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P Gobbi; Refractive Surgery And Super-vision: Chromatic Aberration Limits The Ultimate Visual Acuity . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2061.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To evaluate the limits of visual performance attainable in a human eye model under white light illumination, in presence of an imaging system free of monochromatic aberrations. Methods: The eye model optical system is assumed to be diffraction-limited at the central wavelength λd; across the visible spectrum the only aberration term considered is the chromatic defocus, which is evaluated from published data on chromatic difference of refraction measured in human eyes. The white-light Point Spread Function is computed allowing for diffraction from the exit pupil, photoreceptor discretization, Stiles-Crawford effect and photopic Spectral Luminous Efficiency. Visual Acuity (VA) is estimated through evaluation of the Contrast Sensitivity Function (CSF), by means of a model quantifying the level of photon and neural shot noise in the eye and best-fitting CSF data. Results: The largest computed white-light VA occurs at 2.7 mm pupil size and amounts to VAlogMAR = -0.24 (VASnellen = 20/11.5), with range -0.21 to -0.26 (Snellen: 20/12.3 to 20/10.9) according to the level of retinal illuminance. VA turns out to be limited primarily by diffraction up to 2 mm pupil size, by chromatic aberration from 2 to 5 mm, and by internal noise above 5 mm. Noteworthy, aliasing due to photoreceptor undersampling never occurs in white light conditions, while it is seen under narrowband illumination. Conclusion: In principle, wavefront-guided refractive treatments appear to be limited to a 2 to 3 VA line gain above the 20/20 reference value. Chromatic aberration deserves careful consideration in the design of any refractive procedure and must be properly considered when dealing with monochromatic aberrometry data. Inclusion of ray tracing and monochromatic aberration evaluation will make the present model a powerful tool to predict visual performances in real clinical situations.
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