Lesson 9 - Lens Materials and Treatments

Question 1

Principal axis

Answer 1

This line goes through the center of a lens, perpendicular to the surface of the lens.

Question 2

Optical center

Answer 2

This is the spot where light passes through a curved lens without bending. You’ll find it at the thinnest point of a minus lens and the thickest point of a plus lens. Light striking the lens at any other point will bend, due to the curvature of the lens.

Question 3

Index of refraction

Answer 3

The amount a material is able to bend light. The higher a material’s index of refraction is, the more it bends (refracts) light—and the thinner the lens can be.

Question 4

CR-39

Answer 4

Plastic lenses, lowest index of refraction = 1.498

Question 5

Crown glass

Answer 5

Index = 1.523

Question 6

Polycarbonate

Answer 6

Index = 1.586, impact resistant, UV protection, easily scratched, allows for thinner lenses

Question 7

Trivex

Answer 7

Index = 1.53, superior optics than poly, impact resistant, UV protection

Question 8

Photochromic lenses

Answer 8

Activated by UV, do not darken behind windshield, UV protection. High-power minus lenses will be somewhat darker on the outside edges. Low-power lenses won’t get as dark as high-power lenses.

Question 9

What are polarized lenses? What are their pros and cons?

Answer 9

Polarized lenses are made of polyvinyl alcohol formed into thin sheets and sandwiched between two layers of plastic.

They eliminate the reflected horizontal wavelengths of light that cause glare, so only the vertical wavelengths of light pass through the lenses. They also offer UV protection. Most people who are into sports and outdoor activities love these glasses. Their advantages include enhanced contrast, visual comfort, and improved visibility.

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 10

Blue blocking lenses

Answer 10

Help reduce melatonin blocked by digital devices

Question 11

Mechanical center

Answer 11

This is the point on a lens around which the lens is cut and edged. This is not always the same as the optical center, because not all lenses are symmetrical.

Question 12

The refractive index of air is what?

Answer 12

the refractive index of air is 1

Question 13

When we talk about the refractive index of a lens, we compare it to what?

Answer 13

When we talk about the refractive index of a lens, we compare it to the refractive index of air.

Question 14

High-index plastic

Answer 14

Index = 1.60, 1.67, 1.70, 1.74

Question 15

What are some of the pros and cons of plastic lenses?

Answer 15

On the plus side, plastic is lighter than glass, and it can be tinted. On the minus side, plastic lenses are a little thicker than glass lenses, and they scratch easily.

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 16

What are some of the pros and cons of glass lenses?

Answer 16

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.

Glass lenses have to pass the drop ball test, but 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 17

What are some of the pros and cons of polycarbonate lenses?

Answer 17

The refractive index of polycarbonate is 1.586, which is much higher than plastic. Because of this, labs can make polycarbonate lenses that are much thinner than any plastic lens—as thin as 1 millimeter! In addition, this material blocks 100 percent of UV light.

Better yet, polycarbonate is impact-resistant. (That’s why engineers used it for the windows in 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.

Question 18

What are some of the pros and cons of trivex lenses?

Answer 18

The tri in the name Trivex stands for its three primary benefits: superior optics, impact resistance, and UV blocking.

The optics of Trivex are better than those of polycarbonate because Trivex has a higher Abbe Value. As you’ll recall from an earlier lesson, 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 is 1.53. As a result, Trivex lenses can be very thin—although not quite as thin as polycarbonate lenses. And 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 19

According to the American National Safety Institute (ANSI) standards for lenses, the thinnest portion of a safety lens should not be less than what? What lenses does this apply to?

Answer 19

According to the American National Safety Institute (ANSI) standards for lenses, the thinnest portion of a safety lens should not be less than 3.0 mm. This applies to all lens materials except for polycarbonate and Trivex, since these materials are already impact-resistant!

Question 20

In some practices, impact-resistant lenses are always recommended for what?

Answer 20

In some practices, impact-resistant lenses are always recommended for children’s eyewear

Question 21

In addition to being impact-resistant, polycarbonate and Trivex both protect against what?

Answer 21

In addition to being impact-resistant, polycarbonate and Trivex both protect against UV rays.

Question 22

Polycarbonate is an excellent choice for higher prescriptions. Why?

Answer 22

Polycarbonate is also an excellent choice for higher prescriptions since the lenses made from these materials can be as thin as 1 mm.

Question 23

So how do you choose between polycarbonate and trivex lenses?

Answer 23

The chromatic aberration of polycarbonate is greater than that of Trivex, so the optics of Trivex are a little better. This is a great lens to consider for prescription powers of -3.00 to +3.00, because the lenses in this range will still be fairly thin. Beyond this power range, polycarbonate lenses would be a better cosmetic choice since they can be made thinner than Trivex lenses.

Question 24

For any children’s safety or sports eyewear, ___ and ___ are the only materials to consider.

Answer 24

For any children’s safety or sports eyewear, Trivex and polycarbonate are the only materials to consider.

Question 25

Glasses that aren’t impact-resistant do have to meet certain American National Standards Institute (ANSI) standards. For lenses to be considered for use in eyewear, they must pass that drop ball test. How do lenses pass this test?

Answer 25

glasses that aren’t impact-resistant do have to meet certain American National Standards Institute (ANSI) standards. For lenses to be considered for use in eyewear, they must pass that drop ball test we talked about earlier. To pass the 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.

Question 26

The most popular photochromic brand is? Why?

Answer 26

The most popular photochromic brand is Transitions. Transitions lenses darken when exposed to UV light and become clear after about five minutes indoors. They offer UV protection 100 percent of the time. These lenses are very convenient for people who don’t want to switch to sunglasses every time they go outside. They’re available in all lens designs including multifocals, and they come in several lens materials.

Question 27

There are several sunglass companies that manufacture quality polarized sunglass lenses. List some of the major brands.

Answer 27

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

Question 28

Scratch-resistant coating merely means that...

Answer 28

Scratch-resistant coating merely means that the coating will protect the lens from light scratches; it does not make the lens itself resistant to scratches.

Question 29

Mirror coatings

Answer 29

An option that many people choose for sunglasses and sport lenses is a mirror coating. This coating reflects up to 60 percent of light, dramatically reducing glare. Mirror coatings are quite effective in very bright environments like ski slopes and beaches—and because they don’t reduce contrast as much as polarized lenses, they’re safer for skiers, bikers, and mountain climbers.

Question 30

Antireflective coatings

Answer 30

These coatings prevent light from reflecting off the surface of a person’s lenses. This makes a big difference in how the glasses look. Antireflective coatings also increase the amount of visible light going through a lens from 92 percent to 99 percent. They’re a good choice for people who work on computers, because computer monitors create a lot of glare. Antireflective coatings also work well with high-index lenses, which reflect more light than regular plastic and glass. Sunglasses with antireflective lenses can help reduce daytime glare, while clear lenses with this coating can reduce the glare from headlights at night.

Question 31

Tints

Answer 31

Tinted lenses are made by dipping the lenses into various colored ophthalmic tints. Colored tints often fade with time, but they’re lots of fun for fashion lovers. They’re also handy for athletes, because they can enhance performance. (For instance, some trap shooters like yellow-tinted lenses because they increase the contrast between the sky and the target.)

Question 32

What is a tinting unit? How does it work?

Answer 32

A tinting unit has several stainless-steel bins filled with colored dye. Heated oil beneath the metal bins keeps the tint at a steady temperature. The lenses are lowered into the dye several times until they’re just the right color.

Question 33

Absorptive Lenses

Answer 33

These lenses absorb specific wavelengths of light. The most common types of absorptive lenses you’ll encounter are ultraviolet (UV)-blocking lenses. A lens with a UV-blocking coating can be clear but still block UV rays.

Question 34

Many of your clients will think that all dark lenses block UV light and that clear lenses can’t block UV light. Why is this not true?

Answer 34

Many of your clients will think that all dark lenses block UV light and that clear lenses can’t block UV light. However some lenses are just tinted and don’t have any UV coating—while other lenses have a UV coating but no tint. So tell your clients that it’s always a good idea to have their sunglasses tested in a UV meter to be sure they have a UV protective coating.

Question 35

When you’re talking with your clients about tints, how should you explain the color in glass sunglass lenses?

Answer 35

When you’re talking with your clients about tints, explain that the color in glass sunglass lenses is not a coating. Instead the glass is made in that color, so the color is present throughout the lens material. This color will never fade over time, as it does in dipped tinted lenses made of plastic.