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Glossary: G

Gain

The amount a signal increases while going through an amplifier. Most cameras have optional automatic gain (autogain or AGC). When possible, increased exposure or light levels should be used in lieu of gain, as gain reduces image quality by amplifying noise as well as signals.

Gamma

The exponent of the function relating output signal to input signal in video cameras and monitors. Industrial cameras naturally have a gamma of 1, that is, they are linear in response. Displays, however, can have a nonlinear response corresponding to gamma values between 2 and 3. When used together the camera may incorporate gamma correction to linearize the resulting system.

Gas Laser

A laser using gas as the active medium. The pumping mechanism is an electric discharge, although some high-power forms employ chemical reaction or gas compression and expansion to form population inversion. 

Gaussian Beam

A light beam where the electric field profile in a plane perpendicular to the beam axis can be described with a Gaussian function, possibly with an added parabolic phase profile.

Gear Ratio

Ratio of the number of teeth on mating gears (larger gear : smaller gear). For example, a ratio of 16:1 means that the smaller gear (or pinion) makes sixteen revolutions for every one revolution of the larger mating gear. 

Ghost Image

In optics, a faint second image caused by reflections within an optical component. In paranormal investigation, a picture of a ghost, spectre, or phantom. 

  See also Beamsplitter , Spectrometer

Gimbal

A type of mounting component that allows for adjustable movement about two perpendicular and intersecting axes (i.e. pitch and roll). Ideal for positioning applications where the position of the center of the optic must remain stationary as it is tilted.

  See also Pitch , Roll

Glancing Illumination

Point source illumination similar to directional illumination, except at a sharp angle of incidence.

Glass Code

A reference code for a specific glass type that is represented by three digits, then a slash, then another three digits. It is always present in technical prints to indicate the specifications of the glass substrate used. The first three digits refer to the three numbers after the decimal in the glass' index of refraction. The second three digits refer to the first three significant digits in the glass' Abbe number. For instance, BK7, which has an index of refraction of 1.517 and an Abbe number of 64.2, has a glass code of 517/642. 

Goniometer

A mechanical component that allows for precise angular adjustment of an object about a fixed point located above the center of the mounting surface.

Gradient Index

The variation in the index of refraction of a medium as the temperature changes. This gradient index (dn/dT) can be problematic when operating in unstable environments, especially if the system is designed to operate for one value of n.

Gradient Index (GRIN) Lens

A lens whose material refractive index varies continuously as a function of spatial coordinates in the medium. 

Grating Equation

The relationship between diffracted order, wavelength, groove spacing, and angles of incidence and diffraction. It is used to determine the diffracted angle for a given incident beam.

Groove Density

The number of grooves per area, specified as grooves/ mm or grooves/ inch of a grating. It is used to determine the groove spacing and, with the grating equation, to obtain information about the diffracted orders and incident/ diffracted angles.

Ground Edge

A type of bevel in which the edge of an optical component is ground to remove sharp corners. Compared to a seamed edge, it is used when tighter tolerances are required.

  See also Seamed Edge

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