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Screen burn-in, colloquially known as screen burn, is a permanent disfigurement of areas on an electronic display such as a CRT display or computer monitor or television screen caused by cumulative non-uniform usage of the pixels.
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Causes of screen burn
With phosphor-based electronic displays (for example CRT-type computer monitors or plasma displays), uneven usage of pixels such as the prolonged display of non-moving images (text or graphics), gaming, or certain TV broadcasts with tickers and flags, can create a permanent ghost-like image of these objects or otherwise degrade image quality. This is because the phosphor compounds which emit light to produce images lose their luminance with use; uneven usage results in uneven light output over time and in severe cases can create a ghost image of previous content. Even if ghost images are not recognizable, the effects of screen burn are an immediate and continual degradation of image quality.
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 degree of non-uniformity of sub-pixel usage. It can take as little as only a few weeks for noticeable ghosting 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).
CRT
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 (called "sub-pixels"), mixed in varying intensities to achieve specific colors, and in typical usage patterns such as "traditional" TV viewing (non-gaming, non-converged TV usage, non-Internet browsing, broadcasts without tickers or flags) are used for operations where colors and on-screen object placement approach uniformity.
Plasma & LCD
LCD-type displays are also susceptible to permanent burn-in, but generally less so than plasma-type displays. In the case of LCDs, the mechanics of the generally transient image persistence become so severe that pixels permanently lose their ability to return to their relaxed state. All major LCD manufacturers' warranties exclude coverage for burn-in (permanent image persistence) as a result.
Both plasma-type and LCD-type displays exhibit a similar phenomenon called transient image persistence, which is sometimes confused with screen burn but is not permanent. In the case of plasma-type displays transient image persistence is caused by charge build-up in the pixel cells (not cumulative luminance degradation as with burn-in), which can be seen sometimes when a bright image that was set against a dark background is replaced by a dark background only; this image retention is usually released once a typical-brightness image is put back up on the display and therefore does not interfere with the display's typical viewing image quality.
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.
In many cases the use of a Screensaver is impractical. Most plasma-type display manufacturers include methods for reducing the rate of burn-in by rotating the image slightly, which does not eliminate screen burn but can soften the edges of any ghost image that does develop.
Historical notes
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 leaving the TV set on at the end of the day, which was not recommended by the TV manufacturers.
References
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