Threshold sensitivity of staring thermal imaging devices operating in slant atmospheric paths

Abstract

Subject of study. A method for predicting threshold sensitivity, i.e., the resulting noise equivalent temperature difference of modern air- or ground-based staring thermal imaging devices, was considered. Aim of study. The study aimed to estimate the effectiveness of thermal imaging devices surveying aerial and ground-based objects. Method. The method implied a theoretical computational analysis based on a previously developed model for emission of the atmosphere in arbitrarily oriented paths considering the following: the nominal noise equivalent temperature difference that characterizes the quantum or thermal array photodetectors used in thermal imaging devices; the temperature profile of the atmosphere whose emission, together with background emission of, e.g., the ground surface, significantly affects the external photon noise in thermal imaging devices; elevation of the line of sight of the object; presence and type of clouds; and season. Results. A method was developed for refined calculation of the threshold sensitivity of staring thermal imaging devices determined by photon noise resulting from the radiation of the elements of the thermal imaging device itself, particularly its objective, dark-current noise, readout noise, and spatial (geometric) noise resulting from the residual dispersion of sensitivity of photodetector elements after correction. This threshold sensitivity determines the effectiveness of thermal imaging devices in object detection and recognition against a quasi-uniform background of ground or sky under different conditions, including long slant ground-to-air and air-to-ground atmospheric paths. The investigation results are presented in a form convenient for practical applications, i.e., reduced to equations and illustrated with a calculation example. Practical significance. The engineering method for calculating the resulting noise equivalent temperature difference proposed in this study, which considers the thermal stratification of the atmosphere and corresponds to real operating conditions of thermal imaging devices, is advantageous for adequate assessment of their effectiveness in surveilling ground-based and aerial objects, including low-contrast objects.