Abstract: A beam converting optical system is provided which is easy to position the optical elements, which has a comparatively short total length, and which has good aberration correction. A beam converting optical system converts light beams with an oval cross-sectional shape into parallel light beams with a substantially circular cross-sectional shape. A converting lens is provided which has at least one surface that is cylindrical. The horizontal cross section of the cylindrical surface of the converting lens is essentially non-circular. The converting lens may consist of two similar elliptic cylinder surfaces. The two ellipses are of the shape of a horizontal cross section of the two elliptic cylinder surfaces and have the same focal point.

Abstract: An achromatic optical system for beam transformation is used for example with the optical head of an optical disk apparatus. This optical system is arranged in an optical path which is traversed by a beam emitted from a light source to reach a surface to be irradiated. This optical system includes a first prism arranged on the side of the light source and a second prism arranged on the side of the surface to be irradiated. The first and second prisms are constructed in such a manner that each of the prisms has a beam incident angle which is greater than its beam exit angle so as to expand the width of a beam passed therethrough and that with respect to a plurality of transmitted beams of different wavelengths the deviation angle of the beam passed through the optical system on the whole (the angle formed by the incident beam on the first prism and the emerging beam from the second prism) becomes the same for each other.