Or its 50yr old scrap that is discolored! 
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Thanks! As far as I can tell, keeping it thin and light is not a problem? Polycarbonate is strong, after all.
And yes, I have a pile of microswitches just a few feet away from the build, and I will press some of them into service for interlock use; the only remaining task is designing the mounting hardware.
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On my small laser it’s 1/8" acrylic, which is certainly plenty for a 35ish watt tube. Not sure what ships on bigger lasers, but it seems you might possibly want something a bit thicker for a larger safety factor?
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I have been wondering about this for when I start a build. I’ve seen “K40’s” with red acrylic.
There must be some science behind it though?
My safety glasses look clear (but they are rated)?
I just had the orange at hand… and my build project gcc laserpro x252 90watt tube has a clear poly lid.
At infrared wavelengths longer than 2,800 nanometers and as long as 25,000 nanometers, and in thicknesses greater than 0.118 inch, colorless Plexiglas® sheet is entirely opaque.
Still looking for data on polycarbonate for far infrared. I haven’t seen data much beyond 2000nm. Would prefer PC for strength on the doors!
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I am wondering whether the science behind it is psychology… Definitely useful for blue lasers and people associate it with laser safety in general?
I haven’t seen any evidence so far that any dyes notch transmission around 10μm.
For near infrared from my kiln, I use (trustworthy) green safety glasses that cut near IR, but that’s a very different band.
Here’s what I found for far infrared on PC.
Came from this book. Unfortunately I can’t get full free access with my normal source.
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Thank you @NedMan!
Even half a millimeter is virtually opaque to both the primary 10.4µm and secondary 9.4µm bands. But if those two sources are making equivalent measurements, acrylic has lower transmittivity in the near IR bands which could be generated from the lased material burning, and so is probably better eye protection than PC.
For the laser itself, it looks like I’d be safe with clear acrylic, and probably clear PC.
However, under what conditions does the lased material generate plasma? If it does, I would expect it to also emit broad-spectrum UV.
general purpose acrylic naturally filters all UV light below 345nm (100% UVB), but only 35% of UVA light.
OP-3 is a grade of acrylic that filters 98% of UVA as well. It is also possible to get PC with UV-blocking coatings applied. It’s also possible to get PET film dyed to absorb UVA, UVB, and UVC:
https://www.edmundoptics.com/f/flexible-thin-film-uv-filters/12188/
@Jammy I wonder whether this is the science behind using Plexiglass 2422, which absorbs light at wavelengths shorter than 540nm, for CO2 laser windows?
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That makes sense. Ill probably still go for something tinted though, just because it would be easier on the eye.
Good research sir!
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Plexiglass 2422 is orange; it’s the classic look.
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I’ve been filling my amazon basket with gear… time to make one I think.
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Any details on design? 
I was awake most of last night considering what to make. Its going to be 1000 x 500 probably.
I will draft something in fusion, but I will probably just buy and build as I did with my CNC.
Lots of advantages with Laser Vs CNC, I’ve got fed up with buying end mills mostly.
I will post my experience on here, I love this forum, lots of experience and many people happy to help.
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Interested to hear your thoughts so far! If you name it, you can use a hashtag as a design and build log as I’ve been doing. 
If I didn’t have floor space that happens to be a great fit for my current overall external size and gives me a little flexibility for odd jobs, I would have designed out from a bed size that I could buy easily. 
I definitely think of my CNC router and upcoming laser as complementary.
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Every commercial CO2 laser I know is equipped with clear or gray tinted acrylic. Orange tinted is best for UV diodes, as far a I know.
On our Lasersaur we have gray tinted acrylic and I’m happy about that, because the backflash when cutting can be very bright (white).
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Hmm, gray tint and additional UV blocking… There are inexpensive static films to apply to windows for blocking UV which also have a fairly neutral gray tint. I could add that to transparent acrylic and it might be cheaper than tinted OP-3 which I’m not even sure where I could buy near me. The plexiglass would block the IR very effectively, and the plexiglass and UV film would block UV, and the neutral tint would reduce the glare from cutting.
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A lot of what I’m finding is mirrored sputtered-metal film, like this:
I guess if I mount enough LEDs inside the case that would be OK. But I’m going to keep looking for non-mirrored film options. Here’s a 2 mil PET sheet that has a datasheet available. It has 99% UV blocking and 83% transmittance, and low (less than 4%) reflectance:
It looks like there are lots of brands of UV-blocking PET film. This seems like a good route, and I don’t have to decide right away.
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But UV is the opposite of IR?
Yes, it’s the opposite end relative to the visual spectrum.
It seems clear that both acrylic and PC block far-infrared at the 10.4µm and 9.4µm wavelengths (the primary and secondary characteristic wavelengths of CO2 lasers) essentially completely even in thin samples (3mm / .118"). As a result, I expect this to be sufficient protection from specular reflections of coherent laser light.
However, clear acrylic (and both soda and quartz glass, for that matter) transmits lots of IR in the 700nm - 2800nm near IR range. This range can burn your lens; “glassblower’s cataracts” are the result of glassblowers being exposed to wide-spectrum IR from black-body radiation in molten glass. It’s why I have special safety glasses for my kiln; the same kind used for oxy-acetylene welding and for the same reason.
I expect the “white” light of material being laser-cut to be wide-band light including near infrared, and quite likely UV as well. Both near UV and near IR damage eyes; I think near IR damages the lens and near UV damages the retina (but I’m not a doctor nor do I play one on the web).
Therefore, the idea of blocking both near IR and near UV, as well as attenuating visible light to some degree, is attractive.
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