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Wide-Angle Point Spread Function of the Human Eye: An Optical Procedure to Measure Scatter


Purpose: : Different ocular pathologies may increase the naturally occurring light scattering in the human eye, affecting retinal image quality and visual performance. However, accurate and reliable measurements of the amount and nature of the intraocular scatter are not yet available. We addressed this limitation, by developing an optical instrument and the related methodology for the measurement of light scatter in the living human eye.

Methods: : The experimental setup is based on a double-pass (DP) configuration. Series of disks of uniform radiance having angular dimensions ranging from 0 to 9.1 degrees (radius) are projected on to the eye’s retina and the DP images are recorded by a cooled electron-multiplied CCD camera. The disks are created using a computer-controlled liquid crystal spatial light modulator operated in amplitude mode that is back-illuminated by a filtered Halogen lamp (520 nm ±42 nm). Two spatially separated sub-apertures at the pupil are used for stimulus projection and imaging to avoid back-scatter and reflections. The recorded images of a sequence of disks were analyzed to obtain the radiant intensity at the center of each disk. The derivative of this value with respect to disk radius (in angular units) was fitted to a mathematical model of the wide-angle double pass point-spread function (PSF) and from that the underlying single-pass PSF was computed. The setup was first validated using artificial eyes with known scattering properties and used in a group of normal human eyes.

Results: : The derivative of central disk intensity with respect to disk radius was measurable up to the complete range of the angular dimensions of the projected disks (9.1 degrees of visual field). This provides an indirect measurement of the PSF in a range spanning more than six orders of magnitude. Different features can be obtained from the estimated wide-angle PSF to characterize not only the amount of scatter but also its angular dependence. The outcome of the measurement is sensitive to alignment with the pupil of the eye and since a relatively homogeneous area of the fundus is used for analysis, special care should be taken to avoid blood vessels in the centre of the recorded DP images of the disks.

Conclusions: : A new accurate and objective optical method to measure scatter in the eye was developed and demonstrated. The procedure is sensitive enough to detect consistently differences in the light scatter intensity between normal healthy eyes and can be a significant step towards the complete optical characterization of the eye.

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