LED stage lighting flickers in video

Post » Sat Oct 02, 2010 7:04 pm

We just shot a show at a local theater that, for the first time, used new LED lighting for "stage backdrop" tableau lighting.
The backdrop flickers noticeably in the video. The flickering is visible in the camera's display during the show -- though not to the naked eye -- as well as in the resulting video footage.

More theaters in town are about to go this route.
Anyone had experience with this problem -- and hopefully a solution -- yet?

We have two hypotheses we're trying to investigate:
(1)Changing our shutter speed might make a difference.
(2)The theater might be using their old AC dimmer-packs to control their new LED lighting, rather than switching to DC dimmer packs. And IF that's the case, then the alternating current packs might be producing the problem(?)
User avatar
gandalf
 
Posts: 479
Joined: Wed Feb 21, 2007 12:57 pm

Post » Sat Oct 02, 2010 3:16 pm

An interesting problem. After reading your post I just got off the phone with one of the technicians at Hollywood Lights who said that you can rule out the AC dimmer idea: LED lights use DC for dimming and cannot operate on AC dimmers.

His experience is that there is no consistency relative to flicker with LED lights. He points out that television studios routinely use LED lights with no adverse effect.

Hollywood Lights has installed LED lights in several large venues in the Seattle area and has had several reports of flicker. However, when a second video company comes in to shoot in the same venue there are no problems. One church they installed lights in reports that the house video equipment picks up flicker while a staff member who shot the same service experienced no flicker.

The technician suggests that there might be a difference in camera circuitry (I'm not sure what this would be,) in frame rate or in shutter speed.

So far our company has not experienced flicker and I hope that won't change. I'm interested to see what others bring to this discussion.

Jack

User avatar
Sanctum
 
Posts: 483
Joined: Sun Aug 20, 2006 2:29 am

Post » Sat Oct 02, 2010 9:20 am

I think it may be your shutter speed. Try shooting with a slower shutter. You may need a ND filter if the stage lighting is really bright.
User avatar
Farrah Barry
 
Posts: 517
Joined: Mon Dec 04, 2006 10:00 am

Post » Sat Oct 02, 2010 9:06 am

A quick test if you have the cam handy to test shutter speed is point it at a LCD monitor and adjust the shutter speed to see it it yields the same results as your footage
User avatar
Rudi Carter
 
Posts: 499
Joined: Fri Sep 01, 2006 5:09 pm

Post » Sat Oct 02, 2010 2:03 pm

LED lights don't flicker (I should say PROPERLY installed led's won't flicker). They operate on a direct current which does not fluctuate like ac lighting. You CAN however get LED's at different wavelengths....one of the reasons why you can get different colors. Given that... I would imagine that you can get clashes with shutter speed and LED wavelength.
User avatar
SEXY QUEEN
 
Posts: 460
Joined: Mon Aug 13, 2007 1:54 pm

Post » Sat Oct 02, 2010 3:19 pm

LED lights can and do operate at high frequencies. One dimming mechanism for LEDs uses PWM. The wavelength of the light is quite irrelevant to flicker problems.

Bob.
User avatar
Cat Haines
 
Posts: 447
Joined: Fri Oct 27, 2006 3:27 am

Post » Sat Oct 02, 2010 12:23 pm

Blink,

DC comes in two flavors -- pulsed (rectified) and filtered (smoothed).

If the packs output filtered DC, there shouldn't be any flicker at any shutter speed.

If the packs are not filtered, the effect is the same as square-wave AC, only with a DC bias referenced to a 0 volt (relative) baseline -- IOW, a strobe-light effect at 120hz, presuming full-wave bridge rectification on a 60hz circuit.

We used to build IR LED audio transmitters/receivers for fun. Got 'em working up to 30 ft. in a couple of instances in low ambient light. Running on 9v batteries, they worked great, but on DC adapters the buzz was so bad even heavy capacitive filtering didn't remove it completely. It's the rapid slew rate in LEDs that is the problem.

Edited for spelling.
User avatar
Brandi Norton
 
Posts: 490
Joined: Fri Feb 09, 2007 3:24 pm

Post » Sat Oct 02, 2010 1:04 pm

It's likely the problem is dues to interaction between the frequency of the LED lights and a CMOS imager.
Specifically which camera was used?

I've had this problem with fluro lights on dimmers. No way to get rid of it however it didn't show up us flicker rather as rolling banding.

Bob.
User avatar
Sxc-Mary
 
Posts: 501
Joined: Wed Aug 23, 2006 6:53 am

Post » Sat Oct 02, 2010 4:51 pm

"DC comes in two flavors -- pulsed (rectified) and filtered (smoothed)."

Well.... yes... and no

Pulsed supplies are smoothed in the end. A computer or a car power amplifier uses a pulsed supply but they can't operate on a pulsed dc current.. so it doesn't mean the output is pulsed... Pulsing is simply the method used in the power supply. It keeps one from having to use a massive transformer like they did in early high powered home stereo systems but it's not pulsed in the end because smoothing capacitors and filters are added.

If however you use a cheap power supply with something like a half wave rectifier and very small caps on the output, then you surely will get flicker.
User avatar
Charlie Ramsden
 
Posts: 529
Joined: Fri Jun 15, 2007 1:53 pm

Post » Sat Oct 02, 2010 2:51 pm

Hi,

LED lights DO flicker - on purpose!! Most (if not even all) professional LED driver circuits use PWM, to control the average optical output power. This is essential especially driving white LED's, to keep the color temperature constant, independent of the overall brighthess.

White LEDs (that are actually blue ones with a yellow scintillating phosphor on top) will certaily vary the color temperature (detectable by human eye) if you dim them linearly with adjustable current.

Even if it might sound surprising, primary color leds (R;G;B) also suffer from wavelength modulation versus operating current, even if the mechanism causing it is different.

So PWM is the way to go, and is usually anyhow the simpler (and cheaper) way to implement digitally a brightness control.

High(er) quality LED lighting systems use typically high(er) PWM frequencies, in the range of 500Hz and up to 5KHz, and even higher. Those systems should not pose any problems at normal shutter speeds (less than 1/100th of a second).

The only thing you can do yourself to approach the problem, is to choose a slower shutter speed, if that is possible - and a viable option.

Some of the earlier LED lights did not take this in consideration, they used very low PWM frequencies. Anything above 100Hz is normally flicker free for the human eye. For moving spotlights this PWM frequency should be at least 200Hz or even higher. This can still be problematic for electronic "eyes".

Some LED lighting electronics I have designed had a PWM that was deliberately set at 500.00Hz. This seems to work OK with normal digital cameras, as well as PAL (50Hz) video cameras. This has to do with the very precise PWM frequency, so the beat frequency (compared to 50i) is negligible...thus minimal no flickering is guaranteed.

Hope this sheds some light (pun intended) on the subject...

Christian
User avatar
Dorian Cozens
 
Posts: 505
Joined: Sat May 26, 2007 3:47 am

Post » Sat Oct 02, 2010 11:15 pm

"White LEDs (that are actually blue ones with a yellow scintillating phosphor on top)"

That's wavelength you're speaking of... not flicker. LED's are diodes built with a certain wavelength in mind to produce the different color of light. Every color has its own wavelength and if LED's were built with all the same wavelentgh then they would only emit one color.

Flicker on the other hand is accomplished by varying the voltage and or current. You can do this on purpose... or by accident with a cheap power supply
User avatar
Cat Haines
 
Posts: 447
Joined: Fri Oct 27, 2006 3:27 am

Post » Sat Oct 02, 2010 4:31 pm

"Anything above 100Hz is normally flicker free for the human eye."

And this may or may not be true depending on the application. Home lighting operates at 60Hz but you can't see them flicker with the naked eye. It takes the filament longer than 1/60 second (frequency is the inverse of time) to react so the flicker is more or less absorbed.
User avatar
Gill Mackin
 
Posts: 421
Joined: Sat Dec 16, 2006 3:58 pm

Post » Sat Oct 02, 2010 10:47 am

Not all "white" LED lighting uses "white" LEDs. Quite a lot use R,G,B LEDs to produce white or any color you want.

The mains powered LED light I have use unfiltered full wave rectified mains. They will flicker very badly at 100 or 120Hz. The problem is probably worse than iron ballasted fluro light. With fluros the phoshors have some persistance so the light doesn't entirely turn off during the zero crossing. LED lighting could be much worse, especially the cheap lights that use large numbers of low power LEDs. The more expensive ones using LEDs by Cree or Lumileds use constant current drivers fed from well filtered supplies. They do not flicker.

Bob.
User avatar
M!KkI
 
Posts: 534
Joined: Sun Jul 16, 2006 1:50 am

Post » Sat Oct 02, 2010 1:52 pm

My point is here that some lighting armatures uses "white" LEDs (not RGB matrixes) that are blue leds with a yellow phosphor. These together produce a somehat strange spectrum, a broader peak around yellow, and a narrow blue. This is perceived by the human eye as white, since all receptors (R;G;B) in your retina gets exitation. The color rendering might not be perfect, but anyhow. So far we are not talking about flickering at all.

Now - the problem is that if you would like to vary the light output of such a white LED, you will have a noticeable shift in the color temperature - if you do it by just adjusting a DC current flowing into the LED. The mechanism causing it is outside the scope of this discussion.

Therefore such LED lighting armatures that uses white LEDs (blue with the phosphor) normally drive the LEDs with a constant high current, but PWM modulated. If the brightness is an adjustable property.

Now we come back to the flickering. The selected PWM must be high enough so that the human eye does not see any flicker. 100Hz is good enough under all circumstances, assuming that the light or viewer is NOT moving. 200Hz would be already better with moving objects, 500Hz close to pertfect. But these two frequencies are still too low to produce guaranteedly flicker-free imaging with electronic cameras using higher shutter speeds.

Any tungsten filament based lamps have typically less than 10% of light ripple, even if driven with an AC voltage. Fluorescent lights have a 100% ripple, but at DOUBLE the mains frequency, but stay on for more than 70% of the cycle. So this is not 50/60Hz flicker, it is 100/120Hz flicker. The decay in the phosphors is very short, so short that there is no decay to consider at 120Hz. Ok, there is a decay even at these frequencies, but it produces so little light so that is neglible compared to the peak light effect.

LED lights that are driven straight from mains (a long string of LEDs with a series resistor) are the worst lighting armature you can think of. The leds turns completey off twice each mains cycle, and the dutycyle is very low.

There is an old saying; if you want a cheap and a good one, you have to buy two, one cheap one, and one good one... Its true even for LED lighting...

Christian
User avatar
Kayleigh Williams
 
Posts: 483
Joined: Wed Aug 23, 2006 4:41 am

Post » Sat Oct 02, 2010 8:36 am

One though that did occur to me is that the light output of the cheap mains powered LEDs could well have a duty cycle of around 50%. LEDs have a signifcant forward voltage drop.
I have both iron ballasted fluro lights and cheap mains powered LEDs. I know the fluros produce really bad problems with CMOS cameras at 24p in 50Hz land. I haven't tried the LEDs as yet.

Bob.
User avatar
Austin Suggs
 
Posts: 492
Joined: Sun Oct 07, 2007 11:35 am

Next

Return to Vegas Pro Forum