crt - cathode ray tube electron gun is used to

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CRT - Cathode Ray Tube

• Electron gun is used to send an electron beam aimed at a particular point on the screen. Deflection system is used to make the beam strike the screen on a precise point.
• When electron beam strikes the screen, coated phosphor emits light due to the natural phenomenon called phosphorescence. Output decays rapidly approximately within 10 - to 60 microseconds.
• As a result of this decay, the entire screen must be refreshed at least 30-60 times per second. This is called the refresh rate. If the refresh rate is low, screen will tend to flicker.
• CFF (Critical Fusion Frequency) is the minimum refresh rate needed to avoid flicker. It depends on the persistence of the phosphors and the human vision system.
• Electron beam traces out a path on the screen, hitting each pixel once per cycle. Tracing out of the entire picture once is called a cycle. In case of a raster scan display we call the horizontal path of the beam a scan line. In case of random scan display, the path is a directed set of line segment, which are called vector lines.
• On a raster scan display, refresh rate is fixed as mentioned above. On a vector display, a scan is in the form of a list of directed line segments to be drawn, so the time to refresh is dependent on the length of the display list. For small and simple object displays, some kind of delay is introduced to avoid un-necessary refreshing of the screen.
• In color CRT, shadow-mask method is most common since it produces a wide range of colors. In this method, each pixel consists of a group of three phosphor dots (one each for red, green, and blue), arranged in a triangular form called a triad. The shadow mask is a grid structure with one hole per pixel. To excite one pixel, three electron guns are used one for each of red, green, and blue. Each gun fires its electron stream through the hole in the mask to hit that pixel.