Notes:

To measure the eye's aberrations, a point source of light is imaged on the retina and the reflected wavefront of light emerging from the eye is analyzed by a 2-dimensional array of tiny lenses. This micro-lenslet array subdivides the wavefront into several hundred individual beams, each of which is focussed onto a video sensor. For a perfect eye the reflected plane wave will be focused into a perfect lattice of point images, one image for every lenslet in the array, and the spacing between spots will be the same as the spacing between lenslets.

The key point is that the location of any given spot of light on the video sensor will be determined by the direction of propagation of the light wave as it passes through the lenslet. There are two equivalent ways to think about the diredtion of propagation: the direction of the ray and the slope of the wavefront. Regardless of which way you prefer to think about light propagation, the result is the same: the position of the spot on the video sensor tells you the direction the light was traveling when it passed through the lenslet.

Notice that the direction of propagation is everywhere the same on a perfect plane-wave, but for an aberrated wave the direction of propagation varies across the wavefront. Consequently, this simple device can be used to determine the shape of a propagating wavefront of light simply by analyzing the spot locations recorded by the video sensor relative to the perfect lattice formed by a plane wave.

I might point out that although conceptually simple, this technique has only recently been proven feasible for use with the human eye and only 4 such systems are currently in existence in the world, and one of those systems is here at the IU school of optometry.