We use a device called a Shack-Hartmann wavefront sensor to measure the optical aberrations of the eye. The output of this sensor is used to drive the deformable mirror in our adaptive optics system. The Shack-Hartmann wavefront sensor measures the shape of the wavefronts of light (surfaces of constant phase) that exit the eye's pupil. If the eye were a perfect optical system, these wavefronts would be perfectly flat. Since the eye is not perfect, the wavefronts are not flat and have irregular curved shapes.

Shack-Hartmann wavefront sensors are surprisingly simple systems. The essential components are a light source for probing the eye's optics, an array of tiny lenses (the lenslet array), and a camera or some other means for recording the pattern of images formed by the lenslets in the array. The figure below is a schematic diagram of a Shack-Hartmann sensor. Light from a laser or SLD (superluminescent diode) is sent into the eye and is focused by the eye's optics to a point on the retina. The light reflected from the retina emerges from the pupil as an aberrated wavefront and travels from the eye through a set of lenses onto a lenslet array, which is conjugate with the eye's pupil. Since the lenslet array is in a pupil conjugate plane (an image plane of the pupil), the wavefront shape at the lenslet plane is identical to the shape at the eye's pupil. The lenslet array then forms an array of spot images on a CCD detector.

The next figure illustrates how we can determine the wavefront shape from the Shack-Hartmann spot images formed by the lenslet array. For a perfect eye, the wavefront exiting the pupil is perfectly flat (top) and the lenslet array will form a perfectly regular array of spots on the CCD chip. But for a real, aberrated eye (bottom), the wavefront is distorted. The spots formed on the CCD chip for this eye will be displaced because the wavefront will hit each lenslet at an angle rather than straight on. For each lenslet, the amount of spot displacement is directly proportional to the slope of the wavefront at that lenslet. Therefore if we measure these displacements, we will know how the slope of the wavefront varies over the pupil. From this information we can calculate the shape of the wavefront emerging from the eye. This tells us all we need to know about the monochromatic aberrations of the eye at at particular point on the retina.

The following figure shows some Shack-Hartmann spot array patterns and their associated wave aberrations calculated for four normal subjects. The wave aberration was calculated with a 6 mm pupil diameter and the contour lines in the plots represent one wavelength (0.55 microns) of wavefront height.

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