straylight

Straylight refers to unwanted light within an optical system that reaches the detector via paths other than the intended image-forming rays. This stray light can produce veiling glare and reduce contrast, especially in bright-to-dark transitions. It may originate from scattering off surfaces, reflections between optical elements, diffraction at edges, or light entering from outside the instrument’s field of view.

Common causes include external sources outside the intended field, diffuse reflection from dust or surface roughness, internal reflections between lenses and housing, and diffraction from apertures, baffles, or edges. Materials with fluorescence, or imperfect black coatings, can contribute as well.

The presence of stray light degrades image quality, creating halos, ghost images, and nonuniform responses that bias photometry and limit dynamic range. In astronomy, stray light can masquerade as faint objects or wash out faint structures near bright sources; in photography and microscopy, it lowers contrast and resolution.

Control and assessment rely on optical design and calibration. Engineers use baffles, light traps, and high-absorption coatings; careful surface finishing and edge treatments, and anti-reflection coatings limit reflections. Straylight budgets, ray-tracing analyses, and physical models guide design; testing with off-axis light sources, flat-field and dark-frame calibrations, and stray-light measurements quantify performance.

Mitigation is ongoing across disciplines, from space telescopes to consumer cameras to scientific microscopes. Terminology sometimes writes straylight as one word, but it is commonly referred to as stray light.