HMI Flicker

Richard Lowther · February 1, 2005

I am working on a set as a still photographer and I noticed when I have a shutter speed of around 400th sec or faster I start getting discolored bands on my shots. I notice this seems to happen when they light with magnetic HMIs. I like to use a fast shutter to catch action.Are there shutter speeds I can use that will not give me these bands?

Thanks for you help.

Laurent Andrieux · February 1, 2005

Hi !

It would be great us to be able to see the pictures... Can you post a couple of them here ?

Working at such a speed is not a good idea since HMI don't have the same "remanence" as tungsten has. They act more like flashing lights. In the same time, I wouldn't be astonished if you occured such a phenomenom as one would occure with fluo lights, where you don't have the same quality of light (colors) along a cycle. We have this kind of problem with high shutter speeds and fluos (from what I understand your problem is).

I would think working at 1/125 s should still be fine (because it's close to half a cycle, and you have 2 "flashes" by cycle) , the best being to make tests at different speeds, may be 1/250 could still work....

Gilbert · February 2, 2005

Try using an Electronic ballast instead of a Magnetic ballast for HMI Lighting. With an Electronic Ballast you can shoot at any speed. Make sure the ballast is set for Flicker Free.

Gil

Mei Lewis · April 26, 2010

I think this is probably the same issue as you get shooting standard fluorescent tubes, although they are much worse.

With those, there are 3 problems:

1) The light is greenish. That's easily fixed by setting the correct color temp in camera.

2) The light flickers. Fluorescent (and HMI?) tubes cycle in power and color temp slightly every time their power supply cycles. You're probably seeing this. As the shutter moves across the frame different parts of the frame are seeing slightly different brightness and color temps. For ordinary fluorescents the 'safe' shutter times are half the power supply cycle time, the whole power supply cycle time and any whole number multiple of that time.

E.g. in the UK (50Hz supply) the safe shutter times are 1/100, 1/50, 1/25, 1/10, 1/5 etc.

My guess is that HMIs cycle at much higher rate, so that movie cameras with a typical shutter speed of 1/48th or similar, don't notice it. But at 1/400th you have.

3) The spectrum of fluorescents is incomplete. There are gaps where colors are simply missing from this supposedly white light, and anything in the scene which includes those colors won't be represented quite correctly. I don't know of any proper solution to this. For me, no matter how careful I am about (1) and (2) there's usually a slightly odd feeling to the pictures resulting from this incomplete spectrum.

John Sprung · April 26, 2010

Magnetic HMI's flicker at twice the power frequency, 100 Hz in Europe, 120 Hz here. So, 1/120 sec here gets you a complete cycle's worth of light. Shoot any faster, and you only get part of the cycle. Your options are either to go with electronic ballasts, or shoot at whole number multiples of 1/100 or 1/120, depending on where you are.

Magnetic ballast flourescents also have color variations during the cycle, IIRC, they go yellowish on the trailing edge of the pulse. Perhaps HMI's do the same.

The spectrum from flourescents has spikes, primarily 546.1 nanometer green, but not gaps. The curve from the phosphors has its ups and downs, but no holes. You can diffract it out with an old CD or DVD and look at it.

-- J.S.

Mei Lewis · April 26, 2010

The spectrum from flourescents has spikes, primarily 546.1 nanometer green, but not gaps. The curve from the phosphors has its ups and downs, but no holes. You can diffract it out with an old CD or DVD and look at it.
-- J.S.

I mighty have got that wrong then. There are numerous sources on the internet that say the flourescents spectrum is discontinous (eg http://www.idigitalphoto.com/dictionary/discrete_spectrum) and I'd assumed that was correct because of my own experience trying to color correct properly shot stills under fluorescent lighting.

John Sprung · April 26, 2010

It's actually a bunch of spikes sticking up from a lowish base line. This graph from Wikipedia is quite typical:

http://en.wikipedia.org/wiki/File:Fluoresc...ks_labelled.png

-- J.S.

Mei Lewis · April 27, 2010

Thanks. I guess some people just oversimplify and say it's discrete, wheras there actually is some energy in the 'gaps'.

Looking at that graph I'm not surprised I can't get good color out of it.

John Sprung · April 27, 2010

Actually the energy in the gaps is more significant, and that in the spikes less significant than it might appear on typical graphs. The aperture of the spectrophotometer makes the spikes look wider than they really are, and so does the process of drawing and printing the graph. The spikes come from electrons dropping from one orbit to another in an atom, such as the mercury in the flourescent tube, and that's about as monochromatic as you can get.

-- J.S.

timHealy · May 12, 2010

If the guy is a set still photographer, he'll have no input about what kind of ballast are being used. Having said that, I can't remember the last time time I saw a magnetic ballast on a set.

JD Hartman · May 12, 2010

If the guy is a set still photographer, he'll have no input about what kind of ballast are being used. Having said that, I can't remember the last time time I saw a magnetic ballast on a set.

Last low-budget music video I worked had several mag. ballast HMIs brought in by the DP. Maybe un-common on a feature or commercial, but their still out there and being used every day. Hell of a lot more reliable than an electronic ballast.