Light, photometry and illumination : a thoroughly rev. ed. of ''Electrical illuminating engineering'' (1912)
Barrows, William Edward, 1878-1960 · New York ; London : McGraw-Hill Book Co. · leaf 241
thesolid angle subtended by the source in one case is equal tothat in the other the light flux densities at the two points areequal; when the two angles are unequal the flux densities differin the exact ratio of the angles. Referring now to Fig. 132,let AB represent a surface source having a plane… … Fig. 132.—Theory of equilux sphere. source be a plane or the internal zonal section of an imaginarysphere. The source would be seen as a circular plane whenviewed from any point along a line passing through the center ofthe source and perpendicular to a plane passing through the edgeof the source;…
Light · Photometry · Electric lighting
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