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
Airboxlights.com inflatable LED diffusers for Litepanels
twitter lighting tips @airboxlights

Expansion @airboxlights twitter feed of lighting tips, tricks, and opinions

 I’m Tom Guiney, a gaffer, electrician and DP for fourteen years in New York City, and the owner/inventor of Airboxlights.com, your source for ultralight inflatable softboxes/diffusers for Litepanels.

This blog is intended as a co-blog to my twitter feed @airboxlights, where I put lighting tidbits up a few times a week,  sharing my lighting work experience in the world of commercials, on-air promos, reality shows, corporate videos, movies, tv shows, and music videos.
Expansion on recent tweet:
tidbit: Underused light- molebeam beam projector. Powerful defined tungsten beam. Hard model key? window light? like a tungst xenon.

Here’s a link: http://www.mole.com/lighting/beams/tun_beam/tung_beam.html

Have you used them?  They are awesome!  They put out a powerful super-refined beam with very distinct edges.  Useful in some of the same ways a leko is, except for when you need more than 750 watts, the maximum size HPL lamp.  Molebeams come in 2k, 5k, 10k, and 20k.

A molebeam is great when you need something like a shaft of light effect, but you’re in a tungsten lighting situation.  A bit like a xenon, but without a lot of the obvious problems of xenons, like unreliability, the hole in the middle of the beam (on older ones), and being unable to unplug them in a hurry for fear of exploding bulbs.

Also very useful to bounce into a board that’s rigged up and behind the subject. If it’s too much hassle/time/logistical difficulty to actually rig a backlight, you can generally rig up a piece of silver/white beadboard there, with tape if nothing else, and then hit it with a leko or a molebeam, since they  don’t really spill the way a fresnel does and won’t require any gripping to keep spill off of the front of your subject.

Also useful just as tungsten source to use as a bounce source for key/fill/wherever, not rigged; it’s still a 2k/5k/10k/20k.

Useful for creating crisply-defined shadows as well. 

You can always soften a very hard source, but you can’t harden a mushy source.  Try casting a crisp shadow with a kino, tell me how that goes.

yours
Tom Guiney, gaffer and DP
Airboxlights.com inflatable LED diffusers for Litepanels
twitter lighting tips @airboxlights