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