When verifying a prescription, horizontal prism is unacceptable if which condition is met?

• A. The horizontal prism is >0.67 PD AND PRP >2.5 mm from the specified distance IPD
• B. The horizontal prism is >1.0 PD
• C. The horizontal prism is >0.5 PD AND PRP <2.0 mm
• D. The horizontal prism is >0.2 PD AND PRP >3.0 mm

Answer: (A)

Prism in eyeglasses is not just about the amount labeled on the prescription; its effect depends on where that prism is referenced in the lens and how far the measurement point is from the distance IPD. The prism’s impact can shift with decentration and the position of the prism reference point (PRP) relative to the distance IPD, so verification looks at both the magnitude of prism and the geometry of measurement.

The most appropriate rule here says horizontal prism is unacceptable only when both conditions are met: the prism exceeds 0.67 diopters and the PRP is more than 2.5 mm away from the specified distance IPD. If either factor is not present—prism is smaller than 0.67 PD, or the PRP is within 2.5 mm of the IPD—the verification may still fall within acceptable tolerance. This combined criterion captures when the off-axis measurement and the prismatic demand together produce an unacceptable result.