Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . t 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 width of each section is equal to the distance acrossthe middle of that section. Hence by reading the lengths ofthese horizontal lines and by dividing the s
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . t 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 width of each section is equal to the distance acrossthe middle of that section. Hence by reading the lengths ofthese horizontal lines and by dividing the sum of their valuesby 20 we obtain the approximate value of the mean sphericalcandle-power. If the hemispherical candle-power is disired,consider only the areas corresponding to that hemisphere anddivide the sum of the values of the 10 lines by 10. The use of the Rousseau diagram for showing the relative lightvalues of different sources is shown in Fig. 80. The areas en-closed between the curves a, h, c and d, and the vertical on theleft represent the total flux of light from four sources having. Fig. 86.—The Rousseau diagram applied to curves of Fig. 83. distribution curves as indicated by a, h, c and d, respectively, ofFig. 83. It will be seen that the four areas determined by thecurves a, h, c and d are equal. Since these areas representgraphically to scale the value of the light flux we have the samevalue of the lumens or mean spherical candle-power for each. Macbeths Polar Flux Paper.—In the method of determiningmean spherical intensities just discussed it will be seen that thespherical area surrounding the source of light was divided intozones of equal areas and the candle-power in the direction of thezonal centers of these areas assumed as the average for that LIGHT-FLUX CALCULATIONS 151 zone. This method forms the basis of Macbeths polar fluxpaper, which is the ordinary polar coordinate paper on whichare placed the radial lines representing the zonal centers of 20zon
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
