From Clifford W. Brooks, OD Indiana University School of Optometry Bloomington, IN
In a recent Optical Dispensing News, Rajesh Wadhwa brought up the problems encountered when prescribed prism is used with progressive addition lenses. Many people with prescribed prism have problems with their progressive lenses because the monocular PDs and fitting cross-heights are measured in the empty spectacle frames without the prism in place. When the prescribed prism is added, the eyes do not "point" in the same direction as they did during lens fitting measurements.
To avoid this problem, the person measuring for progressive lenses needs to anticipate the amount the eyes will be displaced by the prism. This amount is about 0.3 mm for every diopter of prescribed prism. The eye will turn in the direction of the prism apex -- away from the base direction.
For example, let's take a patient with 5.00 D of prescribed base-in prism for each eye. The patient is measured for progressives using a pupilometer (or ruler) without this prism in place. Monocular PDs are found to be 30 mm for the left eye and 30.5 mm for the right eye. Because of the way the eyes will turn with the prism in place, the monocular PD measurements would need to be changed to 31.5 mm for the right eye and 32 mm for the right eye.
The same compensation is used for fitting-cross height in the presence of prescribed vertical prism.
From: OPTICAL DISPENSING NEWS Number 27 April 4, 2001
http://www.opticaldispensing.com
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