Abstract: The invention aims to increase an expansion angle in a road surface direction in a short-distance area without setting a detection range one-sidedly and without sacrificing a maximum detection distance.A double-focus lens is integrally formed by disposing a short-focus portion in the vicinity of a center and a long-focus portion on the periphery of the short-focus portion. A light source is disposed at a distance slightly shorter than the focal length of the short-focus portion of the double-focus lens. When light is emitted from the light source to the double-focus lens, light emerging from the short-focus portion has a small expansion angle such as not to diffuse largely. Therefore, the emerging light can be caused to reach a long distance. On the other hand, light emerging from the long-focus portion has a large expansion angle such as to diffuse to some extent.

Abstract: A hologram plate manufacturing apparatus comprising a copying optical element disposed in opposed relation to a photosensitive surface of a hologram plate. The copying optical element is arranged so that its one surface facing the photosensitive surface of the hologram plate has a plane configuration and its other surface has a convex or concave configuration. On the convex or concave surface there is formed a reflection film for effectively reflecting incident light. Also included a refraction index adjusting layer which is provided between the photosensitive surface of the hologram plate and the convex or concave surface of the copying optical element, the refraction index adjusting layer having an index of refraction substantially equal to indexes of refraction of the hologram plate and the copying optical element.

Abstract: A light-guiding device having a light-incident surface and a light-outgoing surface for guiding light incident from the light-incident surface from the light-outgoing surface toward a light-receiving surface of a photodetector for generating an electric signal corresponding to a quantity of the received light. The device comprises a hologram layer having an interference pattern which acts as a diffraction grating to diffract and reflect the light, incident from the light-incident surface, in a predetermined direction. The hologram layer is interposed between first and second total reflection layers.