Evaluation of the thermal contrast of low-temperature ground-based objects

Abstract

Subject of study. The thermal contrast of low-temperature passive terrain objects is the difference in radiant temperatures of an object and background in operating spectral regions of air- and ground-based thermal imaging devices for surveillance. Aim of study. The study attempted to provide the designers of thermal imaging devices with initial data for signature features of ground-based objects. Method. The study employed a theoretical analysis on the physical dependences of the formation of thermal contrast in ground-based objects using the previously published model of thermal radiation of the atmosphere, which, in addition to temperature contrast owing to the different thermal inertias of the object and background upon alterations in meteorological conditions, considers the optical properties of the object, solar and environmental (of the Earth’s surface and atmosphere) radiation reflected from the object, and viewing direction of three-dimensional objects. Main results. A methodological approach and corresponding mathematical model were proposed for the quick engineering evaluation of the main signature feature of low-temperature ground-based objects, particularly vehicles surveilled using thermal imaging devices from the upper hemisphere against the background of the Earth’s surface at different cloud coverage, i.e., their thermal contrast in the spectral regions of 3–5 and 8–12 µm corresponding to the atmospheric windows. Accordingly, we presented the analytical expressions for this evaluation considering environmental thermal radiation, direct and scattered solar radiation reflected by the diffuse spherical surface imitating the surveilled object, and its viewing direction. The possibility of thermal contrast variation in the rather wide range and its inversion was demonstrated even for a fixed temperature contrast. An example of a practical application of the proposed model was considered. Practical significance. The developed engineering method for calculating the thermal contrast of ground-based objects can be applied to quickly estimate effectiveness, including the operating range of air- and ground-based thermal imaging devices, and validate the requirements on the main technical parameters of prospective devices of this type.