Calibrating and characterizing ABC
Calibrating and characterizing ABC
- Subject: Calibrating and characterizing ABC
- From: Henrik Holmegaard <email@hidden>
- Date: Wed, 10 Oct 2001 09:38:58 +0200
email@hidden wrote:
Calibration is a scanner function and should be documented in the scanner
docs.
Automatic calibration occurs when you power up the scanner. For
instance, the mid-range Circon reads 'Lamp Calibration' in the
display panel.
Calibration changes the behaviour of a hardware configuration. The
behaviour is changed to comply with a desired target state.
Calibration is part of your hardware, thus your Heidelberg scanner
has lamp calibration, your Apple monitor has electron gun
calibration, your Hewlett-Packard printer has densitometric
calibration, and your GretagMacbeth Eye-One Pro or iCColor or
Spectrolino has white tile calibration. Calibration is built into the
hardware right out of the box.
Color management is a separate technology, an optional conversion
system you place on top of individual, manufacturer-specific hardware
calibration implementations. If you do not use the calibration your
hardware comes with, the hardware behaviour will drift away from
under the color management system, quite simply. But the two are
still conceptually and practically distinct.
The confusion arises because Heidelberg like Adobe calls a CIE
reference a calibration. Thus if an RGB, CMYK or Gray recipe is
referenced to CIE Lab or CIE XYZ, it is said to be calibrated.
What happens when you click the Calibration Scan to Disk command in
Linocolor 6 is that an 8 bit raster file is placed on the disk that
describes the combination of lamp state, CCD filter state and
photographic media. This lowrez raster file you then load into an
input profiling application that supports dynamic range settings /
variable range settings. And in the input profiling application you
marry the raster file with a file that states the CIE values for the
patches you scanned.
The process of describing how a hardware configuration forms color is
properly characterization, and not calibration. It is my
understanding that the UI will be changed at some point.
Now if I could just get the Adobe side to at the very least quit its
internal PostScript color management jargon and use a consistent
terminology the rest of mankind has a decent chance to understand,
I'd maybe be getting somewhere -:).
The scanner documentation for my F4100's do not reveal that even with the
"Scan as Calibration" option the density is not zeroed, and that the
calibration scan will have a tagged profile if your profiling options are
not turned off. This screws up a calibration scan.
Like I don't know how many other users here on the List and on
European lists, you are assuming that creating a 'calibration' scan
to disk and scanning an image both involve tagging. This is not your
fault, but the fault of the documentation.
Clearly if you want to characterize a scanner setup, you want the raw
RGB values that reflect what the scanner sees when fed a set of
patches with known CIE references. Similarly, if you want to
characterize a printer setup, don't alter the CMYK test chart through
host-based ICC color management or RIP-based PostScript color
management in the printing process. You must not convert the test
chart values, or the process goes south.
Also I suspect you don't understand what a variable range input
profile is, or why you should use one. This again is not your fault,
but the fault of the documentation. I can't remember how many posts
you will find explaining this in the List archives.
Anyway, Linocolor does not let you convert the values. But the UI and
documentation does not say that Linocolor does not let you convert
those values, or even that you aren't supposed to try.
The paedagogical fault is not to give the poor user the grand
strategy, the bird's eye view of what it's all about. If the tactical
moves are also bungled in the description, then the user is ditched
with no concept of direction at all.
I could instance a good many examples much worse than the Linocolor
UI. Both Apple and Adobe have a long list of dark and dreadful UI
sins ... and sinfully missing color management implementations to
boot ... -:).