In what unit are base curves typically designated?

Base curves for lenses are typically designated in diopters. The reason diopters is the correct choice lies in the definition of the base curve itself, which is a measurement of the lens's curvature. Specifically, the base curve indicates the power of the lens, which is inherently related to the shape of the lens surface. Diopters measure the optical power of the lens, where one diopter corresponds to a focal length of one meter.

This relationship is critical in optics, as lenses with different curvatures are designed to correct vision at varying distances. A base curve specified in diopters helps optometrists and optical professionals select lenses that provide the correct vision correction based on the curvature of the eye and the desired visual acuity.

In contrast, the other options do not accurately represent the unit of measurement associated with the base curve. Millimeters, for instance, might be used to describe dimensions such as the physical size of the lens or its thickness, while degrees could refer to angles but are not applicable to the curvature used in prescribing lenses. Centimeters are a unit of length and similarly do not relate directly to optical power measurements. Thus, diopters is the appropriate unit for designating base curves in lenses.