An image is stored in computer memory as a series of numbers which represent pixel values. Higher values mean brighter colors. Lower values represent darker colors. These values are, in general, produced and stored in linear format. In other words, a pixel with an intensity value of 32 would have one-half the intensity of a pixel with a value of 64. However, a computer monitor displays colors by exciting phosphors on the screen and phosphors do not excite linearly. For example, if a computer reads a luminance value from a photographic image and sends it directly to the screen, the displayed color will be dimmer than in the original photograph.
This is where gamma correction comes in: a gamma correction value adjusts for the nonlinearity of phosphor excitation. Too correct for monitor nonlinearities, graphics systems use a video lookup table that adjusts the linear pixel values.