Pupil diameter and the principal ray

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Abstract
Around the line of sight, which is tile principal ray from the object of regard that passes through tile image of the patients pupil its projected on the cornea. This point is not necessarily at the geometric center of the cornea and is found by locating tile center of the pupil while the patient is maintaining fixation coaxially with the surgeon. However, the pupil does not dilate concentrically and its geometric center moves its the pupil diameter changes. We have found a shift up to 0.7 mm in the geometric center of the pupil as it dilates. Therefore, centration of an ablated or a radial keratotonty zone is most efficiently done when tile diameter of the modified corneal optical zone is centered around the line of sight and is superimposed upon tile entrance pupil. This will minimize extension of the edge of the large pupil beyond the ablated zone and reduce unwanted secondary optical effects from degrading vision. *Reprint requests to Stephen L. Trokel, M.D., 635 West 165th Street, New York, New York 10032. © Williams & Wilkins 1992. All Rights Reserved....

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