h a l f b a k e r yA riddle wrapped in a mystery inside a rich, flaky crust
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When working with powerful lasers, the threat of instantly
blinding
oneself, or permanently damaging ones vision is a constant source
of
hesitation and distraction. Goggles are available for various
wavelength ranges, but no goggles cover all ranges, and it is
unclear
what their maximum
wattage rating is even at the rated
wavelength.
Inexplicably, the only ways to make sure you don't do this is to
view
your work using a camera and monitor, which leads to a loss of
precision, and stereo.
What is needed is a simple, lightweight way to take your eye out
of
the line of fire using no electronics.
The eyes are blocked by blinders suspended two inches from the
face. Dished mirrors rest against the cheeks, receiving light and
sending it to the backs of the blinders. Finally, lenses close to the
eye flip the image and assist the eye in focusing on the rather
near
blinder backs.
The user only ever sees a projection of anything, but more
importantly, dangerous light is always traveling away from the
eye.
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When laser light is dangerous it is because of its
intensity, not because it is monochromatic or
coherent. The "screen" in front of the eye, upon
which you reflect stuff, needs to be adjustable in
terms of how much light it absorbs/reflects. If it
reflects enough to allow mostly-ordinary seeing of
things, then it would also reflect a dangerous
amount of intense laser light. |
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Therefore this Idea appears to be fundamentally
unworkable if no electronics is involved, measuring
and rapidly adjusting for light-intensity (or doing
the standard camera/monitor thing). |
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Perhaps a simpler solution would be to install many
very bright lights all through the work area, such
that the NORMAL light-intensity is as dangerous as
the lasers you would be working with --and then
just put on a welder's mask. It nicely reduces the
intensity of light that reaches the eyes, and with
the whole workplace lit-up thoroughly, you will
see ordinary stuff just fine. |
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I wasn't aware viewing a beam's projection upon a matte
surface was just as harmful as the beam entering your eye. |
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a pair of cameras and mini screens on a pair of
goggles would be easy enough. That would work.
This idea wouldn't. |
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The other problem, is that there are many lasers
that you can't see... so your normal aversion to
bright light won't work. The screen-goggles will
work here too. |
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The best defense is to know what you're doing,
and pull the appropriate $10 goggles out of the
bag. |
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Yes, reflections make a laser safer, but it's not an
absolute thing. If the visor is reflecting sufficient
light to enable you to see the general background,
it will reflect sufficient laser light to damage the
eye. |
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And the appropriate goggles tend to be more like
$50-150 bucks, I use them a lot at work for a really
intense LED light source (less focused but brighter
than most lasers) but still, you're right. Since you
can buy them with selective light filtration, you
can still see everything else well enough and just
block the beam. |
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//The best defense is to know what you're doing, and pull
the appropriate $10 goggles out of the bag.// |
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Is there no maximum rating on those goggles? You can just
take a direct hit from a laser of any power, as long as the
wavelength is covered? |
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Yes, welding-mask filters come in different grades of
protection. You will need to use the grade that is
appropriate to the laser intensity that you are
working with. |
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It depends on the glasses, the ones I play around
with are OD7+ (optical density, it's a log scale, so
.0000001 transmittance, and they apparently can't
test past OD7, so it might be even better) in the
wavelengths of interest. That's about the highest
you're going to find. So unless you're playing with
a 50kW laser, they'll protect your eyes from a
direct hit. |
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And if you do manage to stand in the way of a
50kW laser, eye damage is not going to be your
largest concern. |
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Reflection, on the other hand, drops the beam
strength about one order of magnitude for most
black surfaces (90% absorption), possibly two (99%
absorption) for certain extreme black optical
coatings. |
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//.0000001 transmittance, and they apparently
can't test past OD7, so it might be even better// |
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That must be wrong (I mean that they can't test
past OD7). Several methods spring to mind: |
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(1) Normal photographic emulsion will register an
image in a thousandth of a second or less from
daylight via a lens. I imagine a powerful laser
beam would register an image in microseconds.
So, timed exposures on the order of a few seconds
to a few hours via the filter would give you
measurable ODs up to maybe 10 or 11. |
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(2) A photomultiplier counts single photons with
decent efficiency, and has a noise of maybe 100
photons per second. A watt of laser power is
something like 10^18 photons per second; the PMT
would be good down to 10^3 photons per second
of less, giving a measurable OD up to 15. |
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(3) If the filter is homogeneous, measure the OD
of half the thickness; then you know that the OD
of the full thickness will be twice that. |
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If you are working in an application with a specific laser,
specific goggles are sufficient, unless the laser is
malfunctioning, in which case, your narrow range goggles
would simply give you a false sense of security. The worst
part about them is, by definition, you cannot see the beam
you are working with. |
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Now if you are, say, aligning separate heat synced crystals
to an IR pump, goggles really become an expensive and
tedious way to still come up short, safety-wise, especially
when you consider experience makes most users less safe,
not more. |
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A full-range means of protection against all specular
reflection would be a safer bet. |
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//Yes, reflections make a laser safer, but it's not an
absolute thing. If the visor is reflecting sufficient light to
enable you to see the general background, it will reflect
sufficient laser light to damage the eye.// |
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If what you're saying is that in the event of a spectral
reflection, a tiny fraction of the damaging light still enters
the eye, I would agree, and I would call that a feature.
This is probably why matte coatings are common in labs.
Not that it would be hard to sense and trigger the screen to
flip up. The user would still be safe because the beam
would be hitting his cheek reflectors, not his eyes. |
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Don't ask me, optics isn't actually my specialty. It
may just be that they can't measure selective
wavelengths past OD7. All I know is that I have had
multiple optical filter vendors tell me that no one can
measure past OD7, and this corresponds with the
fact that the glasses I was looking at were rated OD7+
in the target band, rather than say 7.5 or 8. |
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(Oh, and three doesn't work, since most filters are
surface coatings). |
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No, I'm saying that no matter what you do, you are
going to have a damagingly bright spot an inch
in front of your eye. If you can see the
background, the diffuse reflection itself
will be dangerous. |
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Lasers, except pumped lasers, do not malfunction
out of spectrum, and if your'e using pumped lasers
you should be using glasses that protect against
the pumping wavelength as well as the target. |
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Also, the proper glasses for any given use are the
ones that leave the beam visible but safe. Which
is why they are available in a range of blocking
powers. |
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All lasers are pumped. And there are more wavelengths to
guard against than pumping and destination wavelength. |
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//The diffuse reflection itself will be dangerous.// |
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In the context of an otherwise un-diffused momentary
specular reflection into the naked eye, I think this idea is
rather novel. Regardless, a simple threshold sensor and a
quick actuator to flip-up the projection surface would solve
this. |
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If the matte reflector were swapped for a mirror, this is
exactly what would happen. The cheek reflectors send the
light to the screens at a 45 degree angle, so the light would
end up on your forehead. |
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This brings to mind what might be an important distinction.
When you look at a laser dot on a matte surface, there are
still angles at which the light appears more intense than
others. |
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My idea is really a lot like a shoebox sun viewer, but meant
only for seeing the sun for a duration roughly equivalent to
that of the blink reflex. The rest of the time it's a camera
obsura in stereo. |
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I'm sorry, I used the wrong term, I meant
frequency
doubled lasers. |
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All other frequencies that might be emitted are
non-
collimated and much less intense. |
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And a perfect diffuse reflective surface would
return light in a perfect hemisphere, with no
angles of reflection being greater than another.
However, since there is no such thing as a perfect
diffuse surface, you will see better and worse
angles. |
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And a sun viewer is a camera obscura. The trick is
that it lets in only a very small fraction of the
incoming light, and spreads it sufficiently that it's
safe for the human eye. That's possible because
the sun is an exceptionally bright source. It can
be possible with a laser, as long as the beam is
diffused well back from the point of impact.
However, it is not possible to do it with a high
intensity laser at the same time as with the room
background, the contrast levels just don't allow it. |
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