Lens Materials
High Index Lenses
Don't Let Your Glasses
Weigh You Down — Choose High Index Lenses
High index lenses are chosen for their lightweight comfort and attractiveness.
High index lens materials are lighter in weight and thinner than their regular
glass or plastic counterparts. The benefit is that if you have a strong
prescription, you are no longer forced into "Coke-bottle" lenses. Just about
everyone can get attractive eyeglass lenses that complement their appearance,
rather than detracting from it.
High index lenses are made of materials that are compressed, or denser, so the same amount of visual correction is taking place using less lens material than traditional plastic or glass requires. "High index" means that the lenses are constructed of a plastic or glass material that has a higher index of refraction. The "index of refraction" refers to the speed that light travels as it passes through the lens material. Currently the highest index plastic lens has an index of 1.71. Conventional plastic has an index of 1.49.
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Why High Index
Lenses Are Thinner
Eyeglass lenses work to correct vision
by bending light as it passes through the lens. The amount that the light must
be bent in order to give you good vision is determined by the prescription that
your eye doctor provides. The higher the number in the prescription, the
stronger that prescription is, and the more the light must be bent in order to
correct your vision. (For example, if your prescription is -5.00, you are very
nearsighted and need a stronger lens than someone with a -2.00 prescription, who
is less nearsighted.) So people with stronger prescriptions traditionally have
had thick lenses — that is, their lenses provided more visual correction than
the thin lenses worn by others.
But lens materials with a higher index of refraction (or IOR) allow light to pass more quickly through the lens, so less of the actual lens material needs to be used. That's why lenses with a higher index of refraction, or high index lenses, can be thinner than traditional lenses.
The Higher the
Number, the Thinner the Lens
High index lenses are available in
either glass or plastic. Different manufacturers make different high index
lenses, and what sets each lens apart from the others is its index of refraction
(IOR). The higher the IOR, the denser the material. All things being equal, a
1.66 IOR material will result in a thinner and lighter-weight lens than a 1.57
IOR material will.
Generally, the higher the IOR, the higher the cost of the lenses will be. The thinnest, most lightweight lenses are typically the most expensive. With high index lenses, it seems that the less you get, the more you pay.
Regardless of the index of refraction, plastic high index lenses tend to be lighter in weight than glass high index lenses, simply because plastic weighs less than glass to begin with.
What Are Aspherics?
Just like high index lenses, aspheric
lenses are also thinner than regular lenses. But the thinness is accomplished
with the lens design, not just the lens material. Aspheric lenses are flatter
than conventional lenses.
In plus lenses for farsighted people, the curves flatten away from the center, for a lens that doesn't bulge out as much. It also does not magnify the eye, so the eye appears more natural. In minus lenses for nearsighted people, the curves steepen away from the center, toward the edge of the lens. This results in a thinner lens edge.
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How to Get Better
Looking Eyeglasses
Complicated mathematical formulas are used in conjunction with the optical
prescription to determine the exact thickness of a lens in a particular frame
for a particular patient. But generally, the way to make sure that the resulting
lenses are as thin as possible is to use the highest index lens material
available, use an aspheric lens design if appropriate, and keep the eyeglass
frame as small and as round as possible. The portion of the lens edge that is
left exposed outside the frame can also be polished clear to make it
unobtrusive.
These steps, along with applying an anti-reflective coating, will result in the best-looking prescription eyeglasses that modern science can provide. If your eye care doctor doesn't offer all of these options to you, make a point of asking about them.
Polycarbonate Lenses
Get Tough with
Polycarbonate Lenses
More durable than regular plastic, polycarbonate lenses are very lightweight and
shatter-resistant. They also have the best impact resistance of any lens
material, making them the lens of choice for sports eyewear, children or active
lifestyles. Polycarbonate lenses are not only thinner and lighter in weight than
traditional plastic eyeglass lenses, they also offer ultraviolet protection and
scratch-resistance. In addition, they are very impact resistant.
Because less of the polycarbonate lens material is needed to provide the same amount of visual correction that a traditional plastic lens offers, polycarbonate lenses are thinner and lighter in weight. Polycarbonate lenses also have built-in ultraviolet (UV) protective properties. Similar to the way that sun block prevents the sun's rays from damaging the body, UV protection shields the eyes from the same detrimental light rays. Other types of plastic lenses must be specially treated to block the same amount of UV, but polycarbonate lenses automatically offer this protection.
Polycarbonate material is manufactured differently from the way that other lens materials are fabricated. An enormous amount of pressure is used to compress the raw material. The resulting product holds up so well under impact not because it is hard and unyielding, but rather because it is flexible and gives slightly under pressure without breaking. Polycarbonate lenses are actually constructed out of material that is identical to what is called "bulletproof glass." Poly has also been used for astronaut helmet shields and space shuttle windshields.
The flexibility or softness of the lens material dictates that a scratch resistant coating be applied to the front and back surfaces of polycarbonate lenses. Manufacturers automatically coat them, and fabrication laboratories add a scratch-resistant coating to any uncoated lens surface before sending the lens to a dispenser. So, any polycarbonate lens you buy already has built-in scratch-resistance.
Impact Resistance
Polycarbonate lenses are safer than traditional lens materials because they
aren't brittle. So they won't usually break if they come into contact with
fast-moving objects such as balls, racquets or BBs. Regular glass and plastic
eyeglass lens materials shatter upon violent contact in the same way that car
safety glass does. Pieces of the lens are sprayed around, which potentially can
cause injury to the eye. That's why polycarbonate lenses, which hold up better
under impact, are required in sports goggles and are recommended for all
children's glasses and some safety eyewear.
Studies have been made detailing exactly what happens when an object traveling at high speed meets a polycarbonate lens versus a traditional plastic lens. Using film that is slowed down and viewed frame by frame, one investigation demonstrated that a fast-moving ball coming into contact with a plastic lens broke the lens and impacted the eye. In contrast, the same ball traveling at the same rate of speed impacted the polycarbonate lens and flexed the lens but did not break it.
(Caution: Do not test polycarbonate lenses by firing BBs or hurling objects at them while someone is wearing them! Polycarbonate is impact resistant, but there is no guarantee of non-breakage.)
Use the Proper
Frame
Polycarbonate's impact resistant nature could tempt you to forgo placing the
lenses into a proper sports frame, using a regular "dress" eyeglass frame
instead to play basketball, racquetball and so forth. The danger in not using a
sports frame is that while the polycarbonate lenses provide eye protection, a
regular eyeglass frame is not capable of holding up to the impact of a ball or
racquet.
It's dangerous to play sports in polycarbonate lenses in an eyeglass frame that is not rated for sports eyewear. This combination defeats the purpose of using polycarbonate lenses for safety. However, polycarbonate is always recommended for children's eyewear, because kids tend to be rough at play even when they are not taking part in an organized sport.
The same is true of safety glasses. Polycarbonate lenses will protect eyes from many hazards in the workplace or at home, but if they are placed in a frame that is not safety rated, the eyeglasses cannot be considered safety glasses under the law. Legalities aside, they may not provide enough protection for the particular activity. If you need safety glasses, consult an eye care doctor who can tell you which frames are safety rated.
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