New concepts in wavefront aberrometry may help further optimize lens prescriptions
Research advances in the area of wavefront aberrometry have yielded new understanding of visual optics, the branch of optics concentrated on the role of the eye and brain on how we perceive images. This could result in a number of benefits, including better lens prescriptions and a way to detect accommodation errors.
“Wavefront aberrometers use a laser beam to put a spot of light onto the retina, and when that light comes back out, it goes through an array of lenses, and each lens isolates a small portion of the beam of light. Depending on where that beam goes, we can figure out the slope of the wavefront, and if we can measure the slope of the wavefront at numerous places along the wavefront, we have all the information we need to be able to reconstruct that wavefront of light,” Larry N. Thibos, PhD, FAAO, told the attendees at the American Academy of Optometry meeting award ceremony.
Having a way to measure aberrations and how rays of light refract through the lens has given optometrists an important advantage in determining prescriptions. Higher-order aberrations no longer need be ignored; rather, they can be taken into account to describe the best possible corrective lens, insofar as the best prescription is the one that optimizes retinal image quality, Thibos said.
However, there is substantial difficulty in accessing retinal image quality, as the only person in the world who can see a retinal image is the patient, Thibos explained.
A solution to this problem, Thibos told the audience, is that if the aberrations can be measured accurately and with enough precision, the retinal image of any object can be computed using the principles of physical optics.
“The principle I offer is that better performance indicates better images; and conversely, better images yield better performance,” he said. “With that rule in place, it becomes possible to do objective refraction.
“So, you take the wavefront aberration map, calculate the point-spread function — which itself is the description of the image – and boil that down to a single number,” Thibos continued. “To do that, we have an image quality calculator that we use to get that single number, then we maximize that number by adjusting the lens. And that should provide the best prescription for that aberrated eye.”
From there the concept can be broadened beyond simply measuring refractive error to measuring the refractive state of an aberrated, accommodating eye, he added.
According to Thibos, because the wavefront of the eye can be measured at any given moment, it can be measured as eyes accommodate and relax, and, as such, the changes in the wavefront can be measured.
“The main change to that wavefront is defocus, but the aberrations of the eye are changing too. In fact, for most people, when you accommodate, the spherical aberration changes sign. So instead of the paraxial focus being further away from the eye, it’s actually closer to the eye. That expands your range of accommodation, if you’re using clearer and sharper as your criteria,” he said in his lecture.
“We wouldn’t have been led to that insight if we hadn’t taken aberrations into account,” he added.
From here, with a method of determining the refractive state of an aberrated, accommodating eye at hand, there is now an opportunity to optimize the retinal image quality even further, Thibos said.
“The patient’s looking at a target, and you adjust the target into a place you want. The eye then accommodates. But is that target in the optimum location, or could we have gotten a better retinal image? Well, we trace our rays and find that we could’ve had a better retinal image if we put the target slightly differently. That indicates an accommodative error,” he said.
“For example, is the retinal image quality maximum for all states of accommodation? Or are we, as the textbooks say, all perpetual laggards and we don’t accommodate enough to improve the image quality to the best it could possibly be?” he said. “How do changes in higher-order aberrations and pupil size during accommodation affect refractive state? How do age-related changes in accommodation affect image quality? Do presbyopic theories employing aberrations work? And if not, why not?”
Thibos was the 2012 recipient of the Charles F. Prentice medal for his work in studying the refractive state of aberrated eyes. – by Daniel R. Morgan