Ocular Aberration Measurements by Utilizing Near–Field Diffraction

Abstract

Abstract: : Purpose: Develop a wavefront sensing technique that is compatible with the ocular retinal reflex, that provides accurate and stable accommodative state measurements and that simultaneously provides pupilometry and lateral eye tracking. In this paper I will discuss a direct measurement of the wavefront shape. Utilizing the near–field diffraction phenomenon of a wavefront incident on a periodic reticle, a wavefront sensor (WFS) can be devised that has high spatial resolution, no subapertures, wide dynamic range of wavefront divergence, adjustable sensitivity, and insensitivity to tilt or vibration. Method: When a spatially coherent wavefront illuminates a linear grating, characteristic intensity distributions repeat along the illumination direction with a period that is determined by the grating and the wavelength. The intensity pattern is a closed–form expression that is derived from the Fresnel diffraction field and is a function of the gradient of the phasefront. From the intensity equation, one can derive a continuous expression of the wavefront phase characteristics. Classical Fourier techniques are used on the diffraction pattern to extract the continuous expression of the wavefront. In one of the discrete planes where the intensity pattern has maximum contrast, one can determine the gradient of the phasefront via the spatial frequency domain and related signal processing algorithms. Phasefront sensitivity is proportional to the distance to the selected high contrast pattern plane. Resolution is dependent on the pitch of the grating. Because the gradient is measured, the measurement is tilt. Results: Measurements on fixed phase plates have been made and compared among a Zygo interferometer, a Hartmann–Shack wavefront sensor, and the WFS described here. This WFS provided closer measurements to classical interferometry than the Hartmann WFS, especially for larger aberrations. Ocular refraction measurements have been made and compared to subjective measurements with outstanding results. Conclusions: The WFS described in this paper may provide the future of wavefront analysis in the optics laboratory, the testing lab and the refraction lane. Tracking of the diffraction pattern, as in Hartmann techniques, does not provide accurate results especially for aberrations of higher magnitude and order.

Keywords: refraction • eye movements • refractive surgery