360 



COLOR AND LIGHTING 



Visual Acuity 



In considering the relation between illu- 

 mination and visual acuity, one is dealing 

 with the discrimination of fine details, or 

 threshold vision. The customary method is 

 to determine the least visual angle at which 

 the direction of the break in the Landolt ring 

 (international test object) can be perceived 

 for a given light intensity. Usually five out 

 of eight correct responses are required. In- 

 tensity is usually measured in foot-candles. 

 One foot-candle (f.c.) is the light intensity of 

 a standard candle at the distance of one foot. 

 The data presented by Feree and Rand (8) 

 show that when the intensity is increased 

 from .001 f.c, the rise in visual acuity is rapid 

 up to an illumination of about 5 f.c. As the 

 intensity is increased further the rise in 

 acuity becomes progressively slower and 

 practically reaches a maximum at about 20 

 f.c. Improvement in the later stages is 

 scarcely noticeable. The varying effects of 

 intensity increase at different levels of illumi- 

 nation is revealed by a 4.89-fold increase in 

 acuity from .001 to 0.1 f.c, and 67.7 percent 

 increase from 0.1 to 1.0 f.c, a 43.6 percent 

 increase from 1.0 to 5.0 f.c, and a mere 8.2 

 percent increase from 5.0 to 20.0 f.c In a 

 later study, Feree, Rand and Lewis (11) em- 

 ployed illuminations ranging from 0.5 to 100 

 f.c For their subjects with normal vision, 

 the increase of visual acuity was rapid up to 

 10 f.c, small from 10 to 25 and very shght 

 from 25 to 100 f.c. Normal eyes receive 

 slight benefit from increases beyond 25 f.c. 



One of the most painstaking studies of the 

 relation between visual acuity and illumina- 

 tion is reported by Lythgoe (23). The illu- 

 mination used ranged from 0.0029 to 1275 

 equivalent foot-candles (e-f.c, i.e., foot 1am- 

 berts). Repeated measurements were made 

 on four subjects. The reflection factor of 

 the test object (Landolt ring) was 1.35 per- 

 cent and of the background, 71 percent. As 

 in the earUer studies, the increase in visual 

 acuity was rapid up to about 5 e-f.c, slow to 

 38, and slight to 1275. Acuity was raised 

 only about one- tenth of a point (2.10 to 2.24) 



when the illumination was raised from 38.8 

 to 208 e-f.c, and another tenth (2.24 to 2.35) 

 when raised to 1275 e-f.c. Employing the 

 Ives-Cobb acuity object, Luckiesh and Moss 

 (17), using 10 subjects, measured visual acu- 

 ity under 5, 25, and 125 f.c. of mercury-arc 

 light and of tungsten-filament hght. Al- 

 though there was a marked increase in acuity 

 as the light w-as changed from 5 to 25 f.c, the 

 slight improvement from 25 to 125 f.c. was 

 not significant. 



The representative data on visual acuity 

 in relation to illumination cited here reveal 

 the following trends: (1) when the illumina- 

 tion intensity is increased from a fraction of 

 a f.c. to about 5 f.c. there is a rapid increase 

 in visual acuity. (2) From 5 to 20 f.c there 

 is a gradual increase in acuity; (3) after a 

 level of 25 f.c. is reached, the rate of increase 

 in visual acuity is small. The curve of im- 

 provement is practically flat from about 40' 

 f.c on. Although shght increases in visual 

 acuity can be obtained by higher illumina- 

 tion, they are of no practical importance. 

 Thus, 170 e-f.c. must be added to 38 to get 

 an increase in visual acuity of one-tenth of a 

 point. An additional 1000 e-f.c are needed 

 to increase acuity another tenth of a point. 

 In general, with normal eyes, little is 

 gained in acuity by increasing the illumina- 

 tion beyond 25 f.c, and there is no practical 

 gain at all when the intensity is higher than 

 approximately 50 f.c. 



Many writers, including Lythgoe (23), 

 point out that the relationship between 

 visual acuity and illumination intensity is 

 logarithmic. Expressing this relationship on 

 a logarithmic scale, however, magnifies mi- 

 nute acuity differences at the higher levels of 

 illumination so that they appear large and 

 significant. This is especially true when one 

 is thinking of practical applications of illu- 

 mination in daily life. There are limitations 

 to prescribing light on the basis of the rela- 

 tion of illumination to visual acuity. These 

 limitations will be considered later (under 

 Brightness Contrast). 



