Stray Light Analysis and Control

Reference: Eric C. Fest, Stray Light Analysis and Control (SPIE Press, 2013).

Use this reference to distinguish unwanted geometric paths, specular ghosts, surface scatter, and calibrated radiometric quantities.

Define the quantity

Record source radiance or power, angular and spectral distributions, surface properties, apertures, detector response, and normalization. A ray count is not generally a power fraction.

For detector irradiance, directional radiance contributes

\[E=\int_{\Omega} L(\boldsymbol{\omega}) \cos\theta\,\mathrm{d}\Omega.\]

Include spectral integration when source, coatings, or detector response vary materially with wavelength.

Separate the mechanisms

  • Direct paths: an unintended opening or line of sight.

  • Ghosts: unwanted paths formed by specular reflections.

  • Surface scatter: angular redistribution described by a BRDF or BSDF.

  • Volume effects: absorption or bulk scattering within a medium.

Check energy accounting, angular sampling, normalization, and convergence of low-probability paths. Compare suppression measures using the same source and detector definitions as the original prediction.