Gamma
Definition
Gamma is the non-linear relationship between signal level and displayed luminance, expressed as a power-law exponent. In HD, ITU-R BT.709 defines the camera transfer function (OETF), with an exponent of 0.45 and a linear segment near black, and ITU-R BT.1886 defines the reference display EOTF, close to gamma 2.4 for a dim production environment. Gamma 2.2 is common for displays in brighter environments and for computer content, whose sRGB standard (IEC 61966-2-1) is close to it. A higher display gamma darkens the midtones and a lower gamma lifts them, so a mismatch between projector, LED wall and monitoring makes the same content look correct on one screen and dull on another.
In the field
Content that looks correct on the control-room monitor but dull on the LED wall usually indicates that the wall is set to a higher gamma than the reference. All displays are set to the same gamma target before content is graded, commonly 2.4 (BT.1886) for dim rooms and 2.2 for brighter spaces.
Related lessons
Sources
- ITU-R BT.709 : fonction de transfert de la caméra (OETF), exposant 0,45 et segment linéaire · itu.int
- ITU-R BT.1886 : EOTF de référence des écrans plats de production HDTV, gamma 2,4 · itu.int
- Wikipédia, sRGB : norme IEC 61966-2-1, courbe proche d'un gamma 2,2 (pointeur secondaire) · en.wikipedia.org
- Portrait Displays, BT.1886 - 10 Questions, 10 Answers : application de BT.1886 en étalonnage · portrait.com