Cathode Ray Tubes monitor (CRT)

Those monitors based on Cathode Ray Tubes (CRTs), similar to those used in TV sets. Below the figure illustrates the basic operation of CRT. A beam of electronic (cathode rays) emitted by an electron gun, passes through focusing and deflection systems that direct the beam towards specified points on the phosphor contacted by the electron beam.

Since the light emitted by the phosphor fades very rapidly, some method is needed for maintaining the screen picture. One way to keep the phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points. This type of display is called refresh CRT. Generally, it is required to redraw 30 to 60 times in a second in order to avoid flicker. If the redraw is below 25 times per second than the picture flickers.

Different types of phosphors are available for use in CRT. Besides color, a major difference between phosphor is their persistence, i.e., how long they continue to emit light after the electron beam is removed. Persistence is defined as the time it takes emitted light to decay to one tenth of its original intensity. Lower persistence phosphor requires higher refresh rate to maintain a picture on the screen without high flicker. A Phosphor with low persistence is useful for animation, while high persistence phosphors are better suited for displaying highly complex, static pictures.

Deflection of the electron beam is done with electric field. The beam passes between two pairs of metal plates: one pair vertical, the other pair horizontal. A voltage difference is applied to each pair of plates according to the amount that the beam is to be deflected in each direction. As the electron beam passes between each pair of plates, it is bent towards the plate with the higher positive voltage.

In color screens, there are three guns, one for each color Red, Green, and Blue (RGB). These cause each dot of the screen to generate red, green or blue light. The combination of these three gives the full color spectrum. The computer directly controls the three guns. CRTs give a bright picture, with good colors but they are bulky and consume a relatively large amount of power.

Old monochrome models display text and graphics as either green on a black background or white on a black background. The foreground and background colors a can be reversed by the software. The monochrome monitors are gradually declining as most of the software developed these days uses many colors.

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