VISIBILITY 



41 



where E = the illuminance at the eye 

 produced by the just visible flashing light, 



Eo = illuminance of the just visible 

 steady light, 



t = duration of the flash (in seconds), 

 and 



a = a constant equal to 0.21 seconds. 

 This function has also been plotted in Fig. 32 

 to show its nature. These data are from a 

 study by Blondel and Rey (4) and are 

 average data for 17 observers. Note that a 



100 







001 



001 



01 



LENGTH OF FLASH IN SECONDS 



Fig. 32. Intensity of just visible flashes of light 

 as a function of the duration of the flash. (After 

 Blondel and Rey, 4) 



flash of light which lasts about a half-second 

 or less must be much more intense than a 

 steady light in order to be seen. A flash of 

 light which lasts a half-second or more is 

 almost as visible as a steady light of the 

 same intensity. 



For the second case, i.e., where the 

 location of the light is not known and the 

 observer must hunt for it, the situation is 

 much worse. Although the lES Lighting 

 Handbook (99) gives an equation for this 

 function, it is questionable whether any 

 function can be trusted to correctly represent 

 the facts. Some of the factors which must 

 be considered are the length of time spent in 



hunting, the area searched, the area on the 

 retina which is sensitive enough to see a 

 light of that intensity, the kinds of eye 

 movements the observer makes, how sys- 

 tematic a searcher he is, and so on. Only 

 one thing can be stated with certainty. 

 The values in Fig. 32 are minimum values. 

 They show how intensity is related to time 

 under the best seeing conditions. If the 

 observer is required to search for a light, a 

 large safety factor must be utilized. 



o 



< 



3 



o 

 :e 



h- 



< 

 O 

 O 



to 



UJ 



UJ 



u 



X 



-4 



-10 



I *-x{ 



TOTAL -6 -4 

 DARKNESS 



-2 



of 



BACKGROUND LUMINANCE IN 



LOG Ti\L. 



Fig. 33. Visibility of point sources as a function 

 background luminance. (After Knoll el al., 48) 



Background Luminance. The visibility 

 of a point source is a function of the back- 

 ground luminance. The data in Fig. 33 

 on this function are taken from the study by 

 Knoll et al. (48). At starhght levels, a 

 point source can be seen if it is very dim. 

 As the background luminance increases, the 

 threshold intensity increases rapidly. 



Luminance Contrast. Our best informa- 

 tion about the visibility of non-luminous 

 areas comes from an extensive study carried 

 out by Black well (2) during the war. 

 Spots of light were projected onto a screen 

 60 feet away from a group of observers who 

 reported whether they had seen the spot. 

 A large number of such presentations were 



