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Karl M. Citek, Francisco De Ayguavives, Eric Johnson, Gabriel Keita; Eye Sun Protection Factor (ESPF): A New Index that Considers Spectacle Lens Coating, Curvature, Configuration, and Coverage. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3072.
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Most spectacle lenses provide adequate protection from ultraviolet radiation (UV) that could be transmitted through the lens, as required by various standards. However, previous research (Hall & Schultmeyer, 2002; Reichow et al., 2006) demonstrates that UV can strike the eye directly from around the lens if it does not offer sufficient coverage, based on lens curvature, size, shape, and placement. In addition, environmental factors, such as weather conditions (Sydenham et al., 1997) and sun angle (Sasaki et al., 2011) can affect how much UV reaches the eye. Finally, some antireflective coatings (AR) actually increase the reflectance of UV (Citek 2008), potentially allowing UV from a source behind the wearer to reflect from the back surface of the lens into the wearer’s eye. While certain indices take into account some factors in addition to transmittance, none consider the contribution and interaction of all factors, especially reflectance.
Reflected UV was measured from the back surfaces of typical prescription lenses (curvatures 1.37-9.12 D) of various plastic materials with AR from 5 major lens manufacturers. Measurements were made at various angles of incidence (8-45 deg), which correspond to turning slightly when facing away from the sun. Transmitted UV was also measured. A mathematical model, Eye Sun Protection Factor (ESPF), was developed as a ratio comparing UV exposure of the eye with and without eyewear, considering the various lens parameters and sun angles. Higher ESPF values represent better overall protection.
Reflected UV for the lenses tested generally decreases as incidence angle increases, from an average of 28.6% (range 8.2-54.4%) at 8 deg to 20.0% (range 6.2-35.2%) at 45 deg; by comparison, non-AR lenses (scratch-coat only) reflect about 5%. Transmitted UV for all lenses tested is less than 0.6%. Based on reflectance and transmittance results only, ESPF ranges from 10 to 61 for the lenses tested. Changes in lens size, curvature, vertex distance, and tilt can change the amount of UV that will reach the eye, either directly or due to reflectance, thus changing the ESPF value.
Various spectacle lens parameters and environmental conditions contribute to the protection of the eye from UV. Prescription and non-prescription eyewear that is intended to be worn outdoors must protect the wearer from UV not only from the front, but from the sides and back as well. ESPF is a new index that takes these factors into account. It can contribute to knowledgeable decisions about lens and frame parameters for eyecare professionals and patients alike.
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