One of the basic and commonly used display devices is Cathode Ray Tube (CRT). A cathode ray tube is based on the simple concept that an electronic beam, when hits a phosphorescent surface, produces a beam of light (momentarily – though we later describe surfaces that produce light intensities lashing over a period of time). Further, the beam of light itself can be focused to any point on the screen by using suitable electronic / magnetic fields.
The direction and intensity of the fields will allow one to determine the extent of the defection of the beam. Further these electronic / magnetic fields can be easily manipulated by using suitable electric fields with this background. In following section we describe the structure and working of the simple CRT.
Simple CRT makes use of a conical glass tube. At the narrow end of the glass tube an electronic gun is kept. This gun generates electrons that will be made to pass through the magnetic system called yoke. This magnetic system is used for making the electronic beam to fall throughout the broad surface of the glass tube. The broad surface of the glass tube contains a single coat of high quality phosphorus. This reflects the electronic beam makes it to fall on the computer screen
A pair of focusing grids – one horizontal and another vertical does the actual focusing of the electronic beam on to the screen. Electronic or magnetic fields operate these grids. Depending on the direction (positive or negative) and the intensity of the fields applied to them, the beam is deflected horizontally (or vertically) and thus, by using a suitable combination of these focusing grids; the beam can be focused to any point on the screen
CRTs for Color Display
This was one the earlier CRTs to produce color displays. Coating phosphors of different compounds can produce different colored pictures. But the basic problem of graphics is not to produce a picture of a predetermined color, but to produce color pictures, with the color characteristics chosen at run time.
The basic principle behind colored displays is that combining the 3 basic colors –Red, Blue and Green, can produce every color. By choosing different ratios of these three colors we can produce different colors – millions of them in-fact. We also have basic phosphors, which can produce these basic colors. So, one should have a technology to combine them in different combinations
Beam Penetration CRT
This CRT is similar to the simple CRT, but it makes use of multi coloured phosphorus of number of layers. Each phosphorus layer is responsible for one colour. All other arrangements are similar to simple CRT. It can produce a maximum of 4 to 5 colours
The organization is something like this – The red, green and blue phosphorus are coated in layers – one behind the other. If a low speed beam strikes the CRT, only the red colored phosphorus is activated, a slightly accelerated beam would activate both red and green (because it can penetrate deeper) and a much more activated one would add the blue component also