| 摘要 |
A cylindrical astigmatic lens is used conventionally in detecting focussing errors in an optical pickup system. The inventive astigmatic lens is comprised of an orthorhombic base, made of a refractive material, provided with two parallel surfaces and a centerplane perpendicular thereto, and two sets of M pentahedrons, made of the same refractive material, placed on one of the two parallel surfaces thereof in such a way that each set of M pentahedrons is symmetric with respect to the centerplane. Each of the pentahedrons consists of three rectangular surfaces and two right angled triangular side surfaces, each of the triangular side surfaces being defined by a height Hm, a base Rm and an inclination. Since the height is the same in all of the triangular side surfaces in the M pentahedrons, the dimension of each pentahedron in the inventive astigmatic lens is defined by Rm, Rm being expressed as <IMAGE> wherein f represents the focus length and lambda is the wavelength of the light source with the inventive astigmatic lens, it is possible to improve the alignment accuracy of the optical pickup system, since the inventive astigmatic lens being planar.
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