Types of Bifocal Lenses
Most bifocal lenses currently dispensed are 1-piece lenses that are made by generating the different refracting surfaces on a single lens blank (Fig 4-25). One-piece round segment bifocal lenses have their segment on the concave surface. One-piece molded plastic bifocal lenses are available in various shapes, including (1) round top with button on convex surface, (2) flat top with button on convex surface, and (3) Franklin (executive) style with split bifocal.
With fused bifocal lenses, the increased refracting power of the bifocal segment is produced by fusing a button of glass that has a higher refractive index than the basic crown glass lens into a countersink in the crown glass lens blank. With all such bifocal lenses, the add segment is fused into the convex surface of the lens; astigmatic corrections, when necessary, are ground on the concave surface.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.