Fresnel lens for optical device has a oblong light source enclosed by fresnel lens covered light exit opening, whereby radius of curvature of fresnel lens is greater than half or half of width of rectangle, which encloses fresnel structure

摘要

Frensel lens has grooves made in concentric circles drawn around the center of lens and bordered by a rectangle (1), which somewhat fits the light exit opening. A part of frensel lens surface has a flat area (2), which is joined by a spherically curved surface (3) with radius of curvature (5), which is greater than half or half of width (6) of the rectangle, which encloses the fresnel lens structure. The boundary line between the curved area and flat area runs over a part of circular portion of the fresnel structure. The grooves on the fresnel structure have equal spacing, when measured in parallel direction of flat area.