Prism

Prism bends light toward its base. The image appears to shift toward the apex.

1 Prism Diopter (Δ) deviates light 1 cm at 1 meter.

Base Directions:

- BI (Base In) — Corrects exo deviations (eye turns outward)

- BO (Base Out) — Corrects eso deviations (eye turns inward)

- BU (Base Up) / BD (Base Down) — Corrects vertical deviations

Fresnel Prism — Thin press-on prism for temporary/diagnostic use.

Prentice's Rule: Δ = c × D

- Δ = prismatic effect in prism diopters

- c = decentration in CENTIMETERS

- D = lens power in diopters

Direction Rules:

- Plus lenses: Prism base is in the SAME direction as decentration

- Minus lenses: Prism base is in the OPPOSITE direction

Compounding Prism:

Resultant = √(Horizontal² + Vertical²)

Horizontal Prism: Split evenly, SAME base direction in each eye.

Example: 6Δ BO total → 3Δ BO OD, 3Δ BO OS

Vertical Prism: Split evenly, OPPOSITE base direction in the other eye.

Example: 4Δ BU OD → 2Δ BU OD, 2Δ BD OS

Vertical Imbalance in multifocals is corrected with slab-off, applied to the most PLUS lens.