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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 for entertainment or security purposes.

Until recently, most computer screens depended on cathode ray tubes (CRTs). CRT images are generated using electron beams which are "launched" from the back of the monitor and "draw" images continuously on the screen. Most computer programs paint images in the screen. Some of these images (letters, pictures, animations, menus) are usually moving or changing, and never stay in the same place for long. But some portions of the screen (like the Start bar in Microsoft Windows, or the typical upper "score" bar of some video games) are always in the same place, sometimes for hours or even days or months.

Particularly with older CRTs, these sorts of images, continuously drawn in the same place for a long time, could damage the screen because the electron rays always hit the same points on the screen. Damage would consist in poor image quality, and those fixed images could remain "burned in" to the same place like "ghost lines" even if the image eventually changed. This is commonly known as "screen burn". This effect could be observed in some older video game machines; after displaying the same image for years on end, "ghost images" could be seen to be "burnt" into the display.

Screensaver programs were originally designed to help avoid these effects by automatically changing the images on the screen when the computer was not in use. They can be usually set up to launch automatically, waiting a specified amount of time after the last keystroke or the last mouse movement made by a user. Then the screensaver switches the image to black, or sometimes produces some animation effects, thus avoiding any "fixed" images. The screensaver remains active until a user enters a keystroke or makes a mouse movement. At that moment, the screensaver closes and the former screen contents are restored, to allow the user to work again.

Modern CRTs are much less susceptible to burn-in than older models. Flat panel displays are used in all laptop computers and are gradually replacing CRTs on the desktop. Flat panels are burn-in resistant because they use a fluorescent bulb or similar light source instead of an electron beam. For this reason, screensavers today primarily are decorative or for entertainment, and usually feature moving images or patterns and sometimes sound effects.

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