A gamma characteristic is a power law relationship that approximates the relationship between the encoded luminance in a television system and the actual desired image brightness. Computer graphics systems that require a linear relationship between these quantities use gamma correction.
A cathode ray tube, for example, converts a video signal to light in a nonlinear way, because the electron gun it contains is a nonlinear device. The light intensity L is related to the source voltage VS by the following formula:
To compensate for this effect, the inverse transfer function (gamma correction) is sometimes applied to the video signal so that the end-to-end response is linear. In other words, the transmitted signal is deliberately distorted so that, after it has been distorted again by the display device, the viewer sees the correct brightness. The inverse of the function above is:
A colour CRT receives three video signals (red, green and blue) and in general each colour has its own value of gamma, denoted γR, γG or γB. However, in simple display systems, a single value of γ is used for all three colours.
Other display devices have different values of gammas: for example, a Game Boy Advance display has a gamma between 3 and 4 depending on lighting conditions.
For a tutorial on gamma characteristics in computer graphics, see http://www.w3.org/TR/PNG-GammaAppendix.html .