Graphics Programs Reference
In-Depth Information
next chapter. The scanner is designed to digitize either film or prints so
the best possible target would be made from the same materials. Scan-
ners (and digital cameras) don't really have a color gamut per say, but
the materials that we will eventually scan do. Therefore, placing a
piece of transparency film with hundreds of colored patches on a film
scanner provides the optimal target for analyzing how the scanner “sees”
film. The same is true for a reflective print. If we have a scanner that
can scan both film and reflective materials, we will need to create a
profile for each type of original material. Not only are the color gamuts
of both different, most scanners that can scan film and print uses two dif-
ferent light sources and behaves differently when scanning each type of
original.
The most common target for producing a scanner profile is known as
the
IT8
(Kodak likes to call them a Q60). Figure 4-1 shows an example
of the reflective target from Kodak and Agfa. The IT8.7/1 is a target made
of film and the IT8/7/2 is made from a reflective print material. Notice
that the Kodak target always has the lovely image of
Shirley
, which plays
absolutely no role in building a profile. She does provide, however, a
somewhat useful skin tone reference and gives the entire profile process
a more human touch.
These targets are available in several sizes of transparency film, usually
35 mm or 4 ¥ 5. The targets are often available in different film types
based on the manufacturer. You could purchase a 4 ¥ 5 target made on
Fujichrome
TM
and one on Ektachrome
TM
(see the sidebar, “Targets of Dif-
fering Film Emulsions”). Included with each target, either film or print,
is a small text file called a
TDF
or
Target Description File
. The TDF, as seen
in Fig. 4-2, is just a text file with all the measurement data of each color
patch in the target. The profile building software has to examine the data
captured from the scanner and compares that to the data of the physical
target. The TDF supplies the measured data of this target to the profile
building software. TDFs can be batch measured or hand measured. With
a batch-measured TDF, the manufacturer creates a number of targets,
measures a representative number, and averages the measurement data
into one TDF. The hand-measured TDF is one that has the actual meas-
ured data from a single specific target. Is there a difference in the accu-
racy of the resulting profile? It is difficult to say since we really have no
idea how many representative samples of each target were measured to
produce the average TDF. I can say with certainly that a hand-measured
target/TDF will cost significantly more money. It has been my experience
that such targets are usually worth the extra expense if every ounce of
profile quality is required.
Hand measuring a reflective target isn't too difficult if you own
your own Spectrophotometer and you are a patient person. Due to its
size, hand measuring a 35 mm is virtually impossible with any equipment
available to the average person. Hand measuring a 4 ¥ 5 transparency
can be done with a few Spectrophotometers but is an agonizingly slow
