622 



DISPLAY SYSTEM DESIGN PROBLEMS 



and limitations of the cathode ray tube contributes to a proper under- 

 standing of other visual display devices. 



As previously mentioned, there will be no discussion of display system 

 circuitry or the construction and operating details of the display devices 

 themselves. A bibliography at the end of this chapter provides the 

 interested reader with appropriate references for such material. 



The cathode ray tube (CRT) is basically a two-dimensional indicating 

 device which paints a picture on a phosphorescent screen. All CRT's 

 contain three major sections: 



1. A combination of electrodes, called an electron gun, for producing and 

 focusing an electron beam 



2. A means for deflecting the electron beam to a selected area of the 

 fluorescent screen 



3. A fluorescent screen which is luminous when excited by an electron 

 beam. 



These items wil 



be separately discussed. 



The Electron Gun. The electron gun includes a thermionic source of 

 electrons (the cathode). This cathode is usually a small capped cylinder of 

 sheet nickel which is oxide coated to provide an electron-emitting material. 

 The cathode is indirectly heated by a filament. The cathode element is 

 surrounded by a cylinder which acts as a tube shield and increases the 

 cathode thermal efficiency. A grid for controlling the electron volume 

 density of the beam is placed in front of the cathode. This electrode takes 

 the form of a cylindrical can completely surrounding the cathode, having 

 a circular aperture in front of the emitting cap of the cathode. 



Anode Aquadag Coating 



Control Grid Second Grid 



Focus Coil Deflecting 



Coil Fluorescent 



Screen 



Fig. 12-8 Magnetically Focused CRT. 



