B
base curve
Nominal surface power (or nominal curvature) of the front surface. NOTE 1: When surface power is quoted, the refractive index assumed in the measurement is also stated. NOTE 2: In rare cases, e.g. high minus-powered lenses, it is acceptable to specify the back surface. NOTE 3: The nominal curvature or radius of curvature may be stated instead of surface power, but this should be clearly identified.