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Laser question

  1. Sep 7, 2009 #1
    If using 314mW red laser and cylindrical optics to project a line 500mm x 2mm,
    will it have brightness as 1mW laser that projects a dot with r=1mm since the area difference?
    1000mm / (1mm^2*3.14)
    Would the projected line of that brightness be dangerous if bounced off the reflective surface and hit the eye since it would be less than 1mW hitting the eye? Would it be dangeous to watch it projected on nonreflective surface, without safety googles?
  2. jcsd
  3. Sep 7, 2009 #2


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    Pretty much, yes.

    For the first one, we have
    (314 mW / 500x2 mm^2) = 314/1000 mW/mm^2 = 0.314 mW/mm^2

    For the second case, it's
    (1 mW / π x 1 mm^2) = 1/3.14... mW/mm^2 = 0.318 mW/mm^2

    Since we wouldn't know the laser's power to 3 digits of accuracy, these can be considered to be equal.

    1 mW can potentially cause eye damage, but if I recall correctly one would normally blink or turn away in time before too much damage occurs at this power level.
    Disclaimer: do not intentionally test this idea!

    No, that would be safe.
  4. Sep 7, 2009 #3


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    350mW is dangerous - especially if this is a cheap import unstabilized laser pointer of unknown real power
    Even from an unreflective surface there is a danger of a reflection from a watch strap or a screw head while you are adjusting something
    If you are going to view this be careful, make sure everything is at waist height and that the laser itself and anything before the cylinder lens is in some sort of enclosure - even a cardboard box will help.
  5. Sep 8, 2009 #4


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    Good point. I was thinking in terms of the beam expanded to a 1000 mm^2 area, but there's always the danger that the beam gets focused down to a smaller spot, which would be hazardous.
  6. Sep 9, 2009 #5
    Thanks for the explanation and security warnings!
    I have another related question:

    This would be used in 3D scanner and on museum objects, to your experience what is the maximum mW rating of projected dot r=2mm that would not cause alternation of the surface/generate heat that could cause that? Also which power would not burn unprotected skin? When a laser protective glasses have that wavelength included in protection range, what is considered as max mW output it protects from?
  7. Sep 9, 2009 #6

    Andy Resnick

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    ITYM having the same *Intensity* (W/area). The brightness also has a component from the beam divergence, and thus may still be different for different lasers (brightness is a conserved quantity).

    Since your eye has a non-zero field of view, in order to make a plausible comparison, you must calculate the total optical power incident onto the pupil- the first laser, hitting a concave surface, may throw more power onto an eye than the second.

    Based on your questions, I would not recommend working around a ~350mW laser of any type without first taking a safety class.
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