Understanding Prism: A Practical Guide for Today’s Opticians

By Jon J. Trutt

Published: 12/31/2025

Prism can feel intimidating, especially when you’re juggling patients, adjustments, and orders all at once. The good news? You don’t need to overcomplicate it. This guide breaks prism down into the basics: what it is, when it’s needed, and how to avoid common issues that lead to remakes.

What Is Prism?

Simply put, prism shifts where light lands in the eye.

Instead of light passing straight through the center of the lens, prism moves the image so the eyes don’t have to work as hard to align. When done correctly, prism can dramatically improve comfort and reduce symptoms like eye strain or double vision.

Prism is commonly prescribed for:

• Eye turns or misalignment

• Binocular vision issues

• Double vision (diplopia)

• Eye fatigue and headaches

• Post-concussion visual symptoms

Prescribed Prism vs. Induced Prism

Not all prism is intentional, and induced prism is one of the biggest causes of patient complaints.

Prescribed Prism

This is prism written directly on the Rx and intentionally measured by the doctor.

Induced Prism

This occurs unintentionally due to:

• Incorrect PDs or heights

• Lens decentration

• Poor frame fit

• Excessive wrap or pantoscopic tilt

• Frame movement after dispensing

Many “prism problems” aren’t lab errors, they’re measurement or fitting issues.

How to Read Prism on a Prescription

Prism is typically written like this:

• 2Δ BO (base out)

• 1Δ BI (base in)

• 0.5Δ BU (base up)

• 0.5Δ BD (base down)

Quick reminder:

Vertical prism must balance properly between both eyes. Even small errors vertically can cause noticeable discomfort.

How Prism Is Created in the Lab

Prism is surfaced by grinding the lens at a precise angle. When ordered accurately, the prism is seamlessly built into the lens.

However, even perfectly surfaced lenses can feel wrong if the frame fit isn’t right.

Why Frame Fit Matters So Much

Before assuming a lens issue, always check:

• Pantoscopic tilt

• Face-form wrap

• Vertex distance

• Crooked frames

• Uneven nose pads or temple pressure

Small fitting issues can shift the optical center and introduce unwanted prism.

When to Slow Down and Double-Check

Take an extra moment when you see:

• Vertical prism over 1Δ

• Prism combined with high prescriptions

• Large PD or height changes from a previous pair

• Progressive lenses with prism thinning

A quick review can save a remake later.

Use the Patient’s Old Glasses as a Clue

Reading the old pair can reveal:

• Existing prism not written on the Rx

• Compensations the patient adapted to

• Past errors that “worked” for them

This step alone can prevent adaptation issues.

AccuLab Tip

If something looks off, call us before ordering. We’re happy to review prism, measurements, or old lenses to help prevent delays and remakes.

Need help with prism orders?

The AccuLab Optical team is always available to review prescriptions, double-check measurements, and answer prism-related questions before your order goes into production.

👉 Reach out to AccuLab anytime through our website or give us a call, we’re here to help! · acculab.net, 800-688-3904, or info@acculab.net.