Abstract: An optical fixed-distance sensing apparatus comprising a light source, a receiving lens, a light-shielding plate and a photo sensor is provided. The light source is adapted to project a beam of light upon an object and get a reflected beam therefrom. The receiving lens is set up at a point along the beam path after the object. The light shielding plate comprises an aperture located at the focal point of the receiving lens. The photo sensor is set up at a point along the beam path after the light shielding plate.

Abstract: A polarization-type laser detection system is provided, which is capable of distinguishing between a reflected light signal from a target object and a back-scattered light signal from a mass of suspended particles in the air. This allows the laser detection system to be reliably operable under bad weather conditions when the air is filled with suspended particles. The laser detection system includes a laser means for producing a linearly-polarized laser beam which is the directed to a specified direction for target detection. When echoed back, the echoed signal is first polarized by an analyzer lens into a direction that is perpendicular to the direction in which the emitted laser beam is polarized. Further, a Fresnel lens is used for focusing the emerging light from the analyzer lens into a focal point where an optical detection module is mounted.