630 



DISPLAY SYSTEM DESIGN PROBLEMS 



compartments vary from very low levels (under 10 foot lamberts) in 

 enclosed areas or under night operation conditions to extremely high levels 

 (over 10,000 foot lamberts) at high altitudes in open compartments. 



Brightness is defined as the intensity per unit of projected area of the 

 surface. It is measured in candles per square foot or more commonly, foot 

 lamberts. The spot brightness of a CRT depends upon several factors: 

 (1) beam current and screen voltage, (2) duty cycle of excitation, (3) dwell 

 time of the electron beam on a particular spot, and (4) type of phosphor. 

 The spot brightness of the phosphor is extremely high at the instant of 

 excitation — many thousands of foot lamberts. The electron beam rests 

 on a particular spot of the screen, however, for a very short time compared 

 with the frame time. The average brightness of the spot, therefore, is but 

 a few foot lamberts. 



Fig. 12-12, shows the transfer characteristic of the electrical-to-light 

 transducer. The lower usable light limit is dependent on the noise level of 



Point of Defocus 



Threshold Level 

 Determined by System 



_50 -40 -30 -20 -10 N°'S^ ^""^ "-'S^^ Ambient 



Bias Voltage 



Fig. 12-12 Operating Points of Intensity-Modulated CRT. 



the video signal and the ambient light level. In this region near cutoff, the 

 light output varies in most tubes as the 3 /2 power of the input signal. This 

 relationship tends to enhance the contrast between signals that are large 

 relative to the noise level; on the other hand, at low S jN ratios, the signal 

 information tends to be depressed by this characteristic. As the ambient 

 light level is increased, it is necessary to raise the minimum threshold of 

 visibility by driving the tube toward zero bias. The upper limit of grid 

 drive is determined by the application of the indicator. This determination 

 must be made on the basis of whether high resolution or high brightness is 



