Is there a minimum thickness for eyeglasses in US?
Sum Lin
by law for eyeglass lenses sold in the US?
I have purchased glasses here and overseas and found that eyeglass
lenses available here in the US are much thicker and heavier.
Even the ultra-thin lenses are relatively thicker than the ones
I got in Japan. At first I thought it was some new technology
but when I asked the Japanese Optometrist he indicated that
the lenses I got were made in the US.
If so, why can't I get it here?
Sum Lin
Email: sl...@esri.com
Specmaker
>I have purchased glasses here and overseas and found that eyeglass
>lenses available here in the US are much thicker and heavier.
>Even the ultra-thin lenses are relatively thicker than the ones
>I got in Japan. At first I thought it was some new technology
>but when I asked the Japanese Optometrist he indicated that
>the lenses I got were made in the US.
>If so, why can't I get it here?
ANSI standards in the US require that glass and plastic lenses be
at least 2.0mm thick in the center. Only polycarbonate, the most
impact resistant material available, can be ground thinner. 1.0mm
is possible, but unless it is requested many Opticians use 2.0mm
or thicker polycarbonate. Overseas, lenses can be ground thinner
than 1.0mm, but it may be at the risk of severe eye injury.
Chris Ward
Owner - Guardian Optical
Serving the Electronic Community
Grieg A. Olson
> I have a stupid question - Is there a minimum thickness required
> by law for eyeglass lenses sold in the US?
>
Yes, there are standards. The requirements actually involve inpact resistance.
I don't remember the details.
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Grieg
Steve Machol
: > I have a stupid question - Is there a minimum thickness required
: > by law for eyeglass lenses sold in the US?
: >
: Yes, there are standards. The requirements actually involve inpact
: resistance. I don't remember the details.
Actually, this issue is fairly complicated, and even confuses many people
in the ophthalmic industry. First of all, the only lenses which have to
meet a specified minimum thickness are lenses which are sold as "safety
glasses". These must be no less than 3.0mm at the thinnest point on the
lens.
Although regular eyeglass lenses do not have a specified minimum
thickness, they nonetheless have to meet the FDA requirements for "impact
resistance." For glass lenses, this means that the lenses must undergo
heat or chemical tempering, then subjected to a "drop-ball" test (a 5/8"
steel ball dropped from 50 inches.)
Plastic lenses do not require special treatment or testing. They do,
however, require that lens manufacturers state under what conditions their
lenses will meet the FDA impact requirements. For each specific lens
material, lens manufacturers do this by "certifying" their lenses will
meet these requirements when processed to a certain minimum thickness and
processing standard. These recommendations are based on exhaustive and
continuous "batch" testing done as part of the manufacturing process. Any
lenses ground thinner than the manufacturers' recommendation will not be
in compliance with FDA impact requirements, unless the grinding laboratory
is willng to conduct their own tests. Since the drop-ball test oftens
mars the surface of plastic lenses, this is not a practical option for
most laboratories.
The upshot of all this is that a "standard of care" has developed
concering minimum thickness for various lens materials. For CR-39 lenses
(the lens material in about 70% of all eyeglasses in the U.S.), all
manufacturers that I'm aware specifiy a minimum center thickness of 2.0mm.
Minimum edge thicknesses depend on overall lens power. Some "high index"
and polycarbonate lenses are certified to meet impact requirements at
1.5mm centers, and, in some cases (very high minus powers), even as low as
1.0mm thickness.
I hope this helps clarify rather than confuse the issue. :->
Regards,
Steve
Sola Optical, USA
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| Steve Machol "Dedicated to Kong who must have been a great bloke." |
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