I'm thinking part of the problem is that we need to also do a calibration for Tungsten light (Type A). I'm guessing that's why the colors shift a bit with different color light, even after correcting for the white balance.
This is likely to be part of the issue.
Denis requested us to use the daylight white balance, but I think what was intended was that we take the photo in daylight and use the daylight white balance. Offline he's indicated we should use the appropriate white balance setting for the light source rather than artificially set an inappropriate white balance setting.
Researching a little further,
CIE states we should be using two different illuminants for colorimetry:
- Standard illuminant A - "standard" indoor tungsten lighting
- D65 - "Average daylight" @ 6500K, similar to what you would see around the middle of the day
These are the light sources used in camera matricies that DNG4PS-2 inherited from dcraw (17 and 21, respectively), so it makes sense that the second item is what Denis was referring to when he meant daylight. Unfortunately this type of lighting isn't that easy to come by unless you get some 6500K fluro tubes (with a CRI of 80+, preferably 90+). Purists wouldn't be satisfied with those but they are the closest that I can find to practical D65 lighting.
My calibration photos were taken under natural light or 5000K energy saving bulbs. From what I've read the CRI of energy saving bubls is ~0 so I wouldn't recommend this route, and I think my better results came from natural lighting.
Bottom line: even lighting across the whole screen is important, and ideally should be natural daylight. The camera should be manually (not automatically) set to the correct white balance and you should publish the lighting and white balance type along with the matrix..
Thanks,
Matt
PS I'm learning all of this color management stuff as I go along. Its amazing how much material there is out there about this!
