What are Aspheric Lenses?

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- GoPhotonics

Oct 15, 2022

An aspheric lens is a lens with one or more of its surfaces having a non-spherical surface profile. i.e., its profile deviates from the structure of various simple lenses like the plano-convex lens, biconvex lens, plano-concave lens, etc. as shown in the figure below. Like spherical lenses, aspheric lenses are also rotationally symmetric. They provide improved optical performances, and better image quality, and eliminate or reduce spherical aberrations that are present in the spherical lenses. Spherical Aberration is an optical effect in which the light rays passing through the lens focus at different points on the optical axis resulting in the formation of a blurred image.

A single aspheric lens can be used to replace a complex spherical multi-lens system. This will allow for designing smaller, lighter, & cheaper optics with a better compact assembly compared to multi-lens systems based on simple spherical lenses. They are widely used in various optics-enabled fields like health care, microscopy, photography, lasers, etc.

Aspheric surfaces can be designed using the mathematical expression:

where c = 1/(radius of curvature) and k = - e2 with e as the eccentricity of the conic surface. Some standard aspheric surfaces that can be designed using the above equation are given in the table below.

Computer-controlled devices along with software are available to easily design and fabricate these aspheric lenses. They can produce very accurate optical surfaces with exceptional quality.

Spherical aberration correction

Ideally, the parallel rays after refraction in a lens should converge to a single point. But in reality, the rays passing close to the optical axis of a spherical lens will experience less refraction while the light rays close to the edge of the lens will experience more refraction. As a result, the light rays close to the optic axis will converge to a point at a larger distance on the optical axis while those close to the edge of the lens will converge at a point closer to the lens. i.e., the parallel light rays from different regions after refraction do not focus on a point. This will result in blurring of the image and it affects the resolution of the image. A solution to this problem is to use an aspheric lens whose surfaces are modified such that each of the incident parallel rays after refraction will converge to a single point.

Aspheric lenses are ideal for bar code scanners, laser diode collimation, fiber coupling lenses, data storage objectives, microscope imaging objectives, medical devices, night vision imaging optics, vision correction eyeglasses & ophthalmic tools, Industrial laser machinery, camera, and OEM or R&D integration applications.