Definitions

  • Base curve: a simple spherical curve; the primary lens curve
  • Lens blank: basic spherical lens; the lab grinds the back of the blank to match the prescription
  • Optical center: a spot on a spherical lens where light enters at a 90-degree angle to the lens plane
  • Segment: the portion of a lens added for reading (bifocal or trifocal); it may be added separately to the lens blank or formed as a blended curve onto the base

Lens and Prescription

To make a lens, the first thing you need is a lens blank. Blanks are made in factories and shipped to individual labs to be made into eyeglasses. The raw lens material is poured into molds that form discs about 4 inches in diameter and between 1 and 1 1/2 inches thick. The bottom of the mold forms a spherical curve on the front face. A small segment with a stronger curve may be placed in the mold to form the segment for bifocals or progressive lenses.

How to Read the Prescription

Most prescriptions have four parts:

  • The base (spherical) strength and type (plus or minus)
  • The cylinder strength and type
  • The cylinder axis orientation (in degrees with 90 degree vertical; an "x" means "at")
  • The strength of bifocal segment ("plus" indicating "in addition") and type

A short form prescription from the optometrist or ophthalmologist might read:

2.25 -1.50 x 127 plus +2.00

This means:

  • A +2.25D spherical base curve (plus lens)
  • A -1.50D cylinder at 127 degrees (a minus cylinder lens is added to the base curve)
  • An additional bifocal segment of +2.00D

Total power of the lens with the cylinder is +2.25 + (-1.50) = +0.75D. At the segment, the power is (+0.75) + (+2.00) = +2.75D. And in case you've ever wondered, OD means right eye and OS, left eye.