 This is because a monochrome monitor is made up of a continuous coating of phosphor and the sharpness can be controlled by focusing the electron beam; whereas on a color monitor, each pixel is made up of three phosphor dots (one red, one blue, one green) separated by a mask.Monochrome monitors were used in almost all dumb terminals and are still widely used in text-based applications such as computerized cash registers and point of sale systems because of their superior sharpness and enhanced readability.Some green screen displays were furnished with a particularly full/intense phosphor coating, making the characters very clear and sharply defined (thus easy to read) but generating an afterglow-effect (sometimes called a "ghost image") when the text scrolled down the screen or when a screenful of information was quickly replaced with another as in word processing page up/down operations.