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Weird light behavior on screen with LEDs.

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Ricky Janzen
Weird light behavior on screen with LEDs.
on Apr 14, 2011 at 4:40:55 pm

The LD at the university I work at recently got some new lights (I'm not too familiar with lighting, but it's some sort of LED strip-uses a few of them to light up the back walls of the stage). Anyway, when he uses certain colors, namely a rich, pure blue, it produces a strange green smear or tear. Seems to happen with rich primary colors only. Same results on different cameras.

What's the reason for this phenomenon and is there anything I can do beyond asking the LD not to use certain colors?

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Mark Suszko
Re: Weird light behavior on screen with LEDs.
on Apr 14, 2011 at 8:09:34 pm

Try looking at this with various camera shutter settings and see if you find a shutter setting that makes it go away. Many LED devices like thise for automobile and motorbike brake lights use rapid pulsing as the brightness/fader mechanism, and they also pulse the LED's to reduce power consumption and heat so as to prolong the life of the LED's. They count on persistence of human vision not to notice this rapid blinking. Maybe something like this is going on in your situation and the particular shutter you use makes it worse.

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Bill Davis
Re: Weird light behavior on screen with LEDs.
on Apr 14, 2011 at 11:01:47 pm

Modern LEDs are a LOT of wonderful things.

Amazingly energy efficient. Light weight. Resistant to breakage.

One thing they are NOT is spectrum equivalent to other light producing technologies.

And the cheaper they are, the more they tend to generate light that skews into the green side of the spectrum. I suspect that in some combinations of multi-colored LEDS inside the lights you're dealing with - this SKEW is happening.

Every major LED video light fixture manufacturer understands this and many actually include solutions from packaging magenta (minus green) filters with their lights, to putting switches or dials on their instruments that allow the light itself to adjust the green spectrum content to match other equally color-deficient sources (like some fluorescent tubes)

Even the new $2000+ Arri LED fresnels have a "plus or minus green" spectrum dial included.

It's just how things are right now.

"Before speaking out ask yourself whether your words are true, whether they are respectful and whether they are needed in our civil discussions."-Justice O'Conner

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