A screensaver (or screen saver) is a computer program that blanks the screen or fills it with moving images or patterns when the computer has been idle for a long time. The original purpose of screensavers was to prevent phosphor burn-in on CRT and plasma computer monitors (hence the name). Though modern monitors are not susceptible to this issue, screensavers are still used for other purposes. Screensavers are often set up to offer a basic layer of security, by requiring a password to re-access the device. Some screensavers use the otherwise unused computer resources to do useful work, such as processing for distributed computing projects.