OCT Improves Lens Development

Using OCT during the manufacture of a new type of optical lenses could improve their development by providing researchers with a better picture of the complete structure that makes up the material.

OCT is a powerful imaging tool with many applications in the biomedical sciences, but applying the technique to optical lenses has proven challenging, researchers say.

Now, scientists at the University of Rochester have tweaked the OCT method to obtain 3-D, high-resolution images of spherical gradient in refractive index (S-GRIN) lenses developed by Eric Baer at Case Western Reserve University in Cleveland. These new lenses — which consist of thousands of layers of compressed plastic bent into a lens-type shape — could be used in lightweight single-lens cameras, ball lenses for solar collectors and night-vision goggles.

S-GRINs offer many advantages to glass for lenses: They are lighter and more easily deformed into different shapes. Plastic optics have their own challenges, however, including consistency and high-quality. Jannick Rolland of the University of Rochester believes that embedding OCT in the manufacturing process can begin to address these challenges.

Kyle Fuerschbach, left, a graduate student at the University of Rochester’s Institute of Optics, and Jannick Rolland, Brian J. Thompson Professor and director of the R.E. Hopkins Center, work on a freeform lens experiment in Goergen Hall. Courtesy of J. Adam Fenster/ University of Rochester.

“I had done the calculation and thought we could do it,” said Rolland, the Brian J. Thompson Professor of optical engineering at Rochester's Institute of Optics. “And sure enough, after several months of work we had 3-D movies of the whole structure of the lenses and also clear pictures of each of the layers. It was amazing.”

Light passing through a lens is bent — or focused — based on the lens’ shape and the lens materials’ index of refraction. S-GRINs have a varying or gradient refractive index of over 0.08, which is unusually high, and can help with chromatic aberration — when different colors of light focus at different points, leading to a poorly focused image.

Rolland and colleagues shined IR light onto the lens using a technique similar to ultrasound and observed how each of the lens’ layers bent the scattered light. By counting how much time the light takes to come back, it is possible to know how deep into the material it scattered from.

“In the same way that in ultrasound we measure the time of flight to the tissue and back to localize the presence of a structure, we can do this with light that has a much smaller wavelength, which means a much higher resolution,” she said.

Being able to see the complete lens structure enabled Rolland and her team to pinpoint some areas of the manufacturing process that could be improved. For example, the investigators saw that some of the layers were thicker than PolymerPlus, the developer of the manufacturing process, had hoped for.

The results were published in Scientific Reports (doi: 10.1038/srep01709).

For more information, visit: www.rochester.edu