lens materials Flashcards
index of refraction
index of refraction is, the more it bends (refracts) light—and the thinner the lens can be. That’s why high-index lenses are so thin and lightweight.
Refractive Indices of Common Lens Materials
Refractive Indices of Common Lens Materials
Plastic (regular—CR-39)
1.498
Crown glass
1.523
Trivex
1.53
Polycarbonate
1.586
High-index plastic
1.74
You’ll often hear people refer to plastic lens materials as what?
CR-39
the name of the formula originally used for plastic lenses.
was what?
CR-39
plastic advantages
On the plus side, plastic is lighter than glass, and it can be tinted.
Lenses made from high-index plastic are significantly thinner than regular plastic lenses. This makes high-index plastic an excellent choice for rimless frames and semi-rimless frames.
High minus-powered lenses are extremely thick at the edge, so it’s smart to recommend high-index plastic for these prescriptions. The thinner you can make these lenses, the more comfortable they’ll be for your client—and the better they’ll look.
Plastic drawbacks.
On the minus side, plastic lenses are a little thicker than glass lenses, and they scratch easily.
crown glass
Optical labs use a special type of glass called crown glass to make eyeglass lenses. Crown glass is very hard and clear, and it’s denser than plastic.
Crown glass is the heaviest material we can use for glasses. Like plastic, it has its pros and cons. It provides excellent optics, so it does a great job of correcting vision. It doesn’t scratch easily, and it’s not affected very much by changes in temperature. However, in addition to its weight, it breaks easily.
As you learned in today’s reading, manufacturers in the United States need to make sure their lenses, including glass lenses, pass a drop ball test (which we’ll talk more about later). This guarantees that the material is hard enough to meet certain standards. But even though glass lenses have to pass this test, they can still shatter! For this reason, we don’t recommend glass lenses in our practice. Anything hitting the glasses may cause glass to shatter into a person’s eye.
the heaviest material we can use for glasses.
Crown glass
pros of crown glass
It provides excellent optics, so it does a great job of correcting vision. It doesn’t scratch easily, and it’s not affected very much by changes in temperature.
cons of crown glass
However, in addition to its weight, it breaks easily.
manufacturers in the United States need to make sure their lenses, including glass lenses, pass what?
a drop ball test (which we’ll talk more about later). This guarantees that the material is hard enough to meet certain standards. But even though glass lenses have to pass this test, they can still shatter! For this reason, we don’t recommend glass lenses in our practice. Anything hitting the glasses may cause glass to shatter into a person’s eye.
The refractive index of polycarbonate
1.586, which is much higher than plastic.
Because the refractive index of polycarbonate is higher than plastic, labs can make polycarbonate lenses that are
what?
polycarbonate lenses that are much thinner than any plastic lens—as thin as 1 millimeter!
polycarbonate blocks what?
100% of UV light.
Better yet, polycarbonate is what?
impact-resistant. (That’s why engineers used it for the windows in the space shuttles.) Obviously, this makes polycarbonate an ideal lens for all children’s glasses as well as safety and sports glasses.
Why is polycarbonate so impact-resistant?
Because it’s fairly soft. However that same softness makes it extremely easy to scratch. That’s why polycarbonate lenses today are manufactured with a scratch-resistant coating.
Polycarbonate is extremely easy to
scratch
polycarbonate lenses today are manufactured with what?
a scratch-resistant coating.
The tri in the name Trivex stands for what?
its three primary benefits: superior optics, impact resistance, and UV blocking.
The optics of Trivex are better than those of polycarbonate because of what?
Trivex has a higher Abbe Value. this means that it creates less chromatic aberration as a person’s vision moves away from the optical center of the lens. Thus Trivex is an excellent choice for people who need good peripheral vision for work or sports. It is also a great lens material for children’s glasses, just like polycarbonate.
The index of refraction for Trivex
1.53.
trivex is less likely to split or crack than what?
it’s less likely to split or crack than polycarbonate.
Trivex is a good material for
Trivex is a good material for high-prescription lenses in drilled rimless frames because it’s less likely to split or crack than polycarbonate. Its UV-blocking properties are also a big asset.
Optyl is used primarily in
sunglasses.
Optyl, is
hypoallergenic.
when optyl is heated,
it can be bent to fit and stays in its new shape.
optyl cons
if the lenses become too hot, they can pop out of the frame. Also Optyl isn’t impact-resistant, and it’s not very thin.
the thinnest portion of a safety lens should not be less than what?
3.0 mm. This applies to all lens materials except for polycarbonate and Trivex, since these materials are already impact-resistant!
for any children’s, safety, or sports eyewear, what are the only materials to consider.
Trivex and polycarbonate
Trivex and polycarbonate are available in
single-vision, bifocal, and progressive designs.
glasses that aren’t impact-resistant do have to meet certain what?
glasses that aren’t impact-resistant do have to meet certain ANSI standards
For lenses to be considered for use in eyewear, they must pass what?
the drop ball test
To pass the drop ball test
a lens must not shatter when a 5/8-inch-diameter, .56-ounce steel ball is dropped onto it from a height of 50 inches.
Photochromics
Photochromics are lens materials that darken when the lenses are exposed to ultraviolet light. They’re made of substances that react to UV light, so the thicker the material or the lens prescription, the darker the lenses will get.
High-power minus photochromic lenses will be somewhat what on the outside edges.
Low-power lenses won’t get as dark as high-power lenses.
Photochromic lenses will not darken behind the windshield of a car.
somewhat darker on the outside edges.
Low-power photchromic lenses won’t get as dark as what
high-power lenses.
Photochromic lenses will not darken behind what?
behind the windshield of a car.
The most popular photochromic brand is ….
Transitions
Transitions lenses darken when
when exposed to UV light
Transitions become clear after how long?
about five minutes indoors.
Transitions offer what?
UV protection 100% of the time.
Polarizing lenses eliminate what?
Polarizing lenses eliminate the reflected horizontal wavelengths of light that cause glare—so only the vertical wavelengths of light pass through the lenses. It’s a little like having invisible Venetian blinds on your glasses! These lenses can make driving much safer. You should also recommend them for people who are into fishing or sailing, because they reduce the glare from the water.
drawbacks of polarized lenses.
For one thing, these lenses can make it difficult or impossible to see the LED screen on a cell phone, GPS unit, or ATM. They aren’t recommended for skiers, because they make it hard to spot small hills in the snow. And pilots who wear polarized lenses won’t benefit when their angle of tilt changes as they fly.
Polarized lenses are made of what?
Polarized lenses are made of polyvinyl alcohol formed into thin sheets and sandwiched between two layers of plastic. There are several sunglass companies that manufacture quality polarized sunglass lenses. You’ll want to become familiar with these brands, which include Maui Jim, Oakley, Bolle, Costa Del Mar, Ray-Ban, Serengeti, and Wiley X.
There are several sunglass companies that manufacture quality polarized sunglass lenses they are?
Maui Jim, Oakley, Bolle, Costa Del Mar, Ray-Ban, Serengeti, and Wiley X.
