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SPIE Professional January 2010

Adaptable Optics

Sharing scientific knowledge from military research labs brings huge advancements in ophthalmic medicine.

By Beth Kelley

SPIE Professional Adaptive Optics

There is a surprisingly long history of military research labs sharing technological developments with civilian medical researchers. Many common medical tools, technologies, and techniques were originally developed for defense purposes.

Adaptive optics, for example, were developed for and are still used extensively in defense applications. But they are also commonly used in ground-based astronomical telescopes, and they are considered a foundation for laser and wavefront sensor design.

David Williams, director of the Center for Visual Science and professor in the Institute of Optics at the University of Rochester (USA), saw the potential for adaptive optics in ophthalmology and studying the human eye early on. His work has led to huge advancements in medical applications for adaptive optics, including a better understanding of how the brain sees.

Eye Has Aberrations

Adaptive optics are a combination of technologies-lasers, wavefront sensors, and deformable mirrors-used together to adjust the phase of light so that image clarity can be improved or a point of light like a laser can be controlled.

Hale Telescope
The 200-inch Hale Telescope uses a laser in its adaptive optics system. The previously classified adaptive optics technology corrects for the effects of atmospheric turbulence and enhances the capabilities of the telescope.

Adaptive optics that were first developed for military applications began being applied exclusively for defense purposes in the 1970s. Information on adaptive optics was largely considered classified, and much of the research and work surrounding adaptive optics did not become declassified until 1991. Once it was, the technology was immediately adopted by the astronomy community to help guide ground-based telescopes and correct atmospheric aberrations. Using a laser "guide star" as a reference is now considered almost indispensable for imaging distant objects in the sky.

Meanwhile, Williams was working in the field of visual psycho-physics, in particular studying how the brain receives and processes information through the eye.

"Throughout my career, I've used optics as a way of probing the visual system," Williams says. For many years, Williams used laser interferometry and other optical technologies to study vision. But he knew even with these technologies, his ability to study the eye was still limited. "I realized if you had a way of correcting the aberrations of the eye, you could do a lot more vision science," he says.

Williams' introduction to adaptive optics was through a teacher, Tom Cornsweet, who was interested in deformable mirrors. Through Cornsweet, Williams learned about successes using adaptive optics for military and astronomy purposes. But no one had successfully used it for vision research. When Williams learned about work being done at the Starfire Optical Range at Kirtland Air Force Base, Albuquerque, NM, he and other vision researchers from the University of Rochester visited the Air Force lab. Their goal was to learn about adaptive optics technologies and whether they might be applied to correcting aberrations found in the human eye.

Bob Fugate, a longtime researcher in lasers, optical sensors, and adaptive optics who retired from the Air Force Research Lab in 2006, recalls the visit. "Dave Williams came down, visited for several days, learned about this technology, and immediately started implementing it," Fugate says.

Technology Transfers Easily

Williams was impressed by what he found.

"Bob Fugate was very helpful in talking to us about the feasibility of translating adaptive optics from his satellite tracking telescope to eye applications," Williams recalls. "In fact, we purchased the first drivers for our first mirrors from him. He was very helpful in helping us realize we could use this technology in an ophthalmic application.

"We borrowed the technology lock, stock, and barrel-stole it might be the better term-from the astronomy community," Williams adds.

After some tinkering with this new technique and adjusting it for smaller-sized applications, Williams and his group became the first to successfully use adaptive optics technology to correct most of the eye's aberrations so as to image the living human eye at high resolution.

SPIE Fellow Bob Tyson, associate professor of physics and optical science at University of North Carolina at Charlotte, says Williams and his team made advancements in their research as a result.

"Researchers had started using wavefront sensors to measure the aberrations of the human eye," Tyson says. "Williams took this research a step further by using full adaptive optics systems to correct those aberrations with a deformable mirror to get clearer images of the retina and other interocular structures."

Gemini Observatory ALTAIR adaptive optics image
Gemini Observatory ALTAIR Adaptive Optics Image
Believe Everything You See

Williams' team was the first to image individual photoreceptor cells in the living eye, and in 1997, he obtained the clearest pictures ever of the photoreceptors with adaptive optics. Williams was able to image and identify the red, blue, and green color cones in the eye, and he found the cones had a completely random geometry.

Fugate says Williams' work was a big revelation into how important the brain is in processing information and making our eyes effective sensors.

"In the retina, there's a very unorganized pattern, and it's left up to the brain to somehow put the color picture together," Fugate says. "I remember Williams calling me up; he just was so excited and he said, 'If I were God, I wouldn't have done it this way.'"

Williams and his team have done more than just image the retina or measure the fluid in the eye. In March 2009, they captured the first-ever images of "dark cells" in a living retina. Dark cells, or retinal pigment epithelial (RPE) cells, lie immediately behind the photo receptors of the eye. Up until now, it had been impossible to image that layer of cells without removing or damaging the eye.

3- and 80-year-old retinal pigment epithelial (RPE) cells in the eye
Comparative diagrams of 3- and 80-year-old retinal pigment epithelial (RPE) cells in the eye. As the eye ages, the RPE cells deteriorate, making it harder for the brain to receive and register light, leading to blindness.
Image courtesy of David Williams.

RPE cells are extremely important in maintaining the health of the eye's photoreceptors, says Williams. If the RPE cells die or are damaged, photoreceptors soon follow. This can lead to eye diseases like macular degeneration or other forms of blindness.

"Macular degeneration affects one in 10 people over the age of 65, and as the average age of the U.S. population continues to increase, it is only going to get more and more common," Williams says. "We know these dark retinal cells are compromised by macular degeneration, and now that we can image them in the living eye, we might be able to detect the disease at a much earlier stage.

"So it's very important to understand what their role is and how they function."

Williams' team took advantage of an existing method, auto-fluorescence imaging, for collecting light just from that layer in the retina. "We combined that method with our adaptive optics technologies," Williams says.

The new technique for seeing only this nearly invisible layer of cells could help doctors identify the onset of many diseases of the eye much earlier. It also may help develop therapies that are effective in treating the disease.

Times They Are A-Changing

When Williams first began his research into adaptive optics in the early 1990s, it was hard to find manufacturers who could produce the optical components on a small scale. The first company Williams' lab approached to design the components they needed said they could not produce a deformable mirror for less than $1 million.

"It was only when one member of that company left and began his own little startup, that we were able to get the first deformable mirror for ophthalmic applications," Williams recalls. Today there are many small companies that create adaptive optics for medical or astronomical applications. Fugate, for example, is now a technology transfer consultant at New Mexico Tech and a partner in the company FASORtronics.

Williams' work was soon noticed by researchers looking into laser-refractive surgery, and adaptive optics became instrumental in improving Lasik corrective eye surgery. Adaptive optics have also been developed for imaging, laser communication, laser resonators, laser propagation, light therapy, and several types of surgeries including Lasik.

There is also current research looking at the incorporation of adaptive optics into microscopes, or in situations where doctors need to look through thick layers of tissue. Some studies have shown it is in fact possible to remove aberrations that arise in the tissue between the site that needs to be imaged and the microscope or other detector.

"Williams' work has been a forerunner in the applications for adaptive optics," Tyson says.

ophthalmic adaptive optics diagram
Diagram courtesy of David Williams.

Researchers in David Williams' lab at the University of Rochester have constructed a new adaptive optics instrument called the fluorescence adaptive optics scanning laser ophthalmoscope (FAOSLO). It has a 904-nm laser beacon, a Shack-Hartmann wavefront sensor (WFS), and a 144-actuator Boston Micromachines MEMS deformable mirror (DM). The scanning system (VS: vertical scanner, HS: horizontal scanner) has an adjustable 0.5- to 2.89-degree field of view. Fluorescence imaging is achieved with an AR/KR tunable laser source and a photomultiplier tube (PMT) for fluorescence light detection. Infrared reflectance imaging of the retina is achieved with a 788-nm super luminescent diode source and an avalanche photodiode (APD) for light detection. The two imaging modalities have independent focus control and can be used simultaneously. So, it is possible to image the cone mosaic in the infrared and the RPE mosaic with autofluorescence (AF) in the visible, for example.

Williams is excited by the development of technology using adaptive optics for ophthalmology, and believes the main application for adaptive optics in medicine will remain in the eye. "Almost on a weekly basis there are new reports coming out about the use of adaptive optics in the eye," he says.

The potential for technology transfer from military applications to civilian purposes, including medicine, is huge, and often without too many adjustments or changes to the technology. As Williams points out about adaptive optics, "It's kind of remarkable how the same technology, with very little modification of the tools themselves, could be used in both domains; both to image stars through the turbulent atmosphere, and image the eye."

Optics In Astronomy And Medicine

An SPIE Newsroom video on adaptive optics used for astronomy applications focuses on the laser guide stars and deformable mirrors at the Hale Telescope at the Palomar Observatory in California and speaks to the technology's promise in human vision applications.

The short video features Antonin Bouchez and Christoph Baranec, two researchers from the California Institute of Technology, which owns and operates the observatory, and Bob Tyson, associate professor of physics and optical science at the University of North Carolina at Charlotte and author of five books on adaptive optics.

Tyson notes that the fluid inside a human eyeball (vitreous humor) acts very much like the disturbed atmosphere that can cause fuzzy images of astrological objects. The adaptive optics technology once used exclusively for military applications is now being used for in vivo retinal imaging by David Williams and other researchers.

See the video at spie.org/palomar

The Lighter Side of Adaptive Optics

The Lighter Side of Adaptive OpticsBob Tyson discusses the development of and many uses for adaptive optics in the 2009 SPIE Press book "Lighter Side of Adaptive Optics."

The book is a nontechnical explanation of optics, the atmosphere, and the technology for "untwinkling" the stars.

While interweaving a fictional romantic relationship as an analogy to adaptive optics, and inserting satire, humor, and philosophical rants, Tyson makes a difficult scientific topic understandable.

The "why" and "how" of adaptive optics has never been more enjoyable.

More information

Beth Kelley 
Beth Kelley is an SPIE editor. 

Have a question or comment about this article? Write to us at spieprofessional@spie.org.

DOI: 10.1117/2.4201001.05

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