Future Optics: Optical design and testing trends - better, faster, cheaper

OSA: How have computers changed optical design?

Daniel Malacara-Hernández: Early ray tracing calculations in the day of Eugen Conrady were done by hand with logarithmic tables. Even when mechanical calculators replaced them, tracing just one ray took many hours and a lot of patience. I designed a few lenses that way as a student at the University of Mexico starting in 1957, and I was tired after tracing just one ray.

When James G. Baker first used the Mark 1 computer for ray tracing at Harvard in 1944, it took two minutes to trace one skew ray through just one surface. In 1953, Robert E. Hopkins and Donald P. Feder started using a more powerful IBM 650 computer at the University of Rochester, which could trace rays much faster. The University of Mexico got an IBM 650 while I was a student, and I tried tracing rays on it.

When I came to Rochester in September 1961, they were doing wonderful things with computers. Rochester and Eastman Kodak were developing optimization techniques that greatly improved the design process.

The later advent of the personal computer led to development of several commercial programs that in less than a second could trace a complete bundle of rays through an optical system for one step in the optimization process. At the beginning, we traced only three or four rays to evaluate a lens—now, we can trace 12–15 rays for each of three colors.

OSA: How will optical design change in the future?

DMH: When I teach optical design, my students always ask, "Why do we have to learn all the processes and theory for the rays when we have a commercial program?" I tell them it wouldn't work if they didn't know aberration theory.

Contemporary programs are quite powerful, but they need human intervention. The designer must pick a starting lens design before beginning optimization. After that optimization, the designer must make changes for a series of several iterations. The user of design software has to know the fundamentals of lens design to make these choices.

Future software will become so powerful that the designer will not have to intervene. That would be wonderful, but that may be 20 or 25 years away, so my students can dream of it. Then, they can concentrate on being creative and discovering new things.

The future will bring new design options and challenges. Free-form surfaces lacking rotational symmetry are quite useful in many off-axis optical systems, including the progressive eyeglass lenses used to correct presbyopia. Yet they have been hard to describe mathematically. Now, Greg Forbes of QED Optics has developed new mathematical descriptions that can use orthogonal polynomials to specify any surface shape. That will allow designers to use free-form surfaces in a wider range of applications.

OSA: What changes do you expect in optical testing?

DMH: My PhD thesis in 1965 was on testing aspherical surfaces, and I have worked on them most of my life. They attracted me because they are difficult to test and to make, and the problems are interesting.

The development of new instrumentation and tools has brought large advances in optical testing. We will continue seeking more dynamic range to test strongly aspheric surfaces, more sensitivity to increase accuracy, and less sensitivity to external noise.

Currently, more than 100 types of tests are used for aspheric lenses. My students ask me why they should have to learn them all, and want to know which test is "the best." But I tell them there is no such test—you need all those methods to test differently shaped lenses. So, another of my dreams is to have "the test" so powerful that it can be used to examine all aspheric surfaces, I think. It will not be in my lifetime, but I hope my sons may see it.

OSA: How will these improvements in design and testing change what we can do with optics?

DMH: The small cameras used in iPhones are built around tiny aspheric lenses. They are quite impressive and produce wonderful images, but it was impossible to dream of such a camera only 10 years ago.

Aspheric surfaces will become even more common as we continue making great advances in fabricating and testing them. The use of aspheric surfaces can improve the quality and reduce the cost of lenses and optical systems. Aspheric surfaces also will help reduce the number of elements in the lenses.

Another exciting area is the use of adaptive optics in areas such as medical examination of the human eye. Ophthalmologists now use an instrument called an ophthalmoscope to look into the retina, which can detect many health problems. It can show blood vessels, but it lacks the resolution to see the rods and cones in the eye, which could yield useful information.

The problem is that the fluid in the eye is moving all the time, causing turbulence that limits the resolution of the ophthalmoscope like air turbulence limits the resolution of a telescope. Adaptive optics has been very successful in improving astronomical resolution, and it can do the same thing for instruments examining the eye.

David Williams at the University of Rochester and several other researchers around the world use adaptive optics to image the eye with resolution so sharp that it can show the rods and cones. Three of my students have been working with him, and I am having this instrument assembled in my lab as we speak. Now, the optics are expensive and and must be assembled on a very heavy 2 × 1.5 m optical table.

My dream is to have a portable instrument for use in the office of an ophthalmologist. You will have to work hard to reach this goal because it is not easy to produce this kind of equipment. But I am sure it will be done in the future. Another dream that I have is that future eyeglasses could use adaptive optics. They might compensate for aberrations in the eyes, or they could change their shape to focus the eye at different distance. That would require an even smaller optical system, but with time it could be done.

I just ordered a flexible mirror that is only 1.5 in. across, but is very expensive. To comply with government regulations, I had to write a letter explaining what I was going to do with it. I hope that in the future you will be able to buy those flexible mirrors anywhere at a cheap price.

Daniel Malacara-Hernández is a professor at the Centro de Investigaciones en Optica (Center for Research in Optics) in Leon, Guanajuato, Mexico. He is a fellow of The Optical Society, the editor of Optical Shop Testing, and co-author of the Handbook of Optical Design.

The Optical Society celebrates a century of innovation

Throughout a century of breakthroughs, The Optical Society has brought together the best minds in optics and photonics to light the future. This series reflects on that history and looks to what innovations lie ahead. For more information, please visit http://osa.org/100.