250mw red to white flashes.

Hey don't look at me, I said it was plasma too. But maybe they're 2 different effects....

plasma is just anything that is REALLY REALLY hot right?

Click to expand...

Wrong, sorry

I don't know how they do it.Not much details to work with....no power level, no wavelength.Maybe the wavelength they use is more likely to be stopped by air...I dunno :-/ Or maybe they have a super smooth focus and get a hell of a lot of irradiance(then again, it is an IR laser....confusing) :-/

Switch said:

Hey don't look at me, I said it was plasma too. But maybe they're 2 different effects....

plasma is just anything that is REALLY REALLY hot right?

Click to expand...

Wrong, sorry

I don't know how they do it.Not much details to work with....no power level, no wavelength.Maybe the wavelength they use is more likely to be stopped by air...I dunno :-/ Or maybe they have a super smooth focus and get a hell of a lot of irradiance(then again, it is an IR laser....confusing) :-/

Click to expand...

Hmm... I'm just getting more and more confused. However, I know how to find the answer. My great uncle, (Donald Bitzer, http://en.wikipedia.org/wiki/Donald_Bitzer ) invented the plasma display( http://en.wikipedia.org/wiki/Plasma_display ), so the next time I have a chance to talk to him, I'll ask him what the phenomenon is.

And yes, Donald Bitzer REALLY is my great uncle. I swear to god that I'm not lying.

laser_freak said:

I know that you guys concluded that it is incandescence, but I personally would rather believe a bunch of Japanese scientists rather that a group of people gathered on a forum.  
...
PS:  I have absolutely no clue how powerful a laser they are using, but I would definitely not want to stick my hand into the cluster of plasma balls.

Click to expand...

You can comfortably believe both, really.

What you see there is probably dielectric breakdown of air, which causes a spark in mid air when you focus a rather powerful laser to a narrow beam. The typically happens at the 'sweet spot' or just before, since that is the point where power density is large enough to produce this effect.

In terms of power, think way, way beyond what any diode laser can do. This effect can, however, be achieved and easily demonstrated with a tabletop pulsed YAG laser. The breakdown of air happens due to exceeding a certain power density level, and works just as well from very short pulses. As these lasers are 1064 nm, it is an eerie sight to see the spark appear in mid air a foot in front of the unit.

... video ..
http://www.youtube.com/watch?v=1HHJhpStza0&feature=related

I've seen it for real, didnt look much like a sphere, more like a spark, but its the same principle.

It is definitely incandescence. I just tested it with my DX 200mW (definitely! ;D). I focused the dot to smaller than a pinprick (actually, I didn't - I just got very lucky to find the ideal distance when it would do so naturally), and aimed it at my desk. Instant white flash of light, from the superheated carbon that my desk was emitting. When you get the focus right, the carbon residue in the smoke itself combusts (immediately after release) and gives that white flash. I couldn't see any smoke coming out when I did it.

Now I'm just waiting for that tiny little blind spot in my eye to go (looking at the reflection off a matt black surface is still bloody bright!).

bob1122 said:

It is definitely incandescence. I just tested it with my DX 200mW (definitely! ;D). I focused the dot to smaller than a pinprick (actually, I didn't - I just got very lucky to find the ideal distance when it would do so naturally), and aimed it at my desk. Instant white flash of light, from the superheated carbon that my desk was emitting. When you get the focus right, the carbon residue in the smoke itself combusts (immediately after release) and gives that white flash. I couldn't see any smoke coming out when I did it.

Now I'm just waiting for that tiny little blind spot in my eye to go (looking at the reflection off a matt black surface is still bloody bright!).

Click to expand...

You know you're supposed to use goggles with near 200mW of green and another 30mW of IR focused in a super small spot inches from your eye : You are going to majorly phuck up your eyesight that way.
About the video Benm posted:How is that laser only 200mJ? Isn't that very little energy?Did he mean MJ or kJ (k is close to m on the keyboard) ? :-/

Is'nt it a red laser? But yeah, use goggles.

Switch said:

[quote author=bob1122 link=1204963819/45#51 date=1205859983]It is definitely incandescence. I just tested it with my DX 200mW (definitely! ;D). I focused the dot to smaller than a pinprick (actually, I didn't - I just got very lucky to find the ideal distance when it would do so naturally), and aimed it at my desk. Instant white flash of light, from the superheated carbon that my desk was emitting. When you get the focus right, the carbon residue in the smoke itself combusts (immediately after release) and gives that white flash. I couldn't see any smoke coming out when I did it.

Now I'm just waiting for that tiny little blind spot in my eye to go (looking at the reflection off a matt black surface is still bloody bright!).

Click to expand...

You know you're supposed to use goggles with near 200mW of green and another 30mW of IR focused in a super small spot inches from your eye : You are going to majorly phuck up your eyesight that way.
About the video Benm posted:How is that laser only 200mJ? Isn't that very little energy?Did he mean MJ or kJ (k is close to m on the keyboard) ? :-/[/quote]


200mW of red, I wouldn't dare do it with a green (the KD 50mW is bad enough).

The reflection is not that bad as to need goggles, but I can tell you for certain that I won't be using this laser on any remotely specular surfaces! It's not as bright, as say, a 1W filament bulb, which I can happily look into for a few minutes. The divergence is atrocious as well, so it's not too much of a problem. This is a burning laser. Note that when it's actually burning, the overwhelming majority of the light it puts out is absorbed straight in by the material. You can tell because the room goes dim all of a sudden. And the white hot light it puts out is just too beautiful to not look at, looks like a glowing diamond ;D

Also, a laser can be 200mJ but what matters is energy density (energy per unit area), not total energy. If that 200mJ is concentrated into a millimetre, there will be some issues, suffice to say. Kinda like a bullet going at 1000mph and you walking at 5mph - you may have the same total energy, but that bullet has a much higher energy density (energy per unit mass in this case).

Green/red, I don't think it matters that much, or you could say that 200mW of IR is nothing : You can still look at the white flash with goggles.

Still 200mJ seems very little, it better be focused to a super tiny spot and the pulse width has to be very short. I think a 1 second pulse of 200mJ can be focused to the tinyest spot you can , it still would be nothing. :-/

It depends on the scale. 200mJ = 0.2J, which is not bad at all. A bullet of mass 0.02kg with 0.2J of KE is going 4.5m/s (not really harmful). You're right, a 1 second pulse of 200mJ total would be nothing. It's still good enough to burn shit though!

Again, you're misunderstanding. Here's the process:

1.) I shine the 200mW red, focused to smaller than a millimetre diameter at my matte black desk.

2.) Momentary reflection of that red.

3.) Roughly a nanosecond later, ALL of that red is being absorbed either by the material or the carbon emitted in the smoke (the latter causes incandescence). This means, NO coherent, monochromatic light, it means the full blackbody spectrum of frequencies, in completely random phase, with a divergence so large as to be irrelevant, etc, etc. Net result - the intensity recieved by my eye is not very far off a moderately diffuse reflection of the sun on a metal object. Meaning, no goggles needed.

You can also achieve this same incandescence with a jet lighter and a thin wooden branch. Or magnesium ribbon (superb).

I know what you're talking about , but it still is a bright spot on your field of view.I mean, it's still bright enough to give you vision spots that (I'm guessing) give you a hard time reading word off the screen for a little while.I dunno, I still try to keep safe from those stuffs. :-/

And I agree, burning Mg is kickass ;D It does look totaly like the white flash if I come to think about it.

Hehe, not really peeved by that dark spot. It's exactly like the spot you get from momentarily looking at a 60W light bulb, won't affect reading.

Anyway, in comparison, I spent a great deal of time etching the back of my hand...

Switch said:

Green/red, I don't think it matters that much, or you could say that 200mW of IR is nothing : You can still look at the white flash with goggles.

Still 200mJ seems very little, it better be focused to a super tiny spot and the pulse width has to be very short. I think a 1 second pulse of 200mJ can be focused to the tinyest spot you can , it still would be nothing. :-/

Click to expand...

Watts=Joules/second. The pulses from that laser are probably about a microsecond long (just guessing, but it seems resonable). Which gives about 200,000W. Give or take a factor of 100 based on pulse duration.

To breakdown air you need like 10^13 Wcm^-2. It says it is a Qswitched laser, so figure the pulses are on the order of 10 ns. For a 200 mJ pulse that is about 20 MW of power, so if it was focused to a spot of about 15 microns it will breakdown the air.


Edit:

I was wrong, you need only 10^11 wcm^-2 to breakdown air (with a several ns pulse). That means a spot size of about 150 microns with the above laser would do it.

Kenom said:

we've all commented how when a red laser is focused perfectly you get white light.  Nobody really knows what causes it but the most popular argument being that your eyes become over saturated with light.  Well, I was playing tonight and saw it even through my laser shades, so I figured I'd film it.  Here it is.

http://s270.photobucket.com/albums/jj105/kenkassidy/?action=view&current=flashes.flv

Click to expand...

This "flash" of light is a very simple phenomenon! you can obtain this with all high power laser, when they are focused to a spot that it's near to the wavelenght of the laser. The light actually is white/yellow and caused by the over-heating of the material hitted by laser: as example if you shine in this way a piece of wood, it became suddenly carbon which literally burns with oxygen in the air making this brigh light. You should repeat this experiment under a clear glass without oxygen(for example burning a match inside first), and you shouldn't see any "flash"(or very few).

But "burning" is the reaction between oxygen and something else , right? If we were to do this in an oxygen free enclosure , no burning would take place at all. :-/

Well, consider what happens when you heat wood in the absense of oxygen: it turns to charcoal rather than ash. It will still blacken, emit fumes, the whole deal basically.

But incandescense has nothing to do with burning, it would work perfectly well without any oxygen presents (a lightbulb proves that pretty much).

I was wrong, you need only 10^11 wcm^-2 to breakdown air (with a several ns pulse). That means a spot size of about 150 microns with the above laser would do it.

Click to expand...

Sounds reasonable... with very small beam diameters and pulse times, the momentary energy density can become very large without too much effort. I'm sure the effect can be demonstrated with tabletop equipment, at it makes a fascinating sight!