Phoropters can be found in nearly every practice that's concerned with correcting vision. However, compared to all of the advanced, computerized technology that ophthalmologists use today, the phoroptor is surprisingly low-tech. It works well—hence its longevity and widespread acceptance—but it's large, time-consuming and requires a skilled technician to operate.
Now Gholam A. Peyman, MD, a long-time pioneer in the development of new instruments and techniques, has set his sights on creating a more advanced phoroptor that may eliminate some of the limitations of today's instrument. Dr. Peyman, a clinical professor of ophthalmology at the University of Arizona School of Medicine in Tucson and Emeritus Professor of Ophthalmology at Tulane University—as well as a member of the American Society of Cataract and Refractive Surgery Hall of Fame—is collaborating on the project with Nasser Peyghambarian, PhD, James Schwiegerling, PhD, David Mathine, PhD, and Randall Marks, PhD,optical engineers in the University of Arizona photonics department. (The new refracting instrument they are developing is the intellectual property of the university.)
Building a Better Mousetrap
"The phoroptor we're all accustomed to contains a number of lenses—plus, minus and cylindrical," notes Dr. Peyman. "The lenses have to be placed in front of the patient one after the other, so the image is constantly interrupted by dialing in the different lenses.
The patient has to make comparisons by remembering how sharp his vision was through the previous lens, which is easy for young patients to do, but not always easy for older patients. Also, because the instrument is based on this kind of technology, the lenses can only measure in steps of a quarter diopter of spherical or cylindrical power. Being more precise would involve increasing the number of lenses in an already bulky instrument. Measuring in smaller increments may not seem terribly important, but it is important if you want to achieve 20/20 or 20/15 visual acuity.
"In addition, the process of measuring visual acuity normally involves finding a refractive starting point using instruments such as an autorefractor or retinoscope and then dialing that information into the phoroptor," he continues. "This requires a skilled operator. All of this extends the chair time required—and higher-order aberrations can't be measured at all.
"My interest is in making visual acuity measurement automatic," he explains. "I'd like to replace this entire system with something smaller, easier to operate and hopefully more accurate, where the patient can see an image such as a visual acuity chart in real time and move a dial or a switch back and forth until his vision is sharpest. That's all the patient would need to do. And that's what we're in the early stages of accomplishing."
The new system Dr. Peyman and his team are developing will be based around a set of smoothly adjustable fluidic lenses; it will also incorporate a modified Shack-Hartmann wavefront sensor to take an initial reading and automatically set the liquid-lens device to an appropriate starting point for the manual/subjective refraction.
Perfecting the Liquid Lens
For the immediate future, Dr. Peyman says the team's focus is on perfecting the fluidic lens part of the system. That will allow the phoroptor to be automated—and be more precise. "We've found that we can replace all of the lenses in the phoroptor with three fluidic lenses—one spherical and two cylindrical," he explains. "These lenses will be able to provide all the refractive power that we need to correct visual acuity routinely in the clinic. They'll make the instrument far smaller than today's phoroptor. And everything will be connected to a computer that controls the refractive power of the fluidic lenses; the wavefront attachment will be flipped in and out."
The fluidic lenses consist of a clear container with flexible silicone membranes on both sides. "The chamber has a port through which fluid is injected or removed in microliter amounts," Dr. Peyman says. "By increasing the fluid, you cause the lens to become more convex; by removing fluid, you cause it to become more concave. This is also the case for the cylindrical lens, which has a different configuration. We've tested the accuracy of the system using an eye model on an optical bench, placing standard spherical or cylindrical lenses in front of the eye to create optical aberrations. We found that our system was able to precisely correct the spherical and cylindrical errors. (See images, above.) Right now, our fluidic spherical lens can correct from plus to minus 20 diopters; the cylindrical correction can be plus or minus 8 diopters. Of course, we won't normally need that big a range. "The advantages of such a system will be numerous," he continues. "A bulky instrument will be replaced with the equivalent of a large pair of glasses; the system will be able to correct spherical and cylindrical error to one-tenth of a diopter; and the refractive adjustment will be smooth, continuous and remotely controllable, either by the patient or a technician. This will eliminate the need to hire a skilled person to do the exam; it will reduce chair time, contributing to office efficiency; and it will be portable and very patient-friendly. Furthermore, it will be possible to use it for rapid screening of a large group of patients—something you can't do with today's phoroptors."
A more precise phoroptor, of course, might call for greater precision in the manufacture of corrective lenses. That wouldn't be an issue for preparing spectacles, but could conceivably be a concern when trying to match a very precise prescription with an equally precise contact lens. Dr. Peyman responds that customization is the direction in which everything is headed. "I'm not an optician, but with contact lenses, one size does not fit all," he notes. "In addition to the refraction, corneal curvature is not uniform for every patient, to say nothing of higher-order aberrations. Optimizing for individual pa tients is not a new idea; even wavefront customized contact lenses are being developed.
"Of course, I can't guarantee there won't be any problems in this respect," he continues. "When you're trying to move forward with new technology, the difficulties you'll encounter along the way can't always be foreseen. But from my current perspective, I doubt that this will be an issue."
Assembling the Puzzle
Dr. Peyman says that for now it's too early to make predictions about when a finished instrument will be- come available and reach the marketplace. However, he notes that there will be far fewer hurdles to overcome in the process of reaching that goal than companies face when developing a new medication or surgical instrument, because this is an external de- vice requiring far less evidence of safety and efficacy. "We've proven the concept," he says. "There's an enormous amount of work left to do before we have a final, marketable instrument, but we're definitely moving in the right direction."