Gamma Correction - Terminology

Terminology

The term intensity refers strictly to the amount of light that is emitted per unit of time and per unit of surface, in units of lux. Note, however, that in many fields of science this quantity is called luminous emittance, as opposed to luminous intensity, which is a different quantity. These distinctions, however, are largely irrelevant to gamma compression, which is applicable to any sort of normalized linear intensity-like scale.

Luminance can mean several things even within the context of video and imaging:

• Luminance is the photometric brightness of an object, taking into account the wavelength-dependent sensitivity of the human eye (in units of cd/m²);
• Luminance (video) is the encoded video "luma" signal, i.e. similar to the signal voltage V_S.
• Luminance (relative) is the luminance signal used in a color-space encoding, relative to a white level.

One contrasts "luminance" in the sense of color (no gamma compression) with "luma" in the sense of video (with gamma compression), and denote luminance by Y and luma by Y′, the prime symbol (′) denoting gamma compression.

Likewise, brightness is sometimes applied to various measures, including light levels, though it more properly applies to a subjective visual attribute.

Gamma correction is a type of power law function whose exponent is the Greek letter gamma (γ). It should not be confused with the mathematical Gamma function. The lower case gamma, γ, is a parameter of the former; the upper case letter, Γ, is the name of (and symbol used for) the latter (as in Γ(x)). To use the word "function" in conjunction with gamma correction, one may avoid confusion by saying "generalized power law function."

Without context, a value labeled gamma might be either the encoding or the decoding value. Caution must be taken to correctly interpreting the value as that to be applied-to-compensate or to be compensated-by-applying its inverse. In common parlance, in many occasions the decoding value (as 2.2) is employed as if it were the encoding value, instead of its inverse (1/2.2 in this case) which is the real value that must be applied to encode gamma.