Forget about glass. It is too heavy.

Trivex would be great if you don't mind slightly thicker glasses. 1.60
Plastic would be about the same thickness as polycarb, but much better
optical qualities. Polycarb does have tendency to scratch easily or slight
fog if inappropriate chemicals are applied.

Getting a high quality (durable) AR coating will make your vision brighter
(it lets more light pass through the lens by reducing reflections).

Anytime you avoid polycarb, your vision will improve.

Glass certainly does seem to have an excellent optical quality, and
it's durable to scratching.  As I recall, there is such a thing as
safety glass lenses.  They are made a little thicker than normal, and
then are tempered with either a heat (baking) or chemical treatment.
They pass all OSHA standards just like safety polycarb and plastic,
although they would be quite a bit heavier for sure.  See your eyedoc.

Glass is very unsafe in any form. It can shatter with less force than it
takes to bruise the skin. All glass lenses are tempered in the US. 99% of
them are chemically tempered which means that they are subjected to a hot
bath in potassium salt to replace the sodium atoms in the lens to pull the
surface tighter and set up tension to hopefully deflect blows. A few are
still heat treated where the lens is heated to nearly the melting point and
then suddenly cooled to room temp by compressed air setting up a compression
pattern almost as good as chemical treating. They still fail far quicker
than any of the other options with razor sharp tiny fragments. Anyone who
has hit an animal bigger than a deer with a car knows the needle like
qualities of  glass shards projected back from american windshields;
eyeglasses will respond to impact in a particularly worse fassion since
there is no laminate later,  even if they are called "safety."  "Z87" Safety
glass lenses differ from regular glass lenses ONLY in being 1 mm thicker and
have had a heavier test ball dropped onto them with an engraving(weakening
the tested surface) stating the lab's initials and whether or not they are
photochromic.

CR-39 plastic or Trivex would brighten up the issue quite a bit. The
differences between optics requires equipment to distinguish, the human eye
sees through all three materials the same since the differences are below
the threshold we can detect without a preconcieved mindset. All lenses
should have AR coatings to make things even brighter and crisper. There are
a few occupations that should avoid the improved clarity of AR, however:
miners, gravel pit workers, drywall installers, etc., your glasses will get
trashed quickly regardless so don't waste the money. Maybe put it on a pair
you don't wear to work.

Eyes behind glass lenses are somewhat better protected than exposed eyes,
but are still poorly protected. A high speed impact on a glass lens would
produce shards traveling faster than the eyelids can close. The system was
not designed to protect against such things.

Carl

I would want to look at the lenses to see if they look clear, free of haze, and
are highly polished.  

Although the light transmission is lower with Poly (89.7% versus 92% for Cr39
plastic), that's just not enough difference to be noticeable. Look elsewhere.  

Safe enough for general use. Many people outside of the U.S. wear glass lenses
with 1mm edges or centers, sometimes without tempering. I'm not aware of a
higher rate of eye injuries related to glass lenses in the U.K. or Germany. Just
don't wear them (or wear over the eyeglass protection) when using a table saw, a
nail gun, or when hunting or playing racquetball.

Your plus three diopter lenses are at least 3mm thick at the center; it would
take a pretty big hammer to break them. (If you try it, wear eye protection!) A
thinner lens with pits or scratches might break if dropped on the sidewalk.
(Glass is very scratch resistant but not scratch proof. Moreover, scratches
negate the tempering process, decreasing their impact resistance.)

Not necessarily. Besides, the shards of glass and the object that impacted the
lens is not going to be stopped by a thin layer of soft tissue.

I've heard, but can't substantiate, that soldiers in the Vietnam war who wore
eyeglasses had fewer eye injuries than those who didn't wear glasses (they were
pretty much all glass lenses back then). Although eyeglasses protect the eyes
from low mass, low speed objects like dirt and tree branches, you should use at
least 2mm thick Trivex or Poly lenses in an appropriate frame for protection
from larger, heavier, and/or high speed objects.  

Hope this helps

Robert Martellaro
~~~~~~~~~~~~~~~~~~
Optician/Owner
Roberts Optical
Wauwatosa Wi.
~~~~~~~~~~~~~~~~~~
"Science is a way of trying not to fool yourself."
- Richard Feynman