A CRT (Cathode ray tube) display unit is a commonly used output device. The VDT (Visual Display Terminal) unit contains CRT for visual display unit displays the data and information received from the computer in alphanumeric characters and graphs. CRT being an output device can also perform limited input function when used in conjunction with a light pen (a pencil shaped device used to select screen positions by detecting the light coming from points on screen).
The CRT screen is similar to TV screen. A CRT is a vacuum tube which has a located electron gun at the back portion that produces electron beam which is directed towards the front of the CRT screen. A coating of phosphor material is made on the inner surface of the screen that emits light when struck by electron beam. The color of the emitted light depends on the phosphor substance used and the CRT display may be multicolor (red, pink, green, blue, etc.) depending on it color display of CRT screen are produces on the principle of color TV screen.
To produce color display 3 phosphors: red, blue, and green are used whose coating is made in such a way that dots of these 3 phosphors in a triangular pattern are spread over the entire screen and due to it closeness they appear as a single dot. By varying the intensity of 3 colors the different combination of red, blue, and green beams 16 types of color can be produced and a much wider variety of color can be produced.
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