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Dave Johnson29
03-03-2010, 6:58 PM
Hi All,

Well, I have some serious forelock tugging and bowing and scraping to do. I got it wrong for brushed stainless steel. I decided to start a new thread as reflection is real with some materials.

As you can see from the new pics, brushed stainless steel does indeed reflect so be careful.

Obviously the rate of thermal conductivity of stainless is not great enough to absorb all of the energy and some is reflected.

It is obviously a lower amount of energy as you can see from the small hole in the pics, it barely charred the edges of the paper. But reflected energy, there is and enough to instantly burn a hole in the paper.

For the second pic, I removed the stainless and brought the focus up to burn the paper at low power and high speed and moved the circle off to the side a bit. I wanted to get a comparison between the diameters of the actual hole traced on to the stainless and the one burned by reflection. As you can see there is not much divergence.

The final test was to tape some paper to the underside of the glass in the lid. This produced a very faint scorch mark for the first pass. I did 5 more passes and third pic shows the pattern emerging. The beam is diffusing probably from the surface of the brushed stainless steel.

The interesting thing is the beam is not diverging much. The burn size is about double the actual circle size traced on to the stainless. That burn on the paper taped to the lid was made about 10" from the stainless surface.

With high thermal conductivity materials it seems reflection is not so much an issue, but low thermal conductivity, take GREAT care. The glass on the lid did get a little warm and I suspect that the scorch would have been a nasty burn on skin even on a single pass.

Darren Null
03-03-2010, 7:13 PM
Told you so. DOOOM! I said. BEWAAAAAARRRREEE the reflections I said.

Seriously, thanks for doing the experiment and posting the results- very informative and much more scientific than the guy who melted his lid. For the record, I think it's thermal conductivity + shinyness. I had some cardboard covered with shiny metallic plastic that I was burning some xmas boxes out of and I tried it shiny-side-up. Very briefly. I couldn't get through it from the back either. Should have learned from that, I suppose, but you have to try these things.

Scott Shepherd
03-03-2010, 7:38 PM
TOLD YOU SO TOOOOOOO!!!!!!!!!!!!!!!!!!!!!!!!!!

Free drinks at the bar for Darren and me tonight! WoooHoooo!

Take that Mythbusters :)

Okay, enough gloating for now. Did we mention we told you so yet?


Well, we told you so!

Dave Johnson29
03-03-2010, 7:54 PM
TOLD YOU SO TOOOOOOO!!!!!!!!!!!!!!!!!!!!!!!!!!


You guys, :)

I just got back from walking the dog and while on the walk I thought, "damn, I should have put 'let the told-ya-sos' begin," in that last posting

Ahhh, hindsight.

And hey, the Mythbusters don't bust everything either. :p

Ahhh well, the beer is cold and my dog loves me.

Scott Shepherd
03-03-2010, 7:57 PM
my dog loves me.

He doesn't have any say so in it. You won't give him the keys to the truck :)

It's not often we get to poke fun at you and have a laugh, so I'm glad it worked out that way :D

I'm liking that cold beer sound.....it's 8:00 here and quiting time, so I'll just tell them "put this on Dave's tab" tonight!

You know, if your Mom was around, she'd have told you not to do it, you'd put your eye out :)

Dave Johnson29
03-03-2010, 8:01 PM
Told you so. DOOOM! I said. BEWAAAAAARRRREEE the reflections I said.


Do not look into laser with remaining good eye. yeah, yeah, I know. :)

It really surprised me how little divergence there is in that 10" of travel though.

I am sure a shiny stainless surface would be even worse or better whichever way you look at it. It would behave much like a front surface mirror. The fine brushed stainless I used was food grade and had protective film on both sides so the surface was pretty pristine and shiny-ish.

Suffice to say I will be a little more cautious now. Although I do think I was pretty safe holding the footballs. :D

Jack Burton
03-03-2010, 8:01 PM
OK I'LL eat my words too! ;)

Cool result! Thanks!

Russ Shoe
03-03-2010, 8:20 PM
yep its true....in my short experience of lasering, was marking a SS flask with thermark.....and noticed a small flame next to my lens...must have bounced back to the plastic and started it a blaze...caught it fast enough though.......yikes!!!

Dave Johnson29
03-03-2010, 8:22 PM
He doesn't have any say so in it. You won't give him the keys to the truck :)


Ha! I know he loves me because when I open the front gate, he is the only one in the truck, keys in it and engine running.

I have noticed he does sit in the driver's seat when I am opening the gate, maybe he is getting the feel of things and one day will be gone as soon as the gate is opened. Maybe I had better start shutting the truck off and taking the keys, before getting out.

Time for beer number two. Put yours and Darren's on my tab, you can trust me. :eek::D

Darren Null
03-03-2010, 8:43 PM
I thought, "damn, I should have put 'let the told-ya-sos' begin," in that last posting
We got you covered there. :D:D:D

Suffice to say I will be a little more cautious now.
Then it's all worth it.

Although I do think I was pretty safe holding the footballs.
It's hard-to-predict invisible stuff that can make you blind. I shall be maintaining my position over here behind the sandbags. :D

OK I'LL eat my words too!
Told you so as well. BEEWAAAAAARRRREEEE the vibes of March, I said. Signs and portents, I said.

That's me gloated out. I shall allow myself 15 minutes of insufferable smugness, then it's back to business......

Paul Brinkmeyer
03-03-2010, 8:48 PM
This has been an eye opener. I would have never guessed.

Now who's going to try to hit the mirror left on the moon by NASA?:rolleyes:

Russ Shoe
03-03-2010, 8:57 PM
Oh, by the way, I was marking the inside of the flask, was curved in, (testing) thats when the plastic around the lens got all melty. On the outside of the flask, didnt seem to do anything, and of course I watched it like a hawk after that! Didnt damage anything that I use, just one of the slots for a different lens, which I dont have.


ya just never know

Martin Boekers
03-03-2010, 9:54 PM
OK, now ya got me thinking.

Maybe Dan can help with this.

Is a CO2 laser Infra red?

If so would you be able to see the beam with standard night vision googles?

If I had a set I'd try it.

Someone has to have a set and can try this for us. If it works that would be very interesting to watch the beam in action!


Marty

Darren Null
03-03-2010, 11:07 PM
If wikipedia is to be believed and I've got all my nanos and microns in the right place, night vision goggles usually inhabit the 'near-infrared' band which is 0.75-1.4µm; whereas CO2 lasers run at 10.6µm which is 'long wavelength infrared'.

You'd be able to see the beam with a thermal imaging camera, but those look a bit pricey.

Rodne Gold
03-03-2010, 11:38 PM
Not surprising you would get a "hot" reflection considering the mirrors on your laser are gold plated polished steel.

Joe Pelonio
03-03-2010, 11:45 PM
Hey, I posted years ago that I have two marks on the inside of my glass door from trying to cut mirror acrylic shiny side up without transfer tape on it.

Dave Johnson29
03-04-2010, 10:10 AM
Maybe Dan can help with this.

Is a CO2 laser Infra red?


Hi Marty,

Speaking of Dan, where is he? I thought he would be at the table with Jack and me and a good old plate of words each. :D:D

Yes it is Infrared and as Darren says, the wavelength is 10.6µm.

I am giving up on "what I know" ;):) comments, but it is doubtful you would see the raw beam with NV goggles but maybe ask here...

http://www.nationalinfrared.com/

Dan Hintz
03-04-2010, 10:19 AM
Don't know how I missed this thread other than it must have popped up between refreshes... I'll read it over when I get a chance, but from all appearances it looks like I was incorrect in my earlier assumptions.

If true, now I want to know how it happens and why...

More comments to follow after I read it through...

Scott Shepherd
03-04-2010, 10:23 AM
Dan, I can catch you up on it.....

Someone asked if the beam reflected.

Several people said it did. Darren, Joe, Me (told you so)!

We were ignored.

We were all in agreement we had seen posts and photos of it.

We were ignored again.

Dave nearly put his eye out proving we were right.

Darren, Joe, and Me told you so :D

There, that should pretty much bring you up to speed :p

Dan Hintz
03-04-2010, 10:50 AM
Dan, I can catch you up on it.....

Someone asked if the beam reflected.

Several people said it did. Darren, Joe, Me (told you so)!

We were ignored.

We were all in agreement we had seen posts and photos of it.

We were ignored again.

Dave nearly put his eye out proving we were right.

Darren, Joe, and Me told you so :D

There, that should pretty much bring you up to speed :p
LOL, smart ash :p I owe you a beer next time I'm in VA...

I've always said, I'm happy to admit when I'm wrong when the data proves it. In the absence of validated data, I can only base my decisions upon my education, my experience, and good old-fashioned intuition. In this case, the data was sketchy, and physics tells us under typical conditions this is not logical... obviously there are non-typical conditions we're not aware of. I'd now like to know what those are so I can make a better-educated estimation in the future.




To answer some earlier questions and to expound on previous info... CO2 lasers are generally in the 10.6um wavelength range and fiber setups are generally in the 1.4um range (it varies, but for our intents and purposes, that's good enough). A typical CCD sensor made from silicon may see a ghost of the fiber beam, but to really see it you'll need a sensor made from materials with a higher wavelength sensitivity... IR goggles (night-vision) uses a different detection method than CCDs, and is within the wavelength sweet spot for fiber, but not CO2.

I also wanted to clarify that heat capacity of the material is irrelevant... only its reflectivity to the 10.6um wavelength matters to how much of the beam is sent elsewhere. Remember, heat is a secondary characteristic of the beam when it hits a substrate that cannot reflect the wavelength of interest, not a primary one. If the substrate reflects that wavelength 100%, there is zero heating of the material. Reflectivity changes based upon the state of the substrate... melt it, and the reflectivity wavelengths shift or outright change completely. Solid Stainless has a very high reflectivity to 10.6um, less than 1%.

Mike Christen
03-05-2010, 7:51 AM
Hi Dan

I would hope that you also use Murphy's law in your calculations and analysis. :eek:

Dan Hintz
03-05-2010, 8:38 AM
I would hope that you also use Murphy's law in your calculations and analysis. :eek:
Never intentionally, but the little bugger somehow manages to get his paws on them anyway...

Bruce Boone
03-05-2010, 6:33 PM
I put some aluminum tape for duct work on the bottom of my carriage after one of my plastic guide wheels burned to a crisp from a reflection once. The tape works great. I made a new wheel on the CNC and haven't had any more issues.