Comparison of modeled atmosphere-dependent range performance of long-wave and mid-wave IR imagers

Abstract

In this paper, the ranges of two specific long-wave infrared (LWIR) and mid-wave infrared (MWIR) focal plane array (FPA) systems are estimated and compared, using MODTRAN to calculate atmospheric transmission and NVTherm to calculate the ranges. The calculations are for a standard tank target, clear weather conditions and a horizontal terrestrial path at mean sea level, under varying ambient temperature and relative humidity (RH) conditions. Detection ranges are more sensitive to temperature and RH variations than the recognition ranges, while identification ranges are the least sensitive. LWIR ranges depend more upon these variations than do MWIR ranges. The relative range ratios are plotted versus the ambient temperature and RH as a contour plot. Typically, LWIR systems perform better at lower ambient temperatures and RHs, while MWIR systems are advantageous at higher temperatures and RHs. The so-called ‘phase curve’ separating the MWIR and LWIR ‘phases’ also depends upon system parameters (FPA and optics). Ambient temperatures and RHs vary diurnally and seasonally, and over kilometer distance scales. Our proposed plot is more flexible than the standard scenarios used in MODTRAN. The advent of dual band imaging systems has not rendered this discussion irrelevant since the more producible FPAs are sequential rather than simultaneous.