What Is Ophthalmic Prism?
A prism is a wedge-shaped piece of transparent material with a thick end called the base and a thin end called the apex. When light passes through a prism, it bends toward the base. This bending (deviation) allows prisms to redirect the path of light entering the eye, which is useful for correcting alignment problems in binocular vision.
Unlike a lens, a prism does not focus light. It shifts the entire image to a new position without changing its size or clarity. This makes prism correction distinct from sphere and cylinder correction.
The Prism Diopter Unit
Prismatic power is measured in prism diopters (symbol: Δ or sometimes written as PD). The definition is straightforward:
1 prism diopter displaces a beam of light 1 centimeter at a distance of 1 meter.
So 3Δ of prism shifts the light beam 3 cm at 1 meter, or 6 cm at 2 meters. The relationship is linear, making calculations simple.
Base Direction Notation
Prism in spectacle lenses is specified with both a power (in prism diopters) and a base direction. The four standard directions are:
| Direction | Abbreviation | Base Points Toward | Corrects |
|---|---|---|---|
| Base In | BI | Nose | Exo deviations (eyes out) |
| Base Out | BO | Temple | Eso deviations (eyes in) |
| Base Up | BU | Superior | Hypo deviation of that eye |
| Base Down | BD | Inferior | Hyper deviation of that eye |
Base directions are always specified relative to each individual eye. When prism is split between two eyes (which is common), each eye gets half the total amount with the appropriate base direction.
How Prism Works in Practice
Consider a patient with esophoria (a tendency for the eyes to turn inward). Looking at a distant target, their eyes want to cross slightly, causing eye strain and sometimes double vision. Placing base-out prism before each eye shifts the image outward (toward the temple), which means the eyes do not need to turn inward as much. The prism effectively moves the visual target to where the eyes naturally want to point.
For exophoria (eyes drifting outward), base-in prism shifts the image nasally, reducing the need for the eyes to converge.
Unwanted Prism: The Optical Center
Every lens with dioptric power produces prismatic effect when you look through any point other than the optical center. The farther from the optical center, the more prism is induced. This relationship is described by Prentice's Rule:
Δ = c × D
where Δ is the prism in prism diopters, c is the distance from the optical center in centimeters, and D is the lens power in diopters.
This is why proper alignment of the optical center with the patient's pupil is critical during dispensing. If the optical center is displaced from the patient's line of sight, unwanted prismatic effect results, potentially causing discomfort, headaches, or double vision.
Prescribed Prism in Lenses
Prism can be ground into spectacle lenses by shifting the optical center or by adding a prism component during manufacturing. On a prescription, prism is written as a power and direction for each eye. For example:
OD: -2.50 DS, 2Δ BU
OS: -2.50 DS, 2Δ BD
This means the right eye gets 2 prism diopters base-up and the left eye gets 2 prism diopters base-down, creating a total vertical prismatic effect of 4Δ between the two eyes.
Key Takeaways
- A prism diopter (Δ) displaces light 1 cm per meter of distance.
- Light bends toward the base; the image appears to shift toward the apex.
- Base-in prism corrects exo deviations; base-out corrects eso deviations.
- Prentice's Rule (Δ = cD) calculates prism induced by decentration.
- Proper optical center placement prevents unwanted prismatic effects.
- Vertical prism is split between eyes using opposite base directions (BU/BD).