Light rays from a single laser beam are aimed into the eye. As they are reflected back out from the retina they are subjected to possible aberrations throughout the eye's optics.

If the eye has no irregularities these light rays will come out of the eye in a plane wavefront, or a straight line. However, when the eye has aberrations the wavefront emerges with a unique shape specific for that eye. In custom LASIK, this shape guides the treatment.