Stray Light (Flare)

Causes of Stray Light

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There are many possible causes of stray light. This page aims to list some of them. In general, these can be grouped into two categories: design issues and tolerancing issues. Design issues are problems with the design of the camera and every camera produced with this design will exhibit stray light. Tolerancing issues are related to the manufacturing tolerance of the optical and optomechanical components that go into the cameras. Each camera will exhibit a different stray-light manifestation. Note that this is not necessarily an exhaustive list of the causes of stray light.

Optics and Optomechanics

Reflections

There are many surfaces within an optical system where light can reflect. These include:

  • Reflection off of a lens surface (ghost reflections).
  • Diffuse reflections off an optomechanical component (e.g., baffle, retaining ring, inside of lens-tube assembly, etc.). These typically lead to veiling glare.
  • Total internal reflections within a lens.

Narcissus is a named reflection type with thermal infrared cameras where the heat of the focal plane reflects off a lens element and back to the focal plane. In this case, the stray light is the focal plane “seeing itself”.

Scattering

Scattering can occur at many places within the camera including:

  • Off of contamination on an optical surface
  • Off of mechanical surfaces
  • As a result of surface roughness
  • Off of filters (e.g., spectral cut, anti-reflective, etc.) within the optical design

Diffraction

Especially when dealing with small apertures, the wave nature of light may lead to diffraction.

Sneak Paths

Sneak paths refer to when a ray of light bypasses the optics and “sneaks” to the focal plane. These are more common in reflective imaging systems where the secondary mirror is not sized correctly relative to the primary mirror.

Focal Planes

Within a focal plane, there may be blooming if a pixel gets overfilled (saturated) with light.

Deviations from optical design

When designing a camera there is the nominal design and tolerancing about that optical design. Stray light may manifest itself from deviations both within and outside the manufacturing tolerances of the camera. Note that these deviations may introduce stray light from any of the other mechanisms (e.g., if the hole in a primary mirror of a telescope is slightly too big may lead to a sneak path).