12-6] CHARACTERISTICS OF ELECTRICAL-TO-LIGHT TRANSDUCERS 629 



tube since the curvature of the phosphor front face does not always coincide 

 with the surface of best focus. 



The major effects limiting spot size are: 



1 . Random direction of the electron emission 



2. Space charge limitations 



3. Halation 



4. Light scattering 



Focus is obviously dependent on the electron optics of the CRT. Because 

 of the random manner in which electrons are emitted from the cathode, the 

 electron beam has a finite width to begin with. This width cannot be 

 improved on by the electron-optics system — in fact, it is increased. 



The space charge effect on spot size is a result of the mutual repulsion 

 between electrons in the beam, preventing the electrons from coming 

 together at one point. As the beam electrons converge to a smaller diam- 

 eter, the mutual repulsion increases, causing the converging beam to 

 straighten out and finally to diverge. 



Halation is produced by reflection of the light rays back and forth 

 between the front and back surfaces of the front glass panel of the cathode 

 ray tube. When the electron beam strikes the phosphor screen, light rays 

 emanate in random directions from the excited spot. These are transmitted 

 at all angles into the glass panel, where multiple reflections occur, some 

 light escaping to the observer in areas adjacent to the excited one. The 

 appearance of this halation, as it is called, is a bright spot surrounded by a 

 region of lower brightness. 



Spot size is also limited by the scattering of light created at one point 

 within the phosphor layer by phosphor particles nearby. 



The presentation resolution is also limited by other parameters such as 

 sweep speed and cathode ray tube size. The restricted space in aircraft 

 limits the size of cathode ray tubes to a 5- to 7-inch screen diameter. A 

 radar display on a 5-inch tube is usually limited to 3.5-4 inches on a side. 

 The usable display size by itself does not limit resolution, but the spot size 

 and pulsed information that must be shown in a given size of display do. 

 The sweep speed, which is determined by the number of discrete pulses to 

 be displayed in a given time and tube dimension, is a more important 

 parameter in limiting picture resolution, as was illustrated in the two 

 examples. 



Indicator Brightness. Target and background brightnesses on the 

 indicator are important parameters in determining target detectability. 

 Variations in the ambient light level affect target detectability since they 

 alter the contrast and therefore the ability to distinguish target from the 

 background. The ambient light levels encountered in airborne radar 



