GB648552A - Improvements in and relating to reflectance measuring instruments - Google Patents

Abstract

648,552. Measuring reflectance photo-electrically. BRITISH THOMSON-HOUSTON CO., Ltd. July 21, 1948, No..19537. Convention date, July 24, 1947. [Class 40 (iii)] To compare the reflectance of a surface with that of a standard for light of a particular wavelength (2357 A is mentioned) a box is provided containing a light source which illuminates the surface under test and a pair of differentially - connected photo-cells, both of which cells respond equally to light of other wavelengths but only one of which responds to light of the selected wavelength receiving light reflected from the surface; the rear of the box is detachable and its inside of known reflectance, which constitutes the standard surface, may be removed and replaced by the surface under test. The box also contains a microammeter 16 indicating the difference in output of the two cells with a parallel rheostat and a starting switch 20 for the lamp 2 which is a U-shaped discharge tube. The light emitted from this lamp is rich in radiation of the selected wavelength. One of the cells has a cover of quartz which transmits easily the selected wavelength, the other a glass cover which does not. In use, the cells are first balanced for wavelengths over 3000 Š by covering both cells' with a glass plate transparent to these wavelengths and adjusting opaque strips 25, 26, to obscure part of the face of either cell. The glass plates are then removed and the rheostat shunting the microammeter is adjusted until the known reflectance of the standard surface is indicated on the meter scale. The back is then slid out of guides 9, 10 and the rear of the box placed against the surface under test when the reflectance of the surface is indicated directly.