About Aspherical lenses

What is an Aspherical Lens??

An aspherical lens is a lens composed of aspherical curved surfaces. The aspherical lens has surfaces expressed by a paraboloid or a polynomial expression, such as an ellipsoid, a hyperboloid, and a quartic curved surface. Common lenses and the necessity and superiority of the aspherical lens will be explained here.
Shapes and Functions of Typical Lenses

Double convex lens:
A lens with two convex surfaces.

left: Image of a nearby object as seen through a lens.

right: Image of a distant object as seen through a lens.

Plano-convex lens:
A lens with one flat surface. 
Positive meniscus lens:

Meniscus means "crescent" in Greek. A lens with one convex surface and one concave surface. A positive meniscus lens has a steeper convex surface and will be thicker in the center than on the periphery.

 

Double convex lens:
A lens with two convex concave surfaces.

left: Image of a nearby object as seen through a lens.

right: Image of a distant object as seen through a lens.

Plano-concave lens:
A lens with one flat surface. 
Negative meniscus lens:
Meniscus means "crescent" in Greek. A lens with one convex surface and one concave surface. A negative meniscus lens has a steeper convex surface and will be thinner in the center than on the periphery.

Necessity of Aspherical Lens

 

A lens is used to magnify a small object or a distant object or to film an object on film, such as a camera. A typical lens has a spherical surface (a concave or a convex surface) or a flat surface shaped from a piece of glass by cutting and grinding.

 

To watch the small object or distant object, a lens with a high magnification is used. However, as the magnification increases, the rays of light passing through the lens decrease. Therefore, the more the object is magnified, the more the produced image becomes less sharp. This principle can be also applied where shutter speed is increased to take pictures of a sports car moving at high speed. Since the rays passing through the lens decreases, the produced image will become dim.

To increase the rays of light passing through the lens, a large-diameter lens is used. However, the larger diameter lens introduces a greater optical aberration.  

 

The aberration is a phenomenon where the light beams that pass through the center of the lens and light beams that pass through the outer margins of the lens are focused at different points. To take a picture as an example, optic aberration is when the center of the picture is in focus but the corners of the picture are out of focus. Because of this drawback, a lens with greater aberration cannot be used without correction.

 

The degree of aberration can be reduced by combining multiple concave lenses and convex lenses though it does not produce a perfect correction.

 

As mentioned above, to increase the rays of light passing through the lens, a larger diameter lens is needed. However, the larger diameter lens introduces greater aberration, so multiple lenses must be combined to reduce the aberration. Indeed, the degree of aberration can be reduced by using this method. However, the method raises other problems such that the combined use of multiple lenses boosts the product price and becomes an obstacle to miniaturization of the product, such as cameras. To this end, a lens with less aberration has been researched and developed. As a result, the aspherical lens was produced. 

 

The aspherical lens is composed of curved surfaces other than a spherical surface (such as a concave surface or a convex surface) or a flat surface. The curved surface of the aspherical lens can be a paraboloid, a hyperboloid, an ellipsoid, and a quartic curved surface (expressed by a high-order polynomial expression). And a non-axisymmetric curved surface like a doughnut or a rugby ball is suggested. The biggest advantage of the aspherical lens is that it can eliminate aberration, which has been a problem with conventional lenses. The aspherical lens does not introduce aberration when manufactured in a larger diameter to increase the rays passing through the lens. Thanks to the advantage, the aspherical lens does not need to combine multiple lenses unlike the conventional spherical lenses. Consequently, miniaturization and cost reduction can be achieved. 

However, the aspherical lens also has a drawback. The drawback is that the aspherical lens is difficult to manufacture by cutting and grinding as with conventional lenses because of the complex shape. For this reason, commercialization of the aspherical lenses has been retarded. To solve this problem, a method to manufacture the aspherical lens by heating, softening, and pressing a near aspherical glass material (a preform) put into aspherical molds is employed instead of grinding. This method is called glass molding. By using glass molding, the aspherical lenses can be manufactured more easily than grinding.

 

However, glass molding also has problems. The problem is that the glass material can be softened only at ultrahigh temperatures and raising the temperature to the ultrahigh point takes time. In addition, the very hot glass material soon impairs the molds. In addition, cooling the pressed glass also takes time. To avoid this, glass (or glass materials) that can be softened at relatively low temperatures suitable for use in glass molding has been researched.

Furthermore, the glass molding material must satisfy many requirements:

  1. The glass molding material must be transparent;
  2. the properties of the glass molding material, such as the refractive index, must not change when the temperature changes;
  3. the glass molding material must resist damage;
  4. the various types of glass molding material must be commercially available;
  5. the phenomenon of the production of a crystalline structure or volatile matter that impairs the transparency of the glass molding material must not occur during molding;
  6. the glass molding material must not contain any substance with high reactivity to the molds; and
  7. the glass molding material must not contain pollutants, such as lead and arsenic. Compared with plastics with respect to the requirements listed above, glass is superior. However, plastic has advantages such that it can be mass-produced at low cost. Against this backdrop, K-PG325 Super Vidron, a glass molding material that can be molten at a relatively low temperature of 325 degrees centigrade, was created. 

Even though glass material that can be molded at a relatively low temperature has been achieved, the glass-molded product is still expensive. One of the price-boosting causes is that the material to make the glass molding material is expensive. The material to make the glass molding material is generally called a preform. Conventionally, the preform is manufactured by grinding, as is the case with a ground preform for spherical lenses. Recently, a method to manufacture a preform by melting glass material has been developed and commercialized. Such a preform is called a gob preform.

Glass Molding Equipment

For the equipment to mold glass materials, the functions to heat and cool the materials quickly are necessary in order to reduce the manufacturing cost. Moreover, functions to adjust the molding temperature and pressure with very high precision and prevent heat distortion are required for the manufacture of high-precision lenses. In addition, the glass molding equipment must have an airtight mechanism to maintain the molding atmosphere, such as atmospheric nitrogen gas or non-oxidative atmospheric argon gas because if oxygen gets into the molding atmosphere (an atmospheric gas used when molding the glass material), degradation of the molds or fusion of the glass and molds may occur. In recent years, optical products having a microstructure (such as a diffraction grating and a micro-lens array) are demanded as new types of glass molding products. To mold such types of optical products with high precision, a function to adjust the pressure of the molding atmosphere as appropriate is essential. Vacuum OSUbeshita is glass molding equipment optimal for research and development and small-batch production.  

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