MINIMUM TEMPERATURE DIFFERENCE DETECTED BY THE THERMAL-RADIATION OF OBJECTS

The spectral dependence of the Noise Equivalent Temperature Difference (NETD) that can be detected from the objects thermal radiation and is limited by photon fluctuations in quantum photodetectors is calculate d. An absolute limit of its specific value (NETD) depends only on one physical variable-the temperature of the object T: NETD(min) = 5.07 x 10(-8) square-root 300/T [K cm s1/2]. This relationship shows that t here is a decrease of the information capacity of thermal images with decreasing object temperature T In the widely used thermal imagery wav elength region below 15 mum the spectral dependence of a specific temp erature difference caused by background fluctuations is expressed with acceptable precision by the relationship: NETD(ideal)(lambda(m)) alm ost-equal-to kT2 x D(ideal)(lambda(m)). This is the so called ''kT2 r ule'' where D(ideal)(lambda(m)) is the specific detectivity of an ide al photodetector with cutoff wavelength lambda(m) and k is Boltzmann's constant. For a particular case, considered in the Appendix, the effi ciencies of thermal imagers sensitive in different spectral regions ar e compared, including atmospheric transmission for horizontal paths of different length.