Patent GB-1150502-A (GB)
Title: Visibility Measuring System
IPC: G01N21/53
Filed: 1966-06-15
Abstract: 1,150,502. Measuring visibility. SPERRY RAND CORP. June 15, 1966 [June 17, 1965], No.26691/66. Heading G1A. [Also in Divisions G4, H1 and H4] Atmospheric visibility is measured by transmitting a light pulse into the atmosphere, detecting the backscattered light and determining one of the characteristics of the backscattered pulse e.g. the time of occurrence 7 of the peak of the backscattered pulse with respect to the transmitted pulse, the width 6 of the peak at half amplitude, or the slope of the trailing edge 5. The source of light may be a ruby laser producing 20-30 nanosecond pulses. The backscattered radiation is detected by a receiver located adjacent the transmitter so that the transmitted and received beams are substantially parallel. The shape of the backscattered pulse depends on the volume of scattering material (fog, cloud &c.) in the radiation path and on absorption by the material as the radiation returns after scattering. Each particular characteristic is determined by a specialized circuit and the outputs of this circuit applied to a computer, again specific to each characteristic, which calculates the extinction coefficient # of the atmosphere. From this coefficient #, the visibility can be measured using Koschmieder's Law (daylight) or Allard's Law (artificial light). In one embodiment, Fig.4 (not shown), in which the extinction is calculated from the width of the backscattered pulse at half the peak amplitude, the transmitted pulse is detected, immediately by one detector (19), and, after scattering and absorption by the atmosphere, by the receiver (9). The pulse is normalized to a predetermined value by an amplifier (10) and clipped by a device (2) which passes the top half of the pulse, and whose output is converted to a rectangular pulse, by a Schmitt trigger (13). This pulse switches on and off a counter (14) which counts pulses from an oscillator (15) during the time it is on, representative of the duration t 2 -t 1  of the pulse at half its peak amplitude. The leading edge of the pulse from the trigger (13) also switches off a counter (18), which counts from the time it directly receives the transmitted light pulse, so that an output representing the time t 1  of occurrence of the leading edge of the light pulse, is produced. The terms t 2 -t 1  and t 1  are applied to the extinction  coefficient computer (17). In the computer, Fig.7 (not shown), these terms are converted by amplifiers (25, 26) to signals representing range r 2 -r 1 , and r 1 , respectively. These signals are applied to summation circuit 30 to produce a term r 2 . The terms r 1 , r 2  are operated on by identical reference backscatter function generators (32, 33) to compare them with reference extinction curve values, producing outputs y 1 , y 2 , respectively, the logarithm of the ratio of which log y 1 /y 2 , is produced by circuits (36, 38, 40). This term is multiplied by the term K/r 2 -r 1 , produced by operation on r 2 -r 1  by an inverse function generator (27) and amplifier (29), and the result compared with a reference source (43) to produce a value of the extinction coefficient for the spacial area between ranges r 2  and r 1 . The visibility is then calculated. In a second embodiment, Fig.5 (not shown), the extinction coefficient of cloud or fog patches at different ranges may be calculated by measurement of the slope of the trailing edge of a logarithmic curve of the backscattered radiation. The log. value of the receiver output is differentiated (67) to measure this slope and a variable delay circuit (73) used to pick out the regions of the atmosphere of interest, which are at different ranges. An extinction computer (77) utilizes signals representing the slope (76) and the range (78). In a third embodiment, Fig.6 (not shown), an extinction computer (56) receives only a signal representing the time of occurrence of the backscattered peak, which signal is produced by twice differentiating the amplified receiver output.