video monitor is the basic output device in any graphics system and its operation depends upon the cathode ray tube (CRT) design. An electron gun emits the cathode ray beam (i.e., a beam of electrons) which allowed to pass through the focusing and deflection systems that forwards the beam towards the phosphorus coated screen. A spot of light emitted at the point of contact which gets fainted gradually, hence some method which retains quality of the picture needed. one method to keep the phosphorus glowing is to redraw the picture again ana again by focusing the electron beam back on the same point. This display tube called a refresh CRT.
The electron gun consists of a heated metal cathode ana a control grid. The source of heat to the metallic cathode is the current applied to the filament.
The purpose of focusing system is to converge the beam in to a small spot, otherwise the electrons repels each other thereby diverging the beam when ever, the beam strikes the screen.
In this method, the electron beam allowed to pass through a positively charged metallic cylinder (act as an electrostatic lens).
Electron Beam Deflection:
Deflection of the electron beam can controlled by the horizontal and vertical deflection plates . for electrostatic deflection, two pairs of parallel plates used, one mounted horizontally to control the vertical deflection and the other mounted vertically to control the horizontal deflection .