Laser MPE meters, but is there any point?

[smokeAndMirrors]

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

Thanks Adam. I've just been quoted around GBP1400 for a thermopile detector and USB widget from Lambda Photometrics - the P-LINK software provides RMS and pulse count measurements which I'm surmising is what I'm after?

OTOH A detector plus oscilloscope for the dynamic (scanning) analysis plus LaserBee meter for the static (with a spreadsheet on my laptop doing the math) comes to about GBP500 so it's a question of how much I value the convenience, I guess?

[buffo]

1400 pounds! Ouch - that's pricey. I think you can do better on E-bay. (But you'll probably have to wait a while to find one.)

Then again, the do-it-yourself method is still expensive. But it will probably be a fun project to assemble nonetheless.

This *is* a money-pit hobby though. This thread is just more proof of that.

Adam

[mixedgas]

[QUOTE=smokeAndMirrors;121291]Thanks Adam. I've just been quoted around GBP1400 for a thermopile detector and USB widget from Lambda Photometrics - the P-LINK software provides RMS and pulse count measurements which I'm surmising is what I'm after?

No, that is a total energy joule type meter if it does pulse count, it does NOT work for long pulses. You need a thermal meter, like a coherent 210 or a fieldmaster with the small head, ie 3 watts or less.



Steve

[drlava]

I note that in this thread, the number 10mW / cm^2 is viewed as the target goal for safe beamshows, and in a group buy thread, there is a scanfail device that blanks the laser if it stops scanning in the 'crowd area' for 2 ms or longer. But 10mW/cm^2 limit knows nothing of scanning speed. A widely held view is that ANY time spent viewing greater than 10mW/cm^2 is too much, regardless of scanning speed.

From the linked article:
"With audience scanning, the exposure is reduced dramatically by the rapid deflection of the beam. For example, a 1 Watt beam scanned over a 3 meter path can reduce the average exposure to a level of 1 milliwatt/cm^2.

However, a safe exposure is not just a function of average exposure. The individual pulse exposure must also be below the MPE."

Later the article states that a scanning 1 watt beam must be blanked within 200 nanoseconds or the exposure levels will exceed Class1 exposure limit.

So, many people are buying this digital 'scanfail' device that will blankn in 2ms, but will this really prevent damage? Based on the 1W/200ns limit, if scaled linearly, the most powerful projector that the 2ms reacting scanfail can protect is .1mW. This can't be correct? Does a 2ms response time ensure safety at any reasonable output power?

[White-Light]

I note that in this thread, the number 10mW / cm^2 is viewed as the target goal for safe beamshows,

I believe under the new ILDA guidelines the upper "safe" limit for nightclubs is now 100mW / cm² (10x MPE).

[Xytrell]

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

I'm more worried about oral or nasal damage, myself. But really, if you even think that you laser might cause skin damage... when scanned?... probably too much power

[smokeAndMirrors]

1400 pounds! Ouch - that's pricey. I think you can do better on E-bay. (But you'll probably have to wait a while to find one.)

Then again, the do-it-yourself method is still expensive. But it will probably be a fun project to assemble nonetheless.

This *is* a money-pit hobby though. This thread is just more proof of that.

Adam

Heh, you think THAT's bad? Just had a quote from Lobo for their LMS-2: over 10,000 Euro (over 4000 for the meter itself, 3000(!) for the software FFS, 2000 for their "special" laptop, and a bunch of other extras).

[smokeAndMirrors]

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.

"Across the back" means between the pins like this?

[mixedgas]

See attached. make all connections as short as possible. Solder the resistor right up against the PD. Twist the leads from the Pd/R combo to the BNC, once a value for R is chosen, encapsulate leads in epoxy for strength. Ideally its in a tiny metal box for shielding.

Steve

[buffo]

10mW/cm^2 limit knows nothing of scanning speed.

This is incorrect. The 10mw/cm^2 limit is *based* on scanner speed. 30Kpps, to be exact. Ok, it's technically derived from the hard MPE of 2.5 mw/cm^2, but it is increased by a factor of 4 because of the scanner's speed.

I'm guessing that you didn't read Patrick Murphy's "Audience Scanning Overview", dated Feb 20, 2009. (Edit: Just remembered that you are not an ILDA member, so yeah, you probably haven't seen it.)

That document spells out where the 10 mw/cm^2 limit comes from. Greg Makov (LSDI) had a hand in developing the methods used. Bill Benner presented an hour-long lecture on the topic at SELEM 2008, and I know a few people videotaped that presentation. Patrick also reviewed the highlights of that presentation at SELEM this year. You might want to check with allthatwhichis or 300Evil and see if you can get a copy of these lectures, as it will probably clear up a lot of confusion.

Nevertheless, for clarity, here is an excerpt from the document referenced above:

By examining the computer signal sent to the galvanometer scanners, the minimum velocity of a moving effect is found to be on the order of 10 radians per second. The maximum velocity is about 1000 radians per second. The minimum distance from a projector is about 1 meter (in a small club or disco), while the practical maximum range is 10,000 meters.

Given that linear velocity equals angular velocity (10 to 1000 radians per second) times range (1 to 10000 meters), the range of possible linear velocities (beam crossing a viewerʼs eye) is from 10 meters per second to 10,000,000 meters per second. Makhov states that this is “a large but manageable area.”

From this, the range of pulse durations can be calculated. Assuming a 1 milliradian divergence – which is tight for laser projectors – the pulse duration is the beam diameter divided by the linear velocity. The range of pulse durations is 2 milliseconds to 1 microsecond. This is mostly within the range of Thermal MPE values ranging from 10 seconds to 18 microseconds. The MPE for the pulse is therefore 1.8 × t3/4 × 10-3 J/cm2.

A table of MPE values for various exposure times is derived:
Exposure time --- MPE
1.0 second ------ 1.8 mW/cm2
0.25 second ----- 2.5 mW/cm2
0.01 second ----- 5.6 mW/cm2
0.001 second ---- 10 mW/cm2
18 microseconds - 27 mW/cm2

Makhov first notes that a person exposed to bright light will have an aversion response. The accepted value is 0.25 second. After this time, the person has blinked and/or turned away. So, 1/4 second is the longest exposure duration needing to be considered. Next, Makhov notes that “from 1/4 second to 1 millisecond, average MPE increases only by a factor of 4” and “from 1/4 second to 18 microseconds, average MPE increases by a factor of 10.”

The result of this analysis is that by scanning the beam, the allowable MPE is increased only by a relatively small factor relative to the average power MPE. Makhov states: “Since the increase in MPE as a function of scanning is small (only 4 to 10 times), scanning parameters are a minor factor. To stay within the MPE, the accessible irradiance is the major factor.”

As for the Scan-fail interlock, let me restate it one more time: Scan-fail devices do not make a show "safe". The irradiance level is what makes the show safe. The scan-fail interlock merely *keep* a show safe by shutting off the lasers in the event a scanner stops moving. They will do nothing to prevent exceeding the MPE if the irradiance of a static beam is greater than 10 mw/cm^2.

Adam