Laser MPE meters, but is there any point?

[smokeAndMirrors]

A number of posters here have strongly suggested that every moment of a laser show that interacts with audience, musicians or crew be measured to assess the risk of ocular or skin damage. And that indeed the professionals both perform this for every show, and document the results. Which I applaud.

A meter to measure laser MPE has been mentioned, and indeed is referred to in many linked documents. Manufactured by "Precision Optical Engineering", which as it turns out are just down the road from me (ok 30 miles or so) in the UK. But you can't buy these meters anywhere, as far as I can tell from Googling. And when I attempted to contact the company by email, explaining who I was, what I wanted and why, I received no response. Further, their website doesn't mention the product. So wtf is that all about?

Further, when reading this article, I'm informed that even if I could get my hands on an MPE meter, there'd be little point due to

* the influence of other light sources, coherent or otherwise
* sampling errors and biases
* detector idiosyncrasies
* measurement is only valid at that time, beam characteristics may vary

So am I being advised to use a device which is nigh impossible to obtain to perform a task which is largely meaningless anyway?

The article suggests a much simpler guideline for safety: power density less than 10mW / cm^2 at the nearest point of interaction. Though I'd still like to know what I measure that with, and why it doesn't suffer from the same problems as above. But more to the point: I really want to do this thing right - safely, so that people can enjoy what I do and I can make some money. But it seems there's no clear guidance on the matter, even from well-meaning articles that purport to give clear(er) guidance.

What would be really useful for me would be someone to give me a relatively short but adequately detailed précis of what they actually do when they're preparing a show - not theoretical, but empirical - to ensure safety.

[mixedgas]

Further, when reading this article, I'm informed that even if I could get my hands on an MPE meter, there'd be little point due to

* the influence of other light sources, coherent or otherwise
* sampling errors and biases
* detector idiosyncrasies
* measurement is only valid at that time, beam characteristics may vary

So am I being advised to use a device which is nigh impossible to obtain to perform a task which is largely meaningless anyway?


You ARE applying too much weight to the critique of the MPE meter.

With a laser set for light regulation, not current, which older ion lasers were more then capable of doing, the MPE meter is pretty valid. So are the modern software approximations.

Steve

[mixedgas]

Further, when reading this article, I'm informed that even if I could get my hands on an MPE meter, there'd be little point due to

* the influence of other light sources, coherent or otherwise
* sampling errors and biases
* detector idiosyncrasies
* measurement is only valid at that time, beam characteristics may vary

So am I being advised to use a device which is nigh impossible to obtain to perform a task which is largely meaningless anyway?


You ARE applying too much weight to the critique of the MPE meter.

With a laser set for light regulation, not current, which older ion lasers were more then capable of doing, the MPE meter is pretty valid. So are the modern software approximations.

LMS-2 is certainly still available.

I have also scanned and posted the PDF of the US FDA'S

"Analysis of Some Laser light Effects for Classification Purposes"

Here before.

Perhaps if you read that you will find MPE meter errors to be less critical then you think. Besides, what the MPE meter does can be done with a good photodiode, a termination resistor, a 50$ used oscilloscope from ebay and a power meter.

Steve

[buffo]

A number of posters here have strongly suggested that every moment of a laser show that interacts with audience, musicians or crew be measured to assess the risk of ocular or skin damage. And that indeed the professionals both perform this for every show, and document the results.

Not quite. Almost no one measures "every moment" of the show. Instead, most of them use the simplified method we discussed in this thread. And that method only requires a single measurement of a static beam before the show starts. (Assuming you have the other safeguards in place.)

A meter to measure laser MPE has been mentioned, and indeed is referred to in many linked documents. Manufactured by "Precision Optical Engineering", which as it turns out are just down the road from me (ok 30 miles or so) in the UK.

I'll bet if you drive to their door and ask them where you can buy one of their meters, they will be only too happy to put you in touch with one of their dealers. Since they're only 30 miles away, it should be a short trip. As to why they don't answer their e-mail, there could be many reasons, but salesperson incompetence is surely high on the list.

Further, when reading this article, I'm informed that even if I could get my hands on an MPE meter, there'd be little point <snip>

That article is 11 years old, and represents the thinking at the time. It has since been superseded by this article. For the record, Greg Makov wrote the original, and when I spoke with him last year on the ILDA cruise, he admitted that they now use the much simpler method detailed in the new article. He even gave a short lecture on the topic. I then watched him actually perform the power measurement before we all judged the shows for the 2008 Pangolin Laser Show competition.

The article suggests a much simpler guideline for safety: power density less than 10mW / cm^2 at the nearest point of interaction. Though I'd still like to know what I measure that with, and why it doesn't suffer from the same problems as above.

With a calibrated irradiance meter, or with a calibrated low-range power meter that has a known aperture size, assuming of course that the beam completely fills the aperture and you're smart enough to divide the power reading by the area of the aperture to get the irradiance figure.

As long as you start with a zero reading, you won't have any interference from "ambient" lighting. And since you only have 1 measurement to take, it's unlike that anything will change while you're taking that reading.

It really is that simple. If you're still confused about the whole thing, I suggest you contact Greg at LSDI directly. He can probably clear up any lingering doubt you may have. I'll bet he can also steer you towards several vendors for the meter you're looking for.

Adam

[smokeAndMirrors]

I'll bet if you drive to their door and ask them where you can buy one of their meters, they will be only too happy to put you in touch with one of their dealers.

I'll bet that, given their chief business appears to be aerospace and defence, I'll be turned away at the gate by some JobsWorth security guard who hasn't a clue what I'm quacking about

That article is 11 years old, and represents the thinking at the time. It has since been superseded by this article.

Splendid.

With a calibrated irradiance meter, or with a calibrated low-range power meter that has a known aperture size, assuming of course that the beam completely fills the aperture and you're smart enough to divide the power reading by the area of the aperture to get the irradiance figure.

And where does one acquire such irradiance meters? Are there good brands? How does one calibrate one's meter?

It really is that simple. If you're still confused about the whole thing, I suggest you contact Greg at LSDI directly. He can probably clear up any lingering doubt you may have. I'll bet he can also steer you towards several vendors for the meter you're looking for.

Adam

Thanks Adam, I appreciate your advice and patience.

[smokeAndMirrors]

3 days later: no response to my email from LSDI; still don't know where to get MPE meters or any of the other meters being touted.

What's a "good photodiode" for this purpose? What sort of resistor - how many Ohms, what material? Where does one get irradiance meters? How do you calibrate them? How does one use the oscilloscope?

[mixedgas]

http://thorlabs.com/newgrouppage9.cf...ctgroup_id=285

FDS100 is a good choice. It has a known size and is fast enough, I have used them before in the lab.

Solder a 470 or 1000 ohm 1/4th watt metal film resistor across the back and then mount it on a BNC connector so it will connect to the oscilloscope.

Or just buy the whole thing, a det36 is overkill but it saves you time packaging it:

http://www.thorlabs.com/NewGroupPage9.cfm?ObjectGroup_ID=1295&pn=DET36A/M


Here, this will work with your laptop and can measure the timing:

http://www.picotech.com/ultracompact-oscilloscopes.html

No promises, but this should work as well:

http://cgi.ebay.com/USB-PC-OSCILLOSC...item3ca6fb4165


Now you just need a good laser power meter and a spreadsheet or calculator.

and spend the money you would have spend on a MPE meter on a good scope and a good power meter.

A bit overkill and biased towards Tek's megadollar scopes:

http://www.tek.com/Measurement/App_Notes/XYZs/

Do note most used 10 mhz crt oscilloscopes on sale for 65$ on ebay will do this, but a picoscope has a good calibrated time base and will give you a better readout of the time. CRT based scopes have x-y mode that will let you preview your show without a laser, too. But since your asking scope questions you might want to start with a USB digital scope, instead of a analog one.

Don't forget you need a cable from scope to connector, 50 ohms coax is the standard, like this first thing I found on ebay:

http://cgi.ebay.com/10FT-BNC-to-BNC-...item3ca6238be9

Steve

[smokeAndMirrors]

OK I feel like I'm getting somewhere now... but having just perused Google and the lobo.de website, the LMS-2 meter is no longer available.

The cheapest laser power meter I can find is lasercheck for the princely sum of $395. The site I went to didn't sell in the UK, and the only UK site that mentions them doesn't seem to sell at all. Ho hum. There are other meters costing $1000 or more. Is that correct?

[mixedgas]

OK I feel like I'm getting somewhere now... but having just perused Google and the lobo.de website, the LMS-2 meter is no longer available.

The cheapest laser power meter I can find is lasercheck for the princely sum of $395. The site I went to didn't sell in the UK, and the only UK site that mentions them doesn't seem to sell at all. Ho hum. There are other meters costing $1000 or more. Is that correct?

Thats about right for a new lasercheck.

I think you can get a lasersbee from here on PL for considerably less then a lasercheck.

Or go find a coherent 212 on ebay.

Steve

[buffo]

A LaserCheck would not be an ideal power meter anyway, since you'd have to first install a 1 sq cm aperture in front of the detector, and then you'd need to take one measurement for each wavelength in the projector. (The LaserCheck's response is not linear for all wavelengths.)

You'd be better off with a low range thermal power meter, though remember that you'll still need to install the aperture...

I just learned that Greg was out of town last week, so you probably won't get an answer from him until he returns.

Adam