Optical Terminology

Single Vision Lens
Single focus spectacle lenses to correct for myopia (nearsightedness), hyperopia (farsightedness), astigmatism, or presbyopia. These are commonly referred to as single vision distance or single vision near lenses, depending on the intended usage.

Multi-focal Lens
Any spectacle lens combining more than one focal point.

Bifocal Lens
A multi-focal spectacle lens with two focal points, generally used to correct for distance vision and near vision. These lenses usually have a “line” which denotes the separation between the distance viewing lens above and the near viewing lens below.

Tri-focal Lens
A multi-focal spectacle lens with three specific focal points: one for distance, one for near and one range for “arms’ length” (intermediate viewing).

“Executive” Bifocal Lens
This is the thickest, heaviest bifocal with a very obvious ledge or shelf-like division line. It gives the widest near viewing area of all lined bifocals.

Progressive Multi-focal Lens
(Varilux®, Zeiss®, Hoya®, etc.)
A no-line computer designed multi-focal lens with a continuous viewing range from near (12 to 16 inches) to intermediate (18 to 48 inches) to distance ( beyond 48 inches). New digital surfacing production techniques allow for a decrease in peripheral side vision distortion. Technology has greatly improved progressive lens design to widen both the near and intermediate portion of the lens.

Prism Lenses
When eye alignment drifts off-center (up, down, or to the side) eye strain, poor focus, or double vision may result. Prism, added to eyeglass lenses when needed, helps align the two eyes allowing the muscles of the eyes to work more easily.

“Half Eyes”
Small single vision eyeglasses used for reading or close work that are worn low on the nose. This enables one to view the distance above the reading lenses without removing the spectacles. Full frame single vision reading spectacles necessarily blur the distance vision and therefore must be removed for clearer distance viewing.

Polycarbonate Lens
This is the safest, strongest, lightest lens material available. “Polys” are recommended for children and people with only one sighted eye. These are the safest of all safety-rated shatter resistant lenses for sports, certain high risk occupations, and for anyone seeking the highest margin of impact resistance. While tough, they remain prone to scratching unless specially treated.

High Index Lens
The index of refraction, an intrinsic physical characteristic unique to each lens material, is an expression of that material’s ability to bend (refract) light rays passing from air into the lens material. Lenses having a high index of refraction bend light more efficiently. Lenses made of such high index materials are much thinner and lighter than standard lenses of equal power. This is especially important in prescription eyeglass lenses for correction of high power refractive errors (i.e., strong “prescriptions.”)

Invisible Bifocal Lens
(“Seamless”)
A small round bifocal, the area of a nickel, in which the demarcation line has been ground down (or actually blurred), to make the lens appear to have no bifocal. This is an optically imperfect lens as the blurred area interferes with a sharp distortion-free image. Strictly for vanity and not utility!

Photochromic Lens
Lens material which incorporates a chemical (usually silver halide micro crystals), which darken when exposed to blue or ultraviolet light. Available in both glass and plastic these lenses are designed to be a convenient alternative to clear lenses. While reducing the intensity and discomfort of intermediate levels of illumination, photochromic lenses usually do not get sufficiently dark for strong sun and do not work well inside an automobile. A darker sunglass is advisable for these purposes.

Anti-Reflective Coating
A multi-layered lens treatment that fuses an anti-scratch, anti-reflective and hydrophobic element into the lens itself. An anti-particulate coating may also be incorporated to enhance the lens and facilitate ease of cleansing. Anti-reflective treatments reduce glare and distortions to provide better vision during low light or night time driving. Less glare reduces eye strain and is especially helpful for computer use. No reflections benefit the wearer’s appearance and minimize the look of thicker lenses.

Scratch-Resistant Coating
A hard quartz-like surface coating designed to protect softer plastic lenses making them almost as scratch-resistant as glass lenses.

UV 400
A chemical bath which treats plastic eyeglass lenses in order to make them virtually 100% ultraviolet-blocking. Just as “UV” rays cause sun damage to our skin, the sun’s “UV” rays can also damage our vision by causing cataracts and contributing to age-related macular degeneration.

“P.D.”
This is the inter-pupillary distance, which is the distance between ones’ pupils. This measurement is essential to center eyeglass lenses accurately, allowing for optimal viewing comfort. Such precision fitting parameters as the P.D. along with other considerations such as base curvature, pantoscopic tilt, though not visible to the casual observer are no less critical to good vision and comfortable viewing than the precision of the glass prescription itself.

Task specific lenses
Specially designed single vision and multi-focal lenses for specific occupations, hobbies, and/or sports participants. Indications include computer operators, building tradesmen, mechanics, sewing, stamp and coin collecting, swimming and cycling goggles, racquet sports, and many more.