Filming At Ultra High Framerates

Hi,

Any one have any idea about at what frame rates you start to notice the 60 hertz of tungsten lights, or if it happens. I spoke to a friend who told me when shooting at 10,000 F.P.S. he saw the lights dim and brighten during the shot, he was using tungsten. I've only done one test so far, at 3,000 F.P.S. with lights, and I didn't see the effect he described. However it is possible that on my test the lights were on different legs, so they would be complementing each other. Strobes are not an option, I wish shooting outside were. Does any one know about this phenomena, or at 3,000 F.p.S. do I not have to worry.

Thanks

Steven Gladstone

While this is just a guess, I would think that 10k and 20k lamps are very unlikely to show any output variation over such short intervals of time...but you could do the 'easy' thing to prevent any fluctuation due to ac by renting a DC generator and running your tungsten lights on DC...no fluctuation from power and quieter to boot...though the camera will be anything but quiet. You might also consider using Xenon lights as they are DC continuous arc sources.

Mark Weingartner

I do not think it is possible to notice the flicker of tungsten lights at any frame rate, since the variation in flicker is so slight. It would not be a function of frame rate, but of the viewer's sensitivity to such a slight variation. Perhaps that variation might increase if the lights were dimmed down ? ...but I doubt that.

Perhaps it was the result of uneven development in the Lab ? ...but that tends to look a bit splotchy.

(Are we certain that these were tungsten units and not electronic ballast HMI's ?)

Mark

I have seen the described fluctuations at slower (relatively) frame rates around 3 to 500 fps. After talking to those in the know (not those who won't admit it) we determined it was the camera that was wavering in exposure. While I prefer not to dis their equipment as I like it and use it all the time, the manufacturer's name rhymes with an extinct bird and an old hair treatment.

Eric (too wimpy to just say it) Swenson

Mark, I don't know about your experience, but I can point a Cine Check at a 60-watt house bulb and read the mains frequency. It would not surprise me that this pulsing could be caught on film, assuming the frame rate was high enough.

Jim Furrer Director of Photography

Yes, Steven you will be able to see 60Hz fluctuations over 2500-5000 fps if my memory serves me right. But the most determining factor is the size of the tungsten source. Tungsten pars or small wattage lamps will be seen but 5k's or 10K's won't due to the nature of the filament size. They simply take longer to respond to the fluctuations. Think of a 10K quickly doused on a dimmer compared to an inky. There's always DC! Hope this helps.

Regards,

Jim Sofranko

Definitly can be seen with small filament lamps at high fps over 2500.

Regards,

Jim Sofranko

Hmmm, I've never heard of smaller filament lamps exhibiting a higher amplitude difference than larger ones...not enough of a difference to matter anyways. But I'll take your word for it.

I must say that I worked on many Photosonics shoots (I was a Photosonics Camera Assistant in New York) and never witnessed this. Then again, at 2,500 to 3,000 fps we usually required 5 & 10K's to expose our shots.

The only flicker that I witnessed were one of three heads on some brand-new flicker-free HMI's. Bad ballast. Needed a reshoot.

Mark

The 60 hz frequency fluctuation is definitely there, but the amplitude difference is quite small. If it were large (as with a magnetic ballast HMI), you'd notice it even if you shot non-crystal at lower frame-rates.

For the sake of clarity, lets say that the (rather long) sustain and decay of a "flickering" & glowing tungsten filament were to approach a 5 % difference in amplitude, then this would be barely noticable on film. We can notice extremely small exposure shifts when they happen very quickly, but not when you have, say, less than 1/10th of a stop shift that is spread-out in slow-mo over a couple of seconds.

I have never witnessed such a problem (we got our cameras up to 3,000 fps with a special Electronic Speed Control)...but Jim Sofranko says that he has definitely seen this exposure fluctuation on smaller tungsten fixtures.

I suppose it's better to run your lights on DC or 3 different phases...or it's time to shoot high-fps shots in the sun only !

Mark "watch out for sunspots" DP

I remember that time in NY and yes, 5K and 10K's were the lights of preference. But a few of the directors at the tabletop houses in the mid to late eighties started to use VNS tungsten pars for high speed Photosonics and macro work where depth of field was important. Much more bang for the amps. The tabletop gaffers during that time (myself included) picked up on this and introduced the idea to many director cameramen. That was when I noticed the sine wave problem with the very responsive par lamps compared to the slower, large filament 5K and 10K's.

BTW-On a similar note a friend who is a tabletop gaffer in NY recently had a flicker problem shooting Photosonics with flicker free HMI's. The problem was attributed to the lamp/age amplitude dilemma that the B&S meter can detect. Now that Photosonics recognizes the problem perhaps they should just include the meter with the camera as Bill Bennett indicated they did for his shoot. It's an expensive item that gaffers always have a hard time in getting a rental. I know that Unilux sends out a Minolta Flashmeter 111 with their system.

Regards,

Jim Sofranko

I had an experience at 300fps, where a practical globe in the frame flickered. It was run off a generator, and don't it wasn't on a dimmer (but now that I think about it I can't remember for certain!) It was puzzling because nothing the gaffer or I could think of really made it even seem possible. The globe was in the shot, none of the other lights illuminating the set showed any irregularity. But, the flicker was an odd one and so slight that I even have the shot on my reel and nobody even notices. My recommendation would be to test or keep sources out of the shot, or run DC on that light. Harry Dawson

This seems pretty reasonable: that the flicker would be fairly subtle.

Now I'm starting to wonder wether some of our Photosonics tabletops were done 3-phase with 3 lights into a piece of diffussion. I know they weren't DC.

I do recall a DP at Ampersand (not Elbert Budin, but the other, younger guy with the dark hair...why do names escape me when I call upon them?) who liked lighting with tight-lensed tungsten Pars (such as the 64's). He liked the hot, messy beam of the older one's with a dead spot near the middle. But I never remember any flicker problems there either...200 to 2,500 fps.

But I suppose the newer, smaller fillaments can really be that responsive, eh ? Good to have that warning.

Mark

Big filaments vs small filaments as regards high speed output variation

Think of the filament as a flywheel (ok, ok, have a mind-relaxing beverage of your choice, and THEN think of the filament as a flywheel.)

The larger filaments take so long to heat and cool relative to 60Hz power (which produces, of course, 120 voltage peaks per second) that they will not exhibit visible variation over the course of a cycle. Smaller filaments may heat up and cool down fast enough to be seen on high speed film. Household bulbs would be amongst the most likely to show this variation . This is the "evil twin" of the characteristic of smaller wattage globes that leads us to use them on flicker boxes for fire effects or lightning, or whereever we need fast modulation.

DC solves the problem,(it never shuts off) as does using multiple fixtures into a common piece of diffusion such that roughly a third of them are powered by each leg of three phase power. Three phase power gives you three sine-waves of AC power offset by 120 degrees from each other so that at any instant in time, at least two of the legs are not at 0 volts. Single phase power, even if it has two hot legs, has two legs of power out of phase with each other by 180 degrees so that they are both at zero volts twice per cycle. Forgive me if I am clarifying things that many people know; as a career gaffer/VFX nerd, I have discovered that even many of my bretheren in the electrical world do not really dig the three-phase thing, and power generation is not something that is covered in depth in all cinematography courses...I mean, that's what your gaffer is for, anyway.

Mark (climbing back down from the soap-box)

However since they are 180 degrees out of phase, wouldn't their fluctuations cancel out? If I used half of my lights on opposite legs. Perhaps I'd have to compensate the exposure some, figuring that the average of the On ( full intensity) and the off ( a little bit less than full intensity) would yield less than what my meter ( at this amount of light and heat now a shrivelled bit of melted plastic) says. Argggghh, this theoretical stuff. Shooting at such high frame rates it's almost as if time stops, like that old Star trek Episode. I like the D.C. Idea, that seems simplest. Just because I started this thread I'd like to say thanks to everyone who has waded in with thoughts, or experiences.

Steven Gladstone

With an oscilliscope (which is basically a device to let you see a picture of voltage over time) you can see in a single cycle of AC that the voltage first goes positive to 120v. and then goes to negative 120v., passing through zero on the way. You can tell the difference between +120v and -120v., but all a filament can tell is that a bunch of electrons are playing through...not which way they are going. Since the "zero points" of the cycle are the same for both legs, the lights will all flicker together. This overall common fluctuation is the worst case for having the exposure variation show up on film. With three phase power, on the other hand, since the three legs are offset by 120 degrees instead of 180 degrees, the overall difference between "brightest" and "dimmest" is much less, and theefore less likely to be perceptible on film. Remember that the challenge here is not to create a certain intensity but rather to create a situation where that intensity does not change too much from frame to frame.

You ain't kidding about the melted plastic bit. On a certain blockbuster about aliens attacking the earth a couple of years ago, I was charged with lighting an area of air extending approx 15 feet above a nine foot long trough from which we were generating steam and debris clouds using, among other things, prima-cord and fullers earth. I used 3 Dino lights (24x1kPAR64) on each side with 5 nine-light Maxi's on a high truss as back-light and 4 maxi's around the front for "soft low-level fill."

Bad day to wear a dark blue shirt.

I got into the killing zone to focus and get readings, and by the time my welding glass was getting warm to the touch, the front of my shirt was getting distinctly hot to the touch. as was my hair. If I had taken one of my old Spectra Pro's out there without a 100x xlide in it, the needle would have wrapped around the high stop.

Oh, the things we do for ART!

Mark Weingartner (a long way from focusing par cans in Central Park with a Texas license plate)

The two legs of a single phase supply are never 'out of phase with each other' . Rather the polarity of the supply alternates between them at the the supply frequency 60Hz for you, 50 for me, and at the mid-point of this alternation there is no potential difference(PD) between the two legs. This is commonly called the 'Zero Crossing Point' The PD rolls serenely up and down either side of this point and peaks once 'negatively' and once 'positively' for each cycle, passing the zero crossing point twice as it goes. If you draw a representative sine wave with a straight line throughout its central axis you will see how this works. The nominal voltage of the supply is the RMS of this sine, 240v in the UK, but the peak to peak voltage is almost 280v! Interestingly, the maximum voltage that you can actually come into contact with at any given moment is the half cycle value; that is 120v for me, and 55v for you. Which is why US supplies are inherently much safer than UK one and all your fittings are of a lighter construction than ours.

Ask your friendly gaffer to explain how the single phase is extracted from your three phase supply!

So the upshot is that single phase supplies are at no volts twice per cycle and its quite possible for lightweight filaments to cool visibly if you are shooting fast enough.

Andy Bowman.