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Glossary of Thermography Terms

Radiation, Thermal

The mode of heat flow that occurs be 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.


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


Absolute temperature scale related to the Fahrenheit relative scale. The Rankin 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 incident 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.


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-sectioned 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, К. (1/r=К)


Clarity or finesses 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.

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