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 lens power required for near vision. In bifocal lenses, there are two segments: the top portion is designed for distance vision, while the lower segment is for near vision, allowing the wearer to see clearly at different distances without having to switch glasses.

The addition is typically expressed as a positive number, indicating how much strength is added to the distance prescription to achieve the proper focus for reading or other close-up tasks. Understanding this term is essential for interpreting bifocal prescriptions and ensuring that patients receive lenses that accurately meet their vision needs.

The other terms listed do not pertain directly to the reading portion of a bifocal prescription. Segment height refers to the measurement for fitting the bifocal segment in relation to the distance portion of the lens. Prism adjustment involves manipulating the lens to correct for specific visual misalignments and refraction refers to the process of determining a person's prescription. None of these terms directly define the near vision aspect of bifocal lenses like the addition does.