L&T: In the Lens Lab

Jul
2002

Those Chameleon Lenses



Those Chameleon Lenses

Simpifying the science behind photochromic lenses

By John Young

Corning invented photochromic lenses before many of you reading this column were born (circa 1960). The first photochromics were glass and were a popular product until the introduction of plastic photochromics in the early 1980s. The initial plastic photochromics didn’t fare well—there was minimal change from clear to dark and these lenses had limited life expectancy—but the product has grown and improved continuously over the years.

Plastic photochromics have, however, always had a common issue: The same ultraviolet (UV) light activating their darkening capabilities also deteriorates the chemistry giving them that darkening capability over time. Glass photochromics never had this problem because they work using Silver Halide chemistry, the same chemistry used in photographic technology. With glass photochromics, the Silver Halide remains “trapped” in the lens and the lens can continue to work (i.e., lighten and darken) for a long period of time.

Because Silver Halide chemistry is impossible in a plastic lens, plastic photochromics use varying types of Spiral Helix chemistry. The Spiral Helix (Indolinospironaphthoxazine, if you want to impress your friends), for instance, has proven to work well when it is “imbibed” into the surface of a plastic lens through the creation of a special resin allowing for maximum penetration of the chemistry into the lens surface. We can realistically expect plastic photochromics manufactured this way to last anywhere from two to four years, depending on the wearers’ geographical location and usage. We are seeing improvements in life expectancy as we continue to test newer products using this process.

The Spiral Helix chemistry is also frequently applied to the lens surface in one of two ways—either via “front surface imbibing of the photochromic chemicals,” which as the name implies involves pushing the chemicals deep into the front surface of the lens, or “dip coating,” which involves dipping the entire lens into the chemicals and surface coating both the front and back sides. In these “dip coated” products, the thicker the coating the better the performance, especially where it concerns longevity. This family of photochromic lenses has a somewhat shorter life expectancy than imbibed plastic products. But coated lenses do perform well and have characteristics similar to imbibed products in areas such as activation, de-activation and temperature dependence. Both coated and imbibed lenses have another advantage as well: When a high-minus lens containing color (i.e., tint) throughout is surfaced to prescription, the center can be lighter than the outer areas of the lens. With coated or imbibed lenses this does not occur.

Finally, some manufacturers cast the Spiral Helix chemistry directly into the lens at the time of fabrication. “Cast” or “in-mass” plastic photochromics have a longer life expectancy than other products because their chemistry “resides” evenly throughout the lens material. Even though the photochromic capability can still deteriorate with UV exposure, there is a larger supply of the chemicals within the lens, resulting in a longer life expectancy based on our tests.

All of these plastic photochromic products activate and de-activate based on the manufacturer’s formula and remain reasonably independent of the type of system used to fabricate the product. Only the longevity seems to be impacted by the manufacturing process. Manufacturers have worked to address this issue with some success. Because of their indoor/outdoor convenience, these products offer many advantages to both consumers and dispensers, especially as they continue to improve.

John Young is an ophthalmic lens expert with more than 25 years experience in the optical industry. He has worked for several lens manufacturers, including American Optical and Essilor, and is the former technical director of the Optical Industry Association. His company, COLTS Laboratories, is a Clearwater, Fla.-based independent lens testing facility designed to provide thorough and accurate quality and performance evaluations of spectacle lens products. His clients include lens manufacturers, wholesale labs, independent research organizations, large retailers and independent dispensers. The lab was the first U.S. ophthalmic testing laboratory accredited by the American Association for Laboratory Accreditation. It is also a Safety Equipment Institute-accredited eye protection/safety test lab. Young can be reached by phone at (727) 725-2323 and by email at
john@colts-laboratories.com.

 

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