Abstract: In order to provide an optical device and a diffusion plate which has high transmittance and is capable of reducing speckle noise, the present it is a diffusion plate 1 in which a plurality of microlens cells 11 are arranged on both surfaces of a transparent substrate 10, the diffusion plate 1 having a first microlens array 12A which is formed on one surface of the transparent substrate 10 and comprises a plurality of concave or convex microlens cells 11, and a second microlens array 12B which is formed on the other surface on the reverse side from the one surface and comprises a plurality of concave or convex microlens cells 11, and the diffusion plate 1 being configured so that light emitted from the microlens cells 11 constituting the first microlens array 12A is incident on the microlens cells 11 constituting the second microlens array 12B.

Abstract: This invention provides a polarization conversion element that is highly resistant to the heat and light that result from increased brightness levels. In said polarization conversion element, inorganic half-wave plates provided selectively on the output surface of a polarizing beam-splitter array have obliquely deposited layers comprising a dielectric material and the side surfaces of said obliquely deposited layers are covered by protective films.

Abstract: This invention provides a polarization conversion element that is highly resistant to the heat and light that result from increased brightness levels. In said polarization conversion element, inorganic half-wave plates provided selectively on the output surface of a polarizing beam-splitter array have obliquely deposited layers comprising a dielectric material and the side surfaces of said obliquely deposited layers are covered by protective films.

Abstract: An optical system and pick-up device having an optical magnification suitable for three different optical disc formats and a light emitting means for emitting a laser beam having a wavelength corresponding to each disc in the optical system of the optical pickup for recording and/or reproducing an information signal to the optical discs. The optical pickup device has a first light emitting unit for emitting a laser beam having a first wavelength or a second wavelength, a second light emitting unit for emitting a laser beam having a third wavelength, a first collimator lens for transmitting the laser beams having the first or second wavelengths, a second collimator lens for transmitting the laser beam having the third wavelength, a first objective lens for transmitting the laser beams having the first or third wavelengths, and a second objective lens for transmitting the laser beam having the second wavelength.

Abstract: The present invention provides an optical pickup device which realizes an optical system having an optical magnification suitable for each disc format between three optical discs and a light emitting means for emitting a laser beam having a wavelength corresponding to each disc in the optical system of the optical pickup for recording and/or reproducing an information signal to the optical discs and which prevents the number of components from being increased to simplify the composition of the optical system, thereby preventing the length of an optical path from being lengthened, the optical pickup from being increased in size.