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 electron 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 a focus 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.
These electronic/magnetic fields can be easily manipulated by using suitable electric fields with this background. In this tutorial, we are discussing 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, there is an electronic gun. 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 fall on the computer screen.
Fig. Basic Design of magnetic Deflection CRT
A pair of focusing grids – one horizontal and another vertical do the actual focusing of the electron 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.
So, we now have a mechanism wherein any point on the screen can be illuminated (or made dark by simply switching off the beam).
Hence, from a graphics point of view, any picture can be traced on the screen by the electron beam by suitably and continuously manipulating the focusing grids and we get to see the picture on the screen “A basic graphic picture” of course, since the picture produced vanishes once the beam is removed, to give the effect to continuity, we have to keep the beam retracing the picture continuously – (Refreshing).
Quality of Phosphors
The quality of graphics display depends on the quality of phosphors used. The phosphors are usually chosen for their color characteristics and persistence. Persistence is how long the picture will be visible on the screen after it is first displayed. Most of the standards prescribe that the intensity of the picture should fall to 1/10 of its original intensity is less than 100 milliseconds.
The color of the phosphor is normally chosen as white, also it should be of small grains so that the resolution of the screen can be high.