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Old 10-05-2010, 12:52 AM #1
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Default Focusability vs. collimation

I have noticed that a lot of cheap non-focusable laser pointers seem to have relatively low divergence, so the dot stays pretty much the same size over all distances. I have one focusable laser, and I can never seem to get its divergence quite as good as the cheap ones. Now, it is 405nm, and I am aware that due to the wavelength, the dot appears to be much larger than it really is at long distances. However, I still notice that to someone standing close to the dot, it still gets bigger.

Some possible reasons for this:
1. I'm just not very good at focusing.
2. The threads of the adjustable lens aren't fine enough, making it difficult to focus accurately and hold the same position for a long time.
3. There is something about the optics or construction of fixed-focus lasers that collimates better than my adjustable lens possibly can.

I have a hunch that #3 is at least part of the reason, and that in general there is a tradeoff: when you get an adjustable focus laser, you can get it pretty well collimated but not quite as good as a typical fixed focus laser. I haven't read anything that mentions this, so I might just be crazy. Can anyone confirm this, or give another explanation?

Thanks


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Old 10-05-2010, 01:58 PM #2
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Default Re: Focusability vs. collimation

These cheap lasers, are you talking about greens, or other 405nm lasers? The greens are DPSS lasers, so they employ different optics, and the beam that leaves the aperture is much smaller than the beams from diode lasers. That said, the smaller the beam at the aperture, the worse the divergence. You can have a really fat beam at the aperture, and it won't diverge much at all. Divergence is also better for shorter wavelengths.

It really is hard to get lasers focused just right at infinity, especially if you're the only one who is there doing it. I tried rigging up a remote webcam to focus a 445nm laser at a distance, but that was pretty hard because it was so bright, and the lens didn't have a focus ring (it was half-threaded). I had to turn the thing on and off between focusing, and the webcam didn't help much at all. I got some decent, but not great, divergence.

On a focusable green I have "infinity" is about at the end of the focus range. I'm not even sure if it can actually reach infinity, but rather stops a little short, so it always diverges somewhat. It may be the same way with the lens in the Aixiz modules, since they're so close to the front of the diode.

As an experiment try having a buddy at a distance help you with the focus. It's really hard to focus anything to a point with just one person, at least at range. Try focusing at a target at least 30ft from you, and even farther if you can. Remember the goggles!
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Old 10-06-2010, 12:08 AM #3
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Default Re: Focusability vs. collimation

The cheap lasers are <1 mW dollar store red lasers. I guess it might be mostly because of the beam size, because those typically don't have very small beams, so it would be easier to focus them really far out and get low divergence. I will have to look into this more carefully.
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Old 10-06-2010, 01:09 PM #4
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Default Re: Focusability vs. collimation

Well, it also may depend on the lens too. Many of the dollar store lasers have lenses that are further away from the diode than the ones in, say, Aixiz modules. This may help the divergence as well, at the cost of reduced intensity since the light from the diode may not be entirely captured by the lens.
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Old 10-07-2010, 01:31 AM #5
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Default Re: Focusability vs. collimation

So would you say that usually, with careful adjustment, you can get a focusable laser to be as collimated as a (decent quality) fixed focus laser?
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