Here's my second take on white balance, color space, exposure and histograms.
On the Nikon D200, the Optimize Image > Custom menu was set to:
Image Sharpening - None
Tone Compensation - Normal (0)
Color Mode - I. This is the default, recommended for portrait. (Even though i suspect that "recommended for portrait" means "rosified", I chose it for the test, because mode II is only selectable if aRGB is chosen for the color space.)
Saturation - Normal (0).
As before, the Color Checker was illuminated by 2 Solux® lamps to EV 10. The starting exposure was according to the Kodak gray card, and the following exposures were increased in 1/3 f-stops.
Either color space (aRGB and sRGB) was shot with AWB and UNI WB, respectively. UNI WB was achieved with the grossly over-exposed image of the Kodak white card.
I forgot to turn off the room lights, which have a greenish cast, but since color rendition was not the issue here, I let it go.
I logged the occurence of the 1st highlight warning and the first highlight clipping of the histogram:
"ETTR choice" is the exposure I would have chosen in the field, i.e. the last one before the highlight warning. In the case of aRGB UNI WB, the 1st highlight warning was so subtle, that I might have neglected it in real life. That's why I include this and the preceding exposure step in this case.
For the comparison of the histograms, I choose the "ETTR choice" rather than the 1st highlight warning this time.
ACR was set to Exposure +0.5, everything else 0, Tone Curve linear. The camera profile is ACR 4.4. I have included the CC image with the histogram to show the highlight warning of ACR.
968: aRGB, AWB, +1.0 EV, in ACR:
968: aRGB, AWB, +1.0 EV, in Rawnalyze:
974: aRGB, UNI WB, +1.0 EV, no in-camera highlight warning, in ACR
974: aRGB, UNI WB, +1.0 EV, no in-camera highlight warning, in Rawnalyze:
975: aRGB, UNI WB, +1.3 EV, subtle in-camera highlight warning, in ACR:
975: aRGB, UNI WB, +1.3 EV, subtle in-camera highlight warning, in Rawnalyze:
982: sRGB, AWB, +1.0 EV, in ACR:
982: sRGB, AWB, +1.0 EV, in Rawnalyze:
990: sRGB, UNI WB, +1.3 EV, in ACR:
990: sRGB, UNI WB, +1.3 EV, in Rawnalyze:
Conclusions (for the Nikon D200) ??
As judged by the unmapped histograms of Rawnalyze,
1-the right-touch of the in-camera histogram is always too late; the highlight warning is the parameter to choose.
2-in either color space, AWB gives reliable highlight warnings.
3-in either color space, UNI White Balance results in highlight warnings too late.
4-Thus, the use of Uni White Balance does not get you "closer to the right" without clipping.
5-In aRGB with UNI WB, ACR gives highlight warning and shows a clipping histogram even when the camera doesn't, and Rawnalyze shows almost 1/3 f-stop of headroom. (#974)
6-A strange observation: in # 974, 975, and 990, ACR places the highlight warning in CC field row 4 column 2 rather than 4/1 (besides the white background wall). This can not be due to uneven illumination. In-camera, the highlight warnings appear as exspected, starting in field 4/1, then spreading to 4/2. (I kept the images on the CF card).
Good light in a happy new year!