One source of differences among observations might be the shape of the
absorption region of the inks, that of the sensitive compound (iron,
dichromate, silver...), and that of the UV emitting source. All are more or
less bell-shaped so that this mix gives back a peak with its maximum in a
different region of the spectrum.
Now THAT would be fascinating--mapping the bell of the three absorption regions...it'd be a great thing to write a grant for.
Thanks for all the info, Alberto.