Adaptive-optics technology originally developed by the US military to clear up images from satellites is now being used in reverse to take the sharpest photographs ever of the human retina. Near term, this could lead to earlier detection of diseases such as glaucoma and macular degeneration. Ten years from now, it may lead to contact lenses custom-made to correct for the tiniest of vision aberrations.
"We can now make out individual cone cells in the retina for the first time," says David Williams, codeveloper of the adaptive-optics-based system and the William G. Allyn Professor of Medical Optics at the University of Rochester (Rochester, NY). "The images have a twofold increase in resolution and their contrast and detail are six times better than ever before."
This imaging capability may prove invaluable for noninvasive examination of the retina, which is the only visible component of the human nervous system. The technology also allows a clearer view of the capillaries in the back of the eye, says Rochester postdoctoral researcher Austin Roorda, making it easier to catch the early signs of diabetic retinopathy, which can lead to harmful bleeding if left untreated.
This imaging technology may also make it possible to give people, even those with 20/20 eyesight, a kind of supernormal vision. For example, the researchers found that eyes with adaptive compensation can resolve fine gratings (55 cycles/degree of visual angle) that are invisible under normal viewing conditions.
Williams and colleagues became interested in enhancing vision with adaptive optics soon after the technology was declassified in the early 1990s. Light passing through the aberrated eye produces dynamic wave-front error much like that caused by atmospheric turbulence in ground-based telescopes. The researchers thought there was potential to correct for the total wave aberration of an eye while coping with the large variation in aberration patterns from eye to eye.
According to Williams, the adaptive-optics-based system measures an aberrated wave front at the eye's pupil formed by light reflecting back when the eye focuses a collimated laser beam onto the retina. The system includes a Hartmann-Shack wave-front sensor to measure the distorted wave front and a deformable mirror to compensate for the eye`s wave aberration.
The wave-front sensor, which Rochester research scientist Junzhong Liang adapted specifically for the eye, is based on a hexagonal array of 217 lenslets, each of which records a single ray of light reflected by the retina to provide something akin to a contour map of light rays traveling through the eye. In a perfect eye, Williams says, all 217 light rays would be parallel, so any deviations would indicate optical distortions. With the help of a computer, the sensor then translates the locations of these light rays into slight adjustments to the shape of the deformable mirror to correct for the retina-obscuring imperfections of the cornea and lens. The mirror basically bends light rays just enough to make them parallel, which allows the light to slice through the optical distortions that normally prevent a clear view of the retina.
Each lenslet in the sensor has a 97-mm focal length and a 0.5-mm diameter. The system magnifies the pupil plane by 1.25 at the plane of the lenslet array, which samples the wave front at the pupil with a 0.4-mm center-to-center spacing across the central 6.76 mm of the pupil. Every lenslet forms an image of the light spot on the retina on a cooled CCD array with 512 × 512 pixels, and the displacement of each image on the CCD gives the local wave-front slope. From this array of slopes, the system then reconstructs the wave front using a least-squares technique.
The system's deformable mirror, from Xinetics Inc. (Devens, MA), has an aluminized glass face plate with 37 lead zirconate-lead titanate actuators mounted in a square array on the back surface. The mirror's stroke beneath each actuator is ۬ µm, allowing an 8-µm shift in the reflected light ray.
The mirror sits in the plane conjugate with both the pupil plane and the lenslet array. Although actuators lie 7 mm apart, the system magnifies the eye's pupil plane 6.25 times at the deformable mirror to produce a 1.12-mm actuator spacing in the pupil plane. The researchers chose the high sampling density of the wave-front sensor to capture most of the higher-order aberrations, even though Williams reports that the mirror cannot compensate for all of them. Williams, Liang, and postdoctoral researcher Donald Miller first reported the work in the November 1997 Journal of the Optical Society of America.