40 



HOW WE SEE 



A study of this magnitude is an enormous 

 undertaking, and the importance of these 

 results must not be minimized. But it must 

 be recognized that this is far from a complete 

 investigation. The parameters of luminous 

 intensity, time of exposure, background 

 luminance, and number of colored lights 

 need to be explored more thoroughly before 

 we shall have the complete story. 



Size and Visibility. As in the case of 

 visual acuity, size is an important factor in 

 visibihty. There are two ways in which the 

 relationship may be illustrated. Fig. 30 

 shows the illuminance on the eye required 



little thought about the kinds of measure- 

 ments these represent. 



Bouma's measurements on the illuminance 

 on the eye represent a certain nimiber of 

 lumens deposited on the area of the pupillary 

 opening. The data of Blackwell, on the 

 other hand, are in terms of luminance, i.e., 

 the number of lumens emitted per unit area 

 of the target. If the lumens emitted per 

 unit area, i.e., the luminance, is held con- 

 stant, the target with the larger area will 

 obviously emit the greater total number of 

 lumens, and so will deposit the greater 

 number of lumens per unit area on the eye. 

 In short, when the appropriate conversions 



SIZE OF SOURCE 



Fig. 30. Threshold visibility for light sources 

 of various sizes in terms of illuminance produced 

 at the eye. (From data compiled by Bouma, 5) 



for threshold visibihty of light sources of 

 various sizes. These data are average 

 data for six experiments on this function. 

 This figure makes it clear that point sources 

 of light are more efficient than large areas. 

 The other way of showing data of this 

 sort is in terms of the luminance of the 

 object. Fig. 31 shows two sets of data 

 caken from experiments by Lash and 

 Prideaux (51) and Blackwell (2). The 

 two sets of data do not agree very well but 

 they show roughly the same function: large 

 areas are much more visible than small 

 areas of equal luminance. Offhand, this 

 might seem to contradict the findings of 

 Bouma cited above. The apparent dis- 

 crepancy can be resolved, however, with a 



^ Oi 3 06 I 3 6 10 30 60 100 



^ DIAMETER OF SURFACE IN MINUTES CF ARC 



Fig. 31. The visibility of light sources at night 

 in terms of the luminance of the source. (After 

 Lash and Prideaux, 51, and Blackwell, 2) 



are made, the data of Fig. 30 agree well 

 with those in Fig. 31. 



Time and Visibility. In considering how 

 time relationships affect visibility, we have 

 to consider two cases: (a) when the observer 

 knows where the light source is and looks 

 in that direction, and (b) when he has to 

 hunt for it. In both cases a steady light 

 source can be seen at much lower intensities 

 than a flashing hght source. 



For the first case, i.e., when the location 

 of the source is known, the intensity of a 

 just visible flash of light bears this relation 

 to that of a steady hght: 



.=..f4-«). 



