Laser MPE meters, but is there any point?

[drlava]

thanks for that, I have already ordered the dvds. Yes, a scanfail can never make a show safe, the safety I was always referring to was in the case of failure.

what do you think of the 10x MPE level?

[buffo]

what do you think of the 10x MPE level?

I think it's a great idea that will never fly here in the US.

My personal experience suggests that the 10X MPE level is still more than adequate for safety. And yes, I *have* had my retinas examined by an opthamologist (for an unrelated issue) within the last year and I do not have any retinal abnormalities, despite being exposed to well over 40X MPE levels routinely from my own projector, plus at least a few incidents that were much hotter than that. (Including one hit from a 275 mw dpss green at about 25 ft that had a 2 mm beam and .8 mrad divergence.)

Note that I do not advocate breaking the law for a commercial show, even though in my heart I feel it would be safe. Rules are rules, and until the MPE is changed, commercial operators are stuck with 10mw/cm^2 irradiance here in the US if the beams are in the crowd. Period.

However, I do not believe the law will be changed any time soon. I think it will take decades, if it happens at all. Call me a cynic, but I think we'll need significant tort reform in this country before you'll get the 10X MPE approved.

Adam

[mccarrot]

Note that I do not advocate breaking the law for a commercial show, even though in my heart I feel it would be safe. Rules are rules, and until the MPE is changed, commercial operators are stuck with 10mw/cm^2 irradiance here in the US if the beams are in the crowd. Period.

Adam

10mw/cm2 can ONLY be used IF you have a proper scanfail. Else you need to stick with 2,5mw/cm2.

[buffo]

10mw/cm2 can ONLY be used IF you have a proper scanfail. Else you need to stick with 2,5mw/cm2.

I know that. I should have said that "commercial operators are stuck with 10mw/cm^2 even if they have everything else they need for an approved audience-scanning variance." In fact, here in the US you're going to need a lot more than just a basic scan-fail board if you want to get an audience scanning variance approved. When you're talking US regs, all sorts of extra things come into play.

Remember that it took Bill Benner and Greg Makov almost 2 years to get their variance approved. And the heart of their application (PASS) does a lot more than the scan-fail unit you've found. For example, PASS actually crow-bars both the power supply and modulation lines for the lasers to ground. Also, it's not a digital device. (Analog devices are safer, especially in the mind of a government regulator.) Then too, they had a massive lens installed on the projector aperture to diverge the beam. (And I do mean massive. Several pounds.)

Honestly, audience scanning here in the US is just not worth the effort for a commercial company, at least in my opinion. It's too much hassle, and the legal risks are enormous. Over in Europe, on the other hand, it's probably doable. (Which is also why everyone that has signed up for the scan-fail board group buy thread so far has been from Europe.)

Still, I would like to have one for my projector at home one day. A little extra safety is always a good thing, even if it's just in your living room! But right now it's a luxury that I can't justify.

Adam

[smokeAndMirrors]

OK guys, first off, thanks for your help so far. I have ordered:

1. The fast photodiode, a snip at £15 inc delivery (plus resistor and will cobble together the rest from household junk )

2. The USB oscilloscope. Its max sample is 1MHz but that seems plenty compared to galvo speeds even allowing for Nyquist.

3. The LaserBee thermopile meter.

I've written an Excel spreadsheet to do the calculations, so all that remains is for the stuff to arrive, be assembled, and I'm off to prep for an upcoming video shoot...

One question. From the article, step 1 says:

"Measure the laser beam irradiance at the closest point of audience access. To do this, project a non-moving beam into the venue."

Now if I have a 600mW (TTL) laser that projects a beam roughly 1cm^2 in area, why is the irradiance not 600mW/cm^2? Lasers aren't inverse-square in intensity, so I'm guessing that the value is due to some complex function of intensity with distance that involves divergence. But assuming the test point is 10m away, the attenuation must still be pretty stupendous to reduce 600 to some safe value?

[lasersbee]

If you need any additional info on the LaserBee I don't hesitate to contact us..
There is a link in my Sig...

Jerry

[mccarrot]

You cannot use the laserbee.

The laserbee is not sensitive enough!

If you put a 7mm aperture in front of the detector, you should measure 3.6mW for 10mw/cm2

The thermopile is not that acurate in the mw range, and only works from 5mW and up.

[smokeAndMirrors]

If you put a 7mm aperture in front of the detector, you should measure 3.6mW for 10mw/cm2

What has 7mm got to do with anything? The meter will measure irradiance which is independent of aperture size (ie it's the power collected over the detector divided by the area of the detector). The analysis for *scanning* suggests a 7mm aperture to emulate the iris, but that part of the calculation is done by the fast photodiode, not the laserbee.

Also the calibration can differ with the used power. I have seen if you calibrate it with a 10mw laser you get wrong readings in the 1W range, and if you calibrate it with a 1W laser you get wrong readings in the mW range, but its good enough to get a indication.

OK well that's... interesting... confusing.

But the most important thing is the sensitivity

OK... it might have helped if you'd said something when mixedgas suggested the Laserbee on the first page of this thread - excuse me if I sound narky but I've just been told that $368 of my hard-earned cash has been spent in error. I've put several posts here asking for specific recommendations, and when I get one and act on it then it gets contradicted

[mixedgas]

Wowa, Lets not panic just yet here people.

lasersbee is fine for old method calculations.

If you want to do this supersensitive point and shoot one shot"new ILDA" method, then you are going to need a megadollar power meter.

If your going to take the time to do it, you can do it old school and use lasersbee. You can also use lasersbee to calibrate the FTS sensor. Just replace the resistor with a 2K trim pot and you can calibrate it for any given wavelength or combination of wavelengths , ie if the color ratios dont change, the calibration is the same and will hold to 2.5% in silicon over two decades or more of range.

I will help you calibrate a few effects the old way.

Besides, most laser power meters are only 2.5-5% accurate anyway at full scale. This is normal and accepted in the industry. I have a medical friend and the FDA spec for medical accuracy is 20%, its the repeatability that counts.

Its not that bad a unit.

I'm not going to get into a pissing match with Mc over methods.

Its not that hard to set up the spread sheet for most effects and just input power and time.

beam arrays are the only major issue, and they can be broken down into single beams.

Steve

[lasersbee]

You cannot use the laserbee.

The laserbee is not sensitive enough! (en not properly calibrated)

If you put a 7mm aperture in front of the detector, you should measure 3.6mW for 10mw/cm2

The thermopile is not that acurate in the mw range, and only works from 5mW and up.

Also the calibration can differ with the used power. I have seen if you calibrate it with a 10mw laser you get wrong readings in the 1W range, and if you calibrate it with a 1W laser you get wrong readings in the mW range, but its good enough to get a indication.


But the most important thing is the sensitivity

Hey mccarrot... I take offence with your insinuations....

I see you're Slamming the LaserBee I LPM again...
and as you usual do in that respect...
you're again talking out of your a$$...

Do you even own a LaserBee I....
Have you personally conducted these "Calibration Tests"..
Please show us your test results....
Have you even read the Calibration Procedures we provide...

Please enlighten us....

Jerry