Looking for some advise

[cosmarchy]

Hi,
I am working on a magic show and was wondering whether anyone can offer some advise.

The basic jist is that that I need to light a cigar or cigarette using a laser from a distance of approx 60m.

My first question is, what is more important, wavelength or power? I believe I need an IR laser as this is indetectable by the viewing audience which limits me to 808nm, 830nm, 980nm or 1064nm. I can get a reasonable sized one in varying power from 100mW to 3W from here (varying models from menu on left side)

Am I right in assuming beam divergence will be an issue to me? The best divergence seems to be around 3mRad (with a 3mm spot) which over a distance of 60m gives a spot of ~18.3mm . Given such a large spot diameter, I'm guessing there won't be enough energy to light anything.

How could I correct this divergence and refocus to this distance? Would something along the lines of a sniper scope work?

If anyone can help on this, I would appreciate it.

Thanks

Cos

 

[Morgan]

A beam expander is what you would need so in some ways it's like a sniper scope, however, this is not really going to get you instant cigar lighting. I also think that is the least of your problems... As you point out, the beam would be invisible, (to all intents and purposes), so how are you going to find it to get your cigar into?

There are probably simpler, safer and cheaper ways to achieve this illusion than firing high powered lasers at your face. Please consider those first.

M

 

[DrSid]

808 is visible on video cameras .. not sure about longer wavelengths ..

 

[Mo13]

I know this is not the answer you want, but you really can't do this trick. Any laser over 5mW can damage the human eye before the blink reflex kicks in, and anything over 100mW WILL damage the human eye almost instantly.

The danger of doing this with an IR laser doubles because nobody can see the dot or beam to even know to shield their eyes. Lighting a cigar from 60m would take at least a 1W laser focused perfectly. That kind of power will cause permanent eye damage in under a second from any accidental reflection. The ONLY way this could ever be safe is if you handed out safety goggles made for the appropriate wavelength to each member of the audience. But that pretty much ruins the illusion.

To do this trick in front of any audience would be extremely irresponsible of you, and I strongly advise against it. I do appreciate the awesomeness of the idea.



To answer your questions, power matters more than wavelength. Focussing the dot at that kind of range would require extremely fine adjustment of a focusing lens which would take a lot of trial and error.

 

[HaloBlu]

Using a laser for this wouldn't just be extremely irresponsible it will land you in jail.
After which your audience members will be filing civil lawsuits.
Even diffuse reflection will cause injury.

 

[Morgan]

I know this is not the answer you want, but you really can't do this trick. Any laser over 5mW can damage the human eye before the blink reflex kicks in, and anything over 100mW WILL damage the human eye almost instantly.

The danger of doing this with an IR laser doubles because nobody can see the dot or beam to even know to shield their eyes. Lighting a cigar from 60m would take at least a 1W laser focused perfectly. That kind of power will cause permanent eye damage in under a second from any accidental reflection. The ONLY way this could ever be safe is if you handed out safety goggles made for the appropriate wavelength to each member of the audience. But that pretty much ruins the illusion.

To do this trick in front of any audience would be extremely irresponsible of you, and I strongly advise against it. I do appreciate the awesomeness of the idea.



To answer your questions, power matters more than wavelength. Focussing the dot at that kind of range would require extremely fine adjustment of a focusing lens which would take a lot of trial and error.

I should think it'd take quite a bit more than 1W at 60m.

Not strictly true. It depends on both. What you're burning will determine how effective a particular wavelength is. 405nm, for instance, burns white paper much more effectively than 660nm for a given power output.

We do agree that the illusion done with lasers would be somewhat reckless though.

M

 

[Mo13]

Not strictly true. It depends on both. What you're burning will determine how effective a particular wavelength is. 405nm, for instance, burns white paper much more effectively than 660nm for a given power output.

M

Really? I never knew that. Thanks for the quick lesson, Morgan. I love learning new things here.

 

[cosmarchy]

I'm really glad people bought up the safety concerns.

I have had these myself but the nature and location of the trick should eleviate a lot of these. The trick is set in the open (my remit was for the edge of a desert for example but i'm sure there are other open areas!!); this should reduce reflections etc.

The laser itself was envisaged being projected approx 15-20ft horizintally above the audience from behind. The trick with lighting the cigar involves it being hoisted up from the stage using mechanism conceiled within the stage surroundings. Part of the team are looking at the logistics of targeting a small spot at the cigar and seem confident they can do it (whizz kids with servo positioners!!).

With the safety aspect aside, pending further health and safety investigations, are there any calculations anyone can point me to regarding reducing the size of the beam over a long distance using a beam expander?

Even if this doesn't get used for this, it would be interesting to know the answers for other projects that may arise

Thanks for your help.

 

[HaloBlu]

The trick is set in the open (my remit was for the edge of a desert for example but i'm sure there are other open areas!!); this should reduce reflections etc.

Diffuse reflection is not reflection off something shiny as you normally think of it. This diffuse reflection is off the tip of the cigar, exactly where everyone will be looking.
You need to obtain a variance for public use of a laser. In many jurisdictions, you will be required to hire a laser safety officer.
What country (or state if in USA) will the performance be in?

Using a CO2 gas laser may reduce reflection. 10600nm that CO2 lasers produce have different properties when it comes to spectacular (shiny) & diffuse reflection.
At a range of 60m your going to need a power range that will push you to CO2 lasers anyway.