Explain UV protection to 400nm


A friend recommended these glasses for use with oxy-acetylene
torches. I like the idea of clip-on flip up lenses for $56, and they
are a pretty shade of purple. They are sold a site for glass workers,
waleapparatus.com Prodid 548083

The site says:

Clip-On Flip-Ups are designed to clip onto existing glasses as
an add-on with Polycabonate lenses. Polycarbonate lenses are
light-weight scratch-resistant polycarbonate lenses that filter
sodium flare similar to glass Didymium. These purple tinted
lenses feature full UV protection to 400nm, and excellent IR

I don’t know what 400nm means. Is that good for jewelers to use?
Does that cover what we do? I am hoping that someone on Orchid knows!

Cynthia Eid

400 nm is the wavelength of the light = 400 nanometers; if it is
used for welding it should provide flash burn protection from the uv


Hello Cynthia,

the 400nm is the longest of the high energy wavelengths of light
that the lenses help to block. Here’s a good chart that shows where
on the spectrum that wavelength falls:

Deep purple light, which is the shortest wavelength most people can
actually see, is about 400 nm, so they are saying that they will
protect you from the ultraviolet emissions (wavelengths shorter than
400nm). Polycarbonate is inherently good at stopping UV - coatings,
tints, or additives to the polycarbonate can help even more with

The wavelength of the energy emitted from a torch or hot object is
directly proportional to its heat. Ultraviolet light comes from HOT
stuff, 4500+ degrees F (oxy acetylene, TIG welders, plasma torches).
Since Oxyacetylene burns at 3840 C (about 6000 F) it definitely is
emitting some UV. Those of us using oxyacetylene a lot should
definitely consider some UV blockers as well as some visible light
blockers, there is a lot of energy that falls into the visible light
spectrum that is not really good for you either. These glasses say
they stop sodium flare (bright purple, that’s why the tint) which is
high energy visible light that comes mainly from working glass:

One of the big problems with UV and other invisible radiation (I
work around fairly high power lasers) is that those wavelengths
bounce quite nicely. The glasses we wear when a big laser is
unshuttered are full on nerd herd… big bulbous wraparounds, because,
while clip-on lenses are useful, especially for small, very bright
oxyacetylene flames like from a smith mini torch, they may be a
partial answer because they only stop the light that is right in
front of you, not the stuff that might bounce in from the sides!
Welders run into that more than we do, because their flare is SO
BRIGHT and bounces off of everything in the vicinity - ask the
welder who has a sunburn on the back of his neck from working with a
white wall behind him!

on the other end of the spectrum…

Infrared (IR) is mainly just felt as heat. If you feel heat on your
skin, your eyes are getting it too, but unfortunately your eyes don’t
have nerves that are sensitive to heat to tell you about it.

Most of the infrared heat from your torch radiates at right angles
to the flame - expanding cylinders of heat, and it doesn’t bounce
around as much as UV does. Usually we as jewelers don’t heat really
big objects, but we DO get really close to our work sometimes… to
see what we are doing.

(geek 7x24, jeweler wannabe)

Hi Cynthia,

I use those glasses. I use the Didymium glass ones, probably the
same, I find it cuts the yellow/orange flame color and I can see the
metal better. I was introduced to them by a glass bead workshop and
thought they might be good for metals too. I like them. I have heard
of other metals people who use them as safety glasses. Since you have
to have safety glasses then why not use a protective one?


Cynthia, I’m no expert on this, but I can tell you this: nm stands
for nanometer, or the visible light length when describing how far in
a spectrum light is visible. When describing light wave length it
is used to determine how “strong” a light is in the infrared
spectrum. Generally speaking, the higher the nanometer length, the
stronger the light is, even though it is less visible to the human
eye. Laser lights are typically described in nanometer strength, and
mW (milliwatt). mW also describes the “strength”, but used more as an
indicator of how far the light will travel.

The didymium glasses protect you from the sodium flare that is given
off by certain types of gases and most fluxes. It also protects the
visible infrared light from reaching your eyes. I like the purple
tint better than the green shade for welders, because you can see
what you’re doing better. However, you can’t tell when you’ve reached
annealing temperature with them (blocks out visible red and infrared
light), so use a Sharpie marker for an indication.

Speaking from personal experience, when I used my didymium headgear
and had a long soldering session ahead of me, I didn’t suffer from
the headaches or eye strain. However, I haven’t been able to use it
since I’ve had to go to bifocals. I’m thinking of taking the lenses
to my local eye doctor and see if some sort of getup can be rigged
for attaching them to my 4X Optivisor.

The price is right, so you might want to give them a try. Or you
might pop in on a glass workers class and ask to observe with a spare

Cynthia and All,

I’m glad you asked the question, a little info on light would be
useful for everyone. A very nice and simplified overview of the
electromagnetic spectrum is on one of our government atmospheric
science web sites at:

The shorter wavelength region of the spectrum is generally measured
in NANOMETERS, one nanometer being one one millionth of a millimeter.
In order to help understand the significance of this wee bit of
science, let me offer the following:

  1. The human visual system is able to perceive light from roughly
    400 to roughly 700 nm wavelength, “our spectrum” from violet to red.
    Visible light is a very small portion of the spectrum.

  2. Wavelengths just longer than red are termed “infra-red” (IR), not
    visible but absorbed strongly and superficially by the skin
    (including the cornea) and sensed as heat. Most injuries from IR are
    simple thermal burns, you don’t have to actually touch that glowing
    piece of hot metal to be burned by it. There is one additional
    possible injury, prolonged exposure to intense IR can result in a
    specific type of cataract development-the glass blowers cataract. You
    won’t get that from what we do in the shop.

  3. The wavelengths just shorter than violet are a segment called
    “ultra-violet” (UV), not visible but absorbed by the skin with
    deeper penetration than infra-red radiation. It’s UV that causes
    sunburn, including burns on the cornea which are known as “welders
    flash burn”.

  4. There is an additional factor to be aware of. The shorter
    wavelengths, UV and visible violet-blue, have the highest rate of
    energy transfer to tissues and can cause trouble. We became aware of
    this risk when we were popularizing the use of microscopes in eye
    surgery in the '60’s and '70’s and few cases of mysterious retinal
    burns were reported, mostly in very long surgical procedures. It was
    determined that the risk is dose related, meaning that along with
    wavelength, light intensity and duration of exposure are the big
    factors. This led to the use of UV-blue filters (didymium glass) in
    the microscope illumination system and caused me to invent the
    “eclipse filter” which Zeiss added to all 'scopes sold for
    ophthalmic surgery.


This does not alter the advise to use welding filters
for very high temperature work (platinum), arc welding, laser
blocking filters for the laser and IR filters for prolonged glass
blowing. There are, of course, lot’s of folks who would love to sell
you another item, or two, anyway!

Dr. Mac