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In visual perception of real life, printed paper, a computer display, etc. contrast between different parts of an image, especially between objects (including characters) and the background enables one to see objects, read text, etc.

It depends on the color of the objects and background, paper and ink, etc. themselves, but also on the lighting.

For the computer display we can additionally distinguish between the properties of e.g. the picture file, and the properties of the computer display and its settings itself, such as how bright it is. For some screens the angle at which one looks at it is also an important factor.

In telecommunication, the term contrast has the following meanings:

  1. In display systems, the relation between (a) the intensity of color, brightness, or shading of an area occupied by a display element, display group, or display image on the display surface of a display device and (b) the intensity of an area not occupied by a display elements, a display group, or a display image. Deprecated synonym: brightness ratio.
  2. In optical character recognition, the difference between color or shading of the printed material on a document and the background on which it is printed.

Source: from Federal Standard 1037C