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

BackgroundANSI 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).MethodsThe 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.ResultsTolerances 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.ConclusionTolerances 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.