resin lens is. Techniques such as laser ablation can also be used to modify the curvature of a lens, but the polish quality of the resulting surfaces is not as good as those achieved with lapidary techniques. Standards for the dispensing of prescription eyeglass lenses discourage the use of curvatures that deviate from definite focal lengths. Multiple focal lengths are accepted in the form of bifocals, trifocals, vari-focals, and cylindrical components for astigmatism. Metrology Measurement technology plays a decisive role in the manufacturing of aspherical lenses. Depending on the manufacturing process and processing status, various measurement tasks are distinguished: form of asphere surface form deviation slope error centre thickness roughness A distinction is made between tactile, i.e. touching, and non-contact measurement methods. The decision as to which method is used depends on accuracy but also on manufacturing state. Tactile measurement Tactile measurement is mainly used between two grinding operations to control the shape of the asphere and to adjust the following operation. A profile gauge probe is used to measure a section across the lens surface. The rotation symmetry of the lenses means that the combination of several of these profiles provides a sufficiently precise knowledge of the shape of the lens. Any damage to the lens surface caused by the probe tip would be removed in subsequent steps. Non-contact measurement Interferometers are used when measuring sensitive or polished surfaces. By superimposing a reference beam with the beam reflected from the surface to be measured, error maps, known as interferograms, are created which represent a full-field deviation of the surface shape. Computer-generated hologram (CGH) Computer-generated holograms (CGHs) represent a method for the interferometric determination of the deviation of the lens from the nominal geometry. These generate an aspherical wavefront in the target shape and thus enable the determination of deviations of the lens from the target shape in an interference image. CGHs must be manufactured specifically for each test item and are therefore only economical for series production. Interferometric measurement Another possibility is the interferometric measurement of aspheres in subareas, with minimal deviations to the best-fit sphere, and subsequent combination of the submeasurements to a full-surface interferogram. These are very flexible in comparison to CGHs and are also suitable for the production of prototypes and small series. Ophthalmic uses Like other lenses for vision correction, aspheric lenses can be categorized as convex or concave. Convex aspheric curvatures are used in