METHOD OF AND APPARATUS FOR MEASURING THE EXTINCTION COEFFICIENT OF AN ATMOSPHERIC SCATTERIN MEDIUM

摘要

1426096 Measuring visibility &c. photo-electrically SPERRY RAND CORP 23 May 1973 [1 June 1972] 24553/73 Heading G1A The extinction coefficient, and thus the visibility of an atmospheric scattering medium is determined by providing a transmitter for generating a pulse of radiation to irradiate the scattering medium, providing a receiver proximate the transmitter having a field of view overlapping substantially the full length of the transmitter field for receiving radiation backscattered from the atmospheric medium, sampling received radiation backscattered at first and second discrete ranges from the transmitter, and compensating the radiation samples for amplitude differences therebetween attributable to the differential range of the respective samples. As shown in Fig. 1b, light pulses are transmitted by a source 13, e.g. optically pumped crystals or injection diode laser arrays, and a lens 12, the backscattered radiation being received by the lens 12 and focused on to a detector 17 by a lens 16. A mask 18 prevents direct light from source 13 falling on the detector 17. As shown in Fig. 4, a control unit 21 controls the pulse rate and duration of the light pulses, and also the opening instants and durations of a pair of gates 22, 23, which thereby sample the output of detector 17 at the two ranges. The outputs of the gates are applied to respective integrators 24, 25 which compensate for amplitude differences attributable to the range difference by integrating respective numbers of pulses received in proportion to the square of the first and second ranges. The ratio of the outputs of the integrators is determined in a divider 26 whose output is applied to a log computer to calculate the extinction coefficient. A plurality of measurements could be made at uniform range increments of e.g. 100 feet. Alternatively the measurements may be made in spaced pairs e.g. 100 feet apart at 300 feet intervals. When a plurality of samples are taken, the measurements may be made either simultaneously or sequentially. In sequential mode, each measurement is stored so that the average or mean value can be calculated. In simultaneous mode, additional pairs of gates and integraters with associated divider and logarithmic units are required. The apparatus can also determine the location of points of discontinuity in the scattering medium.