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Stefan Schramm, Elena Lerm, Patrick Schikowski, André Kaeding, Daniel Baumgarten; Plausibility of a new method for measurement of forward scattered light in the human eye with a Shack-Hartman Wavefront Aberrometer. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1611.
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© ARVO (1962-2015); The Authors (2016-present)
Scatter measurements with Shack-Hartmann (SH) wavefront aberrometers have been widely described. However, the advantage of measuring forward scattered light stands in contrast to the relatively small recordable scattering angle of 1-2°. We developed a novel system that allows for imaging of the point spread function (PSF) of the human eye up to 4°. In this paper, the plausibility of the method is examined.
Our novel SH wavefront aberrometer system is characterized by a variable aperture in form of a Digital Mirror Device, replacing the confocal pinhole in the conventional relay system. With increasing diameter of a circular aperture, the image brightness increases. From the intensity distribution over the diameter, a rotationally symmetric point spread function is calculated by derivation with respect to the aperture area. The function PSF*=A*X^-B analogue to the Stiles-Holladay-Approximation is fitted to the PSF characterized by the viewing angle X, the form factor B and the scattering coefficient A. The intensity at 4° (IS) and the Michelson-Contrast (MC) within the PSF are defined as scattering parameters. Employing the novel system, we examined 14 healthy eyes of 7 subjects in combination with 6 scatter filters Tiffen BPM 1/8 … 3. For comparison, stray light values were measured with the OCULUS C-Quant Straylight Meter and refraction values were obtained with the Autorefractor Rodenstock CX 1000.
Scattering parameter A and IS are significantly correlated (r>0.8; p<0.01) with the filter value and the stray light parameter Log(S) obtained with the C-Quant. MC and B are weakly but significantly correlated with Log(S) (r≈0.4; p<0.01).
Our novel method for measuring forward light scatter in the human eye is an useful extension to SH wavefront aberrometers. It obtains the one-and-half-pass PSF with an angle of up to 4° where scatter is much more present and less affected by higher order aberrations than in normal SH images. Our results show the correlation of scatter and measured parameters. Future work will focus on the examination of cataract patients and the expansion of the measurement range beyond 4° to further reduce the influence by aberrations and fundus scatter.
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