Ophthalmic lenses: accurately characterizing transmittance of photochromic and other common lens materials.
Ophthalmic lens materials are characterized by their spectral transmittance and by derived luminous and average UV-B and UV-A transmittances. Photochromic materials, whose transmittances depend on previous and immediate radiant exposure and other environmental conditions, require special methods of spectrophotometry, which are described here. Nonphotochromic materials are characterized by customary spectrophotometry. Transmittances of seven common ophthalmic lens materials are reported (white crown glass, allyldiglycol carbonate and polycarbonate plastics, and four photochromic glasses). The transmittance spectra of sunlight-darkened photochromics are determined by extrapolation to the instant of removal from exposure by using a sequence of rapidly scanned spectra recorded at precisely timed intervals after removal. Luminous transmittances and average UV-B and UV-A transmittance of all materials tested agree well with published values.
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