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The most common option for correcting vertical imbalance of an anisometropia produces a vertical prismatic effect in the entire lower half of one lens only, beginning at the level of the bifocal segment line. This type of correction is called a slab-off or bicentric grind. A slab-off should almost always be used unless the amount of correction required is less than 1.50 diopters of prism. At less than that, it is difficult to control the appearance and placement of the slab line. Most problems with vertical imbalance do not occur once it is below 1.50 diopters of prism anyway.

Before using 6 diopters of slab off consider using a regular slab-off ( BU ) on one eye and a reverse slab-off ( BD ) on the other. This should create easier acceptance for the patient. It is possible to use a slab-off on any lens, glass or plastic, including a progressive addition lens (plastic only). On conventional slab-offs the lens with the most minus or least plus should have the slab-off and the opposite is true for a reverse slab-off.

Use our slab-off calculator to help you determine the amount of correction required for any prescription. If you have any questions or comments about how to use our slab-off calculator, please call or email one of Robertson Optical Laboratories locations and we will be happy to assist you.

There are several other ways to compensate for vertical prism at the near besides a slab-off or reverse slab-off. When dispensing glasses for correcting vertical imbalance consider these other options. They may be a little more economical for some patients but may not be as convenient or cosmetically attractive as a slab-off

Two pairs of glasses
Dropping the optical center or MRP of a multifocal.
Raising the seg height
Fresnel press on prism
Dissimilar segs
Compensated “R” segs

The amount of slab-off prism present may be verified by comparing the seg areas of the slab-off lens with its partner lens through a lensometer. An easier method, however, makes use of the lens clock. First determine the base curve of the slab-off lens by placing the contact point horizontally across the distance portion parallel to the slab-off line. After noting the base curve, then place the contact points of the lens clock perpendicular to the slab-off line. The center contact point of the lens clock should be placed directly on the line. The difference in these two readings is the amount of slab-off prism ground in the lens.

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