Scrolling light waves from LEDs

[epicsolo]

Hey there,

This is my first post because I can't find any information about this problem anywhere. We shot a scene in a room filled with fog lit by a blue LED rope light and in post I noticed horizontal waves scrolling vertically in the blue light. No other light source caused these waves. Anybody seen this before? Does this problem have a name?

Thanks for any help.

4K, 23.98fps

 

[Pietro Impagliazzo]

Flicker + Rolling shutter?

 

[epicsolo]

Sounds like that could be it. Anybody fixed something like this in post?

 

[Noah Kadner]

Kinda sounds like you live with it or reshoot. Or paint it out frame by frame...

Noah

 

[Brent J. Craig]

AC powered LED lights flicker on camera, and most of them don't flicker at the rate you would expect. For example, you usually can't dial out the flicker by using shutter speeds/ framerates that are safe for HMIs. Someone told me they flicker at half of the power frequency, so 30Hz in North America, but they actually seem more random than that.

Working mostly on TV commercials, this issue comes up every Christmas when people want us to shoot lovely trees decorated with LED lights. You either replace all of the lights with normal incandescent ones, shoot at a very low frame rate (3fps, for example) so you get lots of flashes on each frame, or find a way to power the LEDs with a DC source (without exploding them).

As far as fixing flicker in post, I have heard of it being done, but in expensive FX suites. Flicker plus fog would be quite difficult.

 

[jimhare]

I wonder if you shot 30 frames at 180º shutter if it wouldn't be there because of the 60Hz power.

 

[Antony M]

...or find a way to power the LEDs with a DC source (without exploding them).

I thought almost all LEDs were powered by 9-12V DC. Why do you think DC would "explode them" ?

If they are plugged into the mains, then the power will almost certainly go through a transfomer and rectifier to get it to DC first. If the circuit is not smoothed, then you would expect any flicker to be at the same freq as the mains supply, as it switches from each side of the rectifier.

 

[Brent J. Craig]

I thought almost all LEDs were powered by 9-12V DC. Why do you think DC would "explode them" ?

Because the cheap LED christmas lights and rope lights don't seem to be DC powered. If they were, why would they flicker? Sometimes LEDs are dimmed by rapidly cycling their power, but strings of LED lights are already too dim so that doesn't seem to be the answer.

Why would they explode? Because it seems like every time I try to build something with LEDs I explode a few.

If they are plugged into the mains, then the power will almost certainly go through a transfomer and rectifier to get it to DC first. If the circuit is not smoothed, then you would expect any flicker to be at the same freq as the mains supply, as it switches from each side of the rectifier.

Yup that's what you would expect but it is not what happens. I'm sure we'll be seeing Christmas decorations in stores any day now (it IS September) so buy some LED lights and see for yourself. They are pretty much unshootable.

 

[epicsolo]

I think we'll live with it... Thanks for the help though. There were a lot of different LEDs in the shoot and only one rope light caused the waves. Luckily it doesn't ruin the shot in my opinion. But if there were an easy fix, maybe I'd try it.

 

[Dan Hudgins]

DC

DC

If your lights are 120vac, you may be able to run them off reduced voltage DC, but DC at that voltage is dangerous.

You can make DC with a full wave bridge and a 330VDC photo-flash cap, when you rectify 120VAC into DC you get about 180VDC which can be lethal.

To reduce the voltage you can use a vari-AC or variac before the full wave bridge, start at zero volts and turn it up until the lights come on, go no higher since you may start a fire.

An isolation 120VAC transformer can be put before the VARIAC to isolate the ground to reduce the shock hazard somewhat, but if you tuch both rails of the DC power you will get at least a burn and shock, or even die. I tuched such a charged cap once, it was not pleasant. Observe the left hand in the pocket rule, NEVER TUCH A CIRCUIT WITH BOTH HANDS AT THE SAME TIME, always were rubber soled shoes and always keep your left hand in your pocket and where a leather jacket. That is to keep the discharge from going through your heart and stopping it if you get a shock.

Also if you make such a DC supply, put a bleed off power resistor accross the cap so it will self discharge when you unplug the variac. And put it in a metal box so people cannot poke their fingers at the wires. The use of such an item would invalidate your insurance so if someone gets killed on set you would be liable.