Math Check - Audience scanning - help?

Might be in the wrong forum?

So help me out here guys. If I'm going to do some audience scanning of my friends and family I'm going to want 10x MPE. If I ever do it, it will be with close friends/family at my home. Not saying I'm doing it anytime soon as I have a lot to learn and I do understand the legalities and liability involved but putting that aside for a minute please help me understand if I'm understanding what I've read correctly.

So I found some information from ILDA, which I'm sure 90% of you have seen, and I've also seen it here on the forums I think, that provides a "simple" method that involves measuring the irradiance of a static beam at 100 mW/cm2. In theory that gives you 10X MPE for a 1 millisecond pulse when scanning. Give or/take I understand this is not exact.

OK, so I have a Radiant X4. Its detector head I guess you would call it, is 225mm². So that is 2.25cm². So to make things easy, am I'm looking for around 200mW on my sensor, assuming the entire sensor area is full and a static beam at the closest point of my audience?

If this is entirely wrong please explain as if you were talking to a brick :)

thanks.

Oh I found this handy tool..good stuff, I'm sure most of you have seen it but just in case.

http://lasersafetyu.kentek.com/easy-...c-web-version/

What is your worst case scenario? E.g. What is the exposure time frame in the event of a failure of the scanners?
In my opinion you should really be using this as your exposure time frame and not a 1ms pulse. Of course, if you have a 1ms or faster scan fail device then you're good ;)
If not, then a failure puts your friends and family in the firing line of 40x MPE

If you want to use the 10x MPE levels then do it based on the 250ms time frame e.g. 25mW/cm2

So my method is correct? \o/

The speed of a scan fail device is something I had not fully considered, see this is why I ask these things! My eyeballs, and the eyeballs of my friends appreciate the response.

You did what a lot of people do, and that is to just consider a properly running show, and not the worst case scenario.
One of the best things you could for crowd scanning is get a divergent lens like the Pangolin Safetyscan lenses.

Heh I actually was thinking about picking one up. Probably won't do the set as it will always be the same room. Have not totally figured out how to figure out which one I need yet but I will.

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Originally Posted by demolish50

Heh I actually was thinking about picking one up. Probably won't do the set as it will always be the same room. Have not totally figured out how to figure out which one I need yet but I will.

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Call pangolin. They will help you pick the right lens.

I think I got it sorted. I'm going to pickup a -6 diopter lens as soon as they reply to my email and confirm my thoughts. Pretty small room. There is a nice video out there, https://www.youtube.com/watch?v=ooZ1C5gL7FA. Some demo's, looks pretty damn good. When I get mine and play with it I'll let ya know what I think in case anyone reading this has not played with one.

Nevermind. Doh.

When the 10X proposal was being made, experiments by the proposers (Patrick and Greg) were done at the MPE, at 2X the MPE, 3X and so forth. At the MPE the shows look beautiful. As you move upward from there they become damn bright. Effects start leaving after-images. Even 3X the MPE you're really punishing your audience...

The whole "10X proposal" was formulated just to say to regulators "hey, audience scanning has been going on for decades, and done by guys who had no freakin idea how much over the MPE they were (and testing revealed that these idiots were many many more than ten times the MPE)". If you're willing to make two assumptions: 1) That even at 10X the MPE there is still a safety margin (heads-or-tails chance of retinal damage) and 2) *knowing* how far you are over the MPE is much better than not knowing, then you should be willing to accept the 10X proposal.

Personally I thought the regulators would laugh Patrick and Greg out of the room when this was being proposed at a safety conference, because years earlier, when Greg and I did demonstrations at that same safety conference even at 1X the MPE, not even a single regulator would look into the projected shows. Amazingly the regulators actually bought the argument (knowing is better than not knowing, even if you know you're way over).

Note that not all regulators buy this, and it's not official policy. Also note that I personally would never do shows over the MPE -- for the reason that if anyone *claimed* to have eye damage, they'd have a good reason for claiming it, but to me it's more important to have shows that are actually pleasant to view, rather than performed as a form of punishment... But heck, that's just me ;) .

Bill

I've been toying around with the safety scan lens a bit in my home. That thing is pretty damn cool. I really like it with the static beam around 10-12mW/cm², which I can get in a really short distance out even with a watt coming out of the projector, 10 meters or so. Your right, I could not imagine 10x. I don't have a fancy irradiance meter or anything, just that Radiant X4 so my numbers might not be perfect but I think they are close.

The only complaint I have is the stray "ghost" beam or two that will come out of the lens for some reason. I just can't seem to get the angle perfect.

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Originally Posted by demolish50

The only complaint I have is the stray "ghost" beam or two that will come out of the lens for some reason. I just can't seem to get the angle perfect.

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How did you attach the lens to your projector? Are you using the special Pangolin mount? It will give you 3 axis' of movement, allowing you to get the alignment perfect. Without this mount, getting the lens aligned is a nightmare.

Adam

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Originally Posted by buffo

How did you attach the lens to your projector? Are you using the special Pangolin mount? It will give you 3 axis' of movement, allowing you to get the alignment perfect. Without this mount, getting the lens aligned is a nightmare.

Adam

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Yeah I'm using their mount but but it just seems like there is always a ghost. It seems like dead center has "ghosting" on both x and y that extend past the center point equally. If I go either way it just gets bigger one direction and smaller the other. I hope I'm explaining that right. I've spent hours trying to get it perfect...I'll have to take a few pics one day and show you what I mean. TBH I'm content with, when your viewing a beam show its not really noticeable.

There will always be some ghosting in my experience, but when setup perfectly the ghosting will follow / be aligned well with the scanned effects and so much less visible.

One thing I must add just to clarify regarding using a 2.25 sq/cm as the aperture size on your sensor and calculating your mpe based those measurements, is that your laser dot MUST completely fill or be greater than the size of the sensor in order for that to work. If not, use a smaller aperture over the sensor to measure. But if for example the beam is smaller than the sensor area, then you could potentially be at way higher power density than calculated / measured.

You may indeed already know this, but didn't mention it in your post so I just wanted to stress the importance just in case of a potential misunderstanding. In my experience you much be quite far away and using a high powered safety scan lens in order to get a beam that large so for taking measurements in smaller rooms it may be much more feasible to make a smaller aperture for it

Yes I did :)

"assuming the entire sensor area is full and a static beam at the closest point of my"

With the -6 my beams are that big at around....I dunno 1.5 meters? I have not measured....at 5 meters they are huge...inches.

At 5 meters the white balance is screwed. Since the blue has a different profile then the red/green and the lens really amplifies it at that range I have a smallish green circle, followed by a larger red circle, followed by an even bigger \\ blue shaped beam. Again not really noticeable viewing a show just when toying around.

At the aperture, supposed to be 1mm green, and 1-2mm for the red/blue. I'm not sure how to measure that yet TBH.

Also it seems my numbers are very close to what Bill presented in is video where he discussed the safety scan lenses. Not exact but close enough to make me feel good but if I'm doing something wrong I want to know like know :) so thank you for double checking me.

For sure. I found the initial beam size makes more of a difference than anything else in how much they expand with the safety lenses. Matching up initial beam sizes with a beam expander / telescope very much improves how they match up under the safety lenses. Initial beam size makes way more of a difference than initial divergence or other properties.

But either way is surprisingly not as noticeable when watching a show that would be expected. Even at very large distance with smaller diopter lenses. Only time it really shows up is in still frame pictures.