Stray light

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Stray light is light in an optical system, which was not intended in the design. The light may be from the intended source, but follow paths other than intended, or it may be from a source other than the intended source. This light will often set a working limit on the dynamic range of the system; it limits the signal-to-noise ratio or contrast ratio, by limiting how dark the system can be.[1] Ocular straylight is stray light in the human eye.

Optical systems[edit]

Monochromatic light[edit]

For details on how stray light affects the performance of these instruments, see Ultraviolet-visible spectroscopy

Optical measuring instruments that work with monochromatic light, such as spectrophotometers, define stray light as light in the system at wavelengths (colors) other than the one intended. The stray light level is one of the most critical specifications of an instrument. For instance, intense, narrow absorption bands can easily appear to have a peak absorption less than the true absorption of the sample because the instrument is limited by the stray light level.

One method to reduce stray light is the use of double monochromators. The stray light is reduced to the product of the stray light of each monochromator, roughly from 10−3 to 10−6.

Methods have also been invented to measure and compensate for stray light in spectrophotometers.[2] ASTM standard E387 describes methods of estimating stray light in spectrophotometers.[3] The terms they use for this are stray radiant power (SRP) and stray radiant power ratio (SRPR).

There are also commercial sources of reference materials to help in testing the stray light level in spectrophotometers.[4]


In optical astronomy, stray light from sky glow can limit the ability to detect faint objects. In this sense stray light is light from other sources that is focused to the same place as the faint object.

Stray light is a major issue in the design of a coronagraph, used for observing the Sun's corona.


There are many sources of stray light.[5] For example:

  • Ghost orders in diffraction gratings. These can be caused by periodic variations in the spacing of grooves in ruled gratings, for instance.
  • Light scattered towards a telescope from particles along the optical path to a star.
  • Light emitted by components of the optical system.
  • Reflections from lens surfaces.
    • Anti-reflective coatings are used to reduce stray light.
    • Narcissus [6] - Specifically, thermal radiation from the infrared detector reflected back to itself from lens surfaces.
  • Light scattered from the surfaces of supporting structures within the optical system.
  • Diffuse reflection from imperfect mirror surfaces.
  • Light leaks in the enclosure of the system.
    • This might be the first cause to come to mind, but as this list shows, it is hardly the only source of stray light.

Design tools[edit]

A number of optical design programs feature the capability of modeling stray light in an optical system, for instance:

A designer can use such a model to predict and minimize stray light in the final system.

See also[edit]