364 



COLOR AND LIGHTING 



Then there was a further shght increase at 

 100 f.c, but it was not significant. From 

 100 to 300 f.c. there was a decrease in per- 

 formance. The authors interpret this to 

 mean that 100 f.c. is optimum for exacting 

 visual work. Actually, the increases in per- 

 formance beyond 50 f.c. were neither sig- 

 nificant nor of practical importance. It 

 is noteworthy that the authors found a 

 falling-off in discrimination at the higher 

 levels. Luckiesh and Moss (18, p. 205) dis- 

 covered that the nervous muscular tension 

 at subjects' finger tips as they were reading 

 was less under 10 than under 1 f.c, and less 

 under 100 than under 10 f.c. of hght. Re- 

 analysis (31) of the data indicated that 

 rather rapid changes in tension occurred 

 from 1 to about 10 f.c, smaller changes from 

 10 to around 20 to 25 f.c, and only slight 

 changes from there on to 100 f.c 



Margin of Safety 



It is obvious that visual work should not 

 be done at critical levels of illumination. 

 There should be an adequate margin of 

 safety to provide for individual variation 

 and the like. The addition of about 10 f.c 

 to the critical level will ordinarily provide an 

 adequate margin of safety. 



Adequate Foot-Candles 



Taking into account a margin of safety, 

 performance studies indicate the following 

 as adequate illumination: (a) 15 to 20 f.c for 

 reading 10- to 11-point type on good white 

 paper; (b) 20 to 25 f.c for reading newsprint; 

 (c) 25 to 35 f.c. for discriminating 6-point 

 type. The more severe visual tasks of daily 

 life apparently require 40 to 50 f.c. 



Summary on Intensity 



When considering the intensity of illumi- 

 nation in relation to visual acuity, size of 

 object, speed of vision, brightness contrast, 

 and efficiency of performance, the composite 

 picture is as follows: visual efficiency in- 

 creases rapidly up to about 5 f.c, more slowly 

 to 10, very slowly to 20, and by insignificant 



amounts thereafter, when the object to be 

 discriminated is of moderate size (3 to 6 

 minutes). But when the object is small, 

 vision improves by practical amounts up to 

 40 or 50 f.c. The greater the brightness con- 

 trast, the better the visual efiiciency. Al- 

 though not practically significant, visual acu- 

 ity and speed of vision continue to improve 

 slightly up through and beyond 100 f.c. 

 Certain authors (18, 34) claim these slight 

 gains at high intensities are important. An 

 evaluation of these claims is given by Tinker 

 (31). 



Distribution of Illumination 



In practice, the most fundamental aspect 

 of healthful illumination, and the one most 

 frequently inadequate, is the distribution of 

 light and brightness in the field of vision. 

 For convenience the discussion of distribu- 

 tion may be considered under glare and 

 brightness contrast. When light enters the 

 eyes from any visible light source so that 

 vision is impaired or discomfort and annoy- 

 ance is felt, or both things occur, glare is 

 present. Brightness contrast refers to the 

 variation of brightness of areas within the 

 visual field. 



Glare 



Everyone is familiar with the disagreeable 

 effects of a bright side light or reflection from 

 a highly polished piece of metal shining into 

 the eyes while reading, doing other visual 

 work, or even when no exacting visual dis- 

 crimination is involved. Cobb and Moss 

 (6) studied the effect of a glare source on 

 visibility. The glare source was a 100 watt 

 bulb located at 40, 20, 10 and 5 degrees from 

 the fine of vision and in the vertical meridian. 

 The bulb, located 55 inches from the eyes, 

 produced five f.c. of glare light at the eyes. 

 Visibility was reduced appreciably with the 

 glare source at 40 degrees from the line of 

 vision. This loss became progressively 

 greater as the glare source was brought closer 

 to the line of vision. At the 5-degree posi- 

 tion, 84 percent of the illumination on the 



