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July 5, 2018

What Lens Material Should My Glasses Be Made Out Of?

In the optical industry, there are several different materials available, each with different strengths and weaknesses.  The purpose of this post is to explore the differences and best uses of the various lens options, so you can make more informed decisions when purchasing your next pair of glasses.

To begin, there are a couple of terms we need to identify. The first is called the Abbe value. (Pronounced Abbey) The Abbe value is the measure of how much a certain material disperses (scatters or distorts) light. It is counter-intuitive, but a higher Abbe value means a material is optically clearer, or that it scatters or distorts light less.

The other term that we need to know is called the refractive index. The official definition of refractive index is the ratio of the velocity of light through a vacuum compared to the velocity of light through a given medium. For example, light travels slower through water or glass than it does a vacuum. How does this apply to the optical world and to you?  Simple – a higher refractive index means a thinner lens to get a specific glasses prescription. For example, trivex, with a refractive index of 1.58, is approximately 10% thinner than a polycarbonate lens of the same prescription, with a refractive index of 1.53. Still with me? Ok, let’s move on to the good stuff – what lens material works best for you.

Glass (Crown Glass)

Before World War 2, most optical-quality glass came from Germany. When the war started, optical grade glass was suddenly not available to the allied countries, and thus, Crown Glass was born. Crown glass has many slightly different compositions, all having extremely high Abbe values, making it optically superior to many materials available. However, glass is extremely heavy, which can make wearing glass lenses with higher prescriptions uncomfortable.

It is getting harder and harder to find labs that still produce glass lenses. Why? In addition to their extreme weight, glass is a safety hazard. It is prone to shatter with traumatic impacts – imagine a soccer player taking a soccer ball to the face, or a driver in a high-speed car accident. Eyes have been lost due to shattered lenses.

With the weight and safety issues, why are glass lenses still around, then? Simple… as stated above, they are optically superior to most other materials. Also, they are extremely hard to scratch, making them ideal material for rough environments, such as farmers and construction workers. If glass lenses are right for you, expect delays in finding a provider, and they may require you to sign a waiver stating that you are aware of the safety issues with glass lenses.

Plastic (CR-39)

Plastic lenses, also known as CR-39, were developed in the 1940’s. Plastic lenses are extremely inexpensive, and thus are often the go-to for those seeking an inexpensive and lightweight pair of glasses. However, with the lowest refractive index of any common material, they require the thickest lenses based on prescription, which can be an issue with moderate or high prescriptions.

Plastic lenses are extremely soft, and thus they both warp and scratch extremely easily. They also lack the UV protection inherent in all other lens materials. Thus, if a patient decides to go with plastic lenses, I and my team always suggest both a high-quality anti-scratch coating, and a UV-protection coating. (For more info on the benefits of UV protection in glasses, please see our blog post on Sunglasses Basics, here.)  As a distressing side note, Zeiss, a well-known manufacturer of glasses lenses, recently revealed that 80% of clear (non-sunglasses) lenses, or 4 out of 5 pairs of glasses, do not provide UV protection. (See their article here.)

Polycarbonate

Polycarbonate, or poly for short, is the current industry standard for children’s and safety lenses due to its shatter-resistant properties. After all, polycarbonate was originally designed for military visors and bullet-proof glass.

Poly does come with inherent UV protection, and has a mid-range refractive index, yielding a moderately thick lens. For low to moderate prescriptions, polycarbonate lenses will compare favorably to other materials in thickness and weight.

The major downside to poly is its super-low Abbe value, meaning that polycarbonate is not optically clear. That being said, most patients do not notice much of a difference. However, for those patients who are extremely visually sensitive, the aberrations caused by light scatter, such as chromatic aberration, can be visually distracting, and make poly a less-than-ideal choice for them.

Trivex

Trivex, like polycarbonate, is shatter-resistant, and a good alternative for child or safety lenses, especially in people who are sensitive to poly’s distortions or have slightly higher glasses prescriptions.  Trivex is a newer material than polycarbonate, and we are seeing a trend as the industry moves away from polycarbonate into this newer option in both safety and children’s lenses. A prescription trivex lens is thinner, but heavier, than a polycarbonate lens of the same prescription. Trivex is optically clearer than poly and like poly, provides standard UV protection.

High-Index Materials

In the last few decades, the demand for thinner, more lightweight glasses has increased. While 30 or 40 years ago, people with high prescriptions had little choice but to be saddled with “coke-bottle glasses,” today that demand has given rise to a variety of different thin and lightweight materials. These various High-Index materials are classified simply by their refractive index (1.6, 1.67, 1.7 and 1.74 being the most common), with higher numbers meaning thinner and more expensive lenses for a given prescription.

However, there are trade-offs; super-thin lenses are less optically clear (lower Abbe value) and usually have some level of light scatter and/or distortion. That being said, many patients are willing to accept these trade-offs to not have “coke bottle” lenses.

We hope that this brief overview of the common lens materials is helpful, and will allow you to make the most appropriate choice for your needs the next time you are picking out a pair of glasses. For those who want to dig a little deeper into the technical aspects, we have included a table below with the Abbe values and refractive indexes of all of the materials discussed here – please enjoy!

As always, if you have any questions, comments or concerns, please feel free to either post the comment at the bottom of the blog post, or email us at info@lilacfamilyeyecare.com and we will happily respond.

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