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

• A. The vertical prism is >0.33 PD AND there’s >1 mm difference in the height of the PRPs
• B. The vertical prism is >0.5 PD
• C. The vertical prism is >0.33 PD AND PRP >2.5 mm
• D. The vertical prism is >0.25 PD AND there’s >0.5 mm difference in PRPs

Answer: (A)

Vertical prism in verification is highly sensitive to how the lenses are positioned relative to the wearer's eyes. A small amount of vertical prism is tolerable, but once it exceeds a tight threshold (0.33 prism diopters), you must also check the alignment of the Prism Reference Points (PRPs). If the PRPs—the reference points used to locate where the prism is applied on the lenses—don’t line up vertically by more than about 1 mm, the measured vertical prism can be due to improper lens centering or mounting rather than the prescribed power. In that case, the result is not acceptable, because you can’t rely on the prism reading.

So the unacceptable situation is when vertical prism is greater than 0.33 D and there is more than a 1 mm difference in the height of the PRPs.