I had the chance to take a tour of Walker Manufacturing (home of the high-end commercial mowers) this week. This was my first chance to see electric welding equipment in action, so I took the opportunity to eyeball-jiggle at the bright welding spot. Sure enough, it's time-varying, but it turns out it's varying in two ways:

1. It fluctuates in brightness a lot, as you would expect when welding is taking place.
2. But it also flashes on and off rapidly, probably at 60Hz or 120Hz, which shows that its power is modulated by the A/C cycle.

Obviously, one shouldn't do this for too long; but actually, eyeball jiggling has the effect of reducing the amount of energy reaching any one spot on your retina. As long as you don't stare at it like a deer in the headlights, you'll be fine.

I also had the chance to see a heavy-duty laser cutter in action. It's cool! It takes large sheets of steel, which can be up to half an inch thick and weighing up to 2000 lbs, and it cuts shapes out of them as if they were butter! I was able to observe the bright spot right where the cutter is melting the steel, and sure enough, there was a regular variation in brightness. It seemed quite fast, maybe 200 or 300Hz, though it would need a photograph to allow one to measure the frequency. I'm pretty sure I wasn't seeing the laser itself, but variations in the temperature of the melting spot, maybe. The variations were fairly subtle.

It's always worth jigglin' at stuff - you never know what you might see!