Stray Light Analysis

I was a stray light analyst for several years while at Ball Aerospace. My job was to model real ray paths that wouldn’t be accounted for in a first-order optical design model. During that time, I modeled stray light from optical surface roughness and contamination scatter, reflections and scatter from mechanical structures along the optical path, ghosting between optical surfaces, unwanted diffracted orders, unwanted polarization paths, real properties of multilayer coatings, amplified spontaneous emission, and glints from the sun and moon. There wasn’t much time to publish articles, but I did manage to get one out: Stray Light in Packaged Detectors.

Back then, I used ASAP raytracing software on a near-daily basis for my straylight analysis work and occasionally used Code-V for optical design. Since 2012, Zemax has been my primary tool, mostly for optical design, but also for the occasional stray light analysis. For those purposes, Zemax has been an extremely efficient and robust tool. While I don’t think it’s necessary to budget an exhaustive stray light analysis for every design, I do keep my eyes open for anything likely to pose a risk and I have a pretty good sense of what to look for.

Scatter is rarely a problem, but many optical systems are vulnerable to stray light ghosts, particularly systems designed for the eye and those using high-index glasses. Optical coatings are also common sources of stray light, as they’re designed for a very specific range of wavelengths and ray angles of incidence. The same is true of diffractive elements. Outside those nominal conditions, optical characteristics can change abruptly.

In most cases, stray light can be successfully managed with careful attention to specifications and mechanical clearances. When not, the use of polarizing elements in an optical diode arrangement can resolve many issues.