| 摘要 |
PURPOSE:To correct aberration generated by a wave front converting element efficiently in accordance with the change of wavelength of luminous flux from a light source part by changing optical path length between the wave front converting element and a surface on which the luminous flux is image-formed by said element. CONSTITUTION:An information recording surface 24 of an optical disc is arranged so as to coincide with the focusing point 0' of paraxial rays, so that an air interval 22' is set up to D'2. When laser wave-length is lambda1>lambda0, the focal distance is shortened, so that D'2 is made smaller than D2. However, light 31 diffracted by the most-peripheral part of a lens is focused on this side of the focusing point 0' of the paraxial rays. The figure shows the state converting the thickness of a cover glass from D1 to D''1 to correct said aberration. The change of the cover glass thickness moves the focusing point of the paraxial rays to 0'' and changes air thickness 22' also to D''2. When the change of the cover glass thickness D''1 is proper, the geometrically optical aberration of luminous flux 31' on the most peripheral part of the lens can be turned to ''0'' as shown in the figure. |
