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Some critics have discussed the use of independent,
s
ingle-condition, highly accelerated light- and dark-
fade tests as very misleading.
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These tests are
useful for rapid screening of experimental dyes, but
are prone to possible errors (e.g., reciprocity in light
fade or reaction-mechanism shift in dark fade),
w
hich limits their reliability in predicting print
longevity. They don’t reflect real-world conditions
a
nd require very careful interpretation. This is true
not only because they are accelerated well beyond
the normal light and heat levels found in a home,
but because the data are often reported in isolation.
Running stability tests in a window gives even more
misleading results.
Because thermal stability is so important in the
portrait and social environment, testing and
reporting on light fade only, without considering the
impact of thermal stability, would be relevant only
to consumers who store their prints in a lighted
freezer.
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Predominance of Thermal Stability in the
Portrait/Social Environment
Thermal stability, often called “dark stability,” is
driven by ambient temperature. This is especially
important in the portrait and social environment
where light levels are low. Thermal degradation,
even when prints are on display, predominates.
Note: Temperature can play a role in the commercial
en
vironment as w
ell—
f
or example,
in transmission display materials used on warm light
bo
x
es. Ho
w
ever, the time frame for a commercial
display is relatively short (often three to 12
months). The thermal effects do not become
apparent because the light-fade
effects predominate.
When you see the term “dark stability,” remember that it is
n
ot darkness that causes dyes to fade or D-min to turn
yellow; it is heat. Therefore, even when a print is on display
(unless it’s in that lighted freezer), thermal degradation is
taking place. Dark stability is actually the combined effects
of thermal fade and everything else that is not related to
l
ight fade.
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H
owever, from the early history of color
photography through the early 1980s, thermal fade was
t
he principal mechanism.
Improvements to thermal stability have been
infrequent; but when they do come, they have been
very large:
• Kodak’s introduction of 5-ethyl-4,6-dichloro-2-
amidophenolic couplers resulted in a three- to
fourfold improvement in print stability. KODAK
EKTACOLOR Plus and Professional Papers first
used this new technology.
• The use of non-yellowing pyrazolotriazole (PT)
class magenta couplers virtually eliminated
yellowing of print D-min caused by both
temperature (thermal yellowing) and light (“print-
out” due to unreacted magenta coupler). Kodak
first used this technology in KODAK
PROFESSIONAL PORTRA III Paper.
• It was not until the invention of 2,5-
diacylaminophenol couplers that excellent thermal
stability combined with desirable color hue was
achie
v
ed. Kodak invented these couplers and
patented them in 1997.
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KODAK EKTACOLOR Edge
8 Paper first used them in 1999.
8
R.E. McComb, “Separating Facts from Fiction: Examining Photo Prints,”
PhotoTrade News, February 1998.
9
Op. cit., R.E. McComb.
10
M. Oakland, D.E. Bugner, R. Levesque, and R. Vanhanehem, Proceedings Paper
from NIP 17,
2001, p. 175.
11
U.S. Patent 5686235 (Nov. 11, 1997) and U.S. Patent 5962198
(Oct. 5, 1999).
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