Stray light is

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

in an

optical system Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...

, 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 (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base- ...

of the system; it limits the

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in d ...

contrast ratio The contrast ratio (CR) is a property of a display system, defined as the ratio of the luminance of the brightest shade (white) to that of the darkest shade (black) that the system is capable of producing. A high contrast ratio is a desired aspe ...

, by limiting how dark the system can be.

Ocular straylight Ocular straylight is a phenomenon where parts of the eye scatter light, creating glare. It is analogous to stray light in other optical systems; scattered light reaches the retina, but does not contribute to forming a correct image. One can obse ...

is stray light in the human eye.

Optical systems

Monochromatic light

Optical measuring instruments that work with monochromatic light, such as spectrophotometers, define stray light as light in the system at

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

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 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 monochromator A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is from the Greek roots ''mono-'', "si ...

s. 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 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. Most coronagraphs are intended to view t ...

, used for observing the Sun's corona.

Sources

There are many sources of stray light. . For example: *''Ghost orders'' in

diffraction grating In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structural ...

s. These can be caused by periodic variations in the spacing of grooves in ruled gratings, for instance. *Light

scattered Scattered may refer to: Music * ''Scattered'' (album), a 2010 album by The Handsome Family * "Scattered" (The Kinks song), 1993 * "Scattered", a song by Ace Young * "Scattered", a song by Lauren Jauregui * "Scattered", a song by Green Day from ' ...

towards a

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...

from particles along the optical path to a star. *Light emitted by components of the optical system. **

Infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...

optical systems are, obviously, especially susceptible due to

thermal radiation Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) is ...

. *** one way to reduce the effect of stray IR generated within the system is to move from working with DC signals to a narrow frequency band where the amplitude of the stray emissions is smaller. This can be done, for instance, by modulating the source light entering the system with an optical chopper, and isolating the detected source signal component from the detected stray component with a Lock in amplifier synchronized to the chopper frequency. However, this approach is still limited by the dynamic range of the detector. That is, the stray component must not be so great that it saturates the detector. *Reflections from

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'' ...

surfaces. **

Anti-reflective coating An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the eff ...

s are used to reduce stray light. **Narcissus . - 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 Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle as in the case of specular reflection. An ''ideal'' dif ...

from imperfect

mirror A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...

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

A number of

optical design Optical lens design is the process of designing a lens to meet a set of performance requirements and constraints, including cost and manufacturing limitations. Parameters include surface profile types ( spherical, aspheric, holographic, diffra ...

programs feature the capability of modeling stray light in an optical system, for instance: * ASAP *

FRED Fred may refer to: People * Fred (name), including a list of people and characters with the name Mononym * Fred (cartoonist) (1931–2013), pen name of Fred Othon Aristidès, French * Fred (footballer, born 1949) (1949–2022), Frederico Ro ...

Synopsys Synopsys is an American electronic design automation (EDA) company that focuses on silicon design and verification, silicon intellectual property and software security and quality. Products include tools for logic synthesis and physical desig ...

LightTools *

TracePro TracePro is a commercial optical engineering software program for designing and analyzing optical and illumination systems. The program's graphical user interface (GUI) is 3D CAD-based creating a virtual prototyping environment to perform software s ...

Zemax Zemax is a company that sells optical design software. OpticStudio is its flagship product and a commonly used optical design program for Microsoft Windows. It is used for the design and analysis of both imaging and illumination systems. History O ...

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

References