38 



HOW WE SEE 



the results obtained by Luckiesh and Taylor 

 are probably the more meaningful for most 

 applied seeing problems. When lights are 

 adjusted to appear of equal luminance to the 

 eye, visual acuity at very low luminances is 

 better under red light than under blue light. 

 Further work is needed to discover whether 

 this interpretation by the present writer is 

 correct. In any case, this is another good 

 example of the kind of inconsistency which 

 crops up in visual data because of the lack 

 of a consistent system of low-luminance 



Fig. 

 (Data 



200 



ANGULAR VELOCITY OF TEST OBJECT 

 ( DEGREES PER SECOND ) 



28. Visual acuity for objects in motion, 

 from Ludvigh, 60, 61) 



photometry. Different writers do not al- 

 ways mean the same thing when they report 

 their measurements for visual functions in 

 the region of rod sensitivity. 



Visual Acuity for Moving Objects. Most 

 visual acuity studies have been done with a 

 stationary observer and stationary test 

 object. Yet many practical seeing situa- 

 tions involve motion, either of the observer 

 or of the object being observed. It is 

 important in this connection to make a 

 clear distinction between the perception of 

 motion and visual acuity for moving objects. 



Perception of motion means simply that an 

 observer has seen something move; visual 

 acuity for moving objects implies that the 

 observer can recognize or identify critical 

 details of the object in motion. 



A few studies which have been done in 

 this area show conclusively that visual 

 acuity is greatly impaired if an observer is 

 looking at a moving object. Two sets of 

 data obtained by Ludvigh (60, 61) are 

 shown in Fig. 28. They are for monocular 

 photopic vision and for a constant angular 

 velocity of the test object in a horizontal 

 plane around the observer's head. The 

 earlier data (interrupted line) were obtained 

 with Snellen letters; the later data (solid 

 lines) with Landolt rings. Low (54) has 

 investigated visual acuity for moving objects 

 in the periphery of the eye. He used 

 Landolt rings, with apertures varying from 

 3'27" to 2°17'27", moving at a constant 

 velocity of approximately 15° per second. 

 His results show that visual acuity for 

 moving objects seen peripherally is only 

 about 60 percent as good as visual acuity 

 for stationary objects seen in about the 

 same location. His results, however, are 

 essentially complete only for one position : a 

 point 30° from the fovea. At 45° from the 

 fovea, 32 out of 200 observations were 

 incomplete because subjects could not 

 correctly identify even the largest test 

 object; at 60°, 144 out of 200 observations 

 were incomplete for the same reason. 



This brief summary indicates clearly that 

 here is an area which has scarcely been 

 touched by visual research. Although it is 

 quite likely that they will not yield results 

 of great theoretical importance, additional 

 data on these functions might be of some 

 practical value in the solution of problems 

 dealing with the visibility of rapidly moving 

 targets. 



Visibility 



For purposes of discussion here and in the 

 chapters to follow, visibility means recog- 

 nizing the presence of a light or object 



