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Terms in this set (44)

As the pupil size decreases, the amount of object defocus will decrease, as will the amount of light reaching the retina. Elderly patients often experience a phenomenon called senile miosis in which the size of the pupil is physiologically decreased, affording increased visual acuity secondary to a pinhole effect, along with an increased depth of field. However, the amount of light reaching the retina is diminished, which may cause a paradoxical and offsetting decrease in visual acuity.

A decreased pupil size increases the amount of diffraction experienced. A resulting retinal diffraction pattern caused by a point source appears as alternating light and dark bands of concentric circles in a bull's eye configuration. The measured distance between the center light circle (peak) to the center of the adjacent dark band (trough) is known as Airy's disk. The radius of Airy's disk increases with decreased pupil size. If viewed through a small pupil, two point sources of light will create two retinal diffraction patterns. The two objects must be sufficiently far away from each other to be resolved as two separate objects. The minimum required distance separating the two objects to allow for proper resolution is the radius of Airy's disk (or when the peak of one Airy's disk falls on the trough of the other Airy's disk). Because the radius of Airy's disk increases (as does the size of the diffraction pattern) with decreased pupil size, the distance between two objects must also increase in order for the perception of two point sources to occur.
•Measure distance PDs monocularly (not binocularly)
°Recommended method is to use a pupillometer (most accurate)
•Fit and fully adjust the desired frame on the patient's face
°Pantoscopic tilt, frame height, vertex distance, face-form wrap, and nose-pad alignment
°If the frame is not pre-adjusted, fitting height measurements may be incorrect
•Ensure that the clear demo lenses are in place in the desired frame
°If lenses are not present, or are darkly tinted, place clear (transparent) tape across the eyewire of the empty frame
•Dispenser should be positioned with his or her eyes at the level of the patient's eyes
°The patient should look at the bridge of the fitter's nose
°Draw a horizontal line on the lens or tape through the center of the pupil for each eye (not the lower eyelid margin as for bifocals)
•Place the frame on the specific manufacturer's centration chart (there is no standard progressive lens cut-out chart)
°Line up the bridge so that it is centered on the diagonally converging central alignment pattern
°Line up the marked horizontal fitting lines so that they are aligned with the horizontal axis of the chart
°Mark the previously measured PD for each eye as a vertical line that crosses the horizontal fitting line
•The fitting height of the lenses should then be measured as the vertical distance between the fitting cross and the level of the inside bevel of the lower eyewire of the frame
•Move the frame over to the lens picture portion of the centration chart so that the cross on the lens matches with the cross on the chart
°Verify that the add power circle fits within the boundaries of the frame
°Verify that one of the large overall circles completely encloses the frame's lens shape