Document jBJYgnYOy9JEDY0JryDVNJJ5Q

744 S. K. GUTH AND E. A. LINSDAY Thus, the optimum level of illumination for such a task would be considered asS or 20 footcandles. Obviously, for other more difficult industrial tasks the optimiilr footcandle level will be considerably higher. is As the level of illumination is increased above the optimum, the task beconiJJ easier to perform even though production of useful work may not increase apprec;' ably. Less useless energy is expended by the worker. This is illustrated by^fft upper broken line curve of Figure 1. Obviously, when seeing is extremely diffi$l||| '''v'iiiiSJfS! RESERVE RESOURCES .expWdec, IN ^eless>ork; _ SfEXPENDEO IN USEFUL WORK! "as seeing becomes EASIER-1 Figure 1. A diagramiffAflcYisw of'the expanded concept-of-lighting-and-seeing._E & production ceases to improve appreciably, seeing becomes easier and fewer human' "e are expended in performing useless work. The true measure of human efficiency is th< i resources expended in useful work to the total expended in useful and useless work. we must call upon a large portion of our resources that are involved in seejm seeing becomes easier, our reserve resources increase and the nonproductive'^ decreases. Thus, the worker becomes a far more efficient human seeing machiri^| levels of illumination well above those that will permit him to perform m jijv optimal rate. In other words, a truer index of over-all human efficiency isltMsrMgg] of the resources expended in useful work to the total resources expended in uV and useless work. Tangible benefits from applying this broader concept;'melt greaterAafety, decreased spoilage and rework, decreased lowefTabtih turnover, and so forth.8'4 I. Lighting Terms and Units ' ' - >j : T'h'ere are m'aiiy terms-artd-umts-used-in-illuminating-engineermg it is only necessary to have a clear understanding of certain basic ones. * H. Kuhn, Eyes and Industy. Mosby, St. Louis, 1950. `Value of good lighting in war production (Committee Report), Trane. Ilium. Engli 38, 13 (1943). "Illuminating Engineering Nomenclature and Photometric Standards, Z7.1-1942.' \ can Standards A*s-soc:i-att:io--n, XNTew York, l1O9A4O2. LIGHTING FOR SEEING 745 Sahe lumen is the unit of luminous flux emitted through a unit solid angle Mian) from a uniform point source of one candle. It is used to.represent the pf|ht,output of light sources. fcmdlepower is luminous intensity expressed in candles. An ordinary candle m(HjSftB-inoh in diameter produces about one candle in a horizontal direction. 1W&'jnination.M the density of light flux incident upon a surface. |||he footcandle is the unit of illumination when the foot is the unit of length. *|h;e illumination on a surface, one square foot in area, on whioh is uniformly |uted a flux of one lumen. It equals one lumen per square foot. \'Kotometric brightness, or luminance, is a measure of the light emitted by a bps body or reflected from a surface. It may be expressed in terms of the Sjheing emitted or reflected from a perfectly diffusing source. The term less, alone, usually refers to the sensation that results from viewing surfaces ,, 'hich light comes to the eye; it often is used interchangeably with luminance, |the context usually indicates whether the sensory or photometric meaning is ^jlhe footlambert is the unit of photometric brightness and is equal to the brightness of a perfectly diffusing surface Emitting or'reflecting one lumen Vequare foot. The brightness in fobtlamberts%* equal to 'the|iirumination on a multiplied by its reflectance. . Reflectance is the ratio of light reflected byan object to the incident light. II. .Visibility |ot all things that can be seen are equally visible. Many factors combine to idetails .visible aiid recognizable, but four of them !may be considered as Smental characteristics of visual tasks.0 Perhaps the most;obvious one is the c of an object expressed in terms of visual angle., A telephone pole is r large, yet when viewed from a long distance itoApparent size, is small, 'cuity is a measure of the sensitivity of the, eye in tfems.of the size thresh- lequal to the reciprocal of the angular size in minutes of the critical detail hLbe just barely seen. r-M.:.'.'.: BwHaii'the same print oh a dark gray paper. ' " lljt industrial'tasks involve":m6tibh'.'l'Thus', time is an imporfcSSt'factor. The *?wLmovmg object must be slow ehbughHBat'the eye cari''take^'l'4sual*shap- ,ui^^^^^s^^^^^&^8E^^sia^W)dlyfOtatwf=^eeUap|pea_ Tfr^ilSBut when it is turned slowly, accurate visual impressions canibC registered, ourth fundamental factor is-'brightness. Without brightne3s;,:sight 'Would *;l^Mlible, all objects would-remain invisible, and, in effect;weahSuKhb'e blind. Wf ' ` t B8SV'. Cobb and F. K. Moss, Four fundamental factors in vision, Trane. IUum. Eng. Soc., MWm. Sm at*..