Document Ly2yrB94m6vny1QLvZKykKVb

748 S. X. GUTH AND E. A. L1NSDAY per cent, respectively. The speed of reading was much less for the print on gri paper. However, in order to emphasize the difference in the shapes of the p curves, they have been arbitrarily crossed at 4 footcandles. The speed of readf the high-contrast reading matter does not increase appreciably beyond aboutij footcandles, but there is no indication of an optimum speed of reading for the I'd contrast reading matter in the same range of footcandles. When the illuming! was increased from 4 to 20 footcandles, the rate of reading the low-contrast m||i IgA LIGHTING FOE SEEING 749 S>'`(the maximum available) with no leveling off. In other words, when ii^fesmall, low-contrast tasks are involved, several hundred footcandles are mfjfjlor optimum visual performance. acuity and speed of reading can be considered as indicators of precision ,J^^|*and production, respectively. Ideally, if all workworld tasks could be Kfflj|ll|in the laboratory, it would be possible to determine the footcandle-level jwa&permit the worker to achieve optimum performance. That is, curves such }jd',`,WIhin Figures 2 and 4 could be obtained for each task. Since this-is impracI^Vefelit is necessary to devise a method by which typical tasks can be. visually M)rl|pa and related to fundamental laboratory data. IP^vr'iie.unaided human being cannot make accurate measurements. All he can f qualitatively that things are hot or;cold, soft or hard, and so forth. '*'iWents anc* me-sHrlnS techniqvica are necessary. An instrument that has been be extremely useful is the Luckiesh-Moss -Visibility Meter,17-16 illustrated re 5. Variable density filters are used to reduce a visual task to the thresh- Figure 4. Speed of reading depends upon the contrast between the infcjls and the paper. Curve b is for black print on white paper and curve o is forii; black print on gray paper. The two curves have been arbitrarily represented as 100 at 4 footcandles to show the relatively greater increase in speed for th^,, more difficult task, a. rial increased almost 60 per cent, whereas the rate for the high-contrast nifitTi increased only 17 per cent. . The limited available data on a number of typical workworld tasks sh'gM higher levels of illumination are significantly effective in improving, jpe^f accuracy of .working.18-19 For example, the speed of reading a stecLyerriio_g .001 incji,'continued to increase up to 200 fbofcandles' With -no. optimum had been reached. The speed of discrimination of small j^las^c and steei-on-brass objects increased with levels of illumination up to hu " C. E. Feree and G. Rand, Intensity of light and speed of vision with spe_ ifigi~fftuk~tlons;"Pmts T'ahd~Hp3Vgh^7ffMWrtK;ipr-i?deir^r-79-- (1928). id) "M. Luckiesh and F. K. Moss, Light, vision and seeing, Ind. Med., 7, 636 (19., .. "Lighting for woolen and worsted textile mills (Report of Joint LE-S.-AsS-Md11 mittee on Lighting in the Textile Industry), Ilium. Eng., 44, 364 (1949). ! ' " H. R. Blackwell, Ilium. Eng., 47, 605-606 (1952). ttJrjalrE: BEWV |JU-, Por^a^e Luckiesh-Moss Visibility Meter. This is'a'jusefiil instrument for determining footcandles for equal visibility. 1 > HeEception and, by &eahs<of gh'apprOpriate'scalS,'.iffi|;^equiredHfilter;density a numerical visibility rating; Witte thib'dnstrbfne&Vthe visibility level gjrr-k'is related to the-visibility -of- standard Tabotatbrjfv test objects viewed conditions. -Recent vdatai;have'''shpwn"'tira't;:?tHe,se visibility ratings "KtS^fS^ly relatable to the .absolute,.; thresbbld,,,of, jiiumination?feiandti;,to ..the ........ Luckiesh and F. K. Moss, Visibility--its measurement and significance in seeing, ^WMt7.2?0,43;l (1936). ;!' Luckiesh, A. A. Eastman, and S. K. Guth, Technique of using'the Luckiesh-Moss """"... " :.. SSii'S-.K. Guth, Visibility, Gen. Elec. Rev., 55, 26 (1952). Lu"cvk`"ievs7h*,* S. K. .Guth, and i*A.. nA.,. aEjoa,ssiitumiuaun,, tA\ nneeww ssecaal le of relative-,footcandles. for Wfif;livB|KIieishh-`MMosSs3 Visibility Meetteerr,./IIlifuiim. ERnngo., 45., o2Hn r(1io9s5m). " Guth, A. A,:Eastman, and R. C. Rodgers, Brightness difference--a'basic factor in' - .aw Ipum. Eng., 48, 233 (1953). v