Phosphor burn-in
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Phosphor burn-in, colloquially known as screen burn, is a permanent disfigurement of areas on a CRT display (i.e. a computer monitor or television screen) caused by non-moving text or graphics being displayed continuously for long periods.
[edit] Causes of screen burn
With phosphor-based electronic displays (for example CRT-type computer monitors), the prolonged display of non-moving images (text or graphics) such as a menu bar or spreadsheet grid can create a permanent ghost-like image of these objects. This is because the phosphor compounds which emit light to produce images lose their luminosity with use; the heat generated by a phosphor as the result of having an electron beam constantly triggering it literally burns the phosphor, which results in its darkened appearance (see photo at left).
LCD-type displays can exhibit a similar phenomenon, which is sometimes confused with screen burn but is generally temporary.
Phosphor burn-in is particularly prevalent with monochromatic CRT screens, such as the amber or green monochrome monitors commonly used with older computer systems and dumb terminal stations. This is due in part to the fact that those screens displayed mostly non-moving images, and at one intensity: fully on. Color screens, by contrast, use three separate phosphors (red, green, and blue) per pixel, mixed in varying intensities to achieve specific colors; as a result, it is not typical for one phosphor to operate at full intensity for an extended period of time. Another reason why phosphor-burn in is particularly prevalent with monochrome computer monitors lies in the fact that modern computers are often used for operations where colors and on-screen object placement vary near-constantly (e.g. watching TV, playing a 3D video game, surfing the Internet); older computers and dumb terminals, which did not have those capabilities, were used mainly for routine data operations, where a menu screen, header frame, or grid (such as in a spreadsheet program or arcade-style game) was displayed at one intensity and in one position near-constantly if not constantly.
The length of time required for noticeable screen burn to develop varies due to many factors, ranging from the quality of the phosphors used by the screen's manufacturer, to the length of time that a static image is displayed. It can take as little as only a few weeks for screen burn to set in, especially if the screen displays a certain image (for example a menu bar at the top or bottom of the screen) constantly, and is left to display it at all times (overnight, over weekends, etc).
[edit] Prevention
Screensavers derive their name from their original purpose, which was an active method of attempting to stave off screen burn. By ensuring that no pixel or group of pixels was left displaying a static image for extended periods of time, phosphor luminosity was preserved.
Besides the use of a screen saver, the most obvious way to prevent screen burn is to shut the display off when not in use, and to minimize the number of static images being displayed on the screen while in use.
[edit] Historical note
The most prevalent burn-in image on early televisions was said to be that of the RCA Indian Head test card, which predates the use of the current SMPTE color bars. This was due to the viewer accidentally leaving the TV set on at the end of the day, which was not recommended by the TV manufacturers.
