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29 terms
Photonics Dictionary: G
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gas photocell
A photoemissive cell having an inert gas added to its envelope. Subsequent ionization of the gas increases the responsivity of the photocell.
gas phototube
A phototube having increased response due to the addition of a quantity of gas.
gegenschein-zodiacal light photometer
A photometer used to measure sky brightness and polarization associated with zodiacal light, background starlight, F-region airglow and spacecraft corona.
Glan spectrophotometer
A device similar to the ordinary spectrophotometer but containing particular modifications to provide for the comparison of two sources.
goniophotometer
A device used to measure directional reflectance, with light collection restricted to a narrow range of angles of which the central angle varies.
goniophotometric curve
The graphed curve illustrating the directional reflectance of a sample for different angles of collection.
grown-junction photocell
A photodiode that has been designed so that the bar of semiconductor material has a PN junction perpendicular to its length and an ohmic contact at each of its ends.
GaAlAs laser
A GaAlAs laser is a type of semiconductor laser diode that emits light in the near-infrared region of the electromagnetic spectrum. It is composed of layers of gallium, aluminum, and arsenic, hence...
gain
Also known as amplification. 1. The increase in a signal that is transmitted from one point to another through an amplifier. A material that exhibits gain rather than absorption, at certain...
gain-bandwidth product
In an avalanche photodiode, the gain multiplied by the signal frequency in MHz.
gain-switching
Gain-switching is a technique used in lasers to generate short pulses of light by modulating the gain of the laser medium. This method is distinct from Q-switching and mode-locking, two other common...
Galilean telescope
A refracting telescope that yields an erect image by the use of a positive lens for its objective and a negative lens for its eyepiece.
galvanometric scanner
A galvanometric scanner, also known as a galvanometer scanner or galvo scanner, is a type of optical scanning device used to rapidly and precisely direct a laser beam or other light source to...
gas magnification
The increase in current of a phototube as a result of the gas in the tube becoming ionized.
Gauss lens
A telescope objective with excellent spherochromatic correction, consisting of a meniscus crown and a meniscus flint, both convex to the long conjugate.
Gaussian profile
An intensity distribution (intensity as a function of radial distance from the beam center) which is given by the Gaussian function: where: I0 is the intensity at the beam center r is the radial...
germanium crystal
Germanium crystal refers to a crystalline form of the element germanium (Ge), which is a metalloid and semiconductor commonly used in various electronic and optical applications. Germanium crystals...
germanium detector
A type of photoconductive detector in which germanium, usually doped with boron, gallium and indium, serves as a semiconductor and can detect up to and beyond 100 µm.
glass film plates
An early form of photographic media consisting of glass plates coated with an emulsion.
glossmeter
A photometer for measuring gloss by comparing the specular reflectance to that from a perfect specular reflector.
Golay cell
A thermal radiation detector consisting of a small cell with a blackened plastic front face that bulges slightly when heat is received by it. The bulging tilts a small mirror, which, in turn, varies...
grating beamsplitter
A grating beamsplitter is an optical device that utilizes the principles of diffraction to split a beam of light into multiple beams, often redirecting them at specific angles. It is based on the...
grating monochromator
An optical instrument used to isolate a narrow bandwidth of optical radiation using a diffraction grating as the dispersive element. See also monochromator.
grating spectrograph
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spectrograph
An optical instrument for forming the spectrum of a light source and recording it on a film. The dispersing medium may be a prism or a diffraction grating. A concave grating requires no other means...
grating spectroscope
A spectroscope having a diffraction grating for the resolution of light of various wavelengths.
Gregorian telescope
A telescope with an ellipsoidal secondary concave mirror that reflects rays from a parabolic primary mirror through an opening in the center of the primary, where the image is viewed with an eyepiece.
GRIN lens
A GRIN (gradient index) lens is a type of optical lens that utilizes a gradient in refractive index across its volume rather than having a uniform refractive index like conventional lenses. This...
guide factor
A factor derived by equating the incident light on the subject to the required incident light for suitable photography. The lamp-subject-distance then is multiplied by the aperture and solved in...
gun camera
A camera accurately aligned to a weapons system to provide a photographic record of system performance.
Photonics Dictionary
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