(photometer cube) a device for comparing the intensity of two light fluxes; it consists of two isosceles right-angled glass prisms (1 and 2; see Figure 1) joined by the hypotenusal faces. Over most of their surface the faces are in optical contact with one another, and the light rays (4) and (5) pass through the cube without changing direction. In the area marked (3) the surface layer of one prism is removed; the glass faces there are separated by an intermediate layer of air, so that when rays (4’) and (5’) are incident upon it they undergo total internal reflection. The observer (6) sees two adjoining light fields (one created by the flux of the rays 5; the second, by the ray 4’) and compares their brightness.
Lummer-Brodhun cubes are used in various optical instruments, particulary in photometers, colorimeters, and nephelometers.
M. M. GUREVICH