Re: RelCol Mapping of Middle Gray
Re: RelCol Mapping of Middle Gray
- Subject: Re: RelCol Mapping of Middle Gray
- From: Ken Fleisher <email@hidden>
- Date: Mon, 07 Mar 2005 22:49:50 -0500
actually, I don't think that's true. The white point of Lab is D50 but
D50 is NOT the paper white. The paper white will be a number relative
to D50 (as if measured/viewed under D50) but will probably be
something like 96,1,3.
This is normally true, but not for ICC. The white point for ICC L*a*b*
is the media white point, not D50 or any other illuminant. From the ICC
spec, p.75:
"The relationship between PCS CIEXYZ and PCS CIELAB is given by the
usual set of equations as defined in ISO 13655:1996 but where the media
white point (rather than the illuminant) is used as the relevant white
point."
Ah! I see what you are talking about... OK then yes... sort of. IF you
want to compare measurements from two different paper types to see if
they match (not side-by-side but after the viewer has adapted to each
of them) then yes, you can pull the paper white out of the
measurements using XYZ scaling
That's not what I was describing. In order to compare L*a*b* values
from an ICC profile and L*a*b* values from a spectrophotometer, you
have to use the the same white point in the XYZ to L*a*b* conversion.
The spectrophotometer will use D50, or whatever illuminant you set, for
the white point, so these values will not match ICC values because of
the modification noted above.
I guess you could do that too but is that the typical way of doing the
conversion? I implemented that a while back... I'll have to go check.
I thought you just normalized Y....
It's the correct way if you are comparing ICC L*a*b* and values from a
spectrophotometer. It's not the correct CIE L*a*b* method. I know, it's
confusing!
Ken
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