Arthur Entlich's advice above is well-balanced and very much to
point. In this somewhat belated post, I'd like to add just a few
things. For years now, some manufacturers, like Kodak, and some
academic researchers (see, for example, the Image Permanence Institute
publications) have been advocating a holistic approach to predicting
image degradation. This means taking into account all four major
factors (heat, light, humidity, and pollutants). Further, it's
important that these factors be in balance. That is, predictions should
not be based on one factor at an extreme level (the "worst case"
scenario) and others at more normal levels, as some have done. Further,
providing the data for all four factors will also permit consumers to
adjust the predictions for their own particular environments if they
differ from the median values.
To do this, it's necessary to measure in rigorous scientific fashion
how consumers use prints and base testing on those measurements. Much
of that work has been done in the papers by Douglas Bugner and his
colleagues (and before that, Anderson and co-workers) that have
appeared in the Journal of Imaging Science and Technology. Together
they have measured light levels, temperatures, humidity, and ozone in
typical homes around the world. As reality checks, they've hung
images in real homes to see if they agree with predictions made on the
basis of the factors.
As studies have shown, over 90% of consumer images are kept in the
dark, including people's most precious images. Thus, for many people
dark stability is of primary importance and a product chosen on the
basis of projected high light exposure may well be the wrong one.
Similarly, consumers today often display their ink jet images without
glass covers or mattes. In these cases, atmospheric pollutants may have
the greatest impact and light testing under glass (as some laboratories
practice) may be of little relevancy.
Because natural age keeping tests would not be complete before the
products themselves would be obsolete, all manufacturers, as Arthur
Entlich notes, have to use accelerated tests if they want to know how
their products are likely to perform in the real world. But, as he also
notes, high intensity tests may not cross over to normal keeping; this
is called "reciprocity failure." This is why reliable companies and
laboratories also do their tests at several levels of the factors to
check for consistency within reasonable error limits and publish the
most conservative position. (Not everyone does these tests or does them
correctly.)
Virtually all companies and testing organizations currently use cool
white fluorescent (CWF) as their light source for testing. But the
recent ground-breaking work by Bugner and co-workers has shown that CWF
may not be the best model for home lighting (though it is likely to be
correct for offices and other commercial space). That's why companies
like Kodak are starting to run tests using the more severe filtered
xenon light in anticipation that international standards could move in
that direction.
The CWF vs. xenon issue is also a good example that one size doesn't
fit all in image permanence testing. The mix of factors that influence
image fade in homes, offices, commercial spaces, and museums can be
quite different, and making claims based on the data relevant to one
environment result in bad choices for another environment.
