Hue Angle for Black
Hue Angle for Black
- Subject: Hue Angle for Black
- From: email@hidden
- Date: Wed, 26 Aug 2009 09:16:53 -0700
Greetings,
(FYI, I posted a similar question on the Print Planet list - for those that are concerned about cross posting. Thanks, as
always, to Mike E for your reply)
Can black have a hue angle?
To help me in my G7 qualification runs on our flexo presses, I measure 3 consecutive press sheets and average 6 readings
(3 from each side of the press). I have created an Excel file that enters these 6 Lab measurements as I measure them with
my 528 (using a MAC) and then calculates chroma, hue, dE (under CMC 2:1); and plots L*a*b* to tolerances, and shows an
approximated hue angle view from the average of these measurements.
My challenge is creating the dE for the black. I use the formulas below my name that I received from a colleague. These
formulas work for CMY when compared to the results achieved on Bruce Lindbloom's web page.
However, I am not able to compute a hue angle for black. Since my math education ended with Geometry, I am a little
challenged with these formulas, but I did notice that with black's a* and b* being 0, the formula for dH*ab returns a 0
divided by 0, and I know dividing by 0 is undefinable. My guess is that this is what is "breaking" the formula to compute
hue for black. But once again can black have a hue angle?
I am able to get a tolerance when comparing two blacks using Bruce's site, so I am hoping some one can share with me
how that is computed.
Thanks to anyone that takes a little time to help me and if seeing my Excel file would help, I will gladly send it.
-Bill-
Since as I mentioned, I received these formulas from a colleague (in an Excel file), I had to translate the cell in his file to its
appropriate LabCH values so I could then use them in my file format, which differs from his. The second line in each
formula below is that translation.
SL =IF(B3>16,(0.04097*B3)/(1+0.01765*B3),0.511)
=IF(L#1>16,(0.04097*L#1)/(1+0.01765*L#1),0.511)
SC =((0.0638*E3)/(1+0.0131*E3))+0.638
=((0.0638*C#1)/(1+0.0131*C#1))+0.638
F =((E3^4)/(E3^4+1900))^0.5
=((C#1^4)/(C#1^4+1900))^0.5
T
=IF(F3>164,IF(F3<345,0.56+ABS(0.2*COS(RADIANS(F3+168))),0.38+ABS(0.4*COS(RADIANS(F3+35)))),0.38+ABS(0.4*COS(R
ADIANS(F3+35))))
=IF(h>164,IF(h<345,0.56+ABS(0.2*COS(RADIANS(h+168))),0.38+ABS(0.4*COS(RADIANS(h+35)))),0.38+ABS(0.4*COS(RADI
ANS(h+35))))
SH =+AE3*((AG3*AF3)+1-AF3)
=+SC*((T*F)+1-F)
dC*ab =((H3^2+I3^2)^0.5)-((C3^2+D3^2)^0.5)
=((a#2^2+b#2^2)^0.5)-((a#1^2+b#1^2)^0.5)
dH*ab =((H3*D3)-(C3*I3))/((0.5*((J3*E3)+(H3*C3)+(I3*D3)))^0.5)
=((a#2*b#1)-(a#1*b#2))/((0.5*((c#2*c#1)+(a#2*a#1)+(b#2*b#1)))^0.5)
dL Term =+((B3-G3)/(2*AD3))^2
=+((L#1-L#2)/(2*SL))^2
dC Term =(AI3/AE3)^2
=(deltaC/SC)^2
dH Term =+((AJ3)/AH3)^2
=+((deltaH)/SH)^2
dEcmc2:1 =+(AK3+AL3+AM3)^0.5
=+(L term+C term+H term)^0.5
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