The Extent of the Isoplanatic Patch of the Human Eye

Abstract

 

Purpose:

 

To measure the size of the region near the fovea on the retina over which the wave aberrations remain approximately constant. The extent of the isoplanatic patch is useful in determining effective field–size limits for adaptive optics ophthalmoscopes.

 

Methods:

 

A Hartmann–Shack wavefront sensor was used to measure the aberrations of one eye of 5 young adults. The subjects’ spherical equivalent refractive error ranged from +1.25 D to –6.00 D, and all had less than 1.00 D of astigmatism. The fixation target consisted of an array of 37 locations arranged in a grid pattern spanning 3 degrees from the central location. Six measurements were taken as subjects fixated the location specified in a random order. Wave aberration maps were calculated from the mean Zernike coefficients for 3 different pupil sizes (4, 5 and 6 mm) at each location. The root mean square (RMS) of the difference between each location and the center location was determined for high order aberrations (HOA) only and for HOA and astigmatism combined. The noise floor, based on the average standard error of each Zernike coefficient for the central 9 locations, was subtracted from the RMS difference. Changes in defocus with fixation location were not included because of possible retinal–location–dependent changes in the retinal scattering plane.

 

Results:

 

RMS changes over most of the +/– 3 degree region were less than 0.1 microns. To quantify the extent of the isoplanatic patch, we fit a quadratic surface with zero intercept to the RMS difference values weighted by the average standard error at each location. For the RMS difference based on the HOA, only the coefficients for the x² and y² terms were significantly different from zero (p < 0.05). A similar fit was obtained for the RMS difference surface calculated from the HOA and astigmatism terms. Using a threshold of 0.04 microns RMS, the size of the isoplanatic patch is as follows:  

 

Conclusions:

 

With defocus and astigmatism corrected, the maximum angular field size of the isoplanatic patch of the human eye is around 4.8°, 3.9° and 3.3° for 4, 5, and 6 mm pupil diameters respectively.

 

Keywords: optical properties • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)