642 



DISPLAY SYSTEM DESIGN PROBLEMS 



The plotted data on this phenomenon are usually for threshold values, 

 i.e., a given probability of detection. However, the required contrast will 

 change as a function of (a) probability of detection, (^) the background 

 brightness, (c) size of the spot of light, and (d) the time of exposure. The 

 data are shown in Fig. 12-21. 



Using the data of Fig. 12-21, Fig. 12-22 has been derived. In this figure, 

 the relationship between the required spot brightness of an assumed 50 per 

 cent threshold value is shown as a function of size, background brightness, 

 and the time of exposure to the spot. At the higher background bright- 

 nesses, the effect of the spot size becomes negligible for longer exposures. 

 As the time decreases, the importance of size to achieve threshold becomes 

 more critical. 



Exposure to high levels of brightness immediately prior to spot detection 

 significantly affects the time required for spot detection. The pertinent 

 data are shown in Figs. 12-23a, b, and c. These data are based upon 



Background Brightness 



1.37 Ml 



100 120 



^ 20 40 60 80 100 120 20 40 60 80 100 120 20 40 60 



LIGHT ADAPTATION TIME (sec) 



(a) (c) 



(b) 



Fig. 12-23 Time for Detection as a Function of Target Brightness and Duration 

 of Light Adaptation. 



exposing the subjects to a brightness level of 3100 millilamberts for 5, 15, 

 30, 60, and 120 seconds prior to attempting spot detection. The three sets 

 of curves differ only in the amount of background brightness around the 

 spot. Each figure shows the detection time as a function of spot brightness 

 for a given background brightness and duration of exposure to the high 

 brightness level. These figures suggest that if the exposure lasts as long as 

 60 seconds, an additional 60 seconds has little effect. 



Fig. 12-24 is a plot of the data for an exposure time of 120 seconds and 

 shows the relationships that exist among the variables as a function of 

 contrast. These figures suggest that if the operator is exposed to high 



