626 



DISPLAY SYSTEM DESIGN PROBLEMS 



The usefulness of the cathode ray tube is dependent on the fluorescence, 

 phosphorescence, spot resolution, and secondary emission characteristics 

 of the phosphor material. Some terms commonly used in describing 

 the properties of screen phosphors are luminescence, fluorescence, and 

 phosphorescence. Luminescence is the emission of visible to near visible 

 radiation in phosphors incident to excitation. Some methods of excitation 

 are: {a) by bombardment of electrons (cathode luminescence), ib) by 

 an electrical field (electroluminescence), and (c) by radiation (photo- 

 luminescence). Fluorescence is luminescence occurring during excitation. 

 In cathode luminescence — the method of excitation of all cathode ray 

 tubes — fluorescence is the light emitted during the period of electron 

 bombardment. Phosphorescence is luminescence occurring after bombard- 

 ment has ceased. 



Many nonmetallic inorganic crystals exhibit the properties of lumines- 

 cence when bombarded by an electron beam. However, most do not have 

 the properties of brightness, long phosphorescence, and suitable color 

 necessary in cathode ray tubes. The chemical makeup of most practical 

 phosphors includes a base material and an activator. The base materials 

 are usually the oxides and sulfides of zinc, cadmium, magnesium, and 

 silicon. The activator is mixed in the ratio of 10 to 100 ppm with the base 

 material to give a greatly increased light output. This activator supplies 

 the additional energy levels necessary to make the jump between energy 

 levels (caused by the excited electrons of the beam) small enough to give off 

 light efficiently. 



Phosphors are available in almost every color. A few of the phosphors 

 used in cathode ray tube indicators are listed in Table 12-1. 



Table 12-1 PHOSPHOR CHARACTERISTICS 



'To 1 per cent of peak 



This table indicates the relative time duration of phosphorescence for 

 these phosphors and the decay laws they follow. For many radar displays, 

 long persistence is required because of the long frame time of one antenna 

 scan. Yellow-green sulfides have the required long persistence if they 



