34 



HOW WE SEE 



of the central visual task was, nor is it 

 possible to calculate this value from the 

 information he gives. It appears probable, 

 however, that the central task area was 

 about one or two degrees in size. 



These considerations do not seem to have 

 deterred illuminating engineers from writing 

 specifications for surround luminances. 

 Thus, Luckiesh (67) says: 



1. Brightness-ratios smaller than 1 to 5 are 

 desirable. 



2. Brightness-ratios greater than 1 to 10 

 should be avoided if reasonably possible. 



3. Brightness-ratios of 1 to 100 should not 

 be tolerated. 



The evidence he adduces for this specifica- 

 tion comes from a study in which he investi- 

 gated the minimal perceptible luminance 

 difference as a function of three surround 

 luminances: (a) five times that of the task, 

 (b) equal to the task, and (c) I that of the 

 task. Needless to say, it is ahnost impos- 

 sible to determine maxima or minima in any 

 set of experimental results that contain 

 only three observed points. In another 

 experiment, he studied rate of eye blinking 

 as a function of five surround luminances. 

 Since it is questionable whether the rate of 

 eye blinking has much to do with seeing 

 (1, 88), these data may be discounted. 



Another engineering authority. Crouch 

 (23), states categorically, "Thus he [Lyth- 

 goe] proves that the best acuity is to be 

 expected only when the surrounding bright- 

 ness is equal to that of the task." Lythgoe, 

 however, concluded that best visual acuity 

 occurs when the surround is yw to yo that 

 of the visual task. The data in both Figs. 

 23 and 24 probably support Lythgoe better 

 than Crouch. Fig. 23 shows, as a matter of 

 fact, that throughout most of the range of 

 task luminances (a) the dark surrounds and 

 equality surrounds have about the same 

 effect on acuity and (b) acuity is best for 

 the dimly illuminated surround, i.e., 0.011 

 equivalent foot-candles. It is worth point- 

 ing out, incidentally, that Fig. 23 shows the 



complete data from Lythgoe's report. In 

 his monograph, Lythgoe drew up (in his 

 Fig. 10) the data for only one of the subjects 

 — the one which gave the neater functions — 

 and reversed two of the points at that. 

 Further, Lythgoe did not plot complete 

 functions but only showed the data for the 

 upper range of luminances (from 1.21 

 equivalent foot-candles up). In spite of 

 these criticisms, authoritative publications 

 by illuminating engineers (23, 99) have 

 cited the incomplete and incorrect Fig. 10 

 from Lythgoe's original paper. 



Other studies, occasionally referred to in 

 secondary sources, also turn out to be of 

 little value to the critical scientist. Two 

 papers by Cobb and Moss (13, 14), for 

 example, report data on only two surround 

 conditions: dark surrounds and surrounds 

 equal in luminance to the central task. 

 Although visual performance was better 

 with the equality surround, this obviously 

 does not help us decide whether some other 

 surround condition, e.g., a surround equal to 

 1^ that of the task, as suggested by Lythgoe, 

 might not turn out better. A study by 

 Cobb and Geissler (12) studied only dark 

 surrounds and surrounds equal to 42 

 candles per square meter. Another study 

 by Cobb (10) reports data for dark sur- 

 rounds and surrounds equal to 2.9 candles 

 per square meter. Both are obviously very 

 incomplete and the conclusions made by 

 the authors are cautiously worded. Thus, 

 Cobb (10) states, "Surroundings of a 

 brightness about equal to or less than that 

 of the test object show no consistently 

 better or worse results than dark surround- 

 ings with the identical test object." John- 

 son has reported an extensive investigation 

 (41), not on visual acuity but on reaction 

 time, with task luminance held constant and 

 with six ratios of task/surround luminances. 

 If one selects data for identical subjects 

 under all conditions, his results show that 

 visual performance is markedly poorer 

 when the surround luminance is greater than 

 the central area. When the surround 



