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. Ocular straylight is stray light in the human eye.
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Monochromatic light
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 ability of the instrument to measure light transmission through the sample is limited by the stray light level. One method to reduce stray light in these systems is the use of double monochromators. The ratio of transmitted stray light to signal is reduced to the product of the ratio for each monochromator, so combining two monochromators in series with 10−3 stray light each produces a system with a stray light ratio of 10−6, allowing a much larger dynamic range for measurements.
Methods have also been invented to measure and compensate for stray light in spectrophotometers. ASTM standard E387 describes methods of estimating stray light in spectrophotometers. 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.
Astronomy
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.
Design tools
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.