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A screensaver is a computer program originally designed to conserve the image quality of computer displays by blanking the screen or filling them with moving images or patterns when the computers are not in use. Today, screensavers are primarily used for entertainment or security purposesBefore the proliferation of LCD screens, most computer screens depended on cathode ray tubes (CRTs). Images on a CRT monitor are generated using electron beams which are emitted from electron guns at the back of the tube, and manipulated by electromagnetic fields to form images line-by-line on the phosphorescent screen many times per second. In some situations the images displayed on the screen constantly change, but in other cases some areas of the screen, or the screen as a whole, change very little (the taskbar in Microsoft Windows, for example). When the same image is displayed on a CRT screen for long periods of time, the properties of the exposed areas of phosphor coating on the inside of the screen gradually and permanently change, eventually leading to a darkened shadow or "ghost" image on the screen. Televisions, oscilloscopes and other devices that use CRTs are all susceptible to phosphor burn-in, as are plasma displays to some extent.
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