As far as tonal value gradation is concerned, it is a device-specific characteristic value that manifests itself in different, non-linear gradations in hue, color saturation and brightness in digital cameras, displays, projectors and printers.
Different color gradations mean that the distinctions in dark image areas are different from those in light image areas. The viewer recognizes this by the non- linearity of the gradations. Gamma correction straightens out the non-linearities and thus the gamma curve becomes linear. Gamma is then 1 and the output brightness or the output color tone corresponds to the input brightness or the input color tone.