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- Jun 5, 2012
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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.
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.
