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12-09-2009, 01:25 PM #1
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icen
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Laser + Magnifying lens

Hey guys!

I was just wondering how to calculate laser beam power in mW when its going through magnifying lens (lens like 20x) it's cannot be that simple that if i take laser 20 mW and when it goes through lens 20x the power is 400 mw..

please could you help me calculate what the beam power will be if i take 20 mW laser and lens diameter 1,5cm with 20x magnification. Also how the power of beam is changing when the object is 5cm away from magnifying lens or 1 cm ?

Sorry for my English, that's not my first language and even not a second one :P

12-09-2009, 01:32 PM #2
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Re: Laser + Magnifying lens

I don't have the math for it, but I am willing to bet that if anything, it decreases it's power.
The only thing that I think it does is to possibly shrink the DIA of the beam.

Last edited by iskor12; 12-10-2009 at 02:28 AM.

12-09-2009, 02:29 PM #3
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Re: Laser + Magnifying lens

You are only decreasing the power by going though a medium like
a lens be it Glass or Plastic.

The power of a Laser does not increase going through a focusing
lens it just gets concentrated to a smaller area.. allowing it to burn...

Just like with the Sun and a magnifying glass.. you are just increasing
the power per area.... not the available power..

BTW...
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Last edited by lasersbee; 12-09-2009 at 02:32 PM.

12-09-2009, 03:56 PM #4
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Re: Laser + Magnifying lens

Well i understand that, but how do i know what kind of lens i need to make 20 mW laser burn anything ? how concentrated lens is making the beam ? And will 20x lens make the beam so concentrated so it could burn anything ?

12-09-2009, 04:15 PM #5
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Re: Laser + Magnifying lens

i think so...just get whatever ud like to burn in the focal point of the lens.

When u get the laser across a lens, the rays that (imagine just the outer ones to be parallel) were parallel to "bend" and start goin towards each other. At one point they will meet, and after that point they will stray away from each other, like an 'x'. U want to try to burn things in the intersection of those 2 rays...

if the lens are well made and u point right to the middle of the lens, maybe u can smoke some black leather or dark plastic like the ones for garbagge...but im not the power expert...Just try it that way!

12-10-2009, 06:35 AM #6
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Re: Laser + Magnifying lens

Quote:
 Originally Posted by icen Well i understand that, but how do i know what kind of lens i need to make 20 mW laser burn anything ? how concentrated lens is making the beam ? And will 20x lens make the beam so concentrated so it could burn anything ?
If burning is what you want, then you should buy this

High-Power 200mW Red Laser Pen (Burns Match + Visible Beam Daytime) [E0648] : BestOfferBuy.com, Buy DVD, Shop for PC accessories, Discount MP3 Players, Bargain Deal for Surveillance Equipment, Cheap R4 for NDS, X-sim Unlock

its not exactly 200mW, more like ~160mW~180mW. Also its red, witch is good for burning, because if you use a >100mW green your going to kill your eyes without goggles.

Quote:
 Originally Posted by icen Sorry for my English, that's not my first language and even not a second one :P
that's really badass, knowing 3 languages. What other languages do you speak?
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Last edited by SmurfTacular; 12-10-2009 at 06:37 AM.

01-03-2010, 11:59 AM #7
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Re: Laser + Magnifying lens

mW is an mW it just depends how small spot all the ligth goes
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01-07-2010, 11:38 PM #8
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Re: Laser + Magnifying lens

If basic optics theory is too much for you to handle, you're probably in the wrong hobby. May I suggest a lighter?
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01-08-2010, 05:42 AM #9
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Re: Laser + Magnifying lens

Quote:
 Originally Posted by icen Hey guys! I was just wondering how to calculate laser beam power in mW when its going through magnifying lens (lens like 20x) it's cannot be that simple that if i take laser 20 mW and when it goes through lens 20x the power is 400 mw.. please could you help me calculate what the beam power will be if i take 20 mW laser and lens diameter 1,5cm with 20x magnification. Also how the power of beam is changing when the object is 5cm away from magnifying lens or 1 cm ? Sorry for my English, that's not my first language and even not a second one :P
It wont be 400mw. mw is not a measurement of energy over space. Though a 20mw laser being focused down 20 time smaller than a 400mw "spot", the energy will be equal of both per x, if not exceeding the size of the 20mw scaled down 20 times.

I could be wrong with this, so anyone feel free to correct me if I'm wrong (not bash me...).

Thanks,
Moptsp
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01-08-2010, 05:53 AM #10
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Re: Laser + Magnifying lens

Quote:
 Originally Posted by Cyparagon If basic optics theory is too much for you to handle, you're probably in the wrong hobby. May I suggest a lighter?
WOW way to be a dick about it.

Oh, i forgot, you know every single fact about lasers and optics.

faggot.
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01-14-2010, 08:47 PM #11
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Re: Laser + Magnifying lens

@Moptst:
that's about it, the power in milliwatts is independent of the size of the beam, it's a measure for the total power in the complete beam.
But take a 20mW pointer and a 400mW pointer with the same beam diameter (!), then the 400mW pointer will have a 20x higher power DENSITY. A power density, as in watts per square meter, is just as it says: power divided by the area.
Focus a 20mW laser 20 times smaller, and it's power density in the spot will be the same as in the unfocussed 400mW. But of course this doesn't generate energy, remember that the 20mW laser focused has a 20x smaller dot. Because it's still 20mW, but in a smaller space, it's a 20x times higher power density.

01-15-2010, 12:57 AM #12
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Re: Laser + Magnifying lens

Quote:
 Originally Posted by Bluefan @Moptst: that's about it, the power in milliwatts is independent of the size of the beam, it's a measure for the total power in the complete beam. But take a 20mW pointer and a 400mW pointer with the same beam diameter (!), then the 400mW pointer will have a 20x higher power DENSITY. A power density, as in watts per square meter, is just as it says: power divided by the area. Focus a 20mW laser 20 times smaller, and it's power density in the spot will be the same as in the unfocussed 400mW. But of course this doesn't generate energy, remember that the 20mW laser focused has a 20x smaller dot. Because it's still 20mW, but in a smaller space, it's a 20x times higher power density.
Exactly.

-Moptsp
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01-22-2010, 01:56 AM #13
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Re: Laser + Magnifying lens

One should also note that focusing a laser down to an infinitesimal point is not an easy task.

Most lasers produce Gaussian beams that will be limited by their beam waist. If you start with a beam of parallel rays that is small in diameter your ability to focus will be very limited. If you want to focus to the tightest spot possible you should first expand the beam to a larger diameter then refocus using the converging lens with a short focal length to get the best results.
Feel free to use the math on the wiki page below as a reference.
Gaussian beam - Wikipedia, the free encyclopedia

01-22-2010, 08:28 AM #14
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Re: Laser + Magnifying lens

No one can focus it down to infinity. You would have to be infinitely accurate to do so. It is physically possible for it to be focus down to infinity, though a human wont be able to do it just as you can't count to infinity.

-Moptsp
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01-22-2010, 11:40 AM #15
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Re: Laser + Magnifying lens

Quote:
 Originally Posted by icen Hey guys! I was just wondering how to calculate laser beam power in mW when its going through magnifying lens (lens like 20x) it's cannot be that simple that if i take laser 20 mW and when it goes through lens 20x the power is 400 mw.. please could you help me calculate what the beam power will be if i take 20 mW laser and lens diameter 1,5cm with 20x magnification. Also how the power of beam is changing when the object is 5cm away from magnifying lens or 1 cm ? Sorry for my English, that's not my first language and even not a second one :P
I don't remember my optics equatiosn since I did poorly in that class, but you gotta get the wavelength of the laser, the focal length of the lens, whether or not the beam is collimated etc etc. But all focusing the beam does is increase power per area.

01-22-2010, 01:42 PM #16
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Re: Laser + Magnifying lens

Quote:
 Originally Posted by Dirac One should also note that focusing a laser down to an infinitesimal point is not an easy task. Most lasers produce Gaussian beams that will be limited by their beam waist. If you start with a beam of parallel rays that is small in diameter your ability to focus will be very limited. If you want to focus to the tightest spot possible you should first expand the beam to a larger diameter then refocus using the converging lens with a short focal length to get the best results. Feel free to use the math on the wiki page below as a reference. Gaussian beam - Wikipedia, the free encyclopedia
Nice try, but not entirely correct. The beam diameter is not an important aspect here, we're always using a beam diameter much larger than the mavelength and much larger than the beam waist we want to get.
Note that the wikipedia page is about a beam propagating in free space. It's not that clear about focussing such a beam. The beamwaist is determined by the wavelength, focal length and aperture/initial beamwaist (or Numerical Aperture to combine both) and the constant pi. The higher the NA, the smaller the beam waist, no need to expand the beam first. See the end of here.
So, just take a lens with a high NA that just a little bigger than the laser beam diameter (or you'll end up not using the high NA).

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