When a positive voltage is applied to the screen, the screen is made brighter and when negative voltage is applied the electrons are decelerated and the screen does not glow. The grid controls the speed with the electrons that hit the screen. The line-by-line scanning in CRTs is known as raster scan. The image is painted on the screen by the electron beam that scans from one side of the display to the other. A synchronization (sync) signal brings the beam back to the top row of the display. The electron gun in a CRT emits a flow of electrons towards a screen horizontally, one line at a time, as shown in Figure 38. The screen of a CRT is covered with a fine layer of phosphorescent elements called phosphors, which emit light and create a lit-up dot called a pixel. This is the most common type of display device that is in use for many years, as shown in Figure 37: The display quality of display devices is decided on the basis of their screen resolutions. The display devices commonly used these days are CRTs such as TVs, LCDs as used in laptops, and projectors. A display device is an output device of a computer that presents the information on the screen.

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