Gamma correction

Gamma correction is a point operation performed on the components of color images with RGB representations. Gamma correction is mostly provided for video, both analog and digital.

The goal of gamma correction is to compensate the nonlinearity of the monitor (cathode ray tube) transfer function

L₂ = k ( X ' )^γ ,

where L₂ is the luminance produced by the monitor in the response to the input voltage excitation X '. The parameter k depends on the monitor settings while γ = 2.3 - 2.6 is constant for a given monitor and its value depending on the physical process of image synthesis is similar for various monitors.

The gamma correction is made according to the formula

X ' = X ^1/γ .

The value of γ used by correction does not necessary match the value of monitors. If the overall system characteristic remains still exponential (effective γ > 1), the image will appear sharper but the scene contrast range that can be reproduced is reduced. On the contrary, the images appear soft if the overall characteristics is logarithmic (effective γ < 1).

In analog television systems,

γ = 2.8 for PAL system (Europe),

γ = 2.2 for NTSC system (USA).

Gamma correction additionally allows for video coding in perceptually uniform space as brightness perceived by humans is related to luminance, radiated by a monitor, by the relation coincidentally being approximately inverse to the CRT characteristics described above.