Sunday, April 17, 2011

Laser protection: Cheap is totally fine because if you needed the expensive ones you're screwed anyways.

Laser goggles are rated by optical density:

Turns out this is a base-10 exponential rating. And when it comes to the goggles you do get what you pay for in terms of OD. The two pairs I've decided to try are the dragon-lasers UV to Green goggles and the Thor LG3 goggles.

The Thor goggles are OD 7+ (and $145) while the dragon-laser ones are 4+ (and $25). Of course, if we're dealing with 1W lasers, the cheaper goggles will cut it down to 0.1mW. If a normal laser pointer is 5mW then we've clearly implemented enough protection.

Now the question is, would you ever need such goggles?

If we assume that 5mW is about what the human eye can be expected to tolerate, for someone to need more than 4 OD of protection, the laser must be at least a 5mW * 10,000 = 50W laser. Of course, when the glasses are protecting you they must absorb the energy of the laser. And they're plastic. Which melts, burns, and vaporizes.

So if you take a shot of that 50W laser to the face, how long till it burns through the goggles?

If the laser has a 1mm diameter (pretty normal for a large laser), it's beam will have a surface area of 3.14E-6 m^2 on your glasses. If acrylic of the glasses are about 2mm thick we're illuminating 6.28E-9 m^3 worth of material. If that material has a density of acrylic (118kg/m^3) then we're talking about 7.414E-3 g worth of material. If that material has the specific heat of most plastics (~1.25 J/(g*degC)) then our material will warm at the rate of 9.27E-3 J/degC. Well our laser is 50W or 50 J/s. So our material will be heating at a rate of 5,395 degC per second. If the Acrylic melts at 150 degC or vaporizes at 200 degC...

We'll have about 30ms before such a laser melts right through your glasses and burns your eyes out anyways. So either you've got a laser exposure that's weak enough that the cheap classes can handle it or you've got a laser exposure so strong any of the glasses would just burn away almost instantly.

I guess I really shouldn't have paid for the nice ones before writing this post...

(Also, I have no education in this so I'm mostly piecing this together from internet research. If you're going to put your life or eyes on the line and/or like to sue people instead of thinking for yourself, don't trust anything I've said here.)

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