RE: [ECI-EN] Tone value reproduction limits and Color Management System
by Bestmann, Guenter RD-PN32
Hello Mr. Symonowicz,
In color reproduction it is good practice not to adjust images to
RGB=255 for white and RGB=0 for black. It is best to adjust white to a
RGB which will give a 3% dot value for cyan and 2% for magenta and
yellow (gray balance) and to adjust black that will give a maximum tone
value sum, f.e.320%. This gives a RGB=249 for white and RGB=10 for
black. (All values are approximatly for ISOcoated and sRGB) Highlights
(specular lights) will be in the range from RGB=255 to 250, very dark
colours will be in the range from RGB=9 to 0.
Working in this way will be in accordance with the ISO spec as mentioned
below.
On the other side it is a problem to properly image dots on plates in
the range from 0 to 2-3%. During film to plate conversion these dots
will sometimes disappear. During print runs these small dots are not
very stable and will perhaps also disappear after some 1000 sheets.
A further problem are ICC profiles. Because of the coarse grids (25 to
33 grid points for L, a, b) and the interpolation between these grids it
is not possible to exactly build profiles that cuts dots smaller 3% or
greater 97% in the gray axis with proper gray balance and in the process
colors. Up to now it is not common to cut the CMYKs in the output trc's
of profiles to 3 and 97%.
Hope this helps a little bit.
Kind regards,
Guenter Bestmann
Heidelberger Druckmaschinen AG
Research & Development
-----Original Message-----
From: eci-en-admin(a)lists.transmedia.de
[mailto:eci-en-admin@lists.transmedia.de] On Behalf Of Szymon Symonowicz
Sent: Tuesday, April 12, 2005 9:25 PM
To: eci-en(a)lists.transmedia.de
Subject: [ECI-EN] Tone value reproduction limits and Color Management
System
In ISO standards it can be found information about tone value
reproduction limits depending on screen ruling (12647-2) or print
substrate type (12647-6.2).
For example:
-in ISO 12647-2 for screen ruling between 40 and 70: 3 % to 97 %,
-in ISO 12647-6.2 for coated paper 3 % to 85 %.
I would like to not discuss about this specific values rather about how
to achieve CMYK files with some tone value reproduction limits.
We can get CMYK files in different ways. But my wondering is concerned
in getting CMYK files from RGB files. One way to do this is using CMS:
RGB file through source profile to LAB to (through) destination profile
to CMYK file.
But how to create an ICC CMYK profile that creates separations with some
tone value reproduction limits?
During creating an ICC profile in separations options we can find some
options for example: TIL, black (!) start, UCR, GCR, etc. Why it is
impossible to adjust starts for rest of colors (CMY)?
For example using "isocoated.icc" and sRGB and RGB image after
conversion
(perceptual) the highlights are C 0% M 0% Y 0% K 0% or C 1% M 0% Y 1% K
0%.
Another way to get separations with specific tone value limits is to cut
in RIP highlights and let them start from 3%, but this operation doesn't
compress the image range but cuts the image highlights resulting in
image loss.
Summarize:
How to get correct CMYK files with specific tone value range using CMS?
Question on different subject:
Does anyone know any free software that lets plot given LAB values on
LAB diagram (3D diagram) or AB values (2D diagram)?
Simon
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