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Why is the flat screen's WB shifting on me? [pic]

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Sam Carleton
Why is the flat screen's WB shifting on me? [pic]
on Jul 22, 2009 at 4:01:43 am

I am coming to you all with a photography lighting question. There are two reasons, lighting is equally important in both mediums (video and still), and it has to do with flat screens and projectors in the shot:

Ok, I have been around photography a long time and fully understand the concept of color balance. I know, or at least think I know, that most flat panel displays are balanced for 5000K. The stage lights at church are, of course, 3200K.



What I don’t understand is why in one frame the flat panel display in the background looks green and the next frame, 3 seconds later, looks good, though a little blue.



Do flat screens shift in color balance the way florescent lights shift? Is there another reason for this?



Most importantly, has anyone ever had to go in and balance lighting to work with displays and/or projectors that will also be in the shot? I would like to go talk to the person in charge of the lights and displays/projector and see what might be done to improve things a bit, but I want to walk in having some answers.


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Michael Palmer
Re: Why is the flat screen's WB shifting on me? [pic]
on Jul 23, 2009 at 4:06:03 am

Was the camera set to auto white balance? Is this a digital still camera or video camera? If a video camera were you moving the camera during the shoot?

I was at this years CES in Vegas and one display there really confused one of my Sony XD Cam EX HD video cameras to the point I had to shut it down and do a complete camera reset. There was a display that changed colors rapidly in the shoot and soon the faces of the people I was shooting turned very dark and none of the colors looked like they should. I wonder if I still have these clips as It was another run & gun shoot and I turned over the data at the end of the job.

Good Luck
Michael Palmer


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Sam Carleton
Re: Why is the flat screen's WB shifting on me? [pic]
on Jul 23, 2009 at 12:50:45 pm

I am shooting RAW with the camera set to 3200K and processing the images at 3200K. In such a controlled situations where the lights are a consent, it makes no sense to shoot on Auto WB.

Sam


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Richard Herd
Re: Why is the flat screen's WB shifting on me? [pic]
on Jul 24, 2009 at 4:39:34 pm

Here's my best guess: It's the screen not the camera. There are two refresh rates on the x-y board. Horizontal which (at WXGA rez 1280x768) is either 48.1 Hz or 48.4 Hz. The Vertical refresh at 60 Hz. The RBG pixels are arranged as a bayer pattern, but they all don't receive power simultaneously. It happens so fast that our human eyes hardly perceive it, rarely also will a video camera operating roughly at the same speed as the monitor. But a still camera with a "fast" shutter (faster than the inverse of the focal length) will show it, when comparing frame to frame.

An interesting experiment to try: Shoot a screen with various shutter speeds, see what happens.


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Sam Carleton
Re: Why is the flat screen's WB shifting on me? [pic]
on Jul 24, 2009 at 5:21:58 pm

Your explication makes perfect sense. I know my still camera and single CCD video cameras use the Bayer pattern, but it had not dawned on me that LED/LCD screens do the same, but it makes perfect sense now that you say it. And the fact that the R,G, and B are going to be out of phase, also make perfect sense.

Is there some type of display, like Plasma, that won't have this problem? I am thinking if there is some type of display that does two key things:

1: All three colors are in phase.
2: The physical stuff that is luminance is able to maintain it's is state while the current is out of phase, allowing for high shutter speed on my still camera. (I want to be using 1/320 to 1/500 to freeze things with my long lens)

Sam


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Richard Herd
Re: Why is the flat screen's WB shifting on me? [pic]
on Jul 24, 2009 at 7:26:16 pm

You might be interested in something like 4:4:4 video monitors: http://visionaryforces.com/monitors.html

However, you also need the capability to send it that signal. Capture, edit, deliver via HD-CAMSR
http://visionaryforces.com/decks.html

The luminance value is in the green.

The human visual spatial system seems to require contrast more than color.

What happens at 1/48 or 1/60? Exposing motion pic is frame rate time shutter angle. So 30 frames per second times a 180degree shutter makes a 1/60 shutter speed. I wonder if that would give the image enough time to "mix the colors," Or would it be blurry? The video is cycling at very near 1/60.


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Sam Carleton
4:4:4 projectors...
on Jul 24, 2009 at 7:38:02 pm

I knew about the 4:4:4 cameras, never realized there are also 4:4:4 tape decks and displays, but it make sense.

I assume a 46" to 50" 4:4:4 flat screen is NOT cheap. Now that I think about it, in the four months I have been attending this church, the flat screens always display a static image. Maybe there is an alternative to using flat screens that will be more still camera friendly. I will have to explore that one else where...

The last picture I displayed was from an over head projector. I assume there are 4:4:4 projector, so I have some Q's about that:

1: Is it possible to convert a lower quality signal, I think the cameras are all 4:1:1, up to 4:4:4 just for the projector?
2: Is it possible to adjust and/or pick the white balance of projectors? Ideally I would like to set it to 3200K to balance with the stage lights.

Sam


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Richard Herd
Re: 4:4:4 projectors...
on Jul 24, 2009 at 9:33:06 pm

This is a weird topic for the lighting forum :)

Yep that equipment is expensive.

Yep, you can convert signal to "dual link." A quick search turned up this: http://www.evertz.com/products/7732DVP-HD but I am not certain this will solve your problem, because the electricity still has to cycle through the electronics, the speed of light is fast, but as you know snapping a photo stops time, so there's always going to be some kind of refresh and you can always snap a photo between cycles.

Yep, there are 4:4:4 projectors, check out DCI (digital cinema initiative) http://www.dcimovies.com/

Yep, you can pick the white balance of the projector (probably need the remote for that and the manufacturers typically have a series of hidden menus also. You can call them for support.) Often you can also set the white balance on the computer's video card that's originating the signal.

Another thing to note: When LCDs are on 24/7 (like the ones I'm using here in Tahoe), over time, they seem to retain a charge that shifts the color into green (since there are double the amount of green pixels compared to red and blue, this makes sense). What's worse is turning them off to let them charge down often means they don't come back on because the power board retires to a beach in Cabo.


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