Extracting wavefront error from Shack-Hartmann images using spatial demodulation

To determine whether the spatial demodulation processing of Shack-Hartmann images is suitable for extracting wavefront gradients for ocular wavefront sensors.

We developed a custom software program to implement the spatial demodulation technique. To test the algorithm's performance, we generated simulated spot images and obtained an eye examination image. We generated a collection of simulated aberrated spot images corresponding to: astigmatic wavefront (-5.00 -2.00 x 17), highly aberrated defocus (+/-20.00 diopters [D]), high-resolution defocus (-0.01 D), and third-order aberrations (trefoil and coma). The eye examination image and its measured Zernike coefficients were obtained from a Shack-Hartmann ocular aberrations system. We evaluated the output from the algorithm in terms of comparing the results to the known Zernike coefficients (for the simulated images) or the previously measured Zernike coefficients (for the eye examination image).

The spatial demodulation algorithm was able to correctly recover the aberrations to better than 1/100 (0.01) D for the simulated spot images. The processing of the eye examination image yielded results within approximately 1/4 (0.25) D to the values provided by the Shack-Hartmann system.

From the set of simulated images and the eye examination image used to test the spatial demodulation technique, it appears that the method is suitable for application in ocular wavefront aberrations Shack-Hartmann systems. The method appears capable of accurately processing high levels of aberrations (+/-20.00 D) as well as providing high resolution as evidenced by finding the -0.01 D defocus. The method may be especially well suited for processing highly aberrated wavefronts.