484 



THE INDIA RUBBER WORLD 



April 1, 1921 



line. «ight point, ami witli the maximum contrast — black on 

 white — and with no disturbance from glare or scattered light in 

 the eye, a minimum illumination of 10 foot-candles is required ; 

 20 foot-candles giving even a slightly higher visual efficiency. 

 This is more than rive times the minimum intensity given in 

 the list of operations in cotton goods manufacture, in which the 

 conditions for discrimination are very similar to those in reading 

 print. 



An equally interesting and important fact is the difference in 

 visual efficiency due to difference in the quality of the light, evi- 

 dently due to the difference in the sharpness of the retinal image 

 by light of different color composition, as explained in the pre- 

 ceding article. According to the experiments just referred to, 

 the relative values of the three available sources of light, viz., 

 daylight, incandescent electric light, and mercury-vapor light, at 

 5, 10. and 20 foot-candles intensity, reduced to a scale in which 

 10 foot-candle daylight is taken as the standard, or 100 per cent, 

 is as follows ; 



Daylight Incandescent Electric Mercury \'apor 



5 ft. -can 95.3 91.5 101. 



10 ft.-can 100 97.1 106.2 



20£t.-can 101.4 98.9 107.3 



The experiments by which these values were obtained included 

 not only the discrimination of objects in seeing, but the time 

 required to make a muscular action in response to the visual 

 impression, which is exactly what takes place in all manual labor 

 that it directed by sight. The figures thus represent the actual 

 labor-output values of the different lights and intensities. 



These figures show that for all work requiring close vision 

 and sharp focussing, at least 10 foot-candles illumination should 

 be provided, and that at this intensity mercury-vapor light is 

 9 per cent better than incandescent electric, and 6 per cent better 

 than daylight, measured in labor efficiency. 



In the case of the coarser grades of work, where close vision 

 is not required, no equally reliable data are at hand. Two gen- 

 eral facts, however, will afford some help in forming a judgment 

 in such cases : first, the difference in cost due to a difference of 

 5 foot-candles in illumination is insignificant in itself, and still 

 more so in comparison with the cost of labor ; and second, there 

 is no danger of loss in efficiency from too much light, if it is 

 of the right kind. A minimum of 5 foot-candles may therefore 

 be taken as a safe ligure for all cases of rough work, i.e., where 

 the objects seen are not ordinarily closer to the eye than arm's 

 length. 



There is but one other general case to be considered ; that of 

 spaces in which no work is regularly done, such as storage rooms 

 for raw and finished products, or the intervening spaces between 

 machines. In the former, continuous lighting may not be neces- 

 sary, but only a working intensity required locally on occasion. 

 In the latter, a sufficient intensity to avoid any possil.iility of 

 accidents from imperfect vision is the chief requirement. One 

 foot-candle may suffice, but two is a safer amount. 



There remain, then, only the special cases where unusually 

 exacting work, like die sinking, is done, in which case 20 foot- 

 candles is a fair minimum. 



Knowing the degree of illumination required, the next ques- 

 tion is, how to secure it. This problem involves two factors : 

 the size of the light-unit, and its position in the space to be 

 lighted. These two factors, when taken in connection with a 

 number of conditions which effect the final result, afford an 

 opportunity for endless mathematical calculations; and the vast 

 amount of work that has been expended in this way is chiefly 

 impressive for the inutility of the results obtained. 



When reduced to its lowest terms of practicality, the problem 

 is simple enough. To begin with, we have a light-unit, i.e., a 

 lamp and its accessory apparatus for diffusing an<l reflecting 

 light, which distributes its light in a certain manner, which is 

 usually shown by a curve supplied by the makers. It follows 

 that there will be as many different kinds of distribution curves 

 as there are different kinds of reflectors and globes ; but the 



])roblem has been greatly simplified by the narrowing down of 

 the choice of light-units to two types: the Cooper-Hewitt lamp, 

 and the gas-filled tungsten filament lamp, known in the trade 

 generally as the "Mazda C," equipped with a white-enameled 

 steel reflector, commercially known as an "R. L. M. reflector." 

 This reduction of practical industrial lighting units to two type» 

 is the combined result of the process of elimination by which 

 the fittest survives, and the .American tendency to standardize. 

 The Cooper- Hewitt lamp is regularly produced in one size, 

 though a half-size lamp may be had on special order ; the Mazda 

 C unit may be had in a variety of sizes. The distribution curves 

 of these two are so near alike that they may be treated as iden- 

 lical,_ as shown in Fig. 1. 



"Curves" are now so frequently u.sed to show the relation be- 

 tween variable quantities that a very brief explanation will be 

 sufficient. In these curves the light-source is at the center, or 

 "origin," and the candle-power intensities at different angles are 

 measured off on radii from this point. In the case of the 500- 

 watt, bowl frosted tungsten lamp shown, the intensity directly 

 underneath is 1,.^00 candle-power; at 45 degrees it is 1,300 candle- 

 power, and is cut off entirely at 15 degrees below the horizontal. 

 The curve of the Cooper-Hewitt lamp taking 430 watts is prac- 

 tically the same, the difference being too slight to be recognizable 

 in the illumination produced. 



These curves deal with intensities of /I's/i/; the problem is to- 

 select units of such size, and to place them in such positions 

 that the desired intensities of illumination may be secured. 



The intensity of illumination produced upon a given surface 

 by a given light-unit depends upon three things: (1) the candle- 

 power intensity of the light-beam; (2) the distance of the sur- 

 face from the source ; and (3) the angle at which the rays strike 

 the surface. The first and second of these laws we have already 

 discussed ; let us now examine the third, 



.■\ beam of light. L-.\BCD, from a source at L, will cover the 

 surfaces S, S' and S", which become larger as their inclination 



1-: 



1. Diagram Showing Increase of Surface Covered by Given Beam 

 OF Light at Different Angles of Incidence 



becomes greater ; and as in the case of the distance, or inverse- 

 square law, the greater the surface the less the illumination. Ex- 

 pressed mathematically, the intensity varies as the sine of the 

 angle of incidence. Jf this happens to be beyond your familiarity 

 with mathematics, don't worry ; the figures have been all worked 

 out by the lamp makers, and you can use them for your estimates^ 

 just as the banker uses his interest tables instead of making 

 laborious calculations of his own. 



In figuring illumination the question naturally arises: what 

 surface is to be taken, the horizontal, the vertical, or the surface 

 l)erpendicular to the light-beam? In the actual use of light for 

 seeing things, all three of them, and every position between, 

 come into play; but for the purpose of estimating the size and' 

 position of light-units — and any calculations of Ibis kind are only 

 estimates, or indicators, at the best — the horizontal surface is 

 usually taken as the standard, and is assumed to be 30 inches- 

 from the floor. 



Uniform illumination on this imaginary plane is generally con- 

 sidered the 100 per cent perfect result. But perfection in light 

 distribution is as difficult of realization as perfection in human 

 nature, and far less to \)C desired. The practical object, and one 

 which is easily within reach, is to have not less than the minimum 

 intensity required, at every place in the room, and to avoid too 

 L-iryiL- an excess above this amount. 



