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R'G'B' is gamma corrected RGB.
The gamma correction adjusts the overall brightness of an RGB image by normalizing the pixel's RGB color components from its current integer values of 0 through 255 to real numbers between 0.0 and 1.0.
It then raises the normalized value of the pixel to a specified power, which is otherwise known as the gamma value. It will then transform the new value back to an integer component between 0 a 255.
Any gamma value between 0 and 1 will lighten up an image while anything higher will darken the image.