| 摘要 |
PURPOSE:To eliminate a difference in sphericity and to reduce coma-aberration by regarding the concave surface side of a plano-concave lens as an incident surface where parallel light strikes, forming a Fresnel zone on the plane side, and specifying the radii of rings and the thickness, refractive index, radius of curvature, etc., of said plano-concave lens. CONSTITUTION:A Fresnel zone plate 3 uses the concave surface of the plano- concave lens 19 as the incident surface where the parallel light strikes and has the Fresnel zone 2 on the plane side; and simultaneous equations I -VI hold for radii of rings, and the refractive index, thickness, and radius of curvature of the plano-concave lens 19 are so determined that the evaluated amount of residual coma-aberration is reduced, thereby reducing the spot diameter of a diffraction limit and coma-aberration in a transparent material 8 which has a >=1 refractive index. Thus, the radii rk of rings of the Fresnel zone, thickness d1 and refractive index n1 of the concave transparent plate, and thickness d2 and refractive index n2 of the parallel transparent plate are determined, and consequently the coma- aberration of projected light is eliminated even if the incident parallel light slants to an optical axis, so that a sharp spot is obtained at a convergence point 7. |
