OCT measurement offers route to improved optical lenses

01 May 2013

Greater understanding of “S-GRIN” lens structure boosts design and performance.

Researchers at the University of Rochester have applied a sophisticated imaging technique to obtain the first 3D, high-resolution pictures of a recently-developed type of optical lenses, known as S-GRIN lenses. Using optical coherence tomography (OCT) during the manufacturing process allows them to significantly improve the quality of these new and promising lenses.

The results are published this week in the journal Scientific Reports, published by Nature Publishing Group.

Spherical Gradient in Refractive INdex (S-GRIN) lenses represents recent breakthrough in lens manufacturing, the Rochester scientists believe, and they have many potential applications, including for lightweight single-lens cameras, night-vision goggles and ball lenses for solar collectors.

When light passes through a lens, the amount it is bent – or focused – is determined by the lens shape, and the index of refraction of the lens material. S-GRINs work by having a varying or gradient refractive index. This is known as refractive index gradient. In human eyes this gradient is typically 0.03. These new lenses have a gradient of over 0.08, which is unusually high, and can help compensate for chromatic aberration: when different colors of light focus at different points, leading to a poorly focused image.

S-GRINs consist of thousands of layers of compressed plastic bent into a lens-type shape. By employing OCT, Jannick Rolland, Brian J. Thompson Professor of Optical Engineering at Rochester's Institute of Optics, and her team were able to look at each of these layers individually. They could see all the features of the material down to about 1-2μm, obtaining high-resolution 3D images, giving a more precise characterization of how the lens bends the light.

OCT is a powerful imaging tool with multiple applications in the biomedical sciences. But until now, people had struggled to apply it to optical lenses. Rolland was confident that her team could tweak the technique to make it possible to visualize the complete structure of these lenses.

3D movies

"I had done the calculation and thought we could do it," she said. "And sure enough, after several months of work we had 3D movies [see below] of the whole structure of the lenses and also clear pictures of each of the layers – it was amazing."

Rolland and her team are collaborating with Michael Ponting from company PolymerPlus, which is developing the manufacturing process for this novel lens type, which was invented by Professor Eric Baer at Case Western University.

By being able to show the whole lens structure, Rolland and her team were able to pinpoint some areas of the manufacturing process that could be improved. For example, they saw that some of the layers were thicker than the company had hoped for. The feedback that OCT offered allowed S-GRINs to be improved significantly.

S-GRINs are made from plastic, which offers many advantages to glass for lenses: it is lighter and more easily deformed into different shapes. Plastic optics have their own challenges however, consistency and high-quality among them. Rolland believes that embedding OCT in the manufacturing process can begin to address these challenges.

Rolland, Baer and Ponting are all authors on the paper. Other authors in the paper include Panomsak Meemon, Jianing Yao and Kye-Sung Lee from Rochester's Institute of Optics, part of the Hajim School of Engineering and Applied Sciences, and Kevin P. Thomson from the Institute and Synopsis Inc. This work was supported by the Manufacturable Gradient Index Optics program of DARPA and by the NYSTAR Foundation.

Below is one of the team's OCT videos of the structure of an S-GRIN lens:

About the Author

Matthew Peach is a contributing editor to optics.org.