Dynamics of the eye’s wave aberration

TLDR

The eye’s optics are known to exhibit microfluctuations in focus, yet the temporal properties and origins of its other aberrations remain largely unknown. We used a real‑time Hartmann‑Shack wave‑front sensor operating up to 60 Hz, with a low‑coherence source and scanning system to reduce spatial inhomogeneities, to record and compute temporal power spectra of the wave‑front RMS, Seidel aberrations, and 32 Zernike coefficients in three subjects with natural and paralyzed accommodation. Fluctuations were detected in all aberrations, with higher‑order terms exhibiting similar spectra that fall at ~4 dB per octave and differ from accommodation microfluctuations; if purely corneal or lenticular, a closed‑loop adaptive‑optics system with 1–2 Hz bandwidth could correct them to achieve diffraction‑limited imaging.

Abstract

It is well known that the eye's optics exhibit temporal instability in the form of microfluctuations in focus; however, almost nothing is known of the temporal properties of the eye's other aberrations. We constructed a real-time Hartmann-Shack (HS) wave-front sensor to measure these dynamics at frequencies as high as 60 Hz. To reduce spatial inhomogeneities in the short-exposure HS images, we used a low-coherence source and a scanning system. HS images were collected on three normal subjects with natural and paralyzed accommodation. Average temporal power spectra were computed for the wave-front rms, the Seidel aberrations, and each of 32 Zernike coefficients. The results indicate the presence of fluctuations in all of the eye's aberration, not just defocus. Fluctuations in higher-order aberrations share similar spectra and bandwidths both within and between subjects, dropping at a rate of approximately 4 dB per octave in temporal frequency. The spectrum shape for higher-order aberrations is generally different from that for microfluctuations of accommodation. The origin of these measured fluctuations is not known, and both corneal/lenticular and retinal causes are considered. Under the assumption that they are purely corneal or lenticular, calculations suggest that a perfect adaptive optics system with a closed-loop bandwidth of 1-2 Hz could correct these aberrations well enough to achieve diffraction-limited imaging over a dilated pupil.