and in typical usage patterns such as "traditional" TV viewing (non-gaming, non-converged TV usage, non-Internet browsing, broadcasts without tickers or flags, no prolonged or permanent letterboxing) are used for operations where colors and on-screen object placement approach uniformity. Modern CRT displays are less susceptible than older CRTs prior to the 1960s because they have a layer of aluminum behind the phosphor which offers some protection. The aluminum layer was provided to reflect more light from the phosphor towards the viewer. As a bonus, the aluminum layer also prevented ion burn of the phosphor and the ion trap, common to older monochrome televisions, was no longer required. Plasma, LCD, and OLED displays Plasma displays produced until around 2007 were highly susceptible to burn-in, while LCD-type displays are rarely affected. The wide variation in luminance degradation with RGB-based OLED will cause noticeable color drift over time (where one of the red-green-blue colors becomes more prominent). OLEDs do not need a backlight to light up; each pixel is a self-illuminating LED. The pixels on OLEDs inevitably lose their brightness over time. The longer an OLED pixel is used (illuminated), the dimmer it will appear next to a lesser used pixel. In the case of LCDs, the physics of burn-in are different than plasma and OLED, which develop burn-in from luminance degradation of the light-emitting pixels. For LCDs, burn-in develops in some cases because pixels permanently lose their ability to return to their relaxed state after a continued static use profile. In most typical usage profiles, this image persistence in LCD is only transient. Both plasma-type and LCD-type displays exhibit a similar phenomenon called transient image persistence, which is similar to 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 displayed and does not inhibit the display's typical viewing image quality. Mitigation 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. Modern screensavers