Hi-speed shooting, tungsten filaments, and flicker/pulse

Tweeted on the topic of high- fps shooting today.  More info here:

It is commonly understood that when you're shooting above 120 fps, you can't use small lights, that a 5k is the smallest light that you can use.  Why? In brief, because a smaller, thinner filament can cool and therefore dim quickly, as the AC voltage cycles up and down 60 times per second on a 60-hz system.  To the eye, and a normal framerate, the very brief dim period of out put in between +120volts and -120volts that happens 60 times each second isn't visible at all, the same way you can't see hummingbird wings or helicopter blades.  When you're shooting extremely high speed, these "brief" dim periods are extremely obvious, and the lights that appear steady to your eye appear to be pulsing in a very odd way.  With larger, thicker filaments, like on a 10k or 20k, it takes a much longer time for the heat and light to dissipate out of the metal, and by the time the filament would be noticeably darker, the voltage has already reached it's next peak and the brightness of the filament is restored.

Today it was pointed out to me by a DP I'm about to work with that the "nothing under 5k" rule is actually a conservative overcompensation(a little typical of us gaffers and electricians, I think).  I've pulled maxibrutes (9 x 1k Par-64 bulbs) off of an order because I heard that we were adding a bit of high speed.  Jim Matlosz (Www.dpmatlosz.com) clearly knows what he's doing, if you look at his site, and he said, "Yeah, you get away with it sometimes. 1ks?  Yeah. I've seen them flicker, and I've seen them not. We'll try it." But in any case, he firmly asserted that 2k is the safe smallish light, not 5k.  

Regular tube-shaped kinos are fine when they're bright enough, because kino ballasts ramp up the frequency of the current to about 10,000 hz.  Interestingly, the u-shaped kinos like diva lights, parabeams, and vistabeams are problematic sometimes.  That requires further research; I don't know why they aren't fine and the straight tubes are.  They must use lower-frequency ballasts.

"Flicker-free" HMIs:- those arent really flicker free.  Their brightness still increases and decreases 60 times per second, but they become "flicker-free" by squaring off the top of the AC sine wave.  The amplitude(height) of the wave is increased, but a filter is applied that cuts off the top of the wave beyond a certain point, which gives you an almost completely flat-topped, steep-sided waveform, almost a true square wave.  This results in seeing the maximum brightness for a relatively longer time out of each cycle and having the transition between brightness peaks be relatively brief.

In any case, don't use flicker free HMIs at a framerate higher than 150 fps. Some people say 120, but that might be just staying on the safe side.

yours
Tom Guiney, gaffer and DP
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