Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . aching thephotometer by a small screen. The mirrors rotate about thethe axis of the lamp and reflect to the screen a quantity of lightproportional to the mean horizontal intensity of the source.This method necessitates, of course, the determination of thereflection coefficients of the mirrors, which is accomplished by 140 LIGHT, PHOTOMETRY AND ILLUMINATION means of a standard lamp, the mean horizontal intensity ofwhich has l^een previously determined by the first methodmentioned above. The arran
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Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . aching thephotometer by a small screen. The mirrors rotate about thethe axis of the lamp and reflect to the screen a quantity of lightproportional to the mean horizontal intensity of the source.This method necessitates, of course, the determination of thereflection coefficients of the mirrors, which is accomplished by 140 LIGHT, PHOTOMETRY AND ILLUMINATION means of a standard lamp, the mean horizontal intensity ofwhich has l^een previously determined by the first methodmentioned above. The arrangement of the rotating-mirror apparatus is shownin Fig. 77.^ For experimental purposes it was so constructed thatthe lamp could be rotated independently of the mirrors. Themirrors may be rotated by a motor belted to pulley E. Themirrors are held to the frame by the bolts B and the frameprevented from spreading by the stay rods C. The screen Dintercepts the direct rays from the lamp. Apparatus for Obtaining Values for the Polar Curve.—Indetermining the distribution of light around a lamp which. Fig. 78.—Mirror reflector photometer. must be burned in a certain position, as for instance an arclamp which cannot be inclined without introducing variationsin its candle-power values, it becomes necessary to resort toother means than those heretofore described. A device for this purpose consisting of three mirrors A, B, and Crevolvable about an axis D is shown in Fig. 78. It will be seenthat the beam of light follows a path indicated by the brokenline and is compared on the photometric device shown in theright-hand side of the figure. It will be seen that the mirrorscan be rotated so as to measure the candle-power in a verticalplane. It is obvious that this device can be used for photom-etering any ordinary source of light and can be calibrated bysubstituting a source of known candle-power in the same wayas with the portable photometers. » Bull. Bur. Stand., Vol. 2, p. 419, 1906. P