For many years, CR-39 plastic has been the standard
lens material for eyeglasses. Plastic lenses are half the weight of glass
lenses, have better ultraviolet (UV) light protection, are less prone to
shattering, and can be tinted more easily. With an added scratch-resistant
coating, they are nearly as resistant to scratches as glass lenses.
Polycarbonate Lenses
Polycarbonate lenses
are made of a type of plastic that is more impact-resistant than standard
plastic lenses. Polycarbonate lenses are also much thinner and lighter in
weight. They are considered a high index plastic. Polycarbonate lenses also
have built-in ultraviolet protection. Because of these properties, it is the
lens material of choice for children’s lenses, sport lenses and safety lenses.
However, some people complain that polycarbonate lenses fail to give them the
crispest, clearest vision.
Hi-Index Lenses
Hi-index lenses are
made of a special plastic material that refracts light in a different way than
regular plastic lenses. With hi-index lenses, vision can be corrected with less
material, making the lens much thinner. Plastics are graded in numbers, such as
1.50 or 1.67; the higher the number the thinner the lens. Because they are
thinner, hi-index lenses are also lighter, making them more comfortable to
wear. This is important to patients with high prescriptions, as their glasses
can be made more cosmetically attractive and appealing. Hi-index lens materials
tend to cost more than standard plastic lenses.
Photochromic Lenses
Photochromic lenses
have a special chemical coating that makes them change to a dark tint in the
sunlight and turn clear indoors. Photochromic lenses are great for people who
do not wish to carry a separate pair of prescription sunglasses. It is important
to recognize that these lenses do not darken as well while driving a car. The
windshield prevents most of the UV light from reaching the lens.
Anti-reflective Coatings
Anti-reflective
coating is applied to eyeglass lenses to reduce the amount of internal and
external reflections on a lens. This increases the amount of light transmitted
through the lens, which improves quality of vision. Anti-reflective coating
also decreases unwanted glare and halos at nighttime. It also makes the lenses
appear somewhat invisible and very thin. While everyone could benefit from an
anti-reflective coating, it is especially beneficial for people with high
prescriptions, people who have a decrease in vision at night, and professions
in which cosmetic appearance is important.
Scratch-Resistant Coatings
Scratch-resistant
coatings are applied to the front and back of lenses in the manufacturing
process. Although it is important to realize that no lens is scratch-proof,
this special coating does make lenses harder to scratch when dropped or rubbed
against a hard surface.
Ultraviolet Treatments
Ultraviolet treatment
is applied to lenses to protect against harmful UV sunrays that can accelerate
the development of cataracts and macular degeneration. It is extremely important
to protect eyes from the damaging effects of the sun. UV treatment is easy to
apply to lenses and is often included with the purchase of eyeglasses.
