362 



COLOR AND LIGHTING 



The most extensive study of speed of 

 vision was done by Ferree and Rand (9). 

 They investigated the influence of illumina- 

 tion on speed of vision with test objects of 1, 

 2, 3, 4.2, and 5.2 minutes visual angle and 

 "with backgrounds of 78, 29, 21 and 16 per- 

 cent reflection. The test object was black, 

 reflecting only 3 to 4 percent. Illumination 

 intensities ranged from 1.25 to 100 f.c. The 

 standard Landolt ring was used as test ob- 

 ject. In all cases the time taken to perceive 

 the test object decreased rapidly up to 30 

 or 40 f.c. (up to the knee of the curve of 

 improvement) for the small test objects (1 

 and 2 minutes), and up to 15 to 25 f.c. for the 

 larger test objects (3, 4.2 and 5.2 minutes). 

 In all cases there was a slower decrease for 

 about 5 to 10 f.c. beyond the knee of the 

 curve and then a rather rapid flattening out 

 of the curve. With the lower degrees of 

 background brightness, the improvement in 

 speed extended to higher levels of illumina- 

 tion. These data signify that time of seeing 

 decreases by significant amounts up to about 

 20 or 30 f.c. for the larger test objects and up 

 to 40 or 50 f.c. for the smaller test objects. 

 The reciprocal of the time for perception was 

 used for plotting the curves of change of 

 speed with increased illumination intensity. 

 This magnifies slight differences so they look 

 appreciable in the figures. For instance, the 

 decrease in time of perceiving test objects 2 

 to 5.2 minutes in size as the illumination is 

 increased from 20 to 100 f.c. is only 1.5 ms. 

 (thousandths of a second); for the 1-minute 

 test object, the decrease in time as the light 

 is changed from 50 to 100 f.c. is about 7 ms. 

 Changes of this magnitude are of no practical 

 significance. Arnold and Tinker (1) have 

 demonstrated that the shortest average 

 pause of the eyes to discriminate an object is 

 157 ms. In reading, the average pause 

 ranges from about 200 to 300 ms. They 

 also cite data which indicate that when vis- 

 ual impressions succeed one another, as in 

 everyday life situations, an exposure of 100 

 ms. is necessary for a fairly well cleared up 

 visual impression. These data indicate that 



any decrease of a few ms. is of no practical 

 importance, since the impression (stimulus) 

 must act for at least 100 and more hkely for 

 200 or more ms. for an adequate visual im- 

 pression. 



The data on speed of vision may be sum- 

 marized as follows: (a) for objects that are 3 

 minutes in size or larger and with good con- 

 trast between object and background, speed 

 of vision is near maximum at about 15-20 

 f.c; (b) for small objects on a background of 

 low reflectance, significant decreases in time 

 for seeing occur with illumination intensities 

 up to about 50 foot-candles; (c) the findings 

 agree with those on visual acuity. 



Brightness Contrast 



It would be impossible adequately to 

 specify lighting for a specific situation in 

 terms of the relation of illumination to visual 

 acuity, size of object, or speed of vision. In 

 each of these the brightness contrast between 

 the object to be discriminated and the back- 

 ground is very important. In any seeing 

 situation, the inter-relation of three factors 

 determine ease of seeing: size of details, 

 brightness contrast between details and 

 background, and level of illumination on the 

 task. In general, the greater the brightness 

 contrast, the better the seeing at any level of 

 illumination. Luckiesh (14) presents data 

 which demonstrate that vision improves as 

 the brightness contrast changes from 2.5 to 

 12.5, 25, 50 and 96 percent, whether 1, 10 or 

 100 foot-candles of light are used. 



The most complete investigation of illu- 

 mination and the effects of brightness con- 

 trast was done by Weston (34). The task 

 required the discrimination and cancellation 

 of Landolt rings with a given gap orientation 

 on specially prepared test sheets. There 

 were 15 subjects in Part I and 12 in Part II. 

 In Part I, the percent contrasts were 36.5, 

 68.3, and 91.6; the size of test object 1, 3 and 

 6 minutes (visual angle); and illumination 

 levels 0.8, 4, 20, 100, and 500 f.c. Both ac- 

 curacy and speed are included in the per- 

 formance score. For the 6-minute object, 



