HMI Light Phasing
Hi folks: New guy here but I have been shooting pro glamour photography for pay per view television for nearly 20 years. Here is small sample of my work: http://www.wlstevens.com
I'm trying to solve a mystery that we just started noticing with our HMI lighting. We have been using a DeSisti 1.2k with an oldschool Ari magnetic ballast for about 7 years.
We have just started to notice that the light phases from a warmer color to cooler in about a 5 second cycle. It is not noticable to the eye but it can be seen on video playback. We shoot with a Sony DXC-637 studio broadcast rig into BetaSP. (I know, we need to update but our distributor will only take SD right now.) The phasing also shows up in digital stills which we are now taking with a Nikon D3. The phasing occurs at all different locations. The HMI bulb is fairly new. Maybe a hundred hours on it.
Any clues what this might be?
Firstly, in re your samples..... HELLO!
I'll bet a dollar that what you are seeing is not actually a phasing of the color of the HMI light itself, but rather a phasing of its intensity. I've never ever known an HMI to color phase, unless a globe was just about to go (and yours has low hours). However, even minuscule almost undetectable phases in intensity can cause a visible color shift IF the HMI is accompanied by other light sources. That is, the ratio of the HMI lighting to the amount of lighting from other instruments is changing (even if it's not enough to affect exposure or even see with your eyeballs, it's enough to affect the color balance), and therefore you see a color phase. I haven't had this happen with HMIs, but I've seen it with flos that were out of phase.
Do you get this problem when your HMI is your one and only lighting source, in a completely dark studio setting? Or when it is one instrument mixed with others (or mixed with daylight)?
I would at first think you were getting a dirty out-of-phase power source... but you say you get this phenomenon at all locations, so it's unlikely that's it.
I think the problem is in your ballast... which is unusual, as magnetic ballasts usually go on forever. People don't get rid of them because they are unreliable, they get rid of them because it's like carting around an anvil.
Firstly, I bet you are powering your Betacam with a battery. Try powering it with AC, so both the camera and the HMI are getting the same cycled source (although that won't help you with stills, but I imagine that's less of a problem).
Secondly, look at the ballast. To be honest, I'm not real familiar with magnetic ballasts anymore, it's been years since I used one. Does it have a switch on it anywhere that lets you switch from "24fps" to "Free Run," or some verbage like that? That may be something that's on electronic ballasts only, I'm not sure. If so, try it in the opposite position than it is now.
If that's not the case, I think you'll have to do some switching and swapping to locate the problem source, which is going to require getting another 1200 HMI for testing. I'd suggest that you beg borrow or rent another one and start swapping (you might even be able to do this at the rental house, if they are accommodating, and not get charged anything). Firstly, swap the globe. Then the header cable. Then the ballast itself. Then you might want to try the new (rented or borrowed) head with your old ballast.
I think you're going to find that the ballast is the issue... and that the best fix is going to be upgrading to an electronic ballast. Which, unfortunately, isn't cheap.
Fantastic Plastic Entertainment, Inc.
Todd: Thanks so much for your very helpful input. I was pretty much thinking the same thing; namely, proceed with a process of elimination starting with the ballast. It also makes more sense that the problem is a light output intensity issue rather than colorshift. I will do the troubleshoot and report back on my results and conclusions. Many thanks!
One more thought, Lawrence.... can you adjust the shutter speed on that Sony camera... and most importantly does it have a "clear scan" mode?
If so you may be able to dial out the phasing by tweaking the shutter speed a bit. My guess is that a 60.1Hz clearscan setting would be a good starting point.
Again, that wouldn't help you with the stills, but might solve the video side of the problem.
Fantastic Plastic Entertainment, Inc.