What term refers to the additional prescription power needed for the reading portion of a bifocal lens?

The term that refers to the additional prescription power required specifically for the reading portion of a bifocal lens is "Addition." This term is commonly used in the context of multifocal lenses, such as bifocals, to indicate the extra plus power needed for near vision correction. When prescribing bifocal lenses, practitioners typically assess the patient's distance vision and then determine how much additional power is necessary for the reading segment, which is referred to as the addition.

In bifocal lenses, the distance portion caters to vision at far distances, while the addition is specifically added to assist with close-up tasks such as reading. This additional power is crucial for patients who require assistance with near vision due to presbyopia or other vision changes associated with aging.

The other terms listed have specific meanings in the context of vision correction but do not pertain to the additional reading power in bifocal lenses. Sphere power relates to the overall strength of a lens (correcting nearsightedness or farsightedness), cylinder power addresses astigmatism correction, and axis refers to the orientation of the cylindrical correction. These terms play important roles in a comprehensive vision prescription but do not describe the concept of the addition.