Scatter-based interpretation of infrared stray light from warm windows in optical instruments

We propose a novel interpretation of stray light, which is modeled using a scattering process. For this, a description and simulations based on the bidirectional scattering distribution function (BSDF) are developed. We focus on the particular example of a window closing an optical cavity. Such a window is known to introduce a stray light component that is experimentally very subtle to handle. Our idea is motivated by the uncomfortable observation that for years, the usual estimation of this parasitic contribution (be it for absolute or relative measurements, for visible or IR applications, for field or laboratory experiments, etc.) was based on the rather imprecise and obviously incorrect assumption (nevertheless today a widely used rule of thumb) that the optical window can be approximated by a graybody emitting in its transparency range. This statement obviously goes against Kirchhoff's law of radiation, and can thus not be physically sound. Various typical experimental situations are explored with our model, and a comparison is made with results given by the rule of thumb. This shows some cases of relative concordance. However, the scatter-based model should provide an individually more accurate estimation, as shown by some examples.