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 used 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. ==Sources==