12-9] SPECIAL DISPLAY DEVICES 661 



The advantages and disadvantages of available transparent thin phos- 

 phor films are listed here. 



DISADVANTAGES 



1. No persistent phosphors are presently available. 



2. Efficiency is less than conventional powder because of the limited 

 light-generating ability of the phosphor and the internal light-trapping 

 properties of the film. 



3. The film cannot be deposited on the front face of the cathode ray tube. 



ADVANTAGES 



1. Resolution is improved. 



2. Contrast is improved by reduction of halation. 



3. Visibility increases at high ambient light levels. 



4. There is more resistance to deterioration under electron bombardment 

 because of better heat transfer of the substrate. 



Color Tubes. Not many radar indicators have utilized color tubes in 

 the past. The information obtained by gradation of hues is not definitive 

 enough to warrant use of color in fixed fire-control, surveillance, or bomber 

 defense radar systems. There are indicators, however, where contrasting 

 colors such as red and green could display zones of danger (profilometer 

 indicator) or distinctive signals (difi^erentiation of fixed and moving targets) 

 and would therefore be of great use. 



The color tubes and color phosphors presently available are not ideal for 

 radar applications. A tube which can withstand the rigors of a fighter 

 aircraft environment is still not available. Persistence limitations of 

 single-layer phosphors which also have color purity still provide a deterrent 

 to their use in radar color display tubes. It is anticipated that the develop- 

 ment work being conducted in electroluminescence, in thin transparent 

 phosphors, in the Aiken type of deflection tube, and in color storage tubes 

 will produce several useful color indicators. 



The Shadow Mask Tube. The tubes available today are the outgrowth of 

 industrial developments aimed at the home television market. The most 

 widely used tube, the shadow mask tube, cannot be used in airborne 

 applications since it is sensitive to the position of the indicator with respect 

 to the earth's magnetic field, but it is worthwhile to consider its construction 

 and operation. The tube utilizes three beams, a shadow mask, and a 

 phosphor dot array as shown in Fig. 12-39. In this dot-array color tube, 

 it is necessary for the three beams to converge exactly at all points in the 

 shadow mask plane, as shown in Fig. 12-40, so that the beams strike the 



