Eyeglass frames are commonly made
from metal or plastic. Lenses were originally made from glass,
but many are now made from various types of plastic, including
CR-39 and polycarbonate. These materials reduce the danger of
breakage and weigh less than glass lenses. Some plastics also
have more advantageous optical properties than glass, such as
better transmission of visible light and greater absorption of
ultraviolet light. Some plastics have a greater index of
refraction than most types of glass; this is useful in the
making of corrective lenses shaped to correct various vision
abnormalities such as myopia, allowing thinner lenses for a
given prescription.
People who have more than one vision problem
often need glasses with multifocal lenses. Multifocal lenses, bifocals,
trifocals, or progressive lenses are lenses that contain two or more
vision-correcting prescriptions.
- Bifocals: Bifocals are the most common
type of multifocal lens. The lens is split into two sections; the upper part
is for distance vision and the lower part for near vision. They are usually
prescribed for people over the age of 40 whose focusing ability has declined
because of presbyopia.
- Trifocals: Trifocals are simply bifocals
with a third section for people who need help seeing objects that are within
an arm's reach.
- Progressive lenses have a continuous
gradient (inclined) lens which focuses progressively closer as one looks
down through the lens.
Corrective lenses modify the focal length of
the eye to alleviate the effects of nearsightedness (myopia), farsightedness (hyperopia)
or astigmatism. As people age, the eye's crystalline lens loses elasticity,
resulting in presbyopia, which limits their ability to change focus.
The power of a lens is generally measured in diopters. Over-the-counter reading
glasses are typically rated at +1.00 to +4.00 diopters. Glasses correcting for
myopia will have negative diopter strengths. Lenses made to conform to the
prescription of an ophthalmologist or optometrist are called prescription lenses
and are used to make prescription glasses.
Safety
glasses are usually made with shatter-resistant plastic lenses
to protect the eye from flying debris. Although safety lenses
may be constructed from a variety of materials of various impact
resistance, certain standards suggest that they maintain a
minimum 1 millimeter thickness at the thinnest point, regardless
of material. Safety glasses can vary in the level of protection
they provide. For example, those used in medicine may be
expected to protect against blood splatter while safety glasses
in a factory might have stronger lenses and a stronger frame
with additional shields at the temples. The lenses of safety
glasses can also be shaped for correction.
n the past, eyeglass
lenses were made exclusively of glass; today, however, most lenses are made of
plastic. Plastic lenses are lighter, do not break as easily as glass lenses, and
can be treated with a filter to keep out ultraviolet light, which can be
damaging to the eyes. However, glass lenses are more resistant to scratches than
plastic ones.
As technology advances so, too, do eyeglass lenses. The following modern lenses
are lighter, thinner, and more scratch-resistant than the common plastic and
glass lenses:
-
Polycarbonate
lenses: These lenses are impact-resistant and are a good choice for people
who regularly participate in sporting activities, work in a job environment
in which their glasses may be easily scratched or broken, and for children
who may easily drop and scratch their glasses.
-
Photochromic and
tinted lenses: Made from either glass or plastic, these lenses change from clear
to tinted when exposed to sunlight. This eliminates the need for prescription
sunglasses.
-
High
index plastic lenses: Designed for people who require strong
prescriptions, these lenses are lighter and thinner than the
standard, thick lenses that may otherwise be needed.
-
Aspheric
lenses: These lenses are unlike typical lenses, which are spherical
in shape. Aspheric lenses are made up of differing degrees of
curvature over its surface, which allows the lens to be thinner and
flatter than other lenses. This also creates a lens with a much
larger usable portion than the standard lens.
Rimless
Three-piece rimless and semi-rimless glasses are common variations that
differ from regular glasses in that their frames do not completely
encircle the lenses. Three-piece rimless glasses have no frame around
the lenses, and the bridge and temples are mounted directly onto the
lenses. Semi-rimless (or half-rimless) glasses have a frame that only
partially encircles the lenses (commonly the top portion), which are
held in place most often by high strength nylon wire. A rare and
currently non commercial variation are rimless and frameless glasses
attached to a piercing at the bridge of a wearers nose.
Glazing
Spectacle lenses are edged into the frame's rim using glazing machines
operated by ophthalmic technicians. The edging process begins with a
trace being taken of the frame's eye shape. In earlier days the trace
was replicated onto a plastic pattern called a Former. Nowadays the
process is patternless and the shape is sent to the edger
electronically.
The lens, in the form of a round uncut, is positioned in the correct
manner to match the prescription and a block is stuck to the lens and
that block fits into a chuck in the edging machine. A diamond coated
wheel spins as the edger replicates the frame's eye-shape to the uncut
lens. A 'v' bevel is applied to allow the edge of the lens to fit into
the frame rim.
Lens
Coatings
-
Anti-reflective coating. If glare becomes a problem, consider an
anti-reflective coating applied to new eyeglasses. Anti-reflective
coating will reduce reflections, decrease halos around light, and
create a nicer cosmetic appearance.
-
Scratch-resistant coating and ultraviolet coating. Most eyeglass lenses
today have built-in scratch resistant coatings and ultraviolet
protection.
-
Tinted lenses.
Sometimes a light or dark hint of color on the eyeglass lens can be
beneficial to aid in vision. For example, a yellow tint may increase
contrast and a gray tint may not alter color perception with sunglasses. A
light tint can also hide the signs of aging around the eyes.
-
Mirror coatings.
If you are looking for a purely cosmetic lens that allows the eyes to be
hidden from view, then this is the coating for you. Mirror coatings come in
a variety of colors such as silver, gold, and blue.
