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J. Nam, N. L. Himebaugh, H. Liu, L. N. Thibos; Measuring Light Scatter With a Shack-Hartmann Wavefront Aberrometer. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3947.
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To develop a quantitative method for specifying the amount of light scatter at different locations in the eye’s pupil. A spatially-resolved measure of light scatter may be useful for studies of tear film, cataract, corneal transparency, and refractive surgery.
We assume the mechanism of light scatter by the eye can be modeled as a random phase screen (Goodman, Statistical optics, 2000). Invoking reasonable simplifying assumptions, the point-spread function (PSF) of a single lenslet in a Shack-Hartmann wavefront aberrometer (SHWA) is the convolution of two other PSFs. First is the PSF due to ocular wavefront aberrations over a sub-aperture defined by the lenslet face, and second is the PSF due to light scattered by that portion of the random phase screen associated with the lenslet. This convolution relationship enables a simple rule: the size of the lenslet’s PSF is the sum of the size of the PSF due to wavefront aberrations and the size of the PSF due to scatter. We define "size" in this context as the radial variance of the light distribution in the PSF. Applying this rule twice to account for the double-pass nature of the SHWA enables an accounting of the amount of blur on each pass. The method was validated on a theoretical test case and feasibility was assessed with a human eye before and after tear breakup. The theoretical model subtracted a Gaussian function C*exp(-R/2S) from the wavefront error map, where R is the squared radius of points over the lenslet aperture relative to the lenslet center.
For a range of parameter values (-1<C<2 microns; √S < 3*lenslet diameter), the measured size of the double-pass image produced by each lenslet agreed with expected values with less than 2% error. Measurements on a human eye before and after substantial breakup of the tear film were feasible and indicated a significant, measurable increase of SHWA spot size due to light scatter.
Spatially-resolved measurements of light scatter using a SWHA is a valid procedure that is feasible for quantifying the light-scattering effects of tear film breakup.
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