Aspheric lenses: when the sphere is not enough

Aspheric lenses are the optics of choice for many applications – but they are regarded as complex to produce and therefore expensive. This blog post delves into the specifics of what constitutes an aspheric lens, highlighting the distinctions from spherical lenses, and provides relevant information regarding molded aspheric lenses.

In the beginning was the sphere

Since the first lenses were used systematically as reading aids (from about the 13th century), at least one of the two optically effective surfaces of a lens was part of a spherical surface (sphere), i.e. curved with a constant radius over the entire surface. Not least because of this singular dimension, a manufacturer does not need too much know-how and effort to produce such spherical lenses by grinding a glass body – one of the main reasons why spherical lenses formed the norm for centuries and were synonymous with an optical lens.

Why deviate from the simple spherical shape?

Spherical lenses are familiar to anyone who has ever used a magnifying glass, binoculars or eyeglasses. If spherical lenses work so well in all these optical devices and are easy to make, why deviate from the spherical shape?

Well, due to the shape of its surface(s), a principle-related aberration occurs in the spherical lens: the so-called spherical aberration. This is because light rays that are not incident in the center but further towards the edge of the lens have a different focal point than rays that are close to the axis – and this error is usually dependent on the wavelength of the light. If, for example, a collimated light stream is focused on a focal point via a spherical lens, the diameter of this focal point is small, but not really minimal, because the light rays captured more towards the edge of the lens already diverge again in the main focal point. This principle-related error in the optical imaging performance simply is too large for some demanding applications.

Can this aberration be corrected? Well, if instead of one lens a whole system of several lenses is used (e.g. anastigmats, a "Cooke triplet" or a tessar), the aberration can at least be reduced. However, this involves a lot of effort, many possible sources of error in the positioning and alignment, and always also a loss of light output due to the many optical elements in the optical path. Or we move away from the spherical surface and use aspheric lenses. Now, we will take a closer look at these aspheric lenses. 

Much more than just a radius of curvature

A lens is "aspheric" when at least one of its light-refracting surfaces deviates from the spherical or planar shape. However, most common aspheric lenses do not have a completely free form geometry surface but are at least rotationally symmetrical about the main optical axis of the lens.

Looking at the cross-section of a typical aspheric lens, the radius of curvature of the surface usually increases towards the edge of the lens, i.e. the curvature becomes "flatter". Only with this largely freely formed surface can aspheric lenses avoid or at least reduce aberrations – and especially actually correct spherical aberration completely.

This makes aspheric lenses simply better: in collimation applications they deliver collimated beams of higher quality, in focusing applications a significantly smaller spot size and in imaging applications a higher image quality.

Why not just always use aspheric lenses?

If aspheric lenses are so much better, why not just use them all the time? The short answer: because the surface is much more complex than that of a spherical lens. While a single radius of curvature is sufficient for the mathematical definition of a spherical lens, a rotationally symmetrical aspheric lens is usually defined by a conic section (circle, ellipse, hyperbola, but usually a parabola with a radius of curvature and a conic constant in the parabolic term) plus a correction polynomial with several coefficients for higher-order deformations.

Such a complex surface is, of course, much more difficult to produce in high, consistent quality than the simple spherical variant, and with disproportionately greater effort. For a long time, since the early 17th century to be precise, aspheric lenses have been considered the optimal solution for light refraction – but at the same time far too expensive to manufacture, which is why they were only used very sporadically.

What are molded aspheric lenses?

Traditionally, aspheric lenses are individually grinded and polished from glass preforms in complex processes, for example computer-controlled (CNC polishing), with magneto-rheological finishing or using diamond turning technology.

In contrast, precision glass molding is a much less complex manufacturing process for aspheric lenses. In precision glass molding, optical glass blanks are heated to such an extent that the surface becomes soft enough to be pressed into an aspheric mold.  

It all depends on the mold

To ensure that the lenses produced at the end of the manufacturing process fulfil the required detailed dimensions exactly and reproducibly, this mold must, however, meet high requirements. A lot of effort and a high amount of initial costs are necessary to ensure that the inside of the mold has an extremely smooth surface without even the tiniest indentations or bumps – and with a high degree of durability so that this surface can withstand many pressing processes without any loss of quality. In addition, when calculating and manufacturing the mold geometry, manufacturers must consider that it precisely compensates for material shrinkage when the finished pressed lens cools down.

Extreme pressure on the aspheric lens – and on its price

But once these challenges are overcome and the mold is finished and in place, suppliers can produce aspheric lenses at a much lower cost than those of conventional manufacturing techniques. Because our experienced manufacturing partner uses very advanced glass molding technology, aspheric lenses are produced with consistently high quality in large quantities at an affordable price.

Aspheric lenses offer further savings in applications where a single aspheric lens can replace an entire set of multiple spherical lenses – rendering the overall optical system less complex, more compact and lighter, while often even increasing its optical performance. 

Click here for an overview of our portfolio of precision molded aspheric lenses: standard, infrared and mounted in stainless steel holders:

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