I was curious about the condition for our beloved inkjet papers in a real-life daylight situation. What happens with the paper itself, it's whiteness....
After using one of Bruce Lindblooms calculators I got the differences as deltaE-values (CIE76), for the first value and the one seven months later:
Cotton Rag, 1.8
The Lab-values tell us more what is happening; a very modest shift in Lightness, around 1 or lower, but some huge shifts in the b-axis, resulting in a strong yellowing. Because of rapid fading of the whitening agent (which gives us the blueish reflected light).
And the big surprise, for me anyway, is the low (good)values for the RC paper and the very high (not so good) shift for the Fiber-based paper....
A deltaE value of 8.9 is in fact a very strong yellow cast. In seven months.
Yikes, I feel like we've covered a lot of this ground before, and in my case, I did name names, and I was left with a sense that most of the participants in this forum and elsewhere simply don't care. They like a paper, they buy it, they assume it's going to last long enough to meet their needs because they are using a pigmented ink printer. If not, they will let the cards fall where they may.
That said, Sven's test is indeed a very real world result with logical caveats. It doesn't statistically represent the whole population of OBA-free and OBA-filled papers, nor does it speak directly to more benign interior conditions. But it is real world, and this test exercised commonly encountered conditions of humidity, gas exposure, and thermal conditions for prints that are placed front facing in windows...e.g., think about a photography studio that displays examples of its craft in the front window, usually framed with glass since the prints won't be on display year after year. Sven managed to yellow two premium inkjet papers noticeably, the worst being a premium "traditional fiber print" type, in just seven months. If the print were a high-key bridal portrait and the b* value changed by 7 or 8 units in just seven months, that print would no longer represent optimum quality. If I were the owner of the studio, I'd swap it out for a new print.
Now, based on my own real world print monitoring experience that adds dataloggers to gather actual temperature, humidity, and light levels during deployment of a print at any given location, let me share some educated guesses about Sven's environmental conditions. Sven's window test produced a temperature and humidity cycle that very likely went on a daily basis (when the sun shined) from typical room temperature and RH levels like 21 degrees centigrade (70F) and 50%RH to print surface temperatures in excess of 40C (i.e.> 100F) with concurrent print moisture content levels similar to those reached at equilibrium with environments less than 15% RH. That's a big swing, but it's also real-world. Think about your car interior parked at night and then parked in sunshine. Prints facing forward in a commercial window display experience this behavior. Sven's light exposure dose was somewhere in the range of about 1 to 2 megalux hours of exposure per month. His test ran seven months so we're discussing a total exposure somewhere on the order of 7 to 14 megalux hours. His paper media discoloration is consistent with AaI&A laboratory tests for some papers in the AaI&A database. The worst performers in AaI&A tests are triggering AaI&A conservation display limits due to highlight color changes and OBA burnout in the 8-15 megalux hour exposure range and many OBA containing papers reach the AaI&A conservation display rating (CDR) lower limit in the 20-40 megalux hour range. Sven's media change for the matte and fiber samples would have caused the lower CDR limit to have been exceeded but the cotton rag and RC samples aren't there yet. So, the cotton rag and RC paper samples have measurably changed, but the typical fine art collector of these print samples would not be getting concerned yet. In other words, he or she wouldn't really notice yet. Not so for the matt and Fiber samples. They exhibit definitely noticeable changes and ones to be concerned about if the piece has collector's value.
Based on reciprocity law treatment, and using the industry-standard extrapolation of 450 lux average illuminance for 12 hours per day, 7-14 megalux hours of exposure and Svens' measured paper yellowing would be projected to occur in more typical interior locations in about 3.5 to 7 years from light exposure alone. Add gas fading for uncovered prints and you can find papers beginning to go yellow much much sooner, but even under glass this yellowing would happen in less than 10 years. So, how come industry ratings don't flag these "bad apples"? Well, a delta E = 8.9, even if it is all due to the b* shift upon yellowing, is apparently not enough to trigger the industry criterion for easily noticeable print fading. This level of paper yellowing is deemed acceptable for consumer photos, and it's this consumer toleranced test that is still being applied to fine art print media ratings (except at AaI&A). In the industry's defense, one way to look at it is that we routinely buy new papers that range from bright white to warm white, and as such, they have initial b* values of -10 to about +5. That's a 15 delta E spread, so if we don't mind this media color range when new, why would we mind any specific paper if it changed anywhere within that range from -10 to +5? Ok, Industry defense over. Fine art printmaker's hat on!... because if I desired a bright cool-white paper and carefully printed my image content to suit, I wouldn't want the highlight areas of the print to turn warm white any time soon! That's the funny thing about ratings criteria. It's a matter of personal standards for what is acceptable, and at least for me, that also depends on the application. I don't care if the 4x6 photo I tape on my refrigerator dies quickly. I do care if I just bought a fine art print worth big bucks and it noticeably loses its pristine image quality during my ownership period!