Testing sunglasses for UV protection

#5 11-08-2005, 06:41 AM

I think it is harder to find glass or plastics that are not UV absorbant.

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"The Central Scrutinizer" <bobby_dread[at]hotmail.com> wrote in message
news:1131385765.325757.320560[at]g47g2000cwa.googlegroups.com...
- quote -

> Hey, all.
> I recently bought a pair of nice Serengeti sunglasses off Ebay. The
> price was _really_ good - like less than 40% of retail. It was cheap
> enough for me to wonder if they might be a knock-off.
> The build quality seems really good; the only thing that would bother
> me about them not being 'real' Serengetis would be the actual quality
> of the UV protection they offer. I understand the issue about
> sunglasses with bad UV being worse for your eyes than not wearing
> anything, because they cause your pupils to dilate and let in more UV
> that way.
> Is there a home-brew way I could 'test' the UV protection of these
> things? Maybe some test involving chemical reactions to UV light of
> some compound I could lay my hands on?
> Thanks!!
> BD

#4 11-07-2005, 10:43 PM

- quote -

> The amount you dilate behind sunglasses (say, from 4 mm to 6) might double
> or triple the UV flux through your pupil. But plain old dark CR39 reduces UV
> by 90 or 95%.

Oh, okay - well, if that's the case then I just won't sweat it at all.

Thanks for the reassurance!

#3 11-07-2005, 10:01 PM

"The Central Scrutinizer" <bobby_dread[at]hotmail.com> wrote

- quote -

> > Dark-tinted plastic lenses are pretty good UV filters, even without
> > coatings.
> Interesting; that's pretty much opposite of views I've read which
> prompted me to ask the question.

Depends on who's selling what, in my experience.

- quote -

> I think the argument is that UV light
> is not affected by dark tinted lenses, at least not to the same degree
> as visible light; the net effect is that if you're behind dark lenses,
> your pupils open up, and let in an inordinately large amount of UV
> light, which can basically mean that 'bad' sunglasses can be worse for
> you than no sunglasses at all.

It was a popular urban legend ten years ago. But the math doesn't work.

The amount you dilate behind sunglasses (say, from 4 mm to 6) might double
or triple the UV flux through your pupil. But plain old dark CR39 reduces UV
by 90 or 95%.

Of course all UV wavelengths aren't filtered equally. The optician's meter
will test mostly for UV-A because UV-B and C are generally blocked by
anything.

-MT

#2 11-07-2005, 05:54 PM

- quote -

> Dark-tinted plastic lenses are pretty good UV filters, even without coatings.

Interesting; that's pretty much opposite of views I've read which
prompted me to ask the question. I think the argument is that UV light
is not affected by dark tinted lenses, at least not to the same degree
as visible light; the net effect is that if you're behind dark lenses,
your pupils open up, and let in an inordinately large amount of UV
light, which can basically mean that 'bad' sunglasses can be worse for
you than no sunglasses at all.

Oh well. Interesting regardless. I may call an optician and just see if
they can test the approximate UV protection of the lenses I have.

#1 11-07-2005, 05:52 PM

"The Central Scrutinizer" <bobby_dread[at]hotmail.com> wrote in message
news:1131385765.325757.320560[at]g47g2000cwa.googlegroups.com...
- quote -

> Hey, all.
> I recently bought a pair of nice Serengeti sunglasses off Ebay. The
> price was _really_ good - like less than 40% of retail. It was cheap
> enough for me to wonder if they might be a knock-off.
> The build quality seems really good; the only thing that would bother
> me about them not being 'real' Serengetis would be the actual quality
> of the UV protection they offer. I understand the issue about
> sunglasses with bad UV being worse for your eyes than not wearing
> anything, because they cause your pupils to dilate and let in more UV
> that way.
> Is there a home-brew way I could 'test' the UV protection of these
> things? Maybe some test involving chemical reactions to UV light of
> some compound I could lay my hands on?
> Thanks!!
> BD
All sunglasses by law must have UV protection. That is no guarantee, but it

is unlikely that it lacks the UV protection you want.

11-07-2005, 05:11 PM

"The Central Scrutinizer" <bobby_dread[at]hotmail.com> wrote

- quote -

> Is there a home-brew way I could 'test' the UV protection of these
> things? Maybe some test involving chemical reactions to UV light of
> some compound I could lay my hands on?

Dark-tinted plastic lenses are pretty good UV filters, even without
coatings. It's hard to tan under most any sunglasses.

If you need more reassurance, many opticians have a UV meter for
demonstrating UV transmittance.

The difference between 10% and 0.1% isn't often important.

-MT

#-1 11-07-2005, 04:49 PM

Hey, all.

I recently bought a pair of nice Serengeti sunglasses off Ebay. The
price was _really_ good - like less than 40% of retail. It was cheap
enough for me to wonder if they might be a knock-off.

The build quality seems really good; the only thing that would bother
me about them not being 'real' Serengetis would be the actual quality
of the UV protection they offer. I understand the issue about
sunglasses with bad UV being worse for your eyes than not wearing
anything, because they cause your pupils to dilate and let in more UV
that way.

Is there a home-brew way I could 'test' the UV protection of these
things? Maybe some test involving chemical reactions to UV light of
some compound I could lay my hands on?

Thanks!!

BD

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