Colour Roll With Three Phase

[Gopher]

I've seen this but it was the camera itself that was on a different phase so not quite the same thing but we adjusted the "phase" of the camera as it had a phase adjustment on the back of it. I do know many cameras have this option accessible via an onboard menu. It's the fact that each phase is 120 degrees apart and the colour roll is basically the 2 signals combining to form 1 wave which appears to "colour shift", the pdf link I've attached has a few graphs on it to basically show what your seeing it's the combination of the different phases, http://www.itacanet.org/eng/elec/edu/pt14.pdf

How this helps.. I think the only way to solve this is to try and have the lights moved to the same phase even if it's not the same phase as the cameras, if the cameras have a phase adjustment on them you could use this to smooth out any problems, if not try and get them on the same phase..

[RobOwen]

The cameras seem to adjust their phase automatically, because if only one set are on (regardless of phase) it does adjust itself and the colour is perfect. I think the camera is basically "hunting" to find the correct phasing.

I don't think the problem is criminal, so I might suggest upgrading the lights to 100hz so the cameras cant see the flicker, or just grin and bear it.

I really appreciate all the comments, and will try the suggestions mentioned.

Regards,

Rob

P.S. It's interesting what Miaren said about phase loading, as the rolling gets quicker some times and slower others. The only reason I could think of for this would be that different loading at different times on the phases is causing the change.

[SystemQ]

The cameras seem to adjust their phase automatically, because if only one set are on (regardless of phase) it does adjust itself and the colour is perfect. I think the camera is basically "hunting" to find the correct phasing.

I don't think the problem is criminal, so I might suggest upgrading the lights to 100hz so the cameras cant see the flicker, or just grin and bear it.

I really appreciate all the comments, and will try the suggestions mentioned.

Regards,

Rob

P.S. It's interesting what Miaren said about phase loading, as the rolling gets quicker some times and slower others. The only reason I could think of for this would be that different loading at different times on the phases is causing the change.

Hi Rob.

I have seen an issue similar to this before that was caused by the IR filter of the camera. It may not be the case at this site but it

[Doktor Jon]

RobOwen,

From what you're describing I must confess I'm a bit confused as to what's causing the problem.

The only thing I can suggest is that you do a couple of tests to establish whether the problem is caused by differing colour temperatures from various light sources, or whether it's simply a frequency related problem that causing issues with either the cameras electronic iris shuttering or the auto white balance.

Given that I don't know what cameras you're using, first test I would suggest is to see if the camera/s fiunction correctly with each isolated light source (i.e. do not have more than one light source switched on at any time - if the camera/s produce a correctly displayed image with each individual light source then that is one less thing to worry about.)

If the cameras work correctly with each individual light, see if you can get hold of a colour temperature balancing filter from a photographic shop. If you can buy either a "daylight" ( 5600k) or "artificial light" ( I think it's 3200K from memory), then hold that in front of the lens, it shouldn't matter if you have half a dozen different light sources each with a varying colour temperature ... the filter will balance the colour temperature of the light entering the camera, and may well solve your problem (with the one camera at least).

If there is a suggestion that the issue may be related to differing frequencies interacting with the cameras electronic iris (auto shuttering circuit), try placing an ND x4 or x8 filter in front of the lens. This will force the camera to automatically select a much slower shutter speed (assuming that the camera is not fitted with an auto iris lens), which should offset any problems which may result from a higher shutter speed being used in conjunction with fluorescent light tubes, that are frequency dependent on the power source.

There are probably one or two other things I could suggest, but I'd certainly try the colour temperature filter first to see if that makes a difference.

[james.wilson]

you might also want to turn off awb.

If the phases are not properly balanced it would cause a voltage to be on the neutral. Not good but wont affect the cameras

[Matt the Teckie]

I suspect you're using a DC supply. Seen this a few times (especially in fish and chip shops).

Here's the problem... The light being emitted by the flourescents is never actually white. It goes through a variety of colour temperatures depending on the point of the mains cycle. If the camera's running on a DC supply, the shutter speed is based aound the camera's internal oscillator. Unfortunatly this clock can never be harmonic with the mains frequency (firstly as it constantly changes slightly and secondly because presisely tuned frequencys would be too expensive to built into standard cameras). Since the trigger frequency of the camera is slightly different each time, the overall colour temperature of the scene seems to drift. A good analogy of this effect is where we see the wheels of a wagon in old westerns appear to move backwards although we know they're traveling forwards.

Best cure is to use 24Vac or Mains cameras. Regardless of how many phases are used, if they're all line locked, they will all trigger at one point on the mains cycle. Regardless of what the colour temperature being emitted by the tubes, they'll always be the same. The auto-white balance takes care of the rest.

You'd tend to see this effect more if the scene is predominantly one colour (usually pale colours at that.. hence it's more often seen in chippies).

Hope this helps and happy Christmas everyone.

Matt