Derivation of various temperature detection limits for thermal imagers under field conditions

Abstract

The frequently quoted temperature differences NETD (noise equivalent temperature difference), MRTD (minimum resolvable temperature difference) and MDTD (minimum detectable temperature difference) are actually laboratory figures of merit describing a major property of thermal imaging devices and radiometers. These quantities cannot be simply applied when imaging or measuring in the field since, in deriving the various relations, the atmospheric transmittance has been assumed to be unity and the temperature differences have been assumed to be of the order of one degree centigrade or less. In the field, however, the transmittance encountered is strongly wavelength dependent and often low. The temperature differences might also be of the order of ten degrees centigrade or more. For such field situations the SNR is reassessed in a more exact way than for laboratory conditions, permitting the derivation of practical NETD f, MRTD f and MDTD f. The various derivations and the applicability of the results are dealt with.