I was using a pair of these safety goggles which appear to be quite popular among laser hobbyists:


They claim OD5 protection in the 190-540nm range. I have tested them with a blue 450nm diode. They work well and fully hide the beam and dot for low powers but above 1W I can see a faint yellow/greenish dot that gets brighter as I ramp the power up; at 5W it is quite visible (although still nothing compared to the unfiltered dot, I would imagine).

My question is, shouldn't the dot be completely filtered out, or at least show up as blue if it is just too bright for the goggles to fully attenuate? Why am I seeing a yellow dot being produced by a laser that produces only blue wavelengths? Is there some mechanism in the goggle lenses which diffuses the light and causes it to change wavelength as it is absorbed, or should I be worried about the yellow dot somehow being some unfiltered laser light coming from a supposedly blue laser?

I want to take precautions to correctly protect my eyes when working with powerful lasers so I think getting a better understanding of how these goggles work will help me to achieve this. Many thanks.

EDIT: when I say dot I am talking about the dot that the beam shines on a diffuse surface (wooden in my case). Obviously I am not directly looking into the beam.

  • $\begingroup$ Those goggles attenuate visible light at most by a factor of 100,000 (OD 5 at 190-540 nm). Your eyes have a broader dynamics range, being able to detect just a few photons under ideal conditions. If the diode light has a large bandwidth or the wood reflects at a higher wavelength, you could easily be seeing light at a frequency that's attenuated by a factor not of 100,000 but of 10, for example. See this chart; the higher end of green and yellow light can be expected to pass through almost undetected. $\endgroup$ – Chemomechanics Oct 7 '17 at 3:36
  • $\begingroup$ @Chemomechanics But wood isn't fluorescent or anything, so how can it reflect yellow light from a purely blue laser beam? (of course like you said my assumption that the laser is truly monochromatic may be in error, but a 450nm laser producing 540nm+ light seems like a very, very wide bandwidth for a laser diode) $\endgroup$ – Quix32 Oct 7 '17 at 3:44
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    $\begingroup$ @Quix32 - The laser emission itself may be at 450 nm, but do you know for a fact that these blue laser diodes don't also emit some non-laser light at other wavelengths? I seem to recall being able to see some light from a 532 nm solid-state, diode-pumped laser even though I was wearing laser googles selected for the laser wavelength. But I wasn't seeing 532 nm laser light but apparently light from the diodes used to pump the laser. In the same way, there may be non-laser, non-450nm light being emitted from your blue laser diode. $\endgroup$ – Samuel Weir Oct 7 '17 at 4:08
  • $\begingroup$ Interesting. I'm not an expert in this area, nor am I an experimentalist, but I can't think of any plausible effect in the glass (e.g. Raman scattering, fluorescence) which would account for a frequency shift while maintaining the beam integrity well enough to have a focused spot on the other side. Do you have a different laser that you could try this with? $\endgroup$ – J. Murray Oct 7 '17 at 5:27
  • $\begingroup$ might the wood start burning at higher W? i.e. atomic excitations and deexcitations might produce the yellow ( which is part of the burning light of wood) $\endgroup$ – anna v Oct 7 '17 at 6:00

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