RE: calculating from abscol to relcol from L*a*b*
RE: calculating from abscol to relcol from L*a*b*
- Subject: RE: calculating from abscol to relcol from L*a*b*
- From: "tl" <email@hidden>
- Date: Tue, 21 Apr 2009 06:37:47 -0400
Hi Terry,
" Anybody here know where I can find the formulae for calculating
relative colorimetry from absolute L*a*b* where the L*a*b* for paper
white is known? I know that it's not as simple as subtracting paper
white L*a*b* but I've searched and can't find the math used to come up
with relative colorimetric values."
>From your post, it's not clear what you are really trying to do.
First and foremost, Lab is not an absolute measurement under any
circumstances. It always, by definition, contains an arbitrary white point
to which all subsequent data is normalized. For most users on this list, the
white point is D50. Because of the normalization process, it is possible
to get back to XYZ, but the meaning of the XYZ is very specific and it is
still corrupted by the illuminant. If you are trying to eliminate the paper
white and the illuminant from the measurement, then it is necessary to do
this in spectral space. If you are dealing with Lab data that is in a
profile and trying to compute the equivalent XYZ values from this data, the
reference that Klaus provided is the mechanism. Equation D.7 is the one you
would use, but be aware, because of the normalization (Wrong Von Kries) used
in the Lab conversion, the XYZ values are not necessarily correct.
So Terry, Please tell us what you really want to do, rather than ask for the
formula to do it.
Regards,
Tom Lianza
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