What is the term for the reading portion of a bifocal prescription?

The reading portion of a bifocal prescription is referred to as the "Addition." This term specifically denotes the additional power needed for near vision, which is added to the distance prescription to accommodate for presbyopia—a common condition affecting older adults where they experience difficulty focusing on close objects.

In bifocal lenses, there are typically two distinct areas of vision correction: one for distance and the other for close-up tasks. The "Addition" measures the difference in lens power required for the wearer to see clearly at close range. Thus, when a prescription indicates the Addition, it is identifying how much extra magnification is necessary for reading or other near-vision activities, helping the wearer transition smoothly between different vision needs.

The other terms listed do not apply specifically to the reading portion of bifocals: "Segment" usually refers to the lower part of the bifocal lens but does not specifically indicate the prescription power. "Index" and "Base Curve" are terms that relate to lens materials and the curvature of the lens, respectively, but they do not address the specific prescription power adjustments needed for reading.