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Glossary of Terms

Thermographic Glossary of Terms

A

Absolute Zero
The Temperature that is zero on the Kelvin temperature scale. The temperature at which no molecular motion takes place in a material.

Absorbtivity, (α) (Absorptance)
The proportion (as a fraction of 1) of the radiation energy iminging on a material's surface that is absorbed into the material. For a blackbody, this is unity (1.0). Technically absorbtivity is the internal absorptance per unit path length. The thermography, the two terms are often used interchangeably.

Accuracy (of measurement)
The maximum deviation, expressed in % of scale or reading (depending on the measurment range and instrument dynamics) or in degrees celsius, that the reading of an instrument will deviate from a correct standard reference.

Ambient Operating Range
Range of ambient temperatures over which an instrument is designed to operate within published specifications.

Ambient Temperature
See Reflected Apparent Temperature.

Ambient Temperature Compensation
See Reflected Apparent Temperature Compensation.

Anomaly
Any irregularity, such as a thermal anomaly on an otherwise isothermal sirface. Any indication that deviats from what is expected.

Apparent Temperature
The target surface temperature indicated by an infrared point sensor, line scanner or imager without compensation for measurement (object) perameters

Artefact
A product of an artificial character due to extraneous agency; an error caused by an unacompensated anomaly. In thermography, an emissivity artefact simulates a change in surface temperature but is not a real change. A hot solar reflection or a cold reflection due to narcissus would be examples of artefacts.

Atmospheric Temperature
The temperature of the atmosphere between the camera and the object.

Atmospheric Window (infrared)
The spectral interval within the infrared spectrum in which the atmosphere transmits radiant energy well (atmospheric absirption is a minimum). These are roughly defined as 2-4µm and 8-14µm.

B

Blackbody, Blackbody Radiator
A blackbody absorbs all incident radiation. From Kirchhoff's Law it follows that a blackbody is also a perfect radiator. A perfct radiator radiates the maximum number of protons in a unit of time from a unit of area in a specified spectral interval into a hemisphere that any body in thermodynamic equilibrium at the same temperature can radiate. In Thermography, a calibration reference radiator is referred to as a "Blackbody Radiator", although it is a practial, not theoretical device.

Blackbody Curves
Plots of radiant power spectral exitance vs. wavelength (W/m²/µm) for various temperatures according to the Planck equation. These curves show he maximum amount of energy at any given wavelength that can be radiated by an object due solely to its temperature. Also called Planck curves

Bolometer, Infrared (micro-bolometer)
A type of thermal infrared detector commonly used in spot pyrometers. The micro-bolometer is used in uncooled imaging radiometers.

C

Calibration
Checking and/or adjusting an instrument such as that its readings agree with a standard. Calibration removes instrument systematic error.

Calibration Source, Infrared
A Blackbody radiator of known temperature and effective emissivity, used as a calibration reference.

Capacitance, thermal
This term is used to describe heat capacity in terms of an electrical analog, where loss of heat is analogous to loss of charge on a capacitor. Structures with high thermal capacitance lose hear more slowly that structures with low thermal cpacitance.

Capacity, heat
The heat capacity of a material or structure describes its ability to store heat. It is the product of the specific heat (c_p) and the density (ρ) of the material. This means that denser materials generaly will have higher heat capacity capacities than porous materials. Also called thermal mass.

Celsius (Centigrade)
A temperature scale, formed by the swedish physicist Anders Celsius, which is based on 0°C as the freezing point of water and 100°C as the boiling point of water at standard atmospheric pressure; a relative scale related to the Kelvin scale [0°C = 273.15 K where
ΔT 1°C = ΔT 1K]

Colour
A trm sometimes used to define wavelength or spectral interval, as in two-colour radiometry (meaning a method that measures in two spectral intervals0; also used conventionally (visual colour) as a means of displaying a thermal image, as in colour thermogram.

Coloured Body
See non-graybody.

Conductance, thermal
A measure of the ability of a material of defined thickness and cross-sectional area to conduct heat. Rlated to the material property, thermal conductivity. The inverse of thermal resistance (C=1/R).

Conduction, thermal
The only mode of heat low in solids but can also take place in liquids and gases. It occurs as a result of atomic vibrations (in solids) and molecular collisions (in liquids and gases) whereby energy is transfered from locations of higher temperature to locations of lower lemperature.

Conductivity, thermal (κ)
A material property defining the relative capacity to carry heat by conduction in a static temperature gradient. Conductivity varies slightly with temperature in solids and liquids and with temperature and pressure in gases. It is high for metals (copper has a κ of 380W/m/°C) and low for porous materials (concrete has a κ of 1.0W/m/°C) and gases.

Convection
The form of heat transfer that takes place in a moving medium and is almost always associated with transfer betwen a solid (surface) and a moving fluid (such as air). Whereby energy is transferred from sites of higher temperature to sites of lower temperature

D

Delta T
The temperature difference between two targets or locations. Usually of comparable targets under comparable conditions.

Detector, infrared
A transducer element that converts incoming radiant energy impinging on its sensitive surface into an electrical signal.

Diffuse reflector
A surface that reflects a portion of the incident radiation in such a manner that the reflected radiation is equal in all directions. Also called a Lambertian surface.

Diffusivity, thermal (α)
The ratio of conductivity (Κ) to the product of density (ρ) and specific heat (c_ρ). α = Κ/ρc_ρ [m² sec^-1]. The ability of a material to distribute thermal energy after a change in heat input. A body with a high diffusivity will reach a uniform temperature distribution faster than a body with a lower diffusivity.

D* (detectivity)
Sensitivity figure of merit of an infrared detector - detectivity expressed inversely so that higher D* indicate better performance; taken at specific test conditions of chopping frequency and information bandwidth and displayed as a function of spectral wavelength.

Display resolution, thermal
The precision with which an instrument displays its assigned measurement parameter (temperature), usually expressed in degrees, tenths of degrees, hundredths of degrees, etc.

E

Effusivity, thermal (e)
A measure of the resistance to temperature change of a material. e is a relevant thermophysical parameter for surface heating or cooling processes as well as quenching processes. e = (kρc_ρ)^½: [W m^-2 K^-1 s^½]
Where k = thermal conductivity, ρ = bulk density, c_ρ = specific heat.

Emissivity (ε)
The ratio of a target surface's radiance to that of a blackbody at the same temperature, viewed from the same angle and over the same spectral interval; a generic look-up value for a material. Values range from 0 to 1.0. Also called emittance.

EMI/RFI noise
Disturbances to electrical signals caused by electromagnetic interference (EMI) or radio frequency interference (RFI). In thermography, this may cause noise patterns to appear on the display and in saved images.

Equalization, emissivity equalization
A method for determining the emissivity value in each individual pixel of the image, much used for objects where the emissivity varies from pixel to pixel, e.g. printed circuit boards. The found values are then used for finding the correct temperatures in each point of the image.

Equalisation box
A box, in which a test object can be heated up uniformly to a preset temperature. Knowing this temperature it is possible to calculate the emissivity in each pixel of the image of the object. Much used for accurate temperature measurements on objects with great emissivity variations over the surface e.g. transistors, PCBs.

Exitance, radiant (also called radiosity)
Total infrared energy (radiant flux) leaving an object surface. This is composed of radiated, reflected and transmitted components. Only the radiated component is related to the surface temperature of the object.

F

Fahrenheit
A temperature scale based on 32°F as the freezing point of water, and 212°F as the boiling point of water at standard atmospheric pressure; a relative scale related to the Rankine scale [0°F = 459.67R;
where ΔT 1°F = ΔT 1R]. Used in the USA only. Relation to the Kelvin scale is: 100K = 180R

Field of View (FOV)
The angular subtense over which an instrument will integrate all incoming radiant energy. In a radiation thermometer this defines the target spot size; in a scanner or imager this defines the scan angle or picture size.

Fibre optic, infrared
A flexible fibre made of a material that transmits infrared energy, used for making non-contact temperature measurements when there is not a direct line of sight between the instrument and the target.

Filter, spectral
An optical element, usually transmissive, used to restrict the spectral band of energy received by an instrument's detector.

Flame filter
A filter of a specific waveband used to minimise the effects of flame, enabling the IR camera to "see through" the flame. The specific waveband is a region where the contents of CO₂ and H₂O does not reduce transmittance. The central wavelength is 3.9μm, the bandwidth is less than 0.25μm.

Focal plane array (FPA)
A linear or two-dimensional matrix of detector elements, placed in the focal plane of an instrument. In thermography, rectangular FPA's are used in "staring" (non-scanning) infrared imagers. These are called IRFPA imagers.

Focal point
The point at which the instrument's optics image the infrared detector at the target plane. In a radiation thermometer, this is where the spot size is the smallest.

Frame repetition rate
The image update frequency - how many times per second the infrared imager delivers a complete IR picture.

Full scale
The span between the minimum value and the maximum value that an instrument is capable of measuring at a selected measurement range.

G

Grey body
A radiating object whose emissivity is a constant value less than unity (1.0). over a specific spectral range.

H

Heat transfer
The movement of heat from one point to another by conduction, convection and/or radiation.

Hertz (Hz)
A unit of measurement of signal frequency; 1 Hz = 1 cycle per second.

Herschel, Wilhelm
A German astronomer, physicist and musician, who was active in Bath, UK, where he discovered infrared in the year 1800.

I

Image, infrared
See Thermogram.

Imager, infrared
An infrared instrument that collects the infrared radiant energy from a target surface and produces an (electronic) image of the target exitance. This image can be shown on a screen in monochrome (black and white) or colour, where the grey shades or colour hues correspond respectively to target exitance. Usually does not have any measurement capability.

Image processing, thermal
Analysis of thermal images, usually by computer, enhancing the image to prepare it for computer or visual analysis. In the case of an infrared image or thermogram, this could include temperature scaling, spot temperature measurements, thermal profiles, image manipulation, subtraction and storage.

Imaging radiometer
An infrared thermal imager that provides quantitative thermal images.

Indium Antimonide (InSb)
A material from which fast, sensitive photo-detectors used in infrared scanners and imagers are made. Such detectors require cooling while in operation.

Infrared (IR)
The infrared spectrum is loosely defined as that portion of the electromagnetic continuum extending from the red visible (0.75 μm) to about 1,000μm.

Infrared radiation thermometer
An instrument that converts incoming infrared radiant energy from a spot on a target surface to a measurement value that can be related to the temperature of that spot.

Infrared thermal imager
See Imager, infrared.

Instantaneous field of view (IFOV)
The angular subtense found by the ratio of the detector dimension divided by the instrument focal length; the projection of the detector at the target plane. In a radiation thermometer this defines the target spot size; in a line scanner or imager it represents one resolution element in a scan line or a thermogram and is one measure of spatial resolution. Also see SRF.

Isotherm
A pattern superimposed on a thermogram or on a line scan that includes or highlights all points that have the same apparent temperature or exitant radiation.

J

NA

K

Kelvin
Absolute temperature scale related to the Celsius (or Centigrade) relative scale. The Kelvin unit is equal to 1°C; 0 Kelvin = -273.15°C; the degree sign and the word "degrees" are not used when expressing Kelvin temperatures.

Kirchoff's law
In thermal equilibrium, the absorbtivity of an object equals its emissivity (&aplha; = ε).

L

Laser pyrometer
An infrared radiation thermometer that projects a laser beam to the target, uses the reflected laser energy to compute target effective emissivity and automatically computes target temperature (assuming that the target is a diffuse reflector) - not to be confused with laser-aided aiming devices on some radiation thermometers.

Latent heat
Also called "hidden heat" as heat is added or removed without changing the temperature. The amount of heat required (or released) for a change of phase from solid to liquid and liquid to gas (or vice versa). The latent heat of vaporisation is the amount of heat required to change one gram of liquid to vapour without change of temperature. The latent heat of fusion is the amount of heat to melt one gram of solid to liquid with no temperature change.

Line scan rate
The number of target lines scanned by an infrared scanner or imager in one second.

Line scanner, infrared
An instrument that scans an infrared field of view along a straight line at the target plane in order to collect infrared radiant energy from a line on the target surface, usually done by incorporating one scanning element within the instrument. If the target (such as a sheet or web process) moves at a fixed rate normal to the line scan direction, the result can be displayed as a thermogram.

Lock-in thermography
A term used to describe an active technique for infrared non-destructive material testing, in which the sample is stimulated with periodically varying (sinusoidally) radiant energy, thus generating heat waves through the material. The resulting thermal images are processed to determine flaw depth and severity. The term Lock-In comes from the fact that excitation, image acquisition and analysis are phase-locked to each other.

M

Measurement spatial resolution
The smallest target spot size on which an infrared imager can produce a measurement, expressed in terms of angular subtense (mrad per side). The slit response function (SRF) test is used to measure M.

Medium, transmitting medium
The composition of the measurement path between a target surface and the measuring instrument through which the radiant energy propagates. This can be vacuum, gaseous (such as air), solid, liquid or any combination of these.

Mercury cadmium telluride MCT (HgCdTe)
A material used for fast, sensitive infrared photo-detectors used in infrared sensors, scanners and imagers that requires cooled operation.

Micron (micrometer) (μ or μm)
One millionth of a meter; a unit used to express wavelength in the infrared.

Milliradian (mrad)
One thousandth of a radian (1 radian = 180/π); a unit used to express instrument angular field of view; an angle whose tangent is equal to 0.001; 1 mrad = 0.05729578°).

Minimum resolvable temperature (difference), MRT(D)
Thermal resolution; thermal sensitivity - the smallest temperature difference that an instrument can clearly distinguish out of the noise, taking into account target size and characteristics of the display and the subjective interpretation of the operator. The limit of MRTD is MDTD (minimum detectable temperature difference). MDTD is the MRTD of an extended source target, that is, a target large enough to be fully resolved by the instrument.

Modulation
In general, the changes in one wave train caused by another; in thermal scanning and imaging, image luminant contrast; (Lmax - Lmin)/(Lmax + Lmin).

Modulation Transfer Function (MTF)
A measure of the ability of an imaging system to reproduce the image of a target. A formalised procedure is used to measure MTF. It assesses the spatial frequency resolution of a scanning or imaging system as a function of distance to the target.

N

Noise equivalent temperature (difference), NET(D)
The temperature difference that is just equal to the rms noise signal; a measure of thermal resolution; (thermal sensitivity), but not taking into account target size, characteristics of the display and the subjective interpretation of the operator.

NIST, NIST traceability
The National Institute of Standards and Technology (formerly NBS). Traceability to NIST is a means of ensuring that reference standards remain valid and their calibration remains current.

Non-greybody
An object whose emissivity varies with wavelength over the wavelength interval of interest. A radiating object that does not have a spectral radiation distribution similar to a blackbody and can be partly transparent to infrared (transmits infrared energy at certain wavelengths); also called a "coloured body" or "real body". Glass and plastic films are examples of non-greybodies. An object can be a greybody over one wavelength interval and a non-greybody over another.

O

Objective, objective lens
The primary lens of an optical system. On an infrared instrument, usually the interchangeable lens that defines the total field of view.

Opaque
Impervious to radiant energy. In thermography, an opaque material is one that does not transmit thermal infrared energy, (τ=0).

Optical element, infrared
Any element that collects, transmits, restricts or reflects infrared energy as part of an infrared sensing or imaging instrument.

Oversampling
Collecting samples at a rate higher than the Nyquist critical frequency, f_c = 1/(2Δ), where Δ is the sampling interval. Applies to both time and spatial domains.

P

Peak-hold
A feature of an instrument whereby an output signal is maintained at the peak instantaneous measurement for a specific duration.

Photo-detector (photon detector)
A type of infrared detector that has fast response, (on the order of microseconds), limited spectral response and usually cooled operation; photo-detectors are used in infrared radiation thermometers, scanners and imagers.

Pixel
Abbreviation for picture element. In infrared technology a pixel is a focal plane array element, for scanning systems it is defined by the SRF function or rather associated with a sample, for spot radiometers by FOV.

Planck, Max Karl Ernst Ludwig
German physicist who incorporated quantum physics into the blackbody spectral radiance equation, giving rise to blackbody curves.

Pyroelectric detector
A type of thermal infrared detector that acts as a current source with its output proportional to the rate of change of its temperature.

Pyroelectric vidicon (PEV), also called pyrovidicon
A video camera tube with its receiving element fabricated of pyroelectric material and sensitive to wavelengths from about 2 to 20μm; used in infrared thermal viewers.

Pyrometer
Any instrument used for temperature measurement. A radiation or brightness pyrometer measures visible energy and relates it to brightness or colour temperature. An infrared pyrometer measures infrared radiation and relates it to target surface temperature.

Q

Qualitative measurement
The process of obtaining and interpreting thermal images based on thermal contrast in order to identify areas that correlate to construction or sub-surface anomalies; the purpose is more to determine where a temperature difference exists than to obtain a number for the temperature difference or the actual temperature.

Quantitative measurement
The process of obtaining thermal images with correct temperature readings. Especially useful in situations when the exact temperature or temperature difference of the target determines whether it falls in or out of a determined criteria or range of acceptability.

R

Radian
An angular measurement equal to the ratio of the arc length of a circle to its radius. The circumference of a circle is 2π times the radius. Thus π radians = 180 degrees, and 1 radian = 57.29578 degrees.

Radiation, thermal
The mode of heat flow that occurs by emission and absorption of electromagnetic radiation, propagating at the speed of light. Unlike conductive and convective heat flow, it is capable of propagating across a vacuum. The form of heat transfer that allows infrared thermography to work since infrared energy travels between the target and the detector by radiation.

Radiosity
The radiance from a target surface due to emission, reflection and transmission that is sensed by the IR instrument.
See also exitance, thermal.

Rankine
Absolute temperature scale related to the Fahrenheit relative scale. The Rankine unit is equal to 1 F°; 0 Rankine = -459.67°F; the degree sign and the word "degrees" is not used in expressing Rankine temperatures. It is a non-metric scale, which is used exclusively in the USA.

Ratio pyrometer, also called two-colour pyrometer
An infrared thermometer that uses the ratio of incoming infrared radiant energy at two narrowly separated wavelengths to determine a target's temperature independent of target emittance; this assumes "greybody" conditions and is normally limited to relatively hot targets (above about 150°C). The technique is built upon quite strong simplifications and is reasonably correct only at higher temperatures.

Reference junction
In a thermocouple, the junction of the dissimilar metals that is not the measurement junction. This is normally maintained at a constant reference temperature.

Reflected apparent temperature
The apparent temperature of the radiant energy impinging on an object that is reflected off the object and enters the instrument. Originates from the scene behind and surrounding the instrument, as viewed from the target. The reflection of this background appears in the image and affects temperature measurements.

Reflected apparent temperature compensation
Correction built into an instrument to provide automatic compensation in the measurement for the influence of the reflected apparent temperature.

Reflectivity, (reflectance) (ρ)
The ratio of the total energy reflected from a surface to total incidence on that surface; ρ = 1-ε-τ); for a perfect mirror this approaches 1.0; for a blackbody the reflectivity is 0. Technically, reflectivity is the ratio of the intensity of the reflected radiation to the total radiation and reflectance is the ratio of the reflected flux to the incidental flux. In thermography, the two terms are often used interchangeably.

Relative humidity
The ratio (in per cent) of the water vapour content in the air to the maximum content possible at that temperature and pressure.

Repeatability
The capability of an instrument to exactly repeat a reading on an unvarying target over a short or long-term time interval. For thermal measurements, expressed in ±:degrees or a percentage of full scale.

Resistance, thermal (R)
A measure of a material's resistance to the flow of thermal energy in a fabricated condition, inversely proportional to its thermal conductance, R=1/C. For one-dimensional conductive heat flow, the thermal resistance is the thermal resistivity times the thickness divided by the cross-sectional area normal to the heat flow; (R=rL/A). Note also that thermal resistance is sometimes defined as the resistance per unit thickness.

Resistivity, thermal
A material property that defines its resistance to the flow of thermal energy, inversely proportional to its thermal conductivity, k. (1/r = k).

Resolution
Clarity or fineness of detail; used to denote the total number or pixels in a displayed image; a high-resolution camera can render fine details with greater sharpness than a low-resolution system.

Response time
The time it takes for an instrument output signal or display to respond to a temperature step change at the target; expressed in seconds.

Resistance temperature device (RTD)
A sensor that measures temperature by a change in resistance of the sensor as a function of temperature.

S

Sample and hold
A feature of an instrument whereby an output signal is maintained at an instantaneous measurment value for a specified duration after a trigger or until an external reset is applied.

Scan Angle
For a line scanner, the total angular scan possible at the target plane, typically 90°

Scan position accuracy
For a line scanner, the precision with which instantaneous position along the scan line can be set or measured.

Sector
For a line scanner, a portion of the total scan angle over which measurement is made at the target plane.

Seebeck effect
The phenomenon that explains the operation of thermocouples; that in a closed electrical circuit made up of two junctions of dissimilar metal conductors, a DC current will flow as long as the two junctions are at different temperatures. The phenomenon is reversible; if the temperatures at the two junctions are reversed the flow of current reverses.

Sensitivity
See minimum resolvable temperature (difference), MRT(D).

Setpoint
Any temperature setting at which an activating signal or closure can be preset so that, when the measured temperature reaches the setpoint, a control signal, pulse or relay closure is generated.

Signal processing
Manipulation of temperature signal or image data for purposes of enhancing or controlling a process. Examples for infrared radiation thermometers are peak hold, valley hold, sample hold and averaging. Examples for scanners and imagers are usually referred to as "image processing" and include e.g. isotherm enhancement, image averaging, alignment, image subtraction and image filtering.

Slit response function (SRF)
A measure of the measurement spatial resolution of an infrared scanner or imager. Typically plotted as per cent response vs. angular target subtense (ratio of target slit width to target-to-camera distance).

Spatial resolution
The spot size in terms of working distance. In an infrared radiation thermometer this is expressed in milliradians or as a ratio (D/d) of the target spot size (containing 95% of the radiant energy, according to common usage) to the working distance. In scanners and imagers it is most often expressed in milliradians.

Specific heat
the amount of heat needed to raise the temperature of a unit mass of a material one degree; water has a specific heat of 1 J/kg degree C.

Spectral response
The spectral wavelength interval over which an instrument or sensor responds to infrared radiant energy, expressed in micrometers (μm) - also, the relative manner (spectral response curve) in which it responds over that interval.

Spectrum - (electromagnetic spectrum)
The entire range of wavelength of electromagnetic radiation extending from gamma rays to the longest radio waves and including visible light and infrared radiation.

Specular reflector
A reflecting surface that reflects all incidental radiant energy at an angle complementary (equal around the normal) to the angle of the incidence. A mirror is a specular reflector.

Spot
The instantaneous size (diameter unless otherwise specified) of the area at the target plane that is being measured by the instrument. In infrared thermometry, this is specified by most manufacturers to contain 95% of the radiant energy of an infinitely large target of the same temperature and emissivity.

Spot radiometer
A non-imaging, non-contact device for measuring temperature.

Spot size ratio
The inverse of the measurement field of view. The practical measurement resolution for a radiometer, which takes into account the optical, mechanical and electrical functionality of a device. Relates the size of the target to the maximum distance the viewer can be from the target to the minimum size of the target in order to ensure accurate temperature readings,
(Spot size ratio)(Minimum target size) = Maximum viewing distance.

Storage operating range
The temperature extremes over which an instrument can be stored and, subsequently, operate within published performance specifications.

Subtense, angular
The angular diameter of an optical system or subsystem expressed in angular degrees or milliradians. In thermography, the angle over which a sensing instrument collects radiant energy.

T

Target
The object surface to be measured or imaged.

Temperature
A measure of the thermal energy contained by an object; the degree of hotness or coldness of an object measurable by any of a number of relative scales; heat is defined as thermal energy in transit, and flows from obkects of higher temperature to objects of lower temperature.

Temperature conversion
Converting from one temperature scale to another; the relationships are:
Celsius: 0°C = 273.15 Kelvin
Fahrenheit: 0°F = 459.67 Rankine

Temperature Masurment Drift
A reading change (error), with time, of a target with nonvarying temperature, which may be caused by a combination of ambient changes, line voltage changes and instrument characteristics.

Temperature Resolution
See minimum resolvable tmperature (difference), MRT(D)

Thermal conductivity
See Conductivity, thermal.

Thermal detector, infrared
A type of infrared detector that changes electrical characteristics as a function of temperature. Typicallt, thermal detectors have slow response, (on the order of milliseconds) broadrd spectral responce amd usualy operate at room temperature. Thermal detectors are commonly used in infrared radiation thermometers and in some imagers.
See Bolometers, microbolometers.

Thermal senssitivity
See minimum resolvable tmperature (difference), MRT(D)

Thermal viewer
A nonmeasuring thermal imager that produces qualitative thermal images relater to thermal radiant distribution over the target surface.

Thermal Wave Imaging
A term used to discribe an active technique for infrared nondistructive material testing, in which the sample is stimulated with pulses of thermal energy, and where the time-based returned thermal images are processed to determine flaw depth and severity; also called "pulse-stimulated imaging".

Thermistor
A temperature detector, usualy a semiconductor, whsoe resistivity decreases predictably with increasing temperature.

Thermistor bolometer, infrared
A thermistor so configured as to collect radiant infrared energy; a type of thermal infrared detector.

Thermocouple
A device for measuring temperature based on the fact that oposite junctions between certain dissimilar metals develop an electrucal potential when at different temperatures; typical thermocouple types are:

Thermogram
A thermal map or image of a target where the gray tones or colour hues corespond to the distribution of infrared thermal radiant energy ovet the surface of the target. (qualitative thermogram); when correctly processed and corrected, a graphic representation of surface temperature distribution (wuantitative thermogram).

Thermograph
Another word used to discribe an infrared thermal image.

Thermometer
Any device used for measuring temperature.

Thermopile
A device constructed by the arrangment of thermocouples in series to add the thermoelectric voltage. A radiation thermopile is a thermopile with junctions so arranged as to collect infrared radiant energy from a target, a type of thermal infrared detector.

Time constant
The time it takes for any sensing element to respond to 63.2% (1-1/e) of a stepchange at the target being sensed. In infrared sensing and thermography, the time constant of a detector is a limiting factor in instrument performance, as it relates to responce time.

Total Field of View (TFOV)
In imagers, the total solid angle scaned, usualy rectangular in cross section; usualy called FOV, field of view.

Trancducer
Any device that can convert energy from one form to another. In thermography, an infrared detector is a transducer that converts infrared radiant energy to some useful electrucal quantity.

Transfer calibration
A technique for correcting a temperature measurement or a thermogram for various errors by placing a radiation reference standard adjacent to the target.

Transfer Standard
A precision radiometric measurment instrument eith e.g NIST traceable calibration used to calibrateradiation reference sources.

Transmissivity, (transmittance) (τ)
The proportion of infrared radiant energy impinging on an obgect's surface, for any given spectral interval, that is transmited through the object. (τ=1-ε-ρ) For a blackbody, transmissivity = 0. Transmissivity is the internal transmitance per unit thickness of a nondeffusing material.

Two-colour pyrometer
See ratio pyrometer.

U

Unity
One (1.0).

V

Valley-hold
A feature of an instrument whereby an output signal is maintained at the lowest instantaneous measurment for a specified duration; opposite of peak-hold.

W

Working Distance
The distance from the target to the instrument, usually to the primary optic.

X

N/A

Y

N/A

Z

Zone
In line scanners, a scanned area created by thansverse linear motion of the product or process under a measurement sector of the scanner.