Phasing colours

[Vincent Oliver]

I am just shooting some footage with a set of daylight fluorescent tubes (Kaiser copy stand) and when viewing the footage the colour is oscillating between cool to warm (slight green to slight magenta). It is enough to be a distraction and I will have to re-shoot. Does anyone know why this happens and is there a cure for it.

I am using a Canon XH A1 with gain off and daylight as the White Balance, (it also happens when I set a custom WB)

Thanks

[John Miller]

Originally Posted by Vincent Oliver

Does anyone know why this happens and is there a cure for it.

Have a look at http://www.dvinfo.net/conf/showpost....09&postcount=9

[Vincent Oliver]

Thank you,

I have just spoken with a fluorescent lighting manufacturer and was told that my elderly Kaiser lights were switching on and off at a slower speed which causes them to change colour. I need a system that will switch on and off at a higher rate.

Here goes another £450

[Don Palomaki]

Does anyone know why this happens and is there a cure for it

As noted in the reference above, it is caused by the fact that fluorescent light are discharge lamps and the instantaneous color of the output depends in part on the point you are at in the line voltage cycle. The eye averages it out over the power cycle (half the power line frequency). The CCD averages it out over the period of the exposure. If the exposure is substantially less than half power frequency driving the lamp, the CCD sees only a small part of the cycle (and corresponding color of the lighting), and if the frame/field rate is not matched to the power frequency, it will vary over time at a rate that is based on the difference in frequency.

Possible solutions are to use a slower shutter speed that is more in line with the lamp's power frequency (probably no cost), special high speed lighting (costly) , or incandescent lighting (modest cost). Using clear scan might help, but I've not tried it so I cannot confirm whether or not it does.

When doing copy stand work, I use halogen (incandescent) lighting and white balance for it.

Note that fluorescent room lighting can cause a similar effect at higher shutter speeds.

If using a copy stand that is lit by high speed fluorescent in a room with standard fluorescent lamps, and depending on your imaging system, you may see some interesting interference patterns cause by the two light sources. I ran into this using a document scanner a few years ago.

[Odd Nydren]

It is because all fluorecent tubes actually flicker on and off...but too fast for our eyes to see.

..that means - if you have a fast shutter speed in your camera it will pick it up...as a matter of fact...you probably need it to be rather slow to make it NOT pick up the tube flicker. (in other words, by exposing each frame a longer time, the tube gets to flicker a couple of times for each frame and that way evens out the appearance of flicker)

Also - set your custom WB by zooming in on a white paper and set it on that before you shoot - no need to focus on the paper, just see to it the paper covers all of the frame as the camera usually uses the sum of the whole image to set its WB, and that it is evenly lit without shadows - that way the tint will disappear. (as you tell the camera that whatever the colour of the light...this is a white paper) Currently you probably have it set to daylight wich is very blue...when fluorecent tubes usually are very green - hence the magenta/green tint.

whoops seems several of us responded at the same time hehe - oh well

Good luck! :)

//O.

[Don Palomaki]

For 50 Hz power, using a shutter of 1/100 or 1/50 should be OK.

For 60 Hz power, using a shutter of 1/60 or 1/100 should be OK.

Canon has documented the effect, See the note on the bottom of page 55 of the (NTSC, don't know about PAL versions) manual.

[Vincent Oliver]

Thank you for your replies. The colour change is over a perriod of 30 seconds, it gradually moves from a slight green to magenta and back. This will last for the duration of the take. I have experimented with Custom WB, gain settings etc. and they all still produced the same problem.

Since posting this question I have spoken with a fluorescent lighting company, and they told me it was due to my elderly Kaiser lights which operate at a slower switching speed than a modern system - hence the phasing in and out of colour.

I guess it's going to be another £450 out of my bank account.

[Brad Jordan]

Just shot a wedding this weekend and the bride's room is under flourescent lighting. It seems to be dependent on the quality of the lights/electricity that determines whether or not I get the "phasing" under flourescent lighting. I certainly got it in this particular room. I believe I can fix in post, but man I hate flourescent lighting. Doesn't seem to help custom setting the white balance as it is set on white in a certain cycle of the light phase.

Up until about a year ago, I thought my cameras were going bad (it was happening on my VX2000's) and now I see it on my XH-A1 when in certain flourescent lighting conditions. I certainly wish there was some custom setting I could come up with that would minimize or eliminate this problem.

Any ideas?

[Don Palomaki]

I say again, try a shutter speed that approximates the line frequency; i.e., 1/60 in USA or 1/50 in UK. That should solve the issue for fluorescents and other discharge lamps that operate at power line frequency or faster. If you use an auto exposure mode that allows the camcorder to vary shutter speed, the issue could pop up again.

Try it and if that does not improve the situation significantly, let us all know.

The 30 second variation implies that the the main frequency and the camcorder frame rate were very close, differing by perhaps ~0.02 Hz. Power line frequency averages out to spot on 50 or 60 Hz at the end of the day, but at any given instant could be a bit higher or lower. The 0.02 Hz difference appears to be within expected short term tolerances, assuming the camcorder was spot on 50 Hz.

[Richard Hunter]

Originally Posted by Vincent Oliver

Thank you for your replies. The colour change is over a perriod of 30 seconds, it gradually moves from a slight green to magenta and back. This will last for the duration of the take. I have experimented with Custom WB, gain settings etc. and they all still produced the same problem.

Since posting this question I have spoken with a fluorescent lighting company, and they told me it was due to my elderly Kaiser lights which operate at a slower switching speed than a modern system - hence the phasing in and out of colour.

I guess it's going to be another £450 out of my bank account.

Vincent, the advice on this forum is correct. I saw it before when I used Av mode once (in the mistaken belief that this was the best way to control depth of field). Switching to Tv or manual with a shutter speed of 1/50s immediately fixed the problem. I suggest you test it out for yourself before listening to a guy that quite possibly has an interest in selling you something expensive.

Richard

[Vincent Oliver]

Thank you Don and Richard,

I will try a slower shutter speed later this morning and up-date you with the results.

[Vincent Oliver]

I have just shot a quick test using a shutter speed of 1/60 and another at 1/50. I take my hat off to you, it has cured the problem, there is still a very small amount of colour shifting but it's not obvious enough to worry about.

Here is a composite still showing the colour variation I was getting (see strip in middle) - sorry it's a naff picture, I didn't want to spend too much time on the shot if it wasn't going to work.

http://www.photo-i.co.uk/Board/frame.jpg

[Don Palomaki]

Interesting and about what I would expect. The attached image shows a similar but different effect resulting from interaction between the room fluorescent lights (on in one image, off in the other) and the high frequency lamps on a book scanner. In this case it resulted in a vertical ribbed look. Some folks would pay money for that effect as a plug-in filter <g>.