April 1, 1921 



THE INDIA RUBBER WORLD 



485 



In considering the distribution of light over a horizontal sur- 

 face the most obvious fact is the way in which all conditiotis 

 work together to produce the greatest intensity directly under 

 the unit : thus the candle-power intensity is greatest in that direc- 

 tion, the distance from the source is the shortest, and the in- 

 clination of the surface least, in fact, zero. As we move out 

 from this positiosi the intensity falls off very rapidly ; at a 



Fig. 2. rrB\'F? Sttowing D!stribution of I.h^ht r\ the Two Units Best 



Adapted to Factory Lightinc. The "Mazda C" Lamp With "R. L. M." 



Reflector, and the Cooper-Hewitt 1-amp 



distance equal to the height of the unit it falls to one-fourth, 

 and at twice this distance of one-sixteenth, and at an equal rate 

 beyond this. The foot-candle intensity directly underneath is 

 found by squaring the height of the unit, measured in feet, and 

 dividing the candle-power by this product. Example: If the 

 500-watt Mazda unit is placed 10 feet above the floor, the foot- 

 candle intensity directly below is 150O-r-10O=: 15 foot-candles. At 

 ten feet from this spot it is J4 of IS, or 3^ foot-candles, and at 

 20 feet, \/\h of 15, or 15/16 foot-candle, reckoned by the math- 

 ematical laws which apply in the case. 



These figures are for the simplest case; that is, where a 

 single unit is used. When more units are used to light the same 

 surface the resulting illumination is, of course, the sum of the 

 intensities produced by all the units used. Mathematically, the 

 problem of figuring the resulting intensity becomes greatly com- 

 plicated in the case of a number of units; but practically it is 

 very simple. The amount of light received by units beyond those 

 next to a given unit is so small that it may be left out of the 

 reckoning entirely. For all practical purposes the following 

 rules arc sufficient. When the units are spaced at distances 

 equal to their mounting heights the illumination will be uniform. 

 When spaced at twice the mounting height the minimum illumi- 

 nation, midway between two adjacent units, will be one-half the 

 maximum, or underneath intensity. Spaced at three times the 

 mounting height the minimum will be one-third the maximum. 



From the candle-power, or distribution curves, or the intensi- 

 ties which they represent, and these few computations by simple 

 arithmetic, all the information as to foot-candle distribution that 

 is worth having may be quickly obtained. \'ariations due to 

 deterioration in use, accumulations of dirt on lamps and re- 

 flectors, reflection from walls and other surfaces, will far exceed 

 any discrepancies arising from mathematical inexactness in these 

 calculations. 



Having decided upon the intensity of illumination to be sup- 

 plied, cither on the basis of uniformity, or of a given minimum, 

 it is a simple matter to determine the size of units, and the spac- 

 ing distance required : and from this data the total amount of 

 current, and the ratio of current to floor space, is quickly reck- 



oned. Thus, if a uniform illumination of 10 foot-candles is- 

 desired, it may be obtained by using either the SOO-watt Mazda C 

 units, or Cooper-Hewitt lamps, hung 12 feet high and spaced; 

 12 feet apart. There would then be one unit or lamp to each 

 144 square feet, which would be at the rate of 3J/' watts per 

 sfiuare foot for the Mazda unit, and 3 watts per square foot 

 for the Cooper-Hewitt lamp. If the ceilings are too low to 

 admit of the 12-foot hanging, or if conditions are such that a. 

 lower hanging is practicable, they may be hung at 10 feet, which 

 will give IS foot-candles underneath, and with 20-foot spacing, 

 1]A foot-candles minimum, with a rate of 154 watts per square 

 foot for the Mazda, and 1 watt per square foot for the Cooper- 

 Hewitt. If a 10 foot-candle minimum is desired, the spacing 

 can be reduced to 15 feet, which will give a rate of 2 watts per 

 square foot for the Mazda, and 1.9 watts for the Cooper-Hewitt. 

 Smaller units may be used, but their distribution is the same, 

 and the methods of figuring will be the same, with the sub- 

 stitution of the smaller candle-power values, which are supplied 

 by the lamp makers. 



The method may seetn very haphazard and unscientific to the 

 professional engineer; if so, then there is plenty of opportunity 

 to manipulate mathematical formulas for those who are so dis- 

 posed : but the average factory manager probably would sooner 

 spend his time playing solitaire, which would be quite as valu- 

 able an occupation so far as practical results are concerned. There 

 are several published volumes of which this subject occupies the 

 major portion. Never was so much perfectly good mathematics- 

 squandered to so little purpose. 



There are two ways to lay out a lighting installation : to 

 locate the units with reference to the structure of the building; 

 or to locate them with reference to the machinery or the posi- 

 tions which the workmen occupy. There would seem to be 

 little doubt as to which method would produce the better results, 

 measured in labor efficiency : and yet most new installations are 

 planned by the former method. Having produced uniform il- 

 lumination over the theoretical "working plane" — on which no 

 work may ever be done — the job is considered a fine piece of 

 "engineering" — by the engineer. Architects have a cheerful way 

 of lighting two kinds of the rooms in whicli the manner of light- 

 ing is most important, kitchen and bedrooms, in such a way that 

 it is impossible to see plainly and comfortably what you most: 

 need to see. A single unit is placed in the center of the room; 

 and as the tables, sink, stove, cupboards, etc., are around the 

 walls of the kitchen, and the bureaus and mirrors similarly 

 placed in bedrooms, you invariably "stand in your own light," 

 that is, your own shadow is upon the very thing that you most 

 want to see. And yet the illumination of the room is fairly 

 uniform, and may be of ample intensity, thus fully satisfying 

 "engineering" requirements. The best laid plans of blue- 

 print installations "gang aft a-gley" for similar practical reasons. 



We discriminate in seeing by dilTcrences in brightness, or in 

 more familiar terms, by dififerences in "light and shade." A 

 surface of uniform brightness appears perfectly fiat. A light 

 that was perfectly uniform and perfectly dilTused would be about 

 the worst possible light to work by. The best possible illumina- 

 tion is one in which the shadows are sufficiently light to reveal 

 all details, and yet in which there is a sufliciently strong light 

 from some one direction to bring out distinct contrasts. Noth- 

 ing is more confusing than multiple shadows, — the result of 

 "cross lights," which have long been recognized as serious evils. 

 The ideal method of illumination is, therefore, to light each piece 

 of work in the manner just stated, so as to bring out all the 

 details as sharply as possible, and to handle the light-units in 

 such a way as to avoid glare and scattered light in the eye of 

 the workman. This can only be accomplished by careful con- 

 sideration, and often by actual experiment, of the particular con- 

 ditions arising in each case. We shall study some of these cases 

 in our next article, in connection with typical rubber factory 

 lighting installations, which will conclude these papers. 



