Laser safety without goggles

[musik7]

I have built a 2w 445nm laser and have more than one pair of laser safety glasses so that I am never taking any chances.

My question, is I would like to build some lasers in other wavelengths, especially 405nm, but I would like to make them weak enough so that I can use them the same as I would any other basic laser pointer, without safety glasses. Obviously there is some safety still involved, but not more than you would need to observe with a standard laser pointer.

Does anyone know how low the wattage needs to be in order to be safe without safety glasses? Obviously 5mw and less is safe, but is that the threshold, or is it higher than that? I have seen some 405nm diodes that are 20-50mw, which would be nice to use those, as I think you might be able to see the beam a little better. In the end, I want it to be safe without safety glasses though, even if that means sticking to 5mw.

Thanks in advance for the advice.

 

[NightExplorer96]

You are correct, "5mW" and below is considered safe for use as a laser pointer since your blink reflex is fast enough to prevent damage if you were unlucky enough to receive a hit.

Nevertheless, as I have been told before(asked how birght 5mW of 405nm is), I would highly going agaisnt a 5mW 405nm pointer. The dot itself will be barely(if at all) visible during the daytime and it would get boring rather quickly. A 50mW 405nm pointer will offer you a beam at night if under the right conditions and if properly used with goggles shouldn't be anymore of a danger then a 5mW pointer

 

[musik7]

Thanks, that some good info. That raises another questions though. Why is it then that a 5mw 405nm laser would be almost invisible during the day, whereas your typical red laser pointer works just fine? Is that just differences in the wavelengths? I can see how that would make sense, as violet is a darker color that red, and would take more energy to make it visible.

 

[NightExplorer96]

Thanks, that some good info. That raises another questions though. Why is it then that a 5mw 405nm laser would be almost invisible during the day, whereas your typical red laser pointer works just fine? Is that just differences in the wavelengths? I can see how that would make sense, as violet is a darker color that red, and would take more energy to make it visible.

Correct. In general, the peak wavelength(brightest wavelength) of the human eye during the day is considered to be 555nm. As one moves away from that wavelength the color will start to appear dimmer and dimmer until you hit UV/IR light which technically both are considered invisible(if you have a high enough powered 808nm laser you can see a very dim red dot).

405nm is on the edge of what our eye can see, which means it will appear very very dim when compared to a 532nm let's say at the same amount of power. A typical red pointer(650nm/<5mW) is still barely visible during the day since it is located closer to the peak wavelength then 405nm is

 

[Bionic-Badger]

Something to understand about eye safety: that the "5mW" limit is not "eyesafe". It depends on your eye's blink reflex to save your eyes from damage. "Eyesafe" is in the 0.5-1mW range -- a class 1 laser.

Laser pointers are rated for class 3r or less (5mW). However, this assumes that the eye can detect the brightness of the laser and react in time. For less visible wavelengths, such as 405nm, the power-safe threshold is even lower because the eye may not react in time. For nearly invisible wavelengths like near infrared, the power level should be extremely low (class 1) or the laser not used without goggles ever because of the danger.

So in summary: your 405nm laser has poor visibility and should actually be lower in power than 5mW to be safe. Your eyes simply can't see the output as well or react as well. Try shining a 405nm laser at some bleached white paper and you'll see just how bright it actually is despite looking pretty dim to the eyes otherwise.

Practical matters: it is actually hard to get the 405nm lasers under 5mW anyway. They usually have higher lasing thresholds. That's why practically all 405nm laser "pointers" are not eyesafe at all.

Maybe something you can experiment with is some of those red/green combined pointers, and then blend them together with a cube and mirror (or a dichro) to form yellow. You'll need to make sure the green + red total power is below that of 5mW though.

Edit: As noted above, the eye sees some wavelengths brighter than others. You can check out the luminous efficacy of wavelengths at this site (higher values = brighter).

 

[petters]

Serious question: If you have a pair of goggles rated for say OD4, would wearing two pairs at the same time double the optical density?

 

[musik7]

Thanks Bionic-Badger, that's great info.

Petters- I would guess that to be true, but I don't think that would be possible with 2 identical pairs. I just have a second pair so that I can do laser stuff safely with a second person present. At one point, I had found a calculator for the OD value required for a particular laser output, and even OD3+ is overkill for a 2w laser (not that there's anything wrong with overkill when it comes to safety).

Thanks for all the input everyone.

 

[Bionic-Badger]

petters: The optical density is a logarithmic scale: I_output = I_input * 10^{-x} where x is the OD value, and I_input is the input power and and I_output the output after filtering.

I believe using two pairs of OD 4 goggles would just act like doubling thickness of the filter, adding a multiplier to the value: 10^{-4} / 2 = 0.00005. That makes it about OD 4.3. That really isn't very much, and explains why if you were to look through the two goggles with your eyes it doesn't look all that much darker. What it does do for you is increase the damage the filter can take before failing.

musik7: OD3 is not overkill in some situations. It may protect your eyes, but 1mW can still be uncomfortable, especially for colors your eyes are very sensitive to, like green lasers. When you're doing something like aligning, it's nice to have the light significantly reduced in power. For example, those ARG goggles have OD 6 or 7 for 532nm, but you can still see the dot on the wall pretty well with a 1W green despite it being 1uW in power.