You can drop the voltage quite a bit and they're still visible
44-46V was when the light became visible - but the individual LEDs were very easily distinguished - at 46-52V you could look directly at them without your retinas melting, but they were bright enough that you couldn't see the individual LEDs.
At 52-60V they used increasingly high current and were too bright to look at. No seriously, those were BRIGHT.
At 110-120V they make a lot of smoke and a brief flash reminiscent of the trinity test. They don't smell nice either. I suggest not doing this.
(Ps - I was just wondering at what voltage they died at, they were surprisingly stable up to 110V, then the current draw shot through the roof and that was the end of the test-piece.)
I'll get a low-voltage picture tonight and show a comparison at the various voltages: though I may need to put a welding visor in front of the camera