If you understand how to interpret LAB numbers, then useful information on initial media white point color is reported in the media L*max, media a*, and media b* columns in the AaI&A database:
http://www.aardenburg-imaging.com/cgi-bin/mrk/_4899c2hvd19kb2NfbGlzdC80Note: you can collapse the columns in this database list and reselect only those you want to see. You can also sort on ascending/descending by clicking once or more on the column label for any column you wish to sort on. And you can also filter the list to narrow the items to, for example, a specific paper of your choice.
Additionally there is a column labeled "UV ∆b* influence" This column reports the b* color shift that occurs in the media when the UV component of the spectrophotometer's illuminant is excluded during measurement. Most people with normal color vision can visually "just notice" 1 unit of ∆b* shift, 3 is easy, 5 is very easy. Exhibition Fiber paper samples measure in the 5-6 range. OBA-free papers return values of 0 to 0.3 ∆b*. HN photo rag measures about 1.1-1.9. It has low OBA content but is not OBA free. Thus, the more OBA, the larger this UV ∆b* influence value, and the more color shift you will notice when UV energy is blocked from the illumination falling on the print or when the OBA burns out over time. UVA exclusion can be accomplished with a glazing or it can simply be filtered out right at the light source itself.
Now, as you've noticed the glazing can impart additional color due to color forming impurities in the glazing or other changes in transmission qualities that "reach" into the visible spectrum. UV blocking OP3 plexi, for example, cuts sharply at about 400 nanometers but still keeps preferentially absorbing at least somewhat into the blue region up to about 420. Hence, some will see any print, even OBA free, as shifting more yellow under OP3 plexi, but it will shift even more yellow if OBAs are present in the media. Standard soda lime glass typically has trace iron content that causes it to look a little green. As Ernst noted in another thread about this, "water-white" glass can be specified that has lower trace levels of iron and so appears clearer than standard soda lime. It's easier to find in Europe, but Tru Vue does import it into the U.S. and then coats an anti-reflection layer (not a full UV block). So, all in all, you are on the right track just to make a pragmatic judgement about what bothers you or not by just trying different glazings over the print. Best place to judge is probably in any white or very highlight areas of the print. The most color-clear glazings are typically standard optical grade acrylic or the water white glass.
As for how much any of this OBA and UV blocking glazing interaction becomes an issue for you also depends on how critical the media white and highlight color accuracy is to you in your printmaking. And whether it's important to carry that media whitepoint on into subseqent display environments. If none of this mattered at all to anyone, there'd be no reason for manufacturers to add OBAs, but they've obviously learned that many people like the whole "brighter-than-white" appearance of high OBA-content papers. Yet the typical customer walking into a framing shop probably doesn't even know what OBAs are let alone if the artwork to be framed contains any. And not all framers understand the OBA issue, either. Thus, they may offer the customer "conservation framing" and not inform the customer that a bigger trade-off is going to occur in paper white color if the print has high OBA content. When the framed print comes back, the overall presentation will be what is mostly noticed. Only a more color discerning printmaker or collector will take the time to pin down all these trades between UV content, OBAs, and choice of glazing with a more informed understanding of the issues.
my 2 cents,
cheers,
Mark
http://www.aardenburg-imaging.com
