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Old 02-15-2010, 06:47 PM #1
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Default Spectral Reflections?

Hi all, i have a question.
If i have a green laser pointing from the top to a glass full of water, do these reflections in the water are dangerous with a 10mw laser?
Also, if anyone can clarify me the concept of spectral reflection it will be great.


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Old 02-15-2010, 06:58 PM #2
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Default Re: Spectral Reflections?

I think you mean specular reflection. A specular reflection is one from a reflective surface like a mirror or pane of glass. A diffuse reflection is one from a rougher surface like a painted wall or a piece of rubber. The surface of a glass of water or a piece of crumpled aluminum foil would produce specular reflections, but at 10mW the danger of injury is small.
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Old 02-15-2010, 07:01 PM #3
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Default Re: Spectral Reflections?

Reflectivity - Wikipedia, the free encyclopedia

EDIT: What EF said. Also, 10mW reflected off the surface of normal tap water should not pose eye danger, unless you stare directly into the reflection on purpose. You may want to tell us what "10mW laser" this is however, since some cheaper "5mW" and "10mW" lasers may be far more powerful than what they are rated as. For example, "5mW green laser pointers" from DealExtreme have been measured at powers exceeding 50mW.

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Old 02-16-2010, 01:46 AM #4
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Default Re: Spectral Reflections?

IMO be safe and spend the 30$ or w/e on goggles too protect your eyes!


and with a 10mW u could be safe with some 10$ ones, check the review sections and u will see a good review on some cheepo goggles
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Old 02-16-2010, 02:31 AM #5
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Default Re: Spectral Reflections?

My 50mw makes my eyes feel itchy when I play in the fog for too long.

no permanent damage, just discomfort.
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Old 02-16-2010, 02:41 AM #6
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Default Re: Spectral Reflections?

I'd wager it's more the fog than your laser..
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Old 02-16-2010, 02:45 AM #7
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Default Re: Spectral Reflections?

could be??
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Old 02-16-2010, 02:09 PM #8
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Default Re: Spectral Reflections?

Hello everybody, thanks fo the replies!.
The laser will be pointing to somekind of moving water, inside a translucent container with no cap, iīve made some tests at an expo last year with a 5mW and it seems fine, but i preffer your advices just in case iīm missing something (not my eyes or someone elses ;-)
So... any advices or experiences... what if the user touches the laser without any reflective rings, are those dangerous reflections in the handīs skin? the laser itīs one from aixis (15mW):

532nm 15mw TTL power adj. laser with DC power supply - eBay (item 380204312115 end time Feb-12-10 20:47:59 PST)

I donīt have a power meter to set it up to 5/10mw, i was planning to set the pot the lower i can, ... a power meter itīs about $200 and i canīt afford it right now plus shipping and customs its more than $330 (minimum) + the googles i think almost $400.

Oh my... this is an expensive and pretty dangerous hobby, but i love it!...
If you live in Buenos Aires and want to lend me a hand..., you are welcome to my house when they send me the laser. ;-)
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Old 02-16-2010, 02:19 PM #9
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Default Re: Spectral Reflections?

Quote:
Originally Posted by kiloko View Post
Hi all, i have a question.
If i have a green laser pointing from the top to a glass full of water, do these reflections in the water are dangerous with a 10mw laser?
Also, if anyone can clarify me the concept of spectral reflection it will be great.
spectral reflection is a reflection from a mirror like surface that preserves the laser beam with little distortion.

Diffuse reflection, aim the beam at a rough surface such as fine sandpaper, and you get the idea.

There is not a good explanation, other then experience, of where the line is between the two. It strongly depends on what the material is that is doing the diffusing,. It is possible to have a hazardous diffuse reflection, and in that case, you need to make measurements with a laser power meter to be sure. The line between class IIIB (500 mW) and Class IV 350 mW and up, is thought by many to be where diffuse reflections some distance away from the eye become hazards. In reality, you have to measure the distance to the eye and the power received.

As to your question, your looking at Fresnel reflections, and the strength of the reflection is proportional to the index of refraction of the water.. For lead glass, a similar material, the reflection is 4-5% of the incident power, so for 10 mW , would be less then .5 mW, or class IIA. Your blink reflex, will for the most part, protect you up to 5 mW or so, so the reflections will be painful, but not that hazardous.

This assumes your pointer is actually 10 mW, and the only way to know for sure is to measure it.

You can experience secondary effects though, where a unexpected bright reflection causes you to physically react, loose some muscle control, and do something like fall off a chair, or hit your head on a nearby file cabinet etc..

Your homework assignment is to read pages 1-360 of this:

http://www.army.mil/usapa/med/DR_pub...f/tbmed524.pdf

Steve
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Old 02-16-2010, 07:33 PM #10
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Default Re: Spectral Reflections?

WOW, amazing reply Steve, thank you for that paper, i am currently reading it, very good and informative. Highly recommended!
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Old 02-17-2010, 12:16 AM #11
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Default Re: Spectral Reflections?

I suppose the main difference between mirror reflection and specular reflection is just intent, as far as the terms are used in laser safety. As we all know a good specular reflection is nothing short of a bad mirror, with people using store windows to help comb their hair and all.

Creating a hazardous situation with just 10 mW would be far fetched though - it would require over half of the lasers output to end up in someones eye. When pointing at things that are not intended as mirrors, the chances of that are very slim.

If this is a public display you might still have to take precautions though. While the chances of harmful exposure are minimal to none, it would still be very difficult to prove that someone was not hardmed by your installation if they get hit in the eye and seek damages for discomfort. A true laser burn could be assessed by an optometrist, but seeing spots for quite a while without any visible retinal evidence is not uncommon.
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Old 02-17-2010, 01:21 AM #12
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Default Re: Spectral Reflections?

Hmmm, you made me think

Is there a light, led, set of mirrors or something that ressemble a laser beam, of about 0.5 cm or 0.2 inches in diameter or so... the light will not travel too long, about 3meters or 9/10 foot

I know the laser is beautiful and unique in many ways (visually and physically) but just in case, to have a backup plan...

Any ideas?

How about the laser harp? i saw that it works with high power lasers
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Old 02-17-2010, 12:23 PM #13
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Default Re: Spectral Reflections?

If you want to keep it within an inch wide over 10 feet, you will need a laser to do so.

If its 10 mW, you could consider a beam expander, giving you an inch wide beam to begin with, which would be eye safe since its so large only small portion of it fits the pupil.

A beam expander will however make your laser even less divergent, so the obvious solution would be to have a beam stop somewhere.

Dont use a mirror to bounce back the beam into the laser in any case, this will readily destroy any diode laser, and might not be so good for a green dpss either.
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