Tuesday, June 19, 2007
The Paper Chase - Selecting The Best Paper
One of the least considered, yet most used materials in the photographer's arsenal, a paper's fundamental construction can have a profound impact on how images print
Along with ink characteristics, paper archival properties are a hot topic in the digital printing world. With early inks and papers, the life span for prints literally could be measured in days before color shifting or fading had become objectionable. This certainly wasn't acceptable for the pro making a living from print sales. Along with advancements in ink durability, the display life for prints is now typically in the decades, and in many cases with the latest pigments, over a century according to testing by Wilhelm Imaging Research (in fact, the newest pigment inks and papers from HP are rated over 200 years).
When looking for papers with longer print life, only consider acid- and lignin-free products, such as the photo rags and mould-made papers. Manufacturers normally rate the display life for their papers with particular ink combinations, and a visit to the Wilhelm Imaging Research Website (www.wilhelm-research.com) provides you with a wealth of information.
The OBA Controversy
More and more we're hearing about optical brightening agents, or OBAs.
It's common for many fine-art papers to come in natural and bright white versions. The only difference between these is the addition of OBAs.
OBAs are compounds that work by reflecting ultraviolet light into visible light, which makes the paper appear brighter. They aren't actually changing the color of the paper, only the way it's perceived by the eye. This by itself isn't a problem, of course, but the OBAs degrade over time, losing their ability to reflect ultraviolet light. (Interestingly enough, almost all conventional silver-halide photo papers use OBAs, so this isn't anything new.)
As this degradation occurs, it gives the appearance of yellowing, when in fact the paper is merely reverting to the look of the natural paper. How long this degradation takes and how evenly it occurs depends greatly on how the OBAs are added to the paper. It's less expensive to add them to the coating of the paper where it comes in contact with the inks. There's the possibility of the OBAs interacting with the inks and causing unwanted color shifts.
A far better method, but more expensive to manufacture, is to add the OBAs directly to the paper where they're isolated from the inks. Any change in the ultraviolet reflectance is even across the entire surface this way.
So, how long does all of this change in brightness take? Unlike the print permanence ratings with which most of us are familiar, it's unknown. It's certainly not likely to be as permanent as the print life itself, which is often over 100 years now (and in some cases much longer), and will be significantly impacted by how the paper is stored. When exposed to ultraviolet light, the fading will be faster than it would if stored in an archival storage box.
A standard is used for traditional photo papers, which can be viewed at the Library of Congress Standard for Paper Permanence Website, www.loc.gov/preserv/pub/perm/pp_1.html
Jon Canfield is the author of several books on digital imaging, including Print Like a Pro and RAW 101, and a consultant on printing and color management. Visit www.joncanfield.com.
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