Question about Glass, Plastic, Trivex, polycarbonate lens and safety

I can't find my perscription, but if memory is correct, both eyes are around
+3 with no astigmatisms. My current glasses are polycarbonate lens. Before
them, I had glass. I noticed a drop in "brightness" with the polycarbonate
lens. Would Plastic or Trivex make the lens brighter? How "unsafe" is
glass? My glass lens up until this time were bullet proof i.e. they took a
lot of bumps. And even if a lens was to shatter, would the eyelid not close
in time to protect the eye? I have to think that my eyes behind a glass
lens are infinitely more protected than the guy that doesn't wear glasses of
any kind......

Comments?

Chris

 

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.

 

All this is more than a bit alarmist. I had glass lenses for over 10
years, starting at age 10. They were heavier, and I'm sure they
wouldn't stand up the same abuse as something like polycarb, but they
were not nearly as fragile as you imply.

And there are no needle like shards of glass projected back from
windshields. I had an aluminum boat driveshaft crash through mine a
few years back at 75mph. All of the glass that broke free was
pebbled.

The only real argument against glass lenses is weight. Any face plant
that would break them is going to cause more serious injuries first,
as I can attest from experience (none of mine ever broke).

 

Windshields are made completely differently. They have a lamination in the
middle of two sheets of glass that holds it all together to reduce serious
injury when it breaks.

The reasons why you want glass lenses are not sufficient. Just avoid
polycarb and you will be happy with the lenses.

 

When I face-planted I broke the frame above the right lens (which I
repaired with super glue), but the lenses were fine. I used glass
glasses (photochromic) as motorcycle glasses for at least 15 years with
no mishaps. The bee that bounced off a lens and then into my helmet,
giving me a nasty sting in the process, probably shouldn't count anyway;
bees are pretty soft.

They really WERE heavy, though -- and those were the days when glasses
were BIG.

 

Their photochromic ability was WAYYYY better than what I got with
plastic a few years back. The plastic photochromes were essentially
worthless while the glass ones worked just fine.

BUT the thin polycarb that non-prescription lenses are made of works
quite nicely -- my favorite sunglasses (the ones I wear with my
contacts) are cheap ($5-$7 on sale) polarized ones from Big 5 (discount
sporting goods store) and are light and cover a lot of area, making them
good for wind-in-the-face occasions.

 

Agreed PC has a poor Abbe, but that is an increasing problem
at higher powers and not for plano like Bev's sunglasses.

Did I miss something?

-- Robert

 

And also don't exist in plano polys or near plano ones, which make great
sunglasses. I like the big poly wrap arounds with the Rx inserts...
best of both worlds.

I use polys pretty routinely in kids, esp. in situations where insurance
pays for them but not for Trivex. Kids don't seem to mind the chroma.

re the glass +3 lenses, if his nose can handle the weight, they're
pretty safe.

In wwII, the soldiers who wore glasses often had drilled glass lenses,
1.0 mm ct, no tempering. That's why they are called glasses, after all...

one last item from an old o.d.'s memory: the optically best glasses I
ever wore were glass, with cruxite A tint in the glass and
anti-reflection coating. Far less reflection than any of today's
lenses, just about zero ghosting, and almost unscratchable.

My nose was stronger then...

w.stacy, o.d.

 

Agreed. Thanks for the explanation.

I may be misinterpreting you, but chromatic abberation most
certainly can exist on the principal optic axis. Abbe number [wiki]
is a measure of how different the material's refractive index is
at different visible wavelengths (colors). How far apart the focal
points are for different colors, hence how big the chroma haloes.

Off axis is is worse (prism rainbows), and at higher powers
linearly worse: more defraction means more dispersion.

In photographic lenses, negative and positive elements with
different RIs and Abbes are combined to cancel out as much
of the chromatic abberation as practical.

What I don't know is how the human eye system is wrt chromatic
abberation (RI & Abbe of lens and vitrious humor). I suspect it
is not fully compensated and does corrections by cone placement.

-- Robert

 

Obviously no distance at all. However, neglegible amounts of
light actually pass through the center of a lens. Technically
zero. Most passes through the periphery where refraction is
required. Whence un[der]corrected vision worsening in low light.

Correct. I misspoke -- too much work with the grates!

That is what I'm suggesting, or that the phtosensitive
portions of the cones are biased to compensate for ab.

This is well known and has little bearing on ab.

Certainly these features exist but I'm not sure they have a
positive effect on ab: to the extent it matters (the fovea
is small and hence relatively flat), hemispherical focus
planes require more refraction than equivalent flat-plane.
Aspheric lenses still have chrom.ab and are more used to
fight spherical abberation and intensity fall-off or for
improved off-center use in glasses.

-- Robert

 

1) I wasn't comparing glass lenses to car windshields. The person I
was responding to did, and in doing so made an incorrect claim about
windshields. I was correcting that.

2) I don't want glass lenses. I simply said that there's no
legitimate safety argument against them, and that weight is the main
reason you'd want to avoid them.

 

This is true for the lenses themselves, but not the wearer. You get
color fringes in the center of your vision if you move your eyes
instead of your head.

 

Awesome responses! Thank you,
Chris