Principles of Hartmann-Shack aberrometry.

RAY OPTICS DESCRIPTION Nearly 400 years ago, the celebrated Jesuit philosopher-astronomer, Christopher Scheiner, professor at the University of Ingolstadt, demonstrated the focusing ability of the human eye using a simple device known as the Scheiner Disk.1 Scheiner's experiments showed that if an optically imperfect eye views through an opaque disk perforated with two pinholes, a single distant point of light such as a star will form two retinal images as illustrated in Figure 1. If the eye's optical imperfection is a simple case of defocus, then the double retinal images can be brought into register by viewing through a spectacle lens of the appropriate power. For optical aberrations other than defocus, however, a simple lens won't bring the two retinal images into coincidence. In such cases a more general method is needed for quantifying the refractive imperfections of the eye at each pupil location. This is achieved by using a fixed light source for the central, reference pinhole and a moveable light source for the outer pinhole as illustrated in Figure 2. By adjusting the moveable source horizontally and vertically, the isolated ray of light is redirected until it intersects the fixed ray at the retina and the patient now reports seeing a single point of light. Having made this adjustment, the displacement distances x and y are measures of the ray aberration of the eye at the given pupil point. This subjective aberrometer was first described by Smirnov2 and has been used extensively in visual optics research the past 40 years. To convert the Scheiner-Smirnov subjective technique into an objective aberrometer, we reverse the direction of light propagation by placing a spot of light on the retina. This spot then becomes a point source which radiates light back out of the eye as illustrated in Figure 3. Additional holes are drilled in Scheiner's disk, which astronomers and optical designers call a Hartmann screen.3 Each aperture in the Hartmann screen isolates a narrow pencil of rays emerging from the eye through a different part of the pupil. These emerging rays intersect a video Principles of Hartmann-Shack Aberrometry