Aspheric lens
From Wikipedia, the free encyclopedia
An aspheric lens or asphere is a lens whose surfaces have a profile that is neither a portion of a sphere nor of a circular cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens.
The asphere's more complex surface profile can eliminate spherical aberration and reduce other optical aberrations compared to a simple lens. A single aspheric lens can often replace a much more complex multi-lens system. The resulting device is smaller and lighter, and possibly cheaper than the multi-lens design.
Aspheric lenses are also sometimes used for eyeglasses. These provide clearer vision and a wider unaberrated field of view than conventional eyeglass lenses, and also distort the viewer's eyes less as seen by other people, producing better aesthetic appearance. The complex front surface curves from the center of the lens to the edge. With a prescription for the farsighted, the lens curve flattens toward the edge of the glass, and in the nearsighted, the surface becomes steeper to the lens edge, which offers vision superior to a conventional lens.
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[edit] Manufacture
Small glass or plastic aspheric lenses can be made by molding, which allows cheap mass production. Due to their low cost and good performance, molded aspheres are commonly used in inexpensive consumer cameras, camera phones, and CD players. They are also commonly used for laser diode collimation, and for coupling light into and out of optical fibers.
Larger aspheres can be made by diamond turning, a process in which a computer-controlled lathe directly cuts the desired profile into a piece of glass or another optical material. This is a slow process. A faster and newer technology is deterministic microgrinding, where computer-controlled grinding wheels are used to shape the aspheric profile which is then polished to the final shape. Lenses produced by these techniques are used in telescopes, projection TVs, missile guidance systems, and scientific research instruments.
Another method for producing aspheric lenses is by depositing optical resin onto a spherical lens to form a composite lens of aspherical shape. Plasma ablation has also been proposed, such as by RAPT Industries.
As defined by the adjective, an aspheric lens does not have a spherical curvature. Carrier curvatures are blended from a spherical, into an aspherical curvature by grinding the curvatures off-axis. Dual rotating axis grinding can be used for high index glass that isn't easily spin molded like the CR-39 resin lens is.
[edit] History
In 984, Ibn Sahl first discovered the law of refraction, usually called Snell's law,[1][2] which he used to work out the shapes of anaclastic lenses that focus light with no geometric aberrations.
Francis Smethwick ground the first high-quality aspheric lenses and presented them to the Royal Society on February 27, 1667/8.[3] A telescope containing four aspheric elements was judged superior to a "common, yet very good" telescope used for comparison, and aspherical reading and burning glasses also outdid their spherical equivalents.
[edit] Aesthetic curvature combinations in an aspheric lens prescription
High minus lenses, especially finished in a plastic resin lens, have dangerously curved edges that do not bevel off sufficiently to protect the eye from injury. Serious injury to the eye is often seen from blunt trauma, when the edge of a thick lens has been mounted in a poorly fit frame. Bi-concave lens design is different from the usual plus four base curvature ordered in thin lens prescriptions, but by splitting the curvature in thirds or so, a thinner lens is developed, although costing more, and more difficult to mount into a frame.
[edit] Ray tracing techniques for finding the change of aspheric lens focal lengths
Beginning at a point source of light, diverging rays are refracted through an aspheric lens. Emerging from imaginary space, the collimated light passes through a ring aperture having an open ring center a given radius from the optical center of the aspheric lens. This blocks light rays that are out of focus, but allows the rays of light passing through the correct curvature gradient of the aspheric lens to be refracted with the correct power of diopters.
[edit] References
- ^ K. B. Wolf, "Geometry and dynamics in refracting systems", European Journal of Physics 16, p. 14-20, 1995.
- ^ R. Rashed, "A pioneer in anaclastics: Ibn Sahl on burning mirrors and lenses", Isis 81, p. 464–491, 1990.
- ^ (1668) "An Account of the Invention of Grinding Optick and Burning-Glasses, of a Figure Not-Spherical, Produced before the Royal Society" (PDF). Philisophical Transactions of the Royal Society 3: 631. Retrieved on 2006-11-03.
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