Evaluating Ultraviolet
Transmittance of a Disposable Lens

BY SALENA LEE, O.D.; TINA HAN, O.D.; QUYEN NGUYEN, O.D.,
& MICHAEL G. HARRIS, O.D., J.D., M.S.
SEPT. 1996

This study assesses the Precision UV lens, the first soft disposable with UV protection.

Acute and chronic exposure to ultraviolet radiation has harmful effects on ocular structures. Various RGP and soft lenses contain UV-blocking materials that protect against the sun's deleterious rays. This study evaluated the UV transmittance of PBH's Precision UV lens, the first disposable soft contact lens with a UV blocker. The manufacturer claims that this lens absorbs an average of 90 percent of harmful UVA and UVB radiation.

UV REVIEW

UV radiation consists of three bands: UVA (320nm to 400nm), UVB (280nm to 320nm) and UVC (200nm to 280nm). UVA is closest to visible light and is absorbed by the crystalline lens, but can affect the retinal tissue if the lens is absent. UVB, the most dangerous UV radiation, is absorbed mostly by the cornea and can cause sunburn, photokeratoconjunctivitis and cortical cataracts. UVC is mostly absorbed by the atmosphere and is not harmful to our eyes.

STUDY PROCEDURE

We obtained 20 factory-sealed lenses from the manufacturers of the Precision UV test lens and the Acuvue non-UV-blocking control lens. After cleaning and rinsing each test and control lens with Bausch & Lomb's Sensitive Eyes saline solution, we mounted the lenses in a quartz lens holder. For each lens, we used a Zeiss model DM 4 Dual Beam Spectrophotometer to take three optical density readings at every 10nm increment from 280nm to 400nm. We calibrated the spectrophotometer before measuring each lens to minimize the effect of machine drifting on the accuracy of results. At every wavelength, we averaged the three optical density readings for each lens and calculated standard deviation values. We determined the transmittance of each average value using the formula: Optical density = log 100% transmittance.

RESULTS

We found that Precision UV disposable soft contact lenses effectively reduced transmittance of the ultraviolet spectrum (Fig.1). By applying the Student's t-test of two independent variables to our data, we found a significant difference between the absorbency characteristics of the two lens types (p <0.050). From 290nm to 370nm, the Precision UV lenses absorbed more than 90 percent of the radiation. However, at extreme ends of the UVA and UVB spectrums, namely at 280nm and at 380nm to 400nm, there was less than 90 percent absorption. In contrast, the control disposable lenses absorbed less than six percent radiation for all wavelengths from 280nm to 500nm. Thus, the control lenses showed no difference in absorption of ultraviolet radiation and of visible light.

MEASURING UP

Our results indicate that the ultraviolet-absorbing properties of Precision UV lenses are comparable to those of previously studied RGP and soft lenses with UV blockers. Harris et al. (1994) found that in the range of 280nm to 380nm, rigid gas permeable lenses treated with UV blockers, i.e., Boston RXD and Equalens, absorbed 90 percent and 97 percent respectively. The same 1994 study found that Suntact contact lenses manufactured with UV-blocking material transmitted one percent of the radiation from 280nm to 370nm.

Past studies on transmittance of UV radiation through soft contact lenses showed that non-UV-absorbing types do not provide adequate protection against harmful rays.

With mounting evidence of the harmful effects of UV radiation on the human eye, practitioners need options for providing adequate UV protection for their contact lens-wearing patients. Our study indicates that Precision UV lenses provide an effective disposable lens option when prescribing soft contact lenses to patients who spend a lot of time outdoors. CLS

References are available upon written request to the editors at Contact Lens Spectrum. To receive references via fax, call 1-800-239-4684 and request Document #17. (Be sure to have a fax number ready.)

Drs. Salena Lee, Tina Han and Quyen Nguyen are 1996 graduates of the University of California School of Optometry. Dr. Harris is a clinical professor, an associate dean and the chief of the Contact Lens Clinic at the University of California School of Optometry.