Design For A Large Dynamic Range Shack-Hartmann Aberrometer

The Shack-Hartmann aberrometer, SHA, has been widely used for the clinical wavefront aberration measurement for the human eye. While the refractive error of the eye for almost 85% of the population falls within the range of-6D-+3D, an aberrometer with a large dynamic range is desirable to measure the remaining 15% of the population to replace An aberrometer without any moving part in the system and with a small area detector is developed, and only one snapshot is required for each measurement.According to the design target of our system, some optional approaches are compared to each other. A configuration with demagnification of the local wavefront slope by the afocal optics is used along with a sophisticated SH spots sorting algorithm in the post-processing algorithm to increase the dynamic range of our system. To match the specified size detector, an image relay lens is used between the rear focal plane of lenslet array and the detector. The system then can be capable of measuring large eye pupil over a large dynamic range of diopters.Based on the spatial frequency estimation, a SH spots sorting algorithm is presented. In this model, the search boxes can be relocated according to the main components of the ocular wavefront aberration, namely defocus, astigmatism and spherical aberration. This sorting algorithm can deal with most of the ocular aberration measurement cases, even one or two SH spots are lost in capturing SH spot pattern.In calibration, a significant systematic error of our system varying as a function of input wavefront was observed. We figured out the culprit is the pupil aberration of the image relay lens used in the system, as a result the local shift errors of SH spots are as a function of the lenslet position and the local wavefront slope. Based on the chief ray tracing and the vector aberration theory, a generalized theoretical model is presented for calibration. Herein, the local shifts of SH spot are described by both local gradients of wavefront and the corresponding lenslet positions.The double Zernike expansion is proposed to describe the peripheral ocular aberrations in terms of a global function with both the field and the pupil variables. The estimation methods of the double Zernike coefficients are presented. The double Zernike expansions of some simple optics are investigated and figured with the checkerboard plots. The typical double Zernike modes are compared to the traditional Seidel wave aberration terms, and the spot diagrams of those modes are illustrated.