Prism correction in eyeglasses is a specialized adjustment that helps your eyes work together as a team. Unlike standard lens corrections that focus on issues like nearsightedness or farsightedness, prism addresses problems related to eye alignment. These lenses do not change your ability to focus, but rather shift the light entering your eye to correct where the image lands on your retina. This small, precise adjustment is designed to make comfortable, single vision possible when your eyes naturally struggle to align.
Why Prism Correction is Necessary
Prism correction is primarily prescribed to manage binocular vision problems, which occur when the visual axes of your two eyes do not point to the same spot simultaneously. A common symptom of this is double vision, known as diplopia, where the brain receives two slightly displaced images instead of one unified picture. This misalignment can happen due to issues with the eye muscles, certain neurological conditions, or other factors causing the eyes to deviate.
By redirecting the light path before it hits the retina, the prism essentially tricks the brain into thinking the two images are aligned. This allows the brain to fuse the images into a single, cohesive view without the strain of constantly trying to realign the eyes. Correcting this issue can alleviate related symptoms such as headaches, eye fatigue, and difficulty with activities like reading or driving.
Decoding Prism Notation on Your Prescription
If you require prism correction, your eyeglass prescription will feature a specific section detailing the required strength and direction. The strength of the correction is measured in a unit called a prism diopter, often abbreviated as P.D. or represented by a small triangle symbol (\(\triangle\)). This number indicates the precise amount of image shift needed, with typical prescriptions ranging from 0.5 to 10 \(\triangle\) per lens.
The prescription specifies the required eye using standard Latin abbreviations: OD for the right eye (Oculus Dexter) and OS for the left eye (Oculus Sinister). Next to the diopter value, a two-letter code indicates the direction of the prism’s base.
Directional Codes
These directional codes are:
- BI (Base In, toward the nose)
- BO (Base Out, toward the ear)
- BU (Base Up)
- BD (Base Down)
For example, a notation reading “OD: 1.5 \(\triangle\) BO” means the right eye requires 1.5 prism diopters of correction with the base positioned outward, toward the ear. It is possible to have both horizontal (BI or BO) and vertical (BU or BD) prism correction listed for a single eye.
Understanding the Base and Apex
Every prism has a thickest edge, called the base, and a thinnest edge, known as the apex. When light travels through the prism, a fundamental rule of optics is that the light ray always bends toward the base.
This bending of light toward the base causes the image you perceive to be shifted toward the apex. By specifying the base direction (In, Out, Up, or Down) on the prescription, the doctor dictates where the thickest part of the lens must be positioned to achieve the needed image shift. For instance, a Base In (BI) prism means the thickest part of the lens is toward the nose, which shifts the image outward toward the temple.
This required shaping means that lenses with prism correction may sometimes have a noticeable difference in edge thickness. The prism is “ground into” the lens, meaning the lens surface is constructed with two non-parallel surfaces to create the effect.