Mario Becroft (mailing list) wrote:
it look white--either that or my DTP-22 is reading wrong. But I tried reading some other paper with it, and while most of the papers I tried seemed to have a similar cyan bias according to the DTP-22, there were several which were exactly white--so I guess the papers really do vary and you just can't see it.
Sounds suspiciously like FWA (Fluorescent Whitener Additive) effects. You can get any sort of blue/non-blue shift depending on how the viewing conditions compare to the lamp in the instrument (which is usually incandescent 'A' spectrum) and the level of FWA in the paper, and also depending on the differences between source and destination. If you have a tool for plotting the spectrum, it's quite obvious when there's FWA in the paper - measure white, and it will have a peak at the blue end from the fluorescent emission, and a steep slope into the UV where the absorption is. A paper with no FWA will have no peak, and only a mild decline at the UV end. Many typical viewing conditions are UV poor compared to an incandescent lamp (i.e.., typical fluorescent lamps have phosphor that absorbs UV, a lot of light fittings are covered by normal glass or UV stabilized plastic, which absorbs UV), while other situations (such as sunlight unfiltered by glass, or a true D50 spectrum) are rich in UV compared to an incandescent lamp. Some of the so called "D50" viewing booth tubes have a special UV emitting phosphor to emulate unfiltered sunlight (i.e.. GTI), while others viewing booth lights emit almost none. FWA plays havoc with visual matching as a result. Graeme Gill.