Green lasers: how much power is too much?

In summary: So there is no risk of IR radiation harming your eyes.In summary, the 5mW green lasers on dealextreme.com are powerful and dangerous if used improperly.
  • #1
dotancohen
106
1
I saw some green lasers on dealextreme recently, they have units that go up to 200 mW:
http://www.dealextreme.com/details.dx/sku.1997

This thing looks a bit unsafe to me! My intention is to show the daughters some stars here in our polluted skies, not much else. How much power is needed for that? How much is dangerous? I will be the only one handling the device, so I am not concerned with someone pointing the laser at their friends' eyes (I treat the tool like a firearm).

Thanks for any advice.Moderator note -- Laser pointers are dangerous; please use with extreme care. See warning.
 
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  • #2
Wouldn't it be easier, and more accurate, to take a field trip?
 
  • #3
The laser would be even more important then! There will be enough stars that they won't know to which one's I'm pointing!

The neighbours also like to bug me with "Where is *** now" and "which star is that bright one" so I think that a laser would be great for these instances.

Are the 5 mW lasers adequate? Would I be much happier with a 20 mW or even 50 mW model? Are they really that dangerous when used for their intended purpose?
 
  • #4
There is currently a nice special running at OPT, offering some decent binoculars, a green laser, and a red flashlight for $29.95 plus S&H. If you live in a light-polluted area, this would be a good kit to show people around. Some astronomy clubs are snapping up multiples of these kits as "outreach resources". I just ordered a kit for myself, despite the fact that I already have a nice pair of Nikon binos and some red flashlight/chart-lights. My neighbor's grand-daughters are very interested in nature, and the laser-pointer will be handy to show them around the sky.

http://www.optcorp.com/product.aspx?pid=10245
 
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  • #5
My 5 mW green laserpointer works fine, the beam is extremely bright. On cloudy days you can even shine on a cloud and with binos see a green spot on the cloud!

A few weeks ago I used it to point out the M51 galaxy to a friend who is too lazy to study a detailed finder's chart. I look through my binos and point with my laserpointer exactly at the spot where M51 is and then wait until the friend has the beam in his view. Then I point out a pattern of four stars that surround M51 which are visible in direct vision in the bino.

Then I tell in words where M51 is relative to these stars and I point with my laserpointer again at M51. Then I ask him to use his averted vision in order to see something that looks like an extremely faint cloud where M51 is supposed to be.
 
  • #6
Isn't 200mW approaching industrial cutting power?
 
  • #7
Pengwuino said:
Isn't 200mW approaching industrial cutting power?

If so it is at the very lower end. I have worked with 30W green Coherent lasers for ablating Si. I believe that the same family of lasers go up to 50W. This is in a 3mm beam, we focused the beam to 30Micrometer diameter to cut through Si Wafers.
 
  • #8
Integral said:
If so it is at the very lower end. I have worked with 30W green Coherent lasers for ablating Si. I believe that the same family of lasers go up to 50W. This is in a 3mm beam, we focused the beam to 30Micrometer diameter to cut through Si Wafers.

Yah it definitely wouldn't actually cut anything or engrave, but I imagine it's overkill for just a pointer. Actually looking at the rest of the site, it does seem quite powerful for a pointer.
 
  • #9
Warning, don't buy one of those newwish lasers. They may be rated at 200mW, but only 5-15mW is 532nm green, the rest is infrared caused by the lack of a inrared filter that can harm your eyes bigtime. I would just type in green laser pointer on ebay and get a 5mW one that will look the same only about 8$ shipped. Anything over 5mW is illegal in the USA now which is quite sad. But when you get it you can dissasemble it and turn the potentiometer in it to squeeze out some more juice. But beware as you can burn the diode out easily. Any more questions let me know.
 
  • #10
Thanks, guys. As I understand it green is not a good cutting wavelength. Red might be, though.

Bass, can you tell me more about the infrared filters? I see the infrared warnings, but I do not really understand them. Can I purchase a laser with an IR filter? I don't see them anywhere.

Thanks!
 
  • #11
I see that some of the lasers warn that they have no IR filters. But at a fixed 532nm wavelength, what IR is there?
 
  • #12
The laser work with dpss technology. Double the wavelength is shot into a crystal that is halved to create the green. A lot of IR gets through and a lens with a special coating is used to filter it out. If you really want a good laser that is very very quality then try wickedlasers. You will see the price difference for a quality laser with pure green power is a big difference.
 
  • #13
I did look at their site: there is an order of magnitude difference in price.
 
  • #14
One thing you don't have to worry about is the recoil when you switch the laser on. :smile:
 
  • #16
How is a powerfull green laser going to help you show your daughters stars? you plan on pointing out the star with the laser? Dosen't quite work that way.
 
  • #17
flatmaster said:
How is a powerfull green laser going to help you show your daughters stars? you plan on pointing out the star with the laser? Dosen't quite work that way.
Have you ever used a laser pointer at night? The laser's beam is visible, especially from the rear, and when you give the name of a star and point to it with the laser, It allows the kid to get a point of reference, and reinforce it with a look at a star-chart. They are very handy tools for getting people familiar with the night sky.
 
  • #18
I stand corrected
 
  • #19
Actually the pointer and observer see the laser beam off till a certain height. If you do point it at a star, the observer will actually see what star you are pointing too. That is if the observer is near the pointer.
 
  • #20
bassplayer142 said:
Actually the pointer and observer see the laser beam off till a certain height. If you do point it at a star, the observer will actually see what star you are pointing too. That is if the observer is near the pointer.
Yes, that is trivial. It is a whole lot more effective than pointing your finger, though, and if you have the observer stand in back of you, the accuracy of the pointing is remarkable.
 
  • #21
Count Iblis said:
One thing you don't have to worry about is the recoil when you switch the laser on. :smile:

Actually, knowing the wattage and wavelength, that would be an easy calculation. I don't have a pencil on me though.
 
  • #22
F = (p * hbar)/wavelength? That's just in my head.
 
  • #23
5mW is pretty low power. On a regular night the beam will be very dim. I suggest either buying ~15mW or pot modding the cheap one (best value).
 
  • #24
For light:

momentum = energy/c ---------->

force = power/c
 
  • #25
> you plan on pointing out the star
> with the laser?

Yes, I have done that in the past with a 50 mW green laser. That is what got me interested in the idea on the first place. It is also why I question whether a device an order of magnitude less powerful is adequate.


> Dosen't quite work that way.

In your experience, then, how does it work?
 
  • #26
I believe those ratings are in milliwatts, not megawatts. Metal cutting is not an issue in that case. You may be able to see reflections off clouds, but not off the moon, or [cough] stars or galaxies.
 
  • #27
wickedlasers is way to expensive for me. Does anyone know where I can get a 5-20 mW laser with an IR filter for a reasonable price? Thanks!
 
  • #28
Count Iblis said:
For light:

momentum = energy/c ---------->

force = power/c

That's what I get too after working it.

So the recoil on the 200mW laser is approx 10^-11 N
Not much to worry about.
 
  • #29
Warning: When using laser pointers in the sky to assist with astronomy, one should ensure that the area of sky the laser is shone into is clear of any aeroplanes or other vehicles. In some places it is illegal to shine a laser pointer at aircraft, either intentionally or unintentionally. In all cases, care should be taken when using such an instrument.
 
  • #30
Naturally, I would take into consideration aircraft just as I would take into consideration buildings or other occupied structures when aiming the device. I appreciate the warning, and vow to abide by it.
 
  • #31
dotancohen said:
Naturally, I would take into consideration aircraft just as I would take into consideration buildings or other occupied structures when aiming the device. I appreciate the warning, and vow to abide by it.
And stick with the low-powered laser pointers sold by astronomy supply-houses. They do a good job. You should realize that any kids seeing that neat pointer will want to play with it, so never leave it unattended.
 
  • #32
cristo said:
Warning: When using laser pointers in the sky to assist with astronomy, one should ensure that the area of sky the laser is shone into is clear of any aeroplanes or other vehicles. In some places it is illegal to shine a laser pointer at aircraft, either intentionally or unintentionally. In all cases, care should be taken when using such an instrument.

:smile: reminds me of when my astronomy prof was pointing out a constellation and a few stars...

Oh yes and our university is right next to an international airport.
 
  • #33
This is a fun problem to solve. Suppose you are at the International Space Station and you overfly some country at the night in the Earth's shade (so the Space Station is not lit by sunlight and is thus not visible from the ground). Then you shine your 5 mW green laserpointer toward the ground at some person looking in the direction of you. How bright will the lightsource be to the person in magnitudes?
 
  • #34
laser_fog.jpg


This is a red laser rated at 5mW.

No idea how the green one will work, but with a red one you may forget about using it as a pointer.

To take this picture I had to put the laser directly above the camera, so that the picture is taken almost exactly in the line of laser beam. Otherwise - if you look from the side - beam is invisible.

Then it was possible to take this picture only because there was a light haze/fog - and the beam had something to disperse on. However, that means that the beam is visible when stars are not visible - and vice versa :-p
 
  • #35
Looks like someones house is about to be bombed...
 

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