Alberto,
I appreciate you sharing this information with the list and find it interesting.
In practical application, it is fairly easy to determine the ink blocking ability with a particular alt process, ink set, printer and light source. I use the Color Density Range Palette to do this and find that there are few variations from some common outcomes.
Most of my experience has been with Palladium & Na2. When working with one color of ink, pure yellow ink will block the most, then pure Cyan ink, and least Pure Magenta, however I rarely use those pure inks, but rather a mixture of two inks.
As far as Black ink goes, the current 2400 and 3800 (which I use) K3 ink set has less blocking ability with black ink than the Epson 2200 series had with the Ultrachrome ink set—it was very dense!
In almost all cases, Green ink blocks the best for most UV processes. One exception to this that I found was the 1280 dye based printer, where Red had slightly more blocking capability. Also, the R1800 ink set was a bit weird in that Yellow blocked more than Green, but you could see that the colors like Green and Red and all other colors with that ink set were very puny looking. This may have as much to do with the driver and inks as the color. The R1800 ink set was very different and took a very long time to dry, though the printer was very smooth and had the smallest dot size at 1.5 picoliters.
I should note that I am exposing with a NUARC plate burner with the mercury lamp. I have had similar results with most tube exposure devices except for one using Super Actinic tubes—and it produced really terrible results with the Ultrachrome ink set—very noisy with only a small range in the reds blocking sufficiently to make a negative.
For me, the above is just basic information that I keep in mind, however the proof is always in the actual outcome with any workflow, so I always try to avoid pre-judging what the outcome is going to be and let the system diagnose that for me. In the end it is not necessarily the color that blocks UV the most that is most important—it is the color that blocks just enough to give at or near paper white. Too dense a color presents a problem too.
Thanks again for sharing your work. --
Best Wishes,
Mark Nelson
On May 2, 2009, at 4:36:40 PM, "Alberto Novo" <alt.list@albertonovo.it> wrote:
> Alberto,
>
> What conclusions did you come to from the data? It wasn't clear to me.
I decided to obtain a full specrum of the absorption of my printer's inks in
the range from UVA (320 nm) to visible (900 nm), in order to have a
numerical basis for reasoning about the -supposed- relation between visible
colour and UV absorption. The absorption spectra of the fundamental colours
(Y, M and C) and of the secondary colours (R, G and B) show (with my Epson
R2400 printer on Pictorico) that the UVA inks absorption is in no relation
with the corresponding visible absorption. This was not proven before, but
only hypothesized and variously argumented.
In particular, (again: with my printer, inks, etc.) in the *near 370 nm
region*, which is the most representative for iron and dichromate processes,
the most absorbing hues are green, followed by cyan and black. This last
measurement confirmed me what I observed when printing the colour patches
test, and at first it was really strange, that is that the black ink was
less absorbing than a green shade.
The less absorbing inks are magenta, red and yellow.
I strongly recommend to perform a similar analysis, which can only be done
with a spectrophotometre and not simply a densitometre (which gives an
integrated value over a not ever clearly defined wavelength interval), in
order to characterize printers inks different from R2400 ones. General
conclusions cannot be drawn from the spectra I have posted because they are
R2400 specific (if not experimentally proven analyzing other inks).
Moreover, knowing the absorptions of the inks is only a first step towards
the knowledge of their behaviour, because they need to be compared with the
photon yeld range of the specific process, the emission spectrum of the
lamps, and the filtering effect of other compounds present in the sensitive
formulation.
Alberto
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