Assessment of ocular aberrations at scaled pupil size and reduced Shack-Hartmann spot number

Abstract

Wavefront aberrations describe the optical imperfections of the eye by measuring the complete refractive elements of the eye. However, the reliability of ocular aberration is uncertain under some challenges and issues. Ocular aberration is generally described in terms of Zernike polynomials. However, the Zernike polynomials are pupil size dependent, therefore, the aberration measured at a fixed pupil size cannot be used for another pupil size. One solution to this problem is to use pupil size scaling technique to scale up or down the aberration to a required pupil size; however, the validity of these techniques for clinical data is not available. To tackle this issue, validation of mathematical pupil size scaling formula by comparing the estimates of the Zernike coefficients with corresponding clinical measurements obtained at different pupil sizes is performed. The results show that the estimation of ocular wavefront aberration coefficients either scaling down from large to smaller pupils or scaling up from smaller to large pupils provides estimates that are not significantly different from clinically measured values. However, when scaling up to a larger pupil size, the estimates are more variable. These findings have implications for pupil scaling on an individual basis, such as in cases of refractive surgery or when using pupil scaling to examine a clinical cohort. Another challenge of an ocular aberration for clinical uses is when the spots on the Shack-Hartmann (SH) are missed due to the opacity of eye parameters or some other disease conditions. This issue is addressed by randomly deleting the number of spots from the SH images and comparing the results with the aberration of the original SH image without the missing spots. The results indicate that as high as 50 % of the SH spots can be deleted without affecting the estimation of spherical defocus within typical clinically acceptable limits of ±0.25D. The results are further examined with in vivo measurements of a human eye wearing a spectacle lens with various models of clustered missing spots to simulate loss that might occur with the disease. The findings of this study provide foundational data on measuring the ocular wavefront aberration when only a reduced number of SH spots are available.