Screensaver

A screensaver (or screen saver) is a software utility which displays either a completely black image or a constantly changing image on a computer monitor to prevent a stationary image from "burning" into the phosphor of the screen. Screensavers usually start automatically after the computer has had no user input for a preset time. Some screensavers come with many different modules, each giving a different effect.

History
Before the 1990s, cathode-ray tubes (CRTs) used in televisions or computer monitors were prone to "burn-in"; if the same shapes or patterns (e.g. menu bar or Control Strip) were shown at the same position on the screen for very long periods of time, the phosphor on the screen would "fatigue" and that part of the screen would appear grayed out, even when the CRT was turned off. Some CRTs were later developed with resistance to burn-in or would go into a built-in sleep mode after a period of inactivity.

Early home video game systems of the 1970s, such as the Atari, would change the screen in a lengthy animated loop while idling to prevent burn-in. This philosophy led to the development of the first screensavers, which were little more than display blankers that just set the screen to all black. Feature creep led these tiny (sometimes under 1 KB) programs to begin to add simple graphical effects, such as a starry sky, eventually leading to elaborate animations (often with sound effects) of increasing length and complexity without regard to usefulness.

Screensavers for Macs
After Dark was an early series of popular screensavers for classic Mac OS which included a number of plug-in modules. Some modules could even add self-contained mini-games. The original goal of avoiding burn-in lost its priority, and screensavers began to be developed with their own functionality, such as PointCast, which made no claim to protect the monitor, but simply push media which self-started after the computer has been inactive for a pre-set period of time.

Apple began including its own Desktop & Screen Saver preference pane with the introduction of Mac OS X (now macOS). The Energy Saver pane can be set to actually cut power to the display. The Security & Privacy pane includes the option to require a password after the screen has been put to sleep.