Lens Materials Advantages and Disadvantages

First things first, I would like to apologize for my extended absence.  Halloween season with a 9 year old means lots of time spent on costumes and parties.  Pair that with a small round of being sick and, well, here we are a few weeks later.

Today I’d like to spend a little time discussing the many materials that can be used to make prescription lenses.  There are many solutions to fit the many needs of different prescriptions and lifestyles.  There is no, one single best choice.  But I will break down the advantages and disadvantages on each material to help you navigate the best choice for your needs.

Abbe Value

No, Abbe isn’t a girl who works at the lab.  This is a number used to define how much aberration (distortion and/or color separation) occurs in a lens material.  The lower the number, the more distortion there is inherent in the material.

 

AbbeValues1

AbbeValues2

Now, I know these numbers by themselves won’t mean much to you.  But the essence of these numbers is that glass has the best optical quality available.  The trade off, however, is that glass is also excessively heavy (especially for a higher prescription), and it’s less safe as a rule given the brittle nature of the material.

Herein lies the rub.  The best optics almost always come with a trade off on weight and durability.  The lighter and thinner materials are more comfortable and cosmetically look better for higher prescriptions, but that does come with an optical clarity trade-off.

Index of Refraction

Refraction is the term used to describe how much light is bent when it enters a medium.  When we are talking about glasses, we’re always talking about the lens material.

Refraction_at_interface.svg

The a 1.0 index of refraction would indicate that the light is not bent at all.  Any number of 1.0 is indicating that the light is entering a different material which bends the light more dramatically.  The higher the number, the more the light is bent, and therefore the less material needed to correct for a high prescription.  Thus a 1.74 hi index plastic bends the light more than a 1.50 standard plastic (CR39).

The more the light is bent, the thinner and generally lighter the lens material is, but that extreme bending also means some light is lost to the prism effect (color aberration).  That is to say, lower abbe values separate white light out into its separate color components and you may actually see a small rainbow effect around the edges of things.

2000px-Chromatic_aberration.svg Chromatic_aberration_with_detail

This aberration is why it’s so important to take the additional measurements of not just pupil horizontal position, but also vertical position on a patient with a higher prescription.  These chromatic  aberrations can become very intrusive with hi index materials.

 The Polycarbonate Reputation

Polycarbonate has traditionally had a very bad reputation.  In part this is the same old story as Transitions.  The older versions of the material from 30 years ago were quite inferior.  Lots of aberration, chromatic distortions and very small optical centers.  In part it was the manufacturing process used at the time.  Over the past couple of decades the manufacturing of this material has improved dramatically.  You can still get “bad batch” versions with terrible distortion, but they are relatively few and far between.  I think it does say something to the improvement of the material that I wear polycarbonate almost exclusively in my glasses, and have for the past 10 years. It can be a good choice as long as your prescription isn’t too strong.

Choosing the Material for You

As you can probably guess from all the above material, the general rule of thumb is to go with the highest abbe value (meaning the least amount of aberration).  But this has to be balanced against the power of the prescription, and the impact resistance necessary for lifestyle.

For these reasons, polycarbonate (for better or worse) has become the most common lens material in the optical industry…at least in the USA.  While it does not have the best abbe value, it does have impact resistance.  It has a relatively wide range of prescriptions which can be corrected before chromatic aberration becomes an issue, and it is has a plentiful number of lens design solutions available (Transitions, progressive designs, glare treatments, etc.).  Is it the best solution?  I’d say probably not.  Is it the easiest solution?  It’s the safest bet of protecting the eye and giving decent visual acuity, while still having access to all the different ways of correcting vision needs.

All things being equal, I think I’d prefer to work with Trivex for most moderate corrections, but the limitations on progressive designs and transitions colors available keeps me from going to it frequently myself.  In fact, I wear polycarbonate for my lenses.

However, if your prescription is either above a +3.00 or a -4.00 I generally recommend considering high index to control some of the chromatic aberration and to provide a cosmetically thinner lens.

Take Away

There are many lens materials available, and finding which one best suits your needs comes down to how strong your prescription is, and how shatter resistant do you need your lenses to be…but in the end polycarbonate is often used just because of accessibility and safety, even if it isn’t always the best optical choice it’s still a decent choice for most common prescriptions.

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