U of S | Mailing List Archive | alt-photo-process-l | Re: green-yellow

Re: green-yellow

... this thread has been about how colors (in the visible spectrum) rank as far as blocking UV (in the invisible spectrum) and there's been some difference of observation about the ranking of the colors.
Indeed, the first in this thread was Loris Medici who was asking "How Green (which is Yellow + Cyan) can hold back more UV than Yellow
alone, where Cyan is a poor UV blocker (slightly denser than Magenta as
seen from your tests)?"
Perhaps one response might have been: because adding another UV blocker to that contained in the yellow ink increases the total opacity towards UV.
What I was trying to say in my previous post was that it is a nonsense to infer a UV absorption from the visible color of a pigmented matter, whatever it is. All the arguments about complementary colors (yellow/blue, red/magenta, etc) hold only in the visible range. UV absorption of an organic molecule relies mostly to the presence of double bonds. How many they are -and their UV absorption- is almost independent on the visible color: you can have a perfecly transparent substance in the visible range which is UV opaque. A factor 30 sun oil is not black, only perhaps very faint yellow...
I don't know what molecules are in the inks the different printers and their different models use, but considering also the additives that certainly are in the inks, there are many reasons for believing they all are different enough.
... then the color can't be the determining factor for ranking the UV-blocking characteristics of the inks, and it would probably be helpful not to make categorical statements about the ranking of colors by UV-blocking.
I totally agree.
Instruments are useful for exactly measuring the UV-blocking of a particular ink (I especially like Clay's QTR graphs, which show precisely how much UV a particular ink blocks at a particular percentage) but most of us have to rely on how a thing prints, to judge. Which, come to think of it, is the real goal anyway, isn't it?
I agree again, but I would stress that if one want to know the behaviour of the inks under UV light, the only scientific and not merely deductive way is instrumental.