Stray light
Encyclopedia
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
.
s (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. 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.
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.
A designer can use such a model to predict and minimize stray light in the final system.
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
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
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
of the system; it limits the signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
or contrast ratio
Contrast ratio
The contrast ratio is a property of a display system, defined as the ratio of the luminance of the brightest color to that of the darkest color that the system is capable of producing...
, by limiting how dark the system can be. Ocular straylight
Ocular straylight
Ocular straylight is the unwanted effect of light spreading as seen by human eyes, and caused by the eye itself, not by external factors. It is most easily visualized as the radiation seen spreading from a bright light source against a dark background. The typical practical example is an opposing...
is stray light in the human eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
.
Monochromatic light
Optical measuring instruments that work with monochromatic light, such as spectrophotometers, define stray light as light in the system at wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s (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. 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 glowSkyglow
Skyglow is the illumination of the night sky or parts of it. The most common cause of skyglow is artificial light that emits light pollution, which accumulates into a vast glow that can be seen from miles away and from high in the sky...
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
Coronagraph
A coronagraph is a telescopic attachment designed to block out the direct light from a star so that nearby objects – which otherwise would be hidden in the star's bright glare – can be resolved...
, used for observing the Sun's
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
corona.
Sources
There are many sources of stray light. For example:- Ghost orders in diffraction gratingDiffraction gratingIn optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
s. These can be caused by periodic variations in the spacing of grooves in ruled gratings, for instance. - Light scatteredScatteringScattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
towards a telescopeOptical telescopeAn optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....
from particles along the optical path to a star. - Light emitted by components of the optical system.
- InfraredInfraredInfrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
optical systems are, obviously, especially susceptible due to thermal radiationThermal radiationThermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
.
- Infrared
- Reflections from lensLens (optics)A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
surfaces.- Anti-reflective coatingAnti-reflective coatingAn antireflective or anti-reflection coating is a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection. This improves the efficiency of the system since less light is lost. In complex systems such as a telescope, the reduction in reflections also...
s are used to reduce stray light. - Narcissus - Specifically, thermal radiation from the infrared detector reflected back to itself from lens surfaces.
- Anti-reflective coating
- Light scattered from the surfaces of supporting structures within the optical system.
- Diffuse reflectionDiffuse reflectionDiffuse reflection is the reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle as in the case of specular reflection...
from imperfect mirrorMirrorA mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
surfaces. - Light leakLight leakA light leak is a hole or gap in the body of a camera where light is able to "leak" into the normally light-tight chamber, exposing the film or sensor with extra light. This light is diffuse, although parts within the camera may cast shadows or reflect it in a particular way. For most purposes this...
s 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
A number of optical design programs feature the capability of modeling stray light in an optical system, for instance:- ASAPASAP (software)The Advanced Systems Analysis Program is optical engineering software used to simulate optical systems. ASAP can handle coherent as well as incoherent light sources. It is a non-sequential ray tracing tool which means that it can be used not only to analyze lens systems but also for stray light...
- FREDFRED SoftwareFred Optical Engineering Software is a commercial 3D CAD software program for optical engineering used to simulate illumination through optical systems. Fred can handle both incoherent and coherent light using Gaussian beam propagation. The program offers a high level of visualization using a...
- ZemaxZemaxZemax is a widely-used optical design program sold by Zemax Development Corporation of Bellevue, Washington . It is used for the design and analysis of optical systems...
- TraceProTraceProTracePro is a commercial optical engineering software program for designing and analyzing optical and illumination systems. The program's Graphical User Interface is 3D CAD based creating a virtual prototyping environment to perform software simulation before manufacture.-History:Developed by...
A designer can use such a model to predict and minimize stray light in the final system.