Unlike CRT monitors, LCD screens do not normally experience irreversible burn-in, but they can suffer from image persistence and stuck pixels, where some parts of the screen won't stop displaying a single color. You can reverse most image persistence by forcing the monitor to display a range of colors, or leaving an all-white image on the screen. Some models of Dell monitors have a built-in feature called LCD Conditioning that repairs persistence automatically.
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LCD Conditioning on Dell Monitors
Many Dell monitors provide an image persistence repair feature that operates directly on the monitor without requiring computer software. To access it, check your on-screen display menu in the "Other Settings" section. Pick "LCD Conditioning" and choose "Enable" to run the utility, which cycles the screen through various colors. If you still see image persistence after the cycle ends, you can try running it again -- the process merely displays colors and so cannot harm your monitor.
Fixing Persistence on Other Monitors
If your monitor doesn't have built-in LCD conditioning, you can simulate the process by manually changing between several full-screen images of colors. Alternatively, try leaving up an all-white image for as many hours as the persisting image remained on the screen -- Apple recommends this method for its monitors. In cases of minor image persistence, the problem may not require any intervention, as it will fade during regular use.