Abstract: A projection-type image display device includes a transmissive image display panel having a matrix of pixels arranged at different pitches in different directions perpendicular to each other, and a planar microlens array for converging light emitted by a light source onto the transmissive image display panel. The planar microlens array comprises a substrate and a matrix of microlenses disposed on the substrate for converging the light onto the pixels, respectively, each of the microlenses being of an oblong shape. Each of the microlenses may have a longitudinal axis inclined with respect to an axis of the matrix of pixels to cover the pixel aperture of one of the pixels. The oblong shape of each of the microlenses may be composed of a pair of spaced identical semicircles and a rectangle interconnecting the identical semicircles and having sides of the same length as the diameter of the semicircles.

Abstract: The present invention relates to a process of producing a magnetic disk substrate comprising etching the surface of a glass substrate to form minute irregularities on the surface thereof, by employing the molten salt containing at least a kind of nitrite of cation, which process can prevent the deterioration of the mechanical property and magnetic property of the glass sabstrate for use as a magnetic disk for a long time.

Abstract: In a plate microlens of the invention, wherein gradient index lenses each having an optical axis along a normal to a surface of a transparent base and a refractive index profile gradually changing along a direction of the optical axis and a direction perpendicular thereto are formed integrally in the transparent base; a ratio d/a (where d is a thickness of the gradient index lens on the optical axis and 2a is a diameter of the gradient index lens on a surface of the transparent base) falls within a range of 0.46 to 0.78.

Abstract: In a plate microlens of the invention wherein gradient index lenses each having an optical axis along a normal to a surface of a transparent base and a refractive index profile gradually changing along a direction of the optical axis and a direction perpendicular thereto are formed integrally in the transparent base, a ratio d/a (where d is a thickness of the gradient index lens on the optical axis and 2a is a diameter of the gradient index lens on a surface of the transparent base) falls within a range of 0.46 to 0.78.

Abstract: An incorrect collating preventing apparatus has a photoelectrical detector including at least one optical system which comprises a lens for converting light beams from a lamp into parallel light beams, and optical means for introducing the parallel light beams through a focusing lens to a surface of each signature and for introducing substantially all of the light beams reflected by the surface of each signature to a photoelectric element, the optical means being a reflector with a hole so as to allow the reflected light beams to pass through the hole, or a transparent plate with a reflector portion where the reflected light beams are focused and are directed to the photoelectric element.