The resolving power of a lens is limited primarily by diffraction, and by monochromatic and chromatic aberrations. The basic design form of a lens is generally determined by the intended application. However, after the basic design form of a lens has been determined, the lens designer ordinarily tries to correct the basic design form to optimize performance by minimizing monochromatic and chromatic aberrations. If the basic design form of a lens were left uncorrected, the five so-called "Seidel aberrations" (viz., third-order monochromatic spherical aberration, coma, astigmatism, distortion, and field curvature) would dominate the performance.
The first task of the lens designer in correcting for monochromatic and chromatic aberrations is ordinarily to balance the third-order monochromatic aberrations against the fifth-order and higher-order monochromatic aberrations so that a monochromatic aberration balance is achieved, which optimizes performance for the particular design form. Correction of the basic design form of a lens to minimize monochromatic aberrations is accomplished primarily by controlling the geometry of the design.
In addition to the limitations on performance imposed by monochromatic aberrations, there are also performance limitations imposed by paraxial chromatic aberrations, which are the first-order and higher-order axial chromatic aberrations (also called primary axial color and secondary axial color), and the first-order and higher-order lateral chromatic aberrations (also called primary lateral color and secondary lateral color). A lens can be designed so that axial color and lateral color are eliminated (i.e., so that "color correction" is achieved) at a desired number of wavelengths, provided that compatible optical materials for obtaining the desired color correction are used for the refractive elements of the lens, and provided also that appropriate geometrical parameters for the design form of the lens are found. Elimination of axial and lateral color cannot be achieved, unless compatible optical materials are used for the refractive elements of the lens. Unless compatible optical materials are used, no amount of adjustment of the geometrical parameters of the design form of the lens can result in elimination axial and lateral color.
A technique for selecting compatible optical materials for the refractive elements of a lens so as to make possible the elimination of first-order axial chromatic aberration at a specified number of wavelengths for paraxial rays passing through the lens was disclosed in co-pending U.S. patent application Ser. No. 419,705. A lens designed so that first-order axial chromatic aberration has been eliminated for paraxial rays at particular wavelengths is said to be "color-corrected" at those wavelengths. The elimination of first-order axial chromatic aberration for paraxial rays passing through a lens concomitantly reduces higher-order axial chromatic aberrations for paraxial rays, and also reduces first-order and higher-order lateral chromatic aberrations as well.
Unless compatible optical materials are used for the refractive elements of an optical system, no amount of effort by the optical designer in adjusting the geometrical parameters of the system can result in elimination of paraxial chromatic aberrations. However, if compatible optical materials are used, the optical designer, by creatively practicing his skill, may then be able to develop a design form in which first-order chromatic aberrations (axial and lateral) are substantially eliminated at a desired number of wavelengths, and in which higher-order chromatic aberrations are concomitantly minimized at wavelengths between the wavelengths at which the first-order paraxial chromatic aberrations are eliminated. It is noted, however, that even when compatible optical materials are used, the development of a design form for an optical system that is to be color-corrected at a desired number of wavelengths is generally not a matter of mere routine. In order to design a lens with zero first-order paraxial chromatic aberration at a specified number of wavelengths, it is generally necessary for the lens designer to invent a novel design form, even when compatible optical materials for the refractive elements of the lens are known a priori.
After the design form of a lens that is to be used for a particular application has been developed and corrected with respect to monochromatic aberrations and paraxial chromatic aberrations, a further limitation on performance (to wit, a limitation on resolving power) might still remain due to chromatic variations of the monochromatic aberrations. The seriousness of the effect of the chromatic variation of any particular monochromatic aberration upon the resolving power of a lens ordinarily depends upon the application for which the lens is intended. For lenses used in wide-field applications, the chromatic variation of coma ordinarily imposes the most significant limitation on resolving power.
Until the present invention, there was no general approach known to optical designers for minimizing chromatic variations of the monochromatic aberrations (and particularly the chromatic variation of coma) in a lens.