10.3 Measuring Prism on the Lensmeter

What is Ophthalmic Prism?

An ophthalmic prism is a transparent optical element with flat, polished surfaces that bend light. Unlike spherical or cylindrical lenses, which focus or defocus light to correct refractive errors, a prism does not change the focus of light. Instead, it displaces the image seen by the patient, bending light toward its base and shifting the apparent image toward its apex.

Prism is prescribed to correct muscular imbalances in the eyes (such as strabismus or binocular vision dysfunction), helping the eyes align and work together to prevent double vision (diplopia). Prism power is measured in prism diopters (Δ), where one prism diopter is defined as the power required to deviate a beam of light by 1 centimeter at a distance of 1 meter.

When verifying a lens containing a prism on a manual lensmeter, the target will be displaced from the center of the reticle. The direction of this displacement allows the optician to determine the prism's base direction, which is designated relative to the patient's eyes as base in (BI), base out (BO), base up (BU), or base down (BD).

Reading Prism on the Reticle

The reticle of a manual lensmeter is calibrated with a series of concentric circles called reticle rings. The center of the reticle represents the optical axis with zero prism (0Δ).

• Each concentric ring represents exactly 1Δ of prism power.
• If the intersection of the target lines falls directly on the first ring, the lens contains 1Δ of prism.
• If it falls on the second ring, the lens contains 2Δ of prism, and so on.
• The radial lines extending from the center of the reticle are marked with angles (usually in 5-degree increments) to determine the orientation of oblique prism bases.

To measure prism, the optician must place the lens in the lensmeter and align the patient's pupil position (or the prism reference point) with the lens stop. The displacement of the target intersection relative to the reticle rings indicates the prism power, while the direction of the displacement indicates the base direction.

Determining Base Direction

Determining the base direction of a prism depends on:

1. The direction of the target displacement relative to the reticle center.
2. Which eye is being measured: the right eye (oculus dexter, or OD) or the left eye (oculus sinister, or OS).

Because the optician views the spectacles from the back (as they sit on the patient's face), the nasal and temporal directions are reversed between the right and left lenses:

• For the right eye (OD): The patient's nose (nasal direction) is to the operator's right, and the temple (temporal direction) is to the operator's left.
• For the left eye (OS): The patient's nose (nasal direction) is to the operator's left, and the temple (temporal direction) is to the operator's right.

Vertical prism directions remain the same for both eyes: upward displacement is base up, and downward displacement is base down.

The following matrix represents the mapping from target displacement to base direction:

Prentice's Rule and Induced Prism

Prism can be intentionally prescribed, or it can be unintentionally introduced (induced) due to lens decentration. According to Prentice's Rule, a lens will induce prism when the patient's line of sight passes through a point other than the lens's optical center. The formula is:

P = c * F

Where:

• P is the induced prism in prism diopters (Δ).
• c is the decentration distance in centimeters (cm).
• F is the refractive power of the lens in the meridian being measured, in diopters (D).

For example, if a patient looks 4 mm (0.4 cm) away from the optical center of a +5.00 D spherical lens, the induced prism is:

P = 0.4 * 5.00 = 2.00Δ

To determine the base direction of induced prism, use the "decals" method or follow these rules:

• For a plus lens (which behaves like two prisms base-to-base): The base direction is in the same direction as the decentration. If a plus lens is decentered temporally, it induces Base Out (BO) prism.
• For a minus lens (which behaves like two prisms apex-to-apex): The base direction is in the opposite direction of the decentration. If a minus lens is decentered temporally, it induces Base In (BI) prism.

When verifying spectacles on a lensmeter, if the patient's prescription does not specify prism but the target is displaced from the center of the reticle at the patient's pupillary distance, the lens contains unwanted induced prism. If this induced prism exceeds the tolerances specified by the ANSI Z80.1 standards, the eyewear is non-compliant and must be remade.