https://www.photonics.com/Articles/Optical_System_Optimization_Analyzing_the/a61737 “Percent stray light” captures the stray light characteristics of a system as a single number; it is the ratio of the optical noise power from every conceivable stray mechanism to the signal power from the intended target. In other words, it is essentially the reciprocal of SNR expressed as a percentage. In a well-baffled system, the percent stray light is typically on the order of a few percent. This particular stray light metric is commonly used in applications such as orbiting Earth-resource satellite cameras where the field-of-view is very small and the Earth as a stray light source subtends a large solid angle.
https://amt.copernicus.org/articles/11/4493/2018/