Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . 60 65 Fig. 181.—Candle-power curves, d/h = 6. 5 10 15 20 25 30 35 40 45 50 55Degrees Fig. 183 the full-line curve shows the actual distribution of onemake of arc lamp to-day on the market, the curve being oneobtained by test by the Electrical Testing Laboratories. Thedotted curve is the ideal curve for ilf = 4. It will be seen thatthe agreement of the two curves is admirable up to an angle of 300 LIGHT, PHOTOMETRY AND ILLUMINATION about 62 degrees. The curves AI — A and il/ = 6 would also becomp
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . 60 65 Fig. 181.—Candle-power curves, d/h = 6. 5 10 15 20 25 30 35 40 45 50 55Degrees Fig. 183 the full-line curve shows the actual distribution of onemake of arc lamp to-day on the market, the curve being oneobtained by test by the Electrical Testing Laboratories. Thedotted curve is the ideal curve for ilf = 4. It will be seen thatthe agreement of the two curves is admirable up to an angle of 300 LIGHT, PHOTOMETRY AND ILLUMINATION about 62 degrees. The curves AI — A and il/ = 6 would also becomparatively easy of attainment with the incandescent lampor with the inverted gas mantle. This is of especial importancesince, with these lower values of M, such smaller units giveamply sufficient intensity of illumination except in those businesscenters where night traffic is considerable. Apparent Candle-power200 300 400 500 600 800. < 300 5 10 15 20 25 30 35 40 45 50 55Degrees 60 65 Fig. 182.—Candle-jDower curves d/h = 4:. Smaller values for M than 4 would obviously fulfill all theconditions imposed by the requirements of good will later be shown by our study of glare effect that a valueM = 3 is necessary to obtain best results whenever, on accountof shaded conditions of street or other considerations, it is Apparent Candle-power 500 1000 1500 2000 2500 3000 3500 4000 Candle-power 1 8 M 1 7/ Appar ./7 2000 \ / \ / \ / / /^ / / / / 5 10 15 20 25 30 35 40 45 50Degrees 80 a75 Si 65 55 60 Fig. 183.—Theoretical and actual caiidIe-i)ower curves, d/h = 4i. desirable to mount the units at a height of 15 ft. or less above thestreet. Closer spacing of units than il/ = 3 entails a correspond-ing increase in installation and maintenance costs, and is notdemanded by the illumination limitations of the problem. . PRINCIPLES OF STREET ILLUMINATION 301 Summing up the previous paragraphs, the requiremen
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