Prism glasses are a specialized form of eyewear designed to address issues related to how the two eyes work together. Unlike standard lenses that correct refractive errors, prism lenses shift the image seen by the eye. They are used for vision correction when a person’s eyes do not align perfectly, causing the brain to receive slightly misaligned images. This targeted correction for binocular vision dysfunction ensures the visual system functions efficiently and comfortably.
The Optics of Prism Correction
A prism is a wedge-shaped piece of glass or plastic with a thicker end, called the base, and a thinner end, called the apex. When light passes through this angled material, it is refracted or bent. Light is always bent toward the base, the thickest part of the prism.
This bending creates an effect where the image appears to have shifted toward the apex. The prism redirects the light path to compensate for the eye’s misalignment, rather than changing the object’s physical location. By controlling this shift, the brain receives images from both eyes in a position where it can more easily fuse them into a single picture.
Measuring Prism Value and Direction
A prism prescription requires two precise measurements: the magnitude of the correction and its orientation. The magnitude, or “prism value,” is measured in a unit called the Prism Diopter, often abbreviated as PD or symbolized by a triangle. This value indicates the degree of image shift needed, with higher numbers representing greater light redirection.
The direction is defined by the location of the prism’s Base—the thickest edge. This specification ensures light is bent in the exact opposite direction of the eye’s deviation. Directional abbreviations denote the Base position: Base In (BI, toward the nose), Base Out (BO, toward the temple), Base Up (BU), or Base Down (BD). Prescriptions may include both horizontal and vertical components, and the direction can differ for each eye.
Common Reasons for Prism Prescription
Prism correction is primarily prescribed to treat binocular vision dysfunction, a problem with how the eyes coordinate to form a single image. The most common condition addressed is diplopia, or double vision, which occurs when the eyes are so misaligned that the brain cannot merge the two separate images. The prism effectively moves one image so it falls on the correct part of the retina in both eyes, allowing the brain to fuse them.
Prisms are used for less severe eye misalignment issues known as heterophorias. In these cases, the eyes tend to drift out of alignment, requiring constant effort to overcome. This excessive effort can lead to symptoms like eye strain, chronic headaches, and fatigue, particularly during detailed tasks such as reading or computer work. Conditions such as strabismus, Graves’ disease, or issues following a stroke or head injury are managed with prism lenses.
Adjusting to Prism Glasses
Wearing prism glasses requires an adaptation period as the brain learns to process the altered visual input. Initial symptoms include temporary disorientation, dizziness, or slight nausea. The perceived shift in the image can also cause surfaces, such as floors or steps, to appear tilted, and may temporarily affect depth perception.
This adjustment phase lasts from a few days up to several weeks, depending on the individual and the strength of the prism prescribed. Consistent wear is recommended during this time to allow the eyes and brain to adapt effectively. Patients should focus on moving their head to look at objects rather than just their eyes, especially with stronger prescriptions, to ensure they look through the correct part of the lens. If discomfort persists beyond a few weeks, consult an eye care professional, as the prescription may require reassessment.