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Old 06-07-2012, 03:31 AM #1
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Default Danger of scattered laser light?

I know that scattered laser dangers aren't mentioned until Class 3b. Basically, Class 3b is danger to your eyes from even scattered light, and danger to your skin from direct exposure to beam. Class 4 is danger to your skin and eyes from even scattered light.


But I'm wondering about the scattered (aka diffusely reflected) light dangers.

I have an HeNe 5mW Class 3b optics lab table-top laser. Now its 5mW rating is no different than that of a 5mW Class 3a laser pointer. Also, visually it appears no brighter than an ordinary laser pointer. It most certainly can NOT burn my hand, as the beam doesn't even feel hot. And I've not had any eye damage from looking at the scattered light from even white paper (which of course is highly reflective in terms of diffusely scattered light).

As for Class 4, I don't see how the scattered light can be a danger to skin.
You see I once had a full page sized (8.5 x 11 inch) fresnel lens. I would often take it out on sunny days to the beach where there was sand (so no fire hazards nearby) and would focus the beam to a usable spot (similar in diameter to some laser beams), which was just under 1cm in diameter if I remember correctly. Now at sealevel on a clear day the sun's intensity is about 1000W/m^2. After doing some math involving inch to metric conversions, it turns out the optical power that my fresnel lens was gathering from the sun was about 60 (which is 60,000mW). That's more than 10,000 times the power from a typical laser pointer, and if this beam was from a laser it would be rated well into the Class 4 range. Yet in spite of the potential dangers of looking at the "dot on the wall" (that is to say the scattered light) from a Class 4 laser, my eyes suffered no ill effects from my repeated use of burning holes into pieces of white paper, which just before they chared and darkened, would scatter the light and it looked VERY BRIGHT (similar to shining a flashlight with good batteries at the face from about 1 to 2 feet away, and in fact my face was about 1 to 2 feet away from the spot on the paper where the light was focused). And by the way, this was enough power to set fire to that sheet of white paper (not just char it) in about 1 second (if the wind conditions were just right, not too fast or too slow, at least, otherwise it would put a hole through the paper but make no open flame)! Yet I suffered no eye damage, despite the fact if this was a laser on the job putting out those SAME 60 watts, my boss would have REQUIRED me to wear special goggles to protect my eyes from the scattered light.

Furthermore, the scattered light danger of Class 4 lasers claims that you can suffer skin burns from the scattered light alone. Yet the paper (and various other things) that I would put in the beam just to experiment with the heat damage it would cause, I would do so by holding the fresnel lens in one hand, and the object to burn, melt, etc in the other. Some times my hand holding the object would be no farther from the focused spot of light on the object than about 2 inches away (doggone close to the spot). Yet I suffered no skin burns from the scattered light, and at most maybe felt a slight warmth, and in fact usually did NOT feel it at all (though if I saw a flame appear I would drop the object to avoid burns from the fire).

So it would seem that the legally required warnings on the warning labels on lasers about the dangers of said lasers lasers really are just over dramatizations, and in fact represent EXTREMELY UNLIKELY circumstances.

Basically with any optical device that puts out lots of power, the importan point is DO NOT let the direct beam enter your eye. The scattered beam is not going to be a danger to the eyes or a burn hazard to the skin except for the ABSOLUTELY MOST POWERFUL lasers (such as lasers that output hundreds of watts that may pose a blindness hazard from scattered light to the eyes, and lasers that output thousands of watts that may pose a burn hazard from cattered light to the skin).

Just because something is over 500mW does NOT mean that there is any eye or skin hazard from the scattered light.


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Old 06-07-2012, 03:45 AM #2
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Default Re: Danger of scattered laser light?

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Originally Posted by Videogamer555 View Post
Just because something is over 500mW does NOT mean that there is any eye or skin hazard from the scattered light.
That is absolutely correct. Only specular reflection, or a direct hit, truly represent any kind of danger with handheld lasers. Diffuse reflections from most surfaces, while they may appear extremely bright, are basically harmless.

I remember going through some mindnumbing math myself when I started playing with lasers.

I also experimented with various types of materials, diffuse reflections, and a laser power meter to confirm.

Ultimately the danger only lies with reflective surfaces.

The problem is most people have no clue what surfaces do represent a danger, and which do not. As such, and this being a beginner caution, and the use of laser safety goggles is always advocated.

As well as an overabundance of caution.

The fact still remains that the specular reflection of off something like regular window glass, is 5%+ and can be higher. Same holds true for a lot of glossy surfaces. When you're dealing with a 1W+ laser... even 5% is dangerous, and cause damage depending on other factors.

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Old 06-08-2012, 04:29 PM #3
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Default Re: Danger of scattered laser light?

Sigh. The blind (or soon to be) leading the blind.

Laser power classifications are meant to simplify the process of determining the danger level of lasers. Otherwise you'll have to go through the process of determining the exact conditions for which your laser is safe, which will require wavelength, power, collimation, reflectivity of the surface at specific wavelengths, duration of the exposure, whether the focused dot image is considered a point or extended source, distance to the spot, etc.

For example, what does 500mW mean? Does it mean 500mW concentrated over a beam the diameter of a quarter? Does it mean 500mW of UV light? 500mW of green? Exactly how "diffuse" is the piece of paper you're shining it off of? What about your skin's absorption coefficient at that particular wavelength? How far are you from the point? What angle?

People complain that they can't pop balloons or light matches and have to use a magnifying lens. That alone should show how laser power alone is not the sole determining factor in burning.

So are you then going to buy the ANSI Z136.1 laser safety guide for $100-$200 and run through all the equations as a damn laser safety officer just to prove that your laser, in some specific set of circumstances in your house, falls below the MPE? Or what? You're going to rely on your anecdotal evidence? Your finger didn't feel hot. The piece of paper you were using didn't ignite. The 405nm spot on the wall didn't appear that bright.

The ANSI "OD" rating is another system that is designed to make it simple to understand what safety goggle OD rating is needed for a particular powered laser. Otherwise, you're going to have to consult with a laser safety officer to determine the exact conditions that you're covered. The European EN 207 standard is different and has different ratings for different conditions such as pulse duration, CW, etc. The ANSI standard was purposely kept simple so people could just use it directly.

Videogamer555: the examples above are anecdotal and incongruent. For example, you're comparing concentrated sunlight to laser light in your Fresnel lens story. I'm sure you've been flash-blinded by sunlight reflecting off specular surfaces. So why aren't you permanently blinded when the power concentrations are far higher than your crummy "handheld" you bought from DealExtreme? Laser light is not sunlight. 60W of laser light off a diffuse surface? I'm diving for cover.

Your example with getting skin burns: have you suffered sunburns in the winter? Just because your laser isn't heating you up doesn't mean you can't suffer skin burns. It depends on distance, power, reflectivity of the surface, angle, and MPE.

InfinitusEquitas: you're equally as bad. So now all "handheld" lasers shining on diffuse surfaces are "safe"? What happens when someone makes a cool 5W "handheld" laser? What happens when the person is only 10cm from the spot? Don't make these kinds of statements. They're harmful. They're disinformative. If you've "done the math" then go through the math so people can see what your assumptions are.
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Old 06-08-2012, 05:19 PM #4
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Default Re: Danger of scattered laser light?

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Originally Posted by Bionic-Badger View Post
InfinitusEquitas: you're equally as bad. So now all "handheld" lasers shining on diffuse surfaces are "safe"? What happens when someone makes a cool 5W "handheld" laser? What happens when the person is only 10cm from the spot? Don't make these kinds of statements. They're harmful. They're disinformative. If you've "done the math" then go through the math so people can see what your assumptions are.
Depending on the surface in question... absolutely.

Good question, but we're not there yet. Nor is it a likely scenario, that someone spending over $500 won't do some minimal research.

Months ago... yes. For my own peace of mind. I also verified the results through experimentation.

It really comes down to the type of surface you are dealing with. You'll note I still advocate the use goggles with EVERY post I make
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