Comments About Glass
Glass is the only lens material that can be made into 100% optically correct sunglasses.  For true vision, there is no comparison between glass and other lens materials. 

Glass does not flex or compress.  Whenever lenses are placed into an eyewear frame, they are held in place by pressure of some kind. Plastic lenses flex and that creates distortion; but that cannot happen with glass lenses. 

Glass does not expand or contract in heat and cold in a significant way. No matter what the temperature outside, 100% optically correct sunglasses will remain 100% optically correct.

Glass is the hardest material, and the most scratch-resistant.  Glass is the most durable, and longest-lasting sunglass lens material. However, it is not scratch proof.  Sand can scratch glass, but it requires serious effort or trauma to scratch glass lenses.

Technology has made it possible to create a very lightweight glass lens.  Today, it can be difficult to tell the difference between glass and plastic, even when holding a pair with glass lenses in one hand and a pair with plastic in the other hand.  (Of course, the difference is clear when looking through the lenses.  But the weight of the lens is no longer a dead giveaway as to which is which.)

The danger of shattering was once one of the major disadvantages of glass lenses.  However, all glass lenses pass the "Drop Ball Test" for impact resistance - the same test that is administered to polycarbonate and CR-39 lenses. When today's polarized lenses tend to break in a similar pattern as a car windshield.  The broken pieces of glass mostly stick to the polarizing film that is inside the lens.


These Spotters had been stepped on by the customer.  Notice that the broken lens has retained its original curvature.  The broken lens pieces were still so tightly held by the frame that we had to smash the broken lens out of the frame with a hammer (it took more than a few smacks, too!).   This isn't to say in any way that glass lenses are equivalent to safety glasses.  However, polarized glass lenses are much, much less likely to cause injuries today than they ever have been in the past.

Glass is the most expensive lens material. 

Comments About CR-39
CR-39 is harder material than Polycarbonate, and therefore less likely to scratch.  However, all plastics are softer than glass, and all plastics will scratch much more easily than glass. 

CR-39 is nearly as optically pure as glass, which makes CR-39 the top of the line plastic for precision vision.

Unfortunately, there is no such thing as 100% optically correct CR-39 sunglasses.  All lenses must be held in a frame with pressure.  Pressure applied to CR-39 (and Polycarbonate) causes it to bend and flex.  This creates visual distortion.  Your eyes are instruments of incredible precision - and even a slight imperfection in the curvature of the lens will affect your eyesight. 

Unfortunately, many well known sunglass companies state in their brochures or ads "100% optically correct CR-39."  Or, "100% Optically Correct CR-39 Lenses." 

That is what's known as "marketing slight-of-hand."  Although what they state is true - the lenses are 100% optically correct - what they don't say is that, when the lenses are inserted into a sunglass frame, they are no longer optically correct.  No manufacturer can solve this problem.  Nevertheless, CR-39 is still a better choice for optical clarity than Polycarbonate.

CR-39 can chip in a way that is similar to glass. Although every lens sold in the USA must pass an impact test (called the "Drop Ball Test"), if hit with a powerful impact CR-39 will break. 

CR-39, like polycarbonate, is a less expensive lens material than glass.  Manufacturing plastic lenses is also less expensive than working with glass. 

Today there is only a marginal weight difference between the best polarized glass lenses and plastic.  Without that strong advantage versus glass, CR-39 has been loosing popularity.  CR-39 retains its weight advantage against glass when it is used in making strong prescription sunglasses (above +2.00 and below -4.00 and beyond 4D).  In those cases, CR-39 is the best alternative to glass lenses.