Phosphor burn-in

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Phosphor burn-in seen at an airport terminal. (Better visible when enlarged)

Phosphor burn-in is a permanent disfigurement of areas on a cathode ray tube or plasma display (e.g. a computer monitor or TV screen) caused by still images being displayed continuously for long periods.

1 Causes of burn-in
2 Prevention
3 Historical Notes
4 References
5 External Images

[edit] Causes of burn-in

With phosphor-based electronic displays (including cathode-ray type computer monitors and plasma displays), the prolonged display of a menu bar or other graphical elements over time can create a permanent ghost-like image of these objects. This is due to the fact that the phosphor compounds which emit the light lose their luminosity with use. As a result, when certain areas of the display are used more frequently than others, over time the lower luminosity areas become visible to the naked eye and the result is called burn-in. While a ghost image is the most noticeable effect, a more common result is that the image quality will continuously and gradually decline as luminosity variations develop over time... resulting in a "muddy" looking picture image.

The burn-in problem can become even more pronounced with plasma displays because of the discrete nature of the pixel elements. Some display manufacturers include image rotation or other mechanisms to reduce the rate of burn-in. One manufacturer has introduced a technology called ZeroBurn(R) which can eliminate it altogether.

Plasma displays also exhibit another image retention issue which is sometimes confused with burn-in damage. In this mode, when a group of pixels are run at high brightness (when displaying white, for example) for an extended period of time, a charge build-up in the pixel structure occurs and a ghost image can be seen. However, unlike burn-in, this charge build-up is transient and self corrects after the display has been powered off for a long enough period of time, or after running random broadcast TV type content.

LCD type displays exhibit a similar phenomenon, although the mechanics of the image retention are different. In the case of LCD displays the liquid crystal molecules, which rotate when energized and allow the white backlight to pass through the color membrane, lose their rotation elasticity. In this case they are unable to fully return to their normal rotation state when de-energized. As in the case with plasma displays, this is usually transient and will self correct after a period of off time or dynamic content. However, in severe cases it can become permanent. Also see: LCD image persistence.

[edit] Prevention

The first of what could be noted as active burn-in prevention was a simple blanking of the screen to black after a set period of inactivity. Today this is achieved by sending an all-black video signal, or on video electronics that support this feature, a complete disruption of video signal to the device. Due to many reasons, including not being able to tell if a computer or piece of high-end video gear was still operational without activating something to un-blank the screen, it became apparent that the simple blanking needed to be replaced with something more interesting.

The development of colorful screensavers was initially marketed to the public as a way to reduce the effects of phosphor burn-in by automatically generating random content on the display. How this would eliminate usage of the phosphor screen was not readily discussed by the marketers of consumer screensavers, but it was postulated that this would allow people to see that the equipment was still operational while minimizing the effect of phosphor burn-in. Some screensavers displayed static images, or at least portions of the image were static, thus defeating the original stated purpose of preventing phosphor burn-in. On newer CRT displays, combinations of screensavers and video power off signals are used to not only protect from phosphor burn-in, but to save electric power and the lifespan of the unit's electronics.

For newer LCD displays, screensavers achieve 'burn-in' protection by generating a constantly changing image to ensure that every pixel in the display is being used at approximately the same rate as the others. While screensavers such as this reduce the rate of burn-in, they do not eliminate it, and a blank screen with no video input is still best as no pixels are being used (although the backlight might still be on, and while strictly not interfering with the liquid crystals the lamp does have a powered-on lifespan measured usually in the thousands of hours, and so in this case a video power off signal would be best).

Another method for reducing the rate of burn-in on plasma displays is the inclusion by manufacturers of image-rotation features which periodically move the image around a five to ten pixel radius. While this helps distribute the luminosity degradation a little, it does nothing to solve the root problem.

Plasma displays can also be left on a static snow image channel where there is no signal to get rid of the effects of burn-in.^{[citation needed]}

A new technique employed by a novel software utility equalizes burn-in by monitoring how the screen is used and creating an inverse burn image. By displaying the inverse burn image, previous burn-in can be equalized. This means that bright and dark patches will not be noticeable and uniform brightness across the whole screen is achieved. This is an effective preventative measure. JScreenFix deluxe

[edit] Historical Notes

Because of several manufacturing and construction factors, not the least of which is that faster refreshing phosphor chemical compositions were not discovered and available at the time, early televisions used higher-latency phosphor mixtures in their CRTs. It was not noticed too readily unless the TV station broadcast a still image for some time, at which point the viewer could discern a 'ghost image' remaining in the picture for a few minutes until it faded. Indeed if the on-screen picture included a very bright object on a black background, when the camera panned a smearing effect could be discerned. The time it took for an actual burn-in of an image varied due to several factors. The most prevalent burn-in screen image on early televisions was said to be that of the RCA Indian Head test card that many early TV stations displayed at the end of the broadcast day. 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.

During the early years of computers (before the evolution of the commodity home computer), green and amber screen computer monitors had the same problems with high-latency phosphor compositions as the early TV sets. When a computer operator or technician received a new monitor for distribution to a user (or 'seat'), a manufacturer recommended procedure was performed before the equipment was deployed that was said to 'condition' the phosphor to prevent burn-in (except in the most extreme cases). This procedure consisted mainly of connecting the monitor to a computer running a program that would continuously print random ASCII characters (or equivalent; i.e. EBCDIC) to fill the entire screen. This was left to run for various periods up to a week or more. It was found that with some brands the procedure was best performed with the monitor upside-down. This was thought to to be better because of interactions between the strong electromagnetic forces induced by the electron beam deflection coils in the CRT and the Earth's magnetic field. Although evidently the procedures were widely used in university and business computing departments in the late 1960s through the 1970s, it is not positively known if they had any effect on reducing phosphor burn-in.