Digital BW, The Print

Paul used the raw CIELAB data in two AaI&A light fade test reports to do a very creative analysis of optical brightener activity in two different papers. I'm impressed.

The moral of the story is that the susceptibility of the OBAS to light induced-fading in both papers (and probably gas fade induced fading as well) was comparable, but the one that used a high concentration of OBAs in the top coating layer (i.e, the Red River Paper) took an initial hit that didn't happen to the paper that deployed similarly vulnerable OBAs in subbing layers or paper core only. In other words, the inherent molecular structure of the OBAs used in both papers may very well have been identical, but their location and concentration influenced the final light-induced fading appearance. 

The practical outcome is that papers with high OBA content, especially in the top coat layers, will register strongly negative initial b* values on spectrophotometers or colorimeters using gas filled tungsten light sources and no UV cut filters, and they will appear to the human observer as strongly "cool white" in appearance under any light source with moderate or high levels of UV content.  Light induced or ozone gas-induced "burn off" of the high OBA content fluorescence will then result in easily noticeable "yellowing" on the strongly brightened papers whereas papers that use the OBAS more judiciously will exhibit less apparent "discoloration" over time because the OBAs played a smaller role in initial whiteness of the paper.  

kind regards,

Mark
http://www.aardenburg-imaging.com


--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "pr_roark" <paul.roark@...> wrote:
>
> I've posted a graph of the paper white Lab B values for 2 brightened RC papers tested by Aardenburg Imaging.  The Lab B values in a brightened paper will increase as the OBAs fade, mostly due to oxidation.  This fading of OBAs will appear to make the papers "yellow" relative to their initial tones.  I wanted to see how a premium paper might compare to a paper that is less expensive.  I chose Epson Premium Luster and Red River UltraPro Gloss for the comparison.  See the graph at http://www.paulroark.com/BW-Info/OBA-compare-Eps-RR.jpg 
> 
> I've discussed the results with Mark at Aardenburg Imaging, and he thinks the initial dip in Lab B values for the Epson paper is probably within what might be caused by normal read variances.  I was speculating that maybe Epson had employed some sort of counter-fade strategy (intentional fast fading yellow to offset the initial OBA fade).
> 
> Mark thinks the major factor that distinguishes the results is that OBAs are often put in 2 locations.  The ones under the barrier of the RC papers are better protected from oxidation, which causes the fade.  OBAs are also often put in the coating above the RC barrier, where they are much more exposed to oxygen and fading.
> 
> Assuming that explains the initial differences, it's interesting that the premium and less expensive papers exhibited almost identical OBA fade after the initial 20 MLux Hours of exposure.  It certainly seems that the OBAs under the RC barrier are essentially the same.
> 
> Paul
> www.PaulRoark.com
>