HMI Flicker

[Jon Rosenbloom]

Shot a short film last weekend, a little office comedy. I asked the set dresser to give me a practical desktop lamp, and she provided a two setting (undimmable) halogen lamp. So, I'm stuck w/ this light that doesn't dim, but I had plenty of ND gel to stuff inside, so I didn't worry about the light blowing out. What I did not suspect, was that the dumb thing would flicker. I understand a little desktop halogen won't run on a flicker-free ballast, but if I'm shooting 24fps, true progessive w/ no pull-down, why should it flicker? Isn't that a flicker safe speed? Or, is there actually some sort of pulldown? Could it have something to do w/ my 200 degree shutter?

Edited June 2, 2007 by Jon Rosenbloom

[Jon Rosenbloom]

Stumped every one. The most viable theory I've heard is that the 200 degree shutter might have thrown something out of phase.

[Stuart Brereton]

I understand a little desktop halogen won't run on a flicker-free ballast, but if I'm shooting 24fps, true progessive w/ no pull-down, why should it flicker?

 

Why would a Halogen lamp have a ballast? Are you sure it wasn't some sort of discharge lamp?

[Hal Smith]

If the little desk halogen had two settings then it had some sort of fixed level dimmer inside it - and who knows what the "chop = flicker" rate of the dimmer is on "lo". Its "hi" setting might have been steady but since you mentioned using lots of ND I suspect you were running it on its "lo" setting.

[Michael Collier]

Halogens have no ballast, and rightly should not flicker, ever.

 

I suspect you shot on the lowest setting. I do not know for sure what those switches do (weather resistence or otherwise) but I suspect its a simple diode. When on full the light sees both phases of the AC cycle. When switched to low, it channels power through the diode, which naturaly cuts one phase off. The result is a light that is still at 60hz, but half the cycle the light is recieving no power at all (compared to full cycle where only a brief moment there is no power as the cycle crosses the zero border) I think that might have led to the flicker. Interesting though, I probably wouldnt have thought of it looking at a halogen light.

[Jon Rosenbloom]

"Halogens have no ballast, and rightly should not flicker, ever."

 

So I've been told.

 

I recall using both settings when I was using it as a source for a bounce, though no one was really keeping track. The main lesson of this job is to specify that the practicals be incandescent! (Lesson #2: When asking for set dressing for the walls, specify no glossy maps or photos!)

[Michael Nash]

Many two-setting practical lamps do indeed flicker, and it's visible to the eye, not just at 24fps. I've got a couple at home like this. It's just the way that kind of dimmer works.

[Ken Martini]

Many two-setting practical lamps do indeed flicker, and it's visible to the eye, not just at 24fps. I've got a couple at home like this. It's just the way that kind of dimmer works.

 

Most dimmers basically chop up the 60 cycle into variable square waves. Even though it is tungsten it can flicker as the dimmer is turned down and it is operating on a low cycle such 20 Hz. You can hear the lamp making noise as the filament is vibrating if you get close to the lamp.

 

You can dim tungsten lights without this artifact if you use a variac which is an oversized potentiometer. This device adds resistance and therefore drops the voltage without changing the frequency.

 

The cheap and easy thing to do is keep a bunch of different wattage lamps and some ND scraps with you.

[John Sprung]

The simple two level dimmer just runs the lamp directly from the mains for high, or through a diode for low. On the low setting, you only get power on one of the AC half cycles, and not on the other. So, a small filament can cool fast enough to flicker visibly and on film or video.

 

The variable solid state dimmers work by turning off at every zero crossing of the 50 or 60 Hz. power, and then waiting to switch on again somewhere during each half cycle. That gives you 100 or 120 light pulses every second, not the 50 or 60 of the simple diode dimmer. The variac is actually a variable tap transformer, not a resistor. It only works on AC. There are no dimmers that alter the frequency of the power.

 

 

 

 

 

-- J.S.