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TDI thermal imaging systems are a new generation in the 8 - 12 micrometer range. This device enables the correction of non- uniformity among detector elements by sampling the internal non-uniformity correction target at each scanning cycle. As compared with previous FLIRs generations, these new devices place greater importance on the correction of non-uniformity among the detector elements because of the higher required quality rendered by these devices. This quality is attributed to the improvement in the S/N ratio by the TDI method, imaging of the pupil into the system's cold shield, and the fact that since the systems are not dc-coupled, improper correction of the non-uniformity accepted as an information from the scene. The general analysis relates to all types of optical systems, and not necessarily to thermal systems only. The technical literature referring to this topic, and in particular to thermal issues, is not unequivocal, and there are contradictory estimates regarding the principles underlying distribution calculations. This is surprising in light of its importance to electro-optical systems as a whole, and thermal systems in particular, in terms of the distribution of image brightness on the display, and the device's dynamic range. The present report is divided into two main sections: (1) General physical analysis of the distribution of image illumination according to several types of optical systems, e.g., plane-to- plane imaging, infinity-to-plane imaging, and a scanning system which images infinity-to-plane. (2) Calculation of the distribution of TDI device image illumination at five (5) telescope zoom positions, and for various horizontal scanning positions of the scanner, as compared with detector elements illumination when performing a non-uniformity correction among detector elements by imaging a thermal reference target to the detector' plane. It has been customarily accepted that the distribution of illuminance in an optical system behaves primarily according to a function of cos (omega) , where (omega) equals the FOV angle. Some sources define (omega) as the chief ray angle of the beam (i.e., central ray), relative to the optical axis. A physical analysis will prove such a statement inaccurate. The chief ray angle in optical systems is a function of the location of their pupil, and may even change a sign, but the amount of light incident upon each point on the image's plane depends on the system's effective F/# as a function of FOV, and is not necessarily identical to the chief ray angle. Radiometric calculations based on our theoretical analysis (presented hereunder) showed that pupil distortions and aberrations have a significant impact on the distribution of the detectors' illumination, sometimes even more than the cos (omega) itself.
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