Gamma

Gamma has several meanings. In the video world a television screen’s brightness is not necessarily linearly proportional to amplitude of the picture signal. This is approximated by a power law, the power being referred to as gamma. For a CRT (cathode ray tube) the gamma is generally calculated to be 2.6. This is compensated for in TV cameras by a gamma of 0.45 giving an overall gamma of 0.45 x 2.6 = 1.17 – adding overall contrast to help compensate for domestic viewing conditions. Today, most viewers have panel screens that use one of several different technologies. Usually there is a menu somewhere for selecting a gamma setting to suit your screen for the correct gamma and gamut.

In film, gamma describes the average slope of the D/Log E curve over its most linear region. For negative stocks this is approximately 0.6, for intermediate stocks this is 1.0 and for print stocks 3.0. This gives a system gamma of 0.6 x 1 x 3 = 1.8. This overall boost in contrast is much reduced due to flare and auditorium lighting conditions of cinemas.

With video now available on a wide range of devices there may be a need to re-visit the gamma settings. For example, a digital film master is versioned for mobile phones, and for home TV (where viewers have, LED, plasma and a few CRT displays) as well as for digital and film cinemas. This can be achieved by applying suitable LUTs for each version.

See also: Internegative, Interpositive

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