Progressive lenses, or those which allow the wearer to shift from distance to near vision, entered the eyewear market decades, ago but their design is finally starting to improve.
Progressive lenses made their commercial debut in the middle of the last century. Also known as progressive addition lenses (PAL), the lenses enable eyeglass wearers to transition from distance to near vision without the image jumping when the eyes shift from one distance zone to another. They also have a cosmetic benefit: there is no line as there is with bifocals.
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In the past, producing these lenses was no easy feat. The traditional process required a semi-finished lens with a standard front. The prescription would be ground onto the back surface with a generator. Calculating the geometry to impart on the lens involves complicated mathematics. "You can imagine doing this without Excel spreadsheets to do all of your calculations," said Dennis Fong, OD, clinical instructor at the UC Berkeley School of Optometry. "Everything was originally done by hand and then you see pictures of these old generators that made forms and then you use the forms to create the complex curvature."
Although the process used to create the lens has steadily improved over years, drawbacks remained, including the fact that distortion or aberration is present in the lower periphery of the lens. "That type of distortion bothers people a lot," Fong said. "It can make them feel nauseated. And the higher your prescription, the bigger the difference between the top and the bottom, the more distortion is going to be out there. Those were the negatives in general of the traditional progressive design."
THE ADVANTAGES OF FREE-FORM PROGRESSIVE LENSES
The debut of digital processing and the ability to create free form lenses has enabled optometrists to craft lenses that better suit the needs of the user while minimizing the distortion inherent in progressive lens designs. These free-form lenses are made using a three-axis, computer numerically controlled (CNC) generator to carve a complex surface that looks something like a 3-D topographical map. In addition, lenses made this way have a resolution up to six times greater than traditional progressive lenses.
A recent study performed at the University of California, Berkeley supported the benefits of the free-form lenses, including "statistically significant preferences for the optically customized free-form lenses over the non-free-form lenses." The study also found that subjects "reported a wider field of undistorted vision when looking through the reading zone of the test spectacles." Unsurprisingly, the study found that consumers prefer the lenses over conventional progressive lenses, that they were able to adapt faster to the lenses, and that they reported a wider near-vision zone.
This last point is becoming increasingly important as a growing share of the population is spending a sizable amount of time looking at objects in the near-to-intermediate visual zone, such as iPads, Kindles, smartphones, and computers. This has contributed to the demand from users to widen the intermediate zone in the progressive lens. Most design companies report that one of the major things that users don't like about progressives is the fact that the intermediate zone in the lenses is too narrow. Using digital processing, however, it can be made wider than it has been traditionally.