GB790945A - Device for comparing the intensities of two beams of radiation - Google Patents

Abstract

790,945. Photometric analyes. PERKINELMER CORPORATION. Nov. 11, 1955 [Nov. 12, 1954], No. 32261/55. Class 40 (3). In a device for comparing the intensities of two beams of radiation alternately switched along a single path to a detector at a first frequency and which before reaching the detector are modulated at a second frequency, the two frequencies are in the ratio of 3 : 2 and the periods of interruption at the lower frequency are in the ratio 2: 1. The voltage from the detector is applied to a demodulator the output of which contains a D.C. component proportional to the beam intensity difference. In the apparatus shown, beams of radiation (RB and SB) from a source S pass through absorption chambers 14, 21 containing test and standard materials respectively and are alternately at a frequency 3f directed by a rotary chopper 20 on to the entrance slit 19 of a spectrometer where the beams are dispersed by a prism 24 before reflection on to orthogonal mirrors 27, 28. Between this prism is disposed another chopper operating a frequency 2f and so designed that the beams are cut off for two thirds of each cycle. In this way, the output voltage of the detector has a component of frequency f and this voltage is fed together with a control voltage of frequency f, to a phase-sensitive full-wave rectifier so that the output of the latter contains a D.C. component proportional to the beam intensity difference. The output of the rectifier feeds a servomotor 36 to adjust an attenuator wedge 37 in the path of beam RB until the beam intensities are equalized, the adjustment to the wedge being recorded in an instrument 38 as a measure of the difference in the beam intensities. Other chopper relationships are suggested.