Three colored phosphor dots on the faceplate of a tri-color CRT.

Some tri-color CRTs use vertical stripes of different color phosphors or vertically oriented oblong dots. These dots or stripes are the ultimate determinants of maximum horizontal resolution. When the dots are round, they are also the maximum determinants of vertical resolution. The finer the dot pitch, the higher the resolution, since it is not possible to reduce the size of a black-and-white pixel below the size of one triad. Triad spacing also cannot be optimized for all numbers of scanning lines. Thus, a tube optimized for 1125 scanning lines will not yield optimum performance with a signal of 1050 scanning lines, or vice versa. Neither black-and-white CRTs nor the three single-color CRTs used in most projection TV sets suffer from these limitations as their faceplates are uniformly covered with a layer of phosphor; resolution is ultimately determined by the size of the electron beam and the projection optics. Picture tubes with striped apertures can deal effectively with multiple scanning rates, but still restrict horizontal resolution to the width of three stripes.