when displaying large quantities of alphanumeric data,the readout system makes use of cathode-ray tube most commonly known as CRT.conventionally,actode-ray tubes form tha basis of cathode-ray oscilloscopes and television systems.For generation of characters on the CRTs needs relative simple electronic circuitry.
As we know that some crystalline materials such as phosphor,have property of emitting light when exposed to radiation.This is called the fluorescence characteristics of the materials.These fluorescent materials continue to emit light even after radiation exposure is cut off.The length of time during which phosphorescence occurs is called the persistence of the phosphor.
The screen of CRT is coated with phosphor.when electron beam strikes the CRT screen,a spot of light is produced on the screen.The phosphor absorbs the kinetic energy of the bombarding electrons and emits energy at a lower frequency in a visual spectrum.The intensity and brightness can be realized with different gray scales,and the display can have different colors depending on the phosphor used in the screen.Generally,the phosphor is chosen to be white or green.
A typical CRT display has easy facilities for the control of digit size by control of deflection sensitivity of system (either electromagnetic or electrostatic deflection).The number of characters displayed can be charged with the help of time shared deflection and modulator circuits.
Storage type CRTs facilitate storing a stationary pattern on the screen without flickering display and it possible to retain the pattern for long time,independent of the phosphor persistence.
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