Abstract: An optical pickup for performing a reproduction or recording operation of information on one of different kinds of optical information recording medium, each having a transparent base board in different thickness. The optical pickup includes: a laser beam generator; a light converging optical system for converging luminous flux from the laser beam generator through the transparent base board of the optical information recording medium onto an information recording surface of the optical information recording medium; a changing member for changing an optical path of the luminous flux, being reflected on the information recording surface, to be different from an optical path of the luminous flux, being emitted from the laser beam generator; an photo-detector for detecting the luminous flux being reflected on the information recording surface; and a compensation element, provided between the changing member and the photo-detector, for correcting spherical aberration of the luminous flux.

Abstract: A light control sheet for receiving a light incident on one surface thereof and for emitting the received light from the other surface thereof, includes plural prisms which is provided at least one of surfaces of the sheet. Each of the prisms has a generatrix in parallel to each other, and has two side surfaces formed on both sides with respect to a plane surface which includes the generatrix and is perpendicular to the light control sheet. The two side surfaces have asymmetrical shapes each other with respect to the plane surface, and at least one of the two side surfaces is nonplanar.

Abstract: A plane light source unit includes a tubular light source, and a light guide. The light guide has a light emitting surface and a reflecting surface in addition to a light incident face placed near the tubular light source to receive light emitted from the tubular light source. The light guide has a plurality of projections or grooves arrayed on the light emitting surface to extend parallel to each other along a longitudinal direction of the tubular light source so as to cause the light incident from the light incident face to emerge from the light emitting surface. The height or depth of each of the projections or grooves increases from the central portion to the two end portions in the longitudinal direction of the tubular light source.

Abstract: In a laser beam-scanning apparatus equipped with a rotatable polygonal mirror and an image formation lens system, a lens of the lens system has a cylindrical surface curved in the secondary scanning direction, wherein a radius curvature "rh" on the primary scanning direction section of the cylindrical surface is continuously changed in such a manner that a change pattern in the radius curvature "rh" is non-symmetrical in relation to the optical axis.

Abstract: A magnetic recording medium which comprises a non-magnetic substrate, a magnetic film formed on the substrate and a protective film provided on the magnetic film, the protective film being composed of at least two groups of element, the element in one group being at least one of B and C having a smaller atomic size, and a concentration of the element increasing from the interface side of the protective film with the magnetic film toward the opposite face side throughout the protective film has an improved durability against a magnetic head.