Abstract
Purpose: Objective measurements of scattered light in the living human eye require the analysis of double-pass (DP) retinal images. The inherent wavelength-dependent properties of the retinal reflection may influence these measurements. The aim of this work was to measure the wide-angle Point Spread Function (PSF) at different wavelengths to better understand the role of the fundus optical properties and to identify the optimal conditions for visually relevant scattering measurements.
Methods: The instrument, based on the DP principle, used an extended-source, and allowed the recording of the wide-angle PSF for different wavelengths. A liquid-crystal wavelength tunable filter was used to select six different wavelengths ranging from 500 to 650 nm. Series of uniformly illuminated disks with angular size up to 8.1 degrees in radius were sequentially projected onto the ocular fundus. A technique previously reported (Ginis et al., J. Vision (2012)) was employed to analyze the DP images and reconstruct the wide-angle PSF for each wavelength. The complete series of measurements were performed in 10 healthy Caucasian subjects with different pigmentation. As an indication of the amount of measured scatter, we obtained the value of the PSF for small (0.5 degrees) and large (7 degrees) angles.
Results: For small angles, there was a wavelength dependence that matches the transmittance spectrum of oxy-hemoglobin, what suggests that diffuse light from the fundus was part of the reconstructed PSF at longer wavelengths. This contribution of the fundus is more important for wavelengths longer than 600nm.There was a different behavior depending on subjects’ pigmentation with light-colored eyes exhibiting higher intensities of scattered light and without dependence on wavelength. For larger angles, scatter was nearly independent of wavelength and subjects’ pigmentation.
Conclusions: The central part of the wide-angle PSF in the eye is affected by the wavelength dependent properties of the retinal reflection, which are closely related to subject’s pigmentation. At those small angles, measurements using wavelengths shorter than 600nm might be better correlated with scatter affecting vision. However, the impact of the retinal reflection in the values of the PSF at larger angles did not depend on the measuring wavelength.
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