The Snellen fraction that indicates normal visual acuity. (Also see Snellen acuity, Snellen chart, and Snellen fraction.)
- “A” measurement
The horizontal width of a lens shape’s box measurement. (See box measurements.)
- Abbe value
A relative measure of a lens material’s dispersive power. The Abbe value of commonly-used eyeglass lens materials generally ranges from 20 to 60. Lenses with higher Abbe values have less chromatic aberration.
An optical defect causing blurred or distorted vision. In eyeglass lenses, aberrations are caused by a defect in the lens design or fabrication, or by optical limitations of the lens material. (See chromatic aberration.)
The process in which light energy is converted into a different energy form (usually heat). Sunglass lenses use absorptive tints and filters to absorb excess light and reduce glare. (See polarizing filter.)
Sharpness of vision. Visual acuity is usually measured with a Snellen chart and recorded as a Snellen fraction (e.g. 20/20).
- Add power
In a multifocal lens, the added magnification power prescribed for that portion of the lens to correct the wearer’s near vision.
Acronym for the American National Standards Institute, the organization that makes quality assurance recommendations to the eyewear industry.
- Anti-reflective coating
A vacuum-deposited coating applied to lenses to decrease surface reflections and increase light transmittance. Anti-reflective (AR) coatings make eyeglass lenses more attractive and provide better night vision, compared to uncoated lenses.
- AR coating
See Anti-reflective coating.