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08-06-2012, 12:53 PM #17
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KreAture
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Re: Size of a focused spot

Quote:
 Originally Posted by BShanahan14rulz I'd like to add my thoughts to this thread. Assuming a perfect lens, the size of the spot would be the size of the emitting area on the facet of the semiconductor for a diode-based laser.
I know this is an old thread and an old post, but I'd like to add a comment on this.
The emissive element size is only relevant for focusing to projection. that is projecting a picture of the emissive element. When focusing a "beam" on the other hand, the source element is irrelevant. Difraction limit together with Aperture, wavelength and focal length are the only contributors to the spot width formula. Simplified we can say d = (4*wl)/pi * (f/D) where wl is wavelength, f is focal length and D is diameter of unfocused beam at the lens.

This might really mean that the ideal position for the lens is at the spot infront of the LD where the beam has spread to the diameter of the lens. This gives maximum D. Then the lens should have a short focal length to reduce spot further. Finally short wavelength will focus more than long so a blue laser will give smaller spot than red.

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08-13-2012, 05:15 PM #18
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Re: Size of a focused spot

Quote:
 Originally Posted by KreAture When focusing a "beam" on the other hand, the source element is irrelevant.

Ever tried getting a beam from a fluorescent tube? If you have, you'd know this is utter bullsht.
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08-13-2012, 05:48 PM #19
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Re: Size of a focused spot

Quote:
 Originally Posted by Cyparagon Ever tried getting a beam from a fluorescent tube? If you have, you'd know this is utter bullsht.
I am talking about focusing a beam, and creating a single spot.
Not creating a beam from a emissive element.

Assuming you have a very wide element like a tube you will get a beam that has an odd shape, that's true, and your spot will also have a asymetric shape. However, the size of the lenses and original beam diameter determine the actual size of the spot. You run into other optical issues and at large enough lenses and focual ranges you even get rayleigh-scattering, but the formulas are simple and the actual emissive element size is not part of them.

No industrial lasercutter would ever dream of using those crappy 3-6mm dia lenses used in these toys we build here.

08-13-2012, 09:06 PM #20
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Re: Size of a focused spot

How come we use laser diodes? Why not LEDs? And how come you can focus lasers down to a spot and not LEDs?

08-13-2012, 09:14 PM #21
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Re: Size of a focused spot

Quote:
 Originally Posted by KreAture Can't you read? I am talking about focusing a beam, and creating a single spot. Not creating a beam from a emissive element.
They're pretty much the same thing anyway. If you focus the output from a fluorescent lamp, you cannot get anything you'd call a spot. You only get an image of the lamp.
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08-13-2012, 09:48 PM #22
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Re: Size of a focused spot

Quote:
 Originally Posted by Cyparagon They're pretty much the same thing anyway. If you focus the output from a fluorescent lamp, you cannot get anything you'd call a spot. You only get an image of the lamp.
You ned to collminate it first. If you don't all you do is project a picture of the lamp.

Focusing and Collimating
If you ignore wave optics and look at the ray optics you have your claim that you cannot get a smaller dot than the emissive element mentioned, and it's wrong. You can, but it will be limited by divergence and practical physics in lenses. You can however expand the beam to make it focus to a smaller point, or use a larger lense.

08-14-2012, 08:10 PM #23
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Re: Size of a focused spot

Quote:
 Originally Posted by KreAture I am talking about focusing a beam, and creating a single spot. Not creating a beam from a emissive element.
Quote:
 Originally Posted by KreAture You ned to collminate it first.
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08-14-2012, 09:52 PM #24
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Re: Size of a focused spot

btw, I never focus my lasers to beams. Instead, I focus to projection for a suitably long distance. It's easier for me.

08-19-2012, 09:45 AM #25
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Living Laser
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Re: Size of a focused spot

Quote:
 Originally Posted by BShanahan14rulz How come we use laser diodes? Why not LEDs? And how come you can focus lasers down to a spot and not LEDs?
^
Well from what I know LEDs, the same in flashlights will generate the light but not the heat. There are LED lasers out there with a variety of WL.

In Manufacturing Circuit board they often use 1 CO2 Laser (dont know exact power) and split the beam into alot of individual beams in the form of a Stensile and the beams are focused into fractions of microns (Depends on Circuit Board).

If your asking what is the minimum beam diameter possible and what would power be relative to as if a 10mm 50mW laser condensed into Microns smaller than a hair, the power stays the same put the density of it is increased. Possibly it can be relative to a 500mW -/+ if burning a e.g. Wood and see it burns at the same rate but different radius.

I have not tested any of this nor researched so I may not be accurate.
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Last edited by Living Laser; 08-19-2012 at 09:53 AM.

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