Aspheric lens
[1] Aspheric elements are used in the design of multi-element wide-angle and fast normal lenses to reduce aberrations.
Not related to the optical quality, they may give a thinner lens, and also distort the viewer's eyes less as seen by other people, producing better aesthetic appearance.
[4] Small glass or plastic aspheric lenses can be made by molding, which allows cheap mass production.
Lenses produced by these techniques are used in telescopes, projection TVs, missile guidance systems, and scientific research instruments.
Diamond turning is slow and has limitations in the materials on which it can be used, and the surface accuracy and smoothness that can be achieved.
Dual rotating axis grinding can be used for high index glass that isn't easily spin molded, as the CR-39 resin lens is.
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.
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.
Computer-generated holograms (CGHs) represent a method for the interferometric determination of the deviation of the lens from the nominal geometry.
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.
Convex aspheric curvatures are used in many presbyopic vari-focal lenses to increase the optical power over part of the lens, aiding in near-pointed tasks such as reading.
They are not commercially available from optical dispensaries, but rather must be specially ordered with instructions from the fitting practitioner, much like how a prosthetic is customized for an individual.
High minus lenses cause an image so small that shape and form aren't discernible, generally at about −15 diopters, while high plus lenses cause a tunnel of imagery so large that objects appear to pop in and out of a reduced field of view, generally at about +15 diopters.
The aspheric curvature on high plus lenses are ground on the anterior side of the lens, whereas the aspheric curvature of high minus lenses are ground onto the posterior side of the lens.
Progressive add reading portions for plus lenses are also ground onto the anterior surface of the lens.
The Visby lenses found in Viking treasures on the island of Gotland dating from the 10th or 11th century are also aspheric, but exhibit a wide variety of image qualities, ranging from similar to modern aspherics in one case to worse than spheric lenses in others.
[10] Francis Smethwick ground the first high-quality aspheric lenses and presented them to the Royal Society on February 27, 1667/8.
[11] A telescope containing three aspheric elements was judged by those present "to exceed [a common, but very good telescope] in goodness, by taking in a greater Angle and representing the Objects more exactly in their respective proportions, and enduring a greater Aperture, free from Colours.
Refractive and reflective optical properties can be tabulated as a function of wavelength, to approximate system performances; tolerances and errors can also be evaluated.
In addition to focal integrity, aspheric lens systems can be tested for aberrations before being deployed.
Trilobites, one of the earliest types of animal with sophisticated eyes, had lenses with two aspheric elements.
