xyZ to LAB and dE (2000)
xyZ to LAB and dE (2000)
- Subject: xyZ to LAB and dE (2000)
- From: Steve Kale <email@hidden>
- Date: Sun, 25 Nov 2007 01:59:26 +0000
- Thread-topic: xyZ to LAB and dE (2000)
Hi there
I am trying to construct a spreadsheet to convert numbers from xyZ to LAB
and then calculate dE (2000). I¹m following the formulae on Bruce
Lindbloom¹s site and I¹m bumping into issues. The formulae for the
conversion of xyZ to XYZ to LAB couldn¹t be simpler but I¹m getting very
modest differences between the answers in my spreadsheet and the numbers
generated by his CIE Color Calculator. The values for L* are perfect but
there are very small errors in a* and b*. I can only think I have the value
of reference white slightly wrong, deep in the decimal places. What are the
exact xyZ values for D65? I see that the definition of D65 has moved over
time. What would he be using in his calculator and would this differ from
SMPTE-C Rec601?
I¹m also getting errors in my dE calculations albeit I can¹t for the life of
me find the error. I¹m comparing target SMPTE-C Rec601 R, G, B, C, M, Y and
white values with measures from a calibrated display. My dE for R, G, M, Y
and White are very, very close but not exactly those generated by Bruce¹s
colour difference calculator but Blue and Cyan are way off. It¹s
frustrating because I¹ve been over the formulae a dozen times. Any ideas?
(I know that¹s wishful thinking...)
Regards
Steve
PS: when he writes tan-1(b1/a¹1) I¹m encoding in Excel ATAN2(b1,a¹1). I¹ve
been careful to check the degrees vs radians.
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