LED Par Cans

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povertylevelplayer

Active member
I filmed a concert a few nights ago and the lighting guy incorporated some LED Par lights into the mix. I noticed a slight strobe effect off f the LED's while they were lit using a color other than Red, Blue or Green. It almost looks like a synchro scan issue. All the other lights were fine and I know there wasn't a strobe light being used. Has anyone else experienced this?
 
Sounds like your transformer is going the way of Boot Hill. that said, I'm weary of most LEDs - only really reliable one we've found is the one from CoolLights.
 
If the LEDs are AC powered, I'm not surprised. The power supply should have a smoothing capacitor so the LEDs see pure DC and are lit constantly, not just 1/120th second.
 
Yeah, the LED's were AC powered. I shot at a 180 degree shutter. I had talked to the lighting guy earlier that evening and the cans were only $99 a piece. Like I said earlier, the primary colors looked fine and I don't notice the strobing effect, it seems to do it only when he blended the colors. As long as it's an issue with a random light guys equipment and not my camera, everything's alright. I wish I had more time to set up, I feel that maybe it could be corrected through the scan settings.
 
LED brightness is controlled by pulse-width modulation (PWM) much like controlling the brightness of incanescent lamps using phase-angle control (which amounts to PWM). When the LED instrument is at full-on (whether any color individually, or all at once), then the LEDs are powered 100%. But anything less than 100% means that they are "strobing" on and off, probably at the power mains frequecy (60Hz in your case.) Incandescent lamp filaments have thermal "inertia" so they don't instantly go to black when the power is interrupted very fast (milliseconds). But LEDs are instant on/off, even when their power is "chopped" in millisecond slices. That is the most likely reason you were seeing the "strobing" effect. Lamp supplies (fluorescent, LED, etc) made for film or video production use a much higher modulation frequency so that the effect can't be seen for most typical frame rates. But cheaper lamps made for stage use don't bother with the more sophisticated (and expensive) measures.
 
Thanks, that makes sense. The sad thing is they had the guy there cause they knew I'd be recording. I kinda wish they had just used the house par cans that had the old incandescent bulbs in them. It would've save them some money and me the headache of having to deal with the way this footage looks.
 
I ran into the same thing last month while shading Sony D50's for a medical convention. On tight shots with fabric panels that were lit by led par cans I saw a strobing or pulsing which surprised me as I had never seen it before. Dialed in clearscan on both and it was gone, frequency might have been 59.something but I couldn't swear to it. There are always new and interesting ways to get burned in this business.

mb
 
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