Continuously self-calibrating differential detection system

927,340. Error compensation in radiation measuring devices. BARNES ENGINEERING CO. Sept. 4, 1961 [Sept. 6, 1960], No. 31712/61. Class 40 (3). The invention relates to means for compensating for changes in detector and circuit parameters in measuring devices which operate by determining variations in radiant energy in the infra-red, visible, and ultra-violet, to give an indication on an electrically operating device. It depends for its operation on the interruption of the measuring beam at a given frequency and the interruption of a calibrating beam, also passing through the system, at an even multiple of the given frequency so that signals of the higher frequency can be separated out and used retro-actively to control the response of the detecting means. In one embodiment the invention is applied to an instrument for the detection of small amounts of hydrocarbons in air. The hydrocarbons have an infra-red absorption band at 3.4 Á. Incoming radiation from the test volume is directed by apertured mirror 1 and curved mirror 2 on to the aperture in a mask 3 in front of which is a rotating chopper 4. The chopper disc has two concentric bands moving through the field of view of the aperture; the first has alternate reflecting sections and openings and the second alternates reflecting and blackened sections at double the frequency of the first. The aperture is viewed by two detectors 9, 11 via beam-splitting prismatic reflector 7 and lens 6. A filter 8 in front of detector 9 passes a narrow band centred on 3.4 Á whilst a filter 10 before detector 11 does not transmit in this region. The chopper disc 4 includes means for generating synchronous signals at the double frequency. The outputs from the two detectors are each amplified and applied to synchronous rectifiers and separators. The double-frequency signals from each of the latter are applied in opposition to a potentiometer and the difference output fed back degeneratively to adjust the gain of the amplifier in the channel from detector 9. The potentiometer is adjusted for initial calibration and thereafter variations due to changes in the components are balanced out. The basic frequency signals may be applied to an indicator or recorder responsive either to the ratio or the difference between them. In a modification of the above instrument a single detector is used and the chopper disc has four concentric bands passing through the mask aperture. The two outer bands are viewed through a filter similar to filte