Why is prism split between both eyes instead of placed in one lens?

Splitting prism balances the added edge thickness and weight between both lenses. Placing all the prism in one lens creates significant asymmetry: one lens is noticeably thicker and heavier than the other. Splitting improves comfort, cosmetics, and frame balance.

When should Fresnel prism be used instead of ground-in prism?

Fresnel prism is preferred for high prism amounts (above 10 prism diopters), temporary or diagnostic use, and evolving prescriptions where the prism may change. It is also used when cost is a concern. Ground-in prism is better for stable prescriptions where optical quality and cosmetics are priorities.

How do I verify prism direction in a lensometer?

In the lensometer, the target displacement points toward the base of the prism. If the target moves to the right, the base is on the right. If it moves downward, the base is down. Read the prism amount from the prism scale and confirm both the amount and direction match the prescription exactly.

Prismatic Correction Dispensing: Splitting, Verification, and Fresnel Prism

When Prism Is Prescribed

Prismatic correction is prescribed to compensate for eye alignment problems (binocular vision disorders). Prism redirects light to move the image to where the eye is actually pointing, reducing double vision, eyestrain, and headaches caused by the eyes not working together properly.

Common conditions requiring prism include:

Phorias (latent misalignment) that become symptomatic
Tropias (constant misalignment) that cause diplopia
Convergence insufficiency causing near vision difficulty
Post-surgical or traumatic strabismus

As the dispensing optician, your role is to execute the prescribed prism accurately, choose the best delivery method, and verify the finished product.

Splitting Prism Between Eyes

Splitting prism means dividing the total prescribed prism amount equally between both lenses rather than placing it all in one lens. This is the standard practice for horizontal prism and offers several advantages:

Balanced lens thickness: Prism adds thickness to one edge of the lens. Splitting distributes this added thickness across both lenses rather than concentrating it in one.
Reduced weight asymmetry: Each lens weighs closer to the other, improving comfort and frame balance.
Better cosmetic appearance: Neither lens is excessively thick on one side.

How to Split Horizontal Prism

For base out (BO) prism: Split equally between both eyes, with base out on each side. Example: 6 BO total = 3 BO right eye + 3 BO left eye.

For base in (BI) prism: Split equally between both eyes, with base in on each side. Example: 4 BI total = 2 BI right eye + 2 BI left eye.

Splitting Vertical Prism

Vertical prism is also commonly split, but the base directions are opposite between the two eyes:

If the right eye needs base down: split as base down right + base up left
Example: 4 prism diopters base down OD = 2 BD OD + 2 BU OS

When splitting prism, the combined effect must equal the total prescribed amount. For horizontal prism, both lenses use the same base direction (both BO or both BI). For vertical prism, the base directions are opposite (one BD, one BU) to achieve the same net vertical effect.

Verifying Prism Base Direction

Accurate verification of prism is critical because incorrect base direction can worsen the patient's symptoms. Use the lensometer to verify both the prism amount and base direction:

Place the lens in the lensometer and center the target on the optical center
Read the prism amount from the prism scale or ring
Note the base direction: the image in the lensometer moves toward the base of the prism
Confirm the base direction matches the prescription exactly (BU, BD, BI, BO, or an oblique direction)

For oblique prism (prism at an angle other than 0, 90, 180, or 270 degrees), verify both the horizontal and vertical components separately and confirm they combine to the prescribed resultant prism.

Confusing base direction when reading the lensometer. Remember that in the lensometer, the target displacement points toward the base. If the target displaces to the right, the prism base is on the right side. Double-check against the prescription every time.

Ground-In Prism vs. Fresnel Prism

There are two main methods for delivering prismatic correction:

Ground-In Prism

Ground-in (incorporated) prism is built directly into the lens during fabrication. The lens blank is surfaced with the prescribed prism integrated into its geometry.

Advantages:

Superior optical quality with no image degradation
Cosmetically clean appearance
Durable and permanent
No maintenance required

Disadvantages:

Adds significant thickness to one edge (the base side)
Increases lens weight, especially for higher prism amounts
More expensive than Fresnel
Changes require a complete new lens

Fresnel Prism

A Fresnel prism is a thin, flexible vinyl membrane with microscopic concentric ridges that create a prismatic effect. It is applied to the back surface of an existing lens.

Advantages:

Can deliver very high prism amounts (up to 30+ prism diopters) without excessive thickness
Easily changed or removed as the prescription evolves
Inexpensive compared to ground-in prism
Excellent for temporary or diagnostic use

Disadvantages:

Reduces visual acuity slightly due to the ridge structure
Visible on the lens surface (cosmetically less appealing)
Can collect debris in the ridges
May need periodic replacement as it degrades

Feature	Ground-In	Fresnel
Optical quality	Excellent	Good (slightly reduced acuity)
Cosmetics	Clean appearance	Visible ridges
Max prism amount	Practical limit ~10 PD	Up to 30+ PD
Adjustability	Requires new lens	Easily changed or removed
Best for	Stable, lower-power prism	High prism, temporary, evolving Rx

Fresnel prism is ideal for patients with recently changed prism needs, such as post-surgical patients whose alignment may continue to shift. Once the prism stabilizes, transition to ground-in prism for better optics and cosmetics.

Thickness Implications

Prism adds thickness unevenly to a lens. The base side becomes thicker and the apex side becomes thinner. The added thickness per prism diopter depends on the lens diameter and material:

Approximately 0.5mm per prism diopter of added edge thickness for a standard-sized lens
Higher index materials reduce the added thickness somewhat
Smaller frames reduce the effect since less lens area is involved

For prescriptions combining high power with prism, cosmetic management becomes challenging. Splitting the prism, using high-index materials, and selecting small frames all help minimize the visual impact.

Key Takeaways

Split prism between both eyes to balance thickness, weight, and cosmetics
Horizontal prism: same base direction both eyes; vertical prism: opposite base directions
Always verify prism amount and base direction with the lensometer before dispensing
Ground-in prism provides superior optics for stable, lower-power prism needs
Fresnel prism is best for high amounts, temporary use, or evolving prescriptions
Prism adds approximately 0.5mm of edge thickness per prism diopter