What is the centroide shift of the pupil?
. The center of the pupil, measured at the time when the wavefront measurement was done is not necessarily the center of the pupil when the person lies down under the laser. This is because, under different lighting conditions and different physiologic tone of the pupil, the pupil is a different size and shape. When the pupil dilates or constricts the center of the pupil shifts ever so slightly, because the iris fibers do not contract symmetrically. This results in .1 to .5 mm. shifts in the center of the pupil. Because of these pupil shifts and the movement of the iris it is necessary to reference to a fixed and unchanging structure. This reference is chosen as the limbus. The limbus is the outer most part of the cornea were it meets the white of the eye (sclera). The computer in the laser calculates the centroide shift relative to the limbus. To do this the laser computer compares the center of the pupil when at the wavefront measurement setting to the center of the pupil when under
