Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . _, ^ cos a cos a that in the equation Lh — ia - the values of can be h- h^ determined for various values of a and h, and designated by K,then Eh^KIa and Ia = -^- These values of K for various heights, distances and angles aregiven in Tables 2 and 3 of the appendix together with an illus-tration of their use. Point-by-point Method—Graphical.—The calculation of illu-mination at a point on a plane may be performed graphically,*supplemented by a few simple calculations, without the use ofthe precedi
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . _, ^ cos a cos a that in the equation Lh — ia - the values of can be h- h^ determined for various values of a and h, and designated by K,then Eh^KIa and Ia = -^- These values of K for various heights, distances and angles aregiven in Tables 2 and 3 of the appendix together with an illus-tration of their use. Point-by-point Method—Graphical.—The calculation of illu-mination at a point on a plane may be performed graphically,*supplemented by a few simple calculations, without the use ofthe preceding equations. As already indicated the illuminationdirectly under the source would be £. = i-: m ^Electrical World, Vol. 55, p. 1215. May 12, 1910. 174 LIGHT, PHOTOMETRY AND ILLUMINATION or from the values indicated by the polar curve of Pig. 100 whenthe lamp is placed 8 ft. above the plane 52En = 0^= foot-candle, (85) When the point is not directly under the lamp as at M the graph-ical method is applied as indicated in Fig. 100, based on tlieequation /„ cos a Eh = h^ + d^ (80). Fig. 100.—Graphical solution of the point-by-point method. where Ef^ = the horizontal illumination in foot-candles. la = the candle-power a degrees from the = height of lamp above the working — distance from beneath the source. It will be seen that the polar diagram is cross-sectioned andh and d laid off to scale. The point R on the polar curve isdetermined by drawing the line OM. The vertical componentof the candle-power, /„ cos a, will be represented by OP, inthis case 48 candle-power and E/^ at M will be, ILLUMINATION CALCULATIONS 175 40 Ea = ^^^^^2 =0-3 foot-candle. (S7)8-^ + 10 If the source of light is inclined and the point under consider-ation is not in a vertical plane through the axis of the source oflight the case becomes somewhat more complicated, still thesame equation holds and the solution is simple if certain factsare kept in mind. Consider in this
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
