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Once it is found that either wanted or unwanted prism is present in any given lens and the direction of the base is determined, the power of the prism can then be calculated utilizing a simple formula called Prentice's Rule. It is important to become familiar with Prentice's Rule since it is used many times each day as a routine matter in the optical dispensary. Prentice's Rule
states: Stated Algebraically:
where:
= prism diopters
Applying Prentice's
Rule, F = +3.00 and dec. = 4.0 mm
How do we know the direction of the prism is base out? The optical centers were positioned a total of 4 mm narrower than the actual PD. By referring to the figure D on page 13 which illustrates this situation with a minus lens. It can be observed that the direction of the base of the prism through which the patient is looking is oriented out, away from the nose. |
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