and opaque (reflective or absorptive) for wavelengths outside this bandpass region.
A range of wavelengths or frequencies passed or processed. Typically used in both electronics and optics to designate the operating range of a filter or electronic component.
A unit for measuring gas pressure in the cgs system. one bar is equal to 10 6 dynes per square centimeter. (29.52 in. of Mercury)
Bar-Code Reader
An instrument providing an alternate means of entering data into a computer by reading a series of machine printed parallel lines off of a product label. The ASCII data is encoded in the width of the parallel lines or bars and the spacing between bars.
Barrel Distortion
Image distortion that spatially expands the center of a display and contracts the corners to make rectangular display objects appear barrel shaped.
Barrier Potential
In a photonic detector, the width of the energy gap between the conduction band and the valence band. This sets the maximum wavelength of incident radiation for detector response. Beam A stream or column of particles or rays, a bundle of radiation. The beam size is usually limited by the aperture of an optical system.
Bias (Supply, Current, Voltage)
Electrical current or voltage generated in a low noise, highly isolated supply applied to a photo-conductive detector through selected load resistors so that the detector response to photons (a resistance change) can be measured.
A display device providing a separate display for each eye.
A viewing device made for display images to both eyes simultaneously
An ideal thermal radiator, usually one whose temperature can be controlled, radiating and absorbing the maximum possible thermal radiation for the set temperature. (Emissivity = 1.0, Reflectance = 0.0, Transmittance = 0.0)
Blur Circle
The smallest size focused spot a lens system can produce at its focal plane.
An temperature measuring instrument using a strip thermistor to achieve higher sensitivity than a simple thermistor. Unlike thermistors which are used for contact temperature measurements, bolometers have been used to measure radiation levels.
The alignment of the sensing axis of a detector to the display axis so that the center of the display screen represents straight ahead to the imager.
A term to indicate the relative amount of light intensity available. Brighter is more light, dimmer is less light. In an infrared system, the brightness control may affect actual image intensity or it may change the temperature range displayed. Either effect will change the brightness of a given temperature.
British Thermal Unit (BTU)
The amount of thermal energy required to raise one pound of water one degree Fahrenheit at or near freezing.
A defined, limited and usually concentrated beam of radiation rays. The ray bundle represents the radiation available to an optical system for focusing on to a detector.


The process of adjusting an instrument to read accurately. or in some cases, accurately constructing a table of errors so that the instrument reading can be adjusted off-line or post measurement to correlate the reading accurately to the true value.
The amount of thermal energy required to raise one gram of water one degree-Celsius at 15'C. Case Hardening A method of strengthening the surface of metals or optical materials involving heating the surface and quenching rapidly with air or water; leaving an amorphous layer of material on and just below the surface.

Celsius (Centigrade)

A scale for measuring temperature where absolute cold is -273.2'C, the melting point of water (ice point) is 0 C, and the boiling point of water is 100 C. Centimeter A unit of length measurement in the metric system. one hundred centimeters is equal to one meter ( 2.54 cm. = 1")
As in "Certified Infrared Thermographer".   A level of competence in understanding the laws of thermal dynamics with regard to Infrared Thermography.  A 40 hour course plus an exam that must be passed.
Channel Valance
See shading.
A means of optically blocking and unblocking the detector sensor repetitively. A chopper normally consists of a round, flat, pancake-like blade rotated by a motor. The blade is cut so that a regular pattern of openings and blockages pass in front of the detector.
Clear Aperture
The useful area of a lens. The edges of a lens can not be used due to edge effects in coating and due the mounting hardware interfering with ray transmission. The clear aperture is usually specified to exclude the edges and is usually circular in diameter.
Cold Shield
A means of increasing the sensitivity of a detector by reducing the background noise. A cold shield is built within the detector package and is cooled to a temperature close to the operating temperature of the detector. It blocks non-signal, high temperature background photons from the detector and decreases the level of background noise.
Traveling together in parallel, as in parallel light rays.
Color Scale
An area of the display screen set aside to display the full range of available colors. Used to demonstrate how each color is assigned a temperature or radiation level.
Color Thermogram
A display of temperature differences which has been enhanced by the assignment of discrete color values to each of the thermal intensity levels. This is considered the normal visual display mode for a thermal video system.
The transfer of energy through a solid without motion of the conducting solid as a whole. Conduction Band The energy band above the energy gap in a semiconductor. Electrons with enough energy to reach the conduction band can freely move within the semiconductor in response to externally applied potential gradients.
The relationship between the brightest intensity on a display and the dimmest. In an infrared system this may adjust the temperature range on the display. Increasing and decreasing the display range of temperature will change the apparent brightness difference between two different temperature values.
The transfer of energy through a liquid or gas due to the motion of the medium.
A medium, usually gas or liquid that is either held at low temperatures or generates low temperatures when it expands used to reduce the temperature of another object.
A quantity measurement electricity. one coulomb is equal to 6.3 x 1018 electrons.
Critical Angle
The smallest incident radiation angle at which total internal reflection occurs. This angle is defined using Snell's law, setting the sine of the refraction angle of the lower index material equal to one.
A graphically-based cursor overlay for identifying locations on an image - represented as the intersection of a vertical and horizontal line.
A form of signal interference created when undesired signal radiation or current affects the signal being interpreted. This interference can be from other currents and signals within the electrical module or can be picked up from radio, television, or nearby switching.
Cryogen Cryogenics
Pertaining to extreme cold. A cryogen is a material which will create extremely cold conditions. Cryogenics is the study of extreme cold. In infrared, cryogenics usually refers to the means used to reduce the detector temperature to a useful value. Temperatures less than 200 K could be considered cryogenic.
A device for cooling detectors to cryogenic temperatures and maintaining these temperatures accurately. Operates on the principal of Joule-Thompson expansion. Contains a small orifice. Gas under high pressure is put through the orifice and allowed to expand rapidly. This creates a small field of extreme cold which is placed in the vicinity of the detector array.
A means of identifying location on a video display. Cursors are usually supported by separate display fields than either text or image data displays and can be moved by either graphics input devices or the keyboard. Typical cursor types include a text cursor (underline or blinking block), graphics cursors (dots or crosshairs), and AOI cursors (circles,, rectangles, polygon shapes).
Cut-off Wavelength
The wavelength determined when the transmission of a filter drops below 5%.
Cut-on Wavelength
The wavelength of a filter where the transmission first exceeds 5%.


D-star (D*)
A measurement of the sensitivity of a detector material in terms of signal to noise ratio. D-star is normally expressed either as a blackbody D-star or as a peak wavelength D-star within the practical operating frequency of the detector. The units of D-star are centimeter-square root hertz per watt.
Dead band
The region of signals close enough to the set point of a controlling instrument that they do not initiate a cycle of corrective action. Similar to the noise level of a monitoring instrument.
An increment of temperature measurement value. delta-T Small differences of temperature
Depth of Field
The range of object distances which are in focus around a set focus distance value (object space distance).
Depth of Focus
The range of motion of the focus assembly which around a set focus distance which will not defocus an object at that focus distance (image space distance).
The ability of a detector to sense radiant power - the signal to noise ratio. The inverse of a noise power reading. Detectivity is related to noise since the signal power must be at least as large as the noise power to be distinguished as signal - not noise.
Detector, IR
A device which converts infrared irradiance into electrical energy.
Detector Element
A small area etched out of a detector material (called substrate) which does the actual photon sensing and conversion. Most of the rest of the substrate is inactive and either covered with a plating or not activated.  Many detector elements can be put on one substrate and used for sensing IR radiation.
A vacuum walled container for thermally isolating the contents from the outside. Dewars are used for holding and transporting cryogenic liquids.
Diffuse Reflections
Non-specular reflections of defocused, uniform intensity for a wide range of reflection angles. A perfectly diffuse surface is defined as a Lambertian surface the intensity is constant no matter what the angle of view.
Conversion of analog data into digital data. This can be done manually or automatically. In an infrared system an Analog to Digital converter is used to convert analog radiation signals into digital information which the CPU can process into equivalent temperature information.

Doublet, Lens

A compound lens assembly consisting of two lens elements.
Dwell Time
The length of time that the detector is allowed to observe the same location to create its radiation signal. Normally the longer the dwell time allowed, the less noisy and more sensitive the detector will be.
Duty Cycle
The ratio of active time to total time allowed in repetitive process.


Edge Effects (Optical)
Image or data distortions which can occur at the edges and corners of an image due to optical aberrations building up at the extreme range of the field of view. These aberrations can include geometric distortion (fish eye), vignetting (hot center/dark corners), ray distortion (apparent corner defocusing).
Electromagnetic Radiation
The field effects given off by accelerating a charged particle in a magnetic field. Depending on field strength and speed of acceleration, many types of electromagnetic radiation are created.
Electromagnetic Spectrum
A plot of the range of wavelengths and types of electromagnetic radiation found to exist from subsonic waves to cosmic rays.
The ability of an object to radiate and absorb energy from its surroundings measured as a ratio of the actual object emission to the blackbody equivalent emission.


Pertaining to heat absorption. A chemical reaction is endothermic if it requires heat to complete the reaction.

Entrance pupil
The size of the aperture located on the front surface of the infrared window containing all of the ray bundles required to scan the object field of view. This is the map of the system aperture stop scanned on the Infrared window.
A condition where all of the thermal changes in a system have stabilized. To observe true heat flow characteristics, a gradient must be developed across an object or interface and allowed to stabilize - this is a thermal equilibrium.
To empty. Used in relation to cleaning Dewars or cylinders prior to refilling them with pure gas. Cylinder evacuation is usually done by drawing a vacuum with a vacuum pump and mildly heating the cylinder to expedite the evacuation. Pure evacuation cleaning can take up to two days of continuous vacuum so an alternating cycle of purging and evacuating the container is recommended.
Pertaining to heat generation. A chemical reaction is exothermic if it liberates heat as the reaction proceeds.


F-number /#)
The ratio of focal length to aperture for a lens assembly.
A temperature measurement scale which defines the ice point of water as 32 F and the boiling point of water as 212 F. Absolute zero is -459.7 F.
Far Infrared (LWIR)
Infrared radiation whose wavelength is in the range from 8 to 100 microns.
Field of View (FOV)
The total field measured in angle within which objects can be imaged or measured and displayed by an infrared system.
Filter (optical)
An optical device which modifies the characteristics of radiation which is passed through it. Usually filters either attenuate all wavelengths of radiation a certain controlled amount or modify the optical passband of the radiation - eliminating selected wavelengths or bands while allowing others to pass.
Forward Looking Infrared


The spectral emission of long wave radiation such as visible or infrared illumination by some materials when stimulated shorter wave radiation.
Rate of energy flow expressed as radiant power. Focus The ability of a lens or system to bring image radiation to a point of convergence within the confines of the active detector sensing area.
Focal Length
The distance from the convergent point for the radiation (focal point) to its affiliated principal plane.
Focal Plane
The plane created by mapping the points of convergence for rays which pass through a lens assembly from an object which is moved on a plane perpendicular to the optical axis at great distance from the lens assembly (i.e. collimated rays). For a detector to work properly, all of its active elements must be placed in the same focal plane for such an object. Otherwise, portions of the object space will appear defocused.
The number of cycles an operation occupies per period of time. The normal unit of measurement for frequency is Hertz.
Front Surface Mirror
A mirror whose reflective surface is on the same side as the incident radiation. Front surface mirrors coated for maximum reflection are normally used in infrared optics to maximize the signal strength of the reflected radiation.


Geometric Concentration
The ability of a lens to concentrate radiation on the focal point; the ratio of aperture size to active detector area.
A thermal reference source which maintains an output, not at 100% emission (which would be a blackbody) but at a lesser ratio. Graybody emissions maintain a constant emission ratio over a wide frequency range. This distinguishes them from spectral radiators.
Gray Scale
An area of the screen reserved for displaying an increasing ramp of image intensities to be used as a calibration aid. Periodic intensity levels are labeled with the temperature or radiation value they represent.


Half Width
The full width of the passband of a filter measured between 50% transmission points.
Thermal energy.
Heat Capacity
A measurement of an objects ability to store thermal energy. Heat capacity equals the specific heat of an object multiplied by its density multiplied by its size (or specific heat times mass).
A device for dissipating heat; it absorbs heat by conduction from heat producing devices and dissipates heat by means of convection.
Heat Transfer
The flow of thermal energy from one object to another. By means of conduction, convection or radiation)
Hertz (Hz)
A unit for measuring frequency. One hertz is one cycle per second.
High-pass Filter
A filter which is blocked at low wavelength, whose transmission spectrum extends from a low cut on wavelength up in wavelength to the maximum pass wavelength of the detector.
A term expressing the "color" aspect of a color display; part of the HLS (Hue, Luminosity, Saturation) color model. Without hue, there are only shades of gray.


Ice Point / Ice point reference
The Temperature at which water freezes. A device which creates the ice point temperature very accurately as a reference for thermocouple temperature measurement.
Image Uniformity
See shading
Image Enhancement
Techniques used to modify an image to present an observer with more readily accessible information.
The part of an infrared system which contains the detector, optics and scanning mechanism. The imager must be held and pointed at whatever scene is to be studied. The word imager is sometimes used instead of viewer to mean an infrared system which creates relative radiation images without calibration.
Incident Radiant Energy
Total energy impinging on a surface from the surroundings.
Infrared (IR)
Electromagnetic radiation which occupies the band from 0.7 microns to 100 microns. Infrared radiation is between the visible spectrum and microwave radiation.
The region of the electromagnetic spectrum that covers from 1 to 100 microns.  The term comes from the Latin base of beyond the red, as referring to the fact that beyond the red color of the light spectrum there is still energy but it is invisible to the human eye with out the aid of a infrared imaging instrument.  The discipline of viewing objects based on there thermal properties is called "Infrared Thermography"
Infrared Camera
A optical/electrical device that transfers thermal radiation into a visible image that is displayed on a CRT screen, Monitor, LCD or other display.  Infrared cameras are primarily used Certified Infrared Thermographers in civilian industrial applications and Research and Development in what is called Infrared Thermography.  Infrared imaging systems come in many capabilities and specific uses.  They may only be able to display the infrared image, or they may be able digitally record the image and perform temperature measurements and analysis at the same.  Infrared image cameras primarily work in the 3-5 or the 7-14 micron range.  They should not be confused with Infrared film that is primarily used in photographic camera.  Infrared film works in the .7 to 1 micron range and dose not see the thermal energy that is emitted from objects at close to ambient temperatures.  Infrared film see the reflected energy off of objects that is at temperatures of greater that 1,100 deg. F
Infrared Thermography
The science of viewing and understanding objects based on their ability to give off thermal radiation.  The use of specialized infrared imagers that can allow someone to view the object based on the amount of thermal radiation that the object either gives off or reflects.
Infrared Thermographer
Someone that is trained in the sciences of viewing, measuring and understanding how objects give off their thermal radiation when viewed through an infrared imaging system.  Most Infrared Thermographer today are Certified by certain infrared training programs that they have passed a criteria of proficiency in the science of Infrared Thermography.  Certification alone dose not indicate a level of proficiency or quality when performing services, but it show that the thermographer has taken the time to further his level of knowledge.  Just as in any trade, the level of education, years of experience, degree of professionalism and quality of past work must be taken into consideration.
Infrared Film
Photographic media which can image radiation in the SWIR band from about 0.7 microns to 1.1 microns.
An optical element usually placed on the front of an infrared system that is transparent to infrared radiation but excludes radiation of other wavelengths and protects the internal sensor components.
Instantaneous Field of view (IFOV)
The angle in milliradians derived by dividing the active detector element's size by the system's effective focal length. An effective figure of merit for system resolution can be derived by dividing the field of view by the instantaneous field of view.
A material to reduce heat transfer by conduction. A good insulator has a large R-value and a large thermal resistance; a low thermal conductivity.
A technique used in visual displays to produce high resolution images at update rates slower than the eye flicker frequency. With interlace, a fraction of the total frame lines (called a field) is displayed at multiples of the frame rate. These lines are spread out across the entire display area and with each field update, a different set of lines are displayed. The number of fields required to reconstruct the entire frame is the interlace factor. The lines are usually spaced apart by the interlace factor. Normal television is interlaced by a factor of two fields per frame.
The intensity of radiation impinging on a surface, the rate of impact of radiation.
Areas or lines of constant thermal irradiance.
Areas or lines of constant temperature.
Isothermal Windowing
An image processing feature which allows the user, to enhance a range of isotherm or isoradian levels. This feature allows the user to highlight areas of similar temperature and easily find the hottest and coldest object in an area such as an electrical distribution panel.


A measurement of thermal energy. one joule is one newton meter, 10 7 ergs, or approx. .737 foot-pounds.


The most commonly used absolute temperature scale. The scale is based on the Celsius scale degree increment with 0 K equal to absolute zero cold.
Kinetic Energy
The energy of motion.


Lag (thermal time)
The delay between the construction of a thermal gradient and the achievement of equilibrium.
An optical component constructed of transparent substance with one or two curved surfaces of different curvature which has the ability to change the direction of beam travel. Infrared lenses are used for focusing the detector at a distance of interest and for modifying the size and distance of the focused field of interest.
Level slicing
An image processing function like isotherm windowing. The visual display is modified by changing the image intensity or color of a region of levels which the user selects. Level slicing is usually implemented by blocking out or whitening out all the levels above or below the user selected level.
The region of the electromagnetic spectrum which is visible to the human eye. Usually considered as the region from 0.39 (violet) to 0.77 (red) microns.
Limiting Resolution
The highest spatial frequency which a system can resolve, regardless of target temperature. Ultimately the spatial performance end of the MRT curve. Usually expressed in milliradians. Line scan An instrument scanning and display mode which scans in one direction only but displays those lines side-by-side in the perpendicular direction to create a two-dimensional thermal map of a scene over time or panning motion.
Linear Array
A collection of active detector elements set in a straight line on a plane usually spaced equal distances apart from each other with a single element in the perpendicular direction.
Low-pass Filter
A filter blocked at higher wavelengths, with a cutoff wavelength in the passband of the detector that passes all wavelengths shorter than the cutoff wavelength.
An SI unit for measuring radiant flux. One lumen is produced per solid angle by a point source with one candela intensity.
The portion of a color display which is related to display intensity; how bright a color is perceived by the eye.


micron (micrometer))
A measurement of length in the metric system appropriate for measuring infrared radiation wavelengths. 1,000,000 microns equals one meter.
Mid Infrared (MWIR)
The middle infrared spectrum, usually from 2.4 to 7.0 microns. Milliradians A measure of small angles. Two thousand-pi milliradians can be measured in a complete circle. There are 17.4 mares per degree of angle.
A viewing device made to produce an image for one eye only.
Having the characteristic of steadily increasing. Functions are monotonic if they are continuous, single valued, and steadily increasing.
See FPA - focal plane array.


Narcissus Effect
An optical phenomenon of scanning systems which describes how a detector can look back at itself or view a mixture of active scene and itself for certain angles of scan. The narcissus effect creates blurry cold areas on screen in an infrared system.
Narrow Bandpass Filter
A multi-layer filter usually based on interference effects whose cut-on and cutoff frequencies are very close to each other. In infrared, the half width of a narrow bandpass filter will be 0.5 to 1.0 micron.
Near Infrared (SWIR)
The shortest wavelength infrared radiation band - 0.7 to 1.4 microns.
Neutral Density Filter
A filter which attenuates radiation uniformly over a wide range of wavelengths; used in infrared systems as a temperature range extension filter.
Unwanted signal interference - usually separated into various forms of signal cross talk and random noise generated by means internal to the sensor.


Object plane
The plane created by tracing ray paths backwards from the active detector elements through the focusing lens assembly, through the scanning mechanism to project the active field of focused image area in space. The object plane is an optical description of what the system can focus. Ocular The eyepiece of an optical system.
An instrument to measure electrical resistance.
The characteristic of not passing any incident radiation (Transmissivity = 0). An optical filter is said to be blocked in a waveband if it has less than 5% transmission over those wavelengths.
Operating Temperature Range
The range of ambient temperatures over which a system will function accurately. This range can be defined by either a calibration limit; what range of ambient temperature offsets are allowed in the data calculations or it can be due to a functional limit in the system; the ability to keep the detector cold or it can be set by a catastrophic limit; batteries may not hold their charge below certain temperatures and integrated circuits will not function above certain temperatures.
Overlap (optical)
Redundant scanning of the image field which results in multiple sets of data taken from the same image points. If done in the direction of scan this is usually considered oversampling. If done by laying scan lines adjacent to each other closer than the optical spot dimension, it is considered scan overlap.
To take multiple sets of data redundantly. Usually indicates digitization of a signal at a rate faster than the maximum information frequency of the data. An oversampling rate of at least two is indicated by information theory to avoid aliasing of repetitive information (moire).
Expanding the beam deflection on a CRT so that the active viewing area is larger than the available CRT faceplate. This cuts off the edges of the active raster. This technique is used on most television monitors.


A means used to observe an object which is larger than the field of view of the system. The operator moves the system back and forth across the target to discover thermal areas of interest.
Passive Infrared System
The normal form of infrared system in use commercially. A passive system does not generate a thermal source to illuminate the scene, it is only acted upon. A passive system monitors the flux of photons already being generated by the scene within its field of view.
Peltier Effect
The ability for two dissimilar metals and metal/semiconductors to create a thermal difference when current flows across the dissimilar junction.
A radiation sensitive resistor whose resistance decreases as it is exposed to radiant energy.
A single quantum of electromagnetic energy having momentum hf/c and energy hf. (h is Planck's constant, f is the frequency of the wave and c is the speed of light).
Photovoltaic Detector
A detector which responds to radiant flux be generating current.
Pixel (Picture element)
The smallest location size on a display or in memory. The incremental location of picture information in either horizontal or vertical direction (also called a Pel).
Principal Plane
A mathematical construct useful in analyzing complex optical assemblies. The principal planes are constructed at the effective focal length distance away from the lens assembly focus and serve to represent the mathematical position of the lens assembly assuming it could be constructed as a single thin lens.
Registered trademark of Hughes Aircraft Co. assigned to the Hughes line of commercial infrared system products.
The map or footprint of the system aperture stop system onto a surface. The pupil represents the collection of all ray paths used by the system to form an image. Obstruction of the pupil will lead to image vignetting.
An optical instrument for remote sensing and measurement of spot temperatures or radiation levels


Qualitative Analysis
An analysis of objects or processes which is concerned with deriving structural, material, or relative information. This type of analysis can be done with imagers, line scanners, and viewers; temperature or radiometric data output is not required.
Quantitative Analysis
An analysis of objects or processes which is concerned with measuring temperatures or radiant energy levels by assigning numerical values to the characteristics of the displayed scene.
Quantum Efficiency
The ratio of actual detectivity to theoretical detectivity for a given detector material.
A technique used for stopping one process with another. In metal or glass formation, crystal formation during cooling can be stopped by rapid quenching with air or water.


A measurement of angle. There are 2 r radians in a full circle of 360 angular degrees. Radiance The total intensity of thermal energy (radiant flux) which can be observed from a surface. It is scaled by unit of solid angle of view and by unit of area from the surface. The flux includes all forms of radiation emission, reflection and transmission.
Radiant Energy
See radiance
Heat transfer of energy in the form of electromagnetic waves. Forms of radiation include cosmic rays, gamma rays, x-rays, ultraviolet radiation, infrared, visible light, radio, audio, and sub sonic. Radiometer An instrument which measures radiation levels. Such an instrument can be calibrated in power or temperature.
A temperature scale based on O*R equal to absolute zero temperature. The increment of one degree Rankin is equal to one degree Fahrenheit. 459.70 Rankin is equal to O*F.
Rayleigh Criterion
A measure of maximum lens resolution. The smallest possible blur circle a lens can create will depend on the aperture of the lens and the wavelength of the radiation. for circular optics: R = 1.22 (lambda)/a
Retesting of equipment to return it to an original standard or calibration. In infrared, it is spoken of in two ways. If the infrared system is being used to produce calibrated data, it must be recalibrated to a secondary standard regularly. When high pressure containers are used to hold the cryogen, they must be recertified to hold the required pressure, either every three or every five years. See the notice on the cylinder.
Reflectance (Reflectivity)
The amount of total radiance which can be attributed to reflected energy. Usually expressed as a percentage of total energy.
A measure of accuracy and stability for an instrument. The capability of an instrument to duplicate a previous answer to a previous experiment within a limited margin of error.
Remote Sensing
To obtain a reading or measurement from a distance, without physical contact between the meter and the object to be measured.
A measure of the capability of a system to resolve small objects against the viewing field. Measured as a ratio of angles.
A measure of detector performance, measures the value of the detector output change for a given applied thermal power change.
Same as scan wheel. one method of creating a field scanning mechanism using a reflective polygon, spinning on its axis.


The ratio of pure color to washed out white in the display of a color. 100% saturation is pure color. 0% saturation is pure white, at whatever intensity is encoded in the color.
Scale Factor, Scaling
A factor used to adjust the gain of an object parameter. Normally used in Temperature range adjustment, the gain of the amplifiers is scaled until the desired temperature range is displayed in full contrast from black to white. The units associated with a reading is a temperature scale (F, C, K, R).
Scale Sensitivity
The minimum adjustment allowed in the system per intensity level. A system is normally configured so that the operator can set a scale sensitivity greater than the system sensitivity. If the NET of a system were 0.5*C, the scale sensitivity might be O.IOC or .050C to allow enough signal for offline image improvement features such as averaging to reduce the noise level.
Scan conversion
The reformatting of data, usually image data from one standard to another. In the case of infrared, the sensor output is not compatible in either time nor number of lines nor number of resolution elements to television. The sensor output must be scan converted into a television format in the image processor.
Scan Efficiency
The ratio of active scan time to total scan time. Can be expressed as a function of the ratio of two angles also. Normal scan efficiencies with rotating polygons can be from 25% to 60% (with reimaging optics). Normal scan efficiencies with galvanometer mirrors can be as high as 90%.
Scan Line
One line of infrared data, whether from left to right or top to bottom, as output to the processor for scan conversion. The number of scan lines read by the scanner is the maximum resolution of the system in one direction. This resolution will be limited further by any overlap factor used.
Scan Wheel
A mechanical scene scanning method which involves a number of flat facets incorporated into an aluminum wheel., rotating around an axle through the center of the ring of mirrors.
Seebeck Effect
When a junction of dissimilar metals changes temperature, a current will flow across the junction - creating a measurable emf.@ This is the principal of operation of a thermocouple.
Selective Radiator
A source which radiates thermal energy is specific bandwidths and emits little or no radiation in others. An incandescent light is a gray body radiator while a quartz iodine light is a selective radiator. Hot pipes are gray body radiators while flames are selective (spectral) radiators.
A measure of the minimum amplitude of input signal change to which an instrument will respond. This is a measurement of thermal resolution.
The component which converts radiation into electrical signals. Can be used to refer to the detector specifically, the imager, or the entire system.
A measure of the non-uniformity of signal in an infrared system. Shading is measured by exposing the system to a wide area, constant temperature target and measuring the variations around a mean value. Shading may be identified by source - for example optical shading will be characterized by strong responses in the center and weak responses in the corners (vignetting). Channel to channel variations in response can also be a source of shading (Channel balance).
Specific Heat
A property of materials. The specific heat of a material indicates how much thermal energy (in joules) is required to increase a mass (in grams) of material a small temperature difference (degree C). Specific heat is one of the principal factors in determining heat capacity, conduction rates in a material ' and the thermal time constant of an object subject to heating.
Pertaining to the electromagnetic spectrum, depending on wavelength or frequency, varying based on wavelength.
Spectral absorption
The ability a filter or gas has of transmitting radiation of some wavelength while absorbing materials of other wavelength. The atmosphere is a spectral absorber for relatively long infrared path lengths.
Spectral emission
A source which radiates thermal energy is specific bandwidths and emits little or no radiation in others. An incandescent light is a gray body radiator while a quartz iodine light is a selective radiator. Hot pipes are gray body radiators while flames are selective (spectral) radiators.
Indicating reflections or a shiny surface.
Spot size
The minimum size of object which can be resolved at a given distance by an optical device. Stability A measure of system accuracy and reliability. Indicates how little a system reading moves from an original value.
Standards, Primary
The ultimate physical standard used as a basis for measurements. In the United States, all primary standards are maintained by the National Institute of Standards and Technology. The primary standards are used to calibration secondary standards.
Standards, Secondary
Secondary standards are physical standards and references, which have been calibrated directly off of the NIST primary standard. These standards are normally maintained by the more sophisticated testing laboratories and used to calibration reference standards.
Standards, Reference / Standards, Working
Tertiary standards are the standards that are actually used in plant facilities for the calibration of product. They are periodically calibrated against the secondary standards. The normal working standards used in the infrared field consist of Black body sources, thermocouples and thermocouple meters, RTDs , and thermometers.
Sterling Refrigerator
A component used for cooling detectors to cryogenic temperatures. A sterling cycle refrigerator operates on compression and expansion of gasses like a home refrigerator. However, the refrigerator uses the Sterling cycle and Helium gas instead of freon.


An object in the object plane which the system can focus on and analyze.
Television Line (Video line)
A single horizontal line of display data on a television display. NOT the same as scan line which is the data input: a television line is a single line of output data.
An expression of thermal energy density. How hot or cold an object is.
Temperature Range
The maximum to minimum temperature display capability of a system. It should be specified whether the range under consideration is the range displayed, the total display range capability of the system, or the operating temperature range of the system.
Thermal Conductivity
A property of materials which measures the ability of a material to conduct heat. It can be expressed as power per degree length (watts/meter-C). Metals have a high thermal conductivity (conduct well) while air fibers and plastics have much poorer conductivities.
Thermal Expansion
The change of size which materials undergo as their temperature changes. In tight mechanical tolerance assemblies, the thermal expansion coefficients must be matched to maintain tolerances over an significant operating temperature range.
Thermal Gradient (or Thermal Profile)
A graph of temperature changes over a distance. A thermal gradient is usually expressed and displayed as a straight line -sometimes only vertical or horizontal.
Thermal Radiation
Electromagnetic energy whose natural wavelength fall between .7 and 100 microns.
Thermal Viewer / Thermal Data Viewer
A class of remote temperature sensing systems. This equipment class offers an image of relative radiation levels and a means for obtaining temperature information from the screen. A common alternative is to have a bulls eye target indicator and an alphanumeric overlay display of the temperature and the operator selected emissivity.
Thermal Video System
A class of remote temperature sensing systems. This class of equipment is characterized by a fully calibrated display screen, video output, provision for the standard optical and data handling accessories.
A device which measures temperature. The sensor for the thermistor is a semi-conducting resister whose resistance changes significantly with temperature.
A device which can measure temperatures, usually by contacting the device to be measured. The device is made from a junction of dissimilar metals, as the junction changes temperature, a voltage is created which is read by a previously calibrated meter.
Thermoelectric Cooling
A solid-state device which converts current into a temperature difference between two junctions. It is possible to put thermoelectric junctions in series or parallel to increase either the overall temperature drop or their power.
A two dimensional hard copy record of the apparent scene temperatures displayed on an IR system (usually a photograph of the display).
The study of remote temperature measurement
A meter for measuring temperature.
A number of thermocouples whose junctions are assembled in series to magnify the Seebeck voltage and increase the sensitivity of the reading. Thermopiles have been used in remote sensing pyrometers.
Transmittance (transmissivity)
A measurement of the ability of a material to pass radiation from one side to the other without absorbing or reflecting it. Transmittance is the ratio of transmitted radiant energy to total irradiance.
Triple Point (of water)
The temperature at which all three phases of the material can exist at equilibrium. 0.01'C for water.
A compound lens assembly comprised of three lenses.


The amount that a cooling cycle in a thermal feedback control system cools beyond the set point.


Valence Band
The energy level which contains molecularly bound electrons which can not move within the crystal lattice. For there to be conduction in a semi-conductor, valence electrons much be stimulated to overcome the energy gap and enter the conductance band.
Velocity of Light
The speed at which light travels in a vacuum. 186,280 miles per second.
Velocity of Sound
The speed at which sound travels in air at STP. 12.3 miles per second (nom.).
Vertical Blanking
The time interval during a video signal allowed for the vertical raster to retrace from the bottom of the CRT to the top. During this interval the CRT is blanked from writing on the phosphor.
A method of storing, generating and reconstructing pictures on monitors based on serial electrical signals and raster scanning of electroptical cameras and displays. Used in television.
View Factor
A factor which adjusts the radiant energy transfer values between two objects based on their relative surface geometry.
A small display attached to a camera (or IR sensor) for aiding the operator in adjusting the location and display characteristics of the equipment. A viewfinder reproduces the finally system display at a reduced size and resolution, usually in black and white to help with adjustments.
(Infrared) Viewer
A class of remote infrared sensing equipment. This class of-equipment creates an image of relative radiation levels but does not allow any direct readout of intensity values. This kind of equipment is quite useful for police, fire, and security use. Viewers have been used effectively for many years in the qualitative analysis of maintenance inspections.
The loss of radiation intensity due to the blocking of a portion of the beam bundle between the focused point on the object plane and the focused point on the image plane.
Void (thermal)
An area in a surface which displays significantly different thermal impedance from adjacent areas. This can be due to a different object internal -structure - such as an insulation void in the wall of a building.


A measure of power equal to one joule expended for one second.
The length of distance between cycles on a repetitive event.
Win's Displacement Algorithm
A statement relating the temperature of an object to the wavelength of maximum radiative output. (They are inversely proportional.)
Wheatstone Bridge
An electronic circuit for measuring the resistance of an object. This circuit is of specific value in measuring the resistance change of a thermistor as it tends to linearize the thermistor response.




Copyright 2004 The Professional Thermographers Association
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