Digital Prints

What Are Endpoints, and Why Are They Important?
Many instrument-measured tests currently use densitometric endpoints—the point at which a test sample
fails and the test ends. Initial starting densities are recorded on different neutral gray, pure color, and
minimum density (“paper-white”) patches. When repeated measurements of the patches show that one
has reached its allowed percentage of density change, that patch has reached its endpoint.
Wilhelm Imaging Research uses a unique, endpoint criteria set that is visually weighted. Instead of uniform
percentages across the board, WIR has employed focus groups and other psychometric factors to
determine what changes are more acceptable than others. For example, most people will tolerate a
Caucasian flesh tone going more red or pink but not more green. Also, WIR’s blue density change for
paper-white has a higher limit that allows the paper to go yellower before failing. Wilhelm colleague
Mark McCormick-Goodhart believes that’s because the human eye is adapted to campfire light, and we
tend to accept some visual yellowing.
The selection of the endpoints is very important in densitometric-based testing. If you want a sample to
appear to be more long-lasting, all you have to do is increase the endpoint percentage so that it takes
longer to reach it. That’s why some don’t agree with the industry standard 30-percent or more density
loss. “Our opinion is that the 30–35-percent fade rate is too generous,” says InkjetMall’s Jon Cone.
“While it makes for impressive ratings, it does not meet users’ expectations, especially when you realize
that the threshold in humans for detecting fade is 5 percent.”
ADDENDUM: Wilhelm and McCormick-Goodhart are now working on a new test method that is
based not on densitometric endpoint criteria, but rather on CIELAB colorimetry. This is partly due to
all the complexity in digital printing including multiple ink colors, unique black generation, etc. It is not
known when this new test method will be accepted, although ASTM methods and other testers are
already using variations of spectrophotometric measurements and Delta-E comparisons.
A typical result of dark fading is the formation of yellow stain on color photographic or
inkjet prints. Dark fading is not limited to dark storage, but since it is primarily due to
the instability of the materials, it starts the moment the print is made and takes place independently
and at the same time as any light-caused fading if the print is displayed.
Different types of colorants or media will have different dark-fading rates, and the only
way to know for sure is to wait for it to happen or to test it. Wilhelm Imaging Research
now includes “dark storage stability—including paper yellowing” as an official category
in its Display Permanence Ratings (DPR). In fact, Wilhelm now states that they won’t
provide a DPR of greater than 100 years for any print material unless Arrhenius-accelerated
dark storage stability test data indicates that the print can indeed last longer than
those 100 years without noticeable deterioration.
Lightfastness Tests
All prints will photodegrade in the presence of light. That means that they undergo a photochemical
reaction when exposed to lightwave photons, and they start to deteriorate. The
changes can be in the form of fading, darkening, or changing hue (color). The more resistant
a print is to this inexorable process, the more lightfast it is.
Chapter 5 ■ Determining Print Permanence 157