Spherical components are relatively inexpensive, partly because of improvements in lapping processes that allow the simultaneous processing of multiple lenses. Although time-consuming, lapping is capable of producing high-quality polished surfaces that deviate from the designer’s specifications by as little as a few hundredths of a wavelength. The result is high-precision, cost-effective lenses.
Aspherical lenses allow an expansion in the optical designer’s solution set. In an aspherical lens, the surfaces can be nonspherical. The addition of high-order curvature to an otherwise spherical surface in an aspherical lens permits independent correction or balancing of spherical aberration. That leads to a reduction in the number of lens surfaces needed for aberration-corrected imagery.
Because they can reduce lens surfaces, the use of aspherical lenses can improve transmission and reduce the weight, space requirements, and cost of optical systems. Although their potential for improving performance of an optical design had been known for many years, their use had been limited by the inability to produce them reliably and accurately. Advances in manufacturing process technologies—including deterministic computer numerically controlled (CNC) manufacturing equipment and processes, single-point diamond turning and grinding, polymer molding, glass molding, and precision metrology—have made possible the manufacture and use of these important surface geometries.52 Among the diverse applications of high-precision aspheric lenses (aspheres) are military aerospace systems, optical data storage, photolithography, and astronomy. Lower-precision aspheres have a wider range of application, including photography and video imaging (especially zoom lenses), such medical instruments as endoscopes, telecommunications, and document scanners and printers. At the low end of the market, aspheres find use in such applications as condenser elements for illumination. Asymmetrical aspheres are also becoming important, especially in conformal applications, in which the outer surface of an optical component must conform to the aerodynamic shape of an aircraft or missile.
Processes and equipment available to fabricate optical surfaces, particularly aspherical surfaces, have undergone notable improvement during the last decade. Improvements have been made in both the ability to produce and the ability to measure precision optical components. Improvement has included nearly all aspects