Shadow Mask

The shadow mask is one of the technologies used to manufacture cathode ray tube (CRT) televisions and computer displays that produce color images. Another approach is the aperture grille, better known by its trade name, Trinitron. All early color televisions and the majority of CRT computer monitors used shadow mask technology. Both of these technologies are largely obsolete, having been increasingly replaced since the 1990s by the Liquid Crystal Display (LCD).

A shadow mask is a metal plate punched with tiny holes that separate the colored phosphors in the layer behind the front glass of the screen. Three electron guns at the back of the screen sweep across the mask, with the beams only reaching the screen if they pass through the holes. As the guns are physically separated at the back of the tube, their beams approach the mask from three slightly different angles, so after passing through the holes they hit slightly different locations on the screen. The screen is patterned with dots of colored phosphor positioned so that each can only be hit by one of the beams coming from the three electron guns. For instance, the blue phospher dots are hit by the beam from the "blue gun" after passing through a particular hole in the mask. The other two guns do the same for the red and green dots. This arrangement allows the three guns to address the individual dot colors on the screen, even though their beams are much too large and too poorly aimed to do so without the mask in place.

The red, green, and blue phosphors for each pixel are generally arranged in a triangular shape (sometimes called a "triad"). For television use, modern displays (starting in the late 1960s) use rectangular slots instead of circular holes, improving brightness.