include aspherical lenses, which have advantages over spherical lenses but are more difficult to produce. This section briefly describes the advantages of aspherical lenses and then addresses some of the recent improvements in their production (for a much more detailed description, see Appendix C). The push toward the upper end of the precision scale will drive the need for improvements in optical sources and imaging tools to support the increase in resolution.
Spherical lenses have been the workhorse of optical systems for centuries. The curved surfaces of a lens cause rays of light from a point on a distant object to come to a focus. A single lens with spherical surfaces forms an image that is not a perfect point. (See Figure 7.6.) Optical design has traditionally been a search for combinations of spherical-surfaced components.
FIGURE 7.6 Light rays are focused to a single focal point with an ideal lens (top). A lens with a spherical surface has an associated spherical aberration, and therefore has no distinct focal point (bottom). (Image is exaggerated).