Questions about a small project

[Snilton]

I have a spare 50mW green laser.
Im planning to just put a star grating spun by a mirror ball motor in front of it.

Now for the questions:
Will this be safe to point at audience?
What kind of grating do i need and where can i get it?
Is there any cheap way to make the laser modulate to the music?

[Carl B]

Hi Mate

I doubt this would be safe as a percentage of the beam from the laser passes straight though the grating leaving you with a static beam in the centre of the star that won't be moving. The only way around this is to mask the beam in the centre after the grating but you will still have to check that the power level of the rotating star is safe for audience scanning.

How are the Scanners you bought from me doing? Got them in a case with the laser yet?

Carl

[Snilton]

So if i mask out the middle one it would be ok?

Scanners are working great, havent started building the case yet.
Waiting to get into the workshop in school, have to do some safety course first.

[Carl B]

I'm not saying it will be safe even after masking the centre beam out as the power levels of the rotating star need to be checked before you even think of scanning any audience. I don't have enough experience my self about checking for safe power levels for audience scanning so I don't do it.

Carl

[Laserman532]

not a good idea, unless your fathers a good defense attorney

[Snilton]

how much does the effect fall off for every dot from the center?

[Laserman532]

how much does the effect fall off for every dot from the center?

that question has no answer, too many variables. I suggest shoot your laser through the grating walk out in front of it and hold your eyelid open and shoot the beam into your eye, If it dosnt hurt than it is fine, if it does hurt, it might not be a good idea to do it to others. I dont know how else to explain it

[soforene]

I nominate you as the PL Safety Expert.

[drlava]

LOL
or he could get a laser power meter and measure every beam to verify they are less than 1mW then rotate the grating by 1/inf degrees and measure all of the beams again, repeat until he has rotated the grating by 360 degrees and all is under 1mW.

[buffo]

I doubt this would be safe as a percentage of the beam from the laser passes straight though the grating leaving you with a static beam in the centre of the star that won't be moving.

Carl is correct in that using a grating in this way probably would not be safe, but I felt it important to explain that the MOVEMENT of the beam is not the most important part of the equation when trying to determine if the beam is safe or not.

Moving the beam doesn't change things very much, even if you increase the speed of the beam to absurdly high levels. The key is to reduce the irradiance. (That is, reduce the number of milliwats per square centimeter in the beam...) You can do this by lowering the power of the laser, increasing the diameter of the beam, or both. Once you have the irradiance lowered to the proper range, you can actually have a slow moving beam (or even a static one) and be OK as far as eye exposure is concerned.

The math behind all this is complicated, but as a general rule of thumb, if you are below 10 mw per square centimeter (measured in a static beam), you are OK for most crowd-scanning effects. (This information is taken from the crowd-scanning lecture that was presented at SELEM 2008.)

As for the specific example of a diffraction grating, DrLava is correct: You'd need to verify the irradiance of each beamlet coming off the grating to be certain that *all* of them are below 10 mw per square centimeter. So either break out your power meter and settle in for a long night's work, or find another way to accomplish your effect...

Adam