Optical System

What is Aspherical Lens?

This lens is made of curve shape, not sphere.
The aspherical lens has surface, shown in parabolic surface and polynomial expression. Ellipsoid, hyperboloid, fourth curved surface are examples.
We will explain about general lens, necessity of aspherical lens, predominance and others.

Gentle explanation is here.

What is aspherical lens?

Shapes of General Lens and Performances

Convex Lens

  • biconvex lenses

    Biconvex Lenses
    Two convex surface lenses are combined.

  • plano-convex lens

    Plano-Convex Lens
    Lens on one side is plano type.

  • convex meniscus lens

    Convex Meniscus Lens
    This means crescent shape lens, and one side is convex, another side is concave lens.
    Center part of lens is thicker than periphery.

convex lens

Concave Lens

  • biconcave lens

    Biconcave Lens
    Two concave surface lenses are combined.

  • plano-concave lens

    Plano-Concave Lens
    Lens on one side is plano type.

  • concave meniscus lens

    Concave Meniscus Lens
    This means crescent shape lens, and one side is convex, another side is concave lens.
    Periphery part is thicker than center part.

concave lens

Necessity for Aspherical Lens

What is Lens?

Lens is applied, when magnifying tiny things or distant things, or, when tracing objects on film, such as cameras.
General lens shape is spherical(convex and concave) and plano, and the glass is made by polishing glasses.

General Lens : First Problem

When observing tiny things or distant things, high-powered lens will be applied. But, as the magnification grows, intensity absorbed in lens is getting less. Thus, as magnifying more and more, observation gets difficult.
And, when taking pictures of a sports car which moves in high speed, shutter speed will be necessary to get faster. That is same case.
Captured intensity is getting low. Thus, the captured images are getting dark.

General Lens : Second Problem

For capturing intensity more, applying big diameter lens is fine. But, as the diameter is getting bigger, aberration is getting more.

What is Aberration ?

Aberration means, focal points between the light comes from center part and the light comes from edge of lens are not clear.
For the example in pictures, even though the focus is clear on center part, focus on the edge is not clear. These are the problems.
Because of these problems, lens with big aberration is not applicable.

What should be done for less Aberrations ?

By combination of many concave lenses and convex lenses, it makes possible to capture less aberration, imperfectly.

General Lens : Third Problem

For capturing intensity more, lens with big diameter is necessary.
But, lens with big diameter makes big aberration. Thus, aberration is necessary to get less by combination of many lenses.
It seems that problems are eliminated. But, many big lenses are necessary to be combined. Thus, when the lenses are installed in camera, compactness and weight saving are very difficult and, it costs much, too.
Thus, development of lens with no aberration has been advanced.
Then, aspherical lens was developed.

What is aspherical lens which does not have aberration, of which the problem in the past?

  • Spherical lens
  • Aspherical lens

Aspherical lens is lens, made by sphere surface(convex, concave) or curved surface, not flat.
It also has parabolic surface, hyperboloid, ellipsoid and polynomial.
And, the nonaxisymmetric shapes such as donuts or rugby ball, are proposed.
The biggest feature is that aspherical lens got rid of aberration, of which the problem in the past.
For capturing intensity more, even though lens diameter is much bigger, this lens has no aberration.
Thus, two or three spherical lens were necessary to be combined in the past. But, one aspherical lens is enough for solving this aberration problem now.
As a result, compactness, weight saving, cost reduction become possible.

Problems of Aspherical Lens and Solutions

But, this aspherical lens also has a problem.
Because aspherical lens shape is complicated, it makes difficult to produce lens by grinding such lenses in the past.
Thus, actualization has been late.

Then, glass preform material near finished shape is placed on aspherical lens molding tool. After heating & softening preform material, press it.
This production method is called Glass Molding.
Glass molding method enables to produce aspherical lens easily, easier than grinding in the past.

Glass Molding Pproblems and Solutions

But, glass molding also has a problem.
The problem is that we have to soften glass in very high temperature.
For softening glass, we have to take time for raising temperature. The temperature of softening glass is very high. Thus, tooling deteriorates soon and, we can not apply it.
It also takes time to cool down pressed glass.
Thus, developing glass which gets soften in low temperature is being advanced.

But, glass molding materials are necessary to be transparent, no altering properties such as refractive index is necessary, even though temperature changes. For glass molding materials, abrasion resistance, various kinds of materials are prepared for making into products, on the halfway of glass molding, crystals which lose glass transparency and volatile components, should not be occurred. Substances which easy to react with tooling should not be contained. Hazard substance such as lead and arsenic should not be contained. Many conditions are necessary to be fulfilled with.
In this respect, glass is superior to plastic.
But, plastic has features that became possible for mass production in cheap price.

In such difficult conditions, glass molding material which is dissolved in 325℃ low temperature, K-PG325 Super Vidron is developed.

Glass materials→Preforms→Glass molding→Aspherical lens

But, glass molding products are still expensive, even though glass molding materials which are able to form in low temperature are applied.
One of the reasons is that, materials before glass molding are expensive.
The materials before glass molding are called "Preform" in general.
As a preform, we only used to have [polished preforms] which has same production method as spherical lens in the past. But, as new production method, producing preforms directly from melting is developed and actualized.
This product is called, [gob preforms].

Glass Molding Machine

Heating and cooling in high speed are necessary for glass molding machine, for the purpose of cost reduction.
Otherwise, as the conditions of producing high precision lens, arrangement of temperature and forming load in high precision, no distortion by temperature, are necessary.

And glass molding means, as oxygen goes into molding Die while molding, tooling will be deteriorated or glass adhesiveness with tooling, will be occurred. Thus, airtight should be inside the Die, and, non oxidative such as nitrogen or argon will be necessary to be maintained.
And, as glass molding products, glass molding machine is necessary for optical products (diffraction grating or microlens array etc) which has minute structures, except aspherical glass which we described before.
For molding these optical products precisely, function which can be arranged pressure inside molding Die, are also necessary.

Vacuum osbeshita is optimum glass molding machine for the purpose of R&D and small productions.

Contact us for information and questions
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