Abstract
Purpose: :
Shack-Hartmann (SH) wavefront aberrometry of human eyes assumes an injected probe beam reflects from a single fundus layer. We avoided this assumption with a global reconstruction method appropriate for a thick fundus reflector.
Methods: :
Computer simulation produced SH data images by computing the array of spot images cast by a known wavefront incident on an array of lenslets. A bi-layer fundus reflector makes two wavefronts differing only in defocus that yield two simulated images that we superimposed into a composite "data" image for analysis. Conventional SH analysis locates the centroid of each lenslet’s image, from which wavefront slopes are calculated followed by Zernike modal reconstruction of wavefront phase. We developed an alternative, global wavefront reconstruction algorithm to also recover the axial separation of the bi-layers by iteratively adjusting their separation until simulated SH images matched the original data image.
Results: :
A bi-layer reflector with 0.7 D axial separation produces radially elongated spots with a degree of elongation that increases monotonically with radial distance of the corresponding lenslet from the pupil center. At 3mm radius individual lenslet images separate into a pair of sub-spots with the potential to confound conventional algorithms for spot location based on centroid calculations. Our global optimization algorithm produced simulated images with maximum difference in pixel intensities less than 4 gray-levels out of 256, which was negligible compared with the average spot intensity (105 gray levels/pixel). Derived Zernike coefficients for defocus agreed with the bi-layer model that produced the data image. However, conventional wavefront reconstruction based on centroids recovered only the mean defocus, which represents the midpoint of the bi-layer reflector.
Conclusions: :
Our global wavefront reconstruction algorithm successfully restored the wavefronts produced by both layers in a thick-reflector model of the fundus. The method is potentially useful for recovering other aberration differences associated with thick-reflectors in addition to defocus.
Keywords: aberrations • computational modeling • refraction