Lenses for miniature optical systems like optical mice and cell-phone cameras have to be manufactured in high volume. Cost and size constraints dictate that the lenses do more than simply refract light. Also, these lenses must be producible in complex forms.

The industry has responded by investing in the technology to manufacture lenses at the wafer scale. The resulting products provide optical functionality far beyond the capabilities of traditional lenses while benefiting from the economic advantages of wafer-scale manufacturing techniques.

Lenses 101

A lens is an optical device with axial symmetry that transmits and refracts light. Its material has a refractive index different from the surrounding media, and its two optical surfaces work to converge or diverge a beam. This may seem obvious, since the limits on materials and manufacturing methods only permit a very restricted range of lens shapes and optical functions to be realized.

In many regards, lenses have not changed significantly over the last 3000 years. Materials quality and manufacturing methods may have advanced, but lenses are still essentially round in plane and curved in cross-section.

One of the few exceptions in mass production is “reading stones,” commonly known as eyeglasses. While the earliest eyeglasses were round, to simplify manufacture, modern glasses now come in a variety of shapes. This is done for aesthetics, not for reasons of optical performance or cost. To look good, the shape of the glasses should match the shape of the wearer’s face.

But what if the generic description of a lens could be changed? Going into more detail, a lens is an optical device with optional symmetry that can transmit, block, filter, refract, and diffract light. It is made of multiple materials, each with different optical properties, and has multiple optical surfaces. The range of optical functions that could be achieved with such a component would pave the way for a range of innovative products. But how could such a lens be made?

Lens Manufacture

Traditional lenses are manufactured from either relatively hard materials like glass or, more commonly, from a range of specially engineered polymers. Glass lenses are fabricated in a limited range of shapes or blanks and then machined to final shape by a series of grinding and polishing operations. These processes entail the removal of material from the surface by a tool. Therefore, the surface must be accessible.

The tool has a finite radius of curvature, and the type of motion necessary to conduct grinding and polishing operations limits the complexity of the shape that can be manufactured. That’s why glass lenses tend to be symmetric about the optical axis, with large radii and smooth optical surfaces. Deviation from this simple formula greatly increases cost and decreases throughput.

Mass production of lenses using engineered polymers is frequently accomplished by injection molding. A metal mold contains a hollow lens-shaped cavity that’s filled by injecting polymer in a semi-liquid form. The polymer is allowed to set or cure before opening the mold and removing the part. Injection molding is very attractive as a high-volume manufacturing process because many parts can be produced simultaneously with rapid throughput.

By injecting the polymer under high pressure, it is possible to make an exact replica of the mold cavity, including the surface finish. As a result, the released part can be used as is, without further processing. The mold surface, which is metal, has to be machined by grinding and polishing, very much like glass lenses. Because one mold can be used many times, it is economical to make more complex lens geometries, but they are still limited by the capabilities of the mold-shaping tools.