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Photonics Dictionary: R

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reciprocal megakelvin -> microreciprocal degree
An approximated measure of the smallest change in color temperature detectable by the human eye, defined as the reciprocal of the color temperature times 106. Also called mired or reciprocal...
reciprocal second (Hz)
The fundamental wavelength standard of time or frequency. An atomic standard, it is properly expressed as 9,192,631,770 times the frequency of the transition between the hyperfine structure levels of...
reciprocity failure -> Bunsen-Roscoe law
The law stating that the amount of chemical change produced is proportional to the amount of light absorbed. Actually, the change is also dependent on the intensity of light -- a fact named the...
reciprocity law
With respect to photography, the law stating that the optical density of an exposed emulsion with standard development is a function of just the irradiance and the exposure times.
recombinase polymerase amplification
Recombinase polymerase amplification (RPA) is a molecular biology technique used to rapidly amplify specific DNA or RNA sequences under isothermal conditions, meaning it does not require the...
recombination radiation
The radiation emitted in semiconductors when electrons in the conduction band recombine with holes in the valence band. If population inversion occurs between portions of the valence and conduction...
reconfigurable optical add/drop multiplexer
A reconfigurable optical add-drop multiplexer (ROADM) is a key component in wavelength-division multiplexing (WDM) optical communication networks. It allows for flexible and dynamic routing of...
reconstructed image
An image that appears when a hologram is illuminated by a suitable light source, generally a laser beam.
reconstruction diffraction efficiency
Holographic quantity expressed as the ratio of the reconstructed first-order image to that of the incident reconstructing beam.
reconstruction wave
The coherent wave used to play back a recorded hologram. It is identical to the reference wave used in the generation of the original hologram; part of it is diffracted by the recording medium and...
recording camera
A type of camera that incorporates a tiny mirror that oscillates in accordance with incoming signals. An illuminated slit is imaged by this tiny mirror on a moving ribbon of sensitized paper or film,...
The growth of particular grain fragments in a metal or alloy, at the expense of others, that occurs when the metal or alloy is severely worked, as by cold rolling. Recrystallization results in...
rectangular scanning
A two-dimensional scanning process, in which a slow sector scan, propagated in one direction, is superimposed at right angles upon a rapid sector scan.
A technique used in photogrammetry to ensure parallelism during projection printing. Failure to do this will change a rectangular image on a negative into a trapezoidal image on the print.
In a straight line. When applied to a lens, it indicates that images of straight lines formed by the lens are not distorted.
rectilinear propagation
Straight line travel. This denotes the fact that light travels in a straight line when traveling through a medium with a constant refractive index.
rectilinear scanning
The scanning of a region in a given sequence of slender, straight parallel strips.
rectilinear system
An optical system that is corrected for distortion and spherical aberration and therefore forms the image of a straight line as a straight line.
radian energy distribution
red, green, blue
RGB stands for red, green, blue, which are the primary colors of light used in additive color mixing. The RGB color model is widely used in various electronic displays, such as computer monitors,...
The displacement of spectrum lines, as determined by the increasing distance between, and the relative velocity of, the observer and a light source, causing the lines to move toward the red portion...
reduced focal length
The ratio of the first focal length of a lens to the refractive index of the medium containing the incident light; the ratio of the second focal length to the refractive index of the medium...
rare earth element
reference beam
In holography, the beam of light that is directed from the beamsplitter to the recording medium, where it interferes with the object beam to generate the hologram.
reference point
The point of a chromaticity diagram that represents the chromaticity of a reference stimulus.
reference surface
The surface of an optical fiber that is used as a reference when joining optical fibers. Although the outermost cladding is usually used, the core cladding surface is also a common reference point.
reference white
The light from a nonselective diffuse reflector due to the standard illumination of the scene to be televised.
The ratio of reflected flux to incident flux. Unless otherwise specified, the total reflectance is meant; it is sometimes convenient to divide this into the sum of the specular and the diffuse...
reflectance confocal microscopy
Reflectance confocal microscopy (RCM) is a non-invasive imaging technique that allows for high-resolution, in vivo imaging of biological tissues at the cellular level. It is particularly useful in...
reflectance estimate
The output of spectral channels written as an integral function of known solar irradiance, atmospheric transmittance and camera spectral responsivity, and the unknown spectral reflectance.
reflectance factor
Ratio of the directionally reflected flux to that reflected in the same direction by a perfect reflecting diffuser identically irradiated or illuminated.
reflectance spectrophotometer
An instrument that spectrally analyzes the flux reflected from a material. The reflected flux may be total, diffuse or specular. Often the instrument measures the reflectance factor instead of the...
reflected light meter
An exposure meter that indicates the amount of light reflected from the subject of interest.
reflected ray
The light ray leaving a reflecting surface, indicating the path of light after reflection.
reflected ultraviolet photography
A photographic method used to obtain an image of a subject by means of its reflectance of incident ultraviolet radiation. An ultraviolet source is used to irradiate the subject, while the reflected...
reflecting galvanometer
A galvanometer having a small mirror that is mounted on a moving element and that reflects a light beam onto a scale.
reflecting microscope
A microscope that uses a reflecting objective; often used with ultraviolet or infrared radiation.
reflecting objective
An image-forming system that uses mirrors rather than lenses. Such objectives are frequently used for astronomical telescopes and in ultraviolet microscopes.
reflecting prism
A prism having several plane polished surfaces, some to transmit light, some to reflect light, and some to serve both functions in succession. It acts generally as a group of plane mirrors held in a...
reflecting spectrograph
A solar spectrograph that uses long focus concave mirrors as its collimator and camera element.
reflecting telescope
A telescope that uses a reflecting objective to focus an image of a distant object at a focal point.
Return of radiation by a surface, without change in wavelength. The reflection may be specular, from a smooth surface; diffuse, from a rough surface or from within the specimen; or mixed, a...
reflection coefficient
Parametric measurement for elliptical fiber and cable expressed as a ratio of the two-directional flow of power through the cable at any chosen point.
reflection echelon
An echelon in which the dihedral angle between the faces of the groove is 90°.
reflection factor -> reflectance factor
Ratio of the directionally reflected flux to that reflected in the same direction by a perfect reflecting diffuser identically irradiated or illuminated.
reflection grating
A reflection grating is an optical component that consists of a series of closely spaced, parallel reflecting surfaces or grooves. These grooves are typically ruled or etched onto a flat substrate or...
reflection hologram
A hologram that is illuminated by a source from the viewer's side.
reflection reduction coating
The thin, transparent film made up of specific substances applied to glass-air surfaces for the purpose of decreasing the amount of reflected light.
reflection x-ray microscopy
A means of high resolution study through the application of soft and hard x-rays onto a sample surface in order to obtain image information from 1-10 nm layers and surfaces. The x-ray source emission...
reflective coating
Thin-film coating, single or multilayer, that is applied to a substrate to increase its reflectance over a specified range of wavelengths. Metal film, transparent or opaque, is the oldest form of...

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