In the lab the patient's full prescription gives these exact details:
- The total power (in diopters) the finished lens must have.
- The strength and size of the segment (if needed).
- The power and orientation of any cylinder curves.
- Details such as the location of the optical center and any induced prism that may be needed.
The lab technician selects a lens blank that has the correct segment (called an add) and a base curve that is close to the prescribed power. Then to make the power match the prescription exactly, another curve is ground on the back of the lens blank.
- In most labs the equipment is designed to grind minus curves, so a strong, plus lens blank is usually selected.
- If the base curve is too strong, then a minus curve is ground in the back of the lens, which reduces the total power of the lens.
For example, a very common lens blank is +6.00 diopters. If the prescription calls for a total of +2.00 diopters, a -4.00 diopter curve is ground on the back: (+6.00D) + (-4.00D) = +2.00D. (See the illustration below.) If it is needed, the cylinder curve is also ground at the same time.
If the prescription calls for a minus lens, the +6.00 diopter lens blank can still be used. To create a lens with the strength of -2.00 diopters, a -8.00 diopter curve is ground on the back: (+6.00D) + (-8.00D) = -2.00D.