Increased Dynamic Range of a Hartmann Shack Sensor by B-Spline Extrapolation: Measurement of Large Aberrations in the Human Eye

Abstract

Abstract: : Purpose: To expand the dynamic range of a Hartmann-Shack sensor in order to measure large peripheral wavefront errors in the human eye. Method: The maximum wavefront curvature that can be measured by a Hartmann-Shack sensor is normally limited by the requirement that the measurement spots should not leave the area behind their corresponding lenslet. If the curvature is larger the spots will cross the lens border and this results in an unwrapping problem of associating every spot with the right lenslet. To solve this unwrapping problem an algorithm has been implemented in Matlab code that uses B-spline extrapolation and astigmatic scaling of the lenslet grid. The performance of the algorithm was evaluated by peripheral wavefront measurements on eyes in large oblique angles and by measurements on eyes with large refractive errors. Results: For a certain lenslet array (lens diameters 325 mm, focal length 18 mm) and a pupil size of 8 mm the maximum measurable spherical refractive error was increased by a factor of ten. For a normal eye it was possible to measure peripheral aberrations out to 50° with the same lenslet array. Conclusion: B-spline extrapolation and astigmatic scaling of the lenslet grid is a useful method to increase the mesurement range of a Hartmann-Shack sensor. With unwrapping there is a limit on the maximum curvature of the wavefront that can be measured and not on the maximum wavefront tilt. The developed Matlab code is made available on the internet.

Keywords: 500 optical properties • 519 physiological optics