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Multifocal Designs
Anisometropia is
that condition where the refractive error of one eye differs significantly
from that of the other and where each eye contains the same sign. An
anisometropic condition exists above where the right eye is + 1.00 sphere
and the left eye is +4.00 sphere. Through the distance optical centers
no prism exists. However if the gaze is directed away from the optical
centers unwanted prism is induced. In this example, the patient is forced
to look 10 mm away from the optical centers in order to utilize the
bifocals. In doing so vertical prism is induced in each eye resulting
in a total of 3.0 D of vertical prismatic imbalance. Excessive vertical
prismatic imbalance may result in an inability for the patient to fuse
the two images. Bicentric grinding or �slab off� is one way to correct
for vertical imbalance.
Simply stated, slab off, also called bicentric grinding, is base up prism ground in the reading portion only of a lens, generally a multifocal lens. It is used for the purpose of minimizing excessive vertical prismatic imbalance which may be induced through the reading portion, usually as a result an anisometropia. Grinding conventional base up slab off requires the skills of an expert optical journeyman. It is both costly and time consuming to produce. The reverse slab off described above is molded in the factory, allowing the laboratory to grind the lens in a manner very similar to any other single vision or multifocal lens. Both styles of slab off serve the patient equally well. |
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