Lens Materials: A Guide to different lens materials

In the “old days” glasses’ lenses were made out of crown glass, hence the term glasses.  Glass lenses are still available today, and a few of my patients still insist on glass, but very few.  Glass has some advantages though, namely they don’t scratch easily.  Glass has excellent optics.  But, glass has major disadvanages: glass can shatter, and glass is heavy.  So in terms of safety and comfort, glass has  largly been replaced by newer materials.  To put this in perspective, I probably order 3 to 5 glass lenses in a year’s time.

Refractive Index: Simply means: how much does the material bend light.  The higher the index, the more it bends light which translates to thinner lenses.

ABBE Value: Means how the material “splits” light into different colors, the higher the number the better.  Technically, it means the material refracts (or bends) different colors differently, which is not good.  The effect is that one color, say, blue light is refracted more than red, thus spliting your vision into a rainbow of colors.  Ideally, you want all your colors focused at the same place.

Plastic Lenses (CR-39)

Plastic lenses remain the most popular lens material today.  It has excellent optics.  It’s less expensive than any of the materials.  It’s about half the weight of glass.  With the introduction of scratch coatings, plastic rivals the scratch resistence of glass.  Of course it’s much safer than glass.  It CAN shatter though.  It’s a great material.

Advantages of Plastic:

     Least expensive of the materials. It’s lighter than glass.  It has good optics.  It lasts forever (yes, lenses do age).  Has the highest ABBE value, ie good.  Trivia:  It’s great for welders; sparks tend to bounce off plastic but tend to bury into glass.)  If you are a welder or work around welders, plastic is for you.  Also plastic can withstand practically any solvent (you might think this is not important until your paint solvent turns your lenses opaque.)

Disadvantages of Plastic: Thicker than any of the other materials.   Can Shatter.  Does filter UV radiation but now all.

Polycarbonate

“Poly” was introduced in the 1970’s.  It’s primary advantage over plastic is strength, weight, and it’s thinner than plastic.  It’s used in safety glasses.  

Advantages of Poly:

    Highter refractive index than plastic.  Roughly half the weight of plastic.  Filters all UV radiation.  Very tough.  I’ve never heard of it shattering but can chip and crack.  Of the “thinner” hi-index lenses, poly is the least expensive of the “high index” lenses.

Disadvantages of Poly:

    Less than ideal optics.  A small percentage of people just hate it.  It has a lower ABBE value so it spits color more.  The solvent acetone will melt poly (i.e. nail polish remover)

Trivex

Trivex is one of the newest materials on the market.  It’s main advantage is that is even tougher than poly.  It won’t chip.  It’s the toughest of the materials. 

Advantages of Trivex :

     About the same index as poly, i.e. it’s about the same thickness as poly.  But, it’s even lighter in weight than poly.  It has crisper optics than poly with a higher (better) ABBE value.  So better durability and optics than poly.  I use Trivex in drill mount frames as I’ve never seen it chip.  it filters 100% of UV radiation.

Disadvantages of Trivex

     Not many. It’s simply more expensive than poly.  It’s slightly thicker than poly by about 10%.  So think, slightly thicker but lighter than poly but tougher with crisper optics.

1.67 Plastic

     1.67 Plastic is a high index plastic.  it is thinner than poly and Trivex.  If you want the thinnest lens, 1.67 is for you.

Advantages of 1.67 Plastic

     It’s thin.  There is 1.74 index plastic, which technically is thinner but not by much, but it’s ridiculously expensive.  For a thinnest lens, 1.67 is the best value.

Disadvantages of 1.67 Plastic

     Not as durable as Poly or Trivex.  It’s surface can “craze” leaving a crackled surface.  Optics are not so good.  You will likely see rainbows as it splits colors into a rainbow.

Conclusion

For most prescriptions, good ol’ plastic is the way to go, but if your presription is high with thick edges, or if you need lenses which won’t shatter or break, then you might consider the alternatives.



  







© Richard Randolph 2012