| 摘要 |
PURPOSE:To reduce the number of lenses and weight and make the lens usable for a semiconductor laser of a wavelength close to 8400Angstrom by consisting the lens of the first to fourth lens groups and specifying the relations between radii of curvature and the focal length of the entire system so as to be within specific values. CONSTITUTION:The lens is constituted by the lst group lens of a biconvex lens, the 2nd group lens of a negative lens and the 3rd and 4th group lenses of positive lenses from the photo detector side to the disc side. When the respective radii of curvature are let to be r2 on the 2nd group lens side of the lst group, r3 on the lst lens side of the 2nd group, r7 on the 3rd lens side of the 4th group and the focal length of the entire system is let to be (f), then the lens is so formed as to satisfy the following conditions. spherical aberrations are corrected by specifying these at 0.7<r3/r2< 1.05, the residual quantity of aberrations of the peripheral and intermediate parts are reduced by 0.7<r3/f<1.2 and astigmatisms are corrected for video discs by 0.4<r7/f<0.65. This is the less-element-number and lightweight lens of a 4-element constitution, whose NA is as large as 0.5 so as to be used for semiconductor lasers of around lambda=8400Angstrom and which is able to correct various aberrations satisfactorily. |
