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At what power will the beam damage your eyes?

KMitch

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I like to use my 3W 445nm Sanwu outdoors at golf courses very late at night when no one is around. I aim at the tree line, far away from people or reflective objects several hundred feet away. I love to look at the beam outdoors without eye protection, but of course I never do this indoors or when I'm pointing at something unknown. The intensity of just the beam itself from my 3W blue got me thinking though...at what point does looking at the beam, not the dot, damage your eyes?

Ignoring fog or smoke, we can see the beam of a laser due to Rayleigh scattering off air molecules, and once scattered, the laser light becomes incoherent and subject to the standard inverse square law. That's why the beam becomes invisible when I'm standing orthogonal to it at about 50 feet away. Given that the dot of a green laser is visible in daytime at 1 mW, and that the beam doesn't become visible until at least 1000mW, I imagine that it would take quite a bit of power for the beam to actually damage your eyes.

Also, since the incoherent nature of light scattered off air molecules makes the distance from the beam and the angle of incidence extremely important, let's assume a typical scenario with the laser 1 foot (30 cm) from your eyes, and the angle of viewing at 0 degrees (looking down the beam). To solve this problem, I'm guessing that we'd need to use the density of air, the power density of the laser beam, collision theory for scattering off air molecules, and the inverse square law. I'm probably missing a few things and maybe stated something incorrectly. If anyone has equations for this already, or even a table with damage thresholds (for the beam, not that common hazard table we all know) that would save me the trouble of trying to calculate this for myself. Any ideas?
 
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Why is 1 foot typical? More typical would be viewing the beam at nearly arms length or somewhat bent arm since a bent arm is more comfortable than a fully extended arm. I was at a laser show at a shopping mall many years ago. Inbetween shows one 25W laser was left on. The color was cyan. The minimum perpendicular distance one could approach was about 10 feet, at that distance the beam caused no visual difficulties.
 
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KMitch

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I just chose a conservative number, maybe not "typical" I suppose. Sometimes I might kneel down, balance my arm on my knee, and keep the laser more stable that way if I'm aiming at the tree line far away. Here, I might be a foot away from the laser. I really need to buy or build a tripod :shhh: Thanks for sharing the story about the 25W cyan, that at least helps to give me some idea.
 

Encap

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I like to use my 3W 445nm Sanwu outdoors at golf courses very late at night when no one is around. I aim at the tree line, far away from people or reflective objects several hundred feet away. I love to look at the beam outdoors without eye protection, but of course I never do this indoors or when I'm pointing at something unknown. The intensity of just the beam itself from my 3W blue got me thinking though...at what point does looking at the beam, not the dot, damage your eyes?
Any ideas?

445nm laser dot and beam exposure concern is not just about thermal damage hazard--don't forget about possible "Blue Light Hazard" with a 445nm --
"Blue-light Hazard is defined as the potential for a photochemical induced retinal injury resulting from radiation exposure at wavelengths primarily between 400 nm and 500 nm. The mechanisms for photochemical induced retinal injury are caused by the absorption of light by photoreceptors in the eye. Under normal conditions when light hits a photoreceptor, the cell bleaches and becomes useless until it has recovered through a metabolic process called the visual cycle. Absorption of blue light, however, has been shown to cause a reversal of the process where cells become unbleached and responsive again to light before it is ready. This greatly increases the potential for oxidative damage. By this mechanism, some biological tissues such as skin, the lens of the eye, and in particular the retina may show irreversible changes induced by prolonged exposure to moderate levels of UV radiation and short-wavelength light."

There is a good 2013 article on Blue Light Hazard here: http://www.crizalusa.com/content/dam/crizal/us/en/pdf/blue-light/Blue-Light-Roundtable_White-Paper.pdf


:can:
 
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I can't rep you again yet Encap, but that's good information.

From the article:

Short wavelength visible light, the spectrum from 380 to 500 nm that includes violet, indigo, blue, and some blue-green light, plays a paradoxical role in health and vision. Not only is blue light essential for color vision, recent research has found that light in this band triggers critical physiological responses, including pupil constriction and circadian rhythm synchronization. However, blue light may also be damaging to the eye, and the term “blue light hazard” has been coined to describe the danger this light presents to critical structures within the eye. Blue light can induce formation of toxic reactive oxygen species that cause photochemical damage, leading to the death by apoptosis first of critical retinal pigment epithelial (RPE) cells and then photoreceptors. This slow process, in which damage accumulates over a lifetime, has been implicated in the pathogenesis of retinal degenerative diseases such as age-related macular degeneration (AMD).

So it looks like it's not a problem for casual, safe, viewing of the beam, but looking at the spot and thinking " that didn't hurt me " may not be correct, it could add up over time and cause an acceleration of natural age related vision degradation.

I would think the beam in the air is pretty safe, I know when a raindrop hits the beam and I get that bright flash I think, DAMNIT, but the exposure duration is very short, still I feel like it's possibly not good, but you can record those pretty beams and watch them on your pc, that is safe and I love it.

 
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Encap

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I can't rep you again yet Encap, but that's good information.
So it looks like it's not a problem for casual, safe, viewing of the beam, but looking at the spot and thinking " that didn't hurt me " may not be correct, it could add up over time and cause an acceleration of natural age related vision degradation.

I would think the beam in the air is pretty safe, I know when a raindrop hits the beam and I get that bright flash I think, DAMNIT, but the exposure duration is very short, still I feel like it's possibly not good, but you can record those pretty beams and watch them on your pc, that is safe and I love it.

Thanks RedCowboy. That is a good article albeit a basic one covering fundamental ground for people who have no prior knowledge of the subject.

The article is a good comprehensive overview of current understanding.--Probably should be posted as a new thread to get the information to a broader LPF audience with so many members building and playing with 405nm and 445nm lasers --they should know of the "hazard" and it's effects.

As interesting as the visual blue light hazard can be, I found the section on non-visual function "blue light hazard" intriguing. In particular the section on newly uncovered third type of photoreceptor cell in addition to rods and cone which I am certain most people here have never heard of.

From the article:
Blue light is Vital for life
"These non-visual functions depend on a newly discovered third photoreceptortype that exists along with the rods and cones.
Called intrinsically photosensitive retinal ganglion cells (ipRGCs), these cells contain melanopsin, a photopigment, and, unlike cone cells, they are not concentrated in the fovea. Instead ipRGCs form a photoreceptive network broadly across the inner retina. Because melanopsin is so important to the daily resetting of our biological clocks, the absorption spectrum of melanopsin is sometimes called the chronobiological spectral band. This band peaks at about 480 nm, within the blue range. The ipRGC response to light in thechronobiological band regulates many non-visual physiologic functions in the human body, including circadian entrainment, melatonin regulation, pupillary light reflex, cognitive performance, mood, locomotor activity, memory, and body temperature. Studies have shown that pupil constriction, the eye’s natural defense against exposure to strong light, is wavelength-dependent and peaks at 480 nm.

The exact physiology by which ipRGCs control these functions have not been fully elucidated."

:can:
 
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Benm

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Looking into a beam poiting away from you generally is not dangerous, unless you have very specific conditions that can reflect the beam right back at you (perhaps snowfall).

You'd think there would be some power point where the backscatter gets high enough to be dangerous in 'normal air', but there is also the dielectric breakdown of air where a laser causes a spark in mid air that actually isn't that dangerous to look at.

With continous output lasers such output levels are unlikely to be ever reached, but with q-switched YAGs the air breakdown is certainly possible. It's quite mesmerizing to see a spark form in mid air like a tiny bolt of lightning, but not really dangerous to look at, though some UV protective sunglasses would be a wise precaution.
 

KMitch

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Thanks everyone for posting a ton of very useful information :gj:
 
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This may be too much to be safe, probably why we have not seen a lot done with it.




I may be getting off topic, but this is cool, and as the light energy is spread out safe too.


 
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dennyf

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I have a 532nm 500mW laser that I use out in the country. The trees are at least 80yds away. I love to look a the beam at night and try not to point at closer trees. With that said the beam reflected off a power line and shortly after that I started seeing bright spots in my periphery vision. I put the laser away for a month and my vision got better. Even though I though I was being careful it did it again. If I put the glasses on it defeats the purpose of seeing the beam. So in summary, I did damage my eyes temporarily. I hope this helps someone else.
 

Razako

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I have a 532nm 500mW laser that I use out in the country. The trees are at least 80yds away. I love to look a the beam at night and try not to point at closer trees. With that said the beam reflected off a power line and shortly after that I started seeing bright spots in my periphery vision. I put the laser away for a month and my vision got better. Even though I though I was being careful it did it again. If I put the glasses on it defeats the purpose of seeing the beam. So in summary, I did damage my eyes temporarily. I hope this helps someone else.
How far away was the power line? You're probably completely fine in all honestly.
 

CurtisOliver

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It is better to be cautious than not. But it probably wouldn't of been any worse than looking at a lightbulb. Just be careful.
 

dennyf

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The power line was about 25 feet away. I did shine the laser on some trees about 50 yds, but quickly moved away. All I know is after playing with the laser I started seeing dots; can't be good for my eyes.
 

diachi

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The power line was about 25 feet away. I did shine the laser on some trees about 50 yds, but quickly moved away. All I know is after playing with the laser I started seeing dots; can't be good for my eyes.

Just flash blindness (Like when you look directly at a lightbulb, camera flash etc), diffuse reflections from that distance at that power won't cause permanent damage. Obviously not something to keep doing all the time, but I've been closer (try 10ft on a white wall!) to 1W 532nm diffuse reflections than that plenty of times without issue (so far).
 

Andratos95

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I like to use my 3W 445nm Sanwu outdoors at golf courses very late at night when no one is around. I aim at the tree line, far away from people or reflective objects several hundred feet away. I love to look at the beam outdoors without eye protection, but of course I never do this indoors or when I'm pointing at something unknown. The intensity of just the beam itself from my 3W blue got me thinking though...at what point does looking at the beam, not the dot, damage your eyes?

Ignoring fog or smoke, we can see the beam of a laser due to Rayleigh scattering off air molecules, and once scattered, the laser light becomes incoherent and subject to the standard inverse square law. That's why the beam becomes invisible when I'm standing orthogonal to it at about 50 feet away. Given that the dot of a green laser is visible in daytime at 1 mW, and that the beam doesn't become visible until at least 1000mW, I imagine that it would take quite a bit of power for the beam to actually damage your eyes.

Also, since the incoherent nature of light scattered off air molecules makes the distance from the beam and the angle of incidence extremely important, let's assume a typical scenario with the laser 1 foot (30 cm) from your eyes, and the angle of viewing at 0 degrees (looking down the beam). To solve this problem, I'm guessing that we'd need to use the density of air, the power density of the laser beam, collision theory for scattering off air molecules, and the inverse square law. I'm probably missing a few things and maybe stated something incorrectly. If anyone has equations for this already, or even a table with damage thresholds (for the beam, not that common hazard table we all know) that would save me the trouble of trying to calculate this for myself. Any ideas?
I know this is old but... Have you tried doing it eventually?
 




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