> On or about Mon, 7 Jan 2008 22:43:31 -0900 did "Sibir"
> <cseutter[at]mtaonline.net> dribble thusly:
> > 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.
> 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).

> --
> - Mike

> Ignore the Python in me to send e-mail.

> On Tue, 08 Jan 2008 03:02:49 GMT, "C" <nospam[at]nospam.com> wrote:
> > 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.
> I would want to look at the lenses to see if they look clear, free of
> haze, and
> are highly polished.
> > Would Plastic or Trivex make the lens brighter?
> 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.
> > How "unsafe" is glass?
> 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.
> > My glass lens up until this time were bullet proof i.e. they took a
> > lot of bumps.
> 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.)
> > And even if a lens was to shatter, would the eyelid not close
> > in time to protect the eye?
> 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 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......
> 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

> 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.

> > There are geometric features that reduce chromatic abberration,
> > namely aspheric optics and a hemispherical projection surface.
> 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

> Fact is, the only problem specific to polycarb is color fringes, which
> cannot exist or cause blur at the optical center.

> "Mike Ruskai" <BUTthannydI[at]DONTearthlinkLIKE.netSPAM> wrote in message
> news:40e7o3dcrlvaldakfrgm3ukqd71c8uqbib[at]4ax.com...
> > 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.
> 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.

> So how far apart are red and blue focal points if they pass
> perpendicularly through the center of a lens?

> I don't know what "defraction" is. Bending light at the
> interface between two media is "refraction"

> There may well be some neurological compensation, with
> blue cones growing fields that are displaced a little more
> peripherally and red cone fields a little more central but
> I've never seen that in print. I don't think we can map
> receptive fields that precisely.

> More realistically, you could say that the concentration
> of cones is highest near the center and tapers off in
> the periphery. You could also say that overall resolution
> diminishes toward the periphery.

> There are geometric features that reduce chromatic abberration,
> namely aspheric optics and a hemispherical projection surface.

> 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.

> 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.

> Sorry to descend into sarcasm but I just think polycarbonate
> gets blamed too frequently for problems it doesn't cause.

> Fact is, the only problem specific to polycarb is color
> fringes, which cannot exist or cause blur at the optical center.

> Fact is, the only problem specific to polycarb is color fringes, which
> cannot exist or cause blur at the optical center.