Inkjet profiling ABC_2
Inkjet profiling ABC_2
- Subject: Inkjet profiling ABC_2
- From: Henrik Holmegaard <email@hidden>
- Date: Tue, 19 Dec 2000 15:45:40 +0100
g. If users were left to their own devices (no pun intended -:)), all
this wouldn't be very easy. This is why an inkjet system keys its
printing behaviour to its current media state. When inks and papers
are loaded, a detection mechanism enables a print mode adapted to the
current media combination. Some inkjet systems are expandable through
third party media programs. This takes the guessing out of working
with other media (which are usually expensive).
h. Because of all these factors, CMYK inkjets may have a raw mode and
a 'smart TAC mode' which leaves the black channel intact and cuts
back CMY, while CMYKcm inkjets actually have no raw mode (isn't
useful for six color devices) but still allow a 'smart TAC mode' that
hands the black channel to third party color management software. And
the 'smart TAC' mode is in both cases policed by automatic on-board
densitometry linearization.
This is basically how the HP internal PS RIP DesignJets work.
Inkjets typically have a large gamut volume, which is ideal for
colorimetric proofing. Also they are stable from print to print,
which dye sublimation and toner-based printing technologies aren't,
as more and more will learn by using the MeasureTool in ProfileMaker
that replaces ColorLab, the inhouse tool.
For color proofing, several things are required. The proofing device
must be a PostScript device or there is no control of the CMYK
channels in the profiling software. The proofing device must have a
larger gamut volume and better black and white point than the proof
process for a non-interpretive absolute colorimetric conversion. And
the conversion must be device independent, of course.
In a device independent conversion, the color patches of the
testchart (e.g. K100%) are described using three channel L*a*b
values. This means that the proofing conversion doesn't care about
the original channels of the offset separation. What the proofing
conversion cares about is the colors of the offset separation. In a
mechanical proofing system this is the other way around. You care
about the original channels, and not about the colors.
Thus reseparation is integral to device independent conversion. The
color fidelity will be as good as the custom profiles, and the
percentage values in the objects in the PostScript stream will be
converted to suit the actual inkjet destination proofing device, too.
Loosing the original offset separation channels which the reprosauria
consider a disadvantage, is actually a benefit. And technologies that
do four channel to four channel conversion, device link profiles, a
subset of CMMs, etc., are not upcoming technologies, they are
technologies revived and quite possibly soon forgotten once more.
You can't feed a PostScript inkjet deviceCMYK separated for offset,
because deviceCMYK is imaged by the numbers, and those numbers are
wrong for the inkjet device. You must have a CMYK to CMYK conversion,
either in the layout application, in an ICC color server or OPI
system, in the RIP frontend, or in the PostScript CMS using a CMYK
CSA that retargets incoming offset deviceCMYK, though not to a freely
configurable assumed CMYK source the way an ICC color server does.
What you want to do is configure each device to its maximum gamut
volume and best visual quality rendering, profile that device state,
and leave the color management system to optimize the device values
while maintaining the real colors the same. The old approach is to
separate for offset, and then feed other devices the same values.
This isn't really a good idea, like the notion that a proof is a CMYK
to CMYK device link that doesn't pass through CIELab so you get the
original black channel, or the idea that you load colorants and media
with a gamut volume shrunk at the factory into the proofer, and then
hope this fixed small gamut will match actual offset processes,
rather than boosting the gamut volume of the proofing system as much
as possible, knowing that this way it's likely that more processes
can be accurately proofed.
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Henrik Holmegaard, TechWrite
Stationay +45 3880 0721 - +45 3881 0721
Mobile +45 2178 3959
Toelloesevej 69, 2700 Broenshoej, Denmark
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