Patent ID: 8134692

Claim:
A fog detector placed in a vehicle interior that is segregated from an exterior by a translucent protective wall, the fog detector comprising: an emitting/receiving device that includes a light emitting element for emitting light waves and a light receiving element for receiving light waves, wherein the light emitting element and the light receiving element are placed in such a manner that the light axes of the elements cross at a crossing point of the light axes; a signal processing device that provides a transmission signal having a predetermined frequency to the light emitting element for emitting the light waves and performs a synchronous detection of a received signal corresponding to the received light waves provided from the light receiving element so as to extract a signal corresponding to a reflected light waves reflected from the crossing point of the light axes, wherein the synchronous detection is performed using a reference signal having the same frequency as the transmission signal and the same phase as the reflected light waves reflected from the crossing point of the light axes; and a determination processing device that determines whether or not fog exists based on results of the synchronous detection performed by the signal processing device, wherein the light emitting element and the light receiving element have a characteristic such that a light receiving angle 2θ 1 of the light receiving element and a spread dimension 2θ 2 of the emitted light from the light emitting element satisfy the following formula: y < 2 ⁢ x tan ⁡ ( ϕ 1 + θ 1 ) + 2 ⁢ x tan ⁡ ( ϕ 2 + θ 2 ) y > x tan ⁡ ( ϕ 1 + θ 1 ) + x tan ⁡ ( ϕ 2 + θ 2 ) where “x” refers to the linear distance between an element placing surface and the crossing point of the light axes, the light emitting element and the light receiving element being placed on the element placing surface, “φ 1 ” refers to a light axis angle of the light receiving element relative to the element placing surface, “φ 2 ” refers to a light axis angle of the light emitting element relative to the element placing surface, and “y” refers to a placement distance between the light emitting element and the light receiving element.