Beam reflection experiment

[Dave Johnson29]

Hi All,

Another thread got me thinking. Always a dangerous thing.

It was on the subject of a digital camera and beam reflection so I figured this experiment.

I cut a 1/2" diameter hole in a piece of paper. I used 5% power and max speed 42cm/sec.

I then placed a piece of flat aluminum at a 10-degree angle on the table and adjusted focus for where the circle would trace on it.

I used two 1/4" strips of wood on the aluminum to raise the piece of paper above the aluminum.

I made the circle 15/32" diameter so the beam in the next process would cut inside the 1/2" diameter hole and not burn the paper on the raised side of the aluminum. I'll skip much comment about the fire in the first test where I used the same 1/2" diameter hole for the next step.

I went to the max power 28W and slowest speed 1cm/sec and ran the 15/32" circle.

Now, any reflection from the aluminum would mark a trace on the under side of the paper on the down side of the 10-degree angle, no?

Pic 1 the set up.

Pic 2 the burn.

My case rests. The aluminum absorbed all of the beam, or more than enough if it for any reflection to mark the paper. Bear in mind it only took 5% power and max speed to cut the paper the first time round.

[Jack Burton]

You need to be on Mythbusters.

[Darren Null]

The aluminum absorbed all of the beam

Diffused the beam, might I suggest? Nice experiment; but it may be possible to damage your retinas with less energy than it takes to brown paper.

Very interesting experiment, and not the result I would have expected. But I'm not ready to poke my retinas over the parapet on the strength of it.

[Dave Johnson29]

You need to be on Mythbusters.

Thanks Jack,

Not in their league yet, but as Adam always likes to test things to destruction, I decided to up the ante. In case the angle of reflection at 10-degrees was too close to passing through the 1/2" hole and presenting a false conclusion I decided to advance the plan.

I reduced the smaller circle to 1/4" adjusted the position so the edges of the 1/2" and 1/4" circles almost touched on the down-angle side.

I went up to 30-degrees angle for the aluminum with focus set for the down side of 1/2" hole and still no reflection burn.

Did the same again for 45, same same.

Getting radical, at 60-degrees the beam caught the upside of the 1/2" hole and flames -- again. I stopped there.

[Jack Burton]

I agree about donating my eyes to science...

I find this as about definitive as possible that enclosure glass can not be etched by an errant beam.

[Scott Shepherd]

You can test all you want, but I know what I saw and it was photographs of someone with an Epilog that had messed up their cover. You could see it clearly.

I'm not a scientist, nor do I have the time or desire to experiment with it, but I know what I saw in that photo and it was a lid that had been etched by the reflection.

[Darren Null]

I remember that too, Scott. It could be that it was a 'perfect storm', but the top of the machine was well toasted.

[Dan Hintz]

Dave,

I applaud you effort... I've always been a doubter that the glass would be etched by an errant beam due to the distances involved, but never had much interest in trying out the experiment.

That said, I'm not quite clear on your setup, though it sounds like you're on the correct path for a good test, so I'll lay out my thoughts and you can verify if what I'm saying matches your setup.

1) You should be focusing on the aluminum plate.
2) The plate should be polished to a reasonably mirrored surface. If not, our low power lasers are going to be scattered entirely too much to make a difference at more than a few tenths of an inch (high power lasers have this issue because they melt the metal, and non-oxidized molten metal is very shiny).
3) If the paper is only 1/2" off of the aluminum, you need a much steeper angle than 10 degrees. 30 degrees would be just touching the edge of a 1/2" diameter circle, and 45 degrees would make it (mostly) use all of its power onto the paper (though at a very odd distribution we'll ignore here for the purpose of this test).
4) Crank up the power in increments of 10 and hit it for 1 second until you see a brown mark (or cause a fire).

Once you have that power value, realize the beam will continue to widen by the square of the distance to the cover glass, and I think you'll agree there's simply not enough power density to mark the glass.

EDIT: Steve... any links to those pictures and a clear story behind them? I know our lasers have the power to etch the glass, but the planets would have to align so perfectly to keep that beam confined enough to get the power density needed to do so... or, the beam could be moving [u]really[/y] slow to get the equivalent power density, but then the etching would be a wide swath. Unknown...

[Darren Null]

I've had a looky for the laser damage by reflection thread...no joy. I remember seeing the pic and thinking WHOA! Significant damage.

I did turn these up on my travels though:
http://www.sawmillcreek.org/showthre...lection+damage

All was going well the I noticed a light behind my #3 mirror.I paused the laser and found that I was burning a line across the #3 mirror access door. Apparently it was reflecting back through the lens and off the #4 mirror and just missing the #3 mirror.

http://www.sawmillcreek.org/showthre...lection+damage

I once tried what you're doing now... about a week later I noticed melted distortions on the undersite of the lid of the machine and reduced power. The laser had reflected off the stainless and melted the lid; it has also etched the lens!

I was engraving a surface with a slight curve, so when it was on the normal it reflected and etched the lens, when off axis it missed the lens and started melting the lid.

...both were attempting stainless steel.

[Dave Johnson29]

...both were attempting stainless steel.

I think i have some brushed stainless here. Back in a minute...

Well, maybe more than a minute as it's lunch time for me and the Black dog.

[Dave Johnson29]

Dan,

I am sure a polished surface would reflect some of the beam. I was trying for a real world use on say clear anodized aluminum. I will test the brushed stainless after lunch.

Check my second posting and I was focusing on the point at which the beam was hitting the aluminum on the down side of the angle. My focusing rod has a 1/8" spherical radius on it, so close enough to perfect. The upside of the angle would be out of focus (high) but the angle of the plate would be directing any beam reflection out towards the center of the hole in the paper.

Also the paper was supported on 1/4" wood above the aluminum plate, not half inch. That's why I set it up for 30 and 45 degree in case the Trigonometry was allowing the reflected beam to squeeze through the gap between the 15/32" hole and the 1/2" hole.

I have no desire to stall the laser in the same spot to see when or if it burns through after some duration.

I am satisfied that at 1/4" there is much less reflected energy than equal to 5% power at 42cm/sec that I used to burn the original hole. As you state the beam will widen by the square of the distance so even at 8" the amount of heat, if any is going to be negligible.

Hope the pic makes sense.

[Scott Shepherd]

Dan, I haven't seen the post either, but I did find a point several years ago when Dave joined the forum and asked about lasering a mirror and I responded back then about the post by the person about messing up their lid and I referenced they had posted photos of it. So there are a couple things there....

1) My memory is the same then as it is now, which is quite alarming considering what I have done to it in that amount of time.

2) It was a real photo I saw then and I'm not just imagining things.

I don't know if it was stainless or a mirror. I just recall seeing the photos, as does Darren.

[Dave Johnson29]

Update:

I can't find the brushed stainless. I saw it just a week or so back and for the life of me, I cannot remember exactly where. Not all that strange if you know how I squirrel away metal scraps when I find them.

The only other stainless I have here is perforated sheet and it has a very dull rolled finish. I will keep looking and report the test results when I can.

I am old I forget stuff.

[George D Gabert]

Dave

Worst case would be a mirror. Could you try using that instead of SS or Aluminum??

GDG

[Dave Johnson29]

Worst case would be a mirror. Could you try using that instead of SS or Aluminum??

George,

A back surface mirror is fine as the acrylic or glass absorbs all of the energy before getting to the reflective surface.

A front surface mirror will reflect because we use them for the beam delivery.