Which of the following must be accounted for when measuring prism?

• A. The patient's PD
• B. The orientation of the prism
• C. The location of the lateral PRP, or MRP
• D. All of the above

Answer: (D)

Measuring prism accurately depends on how light is deviated relative to the eye, which is influenced by three factors. First, the pupillary distance must be accounted for so the prism sits in the correct position in front of each pupil; if the prism is decentered relative to the pupil, you’ll get unintended prismatic effects and eye strain. Second, the prism’s orientation matters because the base direction and the axis determine how the light is bent; the same prism power placed at different axes yields different horizontal and vertical effects. Third, the location of the prism reference point, such as the lateral PRP or MRP, sets where the prism power is defined; placing the prism away from this reference point changes the actual deviation delivered to the eye. Because all three influence the effective prism in practice, they must all be accounted for to ensure the patient receives the intended correction.