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Phosphor Burn-In and Screen Savers
When the phosphor dots on a CRT are struck by electron beams, they glow. When a particular image is displayed on a screen for a long time, the same dots are struck by the electron beam repeatedly millions of times. If the same exact image is left on some screens for a very long time, it is possible for the surface of the CRT to become damaged. When this happens, "ghosting" can be seen on the surface of the screen, and you can actually see the outline of the image that was displayed so many times, even when the power to the CRT is off. When this happens the phosphor is sometimes said to be "burnt in". The most common place that this phenomenon is seen is at the airport--older monochrome screens that have had the same flight arrival and departure information displayed on them day after day, year after year.
Screen savers were first invented to address this problem. A screen saver is simply a software program that, after a specified period of inactivity, either blanks the screen or displays a moving pattern on it. This prevents burn in of the screen phosphor that could occur through the same image being on the screen continuously.
The funny thing about screen savers is that they are really unnecessary today. Older monochrome displays were prone to this problem, but it is actually quite rare with modern monitors. Screen savers themselves continue to be popular, but today they are more of a form of entertainment software than a practical utility. Ironically, many screen savers today use their own images that remain stationary on the screen for long periods of time, which means they don't even do what they were originally supposed to do at all.
A screen saver is not a replacement for proper power management features. The monitor doesn't care much about what images it is displaying, so it uses power to display the screen saver image as well. If you are using a saver that blanks the screen entirely or is comprised mostly of low-intensity images then slightly less power will be used because the electron gun will be striking the phosphor using less energy, but this still isn't the same as using DPMS, for example.