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A screensaver (or screen saver) is a computer program that blanks the screen or fills it with moving images or patterns when the computer is not in use. Initially designed to prevent phosphor burn-in on CRT and plasma computer monitors (hence the name), screensavers are now used primarily for entertainment, security or to display system status information. Decades before the first computers using this technology were invented, Robert A. Heinlein gave an example of how they might be used in his novel ''Stranger In A Strange Land'' (1961):〔http://www.technovelgy.com/ct/content.asp?Bnum=184 〕 Many modern television operating systems, media players and other digital entertainment systems include optional screensavers. Some screensavers also offer additional functions like automated workstation password protection during inactivity (i.e., probably absent) user or disk scan with installed antivirus software. == Purpose == Before the advent of LCD screens, most computer screens were based on cathode ray tubes (CRTs). When the same image is displayed on a CRT screen for long periods, the properties of the exposed areas of phosphor coating on the inside of the screen gradually and permanently change, eventually leading to a darkened shadow or "ghost" image on the screen, called a screen burn-in. Cathode ray televisions, oscilloscopes and other devices that use CRTs are all susceptible to phosphor burn-in, as are plasma displays to some extent. Screen-saver programs were designed to help avoid these effects by automatically changing the images on the screen during periods of user inactivity. For CRTs used in public, such as ATMs and railway ticketing machines, the risk of burn-in is especially high because a stand-by display is shown whenever the machine is not in use. Older machines designed without burn-in problems taken into consideration often display evidence of screen damage, with images or text such as "Please insert your card" (in the case of ATMs) visible even when the display changes while the machine is in use. Blanking the screen is out of the question as the machine would appear to be out of service. In these applications, burn-in can be prevented by shifting the position of the display contents every few seconds, or by having a number of different images that are changed regularly. Modern CRTs are much less susceptible to burn-in than older models due to improvements in phosphor coatings, and because modern computer images are generally lower contrast than the stark green- or white-on-black text and graphics of earlier machines. LCD computer monitors, including the display panels used in laptop computers, are not susceptible to burn-in because the image is not directly produced by phosphors (although they can suffer from a less extreme and usually non-permanent form of image persistence). For these reasons, screensavers today are primarily for decorative/entertainment purposes, or for password protection. They usually feature moving images or patterns and sometimes sound effects. One increasingly popular application is for screensavers to activate a useful background task, such as a virus scan or a distributed computing application (such as the SETI@home project). This allows applications to use resources only when the computer would be otherwise idle. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Screensaver」の詳細全文を読む スポンサード リンク
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