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167 terms

Photonics Dictionary: G

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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.

galvanometer

A galvanometer is an instrument used for detecting and measuring small electric currents. It typically consists of a coil of wire suspended in a magnetic field, with a pointer attached to the coil....

galvanometer mirror

A small mirror, sometimes plane and sometimes concave, attached to the rotating coil of a galvanometer to cause a spot of light to move along a scale.

gamma-ray spectrometer

An instrument used to detect and measure the energy distribution of gamma rays. It has been used to chart the radioactivity at the lunar surface.

GaN distributed feedback lasers

GaN (gallium nitride) distributed feedback (DFB) lasers refer to a specific type of semiconductor laser based on Gallium Nitride materials and designed with a distributed feedback structure. These...

gas filter correlation

A technique for measuring the concentration of any gases. Identical infrared beams are alternately chopped, one passing through a reference cell and the other passing through a cell with the gas...

gateable

In detectors, the ability to switch on and off electronically, thus producing the effect of a mechanical shutter.

gated image tube

An intensified charge-coupled device that uses a large negative charge at the grid to switch off the flow of electrons at periodic intervals in order to provide a clearer output image when adverse...

gated integrator -> boxcar averager

An instrument for detecting and analyzing repetitive signals. Using a fixed time delay or "gate," the input signal is sampled numerous times and averaged to increase the ratio of the...

gated pulse

A discontinuous burst of laser light generated by timing or "gating'' a continuous-wave laser.

gauze technique

The masking of all openings of an optical transform -- except the hole at the symmetrical center -- with a thin wire gauze to attain a more precise image.

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.

Geiger counter

Also called Geiger-Müller counter. An instrument designed to detect and measure radioactivity through the use of a gas-filled metallic tube containing a needle electrode. Radiation traveling...

geodimeter

Trade name referring to an instrument that determines surface distances by measuring the length of time it takes for a modulated lightwave to reach a mirror and be reflected to its point of origin.

geometric center

The physical center of the lens; it is on the axis of the lens, halfway between the front and rear vertex. It is sometimes referred to as the mechanical center of the lens.

geometric extent -> etendue

A product of the area of a light beam (normal to its direction of propagation) and the solid angle that the beam includes; flux per unit radiance or luminance. Alternative terms: geometric extent,...

geosynchronous satellite

A man-made satellite that orbits 35,680 km from the Earth at a rate of one orbit per 24-hour period, thereby retaining its initial relative alignment with the Earth.

germanate glass

A type of glass used in near-infrared optical components, in which germanium is used as a cation instead of silicon.

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.

getter

In vacuum deposition, a device that removes contaminants from the vacuum chamber.

Gires-Tournois interferometer

An interferometer similar in construction to a Fabry-Perot interferometer but having a rear mirror with high reflectivity, allowing light to escape only at the partially reflective front mirror.

Glan spectrophotometer

A device similar to the ordinary spectrophotometer but containing particular modifications to provide for the comparison of two sources.

glass dosimeter

A device that detects and measures the quantity of exposure to nuclear radiation. It uses a special glass rod that fluoresces when placed under ultraviolet radiation after its exposure to nuclear...

glass film plates

An early form of photographic media consisting of glass plates coated with an emulsion.

glitter

The specular reflection of individual parts of a surface.

globulite

A crystal of microscopic size having no definite plane faces and having a globular shape. At the time the crystal is formed, the strong surface tension influences the formation of the plane faces.

glossmeter

A photometer for measuring gloss by comparing the specular reflectance to that from a perfect specular reflector.

Golgi Staining Technique

The Golgi staining technique (also called the black reaction) makes it possible to look at the nervous system by rendering neurons visible under light microscopy. After a long process involving...

goniometer

A spectrometer or autocollimator used to measure prism angles.

goniometer eyepiece

An eyepiece having a rotating index or cross wire linked to an external 360° scale to allow measuring of angles in an image.

goniophotometer

A device used to measure directional reflectance, with light collection restricted to a narrow range of angles of which the central angle varies.

graduated refractive index -> graded index

Descriptive of an optical fiber having a core refractive index that decreases almost parabolically and radially outward toward the cladding. This type of fiber combines high bandwidth with moderately...

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 spectrometer

A spectrometer that uses a grating to diffract light into specific wavelengths.

grating substrate

The substrate upon which a diffraction grating will be ruled. It must be dimensionally stable, and the surface must be polished to an accurate flat or spherical form as required by the grating. The...

gravimeter

An instrument capable of precise measurements of the Earth's gravity. This permits the detection of small changes in local height that indicate changes in the Earth's crust and subsurface density.

gray filter -> neutral density filter

A light filter that equally decreases the intensity of all wavelengths of light without altering the relative spectral distribution of the energy.

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.

Grittington test

A method of determining the abrasion resistance of very hard materials by passing a weighted wiper blade across them in a sand-water slurry for a specified number of times.

ground state

Also known as ground level. The lowest energy level of an atom or atomic system. A material in the ground state is not capable of emitting optical radiation. All other states are called excited...

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...

graded index

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 coefficient -> absorption coefficient, absorption cross section

The transition cross section constant coefficient which defines the transition probability of absorption from ground to a higher level within a given atomic or molecular species. The transition...

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-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...

gallium aluminum arsenide

A crystalline semiconductor alloy used as the light confinement layer in both single- and double-heterostructure diode lasers.

gallium antimonide

A binary semiconductor compound used as a substrate or active layer for diode lasers.

gallium arsenide injection laser

A laser system consisting of a planar PN junction within a single crystal of gallium arsenide. The pair of parallel, semireflective end faces produces a Fabry-Perot resonant cavity, whereas the other...

gallium arsenide phosphide

Gallium arsenide phosphide (GaAsP) is a semiconductor alloy composed of gallium (Ga), arsenic (As), and phosphorus (P). It is commonly used in the fabrication of optoelectronic devices, particularly...

Photonics DictionaryG