What is prism in glasses?

Prism in glasses refers to the use of a prism within eyeglass lenses to correct certain types of vision problems. Here's a breakdown:

• Purpose: Prism in eyeglasses addresses double vision (diplopia) or eye misalignment issues (strabismus). It helps align the images seen by each eye, allowing the brain to fuse them into a single, clear image.

• How it Works: Instead of sharply bending light like a traditional lens, a prism subtly shifts the image. The prism bends the light before it enters the eye, essentially moving the image to a position where the eye can properly see it. This reduces eye strain and eliminates double vision.

• Prescription: Prism correction is measured in prism diopters, often abbreviated as "PD" or the triangle symbol (Δ). The prescription will also specify the direction of the prism: base up (BU), base down (BD), base in (BI - toward the nose), or base out (BO - away from the nose).

• Common Conditions Treated: Besides strabismus and diplopia, prism lenses can also help with convergence insufficiency (difficulty turning the eyes inward to focus on nearby objects) and other binocular vision problems. They can sometimes alleviate symptoms like headaches and eyestrain related to these issues.

• Types of Prism: Prism can be incorporated into eyeglasses in several ways. It can be ground directly into the lens during manufacturing, or a Fresnel prism, a thin, flexible plastic prism, can be temporarily applied to the surface of a lens.