| 摘要 |
1337041 Measuring visibility DEFENCE SECRETARY OF STATE FOR 13 Jan 1972 [15 Jan 1971] 2096/71 Heading G1A In an apparatus for measuring visibility including a light source 4 and a detector 6 receiving scattered light the angle of intersection of light beam 5 and the field or view of the detector 6 and the distance of the point of intersection from the detector are kept constant but the point of intersection is caused to scan the atmosphere by rotating the source 4 and detector 6 together on a shaft 1, Fig.1, as shown, or by keeping the source and detector stationary on either side of two mirrors mounted on opposite ends of a horizontal rotatable shaft Fig.2, (not shown). Alternatively, the position of the two mirrors and the source and detector may be reversed with the mirrors stationary. The shaft in the Fig.1 arrangement may be omitted and the source and detector coupled together by the follow-up servosystem so that they move in synchronism with each other. The instrument may be calibrated by rotating the beams to a horizontal position and comparing the measurements made along ground level paths by a transmissometer of conventional type. Since in this invention the detected light intensity varies as a function of fog density which rises to a peak at a particular value of fog density and then decreases for denser fog, there is the possibility of ambiguous measurements and a second detector may be provided and mounted so that it receives light scattered from within the first beam path but from a different part of said beam. The detector's outputs may then be used to eliminate any ambiguity |
