Basic Tips about Aspherical Lenses

The curvature of the lens of this kind of lenses is better. It maintains the good performance of aberration correction to obtain the required performance. The use of aspheric lenses brings outstanding sharpness and higher resolution, while miniaturization of lenses is possible.

Summary:

Aspherical lens is a constant curvature from the center of the lens to the edge, but the curvature of the lens varies continuously from the center to the edge for aspherical lenses. In photographic lenses, many aberrations must be corrected in order to ensure optical performance. If only the aspherical lens is used to correct the lens, the technical requirements of the lens require a combination of lenses. For special advanced lenses, only spherical lenses sometimes fail to correct aberrations to the level of user satisfaction. Calculation formulas in optical design:

Calculation formulas in optical design

Technical principle:
The radius of curvature of an aspherical lens varies with the central axis to improve optical quality, reduce optical elements, and reduce design costs. Aspheric lenses have unique advantages over spherical lenses, so they are widely used in optical instruments, images and optoelectronics industries, such as digital cameras, vehicle lenses, and high-end micro instruments.

Comparative advantage:

A. aberration calibration
The most prominent advantage of aspheric lenses in replacing spherical lenses is that they can correct the spherical aberration induced by the spherical lens in the collimation and focusing system. By adjusting the surface constant and the aspheric coefficient, the aspheric lens can eliminate spherical aberration to the maximum extent. Aspherical lenses (rays converging to the same point, providing optical qualities) essentially eliminate spherical aberrations produced by spherical lenses (which converge to different points, leading to blurred imaging).
Three spherical lenses are used to increase the effective focal length for the elimination of spherical aberration. However, an aspheric lens (high numerical aperture, short focal length) can be implemented, and the system design is simplified and the transmittance of the light is provided.


B. system advantage
The aspheric lens simplifies the involved elements adopted by the optical engineers in order to improve the optical quality. And improves the stability of the system. For example, in the zoom system, typically 10 or more lenses were used (additional: high mechanical tolerance, high additional assembly procedures, improve the anti-reflective coating), but 1 or 2 pieces of aspheric lens can realize optical quality similar or better, so as to reduce the system size, improve the cost rate, reduce the comprehensive cost system.

Molding process:

A. precision glass molding
Precision glass molding is the process of heating glass materials to high temperatures and becoming plastic. They are formed by spherical molds and gradually cooled to room temperature. At present, precision glass molding is not suitable for aspheric lenses with a diameter greater than 10mm. But new tools, optical glasses, and measurement processes are driving the technology. Precision glass molding, although the cost is high in the design stage (high precision mold development), but after the molding, the production of high-quality products will average the cost of development and the price will become acceptable, especially suitable for the needs of mass production occasions.


B. precision polishing molding
Grinding and polishing are generally applied to the production of monolithic aspheric lenses at one time. With the improvement of technology, the accuracy of lapping and polishing is higher and higher. Most notably, accurate polishing is controlled by the computer and automatically adjusted to optimize parameters. If higher quality polishing is required, magneto-rheological finishing will be adopted. Compared with standard polishing, magneto-rheological finishing has higher performance and shorter time. Precision polishing molding requires specialized equipment and it is currently the primary choice for sample making and small batch testing.

C. mixing molding
The spherical surface of an aspherical lens is cast on by using a spherical molding, and a layer of high polymer aspheric surface is cured by ultraviolet light and cover on it.  In hybrid forming, the achromatic spherical lens is used as the base, and the surface is cast with a layer of aspheric surface to eliminate chromatic aberration and spherical aberration at the same time. Fig. 7 is a manufacturing process of a mixed forming aspherical lens. The hybrid aspherical lens is suitable for occasions where additional characteristics are required (simultaneous elimination of chromatic aberration and spherical aberration) and mass production.


D. injection molding
In addition to glass aspheric lenses, there are plastic aspheric lenses. Plastic molding is the injection of molten plastic into an aspherical mold. Compared with glass, the thermal stability and compressive resistance of plastics are poor, and special treatment is needed to obtain similar aspherical lenses. However, the most important feature of the plastic aspheric lens is its low cost, light weight, and easy molding. It is widely used in such fields as moderate optical quality, insensitive thermal stability, and little pressure resistance.

Selection basis:

Various types of aspheric lenses have their own relative advantages. Therefore, for different applications, it is important to choose the right product. Major considerations include batch size, quality, and cost.
A. precision glass molding aspheric lens has the characteristics of mass production and high thermal stability, suitable for a large batch, high quality, and high thermal stability.
B .precision polishing molding aspheric lens has the characteristics of short period sample preparation and no die need, it is suitable for sample production and small batch sample occasions
C. mixed molding aspheric lens with the features a simultaneous calibration of spherical aberration and chromatic aberration features, suitable for wide spectrum, large volume, high quality.
D. plastic aspheric lens has the characteristics of low cost and light weight, suitable for large quantity, the low requirement on quality, moderate thermal stability.
When aspheric lenses are required (without standard products or inventory products), development costs, sample costs, batch prices, delivery cycles and other factors should be taken into account.