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.
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,” says 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,” Dr. Fong says. “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, computers, and the like. 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.
The aforementioned study shows that digital processing has the ability to optimize the lenses’ performance. “The exciting thing for me though is that it is at the infant stages,” Dr. Fong says. The accuracy of digital processing allows the scientists who design these computer-generated lenses “to actually change the curvature differently than we would if we were just using traditional methods” to optimize the wearer’s off-axis views. In other words, the lens will account for the “as worn” position of the glasses, where “the premium designs of digital backed processing optimize for your prescription but also peripheral aberration,” says Dr. Fong. “That is where the state of the art is: taking into account the ‘as worn’ position to optimize the visual experience.”
Dr. Fong gives an example of a person with a lifestyle that is familiar to many of us. The person sits in front of a computer all day during the week, using a lot of intermediate and near vision. The person also needs to drive home, which involves distance vision. And, on the weekend, the person likes to decompress by, for example, skiing or biking, which again involves distance vision. Now, says Dr. Fong, “companies are trying to figure out algorithms for understanding a user’s need by having them take a survey. Then, when they design a pair of glasses for your weekend stuff, it will be optimized for that.” And, the pair of glasses for office work will be optimized for that task. Dr. Fong notes that “you can technically optimize for all of those things with a digital processing because it can change that backed surface to anything they want.” The result is that glasses with the same prescription can now theoretically account for a different frame style and different lifestyle to improve vision during different tasks.
Dr. Fong adds a word of caution, however: “With anything just starting out, some ideas are interesting but they don’t necessarily work.” On the other hand, “with the theory behind it, it should not be any worse than the traditional,” says Dr. Fong. And, as customizations increase so too do the chances that this new approach will be better than the traditional approach.
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