Ditch those bifocals. You might soon be wearing spectacles whose lenses allow you to see clearly regardless of how long or short sighted you are.
With age, the lenses in our eyes often lose the ability to change shape enough to focus light from near objects onto the retina - a condition called presbyopia. This leaves people who were already short-sighted unable to focus on either near or distant objects. Bifocals offer a solution by having two lenses in the same frame, but users must get used to tilting their head up or down to switch focus.
Zeev Zalevsky at Bar-Ilan University in Ramat Gan, Israel, has developed a technique to turn a standard lens into one that perfectly focuses light from anything between 33cm away and the horizon.
It involves engraving the surface of a standard lens with a grid of 25 near-circular structures, each 2mm across and containing two concentric rings. The engraved rings are just a few hundred micrometres wide and a micrometre deep. "The exact number and size of the sets will change from one lens to another," depending on its size and shape, says Zalevsky.
The rings shift the phase of the light waves passing through the lens, leading to patterns of both constructive and destructive interference. Using a computer model to calculate how changes in the diameter and position of the rings alter the pattern, Zalevsky came up with a design that creates a channel of constructive interference perpendicular to the lens through each of the 25 structures. Within these channels, light from both near and distant objects is in perfect focus.
"It results in an axial channel of focused light, not a single focal spot," Zalevsky says. "If the retina is positioned anywhere along this channel, it will always see objects in focus."
Zalevsky has fitted one of his lenses to a mobile phone camera to confirm the extended focus effect, and he has also tested the lenses on 12 volunteers (Optics Letters, vol. 35, p. 3066). He has now co-founded a company, Xceed Imaging, to develop the technology.
The approach is not without its problems, though: the interference pattern tends to cancel out some of the light passing through the lens, which reduces the contrast of images viewed through it. Pablo Artal of the University of Murcia, Spain, warns that if the contrast reduction becomes too large, the brain will struggle to interpret the information.
Zalevsky counters that people wearing the lenses do not notice a loss in contrast because the eye is very sensitive to light at low intensity. "Unlike a camera, the brain has a logarithmic and not linear [response to light] ." He says that the brain adapts to and minimises the reduced contrast within a few seconds.
This is not the only way in which the brain must adapt to the new lenses. Fixed in a pair of glasses, the lenses would not move as the eye looked in different directions, so the focusing effect would be lost in the regions between the circles. But Zalevsky says that the eye learns to fill in the gaps as it moves from one engraved structure to another, generating a continuous effect.
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