In well-baffled optical systems it is expected that improved stray light suppression can be obtained by using a specular black coating for baffle surfaces as compared to a diffuse black coating. In this context, a well-baffled optical system consists of an all-reflective, narrow field-of-view, unobscured reimaging optical design with a field stop and Lyot stop and baffle vanes for trapping stray light. There is a concern that a specular baffle coating will cause problematic focusing of stray light and result in excessive or spatially varying stray irradiance at the focal plane. A computer simulation was undertaken to study these issues using the General Unwanted Energy Rejection Analysis Program (GUERAP). Three different baffle configurations were studied. They were obtained by optimizing for three different baffle coatings. The coatings are a perfectly specular coating, an ideal Lambertian coating, and Cat-a-lac black coating as measured at wavelength 10.6 μm. Each of the three baffle designs was simulated using each of the three coating models. The point source transmittance (PST) was calculated and the specular baffle coating was shown to give superior stray light suppression, particularly at large off-axis source angles. Specular baffle coatings are not recommended for obscured or nonreimaging systems due to the possibility of specular glints propagating to the focal plane.
Edward R Freniere,
Dennis L. Skelton,
"Use Of Specular Black Coatings In Well-Baffled Optical Systems", Proc. SPIE 0675, Stray Radiation V, (3 June 1987); doi: 10.1117/12.939490; https://doi.org/10.1117/12.939490