Approach to white balance implementation?

[Otis Grapsas]

I'm working on a digital cinema camera and I'm implementing the white balance scheme. Ideas on the subject and what would make the implementation more usable in a production environment would be very helpful.

Some thoughts:

Auto white balance using a card, makes color correction in post difficult, because white balance result depends on user experience and the same correction cannot be applied to a large number of shots. Material shot using fixed white balance presets on a scene should be easier to correct because it will have more uniform and less extreme errors.

White balance presets can have any number and be presented in Kelvin for matching standard home, office, studio and natural lighting situations. This can cover 2900K to 7000K.

White balance in camera uses gain on some of the R,G,B elements. So, a scene lit with 100watt household bulb is at 2900K and will need 12dB of gain on blue in a given camera. If an extraordinary quality camera has a 66dB s/n, the blue s/n will deteriorate to 54dB which is a typical broadcast figure for s/n and nothing to write home about.

An alternative is to white balance using filters. This is the engineering approach to the problem that maximises noise performance. Stacking an 80A and an 82B will take 2900K to 5500K which is the natural balance of the above camera. There will be a loss in exposure of course, but if the camera already needed to be stopped down in order to handle the red elements, the camera could still get good exposure with the filter losses, even at 0dB gain. That would require no blue gain and would produce the ideal 66dB s/n on all three channels. In higher lit situations, no gain would be required because the losses can be covered by opening the aperture.

I understand that practical reasons will lead most users to use in camera white balance instead of filters.

 

[Chris Santucci]

Either white balance with your key lighting, OR use the presets and then filter as needed (like it's done with film.) You can do both. Of course, adding filters will degrade the image.

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[Otis Grapsas]

Sorry, perhaps I'm not being clear about this. I'm writing the software for this camera and I am also choosing the internal filtering in the camera sensors, so I have total control of how white balancing will behave and how it will be presented to the user. So I'm basically asking what you would prefer for white balance control in a digital cinema camera.

The current design includes a daylight balanced sensor/filter setup that is white balanced to other temperatures using color gain like most camcorders to presets of lighting using generic values, just like most DSLRs (daylight, shadow, incadescent etc). The camera also has a white balance control that can use a grey card.

I'm thinking of:

1) Keeping the balance to card feature.

2) Removing the named presets and making more presets that will give you the actual color temperature, with a possible short description so the options will be:

1800K, 2900K, 3200K, 3400K, 3800K, 4000K, 4200K, 4600K, 5000K, 5500K, 6000K, 6500K, 7000K, 9000K.

3) Offering a special setting for using a filter in setups that require too much gain in one of the three colors. This does not deteriorate the quality, it actually improves it a lot because you can avoid the gain, and has excellent results with affordable low color temperature lighting equipment. So an option could be:

3400K with 80B to take it to the sensor default 5500K. This improves the blue noise a lot because it reduces the blue gain requirements and is balanced to the filter, so there are no color issues. You can get ultra clean shadows and lots of options in post if you use color filters to balance situations that go too far from the sensor default balance. There are no light loss issues with these sensitive sensors. If you are using lights, there is already too much light and you have to stop down. The camera can actually skip the ND and open the aperture more which is sometimes a plus in the DOF department.