Re: Epson R2400 inks density - DISCLAIMER
Alberto,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.