Visual Display Unit VDU

The display screen controlled by a computer, works very much like a television set. To create a permanent image on the screen, it is necessary to cause illuminations on the screen repeatedly. This is done by scanning the CRT display screen with electron beam repeatedly. Commonly used scanning scheme is called Raster Scan. In this, the electron beam is moved back and forth across the screen constantly starting from left to right and reaching the beam top to bottom of a screen. The electron beam start striking the screen from top left corner, it moves along the first scan in towards right. On reaching extreme right, the beam is brought back to left. It then moves right from the next scan line. Like that it proceeds from top to bottom painting the entire image and scans the entire screen. On reaching the bottom of the screen, the electron beam is brought back to the top of the screen, at the first scan line.

The electron beam moves over the screen continuously, the computer video controller electronic circuit reads out the data to be displayed from display memory and translates the data bits into the video signals that are applied to the CRT to control the flow of electrons towards the CRT display screen. Video controller also sends synchronization signals to CRT deflection system.

CRT is an evacuated conical glass tube with a phosphor coated on the inner front surface called screen. A source of electrons called electron gun at one end of the tube emits electron beam. This beam is directed towards screen by means of the accelerating electrodes and focusing electrode all along the path of the electron beam towards the display screen. This electron beam is deflected by an electromagnetic deflection coil in order to produce illumination at various spots on the display screen. Horizontal deflection coils deflects the beam in the horizontal deflection and the vertical deflection coils deflect the beam in the vertical direction there by it is possible for electron beam in the an various sport of the screen. When the electron beam strike the screen, the phosphor coated on the inner surface of the screen produces illumination. The illumination caused on the screen exists for a few milliseconds due to the persistence of the phosphor material. To hold a permanent image on the screen, it is necessary to cause illuminations repeatedly. This is done by scanning the CRT display screen with electron beam repeatedly. This is known as refreshing the CRT screen.

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