How clearly can an infrared detector see through a hot window?

Abstract

At elevated temperatures, the self-emission from an infrared (IR) window may prevent a detector from seeing through it. Window coatings may be designed to maximize transmitted signal and minimize thermal self-emission noise toward the detector at a given temperature, but the performance of such coatings deteriorates rapidly with increasing temperature. Using a new figure of merit for hot window performance, we assess the effectiveness and limits of window treatments designed to make objects beyond the window clearly observable over a wide range of temperatures. As an illustration, we explore requirements for simple one-dimensional (1D) coating stacks applied to both sides of the window and assess their advantages over an uncoated window operating in the mid-IR range (2–5 μm). An anti-reflection coating is found to maximize transmission through the object-facing side of the window, while a filter coating is best on the detector-facing side, with the latter offering a much larger performance enhancement. To assess how close practical window coatings can come to achieving the desired behaviors of high transmission, low self-emission noise, and large bandwidth at a given temperature, inverse design techniques are applied to optimize 1D stacks of layered IR materials. For windows that must operate over a range of temperatures, this analysis reveals that the best strategy is to design coatings for the highest expected operating temperature.