Pulse visibility factor and its impact on infrared search and track systems

The performance of an infrared search and track (IRST) sensor depends on a large number of variables that are important for determining systems performance. One of the variables is the pulse visibility factor (PVF). The PVF is linearly related to IRST performance metrics, such as signal-to-noise ratio (SNR) or signal-to-clutter ratio (SCR). Maximizing the performance of an IRST through a smart design of the sensor requires understanding and optimizing the PVF. The resulting peak, average, or worst case PVF may cause large variations in the sensor SNR or SCR as the target position varies in the sensor field of view (FOV) and corresponding position on the focal plane. As a result, the characteristics of the PVF are not straightforward. The definitions and characteristics for the PVF to include ensquared energy (best case PVF), worst case PVF, and average PVF are provided as a function of F * Lambda / d_cc (d_cc is the center-to-center distance between pixels, i.e., pixel pitch). F * Lambda / d_cc is a generalized figure of merit that permits broad analysis of the PVF. We show the PVF trends when the target has a finite size but is still unresolved on the focal plane [smaller than an instantaneous field of view (IFOV)]. The target size was constrained to be no less than 2% of the IFOV but also no greater than 100 % to study the effects on the PVF as a function of target size. Finally, we describe the characteristics of the PVF when optical degradations, such as aberrations, are inherent in the sensor transfer function. The results have illustrated that small F * Lambda / d_cc with large fill factor maximized the PVF at the expense of greater variability. Larger F * Lambda / d_cc can reduce the PVF variations but results in a decreased PVF. Finite target sizes and additional optical degradation decreased the PVF compared to diffraction-limited systems.