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TN Pira, P Custer; Increased Radiation Exposure of the Lower Lid From Reflected Ultraviolet Light From the Cornea: A Computational Analysis . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3035.
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Purpose: The incidence of squamous cell and basal cell carcinoma of the lower lid is higher than the incidence on the upper lid. A possible explanation is increased exposure to ultraviolet radiation of the lower lid margin from reflected ultraviolet light from the tear film overlying the cornea. Using principles of optical reflection, the theoretical amount of additional radiative flux can be calculated. Methods: A mathematical model was developed using normal values of corneal curvature and basic principles of reflectivity at an air-water interface. An ideal spherical surface of the cornea was assumed with the lower lid margin at the base of the cornea. Fresnel’s law of reflection was used to calculate the amount of reflected light from the cornea at different incident angles of sunlight. All incoming rays were assumed to be parallel. Only the reflection off the anterior tear film surface was considered. Results: Preliminary data demonstrate a maximal increase in radiation intensity on the lower lid when the angle of incident light is 72.9 degrees above the horizon. This result applies only to the portion of the lower lid directly below the most anterior aspect of the cornea. Most of the reflected rays come from 1.4 to 2.2 mm below the apex of the cornea, where a glancing angle causes the majority of the light to be reflected instead of transmitted. The quantity of additional flux depends on assumptions of lid width and the number of discrete computational corneal segments used in the model, but appears to be at least 50% over the normal incident flux. Conclusion: In theory, there is increased radiative flux to the lower lid from reflected ultraviolet light from the cornea. The higher incidence of carcinoma on the lower lid may stem, in part, from the higher exposure to ultraviolet radiation. However, the amount of reflected ultraviolet light may not be enough to explain the difference in incidence of carcinoma between the lower lid and upper lid. Confirmation of the theoretical results can be obtained by laboratory measurements using a simulated corneal surface or by real measurements on human lids.
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