Uneven refractive powers in the two eyes. Usually  hereditary, but may also be attributed to cataracts and/or trauma. Patients with this condition usually refer to contact lenses, which move with the eyes limiting the prismatic effect that may be caused by a spectacle lens.

Usually helpful on high add and hyperopia prescriptions that are a one piece multifocal lenses (ex/ e-style and progressives). This process creates an equal amount of base down prism to create a matching thickness on top and bottom of the lenses.

Intersecting points of diagonals unifying opposite corners of a box and/or the vertical and horizontal lens bisectors within the boxing system, which distinguish a lens shape.

Also referred to as the fried egg lens; this lens consist of a small round convex lens that is fused on to a carrier lens that fills the eyeglass frame, hence giving a fried egg resemblance. The lenticular lens is used to help correct severe hyperopia patients by condensing the prescription, usually 12 diopters or higher, into a small convex lens ranging between 20mm to 40mm which is then fused onto a carrier lens. However, with technology evolving there are other lenses available like the aspheric lens to accommodate a more ascetically pleasing lens appearance.

An imaginary line(s) on a sphere like surface that intersect a perpendicular axis point. Meridians may be used to help graph the anatomy of a lens, the eye or any other spherical body.

A corrective lens composed of a concave lens fused on to the outer surface of a carrier lens also referenced as a minus lenticular lens. This lens is usually used in the correction of high myopia low vision patients of 12 diopters or higher to avoid aggressively thick lenses. The prescriptions are usually condensed to a 20mm to 30mm in diameter concave lens, but have been known to be customized in some instances, and then fused on to a much thinner carrier lens that fits the shape of the eyeglass frame. These lenses have also been offered as a blended myodisc, which blends the fusion from the concave lens to the carrier lens creating a smoother transition and cleaner looking lens. However, these lenses may create a visual effect commonly referred to as scotoma, which is a ring of distorted visual acuity where the edge of the concave lens and the carrier lens meet, hence limiting the clear visual acuity to a smaller portion in the center of the concave lens.

Retinoscopic Unit:

A unit used to determine and analyze the refractive properties of the eye.

A strong transparent compound applied to plastic and light weight lenses in an effort to minimize scratches to the lens surface. However there is no lens material that is scratch-proof. Most lightweight lenses (high-index, polycarbonate etc.) are made of a dense materials (the higher the index the denser the material), which requires an additional coating to improve its resistance to scratches. The denser the material the harder the coat is required. One thing to keep in mind is that the harder the coating the less the lens will accept tints.

Slab off is usually used to treat Antimetropia, a form of anisometropia, which creates a vertical disparity of at least two or more diopters between each eyes. This condition usually creates a plus sphere for one eye and a minus sphere the other. Considering this condition a prism is placed on the spectacle lens of the weakest plus or highest minus to help counterbalance any disparity, most notable on multifocal lenses.

The percentile of  light relating to one part of the spectrum from transmitted light to incidental light.

A visual RX prescription may have a (-) minus or (+) plus cylinder. Both yield the same visual effect for a patient and technically both are the same prescription, although they look completely different. There is a formula to transpose (switch) a prescription from a (-)minus to a (+)plus cylinder and vice versa, hence the word "transposition". See our Prescription Lenses page for the transposition formula.

Trivex was invented for the military as a polymer that is clearer and lighter than poly. The name Trivex was derived to reflect the combination of the three distinctive characteristics: excellent optics, impact resistance and lightweight nature. However, Trivex is considerably thicker than poly with a 1.53 index vs 1.59 (poly). First introduced to the public in 2003 this lens medium is an ideal material for patients that want rimless, safety, and/or sports eyewear and cannot adapt to poly and/or are looking for the clearest optics/impact resistance.

The point where the optical axis of a lens coincides with the surface of the eye.