Re: Colormatch vs Adobe 98
Re: Colormatch vs Adobe 98
- Subject: Re: Colormatch vs Adobe 98
- From: email@hidden (Bruce Fraser)
- Date: Thu, 31 Jan 2002 10:29:31 -0800
At 12:19 PM -0500 1/31/02, Don Hutcheson wrote:
<snip>
WHITE POINT:
Here's why you should avoid D65 like the plague:
1. Three-and four-band colorimeters are normally 'hardwired' to D50 and
CANNOT measure D65 correctly.
While it's relatively rare for Don and I to have major disagreements,
this is one of them. Some three years ago, I had teh opportunity to
compare various monitor calibrators using a 0.5nm spectroradiometer
as a reference. The X-Rite DTP 92, the Gretagmacbeth Spectrolino
(which is admittedly not a colorimeter), and the Sequel instrument of
the day all came within 100K or so of producing 6500K tristimulus
values on the subject (CRT) monitors.
2. Any D65 color space will give you a blue cast if you convert from it
with Absolute intent, or a yellow-red cast if you convert to it with
Absolute intent.
This is true, but not terribly relevant.
3. All graphic arts viewing booths and viewing standards specify D50 for
color comparisons.
Around the same time I did the monitor calibrator comparison, I also
went through the rather depressing exercise of measuring some dozens
of viewing booths in everyday use in the SF Bay Area. All were
nominally D50, but the measured range went as far south as 3600K, and
as far north as 7500K. The dimmable viewing booths tend to be less
accurate in color temperature than the non-dimmable ones.
4. Virtually every printer, separator, agency, photolab, photographer or
publisher has access to some form of D50 viewing condition and uses that to
make color judgements. Changing to D65 puts you out of sync with your
clients, vendors and industry standards.
See above.
5. The ICC PCS (Profile Connection Space) defaults to D50.
Again true but not terribly relevant unless you insist on using
Absolute renderings inappropriately.
6. All CIE spaces by default assume D50, in fact 'CIELab' should really be
written 'CIELab(D50)'. CIELab(D65) is computed quite differently and there
is no reliable translation between the two without going back to the
original multi-spectra data, which is thrown away in most profiles.
7. Converting D65 samples or profiles to D50 uses a processes (typically a
Bradford transform) which only works properly if all light sources used for
capture and viewing have a full D50 spectrum with 100 CRI (Color rendering
Index.) This is virtually impossible. The transform cannot solve metamerism
errors or work properly for low-CRI sources. For that we would need profiles
containing multi-spectral samples, not just the three L, a, b, variables.
While I agree with the stated premisses, the conclusions drawn are in
direct conflict with my experience, and put far too much weight on
dragging CIE colorimetry into an area for which it was never designed
and is probably ill-suited.
CIE colorimetry is based entirely on comparison of reflective
samples. It was never designed to handle emissive samples. And one
major difference between the way we view reflective and emissive is
that in the former, our eyes play the well-known trick usually known
as "discounting the illuminant." With emissive sources, we cannot do
so, because the image is self-luminous.
You may be able to get a monitor and a light box to both produce
tristumulus values that add up to D50, but you can't get them to
produce remotely the same spectra. I firmly believe that our
perception detects that spectral difference at some level, in ways
that CIE colorimetry, by design, does not address.
Visually matching your monitor white to a bright white but
non-fluorescing paper in the viewing booth is a good idea, but if you
do so, and measure the result with a research-grade instrument, I'll
be you dollars to doughnuts that the result will be much closer to
6500K than 5000K. Recalibrating the monitor to match each paper stock
is a recipe for madness....
Most of today's CRTs made for graphic arts work have a color
temperature out of the box that 's much closer to D65 than to D93,
though that wasn't true in the past. Most LCDs have backlights that
are hard-wired to D65.
Every time I've measured daylight at sea level outside the golden
hour, it's come in between 6300 and 6700K. This suggest to me that
D65 is a more appropriate color temperature for dealing with captures
than D50.
The D50 model breaks down completely when you're dealing with
fluorescent papers or, even worse, fluorescent inks. This is an
admittedly much wider problem than monitor calibration...
But the main reason I advocate D65 is, simply, that it works in all
the installations where I've specc'd it.
My experience is that you'll have a much easier time creating
original-to-screen matches and screen-to-print matches if you
a) calibrate the monitor to D65 white, and
b) move the viewing booth so that it isn't in the same field of view
as the monitor.
Short-term memory is very reliable on color matching. Forcing you to
look from the screen to the sample allows your built-in
white-balancing algorithms, which are better than any man-made ones,
to adjust for any small discrepancies between the two.
None of this is to say that I have anything other than the utmost
respect and affection for my antipodean colleague. This is simply not
an area where we have definitive answers. Do what works for you.
Bruce
--
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