What does BI signify on a spectacle prescription?

The term "BI" on a spectacle prescription stands for "Base-In" prism. In ophthalmic terminology, prisms are used in glasses to help correct vision by altering the way light enters the eye. The designations "Base-In" and "Base-Out" refer to the orientation of the base of the prism.

For "Base-In," the base of the prism is directed toward the nose, which is often used to help correct conditions such as esotropia, where one eye turns inward. By incorporating a base-in prism, the glasses can help align the visual axes of the eyes, leading to improved binocular vision.

Understanding this terminology is essential for interpreting prescriptions and ensuring that patients receive the correct optical aids for their visual needs. Other terms present in the options relate to different aspects of lens design and function but do not pertain to the specific designation of "BI" in relation to prism orientation.