Prism on the Lensmeter
When a spectacle lens contains prism, the target lines (mires) appear displaced from the center of the reticle when the lens is positioned with its optical center on the lens stop. Prism shifts light, so instead of the target sitting centered on the crosshairs, it appears shifted to one side. The amount and direction of this displacement tell you the prism power and base direction.
How Prism Displaces the Mires
On a lensmeter with no prism present, focused target lines sit perfectly centered on the reticle. When prism is present:
- The mires shift away from center in a specific direction
- The amount of shift corresponds to the prism power in prism diopters (PD or Δ)
- The direction of the shift indicates the base direction
The reticle typically has concentric circles or a grid marked in prism diopters. Each ring or division represents a specific amount of prism (commonly 1Δ per ring). Reading the displacement against these markings gives the prism power.
Determining Base Direction
The base direction of the prism is determined by the direction of the mire displacement on the reticle. The rule is:
The base of the prism is in the same direction as the mire displacement.
| Mire Displacement | Base Direction |
|---|---|
| Mires shifted up | Base Up (BU) |
| Mires shifted down | Base Down (BD) |
| Mires shifted toward nose (nasal) | Base In (BI) |
| Mires shifted toward ear (temporal) | Base Out (BO) |
Using the Prism Compensator
Many lensmeters have a built-in prism compensator (also called a Risley prism). This device allows you to dial in prism to re-center the displaced mires. The steps for using it are:
- Focus the target lines (mires) as you normally would for sphere and cylinder
- Note that the mires are displaced from the reticle center
- Rotate the prism compensator knobs to move the mires back to center
- Read the prism power and base direction from the compensator scale
Using the compensator is more precise than estimating displacement against the reticle rings, especially for oblique prism directions that are not purely horizontal or vertical.
Compound (Oblique) Prism
Sometimes prism is prescribed in an oblique direction (not purely horizontal or vertical). For example, a prescription might call for 2Δ base up and 3Δ base in for the same eye. On the lensmeter, this compound prism displaces the mires diagonally.
The prism compensator allows you to resolve the diagonal displacement into its horizontal and vertical components:
- The vertical component (base up or base down)
- The horizontal component (base in or base out)
Alternatively, the resultant prism can be calculated using the Pythagorean theorem and the base direction by the inverse tangent of the component ratio.
Unwanted Prism from Decentration
Not all prism detected on the lensmeter is prescribed. Unwanted prism can be induced by incorrect lens decentration (Prentice's Rule). If the optical center of the lens is not aligned with the patient's pupil, the patient experiences prism at every gaze direction.
When verifying a finished pair of glasses, you should check for unwanted prism by:
- Marking the patient's PD on the lenses
- Placing each lens on the lensmeter at the PD mark (not at the optical center)
- Any mire displacement at the PD position indicates prism at the patient's line of sight
ANSI Prism Tolerances
ANSI Z80.1 specifies acceptable tolerances for prism in finished spectacle lenses. For prescribed prism, the tolerance depends on the amount prescribed. For unprescribed prism (where no prism is ordered), the tolerance is typically:
- 0.33Δ per lens for most prescriptions
- Calculated based on the prescription power and allowed decentration tolerance
Prism exceeding the tolerance should be flagged for rework.
Clinical Relevance
Accurate prism verification prevents dispensing errors that can cause diplopia, eye strain, and headaches. Prescribed prism must match the prescription exactly in both power and base direction. Unwanted prism from fabrication errors must be identified and corrected. Prism verification is a critical quality control step in the dispensing process.
Key Takeaways
- Prism displaces the mires from the reticle center on the lensmeter
- The base direction matches the direction of mire displacement
- The prism compensator re-centers the mires and provides precise prism readings
- Know which eye you are measuring to correctly identify base in vs. base out
- Separate sphere/cylinder measurement from prism measurement for accuracy
- Unwanted prism from decentration errors must be distinguished from prescribed prism