Minimum temperature difference detected by the thermal radiation of objects

Abstract

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 calculated. An absolute limit of its specific value (NETD∗) depends only on one physical variable—the temperature of the object T: NETD ∗ min = 5.07 × 10 −8 300 T [ K cm s 1 2 ] . This relationship shows that there is a decrease of the information capacity of thermal images with decreasing object temperature T. In the widely used thermal imagery wavelength region below 15μm the spectral dependence of a specific temperature difference caused by background fluctuations is expressed with acceptable precision by the relationship: NETD ∗ ideal (γ m ) ≈ kT 2 × D ∗ ideal (γ m ) . This is the so called “ kT 2 rule” where D∗ ideal (λ m) is the specific detectivity of an ideal photodetector with cutoff wavelength λ m) and k is Boltzmann's constant. For a particular case, considered in the Appendix, the efficiencies of thermal imagers sensitive in different spectral regions are compared, including atmospheric transmission for horizontal paths of different length.