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Subject: Color targets for digital imaging

Color targets for digital imaging

From: Tim Vitale <tjvitale>
Date: Monday, August 18, 2003
Many color targets are available.  Of these, the traditional Kodak
Q-13 and Q-14 are the least valuable.  I routinely use the
GretagMacbeth ColorChecker, a 24-patch, 8 x 11.5 inch color target.
It has individual matte surface color patches in a paper frame to
prevent marring of the surface.  They cost about $60-70 online, or
from Calumet.

The value of the ColorChecker is the 6-patch gray scale that is
always neutral, and always the same from target to target.  The
Kodak Q-13 and Q-14 are never neutral and never have a consistent
color cast from patch to patch, or from gray scale to scale, but
they are "good enough" for film. The Kodak Q-60, IT8.7, color target
has roughly 290 color patches but the grays are never neutral and
never have a consistent color cast within a specific gray scale.
However, you can download measured values from a Kodak FTP site for
each target.  Many Color Management tools in the $300-700 range use
the IT 8.7 target (there are several target manufacturers). The
Kodak version costs roughly the same as the ColorChecker.  Kodak
Q-60, 4x5 and 35mm transparency targets are also available; measured
values are available online.

In Photoshop, I use >Image>Adjust>Curves, to set individual RGB
values of the six gray patches to: 243/243/243, 201/201/201,
161/161/161, 122/122/122, 85/85/85 and 53/53/53 (plus/minus 1-2 RGB
units).  This yields a density range of 1.7D at a gamma of 2.2: a
very solid result. These values were developed by Robin Myers (a
ColorSync co-creator)
<URL:http://www.rmimaging.com/information/information_index.html>. I
used them extensively to make inkjet printed surrogates that match
the originals remarkably well.

Artifacts generally have a density range well below 1.7D.  If an
original has deep darks with detail, suspect a greater density
range.  A different linear gamma will need to be applied to the gray
patches, or the artifact's density range will be compressed using
the above calibration.  (You can generate these values in Excel,
using Myers' base values as a guide.  The white will stay the same,
but the grays will have lower RGB values allowing for the denser
darks in the original.)

The ColorChecker is about 8 x 11.5 inches, but you only need about
1/2 x 9 inches in the frame to get the full gray scale.  I use it
for both scanning and digital photography.  Generally, I suggest
labeling files with corrected ColorChecker gray scales as "v0" at
the end of the naming string; thus, those files without the notation
are not corrected.

There is a smaller version of the ColorChecker that costs about
$100-120, but I have not used one.  The digital version,
ColorChecker DC, still has some problems, so it is best to say away
for a while.  The DC has many more patches, some are gloss to
achieve a greater tonal range, but they can give serious reflection
problems in some lighting situations.  The DC costs roughly $300/420
(with calibration). GretagMacbeth suggests using InCamera software
($150) to make ICC profile using the ColorChecker DC or the regular
ColorChecker.  I have done this with success (using the BetterLight
scanning back). A new profile is required for each new lighting
setup.

Tim Vitale
Emeryville, CA
510-594-8277


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                  Conservation DistList Instance 17:21
                  Distributed: Monday, August 18, 2003
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Received on Monday, 18 August, 2003

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