Glossary

A lens designed for two or more viewing ranges for example, bifocal or trifocal lenses.

A multi-focal lens or a lens blank fabricated from a single piece of glass or plastic.

A cam, or template, used in lens-edging equipment to generate the correct peripheral shape and geometric center location. Also called a lens former.

A lens that has zero refractive power.

Plastic photochromic lenses are light-adaptive and darken when they are exposed to ultraviolet rays. The most common brand is called Transitions^® adaptive lenses or generic versions called, photochromic or photosensitive lenses, these lenses provide the wearer protection from the harmful effects of the sun.

A lens that has positive refractive power. It is thicker at the center than at the edge.

Polarized lenses are used in sunglasses and provide wearers with a filter to eliminate the horizontal glare experienced from reflective surfaces, such as water or the road’s surface. Polarized lenses are also capable of being worn indoors to protect light-sensitive individuals from light exposure. These lenses are recommended for patients with eye conditions such as cataracts and age related macular degeneration.

Polycarbonate lenses are comprised of a lightweight impact-resistant material and are used where eye safety is a concern. Additionally, Polycarbonate lenses provide protection from the sun’s UV rays. Popular uses include safety eyewear, sports protective eyewear and children’s eyeglasses.

• Children: Polycarbonate lenses are covered in full (no copayment) for dependent children.
• Adults: Polycarbonate lenses are covered for adults if they are monocular patients and patients with prescriptions +/- 6.00 diopters or greater.
  • Monocular patients see out of one eye.
  • A diopter is a unit of measurement of the optical power of the lens. Convex lenses have positive value (+1.00 to +3.00 for example) and are used to correct farsightedness. Concave lenses have negative value (1.00 to 3.00 for example) and are used to correct nearsightedness. Optometrists usually measure refractive error using lenses graded in steps of 0.25 diopters.

Progressive addition lenses (PAL) provide continuous progression of lens powers between multifocal lenses, resulting in many lens powers to facilitate all viewing distances without the visible line of bifocal or trifocal lenses. They are categorized in the following groups within the Davis Vision formulary, which includes many popular brand names and the latest technology:

• Standard tier
  • Available in traditional designs
  • Wide reading areas with a smooth progression from the distance viewing area down to the reading area of the lens
• Premium tier
  • Available in both traditional and digital designs
  • Wider fields of vision in the distance and reading viewing areas versus stand progressive designs
• Ultra tier
  • Every design is digitally surfaced
  • Greater enhanced visual clarity
• Ultimate tier
  • Best in digital design and cutting edge technology
  • Widest viewing areas for both distance and reading, and every distance in-between

A lens that has only one surface finished.

A lens with one correction, either for distance vision or for near vision, as opposed to a bifocal lens, which has corrections, for both near and distance vision.

• Myo-disc: A lens that will correct high myopia. Grinded on both sides to reduce thickness.
• Lenticular: A lens that will correct high hyperopia. Essentially a lens within a carrier lens to reduce thickness
• Double Grind: also called myo-disc (see above)

A lens that has the same refractive power in all meridians. Such a lens may have rotationally symmetrical aspheric surfaces.

A lens that has different refractive power in the two principal meridians. It is sometimes referred to as an astigmatic or toric lens. It is sometimes incorrectly referred to as a cylinder lens.

A lens supplied by a manufacturer with both surfaces finished and a specific back vertex power or powers. Such a lens has yet to be edged to a specific shape.

A lens that has two distinct curvatures at right angles (90 degrees) to each other. See Lens, Sphero-Cylinder.

A multifocal lens with three different powers in three different positions. Usually, the top (largest) portion is for distance vision, the middle portion is for intermediate distances and the bottom portion is for near vision.

A lens with finished optical surfaces on both sides but not edged for mounting in a frame.

Instrument that can measure the power of a spectacle lens.

See Lens, Lenticular.