Audience Scanning in the U.S. ?

[mixedgas]

Excuse me if i've misunderstood... is that paper suggesting that all that is required to determine safe MPE is a static beam irradiance measurement ?

Yes, if you like your show wimpy and nearly invisible. Ie class IIIa.

It is, however suggesting that with some work, higher powers then IIIa can be safely used if you keep the beam moving, but not too fast, the right measuring tools, the right divergence, and avoid repetitive beam paths.

Steve

[buffo]

Excuse me if i've misunderstood... is that paper suggesting that all that is required to determine safe MPE is a static beam irradiance measurement ?

Yes. It's a more conservative approach than calculating the power at each point in a scanned image, but it's also much easier to measure and calculate. You simply test the worst case (which would be a static beam), and ensure that even with a static beam you are still below the MPE for the most restrictive case for audience scanning, which - somewhat surprisingly - is the multiple-pulse exposure limit. If your static beam is below the MPE limit for multi-pulse, then you are OK for the single pulse *and* the average power limit as well, since they are both higher.

Bill Benner gave a detailed talk on this subject at SELEM 2008, including all the calculation steps to prove that the multiple-pulse MPE was the limiting case. (It was also discussed on the ILDA cruise last fall.) I don't know if anyone ever uploaded the video from Bill's presentation at SELEM or not though. (I'm pretty sure 300evil taped the entire thing, but I don't know what happened to the video...)

Yes, if you like your show wimpy and nearly invisible. Ie class IIIa.

I don't think the show is wimpy at all, Steve. I've seen this technique used twice, and the results are *quite* impressive. Yes, you are taking a static measurement, and yes, the irradiance is low. (around 10 mw per square cm.) Class IIIa doesn't really explain it though, since that specifies the total power of the beam while the MPE is measured in irradiance (power per unit area). But yeah, it is a low number. The difference is that the beam diameter is larger. (Quite a bit larger actually - up to 2 inches in diameter.)

Surprizingly, the large beam isn't really that much of a factor. We've all been conditioned to think that a good laser beam show has to have a pencil-thin beam, but it turns out that even a 2 inch beam still looks pretty good. (Bill Benner has posted this more than once. You really need to see it with your own eyes to believe it though.) The key is that by expanding the beam, you spread the power out. If you can get below the 10 mw per square centimeter threshold, you can safely (and legally) scan the crowd here in the US.

At FLEM 5, I gave out some large acrylic lenses to the attendees. They were rated at negative 2.75 power. (They were double-concave lenses.) If you put one in front of your projector, it will cause the beams to diverge fairly quickly. Once you get 20-25 feet away, the beam is expanded enough that you'll be below the 10 mw per square centimeter limit for irradiance. Viewing a beam show at this distance is still visually impressive, but there is no discomfort when the beam scans over your eye. (At least not for the lower power projectors - around 500 mw total output - that most of us have. Obviously a more powerful projector would need either a stronger lens, or you'd have to stand further back.)

Mind you, I'm not implying that such a lens by itself is enough to land you an audience-scanning variance from the CDRH. You'll still need a scanner-fail safety system, a secure means of fixing the lens, irradiance measurements at the venue before each show, etc, etc... But for the average hobbyist, it does make things a little safer for your garage or basement laser show!

Adam

[colouredmirrorball]

Can anyone give some sort of equation that gives the link between power and scan speed? Eg. if I have 5K pps scanners, what would be the max. power per 1 cm² and what would the max. power be at 20k etc.?

[mixedgas]

Can anyone give some sort of equation that gives the link between power and scan speed? Eg. if I have 5K pps scanners, what would be the max. power per 1 cm² and what would the max. power be at 20k etc.?

Its not that simple, it depends on speed, angle, distance, beam size and divergence and exposure time, and its cumulative. But the mayth itself is 9th grade algebra.

here in this thread, are sample calcuations as attachments.
http://www.photonlexicon.com/forums/...highlight=cdrh

Steve

[buffo]

Can anyone give some sort of equation that gives the link between power and scan speed? Eg. if I have 5K pps scanners, what would be the max. power per 1 cm² and what would the max. power be at 20k etc.?

As Steve mentioned, it's not so simple. There are numerous factors to consider, many of which are non-linear. This is why the MPE calculations are so hard to begin with. If you read through the derivation of the 10 mw/cm² limit (see the ILDA website link above), you should be able to folow the math though, as it's mostly just algebra. From that, you can substitute the different numbers for the slower scanning speed and re-calculate. But this is going to be a pain in the rear.

That's why *most* audience scanning shows prefer to use the 10 mw/cm² limit for a static beam - it's just ONE measurement to take. Then, so long as you are running at 30K and have all the other requirements (including scan-fail protection), you're golden.

Adam

[mixedgas]

For the irradiance measurement you can use a laser power meter and a b/w CCD camera with the lens removed.

Anybody up for a bit of writing code? If you have a 1/2" or 1/4" ccd camera, good quality and NOT a webcam, and you can digitize the video, you can measure your beam diameter, provided you know the part number of the CCD chip so you can get the pixel size and spacing.

You get a nice variable attenuator, so you can run the laser at full show power. This can be as easy as getting a piece of AR coated glass to pick off a sample of the beam, and then use crossed polarizers to knock it down to not swamp the camera with laser power. I have the fancy variable transmission chrome on glass wheel I ripped out of a old laser typesetter, either way works just fine. Dont burn the CCD by imaging the "waist" aka the smallest part of the laser beam where there is a crossover of the beam onto the CCD Dont use a color camera, the Bayer pattern interpolation will mess you up.

You make sure you have the CCD running linear by checking the histogram function in the digitizing software, histogram shows a plot of number of light values from dark to light over the range of the camera This step is optional if your careful and know what your doing, or you watch the raw video for clipping on the same oscilloscope you should already have for measuring your audience scanning.

You center the beam on the raw CCD, and this is a good time to see if you have a IR beam with the visible beam, on DPSS oftten the IR will show before the visible light as you modulate the laser. You take a frame into the frame grabber and save it to disk.

Next up, you move the ccd back a known distance by mounting it on a rod or a good stand. You recenter the beam, and take another shot. 50 to 100 cm or more is popular in commercial gear that does this.

Take a second shot and save it to disk.

You now take the number of pixels from the 13% power point and 82% power points across the beam in each grabbed frame, and its math time.

That can be done manually in federally funded freeware scientific image software known as Image J or Scion Image, in fact if you like cameras, get Scion or Image J anyways, you can do some really wicked stuff with them, including FFT for making your own crude gratings with a laser printer or film.

now that you have two diameter measurements in linear space, its math time. I need to look up the math.

Anybody game for some coding?

I have a card and commercial software that does this, and if it still works, I'll bring it to SELEM.

Steve

[White-Light]

Can anyone give some sort of equation that gives the link between power and scan speed? Eg. if I have 5K pps scanners, what would be the max. power per 1 cm² and what would the max. power be at 20k etc.?

As I understand it, it has got nothing to do with the scanners speed in the points per second sense, its to do with the minimum scanner velocity. ie. Whatever patterns are scanned they need to be arranged so that the scanner velocity NEVER drops below the specified limit.

A scanner may be able to do scans at speeds both below and above the specified velocity but in order to keep the show within safe limits, you need to use the your software to keep the velocity CONSTANTLY above the minimum specified speed. Currently this means using an oscilloscope to measure scanner velocity for your chosed display patterns although total software control may be on the horizon.

Also, another point, doesn't this method also apply to the proposed 10x MPE standard ie. if ILDA pass the proposed standard you'll be able to scan using the simply method with an exposure of 100 mw cm² from a static beam provided the minimum velocity requirements are satisfied for each pattern deployed. I think Steve ILDA have already indicated this standard won't produce dim beams but will require warning signs as there's an increased if acceptable risk.

[taz_]

I cant believe there is such a matter on scanning the audience. when im out in london, places like heaven have a full RGB scanner and it can scan the audience, and also can project 1000s of dots into the audience to, at eye level. theyv had it for a couple of years. doesnt this pose as a threat?

[colouredmirrorball]

As I understand it, it has got nothing to do with the scanners speed in the points per second sense, its to do with the minimum scanner velocity. ie. Whatever patterns are scanned they need to be arranged so that the scanner velocity NEVER drops below the specified limit.

Yes I am aware of that.
Can any software prevent too low scan speeds?
Maybe software producents can add a function to their software that if there are less than x points in a frame, the software creates automatically extra points in the frame (blanked of course) so that the scan speed is safe again.

Thanks for the documentation, Steve. Unfortunately I am not going to read all that. It is just too much work, trying to understand math above my current level in a foreign language. I have to study a few more years in order to fully understand this kind of math.

[mixedgas]

I cant believe there is such a matter on scanning the audience. when im out in london, places like heaven have a full RGB scanner and it can scan the audience, and also can project 1000s of dots into the audience to, at eye level. theyv had it for a couple of years. doesnt this pose as a threat?

If done with calculations and measurements, NO. If just "DONE" with no science, yes, it can be dangerous. You can buy exposure meters, but they are expensive. The one they showed us at a ILDA meeting was 3000$ or so.

some light reading: http://www.pangolin.com/resguide09a.htm

Steve