Older Computer Models
Cathode ray tubes produce images on the screen by hitting on the pixel grids on the back of the screen. The entire screen in a CRT is divided into several grids by intersection of virtual horizontal and vertical lines. Each intersection is termed a grid so the first grid becomes 0H, 0V. Each grid has three dots in a colored display: red, green and blue. The early monochrome displays (green ones) had a single dot per intersection. Any image or text produced on a CRT based display has to be a combination of these dots.
The back of CRT screens are coated with phosphorous materials so when hit, it produces light. When the information about the image to be produced is sent to the CRT, the electron gun contained in the CRT scans the entire screen and employs a cathode ray (a beam of electrons) to hit the predetermined dots to produce the display. Once illuminated, the dots fade away fast. To keep the image constant on the screen, the electron gun strikes the phosphor points again and again on the same dots. The refresh rate could be as high as 60 hits per minute.
Heat causes illumination and thereby, production of images on CRTs, if the same image has to be displayed for a long time, causes the phosphorus to burn, creating permanent damage to the screen. You can identify a burned out CRT as it displays a ghost image even when the monitor is turned off.
To avoid constant hitting of the same phosphor dots, screen savers are used. Because the screen savers contain movements, the cathode rays strike different phosphor dots and helps in preventing burn-out.