The ISO 3664 lighting standard was meant to give a more realistic simulation of real-world viewing conditions by increasing the amount of activity in UV spectral regions. Mission accomplished; but it has also revealed long-standing issues in proof-press matching that had previously been hidden.
Color engineers don’t particularly like UV because it can create serious complications in measurements: this is one reason why calibration and linearization systems for proofing devices so often make use of instruments that filter out UV light before it can reach the surface of the proofing substrate. Using light stripped of its UV component eliminates the “black light” effect that can cause papers with optical brighteners to “glow in the dark” and take on an additional bluish brightness.
For similar reasons, GRACoL has long specified paper with essentially neutral whitepoint and no optical brighteners (chemical agents that absorb light in the UV range and reemit light in the visible range).
As GRACoL and standards-based printing took hold in the United States, manufacturers of proofing papers took considerable pains to supply the market with the proofing papers that matched white point aims specified by GRACoL…..and for a while everybody was fairly happy. But soon, perceptive viewers began to notice that the wonderfully GRACoL-compliant papers that manufacturers had created at considerable expense simply didn’t match the ultra-bright, OB-using, blue white papers that had become so popular with print customers.
Paper companies had discovered the preference for a bright blue look, and the preference for blue is nothing new. Mrs Stewart’s bluing has helped keep laundered fabrics looking white since 1883. Her original product used Prussian Blue dye to give white clothes their fresh bluish white look. Modern printing papers use UV activated optical brighteners to give a bluish shift and at the same time boost brightness.
ISO 3664, which specified increased levels UV activity in D50 graphics arts lighting, gave a standard that was closer to “real” daylight, but at the same time exposed even more strongly some of the problems being caused by the mismatch between OBA-free proofing papers and OBA-laden printing paper.
At first blush, it seemed as if switching back to proofing papers containing OBAs would be an easy way of matching proof appearance to final print paper output, but unfortunately, this is not the case. As the illustration shows, different OBAs may fluoresce at different hues, from deep purple to a pastel bluish white, and there is no standard for for appearance of OBAs. What this means in practical terms is that there is currently no objectively verifiable way for accurate matching of OBA containing proof papers to OBA containing printing papers.
ISO 3664 has given us a lighting standard closer than ever to actual daylight, but at the same time it has revealed more strongly than ever before existing issues with proof press matching. Until this dilemma is resolved, we may all be forced to reevaluate the best methods for matching proof and press sheet.
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