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Thread: Red beam visibility.. how much power?

  1. #1
    Join Date
    Feb 2007
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    Default Red beam visibility.. how much power?

    I'm experimenting with an LD from a DVD burner. It puts out 100mW at about 140 mA. At this power, the beam is showing no signs of being visible.. which surprised me.

    For comparison, I have a green pointer from Atlas Nova. When I first switch it on, Mr. Lasercheck says it's doing 20 mW. That beam is clearly visible, and the spot is frankly dazzling.. can't look at it for any length of time. If I keep it turned on, the power drops pretty quickly, but the beam remains visible.

    How much red power would I need to get the experience of similar brightness? I have 5 times as much power as the green, but clearly it needs still more.

    Also, how much is the beam visibility affected by the collimation? I think my greenie has better beam quality than what I get from the red LD. I'm using one of those brass collimators from meredith instruments for the red.

  2. #2
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    Default

    You should see the beam in the darkness...
    The power density of your beam is important too. A 100mW 1mm diameter beam is more visible than the same power but 5mm. And the pointer beam diameter is small at the aperture, but I don't know about your red.

  3. #3
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    Quote Originally Posted by sbk View Post
    You should see the beam in the darkness...
    The power density of your beam is important too. A 100mW 1mm diameter beam is more visible than the same power but 5mm. And the pointer beam diameter is small at the aperture, but I don't know about your red.
    Maybe as you say, it's a power density problem. Perhaps I could try a collimator with a shorter focal length. The one I am using is 4.6mm, which is already pretty short though.

  4. #4
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    Cool

    Another thing to consider is that the diode might be lasing down around 670 nm rather than 650 nm. Either way, you're way down on the eye's sensitivity curve with regard to red.

    At 647 nm, you need roughly 5X red to balance your green. At 670 nm, you're going to need a lot more. (How much exactly? It depends on whom you ask. Anywhere from 9X to 16X.) Sam's laser FAQ has a chart that may help some.

    Then too, SBK's point about power density is crucial. If you've got 5 times as much red as you have green, but the red beam is 2x wider than the green beam, then the green beam is still going to appear brighter.

    And the bottom line is that color balance is very subjective. What looks balanced to you might look different to me or anyone else. So shoot for something that looks "good" and go with it.

    Adam

  5. #5
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    Just re-read the specs on my collimator. It says the beam is about 5mm dia. (Makes sense, because fl is almost 5mm). Can anyone recommend a source for a collimating lens with a much shorter focal length?

    I find plenty of lenses by searching with google, but they all seem to be rather expensive. Maybe I'll have to recycle one from a dvd player.

  6. #6
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    CAY033/040.
    Acrylic asphere, single lens. Fits a 4mm bore. Focal length 3.3 mm. Cheap. Lots of suppliers too, Google it's name, it should turn up lots of results.

  7. #7
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    Ahh that's the ticket!

    Lemme see if I can scrounge some of those from somewhere. Better get myself a 4mm drill as well.


    Thanks, Doc.

  8. #8
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    Well, I tried again, with a shorter lens, but still not much difference.

    If I put my head near the spot, and look back toward the laser, I can just make out the beam. If I put my head near the laser, and look towards the spot, I can't see it, and I can't see it from the side.

    Now I'm wondering if this is because red light scatters so much less than green that even with 5 times the power, I don't see the scattering. This would explain why I can see it when it's coming towards me, but not when it's moving away from me. Under the same conditions, the green 20 mW is clearly visible from any direction.

  9. #9
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    Could be. You're right about red scattering less. Try some glycol fluid fog, that has larger droplets than haze. Or fry something to eat. That makes haze AND (initially) larger droplets. Red shows up well with that.

    One of the things that makes it so hard to quantify the amount of red power to make good white light in beam shows is that particulate size in the air strongly affects scattering.

  10. #10
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    Quote Originally Posted by The_Doctor View Post
    Could be. You're right about red scattering less. Try some glycol fluid fog, that has larger droplets than haze. Or fry something to eat. That makes haze AND (initially) larger droplets. Red shows up well with that.

    One of the things that makes it so hard to quantify the amount of red power to make good white light in beam shows is that particulate size in the air strongly affects scattering.
    Oh man.. last night we had potatoes fried in pig fat. If only I'd had the presence of mind to bring my red laser into the kitchen.
    The potatoes were good.. but if I ever eat that again, my doctor will personally shoot me. It certainly made haze, and lots of droplets.. in fact, we're still cleaning them up.

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