2.3 Prism & Prentice's Rule

ABO Advanced Opticianry Certification ABOC-AC

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1 Introduction & Exam Strategy

1.1 Exam Format, Scoring & Eligibility 1.2 How to Prepare & Optical Math Foundations

2 Optics I: Fundamentals

2.1 Geometric & Physical Optics 2.2 Vergence, Lens Power & Focal Length 2.3 Prism & Prentice's Rule

3 Optics II: Prescriptions & Applied Optics

3.1 Prescriptions, Cylinders & Transposition 3.2 Lens Thickness, Magnification & Effective Power 3.3 Aberrations, Polarization & Absorptive Optics

4 Ocular Anatomy & Physiology

4.1 Anterior Segment 4.2 Posterior Segment & Visual Pathway 4.3 Ocular Physiology, Accommodation & the Tear Film

5 Ocular Pathology & Refraction

5.1 Refractive Errors & Binocular Vision 5.2 Ocular Pathology & Systemic Disease 5.3 Refraction Concepts & Low Vision

6 Ophthalmic Products

6.1 Frames and Lens Materials/Designs 6.2 Coatings, Tints, Photochromic and Low-Vision Aids

7 Instrumentation

7.1 Lensometer & Lens-Power Measurement 7.2 Optical Instrumentation & Verification

8 Dispensary Protocols, Laws & Standards

8.1 Fitting, Adjusting & Measurements 8.2 Troubleshooting Patient Complaints 8.3 ANSI, FDA & FTC Standards

2.3 Prism & Prentice's Rule

Key Takeaways Explain This Topic AI

One prism diopter (1 Delta) deflects a ray of light 1 cm at a distance of 1 meter.

A prism bends light toward its base and displaces the image toward its apex; base direction is named base-up, base-down, base-in, or base-out.

Prentice's Rule is P = c x F, where c is decentration in centimeters (mm/10) and F is lens power in diopters.

Oblique prism resolves into components with H = P x cos(theta) and V = P x sin(theta); components recombine as P = square root of (H^2 + V^2).

Prism can be produced by decentering the optical center (only where power exists) or by grinding prism into the lens (required for plano lenses).

Last updated: November 2026

The Prism Diopter

A prism is a wedge of transparent material, thick at the base and thin at the apex , that bends light without focusing it. Prism strength is measured in prism diopters (Delta) . By definition, one prism diopter deflects a light ray 1 cm at a distance of 1 meter (2 Delta deflects 2 cm at 1 m, and so on). Prism does not converge or diverge light — it simply shifts the ray laterally.

Base Direction and Image Displacement

A prism bends light toward its base but displaces the perceived image toward its apex . Because the eye assumes light travels in a straight line, the object appears shifted toward the thin edge. Base direction is specified as base-up (BU) , base-down (BD) , base-in (BI, toward the nose) , or base-out (BO, toward the temple) . Prism is prescribed to correct binocular misalignment (phorias and tropias) by moving the retinal image to where the deviating eye is pointing.

Prentice's Rule

Whenever a patient looks through a lens away from its optical center (OC) , the lens acts as a prism. The amount is given by Prentice's Rule :

P = c x F

where P is prism in prism diopters, c is the decentration in centimeters (millimeters divided by 10), and F is the lens power in diopters (use the power in the meridian of the displacement).

Worked example 1: a patient's pupil sits 4 mm below the OC of a +5.00 D lens. Convert: c = 4 mm = 0.4 cm. P = 0.4 x 5.00 = 2.0 Delta . In a plus lens the base is toward the OC (the thick center), so looking below the OC yields base-down prism. Answer: 2.0 Delta base-down .

Worked example 2 (PD error, from real dispensing): a pair of glasses is made to a 66 mm PD, but the patient's true PD is 62 mm, with -4.00 DS in each eye. The total error is 66 - 62 = 4 mm, so each eye is decentered 2 mm (0.2 cm) temporally from its OC. Prism per eye = 0.2 x 4.00 = 0.8 Delta . In a minus lens, looking nasal to the OC induces base-in prism, giving 0.8 Delta base-in per eye (1.6 Delta total) — enough to cause asthenopia in a sensitive patient.

Prism by Decentration vs. Ground-In Prism

There are two ways to deliver prism:

Decentration: intentionally offset the OC from the pupil. This only works when the lens has power (P = c x F needs a nonzero F), and the achievable amount is limited by the blank size — high prism demands increase minimum blank size .

Ground-in (worked) prism: the lab grinds a wedge into the lens surface. This is required for plano lenses (F = 0, so decentration produces no prism) and for prism amounts too large to reach by decentration alone.

Splitting Prism Between the Eyes

When a large horizontal correction is needed, the total prism is usually split equally between the eyes to balance thickness and weight; an optician checks with the prescriber before splitting prism written for one lens. Worked example: a pair carries 4 Delta base-out in each eye . Each base-out prism moves its eye's image toward the nose, so each eye converges, and the amounts add: total = 4 + 4 = 8 Delta base-out. Splitting the prism keeps each lens thinner than putting the full 8 Delta before one eye.

Resolving and Compounding Oblique Prism

Prism prescribed at an oblique base angle is resolved into horizontal (H) and vertical (V) components:

H = P x cos(theta) and V = P x sin(theta)

Worked example: 3 Delta base at 30 degrees. H = 3 x cos 30 = 3 x 0.866 = 2.6 Delta ; V = 3 x sin 30 = 3 x 0.500 = 1.5 Delta . Check by recombining (the Pythagorean/compounding formula):

P = square root of (H^2 + V^2) = square root of (2.6^2 + 1.5^2) = square root of (6.76 + 2.25) = square root of 9.01 = 3.0 Delta , confirming the resolution. The base direction is recovered from angle = arctan(V/H).

Vertical Imbalance in Anisometropia

Prentice's Rule also predicts a clinically important side effect. When the two eyes have unequal distance powers (anisometropia) and the patient looks down to read , each eye picks up a different amount of vertical prism, producing vertical imbalance . Worked example: a patient reads 8 mm below the OCs with OD +1.00 D and OS +3.00 D. Prism OD = 0.8 x 1.00 = 0.8 Delta; prism OS = 0.8 x 3.00 = 2.4 Delta. Because both are plus lenses, both are base-down, but the difference is 2.4 - 0.8 = 1.6 Delta of net vertical imbalance. Most people adapt to small vertical differences; for a patient who cannot, the classic fix is a slab-off (bicentric grind) , which adds base-up prism to the reading area of the more minus (or less plus) lens. With modern plastic lenses, a reverse slab adds base-down prism to the more plus (or less minus) lens instead. Related lined-multifocal artifacts include image jump , the sudden displacement as the line of sight crosses the segment top; flat-top segments minimize jump because the segment optical center sits near the segment top.

Base direction Effect on the eye's image
Base-in (BI) Displaces image temporally; relieves exophoria
Base-out (BO) Displaces image nasally; relieves esophoria
Base-up (BU) Displaces image downward
Base-down (BD) Displaces image upward

Because induced prism, splitting, and oblique resolution all trace back to Prentice's Rule, drilling P = c x F until it is automatic is the highest-yield preparation for the Optics domain.