fluorescent lights and white balance

[Shawn Mielke]

Hey all!

An observation, and the perhaps obvious follow-up question.

I've been meaning to ask about this, and i feel like it's been addressed before, but...

Fluorescent lights and white balance... When I'm shooting in shutterspeeds higher than the normal 60 under fluorescent lights, the white balance oscillates between extremes, slowly, and frustratingly.

Alright, I guess there are two follow-up questions:

What's the deal?

Can I make this STOP, and how?

Thanks!

Shawn

[Mike Rehmus]

Do you mean in auto or manual, Shawn?

[Shawn Mielke]

Manual everything, Mike, unless there's some hidden menu function that I have yet to uncover. I'll go spelunking, see what I can find in the manual as well the camera menus. But, yeah, manual white balance. It seems like the faster the shutterspeed, the more pronounced the oscillation.

[John Laird]

Check your 170 manual, I seem to remember something about that being mentioned in there. I believe they said it was a known issue.

John

[Romesh Dodangoda]

i get it aswell on a DSR250

[Giroud Francois]

that is normal.
Fluo lights are a bit strange because it is not a a linear spectrum emission but looks like bars.
So you can get a correct rendering for one color and a totally wrong one for another color.
Additionally, the gaz in the tube is excited at the frequence of your main supply (60hz for usa, 50 for europe). The light is ok only during a short periode, in between there is few chance to get anything usable.
that is why professional fluo light for cinema or video uses high frequencies (kHz) and are usually mounted by pairs, each side of the pair being out of phase with the other.
That is something you can manage with regular tubes, ask your electrician how to mount them dephased.
if you use several tubes, a good idea is to mix different models having different spectrum response to fill these "holes".

[Shawn Mielke]

Huh. Good stuff, thank you. So, without running and trying just now, I could turn on more than one fixture in a space/shot and perhaps have the total fluorescent lighting "dephased"? My concerns aren't so much my personal light kit, although that shall need to be addressed in time, but more being in the real world, covering something under normal space flos that I haven't much control over, beyond in-cam compensation. Staying from shutterspeeds faster than 60 shall be my default solution, but having the option...you understand what I am trying to say.
I think I hear my manual calling me, scusi.

[Shawn Mielke]

Bottom of page 46 of the PD170's manual:

"When shooting under fluorescent light or light bulbs, a rare phenomenon may happen in which the screen lights up brightly depending on the shutter speed (Flicker phenomenon)."

Right. I was on my way back to this thread to update my observation when I thought to look up anything remotely relevant in the manual, and there it is.
What I was going to say as the update of my observations is that it might not be a white balance issue per se. In going back to shooting under flos, I noticed the oscillation from cool bluish to warmish again, but then I also noticed that the zebras were disappearing and reappearing too, suggesting a change in light level, which isn't actually immediately obvious when looking at the image . So what's that mean about the cool to warm oscillation? Is that a temperature issue? As the light level goes down, the color warms up, which also affects the nature of the white balance? Am I making any sense? I'll have to see what John Jackman's book on lighting has to say about any of this.

[Mike Rehmus]

The light levels are not changing.

Think of the situation this way:

The light from the flos is actually turning completely off 120 times per second (voltage goes through 0 volts at 2x the 60 cycle rate). OK, they don't really turn off because the phosphors that coat the insides of the tube have what is known as 'Persistence,' and that causes part of the problem.

Now there is the camera, hammering away at the scene and snapping a picture every so often. If the snap happens to occur during one of the periods in which the flo input voltage is low, then it will think the room is dimmer than it really is on-average. OTOH, should it snap the picture when the flo input voltage is high, it would think the room is brighter than it actually is on-average.

Compounding this issue is the makeup of the phosphor in the flo's. To achieve a broader spectrum than a single phosphor can achieve, the manufacturer mixes in a few different phosphors. Each one of those phospohors react differently to the electron bombardment in the tube. Some are emitters of very short duration, some of substantially different duration. So the output of the tube may be dominated by different phosphors at different times in the input voltage cycle.

Which is another way of saying that the tube puts out different colors depending on where the input voltage waveform happens to be when the picture is snapped. And white balance cannot function under these conditions because the camera is always snapping a picture under different color conditions.

When one runs at a shutter speed of 60 fields per second (in a 60 cycle environment), the camera, which is amazingly stable with regard to timing, is sampling the light from the flos's who are powered with a supply that is amazingly stable. So the camera samples (snaps) the picture in essentially the same time on the power cycle into the flo's and the color/output levels appear to stay constant.

[Shawn Mielke]

Thanks for that, Mike. Do dv cams behave abnormally with any other light types, or vice versa?

[Mike Rehmus]

Oh yes. For some reason, schools like to use lights in gymnasiums that I can only describe as anti-video. For most cameras including the Sony's they cannot completely compensate for the light and the final white balance HAS to be done in the NLE. When one finds this situation, make certain you image something white so you can 'fix' the problem in your NLE.

Sodium street lights (the one that make red cars look black or orange) are another problem.

Any line-spectra source will probably cause some problems unless they are run at high frequencies like the flos made for video. Even then, the tubes really need to have a good phosphor mix.

Grow lights work fairly OK, Ott lights work OK too.

[Shawn Mielke]

Just to add to my findings in the way of DV cams, shutterspeeds, and various sources of light, I was messing around the other night, shooting my girlfriend as she sat reading in her comfy chair (if patience is a virtue, then she's a saint), using two light sources, each pointing at her face from opposite sides: one incandescent bulb with a shade on it (warm), and a signalless tv screen (blue). It made for some strange lighting but it was a lot of fun to then tweak the 170 in all sorts of ways for different looks. I then decided I wanted just the blue light on her, and turned off the lamp. Well, it was pretty cool, and I adjusted two things from there: I switched into progressive scan mode, and I quickened the shutterspeed. Two things struck me.

1.This blue tv light on her face and neck began to do this ebb and flo thing, and kind of modulate with a pattern. Difficult to describe and, well, kind of astonishing.
It may be that black bars were rolling over the screen, and causing this modulation on her face. Whatever the case, it's a great effect and would be difficult to emulate in other ways, I would think.

2. The feature of her face were very soft, like the camera was out of focus and not to be focussed. I believe I was using auto focus, thinking that because there was nothing else in the frame was illuminated whatsoever, using AF would be safe bet. The AF wasn't hunting or anything, she was just...soft. Perhaps it has something to do with this particular kind of light.

I am definitely going to play more with blue tv light. Freaky! :-]

Shawn

[Jimmy Martin]

I ran into this problem during my last wedding shoot with the VX2000. All manual mode. white card balanced.

Will leaving the shutter speed at 1/60 sec prevent the colors shifting from cool to warm? Is that the solution for now in uncontrolled environments?

And is there anything I could do in post to correct what I already shot so it looks normal, without going to black and white?

[Andre De Clercq]

Appart from Mike's mains phase and phosphor issues which I covered here some two years ago in my post 582. there is also something which cannot be corrected, even not in post, and this relates to the so called CRI (color rendition index) of lightsources. This often results in a perfect WB but some colors remain unacceptable because the spectrum doesnt contain the specific color energies.

[Tom Bolia]

Originally Posted by Mike Rehmus

The light levels are not changing.

Think of the situation this way:

The light from the flos is actually turning completely off 120 times per second (voltage goes through 0 volts at 2x the 60 cycle rate). OK, they don't really turn off because the phosphors that coat the insides of the tube have what is known as 'Persistence,' and that causes part of the problem.

Now there is the camera, hammering away at the scene and snapping a picture every so often. If the snap happens to occur during one of the periods in which the flo input voltage is low, then it will think the room is dimmer than it really is on-average. OTOH, should it snap the picture when the flo input voltage is high, it would think the room is brighter than it actually is on-average.

Compounding this issue is the makeup of the phosphor in the flo's. To achieve a broader spectrum than a single phosphor can achieve, the manufacturer mixes in a few different phosphors. Each one of those phospohors react differently to the electron bombardment in the tube. Some are emitters of very short duration, some of substantially different duration. So the output of the tube may be dominated by different phosphors at different times in the input voltage cycle.

Which is another way of saying that the tube puts out different colors depending on where the input voltage waveform happens to be when the picture is snapped. And white balance cannot function under these conditions because the camera is always snapping a picture under different color conditions.

When one runs at a shutter speed of 60 fields per second (in a 60 cycle environment), the camera, which is amazingly stable with regard to timing, is sampling the light from the flos's who are powered with a supply that is amazingly stable. So the camera samples (snaps) the picture in essentially the same time on the power cycle into the flo's and the color/output levels appear to stay constant.

Mike, am I to presume that you are saying that the white balance should function properly if the shutter speed is at 60?

My church pulpit has this cross behind the stage, and for some reason, it is lit with flourescents behind it (tacky imo), but it was already like that when we bought the church. Anyhow, I set everything on my PD-150 in manual mode, and my shutter speed stays at 60, and I always white balance before service begins. Whenever framing cross shots, I always get major fluctuations on the camera (normal to dimming to normal light). Is there another resolution to this problem, other than turning off those blasted lights? I think some of the church members think it looks "cool" for some reason. :o(