Prismatic Lenses for Double Vision & Binocular Dysfunction: The Clinical Guide

Standard eyeglass lenses have curved surfaces designed to bend light inward or outward to correct focus. Prism lenses, by contrast, possess non-parallel flat or wedge-shaped planar surfaces that shift the visual image without altering its magnification.

According to the basic optical law of prisms, light passing through a prism is always bent toward the base of the wedge, which causes the perceived image to shift toward the apex. By positioning this wedge correctly, an optical laboratory can steer light directly onto the fovea of a misaligned eye.

On your optical prescription, prism is measured in Prism Diopters ($\Delta$) accompanied by a base direction:

• Base-In (BI): The thick base points inward toward the nose. Prescribed for exophoria / convergence insufficiency (eyes tending to drift outward).
• Base-Out (BO): The base points outward toward the temple. Prescribed for esophoria (eyes tending to cross inward).
• Base-Up (BU) / Base-Down (BD): Prescribed for vertical ocular misalignments (hyperphoria / hypophoria), where one eye sits or aims slightly higher than the other.

For temporary diagnostic trials, doctors often apply a flexible plastic vinyl sheet called a Fresnel Prism to the outside of your lens. While functional, Fresnel prisms degrade visual contrast and create a noticeable ribbed texture.

For permanent everyday wear, true ground-in laboratory prism (surfaced directly into the lens blank by ELLASUV's CNC diamond generators) incorporates the precise prism diopter into the optical blank itself, maintaining 99.8% clear dielectric transparency with zero cosmetic lines or visible ridges.