Refresh type output devices in Computer Graphics

CRT stands for cathode ray tube. CRT it is an elevated glass tube. An electron gun at one side of the tube produces a beam of electrons which is directed towards the front of the tube or towards the screen. The inner side of the screen is coated with Phosphorus substance which gives off the light when it stopped by electrons.

  1. Standard CRT:
    In standard CRT the electron gun emits electron beams which fall on the phosphor coating due to which a glow occurs which display the text or a picture. afterglow the light gets filled for continuous viewing the image is therefore refreshed at regular intervals so frequently that the changing image appears as a continuous picture to the human eyes.



  2. Beam penetration CRT:
    A normal CRT with phosphor coating generate images of only one colour a colour CRT for line drawing displays images on multilayer phosphor. Colours are achieved by controlling the beam accelerating potential. The screen is coated with a layer of green phosphor over which area of red phosphor is deposited.

    When a low potential electron beam strikes the screen only the red phosphorus gets excited thus producing a red trace. A higher velocity beam will penetrate into the green phosphor increasing the green components of the light output by varying the beam potential different combinations of Red and Green lights can be produced.

  3. Shadow mask CRT:
    The majority of colour television sets and computer monitors used Shadow mask colour CRT which has phosphor coated screen behind the screen there is a metal plate with small holes arranged in a specific pattern. It uses three electron guns for red green and blue components of the light output.

    The guns are arranged in a triangular fashion the deflection system operates on the three electron beam simultaneously bringing them to the same point of focus on the shadow mask all the three beans pass through a single Hole in the Shadow mask and strikes the phosphor at the described point.


    Figure – Colour CRT Shadow mask techniques



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