The advantages of aspheres in optical designs, such as better optical performance and a reduced number of elements, as well as the difficulties in fabricating them, have been known for several years [1, 2]. New design approaches  based on aspherical shapes and varying production lots from 10 2 to 10 6 lens elements require rapid, cost-effective fabrication processes. Optic designers will never make use of aspheres in their designs if not convinced that the fabrication processes will deliver the required number of lenses in time. The economic fabrication process is one of the key elements for making aspheres attractive for the optical designer.
In Fig. 5.1, past progress in generating and polishing is shown. Major milestones are the realization of the copy principle, the application of the computer, and the invention of fine correction methods without a polishing pad. The different kinds of polishing and generating are explained in the following sections.
As an introduction, we start with the principle of manufacturing spherical optics, which is based on the identity of the surface functions of complementary bodies, here tool and workpiece, as shown in Fig. 5.2. The lens (above) is moved over the rotating, full-sized stiff tool (below). The tool, which is in contact with the overall surface of the lens, generates a negative copy of its shape on the lens.
Using the same kind of relative movement between lens and tool, the 2D symmetry of the surface is lost for aspheres. In this case the area of contact between a stiff tool and the workpiece is reduced to a line or even to a point contact; for example, the meridional line, as an axis of symmetry, can be used as a line of contact.
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