Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . FiG. 32.—The Lummer-Brodhun equality of brightness photometer. ing through the telescopic tube 0 directed normally to B, clearlyviews a divided field illuminated partly by one source and partlyby the other. In the equality of brightness photometer the rays from I^ passdirectly through the central part of the prisms illuminating the THE PRINCIPLES OF PHOTOMETRY 89 central portion of the field as indicated in Fig. 32. The raysfrom I^ pass in the same way into the prism B, the central rayspass dire
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . FiG. 32.—The Lummer-Brodhun equality of brightness photometer. ing through the telescopic tube 0 directed normally to B, clearlyviews a divided field illuminated partly by one source and partlyby the other. In the equality of brightness photometer the rays from I^ passdirectly through the central part of the prisms illuminating the THE PRINCIPLES OF PHOTOMETRY 89 central portion of the field as indicated in Fig. 32. The raysfrom I^ pass in the same way into the prism B, the central rayspass directly through the prisms while the outer rays are reflectedby the prism B and constitute the outer portion of the field ofview. The paths of the light rays viewed through the eye-piece. Fig. 33.—The Lummer-Brodhun contrast pliotometer. are indicated by the full lines. Those shown dotted from I^ arereflected by the prism B out of the line of vision while thoserepresented by the dotted line from I^ pass directly through theprisms out of view. With this arrangement one observes a two-part illuminated field as shown shaded at 0. 90 LIGHT, PHOTOMETRY AND ILLUMINATION In the co7}trast photometer the piinciple is the same but thefield is divided up as illustrated by the shaded part of Fig. principle can be readily understood from the previous de-scription. It will be seen that the parts of the field a and d areilluminated by the source 7^ while h and c receive light from thesource I^. Two thin pieces of glass g and g intercept some of thelight illuminating the fields b and d thus giving the contrast effectwhereby this photometer receives its name. A photometricalbalance is obtained when the intensity of a and c and of b and d mmmmmm B Fig. 34.—The Marten photomet
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
