I am trying to construct a spreadsheet to convert numbers from xyZ to
LAB
and then calculate dE (2000). I’m following the formulae on Bruce
Lindbloom’s site and I’m bumping into issues. The formulae for the
conversion of xyZ to XYZ to LAB couldn’t be simpler but I’m 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?
I’m also getting errors in my dE calculations albeit I can’t for the
life of
me find the error. I’m 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
Bruce’s
colour difference calculator but Blue and Cyan are way off. It’s
frustrating because I’ve been over the formulae a dozen times. Any
ideas?
(I know that’s wishful thinking...)
Regards
Steve
PS: when he writes tan-1(b1/a’1) I’m encoding in Excel ATAN2(b1,a’1).
I’ve
been careful to check the degrees vs radians.
