Revisions to tolerances in cylinder axis and in progressive addition lens power in ANSI Z80.1-2005.

Background: ANSI Z80.1 (American National Standards Institute) is a voluntary standard that provides tolerance guidelines for ophthalmic lens parameters. The recently approved ANSI Z80.1-2005 changes tolerances for cylinder axis for all ophthalmic lenses and for distance power for progressive addition lenses (PALs).
Methods: The ANSI Z80.1 committee analyzed the amount of unwanted residual cylinder created by rotating a cylinder axis away from its desired orientation for a range of cylinder powers. The limits of axis orientation error that keep the residual unwanted cylinder error within the tolerances for cylinder power were determined. For PALs, a tolerance for distance power error was sought that considers the currently followed standards for front surface power error and the tooling steps used for the rear surface.
Results: Tolerances for cylinder axis error were changed to +/-14 degrees for 0.25 diopter (D) cylinders and to +/-7 degrees for 0.50 D cylinders. Tolerance for error in back vertex power of PALs was changed to +/-0.16 D for both sphere and cylinder.
Conclusion: Tolerances for cylinder axis for low cylinders have been made in ANSI Z80.1-2005 to be consistent with tolerances in cylinder power. Changes in tolerance for power errors in PALs have been made to recognize fabrication challenges.