GB1242479A - Optical transmission responsive apparatus - Google Patents

Abstract

1,242,479. Measuring visibility photoelectrically. COMPAGNIE DES COMPTEURS. Sept.5, 1969 [Sept.5, 1968; Dec.23, 1968; March 28, 1969], No.44094/69. Heading G1A. In a system for measuring the transmissivity of a sample, e.g. the atmosphere, a first light source transmits light to a first detector along a path including the sample and to a second detector along a reference path and similarly with a second source which affects both the second and first detectors. The outputs of the detectors are compared to provide a measure of the transmissivity of the sample. As shown, sources 1a, 1b which are luminescent diodes supplied at different frequencies by oscillators 2a, 2b illuminate photodiodes 5b, 5a along paths 6a, 6b respectively. The photo-diodes 5a, 5b are also illuminated along optical fibre paths 7a, 7b by sources 1a, 1b respectively. The resulting signals are amplified at 8a, 8b and supplied to discriminators 9a, 9b; 10a, 10b which provide four signals A, B, C, D for unit 15. The transmissivity of the sample is determined from the ratio B Î C / A Î D. The equipment at the ends of the transmission path is constructed as a unit, Fig.2, and comprises a metal tube 21 with a luminescent diode 1 mounted at the focus of lens 3. The detector is a photo-diode or phototransistor mounted at the focus of parabolic mirror 4. An optical fibre path 23 transmits a constant portion of the transmitted radiation direct to the detector. A colour filter 22 is provided to shield the detector and the whole assembly is mounted in a housing to protect it from solar radiation and weather. In a modification discrimination between the output signals is provided by pulsing the light sources and using synchronously operated gates to analyse the detector outputs and provide the A, B, C, D, signals, Fig.3 (not shown). Alternatively polarizing filters can be provided in the light paths that pass through the sample, Fig.6 (not shown). In a further embodiment, Fig. 5 (not shown), only a single path through the atmosphere is provided between a first light source and first detector. The paths between the second light source and first and second detectors and the first light source and second detector being provided by optical fibre paths. It is stated that the positions of the sources and detectors can be inter-changed and the source could comprise a coherent light source or filament or flourescent lamps or a discharge tube.