INTRODUCTION. Several patients returned spectacles to our dispensary, indicating that the clear lenses had yellowed after less than 2 years of continuous wear. The lenses were made of Trivex (PPG), a proprietary monomer introduced in 2001. We hypothesized that exposure to ultraviolet radiation (UV) caused the lenses to yellow.
METHODS. We purchased 3 pairs of single vision lenses of each of Trivex, CR-39, and polycarbonate in + 3.00 D, -3.00 D, and plano with scratch-resistant coating only; and a fourth pair of each material in -3.00 D with anti-~eflective coating. One lens of each pair was the test lens, the other was the control. We used a UV curing oven (Dymax), whose output at 365 nm is about 2.34 times greater than the maximum solar irradiance at midday. Assuming peak UV of 4 hrs per day, test lenses were exposed for periods totaling 48 weeks of simulated environmental exposure. Spectral transmission was measured at the center of each lens with a Lambda 20 UV/VIS Spectrometer (Perkin Elmer). Data were analyzed for visible, UV, and infrared (IR) using algorithms described in US and international standards.
RESULTS. All test lenses increased in optical density with increased UV exposure; thicker lenses and ARC lenses increased less. Trivex initially changed sooner than the other materials (at least 1% change in all lenses by 12 weeks of simulated exposure), but CR-39 increased the most (up to 9.3%) after 48 weeks of simulated exposure. Shift in color for all lenses was consistently toward 568-570 nm ("yellow"), and saturation increased most quickly and greatest for Trivex (up to 12.8%), followed closely by polycarbonate. All CR-39 increased in UV-A transmittance, while Trivex remained constant, and polycarbonate actually decreased. All CR-39 also increased in UV -B transmittance and decreased in IR transmittance, while the other materials remained constant.
DISCUSSION. Short-term, high-intensity UV exposure can simulate environmental conditions. Clear plastic lenses are susceptible to yellowing, darkening, and reduction of UV absorption, based on material and lens thickness. These changes are reduced, but not eliminated with antireflective coating.
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