Introduction
Progressive lenses require careful evaluation, and one of the most important checks an optician performs is confirming that the lenses do not contain unwanted prism. Even small prism errors—especially vertical ones—can cause immediate discomfort for patients, leading to symptoms such as headaches, eye strain, or nausea. Because progressives contain multiple viewing zones and lack a single uniform power, accuracy at the Prism Reference Point (PRP) becomes essential during verification.
As opticians, we often troubleshoot non-adapt issues, and many of these problems trace back to prism values that fall outside of ANSI tolerances. This guide explains what unwanted prism is, how to find and verify the correct measurement points, and how to assess the results step-by-step. With a consistent process, you can quickly determine whether a pair of progressives meets quality standards or needs to be remade—ensuring smoother dispensing and a better visual experience for your patients.
Part 1: What is "Unwanted Prism"?
Unwanted prism is a mistake in eyeglass lenses that can make objects appear shifted up, down, or sideways. If the shift is too big, it can cause headaches, dizziness, or nausea for the person wearing the glasses.¹
The Biggest Problem: Up and Down Errors
The biggest concern is always vertical prism error. Your eyes can naturally compensate for small horizontal shifts, but they struggle with even tiny vertical differences. If one lens shifts an image upward while the other shifts it downward, the wearer’s eyes begin to fatigue almost instantly. This is the leading cause of severe symptoms like nausea.¹
Part 2: Finding the Measurement Spots
Progressive lenses must be checked at specific points because the power changes throughout the lens. These points are indicated by factory-etched markings.²
The Main Check Spot (PRP)
The most important point for checking prism is the Prism Reference Point (PRP).³
How to Find It:
All progressive lenses have two tiny, hidden laser-etched reference dots on their surface. These dots are exactly 34 mm apart.⁴
The PRP is located exactly halfway between these two dots. This is where distance prism is measured.⁴
Part 3: The Rules for Success (ANSI Standards)
The American National Standards Institute (ANSI) establishes limits for how much prism error is acceptable. If a lens exceeds these limits, it has failed quality control.⁵
|
Measurement |
What We Check |
Pass/Fail Limit |
|
Vertical Prism Imbalance |
The difference in up-and-down shift between the two lenses. |
0.33 Prism Diopters (Delta) |
|
Horizontal Prism Imbalance |
The difference in side-to-side shift between the two lenses. |
0.67 Prism Diopters (Delta) |
The vertical limit of 0.33Δ is especially strict—less than the thickness of a pencil line. Any lenses failing this test must be remade.⁵
Part 4: Step-by-Step Check at the PRP (Distance Vision)
This process confirms lens quality and verifies whether prism thinning was performed correctly.
Part 5: Diagnosis and Fixing Errors
If your results fall outside ANSI tolerances:
Even small vertical errors above the 0.33Δ threshold can cause symptoms and must be corrected.⁵
Conclusion
Accurate prism verification is essential for dispensing high-quality progressive lenses. By understanding how unwanted prism affects binocular vision, knowing how to locate and use the PRP, and applying ANSI guidelines consistently, opticians can quickly identify can whether a pair of lenses meets required standards. A structured, repeatable approach not only reduces remakes and troubleshooting time but also ensures patients enjoy comfortable, stable vision through their progressives. Precision in prism measurement ultimately leads to higher satisfaction and greater confidence in the eyewear you deliver.
Standout Features of Visionix Automatic Lensmeters
**Medical procedures, case studies, and practices mentioned in this content may vary based on regional standards, local regulations, and the discretion of providing healthcare professional. What may be considered appropriate and ethical in one country may differ in another.
About the Author
Edward Jiongco, ABOC, NCLEC, LDO (NV) is a Nevada-licensed optician certified by the American Board of Opticianry and the National Contact Lens Examiners. He currently serves as Territory Sales Manager – Finishing for Visionix, where he supports optical practices in improving profitability, workflow efficiency, and in-house lens finishing processes. Edward is also a Board Member for the College of Southern Nevada’s Ophthalmic Dispensing program, helping provide vision care to underserved communities and mentor opticians in the profession.
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