Prism Tolerance Overview
ANSI Z80.1 establishes tolerances for prismatic effects in finished spectacle lenses. Prism tolerances address two separate concerns: unwanted prism that should not be present in the lens, and prescribed prism that must be fabricated accurately.
Unwanted Prism Tolerances
When no prism is prescribed, the optical center of the lens should align with the patient's pupil so that no significant prismatic effect occurs. However, minor optical center positioning errors during manufacturing can create small amounts of unwanted prism.
Vertical Prism
The tolerance for unwanted vertical prism is 1/3 prism diopter (0.33 PD) per individual lens. Vertical prism is measured independently in each lens because it cannot be compensated by head position the way horizontal prism partially can be.
Vertical prism imbalance between the two lenses is particularly problematic because the eyes have limited ability to compensate for vertical misalignment. Even small vertical prism differences can cause diplopia, eyestrain, and headaches.
Horizontal Prism
The tolerance for unwanted horizontal prism is 0.67 PD total across the pair. This is calculated by summing the horizontal prismatic effects of both lenses. For example:
- Right lens has 0.25 PD base in + Left lens has 0.30 PD base in = 0.55 PD total base in (within tolerance)
- Right lens has 0.40 PD base in + Left lens has 0.35 PD base in = 0.75 PD total base in (exceeds tolerance)
PD Tolerance and Its Prism Relationship
The ANSI tolerance for total PD is 2.5mm. PD errors directly create unwanted horizontal prism through Prentice's Rule (Prism = Decentration x Power). A 2.5mm PD error on a -4.00D lens creates 1.0 PD of unwanted prism, which exceeds the horizontal prism tolerance.
This means that for higher prescriptions, PD accuracy becomes more critical because even small positioning errors produce significant prismatic effects.
Prescribed Prism Tolerances
When prism is intentionally prescribed, the tolerance for accuracy is 0.67 PD from the prescribed amount. The actual prism in the lens should be within 0.67 PD of the prescribed value.
For example, if 3.00 PD base up is prescribed, the measured prism can range from 2.33 PD to 3.67 PD base up and still meet the standard.
Verifying Prism in the Lensometer
To check for unwanted prism:
- Place the lens in the lensometer and center the target at the prescribed optical center location
- Read the prism scale to determine any prismatic displacement
- Note the direction (base in/out for horizontal, base up/down for vertical)
- Compare to the tolerance: 0.33 PD per lens for vertical, 0.67 PD total for horizontal
For prescribed prism:
- Center the lens in the lensometer at the prism reference point
- Read the prism amount and base direction
- Compare to the prescribed prism
- The deviation must be within 0.67 PD of the prescribed amount
Key Takeaways
- Unwanted vertical prism tolerance: 0.33 PD per individual lens
- Unwanted horizontal prism tolerance: 0.67 PD total across the pair
- Prescribed prism tolerance: 0.67 PD from the prescribed amount
- PD tolerance is 2.5mm, which connects to prism through Prentice's Rule
- Vertical prism differences are harder for patients to tolerate than horizontal differences
- Verify both prism amount and base direction during lens verification