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What is Aspheric Lens?

Aspheric lenses have non-spherical shapes, and have a more complex front surface, such as ellips, parabola, hyberbola, quadric, as well as toric which resembles a section of the surface of a rugby ball or a doughnut.
In this page, we are going to cover how conventional spherical lenses work and the advantages of aspheric lenses.

For an introduction to aspheric lens, click the button below.

Dr. Nazetaro’s Lesson
”Basics of Optical Glass”

Types of Simple Lenses and How They Work

Convex lens
  • Biconvex
    Curved outward on both sides
  • Plano-convex
    Flat on one side and curved outward on the other side
  • Convex Meniscus
    Meniscus means a crescent moon or an object shaped like it. Curved inward on one side and curved outward on the other side more strongly. Thicker in the middle than they are at the edges.
Images formed by lenses
With convex lens
Concave lens
  • Biconcave
    Curved inward on both sides
  • Plano-concave
    Flat on one side and curved inward on the other side
  • Concave Meniscus
    Concave meniscus is a lens curved inward on one side and curved outward on the other side less strongly. Thicker at the edges than they are in the middle.
Images formed by lenses
With concave lens

Why Aspheric Lens is Needed?

What is Lens?

Lenses are used when magnifying tiny or distant objects to help us see more detail. Also, a camera lens is used to make images of objects either on photographic films or on other media.
Traditional simple lenses are spherical lenses, one or both sides are concave/convex or one of the surface is flat, and their shapes are often made by grinding and polishing.

Disadvantages of Spherical Lens

Higher zoom lens is used when you shoot something small or from far distance. As you zoom to higher magnifications, the image dims since the amount of light entering lens decreases the more you zoom in. The same applies in the case when fast shutter speed is needed, such as photographing high-pace sport. The faster the shutter speed, the shorter the time image sensor is exposed to light, and the darker the resulting photograph.

The larger diameter lens will allow more light to be gathered. However, a larger diameter lens tends to be thicker than a smaller diameter lens, making it more likely to create aberration.

What is Aberration?

For lenses made with spherical surfaces, rays which are parallel to the optic axis but at different distances from the optic axis fail to converge to the same point. If the center of the image stay in focus an bright, the edges of the field apprear blurry and dimmeter.

How is Spherical Aberration Corrected?

Spherical aberration is typically minimized by combination of multiple lenses into an optical assembly. Also, by using fewer aspheric lenses instead of a greater number of conventional spherical lenses can reduce or eliminate the aberration.

Aspheric Lens Which can Reduce or Eliminate Spherical Aberration

Aspheric lens has a non-spherical lens surface. The main advantage of aspheric lenses is its ability to correct for spherical aberration. Aspheric lenses allow optical designers to correct aberrations using fewer elements than conventional spherical optics because the former gives them more aberration correction than multiple surfaces of the latter. Given that, smaller amount of aspheric lenses can be substituted for many spherical lenses to achieve similar or better optical results, while reducing system size, simplifying the assembly process, and yielding imaging lenses that ultimately cost less and outperform assemblies made of traditional spherical components.

However, aspheric lenses are not free from problems. Aspheric lenses tends to be more difficult to be manufactured by conventional fabrication prosess such as grinding and polishing, since aspheric lens elements are more complex than spherical ones. Consequently, aspheric lenses had not been widely applied.

As an alternative approach, aspheric lenses can be manufactured by glass molding process: a preform or near-net-shape glass is introduced to heated molds within a molding machine, pressed by two mold halves, then the formed lens is cooled down and released from the molds. Glass molding is as an effective approach to produce precision optical elements with complex shapes at high production efficiency. Once the mold is finished, the incremental cost for each lens is lower than that of standard manufacturing techniques for aspheres, making this technique a great option for high volume production.

Glass molding had an issue that arise from the very high-temperature for softening of a glass, which can deteriorate the molding easily and shorten the service life of molds. Requiring high temperature also means it takes time to heat and cool down the mold. Thus, the development of low softening temperature optical glasses for molding had been expected for a long time.

Glass material for molding has additional requirements, such as transparency, excellence in scratch resistance, stability in optical properties in temperature changes, the properties include refractive index, no crystalization or volatile substances occurs while forming, not containing a material which can react with molds, and are free from pollutants, such as lead and arsenic compounds. Glass lens has advantages over the plastic lens on the aspects as shown above, as well as hardness, refractive index, light permeability, stability to environmetal changes in terms of temperature and humidity, although plascic lens can be mass-produced at a low cost. Furthermore, for the convenience of users, providing a wide variety of glass materials for molding is important to meet customers’ needs.

Considering these requirements, SUMITA successfully developed a new glass material for molding, ‘K-PG325 Super Vidron’ with low softening temperature at 325 ℃ (617 ℉) in 2002. Since then, SUMITA has been developed a wide variety of glass materials for molding.

Also, a preform has improved. Conventionally, a lens preform, shaped in ball, disc or near-net, generated out of raw glass by grinding and polishing processes. A gob preform, a firepolished preform produced directly from the melt without any additional surface processing, has developed and commercialized. For many years, SUMITA has been a reliable supplier for precision gob preforms made of glass materials for molding.

Glass Modling Machine

In order to cost effectively manufacture of the lens, heating and cooling cycle is optimized for the fastest possible cycle time. There is a series of additional requirements which must be considered to produce high precision molded aspherical lenses, including control of temperature and pressing load in a high accuracy, and the uniformity of temperature in glass, since non-uniformity of temperature in glass will cause distortion.

In the glass forming process, it is necessary for the molding system to purge of oxygen and filled with inert gas, such as nitrogen and argon, in order to avoid detrimental reactions caused by oxygen including a deterioration of molding die and contact-induced glass sticking.

Recently, SUMITA manufactures not only molded aspheric lenses but also molded diffraction gratings, microlens arrays and other surfaces microstructures. The surface profile of the molded lenses can be precisely controlled by changing the applied gas pressure. SUMITA’s ‘Vacuum Osvvesita’ is the optimum glass molding machine for research and development and a small lot production.