What does base temporal and nasal mean when applying a prism foil?

Would you like to apply a prism foil to your glasses, and does your prescription say “base temporal” or “base nasal”?
On this page, we explain what these terms mean and how to correctly apply the prism adhesive foil to your lens.

Base Temporal Prism Adhesive Foil

The word temporal comes from the Latin word tempora, which means “temple” (the side area of your head). If a prism adhesive foil is applied to a lens with the indication base temporal, it means that the base (the thicker part) of the prism points towards the temple (temporal side). This causes the light to bend toward the nasal side of the eye. In other words: the image is optically shifted in the temporal direction (away from the nose, outward).

In practice:

• Right eye, base temporal → prism base points to the right temple → image shifts to the left.
• Left eye, base temporal → prism base points to the left temple → image shifts to the right.

On our prism adhesive foils, the word base is printed at the bottom edge of the foil.

A prism with base temporal is, for example, used to compensate for an outward eye deviation (exophoria or exotropia). It helps align the images on the retina of both eyes so the brain can perceive a single image again.

Base Nasal Prism Adhesive Foil

If your prescription says base nasal, this means the base of the prism (the thickest part of the adhesive foil) should be placed on the nasal side of the eye. The prism foil must therefore be applied with the base pointing toward the nose.

• Right eye, base nasal: adhesive foil with the base pointing toward the nose of the right eye
• Left eye, base nasal: adhesive foil with the base pointing toward the nose of the left eye

On our Fresnel prism adhesive foils, the word base is printed at the bottom edge of the foil.

Would you like more information on how to correctly apply the prism adhesive foil to your glasses? Please visit the following page: How to Apply Prism Foil to Glasses.