Ellipsometry of human tears.
PURPOSE
The outer surface layer of tears is presumably composed of lipid. The thickness of this layer is considered critical to retard evaporation. Prior thickness measurements differ widely. Advances in ellipsometry have availed more precise and accurate measurements for thin films. The range in thickness of the surface layer of tears was studied by ellipsometry to uncover the source of prior discrepancies.


METHODS
Tear surface layers of normal and dry eye subjects were measured by in-vitro ellipsometry. Lateral and Z resolutions of ∼1 μm and 0.1 nm, were achieved respectively. Thicknesses were derived from matrices and a Levenberg-Marquardt multivariate regression algorithm to Fresnel equations for multi-layered films.


RESULTS
Ellipsometric measurements of pooled and individual human tears in-vitro revealed a larger overall range (0-500 nm) of surface film thicknesses than previously reported by any one study. Each sample showed thin areas (0-2.6 nm) with interspersed thicker regions (∼200-500 nm). Repeat measurements of a single donor collected at weekly intervals showed a broad range of surface thicknesses within and between samples. Thickness measurements from a dry eye subject overlapped that of normal subjects.


CONCLUSION
The data show that published disparity in surface film thickness may be attributable to limitations of prior methodologies. The range and overlap of surface film thicknesses challenge less rigorous methodologies that claim to segregate normal and dry eye.
['Glasgow|Ben J|BJ|']
D015352:Dry Eye Syndromes D006801:Humans D007368:Interferometry D050356:Lipid Metabolism D008537:Meibomian Glands D013666:Tears
Ellipsometry dry eye Interferometry Lipid layer thickness Meibomian lipid Reflectometry Tear thickness