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Glossary of Thermography Terms

Sample and Hold

A feature of an instrument whereby an output signal is maintained at an instantaneous measurement 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.


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.


See minimum resolvable temperatures (difference), MRT (D).


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.


(Electromagnetic spectrum) – The entire range of wavelengths 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 incident radiant energy at an angle complementary (equal around the normal) to the angle of incidence. A mirror is a specular reflector.


The instantaneous size (diameter unless otherwise specified) of the area 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.

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