Sanity Checking some Cheap Laser Safety Glasses

I own a few unsafe laser pointers, but nothing too crazy. I am still sorely lacking some laser safety glasses, so I bought these cheap ones on Amazon which I am, of course, very suspicious of.

Trust, but verify and also probably do not trust.

If you’re working with high power lasers in excess of 100mW, pay good money for good glasses specific to the right wavelength. Don’t trust these.

That said, they DO seem to pass the test with my lighter-weight <100mW lasers.

So why did I buy these? I actually have a specific experiment/use case in mind at 405nm. More on that below.

Testing

I tested with 3 lasers:

  • 405nm blue/violet ~40mW
  • 520nm direct green ~10mW
  • 532nm DPSS ~90mW, ish.

I tested power with and without glasses present: <5mW getting through counts as a success. I also tested firing the laser through the glasses onto paper: if you can’t see a dot at all, that’s good. If you can, some laser is obviously getting through. This will be a lot more sensitive than power.

At 405nm, a bunch of laser light gets through the glasses, BUT power is reduced to theoretically safe levels (<1mW).

At 520nm, almost no light gets through, and power is again reduced to <1mW.

At 532nm, you should not trust these glasses, mostly because you shouldn’t trust the laser itself. Cheap DPSS lasers tend to emit significant power in both the visible spectrum they state AND the infrared spectrum, because they tend to omit (kind-of-expensive) IR cut filters. I tested my DPSS laser with and without an IR cut filter. We can determine that these glasses block SOME of the IR this laser emits, but not all of it, and a still-dangerous 19mW gets through. With an IR cut filter in place, these DO block most 532nm light, down to the <1mW level. On the surface test, we can see that a bunch of 532 still gets through, just probably a “safe” level.

My Experiment

I bought these specifically to use at 405nm, in the hopes they’d block all 405nm light (which the red color would at least kind of suggest).

405nm diodes are useful because they brilliantly light up anything fluorescent. That’s super useful for leak detection and that sort of thing, where a fluid has been doped with a fluorescent chemical that will light up under UV or near-UV illumination.

The problem is, at least in my experience, the dopant itself usually isn’t that visible, and at 405nm, reflections of your light source itself will drown out any secondary emission from the fluorescent thing. Separating the illumination from the fluorescence is just pretty hard. We could use a deep UV source so that there’s a TON of light to fluoresce, which we can’t see at all, but that’s a little tricky in its own right.

What if we could wear some glasses that would make the laser dot or LED spot completely invisible, UNLESS it’s hitting something that fluoresces in a different spectrum?

Unfortunately, as we’ve shown, these glasses block a lot of laser POWER but the spot is still clearly visible. Pesky high dynamic range of the human eye. So we’ll need glasses better suited to blocking 405nm light for this experiment.

Update

I went ahead and bought a less-cheap pair of glasses to test as well. The brand “Eagle Pair” appears to be reputable, and this pair that quickly surfaced on Amazon claims to block not only 190-540nm covering all my own lasers, but ALSO 800-1700nm infrared that may be leaked by cheap DPSS lasers.

To make a long story short, they work quite well:

Demo image showing Eagle Pair glasses blocking all measurable laser light from a 405nm, a 520nm, and a 532nm laser which also leaks lots of 808nm and 1064nm light.
Blocks all the measurable light at 405nm, 520nm 532nm, and higher near infrared.

It’s also not surprising, but it IS interesting that the green dots are both decently visible through the glasses, where the 405nm dot is arrested completely. Unfortunately, the naked eye can still see the 405nm laser dot when LOOKING through the glasses so these don’t completely enable my experiment. But they DO do a better job than the redder glasses above.

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