What Is Base out Prism

When the PD is narrow on a minus lens, base out prism is induced. When the PD is narrow on a plus lens, base in prism is induced. Normally, when prism is ordered horizontally, both lenses are base in or base out. However, when prism is ordered vertically, one lens is base up, and the other, base down.

What is base in and base out prism?

Usually horizontal prism is either base in (BI) in both eyes or base out (BO) in both eyes. Usually vertical prism is downward in one eye and upward in the other. When the prism indicated by the Rx varies from what is “usually” done, a call to the prescribing doctor may help you avoid making an error in ordering.

What is prism base in glasses?

Base. Depending on your double vision, the prism is placed vertically, horizontally or diagonally in one or both lenses of your eyeglasses. The “base” is the thickest edge of the prism and tells you where the light is redirected.