Sensitometry
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Sensitometry is the scientific study of light-sensitive materials, especially photographic film. The study has its origins in the work by Ferdinand Hurter and Vero Charles Driffield with early black-a
Fig.1. Monochrome Telefilm Transmission.
Fig. 2. Combinations of gamma values in film chain.
Sensitometer from the Institut d'Astrophysique de Paris (Sorbonne University).
Gamma correction
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Gamma correction or gamma is a nonlinear operation used to encode and decode luminance in video or images.
Sensitometry
…region of the HD curve which approximates to a straight line, called the "linear" or "straight-line" portion; the slope of this region is called the gamma. The low end is called the "toe", and at the top, the curve rounds over to form the "shoulder". At extremely high exposures, the density may come back…
The effect of gamma correction on an image: the original image was taken to varying powers, showing that powers larger than 1 make the shadows darker, while powers smaller than 1 make dark regions lighter. This is not the actual gamma the picture has, though.