By “it’s just Boris“
Okay, a brief primer on laser eye protection … caveat, I am not a laser health physicist, just a laser user who’s had some laser safety training classes.
Critical parameters: wavelength, power, and laser type (CW or pulsed). These let you determine the level of attenuation (optical density) needed to knock down that laser’s output to the level it’s safe for your eye.
Wavelength; aka color. The laser wavelength is usually given in nanometers (nm), or microns (um); there are 1000 nm per micron. The average human eye can see from around 740 nm (or 0.74 microns), which is deep red, to around 380 nm – deep violet. The eye can transport and focus light down to at least 1.064 microns (or 1064 nm), but you can’t see it, because the rods and cones don’t work for that long of a wavelength. This is important because even though you can’t see near-infrared laser beams, the eye still will focus them and if it’s powerful enough it can burn your retina.
Optical density. This is how much attenuation a set of laser eyewear will provide at a given wavelength, or over range of wavelengths. It’s a log scale. OD 0 is no attenuation. OD 1 is a factor of 10 attenuation. OD 6 is a million times attenuation, and so forth. The link I pasted above, to ThorLabs, has some plots of OD versus wavelength for their standard laser eye pro.
Visible light transmission, or VLT. Given as a percentage usually. This is the “average” brightness reduction you will see when you put on the glasses. Generally, the more wavelengths a set of eye pro blocks, and the higher the OD rating, the less the VLT is going to be. You can think of this as sort of a rating of how good the eye pro would be as sunglasses – lower VLT = darker. The lower the VLT, the more problematic using them is going to be at night.
There are different standards for what’s eye safe, depending on the type of laser: CW (continuous-wave, or always lasing); pulsed; and ultra-short pulse (USP). You calculate safe average power limits for CW lasers; for pulsed lasers, it’s energy per pulse, and for USP it’s even more complicated because the ultra short pulses tend to spread out over the spectrum. Fortunately, for the near future, we’re not that likely to run into a pulsed or USP laser on the street – too big and bulky for convenience.
Which brings us to power ratings. For CW lasers, power output is usually given in watts (W), or milliwats (mW); 1000 mW = 1 W. That’s – and this is very important – at the wavelength the laser is rated for.
Remember that discussion above about 1064 nm light? Many of the laser pointers that work at wavelengths shorter than, say, red, will actually start somewhere in the infrared and “upconvert” the light into the visible spectrum, because it’s easier to make high-power laser diodes in the infrared than in the visible. A green laser pointer will almost certainly work by taking 1064 nm light, passing it through some nonlinear optical elements to halve the wavelength to 532 nm – green light. Blue/violet laser pointers often play a similar trick; to get blue/violet, use a “tripling” crystal to get blue/violet light (354 nm) from 1064 nm.
The conversion is usually on the order of 10 – 50% efficient, so there’s usually lots of light at 1064 nm (or whatever the starting wavelength was) left over. A good laser will include something called a “dichroic” to dump that somewhere safe. Dichroics aren’t cheap. I’ve not worked with any of the cheap-but-powerful pointers yet, but I wouldn’t bet they have a good, or any, dichroic system. Which means the “waste” infrared laser light will likely come out the end of the pipe.
Takeaway: any eye pro you buy as defense against cheap laser pointers, should have protection in the infrared as well as in the visible.
For a CW laser, the optical density you need will depend on the laser’s output power. I use this one as my go-to quick-check:
As an example, let’s consider what a we’d want to block out a fairly hefty blue laser – say, 5000 mW at 450 nm.
According to that site, we’d need an OD of at least 3.7 to protect our eyes from that laser. (If it were a 50-W laser, we’d need an OD of at least 4.7. Which makes sense – we increased the power by 10x, so we need 10x the attenuation, so the OD went up by 1.)
Now, let’s assume that 5-W blue laser is starting as a 900-nm laser, which we can’t see; that it has a 33% conversion efficiency; and that it has no dichroic. That would mean the laser had to start with 15 watts at 900 nm, half of which is converted to blue; so we’re left with another 10 W at 900 nm.
Plug those numbers in, and we find we need an OD of at least 4.01 at 900 nm.
So, for that particular 5-W blue laser, we’d want to look for eye pro rated at:
OD 4.01 @ 900 nm
OD 3.7 @ 450 nm
Note, these are minimum OD ratings; higher provides more attenuation.
One last note for now, fitment and coverage is very important. If you can see around the edges of the laser goggles, a laser beam can “see in” to your eye. I wear prescription glasses, so tend to prefer “goggle” types that fit over my regular glasses … and by the way include coverage on the top, bottom, and sides as well. It won’t win any fashion prizes (except maybe in the labwear category), but it works well.