ABSTRACT
The rays that pass through L7 and L8 are reflected by mirror M 10 and, as noted in connection with Figure 7, the Purkinje images, along with portions of the eye in the immediate vicinity of the pupil, are imaged at the surface of mirror M 11 • In fact, the output optics from this point on are similar to those described in connection with Figure 7, except for the automatic focus system described in the next section and the drive motor system attached to lens L12, which is described below (Sec. g). b. Automatic Focus Beam splitter BS2 is a 50/50 beam splitter that reflects IR radiation to the focus servosystem. This radiation passes through aperture A2, and the first Purkinje image is imaged onto split-field photodetector P 2• Because A2 is an off-axis aperture, there is lateral shifting of the image with axial motion of the subject's head or eye. When the subject's eye is properly located axially, the first Purkinje image is symmetrically positioned on split-field cell P 2• However, if the focus is either ahead of or behind P 2 as a result of axial eye motion, different amounts of light fall on the two halves of the cell, and this difference produces an error signal to servomotor Mp. MF responds by moving L7 (together with L 6 and stop ST2) in a direction that brings the image back into focus on the surface of P 2 and thus refocuses the entire optical system of the eyetracker. c. Autostaging During the search mode, the first and fourth Purkinje image servomotors are driven to create a raster scan in an attempt to capture their respective images, and the focus servomotor is simultaneously driven back and forth; simultaneously, the operator manually controls the stage_motors (X, Y, and Z) for an approximate visual alignment of the instrument to the subject's eye. After the first Purkinje image servos have been locked, the X, Y, and Z drive motors automatically move the instrument in three dimensions to center all three servo systems (i.e., H1o V~> and focus). The instrument is now optimally aligned to the subject.
