In a broad sense, aspheric surface is other surface which does not include spherical surface and plane surface. From the perspective of application, aspheric surface can be divided into axisymmetric aspheric surface, aspheric surface with two symmetrical planes and free-form surface without symmetry.
Aspheric surface classification
Aspheric surface is usually divided into quadratic aspheric surface and higher aspheric surface. Quadratic aspheric surface is the most widely used in optical system, which has a special position compared with other types of aspheric surface. Quadratic aspheric surface can be divided into two parts: aspheric surface with a pair of non image handicaps and aspheric surface without non image handicaps. The former is widely used in various optical instruments and is the most common aspheric surface. The latter is widely used in optical systems to form distorted images. The aspheric surface of higher degree can be divided into the surface of monotone meridian curve and non monotone surface. The classification of aspheric surfaces is shown in Figure 1.
Figure 1. Classification of optical aspheric surfaces
free-form surface
The general expression of free-form surface: z = ∑ AIJ xiyj I, j = 0,1,2... N. For this kind of free-form surface, although it has lost the characteristic of aspheric axis symmetry, it is still around a coordinate origin. Now there are rules to follow.
Another kind of complex free-form surface, which can only be defined by three-dimensional point coordinates, is widely used in progressive multifocal glasses to achieve the purpose of power distribution according to a specific law.
Advantages and application of aspheric surface
Aspheric lens, the radius of curvature changes with the central axis, to improve the optical quality, reduce the optical components, reduce the design cost. Compared with spherical lens, aspheric lens has unique advantages, so it has been widely used in optical instrument, image and optoelectronic industry, such as digital camera, CD player and high-end micro instrument. It has the following functions:
a. Spherical aberration calibration
Aspheric lens is used to replace spherical lens, the most significant advantage is that it can correct spherical aberration caused by spherical lens in collimation and focusing system. By adjusting the surface constant and aspheric coefficient, aspheric lens can eliminate spherical aberration to the greatest extent. Aspheric lens (light converges to the same point to provide optical quality), which basically eliminates the spherical aberration caused by spherical lens (light converges to different points, resulting in blurred image). Three
Spherical Lenses are used to increase the effective focal length to eliminate spherical aberration. However, an aspheric lens (high numerical aperture, short focal length) can be realized, and the system design and light transmittance can be simplified.
Fig. 2. Aspheric lens with spherical aberration
b. System advantages
The aspheric lens simplifies the elements involved by optical engineers in order to improve the optical quality, and improves the stability of the system. For example, in the zoom system, usually 10 or more lenses are used (additional: high mechanical tolerance, additional assembly procedures, improve antireflection coating). However, one or two
Aspheric Lenses can achieve similar or better optical quality, so as to reduce the size of the system, improve the cost rate, and reduce the overall cost of the system.
