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)
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
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
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
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).
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.
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.
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.
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.
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.
The ratio of the vertical length to the horizontal width.
For a television display the aspect ratio is 3:4.
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.
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
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
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.
The average temperature of the environment around an
object under study. The background temperature creates the radiation level
available as reflected energy.
A filter that is transparent for a range of wavelengths
and opaque (reflective or absorptive) for wavelengths outside this bandpass
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)
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.
Image distortion that spatially expands the center of a
display and contracts the corners to make rectangular display objects appear
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
A viewing device made for display images to both eyes
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)
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.
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. =
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.
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.
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.
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.
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.
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.
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
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.
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
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).
The wavelength determined when the transmission of a
filter drops below 5%.
The wavelength of a filter where the transmission first
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.
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.
A device which converts infrared irradiance into
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
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
A compound lens assembly consisting of two lens elements.
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.
The ratio of active time to total time allowed in
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).
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.
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
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.
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.
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
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.
The distance from the convergent point for the radiation
(focal point) to its affiliated principal 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
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.
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.
The full width of the passband of a filter measured
between 50% transmission points.
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
The flow of thermal energy from one object to another.
By means of conduction, convection or radiation)
A unit for measuring frequency. One hertz is one cycle
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.
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
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
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
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.
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.
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
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
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.
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.
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.
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.
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
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.
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.
A measurement of length in the metric system appropriate
for measuring infrared radiation wavelengths. 1,000,000 microns equals one
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
Having the characteristic of steadily increasing.
Functions are monotonic if they are continuous, single valued, and
See FPA - focal plane array.
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
Near Infrared (SWIR)
The shortest wavelength infrared radiation band - 0.7 to
Neutral Density Filter
A filter which attenuates radiation uniformly over a
wide range of wavelengths; used in infrared systems as a temperature range
Unwanted signal interference - usually separated into
various forms of signal cross talk and random noise generated by means
internal to the sensor.
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
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
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
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.
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).
A detector which responds to radiant flux be generating
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).
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
An optical instrument for remote sensing and measurement
of spot temperatures or radiation levels
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.
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.
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
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.
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
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.
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.
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).
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.
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.
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
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
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.
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.
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
A measure of the minimum amplitude of input signal
change to which an instrument will respond. This is a measurement of
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).
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.
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.
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
Indicating reflections or a shiny surface.
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.
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.
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, 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.
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.
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.
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.
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.
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
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.
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
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.
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
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.
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.).
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.
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
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
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.
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
The length of distance between cycles on a repetitive
Win's Displacement Algorithm
A statement relating the temperature of an object to the
wavelength of maximum radiative output. (They are inversely proportional.)
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.