Les wrote:

Is there any rule of thumb on how close I can be and still safely watch
(not a casual glance) arc welding without any eye protection? Does
this distance vary by amperage or type (stick, wire, TIG, etc.)?

Seattle is a town with lots of shipyards. When you go by in a car or ferryboat
you see welding arcs, lots of 'em. The intensity of the light drops off as the
cube of the distance to the arc. Think about it this way: the arc puts off a
certain number of high energy photons, and the light heads off in a sphere whose
surface area is 4/3PI*r^3, and the light intensity at any radius is proportional
to the subtended entry aperture of the viewer, in this case your pupils.

My guess is you're completely safe at 100 feet, but I personally wouldn't stare
at an arc at that distance. From a mile, I'd stare at it a lot, like a star.

Leo Lichtman wrote:

The eye is just like a camera--the farther away the smaller the image, but
the brightness stays the same. By standing farther away, you are damaging a
smaller area on the retina. How large an image are you willing to damage?

The above statement is rubbish. Perhaps Leo has never looked at the stars, which
would indeed destroy retinal tissue if he were close enough, but to my knowledge
throughout human history nobody's eyes have been hurt looking at the stars,
except for the sun, which is too close. The sun is a bit more energetic than a
welding arc, though ..

GWE