Aspheric Lens

What is an Aspheric Lens?

An aspheric lens is designed for aberration correction. With the help of aspheric lens, the image formed is distortion free.

An aspheric lens is a lens whose surfaces profile neither a portion of a sphere nor of a circular cylinder. Since it is not spherical, the conventional processes no longer apply to making aspheric surfaces. This is why aspheric cost many times what spherical surfaces do.

In optics, a lens assembly that includes an aspheric element is often called an aspheric lens.

History

Early attempts at making aspheric lenses to correct spherical aberration were made by René Descartes in the 1620s, and by Constantijn Huygens in the 1630s. Francis Smethwick ground the first high-quality aspheric lenses and presented them to the Royal Society on February 27, 1667. A telescope containing four aspheric elements was judged superior to a "common, yet very good" telescope used for comparison, and aspheric reading and burning glasses also outdid their spherical equivalents.

Moritz von Rohr is usually credited with the design of the first aspheric lenses for eyeglasses. He invented the eyeglass lens designs that became the Zeiss Punktal lenses.

Theory

Technically, an aspherical lens is a lens whose curved surface does not conform to the shape of a sphere; lenses are usually ground or molded with spherical surfaces; because a spherical surface lens has difficulty in correcting distortion in ultra-wideangle lenses or coma in large-aperture lenses brought about by spherical aberration, an aspherical lens is used. Simplest way to illustrate what an aspherical lens element is all spherical aberration of a spherical lens and convergence of parallel light rays with the use of an aspherical lens element.

Rotationally symmetric polynomial aspheric surfaces are described by a polynomial expansion of the deviation from a spherical (or aspheric described by a conic) surface. The even asphere surface model uses only the even powers of the radial coordinate to describe the asphericity. The model uses the base radius of curvature and the conic constant. The surface sag is given by

where c is reciprocal of radius of curvature, k is the conic constant, r is the radial ray coordinate in lens units, and the coefficients i are indefinite.

Because of their uniaxial character, aspheric surfaces are much more difficult to fabricate than ordinary spherical surfaces. A strong paraboloid may cost an order of magnitude more than the equivalent sphere; ellipsoids and hyperboloids are a bit more difficult, and nonconic aspherics are more difficult still.

Applications

Aspheric lens can be used in a variety of applications, such as in zoom lens, astronomical telescope, DVD pick up, high power laser collimators, LED lens, glasses, highquality magnifier and so on.

Benefit

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. These lenses are small, lighter and in general, better than similar lenses which only employ spherical elements.

As magnifier, aspheric lens enhances image quality and minimizes distortion throughout the viewing area, it reduces distortion at wide angles, improves corner resolution. You can get more detail and high-resolution of the image.

Material

Nowadays, some optical plastics (e.g., PMMA) are used for sphere lens and aspheric lens. A great deal of effort was made to develop plastics for optical systems during the Second World War, and a few systems incorporating plastics were produced. Since then, the technology of fabrication of plastic optics has advanced significantly, and today, in addition to novelty items such as toys and magnifying glasses, plastic lenses can be found in a multitude of optical applications, including inexpensive, disposable camera lenses, many zoom lenses, projection TV lenses, and even some highquality camera lenses.

The low cost of mass-produced plastic optics is one important factor in this popularity; another is the ease of production of aspheric surfaces. Once the aspheric mold has been fabricated, an aspheric surface is as easy to make as is a spherical surface (in marked contrast to glass optics). The rule of thumb that the introduction of an aspheric surface allows the elimination of an element from the system attests to the value of optical plastic materials. This aspheric capability largely offsets the unfortunate fact that the number of suitable optical plastics is very small and that there are only relatively low index materials in that number.