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A shadow mask is a metal sheet with holes punctured in a regular pattern that is present inside a color monitor. It is a part of a cathode-ray tube (CRT) setup where the electron beam, after generation from an electron gun source, is directed toward the screen to create images. The beam is focused by making it pass through the shadow mask.
A shadow mask directs the electron beam by absorbing the electrons that are going in the wrong direction so that the beam only hits the desired points and the resulting picture is not blurred. CRTs have three electron guns — red, green and blue — present for color display. These guns direct their beams to the shadow mask which allows them to pass, if the beams fall on a hole. Since the guns are physically discrete, the beams reach the shadow masks from slightly different levels. It is the task of a shadow mask to direct each beam on its respective dot on the screen and form an intelligible picture on the screen.