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Photonics Dictionary: R

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R
resistance; roentgen; Rankine
RA
reduced aperture
Ra
arithmetic average
Rabi oscillations
Rabi oscillations are a fundamental phenomenon in quantum mechanics that describe the periodic and reversible exchange of energy between two quantum states when subjected to an oscillating external...
RACE
research into advanced communications in Europe
racemic
Inactive optically, but having the capacity for resolution into forms of opposed optical activity. The term is derived from racemic acid, an optically inactive, tartaric acid.
rad
radian
rad
A unit of energy absorbed from ionizing radiation, equal to 100 ergs per gram of irradiated material.
radar
radio detection and ranging
radar display
The spontaneous visual presentation of radar information by electronic traces on a cathode-ray tube.
radargrammetry
The analysis of the photographs taken from the radar display of a survey aircraft and used when recording terrain that is obscured by clouds.
radiac
An acronym for radioactive detection, identification and computation. The term refers to the detection and measurement of the intensity of emitted nuclear radiation.
radial astigmatism
The astigmatism in a lens system that results when light enters the system at an oblique angle.
radial distortion
An alteration in magnification from the center of the field to any point in the field, measured in a radial direction from the center of the field. Some radial distortion is inherent in most optical...
radial distribution method
A statistical analysis of facts obtained when the intensity of x-ray diffraction is calculated at different angles. In this way, the interatomic distances of gases and liquids can be deduced, with...
radial gradient
In gradient index optics, a gradient profile in which the index varies in a direction perpendicular to the optical axis. Radial gradient lenses can focus light even when the lens surfaces are flat,...
radial grating
A grating in which the wires or rods are set radially within a circular structure.
radial runoff
The deviation from the ideal case where a circular variable filter is located at a given wavelength along a radial line (or constant angle of rotation) regardless of the distance from the center of...
radial velocity
The velocity from object to observer, directed along the line of sight.
radially variable reflectivity optic
A reflecting optic whose reflectivity is a function of the radial distance from the optic axis; can be used to convert a Gaussian beam to one of uniform cross section.
radian
The unit angle, within an arc of a circle, equal to the radius of the circle (180/p°, numerically).
radiance
Radiant power per unit source area per unit solid angle. Usually it is expressed in watts/m2/steradian.
radiance factor
Ratio of the radiance of the specimen to that of a perfect reflecting or transmitting diffuser identically irradiated.
radiant
Pertaining to electromagnetic radiation, with the contributions at all wavelengths of interest weighted equally.
radiant efficiency
The ratio of the radiant flux emitted by a source to the power supplied.
radiant emittance
Radiant power emitted into a full sphere (4p steradians) by a unit area of a source; expressed in watts per square meter.
radiant energy
The energy passed on as electromagnetic radiation; e.g., radio, heat or light waves.
radiant exitance
The radiant flux per unit area emitted from a surface.
radiant exposure -> exposure
In optics, the total radiant energy incident on a surface-per-unit area. It is equal to the integral over time of the radiant flux density. Also known as radiant exposure.
radiant flux -> radiant power
The time rate of flow of radiant energy, expressed in watts (W), and carries the units of Joule per second (J/s). The prefix often is dropped and the term "power" used.
radiant flux density -> irradiance
Radiant flux incident per unit area of a surface. Also called radiant flux density.
radiant heat
Infrared radiation emitted from a source that is not heated sufficiently to give off visible radiation.
radiant intensity
The radiant energy emitted within a time period per unit solid angle, usually measured in watts per steradian.
radiant power
The time rate of flow of radiant energy, expressed in watts (W), and carries the units of Joule per second (J/s). The prefix often is dropped and the term "power" used.
radiant reflectance -> reflectance
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...
radiation
The emission and/or propagation of energy through space or through a medium in the form of either waves or corpuscular emission.
radiation angle
Half the vertex angle of the cone of light emitted by a fiber.
radiation counter
An instrument used to recognize and identify incident radiation by the ionizing or stimulating properties of the radiation.
radiation detector
Any of the many devices used to detect the presence of radiation from a specific region of the electromagnetic spectrum.
radiation dosimetry
The detection and measurement of nuclear and x-ray radiation.
radiation length
The average length in a specific material in which a relativistic charged particle will lose 67 percent of its energy by bremsstrahlung.
radiation mode
A mode in an optical waveguide whose fields are transversely oscillatory everywhere external to the waveguide. It exists even in the limit of zero wavelength.
radiation pattern
Relative power distribution as a function of position or angle.
radiation pressure
The force exerted on a surface by radiation.
radiation pyrometer
Also known as radiation thermometer. A pyrometer in which the radiant intensity from the source to be measured is used to calculate its temperature. The usual detector is a thermocouple, thermopile...
radiation temperature
The temperature of a complete radiator that has a total radiant emittance identical to that of an unknown resource.
radiation thermocouple
A thermocouple that is used in infrared spectroscopy to detect a sample's infrared emittance. See thermocouple.
radiation trapping
That process by which radiation spontaneously emitted by a volume of optical materials is resonantly reabsorbed within the same volume before it escapes. This effect is manifested in a reduction in...
radiation-monitoring film
The film used in photographic dosimetry to record the types and amounts of ionizing radiation, such as x-rays and gamma rays, present in an area for a set interval of time. The negative produced by...
radiation-shielding windows
Plates of glass containing as many heavy metal oxides as can be dissolved in the glass without causing devitrification. The glass is yellow, very heavy and has a high refractive index. It is almost...

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