Cathode Ray Tube

The CRT is probably the most well-known visual display tech­nology. It is the major component of television sets and most computer monitors. The CRT dates back to 1897, when Ferdinand von Braun invented the so-called Braun'sche Rohre (Oscilloscope). In 1998, 250 million displays for televisions and computers were produced worldwide. 90% of which were CRTs (Blankenhach, 1999).  A CRT consists of a glass bulb, a cathode, an electron gun deflection coils, a mask, and a phos­phor coating. The major component of the cathode ray tube is an evacuated flask At the back of the flask a heated cathode is charged with high voltage in the range of 10 kV to 30 kV. This cathode emits negatively charged electrons, which are attracted and accelerated by an anode that is located in front of the screen. The electron beam is diverted by an electromagnetic held, built up by the deflection coils, and thus directed toward the screen. A screen mask is attached in front of the ground glass plate, so that the electron beam is focused and then steered on the phos­phorus layer, placed on the front surface. Therefore, the outside areas of the beam are disallowed from wrongly hit­ting adjacent phosphorus dots that would lead to the chromatic distortion and blurred representation.  The display screen masks are either Trinitrons, slot mask screens or dot mask screens.

The electron beam is directed line-by-line across the screen and stimulates the phosphor coating to light up for a brief mo­ment. The intensity of the light is modulated by the voltage applied at the cathode. The more voltage applied, the brighter the light shines. Color representation is obtained both on the type of phos­phor used and the electron beam. The phosphorus layer is made of a triad in the form of an array, consisting of a dot of each of the colors red, green, and blue. Color monitors therefore have three electron guns. The electron beam of each falls only on one col­ored dot in each triad (Bosman, 1989). Three dots, of one triad each, are 1 pixel. With different light intensities, almost every mixture of colors is possible.

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