lens materials

Question 1

index of refraction

Answer 1

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.

Question 2

Refractive Indices of Common Lens Materials

Answer 2

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

Question 3

You’ll often hear people refer to plastic lens materials as what?

Answer 3

CR-39

Question 4

the name of the formula originally used for plastic lenses.

was what?

Answer 4

CR-39

Question 5

plastic advantages

Answer 5

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.

Question 6

Plastic drawbacks.

Answer 6

On the minus side, plastic lenses are a little thicker than glass lenses, and they scratch easily.

Question 7

crown glass

Answer 7

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.

Question 8

the heaviest material we can use for glasses.

Answer 8

Crown glass

Question 9

pros of crown glass

Answer 9

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.

Question 10

cons of crown glass

Answer 10

However, in addition to its weight, it breaks easily.

Question 11

manufacturers in the United States need to make sure their lenses, including glass lenses, pass what?

Answer 11

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.

Question 12

The refractive index of polycarbonate

Answer 12

1.586, which is much higher than plastic.

Question 13

Because the refractive index of polycarbonate is higher than plastic, labs can make polycarbonate lenses that are
what?

Answer 13

polycarbonate lenses that are much thinner than any plastic lens—as thin as 1 millimeter!

Question 14

polycarbonate blocks what?

Answer 14

100% of UV light.

Question 15

Better yet, polycarbonate is what?

Answer 15

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.

Question 16

Why is polycarbonate so impact-resistant?

Answer 16

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.

Question 17

Polycarbonate is extremely easy to

Answer 17

scratch

Question 18

polycarbonate lenses today are manufactured with what?

Answer 18

a scratch-resistant coating.

Question 19

The tri in the name Trivex stands for what?

Answer 19

its three primary benefits: superior optics, impact resistance, and UV blocking.

Question 20

The optics of Trivex are better than those of polycarbonate because of what?

Answer 20

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.

Question 21

The index of refraction for Trivex

Answer 21

1.53.

Question 22

trivex is less likely to split or crack than what?

Answer 22

it’s less likely to split or crack than polycarbonate.

Question 23

Trivex is a good material for

Answer 23

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.

Question 24

Optyl is used primarily in

Answer 24

sunglasses.

Question 25

Optyl, is

Answer 25

hypoallergenic.

Question 26

when optyl is heated,

Answer 26

it can be bent to fit and stays in its new shape.

Question 27

optyl cons

Answer 27

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.

Question 28

the thinnest portion of a safety lens should not be less than what?

Answer 28

3.0 mm. This applies to all lens materials except for polycarbonate and Trivex, since these materials are already impact-resistant!

Question 29

for any children’s, safety, or sports eyewear, what are the only materials to consider.

Answer 29

Trivex and polycarbonate

Question 30

Trivex and polycarbonate are available in

Answer 30

single-vision, bifocal, and progressive designs.

Question 31

glasses that aren’t impact-resistant do have to meet certain what?

Answer 31

glasses that aren’t impact-resistant do have to meet certain ANSI standards

Question 32

For lenses to be considered for use in eyewear, they must pass what?

Answer 32

the drop ball test

Question 33

To pass the drop ball test

Answer 33

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.

Question 34

Photochromics

Answer 34

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.

Question 35

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.

Answer 35

somewhat darker on the outside edges.

Question 36

Low-power photchromic lenses won’t get as dark as what

Answer 36

high-power lenses.

Question 37

Photochromic lenses will not darken behind what?

Answer 37

behind the windshield of a car.

Question 38

The most popular photochromic brand is ….

Answer 38

Transitions

Question 39

Transitions lenses darken when

Answer 39

when exposed to UV light

Question 40

Transitions become clear after how long?

Answer 40

about five minutes indoors.

Question 41

Transitions offer what?

Answer 41

UV protection 100% of the time.

Question 42

Polarizing lenses eliminate what?

Answer 42

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.

Question 43

drawbacks of polarized lenses.

Answer 43

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.

Question 44

Polarized lenses are made of what?

Answer 44

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.

Question 45

There are several sunglass companies that manufacture quality polarized sunglass lenses they are?

Answer 45

Maui Jim, Oakley, Bolle, Costa Del Mar, Ray-Ban, Serengeti, and Wiley X.