# Colour management and calibration!

Edmund Humenberger (ed@atnet.at)
Fri, 14 May 1999 09:56:08 +0200

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I am by no mean prepress expert, I just saw some problems of
colour handling in computers and how these can be handled.

Scanning and printing colours in general is a big problem!

The result depends on a lot of factors (papertype, machine, plating....)
So colour handling is seen as an art by professionals.

I saw one solution by a researcher in Gemany. He just calibrated the
complete production circle.

->Computer------>Printing to paper---------> Scanning-----
| |
----------------------------------------------------------

As Printing and scanning and colour room conversion (RGB<>CMYK)
(or Separation to Hexachrome 6 colour printing) is changing the colours,
he inserts Calibration filters, which are called "colour profiles in
the press industry.

There can be one colour profile for the whole process of scanning and
printing.

->Filter---Computer--->Printing to paper---------> Scanning-----
| |
----------------------------------------------------------------

You just print a refernece sheet on your printing process (which
produces
wrong colours), scan in this printed wrong colours and add the errors
of scanning and then produce a colour profile which is the result of
comparing the original refernece data and the scanned data.
The calibration process produces a Neuronal network, which translates
the wrong scanned colours to the right colours.

As a result, when you scan now an original photo, the photo is
fed through this neuronal network, which correncts the error of
scanning,
and adds the appropriate adjustments to the data so that the resulted
printed image is the same as the original.

The disadvantage of this is that you always have to calibrate a
combination of scanner and printer.

To have a calibration of each equipment, you would need to
scan a reference sheet of colours and calculate the
error which is produced by the scanning, and then recalculate
what error is produced by the printing.

In this process the colour on the Monitor are not correct.
Calibration of the monitor must be done by
adjusting the displayed colours to the reference colour card
and calculating afterwards the colour profile. this is
a lot of work, or you do have a very expensive colour measurement
equipment which measures thedisplayed colour on the monitor.

I suggest such a colour manangement infrastructure for SANE, but
where the colour transformation can be encapsulated in a module so
that different Colour transformations can be implemented.

Yours

Ed

PS: I saw an implementation of the Neuronal Network Colour correction
system. The result is really amazing. The printed copy looks
identical the the original, just printed on an inkjet printer.

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