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Focus effect in Thermal Imagers

Technical Guide 29

Thermal imagers are being increasingly used quantitatively to make absolute temperature measurements, and thus require calibration by an accredited laboratory. However, calibration of these instruments is complicated by the fact that, for some thermal imagers, the radiometric accuracy is not retained when the focusing lens is adjusted. This impacts on the user, as the temperature readings will only be accurate if the focus setting matches that used by the calibration laboratory. This technical guide describes the origin of this focus effect, and gives advice to the calibration laboratory on how to identify whether the problem exists and what steps to take to deal with it if it does.

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