Document JevqE92w2eYyRQ7wooz19ZK6

746 S. K. GUTH AND E. A. LINSDAY Considerable research has provided us with comprehensive data on the rela) tionships among these four fundamental factors.6-9 Figure 2 illustrates how co trast and size vary for three brightness levels. The curves represent the borderli between visibility and invisibility! Small objects must have high contrasts in ord to be seen and, conversely, low-contrast objects must be relatively large in si! As the brightness level is increased, either the size of the contrast, or both, m be decreased. Similarly, when considerable time is available for seeing, objed can be small in size or low in contrast. However, in the case of many typfd .7....\...\.\\ \ >\ \\ "V "V \\ SUI RA-THR ESHOL O VI HBIU TY \ \ N \ \N IMigM -- \\ v> v \ s a ` invisibility X __ ?r I**-- llllllll |lj|lli ' + f\ r , Ji / size or object (minutes) Figure 2. The relationships among size, contrast, and brightness. The invisible region ajjn| the brightness level is increased. visual tasks, size, contrast, and time are generally fixed. Therefore, it is af,.^ that the most universally controllable factor is brightness.10 Increased brigHcb -is-obtained-'by-hdgher-footcandle-lev.els.______ "X.'MTTM A familiar method of presenting data on the effectiveness of illumiSatioi! - 'H. R. Blackwell, Brightness discrimination data for the specification of quafi&C illumination, Ilium. Eng., 47, 602 (1952). . ,* ______ a H. G;-Weston. The Relation between Illumination and Visual Efficiency--The Frj ^55e7jointReport_onTn3ustnarHeaIlirBoar9-a5d"nilinsifettimrItese!rrclr;<^omiS^^~' No. 47-105, H. M. Stationery Office, 1935. ' j *H. C. Weston, The Relation between Illumination and Visual Efficiency--Thr Brightness Contrast, Medical Research Council, Industrial Health Research BoaiJ.j No. 87, H. M. Stationery Office, 1945. '1? "M. LucEesh, Light, Vision and Seeing, Van Nostrand, New York, 1944. . ag . r'Si''../v v**\r.ivr-- LIGHTING FOE SEEING 747 s"ijf visual acuity, or the ability to distinguish fine details. Some typical results ffivpi in Figure 3 for black objects viewed against white and gray backp^,The relative visual acuity obtained with a black-on-white test object _G[,;foptcandles is assigned the value of 100 per cent. However, it must be nfliered that the majority of industrial tasks are not black on white, but range IfgSt- gray-on-white to dark gray-on-black. Thus, the right side of Figure 3 Ju FOR A BLACK TEST-OBJECT (DRF-3%) ON A WHITE BACKGROUND (ORF-SOX) [GRAY BACKGROUND (0RF=8X) BRIGHTNESS-GONTRAST-98X BRIGHTNESS-CONTRAST* MX -W .57, IBHriB |..'FC'7^ 10'FC'.i'100`Fe; >T, "FC': TO:FCJ;*fjOO'FC'? ILLUMINATION `ON VISUAL TASK (FOOTC'AND.UES) [ire 3. The contrast between an object and its immediate background is an important, i neglected, factor in visual acuity. For a given footcandlh'leyel, visual acuity is con- 1. less when the contrast is low (DRF == diffuse reflection factor). U-Wsi' . . . , v .,\ , .!- i , j'CUlarly significant. With 10 footcandles, if the cBiitlilst is decreased from |p,:per cent, visual acuity is reduced about 34 per c|SS"In. other Words, for a ipbtcandle level a lafger 'dbject'ih^ffe'd^ired'Idf^l^^f%onffi'S;vFven with ^^IS'^an'-'t^'^romld^lth^O White task. TrVisual acuity is to'be used as a basis for prescribing illumination, it is ary to have data for a wide variety of contrasts. V',j. ihilar results are.obtainedwhen ^peed of reading is the criterion. Iii Figure 4 . *t61tlt^ftSIe^^l?^ltmgiblftekr6n-w^e^fTOrviri^):^i3::blackvon=-- (Mirve o) printed material.12 The reflectances of the papers were 80 and 23 4-,'fLucKesh, Visual acuity and visual tasks, Ilium. Eng., 39, 415 (1944). ,M. LucEesh, A. H. Taylor, and R. H. Sinden, Data pertaining to visual discrimination haired illumination intensities, J. Franklin Inst., 192, 757 (1921).