On the perils of LED lighting and video…

For the last couple of years, we’ve used LED Christmas lights in our sanctuary. Considering how many we have (hundreds), the electricity savings are probably non-trivial.

All our LED strings in the sanctuary are plugged into either a stage dimmer or, where a dimmer port was unavailable, an Elation UniBar hooked into an RC4 Magic wireless DMX receiver (with the transmitter wired into DMX up in the catwalks). This allows us to control the Christmas lights along with the rest of the theatrical lighting via the Hog. It’s a very nice setup.

The other day, when Frank was running the stream, he saw the Christmas lights were fading in and out in sequence, and called up to the Penalty Box (the plexiglass-wrapped area at the back of house where the lighting operator and worship producer sit) and asked them to quit playing with the lights. As it turns out, they weren’t and the lights were all on. Mysteriously, they were fading in and out in sequence on the wide shot camera. When we looked at them on one of the other remote cameras, everything looked normal.

Then it hit me. I went to the remote control on the wide camera and cranked down the shutter speed, and lo, the lights gradually came together until they were all on. This is what it looked like:

Most stage dimmers operate by switching the AC cycle on and off via pulse width modulation. LEDs then only show one half of the AC sine wave, making them strobe, effectively reproducing the square-wave pulses that are modulating the dimmers. What We were seeing on the cameras was a beat frequency of the camera’s shutter speed and the strobing of the LEDs. You don’t see this on incandescent lighting because of the thermal persistence of the filaments. But why were the lights cycling at different times? Each one was connected to a different dimmer circuit, and those circuits are spread among the three AC phases coming into the dimmer room (which has a monster 2000-amp breaker).

So, if you’re shooting video of anything that has LED lights in it, make sure your shutter speed is at 1/60, or the lights are going to start acting strangely.

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