Re: Metamerism

Judy Seigel wrote:

>
>Thanks Bill -- very interesting and for whatever reason more info on the
>topic than on the long-ago Epson list, or anyway new info. Now, however,
>since you know so much -- why is this suddenly such an issue? Are inkjet
>inks more metamerism-prone than other colors ?
>
I'm not Bill, but I'll answer anyway..

Yes and no..

The color pigments currently used in pigment-based inksets are, in many
cases, encapsulated in epoxy resins. The encapsulation increases the
metameric properties of some of the pigment particles. In other cases,
the pigments themselves tend to be metameric. Part of this is the
physical nature of a pigment when compared to a dye. The pigment has a
physical size and nature that actually reflects light differentially,
whereas a dye simply works by additively coloring the media, without
changing the physical reflectivity substantially. Additionally, the
refractive qualities of the epoxy encapsulation can increase the
metamerism at select wavelengths - as the refraction of light passing
through the epoxy differs by wavelength.

One of the most metameric traditional pigments is Cobalt Blue. Take a
photo of a gorgeous oil that utilizes Cobalt Blue, and almost invariably
it appears purplish. This is because the cobalt blue converts near IR
light reflected onto it to a more visible magenta tone, so when scanning
or photographing artwork incorporating cobalt blue pigments, one needs
to block as much IR light as possible from falling onto the artwork.
Or.. if the image is captured via a digital process, one should also
block as much reflected near IR as possible by adding a
blocking/hot-mirror filter, as digital capture has heightened
sensitivity to IR than does the human eye. There is a whitepaper on this
on the BetterLight website, explaining all the work that has gone into
simply allowing BetterLight digital backs to properly image artwork
containing cobalt blue pigments.

The problem with dyes is that they tend to be more fugitive, so OEMs
have moved towards pigments.. Pigments are more stable but they bring
their own problems..

All that said, as the newer generations of pigments come out, like the
Ultrachrome HG inks in the R800, and as pigment capsule and particle
combination sizes drop substantially, metamerism is decreasing as a
result of both physical factors and pigments that are themselves less
metameric in the first place..

> In printmaking, color
>photography, painting & drawing, etc., you knew colors looked different in
>different lights, but it didn't seem to be such an issue. Is metamerism
>worse, or more uneven, in inkjet?
>
>
It's just a fact of newer pigments and ink formulations.. Oils, litho,
and traditional printmaking evolved over much longer timespans... So,
the bad boys were weeded out.. Problematic inkjet pigments are being
weeded out, and new ones developed quickly, but it takes time..

As an aside, there are dye-based inksets on the VERY near horizon that
have archival qualities similar to the best of the current pigments.
The dye-based nature of these inks avoids many of the problems inherent
in pigment-based inksets.

-- 
 
Keith Krebs
"Just some guy," caretaker of the Multiverse's largest EPSON printer 
User Community (highly recommended by Vogon Poets and MegaDodo 
Publications), at:
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and  the Multiverse's largest Canon printer User  Community at:
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"For the rest of you out there, the secret is to bang the rocks together 
guys"