12-6] CHARACTERISTICS OF ELECTRICAL-TO-LIGHT TRANSDUCERS 631 



the dominant requirement. It is hard to satisfy both requirements because 

 driving the signal toward zero grid not only increases brightness but also 

 increases the spot size. This is so because of the phosphor characteristics 

 and the mutual repelling characteristic of the electrons in the beam. The 

 electrical-to-light curve also changes as the bias is reduced. More of the 

 beam is intercepted by the apertures of the electron-gun structure and so 

 the electrical-to-light relationship is determined by the particular electron 

 optics of the tube. 



The CRT as an instrument has a large light-contrast capability. From 

 complete cutoff to zero bias, the CRT is capable of better then 10 shades 

 of gray (it is assumed that each discernible increase in gray intensity 

 represents a 3-db increase in light output). In a radar system, however, 

 with the defined lower limit specified by the noise level of the video signal 

 and the upper limit by defocus, a reduced contrast ratio can only be 

 obtained in a low light-level surrounding. This contrast represents a 

 30-130-to-l range of signal brightness. 



Insofar as detectability is concerned, the quantity of importance is 

 contrast: 



D 



Contrast = ^ --^ (12-4) 



dn -r Da 



where By; = mean brightness of the background noise 



Ba = mean ambient brightness level 



B = brightness of the signal spot above the background noise level. 



Environmental conditions and target brightness levels yielding the same 

 contrast will have equal detectability. Thus when the ambient light level 

 Ba is low compared with the noise light level Bf^, little advantage is gained 

 by altering the mean presentation brightness (except as the signal-to-noise 

 ratio may be affected by CRT nonlinearities). On the other hand, when Ba 

 is large compared with Bf^, increasing the target brightness will provide 

 increased contrast and hence detectability. Thus, high light output levels 

 are required of the CRT under high ambient light conditions. A limit is 

 quickly reached, however, incident to the upper limit of spot defocusing. 

 The dynamic brightness range is decreased as the CRT brightness is 

 increased. 



Signal Display Integration. One of the more important functions 

 of the display device is to serve as an instrument of pulse correlation or 

 integration to improve signal detectability. When pulses are painted on the 

 display screen alongside one another, the process of assimilation of this 

 information is frequently called correlation. When they are superimposed 

 one on top of another, the process is called integration. The intensity- 



