I have been following that discussion, but don't yet fully appreciate the significance. I was sad to see that IGFS, which I know is one of your favorites - for good reason, does actually have OBAs.
I need to re-read some of that thread, but is the LILIS effect limited to papers with OBAs, or papers with baryta coating?
Light induced staining/discoloration (LIS) of media can be due to many different chemical reactions in the media. Perhaps the most well known one is the rapid paper yellowing that occurs in cheap wood pulp papers where the lignin content has not been removed nor the paper acid neutralized. So any media, with or without OBAs, needs to be evaluated for resistance to yellowing and discoloration, but in general OBA-free papers are indeed more resistant to yellowing over time. However, early production lots of Museo Silver Rag paper are an example of an OBA-free cotton rag paper that also had some yellowing issues. Therefore, although I'm optimistic the Moab Juniper paper will have stable media white point properties, it nevertheless needs to be tested to know for sure.
Papers with OBA "yellow" over time simply due to loss of fluorescence, but the belief that the faded OBA degradation by-products merely remain colorless over time is also suspect. The LILIS effect is a particularly novel media yellowing reaction involving the OBA content and very possibly related to the use of TiO2 whitening agents combined with those OBAs as well. RC photo media are especially prone to high levels of the LILIS effect because they typically have significant OBA content, and the OBAs are usually embedded right in the PE-TiO2 layers of the RC photo media. OBAs are also typically present in the RC paper core as well. The LILIS effect is novel because it has a clear reciprocity law failure (RLF) characteristic. The famous reciprocity law in photography states that light intensity x time = exposure, so when high versus low intensity levels along with their respective amounts of time needed to produce equivalent exposures don't produce the same results, that means the reciprocity law failed in that situation. Thus, the build up of yellowish stain on display due to the LILIS effect depends on the average illumination levels on display which is why I have taken to calling it a low-intensity light induced staining effect. Perhaps a more accurate description would be "light induced discoloration with RLF". The RLF aspect of the problem makes it difficult to simulate properly in high intensity accelerated light fade studies, and consequently, it has been going under reported in many of the published print longevity ratings. More research is definitely needed, but in the meantime, the printmaking community needs to be informed that virtually all of today's RC photo type papers (and some non RC papers as well) exhibit the LILIS phenomenon. The only RC paper I've tested to date that doesn't seem to be affected by LILIS (so far in testing) is Epson Proofing Paper White Semimatte. EpPPWSM seems to be an exception to the rule because it does not have any OBAs incorporated into the PE-TiO2 layer. Almost every other RC photo paper on the market, including all the popular new "metallic" RC papers, employ OBAs in the PE-TiO2 layers.
In the day when color RC photos were comprised of rather fugitive dyes that would fade badly in a few decades, it could have been argued that RC media yellowing was not the weak link in the "archival" properties of the print. However, with today's high stability pigmented ink sets, the battleground for the most "archival" prints is now clearly shifting to the respective properties of the media. I have attached a sample page which shows the LILIS effect in the Aardenburg 30 patch color target printed on Epson Premium Luster RC photo paper using a very high stability ink set (HP Vivera pigments on an HP Z3200 printer). For comparison, I have also included the results from the same printer and ink set but printed on Hahnemuhle Photo Rag Baryta (a non RC paper containing no TiO2 and no OBAs). The exposure dose was 140 Mluxhrs (equals 70 "Wilhelm years" on display) and then the samples were retired to dark storage. They were measured after nine months in dark storage. At the high intensity illumination level of the light fade unit the LILIS effect was suppressed and was not present in the EPPL paper immediately after removal from the light fade test unit, but serious yellow discoloration is now forming rapidly in dark storage. At nine months the b* value of the EPPL paper's white patch was 6.0. I just measured again today (another month in the dark) and it has increased further to 7.3. The yellowing reaction is apparently not yet near completion. I don't know where it will end up, but it's not acceptable, IMHO, even now.
Anyway, take a look at the two papers' performance in the attachment below, and see if the LILIS effect is something for serious printmakers to be concerned about. As a printmaker trying to make long lasting prints which are highly resistant to change over time, I personally think this amount of media yellowing is a serious issue, and I want to be able to evaluate media yellowing properties better than I have to date
kind regards,
Mark
http://www.aardenburg-imaging.com