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J. M. Zavislan, D. Brown, T. Brown, G. Yoon, J. V. Aquavella; Measurement of Lipid Layer Dynamics Using Imaging Ellipsometry. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5678.
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© ARVO (1962-2015); The Authors (2016-present)
To develop non-contact metrology that dynamically measures the refractive index and thickness of the ocular lipid layer using spatially-resolved polarization-sensitive imaging.
We have developed and built a prototype imaging ellipsometer combined with a chin rest that can measure the change in polarization of light (polarization parameters ψ and Δ) that is specularly reflected from the ocular tear surface. The addition of polarization sensitive detection augments the information of traditional tear interference imaging, which is sensitive to the optical thickness of the lipid layer (the product of the refractive index and physical thickness), and allows for the independent measurement of both the lipid layer refractive index and physical thickness. Our prototype imaging ellipsometer measures ψ and Δ across an annular region (2 mm inner diameter to 7 mm outer diameter) of the ocular surface at 32 measurements per second for up to 45 seconds in red, green and blue regions of the visible spectrum with a lateral resolution at the cornea of 60 µm. The imaging ellipsometer can be operated in a climate controlled room where the relative humidity can be continuously varied from 20 % to 90% and the temperature from 60 °F to 95 °F.
The time evolution of the refractive index and physical thickness of the lipid layer can be extracted from ψ and Δ maps. The prototype ellipsometer has standard deviation of ψ and Δ is less than ±1° and ±2°, respectively for both the green and the blue channels. The standard deviation of ψ and Δ is less than ±2° and ±4°, respectively for red channel. The red channel has higher uncertainty because the average radiance of the source is lower in the red. This uncertainty in ψ and Δ maps into lipid refractive index uncertainty of =±0.025 and lipid thickness uncertainty of =±7 nm assuming an initial lipid thickness of 150nm and a nominal lipid index of 1.482.
Ellipsometric measurement of the lipid layer provides information to dynamically extract both the lipid layer refractive index and its physical thickness. The imaging ellipsometer we have developed will allow us to resolve both spatially and temporally changes in the lipid layer refractive index and physical thickness. Using this capability we plan to objectively characterize the role of the lipid layer in dry symptoms and the interactions between lipid and aqueous layer throughout the blink cycle.
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