The inside of the CRT display screen is coated with phosphor. This material is capable of emitting light when struck by radiation. The layer of phosphor, illuminates as the electron stream hits it, causing the desired image to appear on the display. The persistence of the illumination of the phosphor helps to diminish flickering. This persistent glow gives rise to a condition called "burn-in". Phosphor burn-in occurs when the display is kept on for long periods of time. The phosphor becomes "fatigued", remaining partially illuminated even when the pixel is not irradiated. This produces ghost images on the display screen. To prevent burn-in, a utility called a "screen saver" is used to darken the display. A screen saver will activate automatically, after a designated time, if no input is received from the user. This allows the computer to run for extended periods without risk to the display. A screen darkened in such a way might be mistaken for off, so many screen saver programs utilize attractive, dynamic graphics. The moving graphics cycle the illumination of individual pixels.
