The Ophthalmology
Survival Guide
Prism Diopters
Prisms refract light but do not add convergence or divergence. They simply redirect the light passing through, shifting its path toward the base of the prism. The amount of that shift is a measure of the dioptric power of the prism.
Survival Tips!
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Prism diopters are the unit of measurement used to document the magnitude of strabismus.
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By definition, the path of a light ray is shifted 1cm off of its original path by a 1 prism diopter (pd) prism at a distance of 1m away. ​​
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This property allows appropriately oriented and powered prisms to compensate for vertical and horizontal misalignment when placed over the deviated eye.
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Remember:
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Survival is more dependent on accurate strabismus detection than exact measurement with prisms.
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A prism diopter is different from a lens diopter, which is a unit of refractive power of a spherocylindrical lens.
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If eye misalignment is torsional (around the antero-posterior axis) then prisms cannot be used for measurement and other techniques are necessary.