2. Shape and wave defects

Optical surfaces have always been produced by lapping, which results in components with excellent surface finish and very little waviness. On the other hand, this method can give rise to shape defects (figure 2 ). The most common of these are axially symmetrical defects known as concave or convex crowns, or asymmetrical saddle-horse deformations, often due to the lens being fixed under stress. These shape defects affect image quality, and their influence can be simulated using aberration theories. Over the last few decades, research has also been carried out into new ways of surfacing spheres and, above all, aspherical surfaces. A wide variety of methods are used, including vacuum evaporation, ion bombardment, polishing, grinding and turning. Each has its own field of application....