Not all paper is created equal. In fact, extensive technological research has gone into designing paper that can capture the sharp, bright colors that we have come to expect from photographs. Put a sheet of normal paper into even the highest-quality printer and you'll probably end up with a dull, blurry image. But use paper that is specially designed to interact with the printer and its inks and you can end up with a amazingly realistic photo. Here's a look at some of the features that make such prints possible.
Some papers soak up ink (also known as dye) like a sponge; others hardly let the ink past the surface. You might think deep ink penetration would produce a better image, but in fact the opposite is the case. When inks lie near the surface, a photo has much greater "color saturation." That is, colors will appear more vivid and intense. When you stop to think, this makes perfect sense, since more of the ink remains on the surface and thus visible to the eye.
That's not the end of the story, though. If the inks don't penetrate the paper at all, it can be easily blemished or marred. Imagine printing on a solid piece of metal: after the printing process, the ink would just wipe right off. So paper manufacturers must achieve a balance, preserving vivid colors but also making sure inks adhere to the paper and remain permanent.
Dye penetration has to do with the porousness of paper, and the degree to which it can drink up ink. But ink and paper have an even more complex interplay, called "dye and medium interaction." Usually a closely guarded secret of paper manufacturers, this interaction is generally chemical in nature, and affects the permanency of an ink's colors. If ink and paper are designed in regard to each other, their physical interaction will produce a more permanent combination--one that resists fading and other forms of color degradation. That's why, if you want the best-quality, longest-lasting prints possible, it's a good idea to use inks and paper that are designed to work together.
Closely related to dye penetration, dot growth concerns not how deep ink penetrates, but how wide it spreads. Like dye penetration, creating the proper level of dot growth is a balancing act. Some dot growth is necessary for several reasons. First, when two adjacent dots "grow" into one another, they will have an area of overlap where the colors actually mix.
This is essential in creating a realistic image, for instead of creating many dots each of a separate and distinct color, the blending creates smooth transitions--closer to the way colors exist in nature. This is known as "continuous tone" printing. There's another advantage to dot growth, because when the dye spreads, it ensures complete coverage of the paper. If the ink did not spread at all, the image would be full of what is known as "white-space errors." That is, the white of the paper would show through the image in certain areas. Such errors could take the form of graininess, with white dots spread throughout the photo, or the errors could show up as "banding"--tiny lines of white between lines of colors.
On the other hand, too much dot growth can also compromise a photo's quality. Think of writing with a felt tip pen on very porous paper. The ink will actually "bleed," making your writing blurry and perhaps even illegible. Furthermore, dot growth can compromise the accuracy of colors. As dots bleed into each other, the colors mix too much and form combinations that did not exist in the original image. Again a proper combination of ink and paper is necessary for the best prints possible. Why? Because both the viscosity of the ink and the porousness of the paper effect dot growth. Watery ink will spread farther than a thicker ink. But such an ink may spread too far if you use porous paper.
In order to produce a wide range of colors, a printer must mix three, four, or six different inks--in some cases even more. If the paper is too thin, the ink will penetrate all the way to the other side of the paper, which can: compromise color accuracy; weaken the paper, making it more susceptible to tears (think what happens to paper when soaked in water); and create an uneven surface. Thus, the best paper will be thick enough to accommodate the highest amount of ink a printer might apply in a single area.
Most digital photographers want their prints to resemble traditional photographs. So while a wide variety of papers are available, the most popular recreate the thickness, strength and glossy surface of traditional photographic paper. Such paper also tends to produce the best images as well. Why? First, because a glossy surface tends to produce more vivid colors, as inks remain near the surface (remember dye penetration?). Second, extra thickness means the paper can accommodate more ink per dot.
However, you can experiment with a number of different papers, depending on the image you're printing and the effect you want to create. You might try a matte (i.e. non-shiny) finish for a more subdued image. Or you could use papers that are highly absorbent, for example linen or even watercolor paper. Such papers may produce less accurate images, but they can yield interesting, surreal effects. You might even try printing on a transparent mylar base, making your photos curiously translucent.
You may have noticed that paper is often rated for whiteness: the whiter the paper, the higher the number. If you're printing photographs, whiteness is less important, since very little of the original paper will show through. Why? Because even areas that appear very light probably are probably not pure white, and are thus covered by at least a little ink.
Good paper, good inks, and a good printer: all three are necessary to make high-quality prints. But that's not quite enough. In addition, all three have to work well in concert. Take dye penetration, for example. If you use inks that are too watery for a certain paper, colors may penetrate further into the paper than desired. On the other hand, if the ink is too viscous, it may not soak in enough. The same goes for dot growth--watery inks may bleed, leading to distortions, while ink that is too thick may not spread enough, leaving white-space errors. And finally, consider the printer itself. The wrong ink could clog the system, and paper that is too thick could either jam or not advance properly, causing more errors in the printing process. This is why manufacturers, such as Hewlett-Packard with Photo Smart, supply a range of products--printers, ink and paper--that are designed to work together.
As you can see, digital printing is a complicated process with an endless set of variables. Usually, the printers themselves get all the technological glory, with all their finely wrought moving parts and digitized driving mechanisms. But that's only one aspect of the printing process. The interaction between paper and ink is also an integral factor in printing, and can make or break the quality of your digital photos.