What is the difference between the power of the upper segment and the power of the lower segment in a bifocal lens called?

The term referred to in the question is known as the "Add." In the context of bifocal lenses, the "Add" represents the additional power provided by the upper segment, which is generally used for distance vision, and the lower segment, which is designed for near vision tasks.

The "Add" is crucial for multifocal lenses as it denotes the difference in power between the two segments, allowing the wearer to transition smoothly between viewing distances. It is particularly important in understanding how well the lenses will serve their intended purpose, addressing vision needs at both distance and near.

This term is standard in optical terminology and is essential for eye care professionals when prescribing bifocal lenses, ensuring that the appropriate levels of addition power are accurately calculated based on the patient's specific visual requirements. Other options may involve terms that are not standard in the description of bifocal lens design or may serve different functions within optics.