Using Laser for Commercial Purposes, Need your Advice

[Midwakh]

Hello,
I am not really a laser hobbiest, but need some advice about a laser we are going to use for commercial/industrial purposes. So just a brief summary of what I’m talking about, we are going to use a laser to measure straightness of a crane runway beam, basically we shoot the laser parallel to the beam and measure how far the laser is from the beam at certain intervals.
To get a laser dot accuracy required (dot size of about 20mm dia over 200m) we were recommended by the supplier to buy a class 3B laser, 40mW, 635nM green laser. So here are the questions:
1) Is a laser of this class as dangerous as the internet says it is? (Basically is it a “your crazy to even consider using a laser this powerful”, or “it’s safe as long as you wear proper eye gear”
2) How dangerous is the reflective light off the laser? We are going to shoot the laser on glass so there is huge possibility of reflection
3) Will I be able to see the laser beam while wearing the safety glasses?
4) Once I bought the safety eye gear, is there a way to check that it works fine and filters properly
All your opintions/answers would be highly appreciated.

 

[aryntha]

First of all, 635nm is red, not green. So I'm not sure why they told you that... But green (532nm) is the most visible. Meaning, it'll be the most visible at the least power and is therefore theoretically "safer". So for something like this, you do want to go with green. (532nm.)

1) 3B 40mW is "it's safe as long as you wear proper eye gear". Note that at the far field, a 20mm dot at 200m will be far safer than near field, a 1mm dot at say, 1m. The energy will have spread out, and it'll be less dangerous.

What you want to figure out is the "NOHD" (Nominal Ocular Hazard Distance), essentially the 'safe direct hit distance' for a given power. There used to be a calculator for this on the web (EasyHaz) but it seems to no longer be functional. But, to make a long story short, the NOHD for 50mW of 532 at 1.5mRad of divergence is around 50 meters, so that wouldn't be a bad number to use. Note that this is for a DIRECT hit, not a reflected hit. So, at 200m, you're "probably ok", but wear your goggles anyways during setup of this measurement, and when you know your face will be close to the beam.


2) Depends on the distance. Reflection off of glass can be anywhere from 2-15% depending on the angle. If this is reflected off of glass 200m away, you're way past the NOHD of 50m, and so long as the glass isn't a mirror, your transmissive/reflective losses are going to bring it down to a non-issue anyways. That said, don't have someone standing right next to the glass surface anyhow.

3) No, you won't - at least not very well. But again, reflected light will NOT be an issue at those distances. Wear the glasses during SETUP, and look at what you must once everything is staged. Keep in mind the NOHD distance of 50m for a direct hit. Reflected laser light off of non-specular objects at these distances will not be an issue.

4) With a laser power meter, yes, but instead of going that route it should block most of the visibility of the dot (but not all of it). Again, use the eyewear when aligning and staging, and then after that, when viewing from a distance, you won't have any issue. For this purpose I'd say get OD 1 or so eyewear. OD1 will bring 40mW to 4mW, allowing it to still be visible, but making the possibility of damage from a direct reflection when close by much lesser.

To be completely honest, 40mW of green , so long as you're respectful of it when working close up, will not be an issue at the distances you are talking about. Your concern is good - it gives you a healthy respect for lasers, but it also isn't really a lunatic fringe sort of thing to do. Do not be afraid of the reflections (illuminations) of matte objects at those distances (such as off of the crane beam). The "dot" of the laser will not hurt you at 200m. Just watch for nearby shiny metal surfaces, and be aware when close to the laser. -- which is why you'll want to wear the eyewear during setup, in case a mistake is made and a stray reflection happens.

And of course, make sure anyone down-beam is far enough away; again, illuminated surfaces are not a problem, but a direct hit in someone's eyes from closer than 50 meters could be a problem.

 

[Midwakh]

Thanks Aryntha!

Your response was detailed and exactly what we needed to point us in the right direction for our risk assessment.

Im glad I asked here, still waiting on our HSE people to give us an answer... Im more confident now using the laser after speaking with people who actually use lasers!

Much appreciated.

 

[magmabeam]

I'm no engineer but don't cranes have lots of flexibility so they don't snap?

 

[aryntha]

This is likely to measure straightness before it's flexing, not during.

 

[FrothyChimp]

21CFR1040.11 (b) Surveying, Leveling, Alignment (SLA) laser systems cannot exceed 5mW

CFR - Code of Federal Regulations Title 21

 

[Midwakh]

Its true cranes are designed to be flexible to accomodate for an acceptable level of runway (rail) misalignment and manufacturing tolerances, but we suspect the rail we are going to measure straightness against was installed beyond the acceptable limit, thats why we need lasers to help us prove our case.

 

[Midwakh]

Ok now I get what your question was magmabeam, we are measuring the straightness of the rail not the crane itself.

Thanks for the link Frothychimp, good to quote.

 

[Bluefan]

1) Class 3b lasers are dangerous and require laser safety eyewear, which defeats the purpose of the higher power, detectors can do with much less anyway. A 5mW 532nm lasers is more than visible enough.
2) Class 3B lasers are dangerous, both direct and their reflections off a glass surface.
3) The beam won't be visible, but that's not possible safely anyway. In daylight you'd need insane power levels for that, at night just dangerous power levels.
4) Buy professional certified laser safety eyewear, that way you can eb sure it works.

You'd need a low divergence laser in order to have a small dot 200m away. A 20mm beam diameter can give ~0.1mrad divergence, that would mean the dot is 20cm big at 200 meters. The easiest way is to get a quality 5mW 532nm green laser module and a beam expander. A good adjustable mount would be needed too and some way of measuring the deviation of the crane beam, like a 4 quadrant position sensitive detector.