Does prism ballasting incorporate BD or BU prism?

• A. BD prism
• B. BU prism
• C. Both
• D. Neither

Answer: (A)

Prism ballasting uses gravity to fix the orientation of a prism inside a spectacle lens, so you place the ballast so the prism naturally settles with its base downward. The heavier portion of the prism in the bottom of the lens keeps it from rotating when the wearer moves, providing stable alignment. A base-down prism is therefore the correct choice because gravity helps maintain that orientation, whereas a base-up prism would be unstable under normal head movements since gravity would not reinforce the intended base-up position. Using ballast to achieve a base-up orientation would require counteracting gravity, which isn’t how ballasting is designed to work.