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Old 12-02-2010, 02:06 PM #1
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Question Wavelength Interference Question

Alright, the way I learned it, the reason Flashlights Diverge as much as they do is because of the number of wavelengths present, and them interfering with one another caused them to "spread out". So I was thinking yesterday, why is it that Scanners work as well as they do. When you mix a bunch of lasers together, in aiming to get a color, why doesn't throwing multiple wavelengths together cause a flashlight-esque effect?

Is it merely due to the tight divergence of the products causing a tight enough beam that it is not apparent to a viewer? Would an RGB scanner set to project a White Beam have worse divergence than each of the lasers that contributes to the color mix?

Just a question I had. I have never owned a laser projector, so I cannot test it out, but it seems to me that mixing wavelengths would be detrimental somewhat to the overall specs of the resulting beam.

Thanks for any and all input on this matter,
Isaac


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Old 12-02-2010, 02:48 PM #2
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Default Re: Wavelength Interference Question

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Originally Posted by wannaburnstuff View Post
The reason Flashlights Diverge as much as they do is because of the number of wavelengths present
That's not right at all. If that were the case, a gel filter in front of the light would drastically improve the divergence.

It's because there's a tungsten filament heated to 3000C that emits light in all directions. You can reflect some of the light to go in the same direction, but the divergence is proportional to the emitting area.

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Last edited by Cyparagon; 12-02-2010 at 02:51 PM. Reason: Additional explicating
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Old 12-02-2010, 03:00 PM #3
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Default Re: Wavelength Interference Question

Damn all the misinformation out there!

So the Divergence in this case is proportional to the emmiting area? So if you had a smaller divergence you would have a smaller divergence?
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Old 12-03-2010, 09:56 PM #4
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Default Re: Wavelength Interference Question

I'm sure what you meant to say is that with a smaller emitting area, all else the same, you get lower divergence.

I have two Cree XR-E LEDs. One contains an EZ-Bright EZ1000 (1mmx1mm)and the other contains an EZ900 (0.9mmx0.9mm).

The EZ900 creates a noticeably smaller spot than the EZ1000 with the same reflector focused to optimal focus.

From my experience, what you say is true: with a smaller emitting area comes lower divergence.

This is probably why laser diodes do what they do. Instead of a square 1mmx1mm emitting area, it has an emitting area not much bigger than a bacterium. Thus, it would be much easier to collimate.
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Old 12-03-2010, 10:47 PM #5
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Default Re: Wavelength Interference Question

Quote:
Originally Posted by BShanahan14rulz View Post
I'm sure what you meant to say is that with a smaller emitting area, all else the same, you get lower divergence.

I have two Cree XR-E LEDs. One contains an EZ-Bright EZ1000 (1mmx1mm)and the other contains an EZ900 (0.9mmx0.9mm).

The EZ900 creates a noticeably smaller spot than the EZ1000 with the same reflector focused to optimal focus.

From my experience, what you say is true: with a smaller emitting area comes lower divergence.

This is probably why laser diodes do what they do. Instead of a square 1mmx1mm emitting area, it has an emitting area not much bigger than a bacterium. Thus, it would be much easier to collimate.
You're mixing up some ideas.

Flashlights, including LEDs, aren't coherent, so they're a whole different animal from laser diodes. The non-coherent light spreads much faster, in large part because it's not coherent. You have all kinds of other factors in play to increase the divergence, not just diffraction. Chromatic aberration is indeed a reason that it is more difficult to focus something like a flashlight to a smaller spot, but there's more to it.

As far as laser diodes, and anything else that is actually diffraction-limited, emitting area is actually inversely proportional to divergence. When you think of the emitting area of a laser diode, the divergence of the beam actually increases as the size of the emitting area decreases. Same reason why a beam expander decreases your divergence.

Keep reading, guys.
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Old 12-04-2010, 03:34 PM #6
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Default Re: Wavelength Interference Question

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Originally Posted by pullbangdead View Post
When you think of the emitting area of a laser diode, the divergence of the beam actually increases as the size of the emitting area decreases .
Before optics, yes. But flashlights are pretty crappy without optics, and so are laser diodes. A red DVD diode has both a lower divergence (after optics) and a smaller emitting area than a 445nm diode (after optics)
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Old 12-04-2010, 06:48 PM #7
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Default Re: Wavelength Interference Question

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Originally Posted by Cyparagon View Post
Before optics, yes. But flashlights are pretty crappy without optics, and so are laser diodes. A red DVD diode has both a lower divergence (after optics) and a smaller emitting area than a 445nm diode (after optics)
Of course when you bring multiple other factors into it it's no longer a simple relationship. Bringing the astigmatism, elliptical beam, multimode behavior, and optics into it changes the equation considerably. As it should.

As I said, they should keep reading, because there are indeed many more factors beyond the emission area. I was merely correcting the misconception about the relationship between emission area and divergence.
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