GB665942A - Improvements in photoelectric meters - Google Patents

Abstract

665,942. Photo-electric photometers. THOMSON, C. L. Aug. 14, 1950 [Aug. 12, 1949], No. 21005/49. Class 40 (iii). A combined meter for determining quality and intensity of visible or non-visible radiation comprises at least two filters of substantially differing wavelengths, a photo-cell or cell area associated with each and a switch for switching the output of either or both said cells to a galvanometer. As shown, radiation is received on a sloping light diffusing screen 5, behind which are situated a pair of filters 6, 7, one responsive to red or red-orange light and the other to blue or blue-green. These filters may be of dyed gelatine and a proportion of white light may be allowed to pass by perforating them. Each filter has associated with it an area 8, 9 of a divided photo-cell 10, the positive pole of which is connected direct to a galvanometer. In the first position of the switch (as shown) cell-area 8 only feeds the galvanometer via potentiometer 15, in the second position cell-area 9 only. In the third and fourth positions the cell outputs are paralleled to the galvanometer but a different one of shunting resistances 17, 18 is connected across the cell in each case. In measuring colour temperature the switch 14 is set to the first position and potentiometer 15 adjusted to bring the instrument pointer 3 to a fixed mark; the switch is then set to the second position when colour temperature may be read directly. To investigate discontinuous spectra more filters and switch positions may be provided. The filters themselves may be light-diffusing eliminating screen 5. Thus red and blue etched glass elements may be cemented edge-to-edge, or dyed opalescent plastic members such as polymethyl methacrylate filters may be employed.