Infrared Thermal Imaging Terminology

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Struggling with Infrared Thermal Imaging (IR) terminology basics? We’ve compiled & explained the top IR terms.

Infrared Radiation (IR Radiation)

This is heat radiation beyond the visible spectrum. IR thermal used in the field generally detect IR wavelengths from 7 to 14 micro-meters, wavelengths slightly longer than the color red. IR radiation equals the sum of emitted, reflected, and transmitted radiation.


Emissivity is IR radiation emitted from a body indicating the heat of that body. Emitted IR radiation equals absorbed IR radiation.


Refelctivity is ambient IR radiation reflected from a body unrelated to that body’s own heat.


This is IR radiation transmitted through a body such as a gas or liquid and some solids such as glass or plastic films. For most solid bodies, transmissivity is zero.

Distance to spot ratio (D:S)

Distance to spot ratio  (D:S)is the size of the measurement spot on the target relative to the distance the imager is from the target.

Field of View

Field of View is the spot size relative to target area. Spot size should be contained within the perimeter of the target, centered on the area of interest.


Infrared Cameras are focused to obtain crisp IR radiation temperatures.

Minimum Focal Distance

The minimum focal distance is the closest distance to target that an imager can be focused and used.

Environmental Conditions

Thermal Imagers are calibrated to operate within a certain temperature range. Particulates in the air (steam, dust, smoke) between the imager and the target will distort the readings.

Qualitative Temperatures

Accuracy of temperature is secondary to relative temperatures. This is the most common use of thermal imagers.

Take a look at our range of thermal imaging cameras. We are official distributors for leading thermal imaging camera manufacturers FLIR and FLUKE. We also offer a comprehensive Thermal Imaging training course contact us for more information.

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