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Strong laser pointers - facts?

  1. Aug 2, 2009 #1
    Recently, i obtained a 100mW green laser pointer, and the thing gets me confused.

    On one side, the beam is bright enough to see from some distance at 90* angle to it, and the point is visible on a hillside some miles away.

    On the other hand, it doesn't burn anything, like advertisements often say. Even more, pointing it on a thermistor of a digital thermometer gives no temperature increase whatsoever.

    The beam looks like in the commercials, but the match-lighting part is wholly missing.

    So, are there lies somewhere, or am i missing some parameter?
    What really is and is not possible with these things?

    EDIT: Also, is it normal for the pointer to work only for about a minute at a time?

    At start, it gains brightness in discrete steps, over about a second, then work at full for about a minute, then becomes dimmer, in discrete steps again.
    Trying to lit it up right after gives only fractional power - it does not reach the highest steps.
    If you wait a few minutes, it works fine again.

    Is that a normal behavior?
     
    Last edited: Aug 2, 2009
  2. jcsd
  3. Aug 2, 2009 #2

    russ_watters

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    Sounds like maybe it gets hot and shuts down, but I don't know.

    Anyway...does 100 mw sound like a lot of heat to you? It is 1/1,000th of a normal light bulb and 1/10,000th of a hairdryer. No way it could light a match.
     
  4. Aug 2, 2009 #3

    negitron

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    Eh, think power density. This is a laser, so all that energy is concentrated on a few square mm. Also 100 mW is the optical power output, not the consumption of the device as a whole, as it is for light bulbs and hair dryers. A 100 W light bulb is only putting out about 20 W of luminous power.

    Yes, a properly-function 100 mW green laser should be able to ignite a match within a few seconds.
     
  5. Aug 2, 2009 #4

    mgb_phys

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    Possibly in the ads they don't use safety matches, they use lifeboat/survival matches
     
  6. Aug 2, 2009 #5

    negitron

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  7. Aug 2, 2009 #6

    Lok

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    Even a less powered laser would be able to ignite matches but you have to focus the light with optics. A good lens would do.

    A light bulb has a 2 % efficiency so it's a 2 W lightsource. But a lens only captures portion of it's light depending on distance so you can focus very little energy ( < 20mW ).

    Lasers are not that special at heating things, it's just easy to focus them.
     
  8. Aug 2, 2009 #7

    Lok

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    How much of it's light is reflected, thermistors have near infrared (heat) absorbing surfaces, light is usually neglected.
     
  9. Aug 2, 2009 #8
    I believe your laser has an infra-red filter.
     
  10. Aug 2, 2009 #9

    negitron

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    What are you talking about?
     
  11. Aug 2, 2009 #10

    mgb_phys

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    Most green lasers are frequency doubled near IR.
    Generally you diode pump a 1064nm laser with a 900nm diode and then frequency double that to give 532nm to replace an Argon ion.

    You have to be careful to filter all the 900nm, especially in low light fluorescence applications - all of which ahas got nothing to do with thermistors or lighting matches!
     
  12. Aug 2, 2009 #11
    To see just how powerful these laser pointers are, solve this problem:

    Someone at a distance R from you shines his laser pointer in your direction. How large must R be such that you can barely see the light (assume that you can just about see stars of magnitude 5)?
     
  13. Aug 3, 2009 #12
    No luck. I tried a lens, and was able to get the beam to a tiny point that made some polished-looking marks on black duct tape, but failed at lighting a match.

    Nothing like what they show in the video posted on this thread.

    It's about the size of the beam, black and polished. Does indeed reflects a lot of light.
     
  14. Aug 4, 2009 #13

    Lok

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    Last try. Color the match with a black ink or black soot ( harder to do ) or with graphite. And again the lens.

    If this does not work then I'll start a marketing firm as they really know what they are doing.
     
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