The holes prevent the beams from accidentally illuminating adjacent pixels, thereby offering a more precise image. But shadow masks limit how much of a beam actually strikes the phosphor compound and also restrict the strength of the beam. These limitations can make the display too dim. Some phosphors will be prevented from glowing, and if the beam is too strong, the temperature of the mask increases and the mask expands slightly, producing fuzziness. The three electron beams for each pixel are guided through a single hole in the shadow mask, each striking a different-color dot on the display face.
Aperture grilles consist of a phosphor pattern of stripes rather than dots on the display face.
Like the holes in the shadow mask, the slots both restrict and position the electron beams as they make their way to the display face. Instead of dot pitch, aperture grille monitors refer to screen pitch , the distance between successive stripes of the same color, and slot pitch , the distance between the slots. The only real display issue with the aperture grille is that it is held in place by thin wires running horizontally behind the display, which are often visible to the user. Aperture grilles can also loosen over time, causing some distortion. The newer NEC and Panasonic displays offer neither a shadow mask nor an aperture grille, but a kind of hybrid called a slot mask .