So I was out walking the dog this morning when I noticed the brilliant spark of an arc welder at a construction site about half a mile away. It being rather early, I stared stupidly at the spark for a time before higher brain functions kicked-in and reminded me that looking at welding can hurt your eyes. But then it occurred to me that, like many harmful things, the retina-burning death waves of an arc welder must have a limited effective range. Otherwise, anyone anywhere with line-of-sight to any number of construction projects would be in danger, right?
Now, given that:[ul][li]I know essentially nothing about welding[*]I have enough physics under my belt to possess a passing familiarity with the inverse square law and understand terms like “wavelength” and “intensity”[/ul][/li]…for a “standard” arc welding rig, roughly how far away would I have to stand to be secure in the knowledge that I can enjoy the delightful sparklies without worrying about whether or not I’m burning the unprotected eyes right out of my head?
Pages like this one from the National AG Safety Database or this one from the Consumer Product Safety Commission make ominous pronouncements like these:
So one leads me to believe that, unless you’re under the horizon, you aren’t safe. The other takes the less extreme- but far more vague- stance that the danger might reach you-- even at “quite a distance.”
Similarly, a search of the SDMB revealed this thread, which sort of gets around to what I’m asking, but then veers off towards the Vale of Vaguery once more. I’m not looking for a mathematical proof or anything, but an equation or two might be neat. Things like: How much power (in the optical sense) is given off by the spark? How much is needed to cause permanent eye damage? For how long?
Arc welding emits copious amounts of both long- and short-wave UV which can cause eye damage and something akin to sunburn. UV, like all EM emissions, obey the inverse square law which says that the energy density drops off with the square of the distance. So, at a half mile away, you got about 1/700,000th the UV exposure that the welder at about a foot away did. I wouldn’t worry about it.
I can’t pin down a distance, but I’d bet that you can get an eye burn at several hundred ft. It isn’t necessary to look directly at the arc either, the reflection can do it, especially off a light colored surface, like a wall, it also doen’t take very long, a second or two can do it. If you are exposed, you’ll know it in a short time, maybe an hour or less. Feels like you got a piece of gravel in your eye, definitely not pleasant and it lasts for hours, maybe even a day or more, depending on exposure.
This site give 50 feet as a possible ultra violet limit. If you want to be really safe, wearing a belt and suspenders, say 100 ft. Your half mile should not be a worry.
If eye damage can occur as far as 50 feet, why aren’t construction sites required to shield a welding site? If our dear OP could see from 1/2 mile away :eek: that means that people innocently strolling right by who glance over for a second would have a clear shot as well and might never know what hit them.
There are shielding requirements. My experiences have been in and around construction sites. Not in areas where the general public would normally be exposed. I suppose 50 ft. sounds reasonable but, having experienced the pain, I’m extra cautious when I spot arc welding going on.
One point to remember when thinking about this is that the fovea is very tiny, but very important. According to Wikipedia:
“The fovea is less than 1% of the retina but takes up over 50% of the visual cortex in the brain. The fovea sees only the central two degrees of the visual field, which is roughly equivalent to twice the width of your thumbnail at arm’s length.”
The problem with this should be clear: if we’re talking about gazing in the general vicinity of welding arcs, then it won’t be too big a deal, but if we stare straight at it, even if it only damages a very tiny part of the retina, it will be damaging the most important part. Try reading something while focusing an inch above/below the text to get an idea of how important this area is.
Unfortunately, it’s hard not to stare directly at a welding arc if one isn’t in the perceived danger zone.
Well I don’t know about shielding for construction sites in general, but it might assuage your concerns in this particular instance to know that this is construction in the mode of suburban sprawl and the sidewalks haven’t quite made it out that far as yet. We’re talking real “leave no open land undeveloped” Manifest Destiny-style development. They aren’t just building a mall-- they’re frontiersmen. The only people walking around with 100 feet of this place ought to know better than to stare.
…Anyhoo, thanks all for your replies. I’m off to enjoy some retina-safe welder watching. People have to have hobbies, right?
And that’s only the energy along a straight path. The UV from the source is going off in an omnidirectional bearing, so not only is it weaker, at 1/2 mile away, you’re getting a lot less of it into your eyes.
Dude, that’s not in addition to the inverse square law but the reason for it - the omnidirectional emissions are spread over the surface of a sphere of area 4pr[sup]2[/sup] if you’re at distance r from the point source. You’re getting (pupil area)/(sphere area) into your eye and (sphere area) is proportional to the square of the distance - hence the amount that’s getting into your eye is inversely proportional to the square of the distance.
If you’re close to the arc, don’t assume that shielding yourself from the direct path of the light is adequate. It reflects pretty darned well. This is coming from someone who learned, as a teenager, that a white tee shirt will reflect enough light up into a welding helmet to give your face a light sunburn and give you a little headache at the end of a long day of welding hay rings.
Another culprit for reflecting are shops inside metal buildings where the walls were insulated and the insulation has a white paper backing. That has the potential for making your eyes a little sore at end the end of the day even if you’re carefull to look away from the arc while someone else is doing the welding.