Document oegXzmDGVxNknew61naxL4rrg

750 S. K. GUTH AND E. A. LINEDAY One logical objective of lighting, is to make various objects and visual tas as nearly equal in visibility as possible. Therefore, a more important applicatio of the L-M Visibility Meter is to determine the footcandles necessary to provid equality of visibility.22 That is, the levels of illumination are determined wit'' which various visual tasks can just be seen through a given density of the filters; It is widely used to study the visibility of school, office, and industrial tasks. Sine;; it is portable, the instrument can be taken into an industrial area and the ta^'' evaluated where it is being performed and under the actual viewing conditions Table 1 illustrates a few of the results obtained with the L-M Visibility Mete! TABLE 1 Results Obtained with the Luckiesh-Moss Visibility Meter Task Footcandles for equal visibility Reading Typing on white paper Magazine text Newspaper text ^ Handwriting in pencil Shorthand notes Bookkeeping Drafting Medium grade assembly and inspection Metal buffing Sewing, white thread on white cloth Metal finishing--surface grinding Sewing, black thread on black cloth Precision die making 15 30 60 60 90 120 120 200 250 THTo 500 over 1000 over 1000 Some of the tasks require what appear to be unreasonably high levels of illumin. tion. For example, drafting requires eight times as much light as reading . on white paper, and the industrial tasks require still more. When the analyzed in terms of size, contrast, and time, it is evident that such footcandl; levels are required in order for the tasks to be equal in visibility. Furthermore^' values are in agreement with experimental results such as shown in Figures ,3,;ary.l The-footcandle -levels-should-be-considered-as-relative-rabher-than-recommen levels of illumination for the performance of these visual tasks. It is obyip.tus||] the more difficult, low-contrast tasks require considerably higher footcandle^leVf than the larger, high-contrast tasks. In the present discussion, the practical*! of attaining,these higher, footcandle. levels is not..of.importance., TheintsTfiat-Q' illuminating engineer lies in developing a single standard for lighting practic does not seem logical that light for seeing industrial tasks should be recommcn LIGHTING FOB SEEING 751 "^different basis than for office tasks just because it may be more difficult to ^Visibility does provide a uniform basis for evaluating different tasks and iglgcminmg the amount of light needed. Many industrial tasks , have been ffe?l in this manner and .the results used to determine recommended footcandle III. Ease of Seeing g'roviding light for perception is only part of the over-all problem. The eye ||,a single unit of the entire human organism. It is influenced by body fatigue "also, it influences general body fatigue. Unfortunately, fatigue is difficult to '^because of the innumerable factors concerned.2' It is important .to adjust |e environmental factors so that they combine to provide the best possible conditions. Therefore, in the case of seeing, the footcandle level should be ILenough that the worker can perform his job for prolonged periods with ffimum expenditure of energy. lany attempts have been made to detect and appraise visual fatigue by I|irements of visual functions, such as visual acuity, speed of perception, and Eyen where work periods were long and the workers expressed definite rtions of fatigue and discomfort, the decrement in such functions generally neen insignificant. Therefore, it has been necessary to explore the possibilities |her more sensitive criteria that will more adequately evaluate poor seeing [Jtions. e'of-thFTesultsT3btaiired'tvith"ffi6i^ensit'ive"cntena 'based upon certain |ntary responses are presented in Figure 6 for three levels of illumination.28 of these investigations the visual task of reading a well-printed book was Mmed by young adult subjects possessing average normal vision. Thus, the H may be considered conservative, since greater differentials certainly could qected from older subjects or those having subnormal eyesight. |he data summarized in Figure 6 relate directly to easeiofi,seeing as appraised |;|ptegrated physiological effects of visual effort. The close correlation among ffenteria indicates that the favorable influences of illumination upon the ^processes and upon , ease qf,peeing extend to levels of -illumination at least gpls 100 footlamberts. Studies Of this sort indicate that the broader concept fa~`tJe"dJ in Figure 1 is sound.. If the task of reading involved a gray surface ps/that used for obtaining.curve a of: Figure. 4), several hundrfegtjootcandles &Mfejequiredsto^produce..the^sarne'l-brightness conditions-and"&ferif-RopiTm. jSrfc&i : 4 ~ mm,rffwSer'can Standard for Industrial Lighting, All.1-1952. American Standards Association 1952. ...; IBs: H. Bartley and E. Chute, Fatigue and Impairment in Man. McGraw-Hill, New York Iff' I',*1h and F, K. Moss, The Science of Seeing. Van Nostrand, New York| 1937. .