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Dye versus Pigment Ink Printers

Not too long ago, only a few exotic desktop inkjet printers used pigment ink. But now that new pigment based models from Canon and HP have joined the lineup of pigment printers resulting from Epson's pioneering efforts, the choice of dye versus pigment ink has become increasingly unavoidable for those shopping for a new printer. If this might include you, which type of ink should you go with?

Traditional dye-based printers use water-soluble colored liquids to render an image on paper. Tiny droplets are laid down that get absorbed by the paper. By contrast, pigment ink is composed of a neutral carrier liquid with suspended colored particles. Whereas dyes are generally derived from organic compounds and deteriorate rapidly when exposed to UV light and other environmental factors, the colored particles used in pigment inks are inert and will require much longer to decompose or fade to any significant degree.

But pigments haven't been a panacea for the creation of inkjet ink. Pigment inks are much more expensive to manufacture as are the print heads used to dispense them. The individual pigment particles are solids yet still have to pass through the tiny picoliter-sized holes in the printer's print head. Early pigment printers had bad reputations for frequent clogging but newer models have mostly solved this potential problem. From the beginning, it was important to allow the print heads to park properly when turning off the printer so they would seat themselves as the engineers intended. Turning off the printer with a switched power strip prevented this from happening and promoted clogs.

It was also challenging for early pigment inks to produce the wide gamut of colors possible when using dyes. A lot of late night hours in Epson's and other printer manufacturer's research labs have improved matters considerably. The inclusion of additional ink colors as well as the use of specialized processes such as resin encapsulation have all but eliminated gamut differences. It used to be that one had to choose between the wider gamut of dye ink versus the improved permanence of pigments, but not any longer.

But the hardest problem for the developers of pigment ink to solve was a phenomenon known as "metameric failure," and explaining it will require a bit of typing. Bear with me if you've heard the term "metamerism" bandied about by printing geeks and want to know what they were taking about, or feel free to skip this part otherwise since this too isn't really much of an issue with current pigment printers.

When we look at an object, we see it as being some particular color. Certain wavelengths of light that fall on it get absorbed and others get reflected to some degree. The average of this response across the visible spectrum determines the color we perceive. The phenomenon wherein two objects with different spectral responses both average out to be the same visible color is known as metamerism. Think of it much the same way that the average of 10 and 20 is the same as the average of 5 and 25. Add them up and divide by two and both sets of numbers average out to 15. The color we see is to that 15 average much as the spectral response is to either of the sets of numbers that go into that average. You can come up with a great many sets of numbers that all average out to 15 just as you can come up with a great many spectral responses that all average out to the same visible color.

That is, so long as the lighting conditions remain constant. If the color temperature or spectral distribution of the light source changes, all bets are off. If a certain wavelength that an object reflects is now present in the light source to a greater or lesser degree relative to other wavelengths, its contribution to the overall color of that object will have changed. What we will see is a color shift, something quite expected since the color of the light changed. If the color of a second object though came from averaging a slightly different spectral response, its color may shift differently from that of first. This is known as metameric failure: two objects that appear to be the same color when viewed under one light source, but appear as different colors under another light source. This is a normal property that we see around us all the time. You go to the department store and buy something because its color matches what you are looking for, but when you take it home, the color turns out to be all wrong. That's because the light source in the store was different than at your house. Car companies love metameric failure since it enables them to develop those new fangled car colors that shift under different lighting. Is that new car blue, or is it green? It's both of course, depending on the light source.

So how does metamerism fit in with inkjet printing? Basically because it affects pigment ink far more than it does dye based ink. While dye ink soaks into the paper, pigment ink dries on the surface and does not absorb since the color is a solid. If the individual particles of pigment are not completely uniform, they can end up laid down differently across the page and will thus reflect light differently. And unlike dye inks, pigment granules don't mix together when wet on the page so any differences in spectral uniformity will contribute to metameric shifts.

Thankfully, continued research has yielded improvements in manufacturing techniques that have all but eliminated metamerism as an issue with pigment ink. While all this does cost money, the increasing adoption of digital photography has caused the inkjet printer market to explode and nothing helps drive costs down as does sales volume. If photographic printers were still selling in the low numbers they did only a few years ago, they would have to cost a lot more in order to pay for all that R&D. The sheer number of us photographers trying to print at home is what has made this desktop printing thing affordable. And high tech pigment inks and printers are the result. While the ink and print heads do cost more to develop, those costs are spread among much greater numbers of printers than they once were.

My first pigment printer was the original Epson 2000P back in the year 2000. I suppose that's why they named it that now that I think about it. When printing on archival paper, its output was rated to last up to 200 years without fading. At the time, all the available dye-based inkjets were doing good to last even six months. Printing on the 2000P with the supplied ICC profiles though was an exercise in frustration both from the limited gamut and the issues with metamerism. It wasn't until I spent money on custom profiles that I was able to get results I was satisfied with. But once I did I was hooked. It was great knowing (and being able to tell customers interested in buying prints) that they would stay looking as good as they did when they came out of the printer for years to come. Since then I've upgraded printed more than once, but always to a new pigment ink model. Printer manufacturers have made strides in producing more stable dye inks, but for truly professional results, pigment inks are the way to go.


Date posted: July 22, 2007

 

Copyright © 2007 Bob Johnson, Earthbound Light - all rights reserved.
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