Compensating for Vertical Anisometropic Imbalance by the Positioning of Segment Centers

Prismatic imbalance produced in the reading area by an anisometropic spectacle correction can be offset by various methods. If a reading addition is present, one method is to provide differently shaped segments and allowing their optical centers to be separated vertically while the segment tops remain in horizontal alignment. Traditionally, to apply this method, the relative prismatic difference at the reading point is first determined by a thin lens application of Prentice's rule. This rule is then applied a second time to determine the placement of the segments that will offset the prismatic difference produced by the major lenses. However, the conventional application of Prentice's rule for determining the fusional demand in a particular area of the spectacle field often produces very large errors, which will affect the calculation of the placement of the segments. This article develops and demonstrates more accurate methods for applying the principle of compensating segments.