Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . phereto their respective altitudes. The values of the altitudes of zonessubtended by equal angles are laid off to scale along the verticalaxis of the diagram. These may be determined graphically, asshown in Fig. 85, where the sphere is divided into 15 degreezones and the zonal boundaries projected on the vertical or bylaying off horizontal lines at distances, to scale, from the center LIGHT-FLUX CALCULATIONS 149 line equal to the cosines of the various angles. For every 10degrees these values ar
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . phereto their respective altitudes. The values of the altitudes of zonessubtended by equal angles are laid off to scale along the verticalaxis of the diagram. These may be determined graphically, asshown in Fig. 85, where the sphere is divided into 15 degreezones and the zonal boundaries projected on the vertical or bylaying off horizontal lines at distances, to scale, from the center LIGHT-FLUX CALCULATIONS 149 line equal to the cosines of the various angles. For every 10degrees these values are as follows: Table 25 Cosine Values for Constructing the Rousseau Diagram 0 10 20 30 40 50 60 70 80 90 Cosine .985 .939 .866 .766 .643 .500 .342 .173 0 If, on the horizontal lines drawn from the terminals of the suc-cessive altitudes, we lay off to scale the values of the candle-powerin the direction of the corresponding angles, then the area enclosedby the curve, determined by these values of the candle-power atvarious angles, and the vertical, will represent the total luminous. Fig. 85.—Construction of Rosseau diagram. value of the source of light and the proportional jDart of the totallight in any zone will be clearly shown. The value of this areain terms of the product of the scales to which the curve wasplotted divided by the sum of the ordinates or altitudes will givethe mean spherical candle-power and this value multiplied by47r will give the flux in lumens. In the same way, areas corre-sponding to any zone or zones divided by the altitude or altitudescorresponding to the area will give the mean candle-powerthroughout the respective portion of the spherical area. Toobtain the value of the mean spherical or mean hemisphericalcandle-power without a planimeter, the area enclosed by thecurve may be divided horizontally by 20 lines bisecting areas ofequal heights. Without appreciable error we may assume that 150 LIGHT, PHOTOMETRY AND ILLUMINATION the average
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
