Identifying sources of verification errors in progressive addition lenses

Area of Science:

• Optometry
• Ophthalmic Optics
• Lens Manufacturing

Background:

• Discrepancies in spectacle lens prescription accuracy between opticians and laboratories are common, particularly with progressive addition lenses (PALs).
• Despite design advancements, PALs present inherent challenges affecting prescription accuracy.
• This study investigates whether lens design, laboratory errors, or verification methods contribute to these issues.

Purpose of the Study:

• To determine the primary cause of progressive addition lens (PAL) inaccuracies.
• To differentiate between lens design flaws, laboratory surfacing errors, and verification procedure faults.

Main Methods:

• Six Varilux Comfort Orma Supra PALs were surfaced to plano distance power and verified using manual and automatic lensmeters.
• Lenses were re-surfaced with varying sphere and cylinder powers, then re-verified.
• Verification involved both correct and incorrect methods to assess positional tolerance and identify potential errors.

Main Results:

• Lens power measurements at the Distance Reference Point (DRP) center closely matched prescribed values across measurement techniques.
• Deviations as small as 2 mm from the DRP center during verification can significantly skew results.
• Incorrect verification methods can lead to erroneous assessments of lens accuracy.

Conclusions:

• Proper verification procedures are essential for accurate progressive lens assessment.
• Failure to follow correct verification protocols can result in progressive lenses being incorrectly flagged as defective.
• Emphasizes the critical role of meticulous verification in ophthalmic lens practice.