Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . ce the edges of the trans-lucent portion are more sharply defined than those of the Bunsen,less difficulty is experienced in detecting slight inequalities in theillumination of the two sides of the screen. The Lummer-Brodhun photometric device consists of a purelyoptical combination for viewing the two sides of the screen. Adiffusing screen, s s. Fig. 32, of high reflecting power is placed in,and with its plane normal to, the photometric axis. This screenis viewed on both sides by means of the o
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Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . ce the edges of the trans-lucent portion are more sharply defined than those of the Bunsen, less difficulty is experienced in detecting slight inequalities in theillumination of the two sides of the screen. The Lummer-Brodhun photometric device consists of a purelyoptical combination for viewing the two sides of the screen. Adiffusing screen, s s. Fig. 32, of high reflecting power is placed in, and with its plane normal to, the photometric axis. This screenis viewed on both sides by means of the optical device whichpresents both sides of the screen to the eye as adjacent or con-centric fields. The Lummer-Brodhun photometer is built both as an equal- 88 LIGHT, PHOTOMETRY AND ILLUMINATION ity of brightness and as a contrast photometer. Thearrangement of each is essentially the same. The light from thetwo sources, reflected from the sides of the screen s and s, fallson the mirrors m^ and m^ and is reflected along a normal to thesurfaces of the triangular prisms A and B. The observer, look-. 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