Patent GB-1512245-A (GB)
Title: Detection of backscattered radiation
IPC: G01N21/53
Filed: 1975-08-11
Abstract: 1512245 Measuring scattered radiation HSS Inc 11 Aug 1975 [19 Aug 1974] 33410/75 Heading G1A In apparatus for measuring back-scattered (including reflected) radiation along a predetermined path in a medium, an optical arrangement has a substantially constant acceptance angle, and therefore a uniform sensitivity to back-scattered power, irrespective of the point of origin of returning rays within a predetermined range along the path. The apparatus may further include means for determining the transmission T r  of the medium  at some point of interest distant r along the path, given by where L is the range of the apparatus, and P x dx is the detected power received from the medium in an element of path length dx at distance x. In a first embodiment, Figs. 1 and 2, a narrow substantially parallel laser beam B 1  is directed by a semi-transparent mirror 8 along a path A (e:g. an airport landing path). Back-scattered radiation, e.g. due to fog, transmitted by the mirror 8, is refracted by a lens 14 of pupil diameter D towards a hole H of diameter d in a plate PS arranged at the principal focal plane of the lens 14, a photodetector 18 receiving all the light transmitted through the hole H. The range L is the distance of that point from which all light falling on the lens 14 is refracted to fall just within the hole H. Within that range, that proportion of light falling on the lens 14 which is refracted through the hole H varies directly as the square of the distance e.g. x 1  and thus lies within a cone of constant solid angle subtended at x 1 . The total energy received from all points within the range L therefore represent # o <SP>L</SP>P x dx. In a second embodiment, Fig. 9, a constant solid-angle is defined by a bundle of tubes or  filaments 60 each of predetermined diameter and length, and having optically absorptive side-walls, placed adjacent a focusing lens. In a third embodiment, Fig. 11 (not shown), a pair of lenses constituting a telescope directs light to a Fabry-Perot interferometer arranged to have an orthogonal transmission peak at the laser wavelength, the acceptance angle being defined by the next adjacent interference order. The required integrals for computing T r  may be obtained by a split-beam system Fig. 7 including two plates 50 each having an annular hole defined by a concentric obstruction 52. The first plate 50 admits light to a first detector 18 for determining # o L Pxdx . The second plate is followed by a lens 32 arranged to image the lens 14 on a second detector 35 at a plane P. A concentric obstruction at plate P is arranged so as to intercept any light returned from any point within the range r, so that the detector 35 supplies a signal representing #<SP>L</SP> r Pxdx. In a second embodiment for determining #<SP>L</SP> r Pxdx Fig. 5 (not shown) similar to the apparatus of Fig. 1, the integral #<SP>r</SP> o Pxdx is first determined; then the integral #<SP>r</SP> o Pxdx is determined by temporarily inserting a dull black body to intercept all light returned from points beyond distance r; the required integral is finally calculated by subtraction.