| 摘要 |
PROBLEM TO BE SOLVED: To highly accurately and stably measure a visibility distance without being influenced by a stain of an optical system, a measurement environment and a secular change. SOLUTION: The beam light 3 having the expansion with the optical axis 13 as the center is irradiated from a light emitter 1, a light receiver 2 is arranged in the direction for inclining to the optical axis 13 of the light emitter 1 in an irradiating area of this beam light 3, and the beam light incident on a condenser lens 24 from the different directions is individually received by two light receiving parts 21, 22 adjacently arranged in the light receiver 2 to calculate a visibility distance on the basis of the light receiving level.
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