The present invention is in the field of graded-index optics and particularly relates to a singlet graded-index lens useful as a microscopic objective for video-disk lens pickup systems or the like. For applications such as video-disk lenses, it is important that the lens exhibit very low monochromatic aberrations, particularly low spherical aberration, while at the same time being inexpensive. While multi-element and aspheric lenses have been considered for this application, a spherical singlet lens exhibiting the required low aberration could be much more economical to produce.
Graded-index lens designs have been proposed which reduce third-order aberrations. For example, U.S. Pat. No. 3,729,253 to Moore et al. describes spherical lenses with radial refractive index gradients which exhibit reduced third-order spherical aberration, coma and astigmatism. These lenses can be used with an appropriately placed stop to provide an optical system wherein third-order distortion is also reduced. However, the lenses specifically described by Moore et al. are not suitable for video-disk lenses because of their excessively high fifth-order spherical aberration which seriously degrades the axial imaging characteristics of the lens. For video-disk applications, all orders of spherical aberration must be near zero to obtain essentially defraction-limited axial imaging capability.
K. Kikuchi et al., Applied Optics, 19(7), 1076-1080, describe lens designs for video-disk applications with reduced spherical aberrations. However, their solution requires adjustment of the refractive index profile of the lens to a very high degree. As is known, the refractive index profile of a gradient index medium wherein the gradient is radial, i.e., varies with distance from an optical axis through the medium can be defined by a power series expansion in radius r as follows: EQU N(r)=N.sub.00 +N.sub.10 r.sup.2 +N.sub.20 r.sup.4 +N.sub.30 r.sup.6 . . .
wherein N(r) is the refractive index of the medium at radius r from the optical axis and N.sub.00, N.sub.10, N.sub.20 and N.sub.30 are the zeroth-, second-, fourth-, and sixth-order coefficients of the power terms. In accordance with Kikuchi et al. the correction of spherical aberration to obtain improved axial imaging requires control of the sixth-order coefficient N.sub.30 of the power series expansion defining the refractive index profile of the medium. This degree of control over the refractive index profile is difficult to attain.
It is therefore a principal object of the present invention to provide designs for video-disk lenses or the like wherein reduced spherical aberration is obtained without the need for such critical index profile control.
Other objects and advantages of the invention will become apparent from the following description thereof.