Will different sized segments work for anisometropia?

  View PDF

THIS ARTICLE IS BEST READ ON A PDF.

Different sized segments are a potentially good way of removing the differential prism. This was seen as a practical solution in the days of fused bifocals. A now discontinued lens was the R5/R9. It was a deep ‘B’ style fused Univis lens. Placed one way up the seg top was 5mm from the geometrical centre of the seg and, upside down, 9mm from the seg top to the geometric centre (Figure 1). This means it could contribute a different amount of base-down prism, dependent on its orientation. With the 9mm seg top uppermost, it will exert more base-down prism than if it was the other way up. But the amount of prism removed by using R5/R9 is quite small, and this solution is no longer used.