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Shutter angle doubt


Matias Nicolas
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Well, they can, though it's usually an artefact of the backlight rather than the LCD panel itself.

What makes me comment, though, is that LCD video walls are rare. What we might be talking about is LED video walls, which use a fundamentally different technology and can absolutely flicker terribly. Can we clear that up?

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That works when the lighting flickers at the local mains frequency, which assumes the use of magnetic ballasts for fluorescent or other types of discharge light or, sometimes, LED lightbulbs without much circuitry in them. Magnetic ballasts have largely been replaced with electronic ones, to the point where electronic ballasts are also increasingly obsolete, with discharge lighting itself is being rapidly subsumed by LED.

It's certainly not impossible to find street or industrial lighting that flickers at 60Hz, but a gas discharge light which does will likely be a rust-encrusted, decades-old installation at this point. I have some metal halide lighting which uses electronic ballasts and I've measured those at 150Hz (though they're square wave devices, like an HMI ballast in "flicker free" mode, and they are indeed highly flicker free - a casual test with my cellphone's 900fps mode reveals not a twitch).

LED lightbulbs can sometimes flicker at the mains frequency or at double the mains frequency. They may also use much more elaborate drivers that can either cause no flicker, or flicker at almost any frequency the designer found convenient. Sometimes they may flicker very quickly, quickly enough to create venetian-blind effects on rolling-shutter cameras.

LED video walls invariably use pulse width control and will more or less all flicker unless special measures are taken to synchronise with them. I've been around a few virtual production stages recently and taking photos of them on a cellphone generally requires trimming the exposure to create a slow enough shutter speed for reasonable integration of several PWM cycles. Either way, again, the electronics that drives them has nothing to do with the mains frequency.

The sad reality is that classic ideas around picking specific degree shutter angles to avoid lighting flicker are as obsolete as iron-ballasted fluorescent tubes at this point.

Image below shows a virtual production stage video wall shot on a cellphone. Exposure time was 1/160s. Overlook the moiré; the horizontal bands are the residual timing error between the display drive and the shutter timing.

image.thumb.jpeg.3c8b497e08f1a51584a6ff8e172b5939.jpeg

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