Purchase this article with an account.
Jayoung Nam, Larry N. Thibos, Arthur Bradley, Nikole L. Himebaugh, Haixia Liu; Using A Shack-hartmann Wavefront Aberrometer To Measure Forward Light Scatter Caused By Tear Film Breakup. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5276.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
At ARVO 2010 we presented a quantitative method for using a Shack-Hartmann wavefront aberrometer (SHWA) to measure the amount of light scattered by micro-aberrations within the eye’s optical system at different pupil locations. We have applied this new, spatially-resolved method to quantify optical changes of fine-scale micro-aberrations of the tear film during tear break-up (TBU).
Forward light scatter was modeled as a random phase screen (Goodman, Statistical Optics, 2000) added to the eye’s wavefront aberration map. Because the tear film is thin, we can assume light arrives and leaves at the same location in the pupil, but localized variations in tear thickness over the apertures of individual lenslets causes light rays to scatter in random directions. We model this light scatter as a product of deterministic and stochastic pupil functions (which implies additive phase errors) for each lenslet in the SHWA. Under reasonable assumptions, the average point spread function (PSF) of a single lenslet is the convolution of the PSF for deterministic macro-aberrations with an average PSF for the stochastic micro-aberrations. Their combined blur, as quantified by radial variance V, is the sum of radial variances for macro-aberrations and micro-aberrations. Applying this rule twice for a double pass optical system, we estimated PSF blur from the 2nd pass micro-aberrations only as: V(PSF from 2nd pass micro-aberrations) = V(PSF from double pass total aberrations) - V(PSF from 2nd pass macro-aberrations) - (L+U)/2. L and U are lower and upper bounds for the first pass blur and can be estimated from the measured double pass PSF blur.
The spatial patterns of light scatter topographically match the spatial patterns of TBU. On average the gross macro-aberrations contribute about 13% of the double pass total blur before and after substantial breakup of the tear film. For the 2nd pass only, macro-aberrations account for only 30% of the blurring of individual SHWA spots. The remaining 70% of the 2nd pass blur is caused by unresolved micro-aberrations. For this method to be effective, camera resolution must be less than 1 arcmin/pixel.
Radial variance is a convenient measure of the spatially-varying increase of light scatter during TBU. The fine micro-aberrations that are detected, but not resolved, by SWHA are a major component of the eye’s total aberrations.
This PDF is available to Subscribers Only