Adobe Camera Raw: calibration and color accuracy

Question

"In this article I report the accuracy reached with the above scripts in my calibration effort, some validations in different lightning condition and the impact on new color balance caused by the adjust sliders and the tonal curve movement."
If someone has an interest.
Link to Article
Ciao
Marco

Answer 1

Chris,
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The ICC uses LAB D50, as does Photoshop.
Marco,
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(prof. Boscarol) on his forum:
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1.Lab is not absolute and there are infinite Lab.
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2.The illuminant E is a theoretic reference illuminant
I must disagree with the implied conclusions in both statements, but with a friendly, not confrontational tone. And, Adobe and other software may be assuming Lab is D50. I cannot and would not dispute that.
What I said is that if you follow CIE math, the color values in Lab mode will be at Illuminant E. I stand by that conclusion. But there is little in the way of enforcement when it comes to standards.
Lets start with the CIE color values from measurement to numbers. This is the illuminant times the subject (measured) times the standard observer. It requires matrix arithmetic because the operations have to span the visible spectrum. So it is a little more involved that simple multiplication, but that is not conceptually important at this point.
The light source may be defined by a standard such as D50 or D65 or it can be provided in a custom set of tables (from measurements). The subject (raw image) values are adjusted for the measurement instrument to represent illuminant E (equal energy). This would be the spectral response of the color filters and such in an image sensor. The standard observer values are provided by the CIE, again at illuminant E. The resulting XYZ values are at the white point of the illuminant used for the source light.
Before moving on, the image sensor spectral response tables are too often not available. Thus assumptions are made that make calibration so bloody difficult. Enough said.
These XYZ values are then transformed to Lab values. The math in this step effectively removes the original light source from the XYZ values, resulting in illuminant E. This is how it is described in the literature, Berns, Hunt, and Wyszecki. If one adheres to the rules, Lab will always be illuminant E. XYZ values without a white point definition are as meaningless as RGB values without a profile definition.
But there is no enforcement body. I have seen lots of code that does not adjust for the XYZ white point at all. In that case, there would be an infinite set of possible Lab white points.
If you assume Lab is always D50, D65, or whatever floats your boat the transforms from RGB to Lab to RGB again would not be compromised. The ICC does define a white point in each ICC RGB profile. So if the input colors are correct, the output colors will be correct. It isnt too important what white point is used for the intermediate step.
But if you take a file in Lab mode from some other source, the white point would be very important. If I give a Lab file to Fred in E and Fred assumes it is D65, color conversions will be wrong. If Fred creates one at D65 and hands it to Adobe, chaos reigns. The ICC does not have a profile for Lab mode that I am aware of. So I dont know that the ICC attempts to trump the CIE as suggested. If someone knows of a verifiable reference for this, I would love to hear of it. The TIFF metadata does have a tag for white point. But I have never seen it used in a Lab mode file, including Adobes.
Illuminant E is no more or less theoretical than any other Standard Light Source. But it is at the core of all color matching algorithms and the basis of the standard observer target values. It is most often simply referred to as the equal energy light source so it might not ring a bell like D50 or D65.
The conversions between Lab and LCh and the algorithms for Delta E 2000 color differences are all implicitly dependent on Lab values at illuminant E.
I rest my case.
Cheers, Rags :-)