Certified Ophthalmic Medical Technologist (COMT) Practice Exam

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What is the formula for calculating induced prism according to Prentice's Rule?

Induced prism (PD or D) = Lens power (D) + OC displacement (cm)
Induced prism (PD or D) = Lens power (D) / OC displacement (cm)
Induced prism (PD or D) = Lens power (D) X OC displacement (cm)
Induced prism (PD or D) = Lens power (D) - OC displacement (cm)

The correct answer is: Induced prism (PD or D) = Lens power (D) X OC displacement (cm)

The formula for calculating induced prism according to Prentice's Rule is based on the relationship between lens power and the distance from the optical center (OC) of the lens when it is displaced. The correct formulation states that induced prism (measured in diopters) is equal to the lens power (measured in diopters) multiplied by the displacement from the optical center (measured in centimeters). This relationship arises because prism occurs when light passes through a lens that is not centered on the optical axis, causing the light to deviate at an angle corresponding to the degree of lens power and the amount of displacement from the optical center. Therefore, the more powerful the lens or the further the displacement from the optical center, the greater the induced prism. Understanding this principle is essential for practitioners in determining how much prism correction needs to be applied, especially in cases involving strabismus or other binocular vision disorders.