title: Identifying sources of verification errors in progressive addition lenses.
abstract: BACKGROUND
  Occasional disagreement over spectacle lens prescription accuracy between the optical laboratory and the practitioner is to be expected, especially with a progressive addition lens (PAL). The PAL continues to evolve in design, yet retains some unique, inherent problems. The purpose of this study was to determine if the source of these problems results from the lens design, laboratory error, or verification procedures.
  
  
  METHODS
  Six Varilux Comfort Orma Supra progressive addition lenses were surfaced to plano distance power, and then verified using manual and automatic lensmeters. The same lenses were then re-surfaced for various sphere and cylinder powers, and verified in the same manner. The results of each trial were spherocylindrically averaged, and compared to the desired result. The lenses were also verified by several incorrect methods to investigate positional tolerance during verification and possible verification errors.
  
  
  RESULTS
  The averages for the lenses surfaced to the "prescribed" power were not far from the expected power by all measurement techniques when measured at the central DRP location. Positional errors of as little as 2 mm from the DRP center, however, can lead to erroneous results.
  
  
  CONCLUSIONS
  Unless correct verification procedures are carefully followed, progressive lenses can be mistakenly identified as being in error when they are not.
authors.affiliation: Indiana University, School of Optometry, College of Optometry, Bloomington, Indiana 47405, USA. kollbaum@indiana.edu
authors.forename: Pete S
authors.initials: PS
authors.lastname: Kollbaum
authors.forename: Clifford
authors.initials: C
authors.lastname: Brooks
mesh_terms: D005139:Eyeglasses / Q000592:standards*; D006801:Humans; D055095:Optics and Photonics; D055656:Prescriptions / Q000592:standards*; D011474:Prosthesis Design; D015203:Reproducibility of Results