- I photograph the passport image in even lighting, Adobe RGB color
- open it in ACR
- save it as a DNG
- use the DNG editor to create a profile with no edits
- apply that profile to the original image in ACR
- create a tone cure in ACR to match the values from the XRite site
- open the result in Lab mode in CS5 using Adobe RGB and look at the patches
That is what I have been doing as well, except it is best to use ProPhotoRGB as the working space, since Photoshop would have to use the Bradford transformation to convert from the D65 of Adobe RGB to the D50 of L*a*b. A bit depth of 16 would help to prevent rounding errors. According to Danny Pascale
, the Bradford transform produces an average ΔE of 1.4.
Essentially I'm using the DNG editor to build a profile, then applying that profile back on the same image used to generate the profile which I would think would result in an accurate color result even if the tone curve was wrong.
The result is acceptable in most cases, delta E of around 3-6 compared to the XRite published values but several are over 10 off even if I force the L values to be the same as the published values.
Since luminance is included in ΔE, failure in controlling luminance will cause ΔE to increase. BTW, how are you calculating ΔE? The CIED2000 is said to be the most accurate: i.e. correlates with observed differences in color. The various formulas are discussed by Norman Koren in the Colorcheck
documentation. I find that program to be invaluable in evaluating the accuracy of the profiles (the DigitalDog would cringe on reading this
). The CIED2000 usually gives smaller ΔEs and makes the data look better as well as better correlating with observed differences (which I have not checked).
Here is the best that I can do with my D3 and the DNG profiler. The results might have been better had I constructed a dual illuminant profile for my tests which used Solux illumination which is about 4700K. The Passport results are slightly worse.
I did try all this using the XRite software as well and the results were generally better although there were still errors > 9 in patches different than the ones with large errors from the DNG editor.
Overall it does look like the biggest errors may be the DNG editor using patch values that aren't the same as the XRite Passport but there is still some kind of residual error in both. Anyway, if the results in values are noticeably off, perhaps there needs to be a way to tell the DNG editor what target it's looking at.
Certainly for many uses, accuracy isn't really what's wanted, but that's not the case for some applications, and as it stands I don't see how one can generate an accurate profile with either tool. There's no editing in the XRite software, and because of the way the UI works in the DNG editor I don't see how it would be at all reasonable to correct all the patches by hand.
The Colorchecker probably is not introducing that much of an error unless it is old or improperly stored. XRite recommends replacing the chart every 2 years. Here are the ΔEs that Danny Pascale found in evaluating 20 charts.
One must realize that perfectly accurate color is not possible with current digital cameras, since the respond differently than the human visual system to the various colors. In constructing a 3x3 matrix, the profile maker tries to minimize error, perhaps using a least squares approach, or perhaps favoring accuracy in various memory colors such as blue sky, foliage, human skin, etc. A landscape profile would be different than one optimized for portraits.
It would be interesting to learn what other users were obtaining with the DNG profiler and the Passport software.
If you need accurate reproduction under controlled conditions, it might best to use the Colorchecker SG (which uses many more patches) and make an ICC profile. This would entail considerable expense and you would have to use a raw converter (such as Capture One) that uses ICC profiles.