A perforated metal sheet inside a color monitor. Most color monitor screens use
cathode-ray tube (CRT) technology in which electrons are fired from an electron gun onto a phosphor coating on the screen's faceplate. The phosphor converts the kinetic energy of the electrons into light and is illuminated in tiny red, green and blue dots, which comprise the image that one sees when looking at a monitor's screen. The phosphors in a group are packed so closely together that the human eye can only perceive them as a single colored
pixel. Before the electron beam reaches the phosphor dots it passes through the
shadow mask, a perforated metal sheet that ensures that the electron beam hits only the correctly colored phosphor dots and does not illuminate more than one dot. Essentially, the shadow mask "masks" the electron beam, thereby forming a smaller and more rounded point that can hit individual phosphor dots. The shadow mask absorbs electrons that are directed at the wrong color phosphor.
