Oh, while we're on the subject, correct me if I'm wrong, but aren't the fluorescent agents, used for OB, absorbing UV, which we pretty much can't see, and the "agent" emits light at a lower frequency, to a part of the spectrum we do see -- which happens to be blue? Obviously there are phosphors out there that can fluoresce across the full spectrum(remember CRTs). So, is it the cost that prevents OBs from being full spectrum? Or some kind of energy issue ( i.e. CRTs used high energy electron beams to excite the phosphors)?<
I doubt there are paper friendly and affordable OBAs (FWA, Fluorescent Whitening Agents) available that absorb UV and emit the absorbed energy evenly throughout the visual spectrum. Even if available there will not be much need for it. The light reflection of pure cellulose as used in papers is already high at the red end of the visual spectrum and can be improved with whitening agents like TiO2 and Barite. 98% reflection at that warm end is not uncommon. Without OBAs it is however hard to achieve a completely neutral paper, even the best OBA less papers have a Lab b 2 at most. To bring it to neutral, b 0, little amounts of blue emitting OBAs are needed, usually stilbene dyes, other whitening agents and pure cellulose are then still needed to keep overall white reflection high and even over the visual spectrum. To give papers the often preferred cool white reflection b -2, -5, -10 there is more OBA needed. If papers are not judged on their Lab values, spectral plots of the light reflection or whiteness but on their brightness (which is measured on the blue 457 nanometer wavelength) then cheap blue emitting OBAs often disguise the low light reflecting properties of the basic paper, a spectral plot will show a drop of the light reflection in the green-yellow-orange range, say around 570 NM. With both OBAs and bad quality cellulose fibers prone to color shifting in time, the brightness will not last and the whiteness will become even worse. Within the blue emitting OBAs there are differences in quality due to their resistance to UV, visible light and gas fading that will influence their fading or color shifting in time. The distribution of the OBA dyes within the paper structure has an influence as well, little of it and only on top of the paper coating gas fading will take its toll. Spectral plots tell a lot: http://www.pigment-print.com/spectralplots/spectrumviz_1.htm Good fade resistance testing too if it includes the paper whites: http://www.aardenburg-imaging.com/ -- Met vriendelijke groet, Ernst Dinkla Grafische Techniek Quad, piëzografie, giclée www.pigment-print.com