Apparent Temperature Difference
The apparent temperature difference method of defining infrared signature gives the physical temperature difference (e.g. in kelvin) between the object of interest and the immediate background if the recorded radiance values had been measured from perfect blackbody sources. Problems with this method include differences in radiance across the object or the immediate background and the finite size of the detector's pixels. The value is a complex function of range, time, aspect, etc.
Read more about this topic: Infrared Signature
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