Thermal Imaging Performance of Passive Infrared Scanners

An analytical base for assessing the performance of thermal mapping systems suitable for remote environmental sensing is provided. For systems that operate in a pulsed mode and are limited by photon noise, the signal-to-noise ratio can be expressed simply in terms of an effective signal irradiance which involves the whole spectral region of interest, and a single number which is the system's noise-equivalent irradiance (NEI) at the peak-response wavelength of the detector. A compact expression of the NEI is derived for background fluxes representative of situations encountered with contemporary radiation-shielded detector packages operating in the earth's atmosphere. In conjunction with the definition of an effective signal irradiance for extended targets, the concept of a figure of merit for passive thermal-imaging equipment is introduced, and implications for system tradeoffs are discussed. The analysis is applied to evaluate an 8- to 14-μm airborne scanner from the point of view of angular resolution and thermal sensitivity; limitations imposed by optical diffraction effects, detector time constant, and detector size requirement are also considered. Angular resolutions of, say, 0.1 to 0.2 mrad exemplify the capabilities of the current generation of infrared imaging sensors.