The use of optical brightening agents (OBAs) in photo papers and fine art print papers is fascinating. OBAs are also sometimes referred to as fluorescent whitening agents (FWAs). OBAs are mostly colorless compounds that paper and pulp manufacturers add to enhance paper brightness and whiteness. They work by absorbing invisible ultraviolet energy and re-emitting a portion of this energy as visible light in the blue wavelength region of the spectrum. One of my goals for Aardenburg Imaging and Archives is to generate some meaningful test results concerning the longevity characteristics of OBAs in inkjet papers and how much UV exposure is required to degrade the OBA performance over time. It is known from historical use of OBAs in photo papers that OBAs can function for decades if not exposed to high levels of natural daylight or other UV-rich light sources, but their performance can degrade nonetheless if care isn’t taken to control the display conditions. The paper yellowing (or loss of blue to be more precise) which is caused by the “fading” of optical brighteners is generally not great enough to trigger the endpoints typically used in industry light fastness tests. Thus, the consumer is given print display life estimates that essentially ignore any early stage degradation of OBAs. Yet loss of OBA fluorescence is a loss of one of the key paper attributes that attracts people to choose a particular paper in the first place. Many artists and museum curators would not be so quick to dismiss the role of OBAs in valuable artwork.
What prompted my interest in OBAs was the recent introduction of a new category of inkjet papers aimed at the high-end photographic and fine art printmaking audience. These new papers are often described as very closely resembling the look and feel of traditional fiber-base photographic papers. Crane Museo Silver Rag, Hahnemühle Fine Art Baryta, Harmon glossy FB AI, Innova F-type Gloss and F-type Warmtone, Ilford Gold Fiber Silk Paper, Epson Exhibition Fiber Paper, are examples of this new retro look just to name a few. Some of the new papers also contain OBAs at a significant level, and their paper surface color is impressively bright white. For inkjet papers that do contain OBAs, the amount and location of the incorporated OBA varies significantly. A blacklight lamp source commonly available at hardware or home improvement stores can help the end-user to identify on a relative basis how much, if any, an inkjet paper surface depends on OBAs to achieve its initial color. OBAs can often be found in the image receiving layer, in the paper core, in anti-curl layers on the back side of the paper, and in combinations of these regions.
Two papers belonging to this new fiber-based photo category of inkjet photo papers are shown in figure 1. On the left is Epson Exhibition Fiber paper which contains a relatively high level of OBAs, and on the right is Crane Museo Silver Rag paper that does not contain any OBAs. The prints were first photographed under illumination from a pair of Solux 4700K lamps as shown in figure 1. They were then photographed again (see figure 2) but under the combined UVA irradiance from a Phillips F36T8BL8 blacklight and the reduced illumination from the Solux lamps (i.e., by moving the Solux lamps further away from the prints). The Solux lamps possess about 6-8% UVA energy content compared to their visible energy content which is enough to activate some OBA fluorescence. However, the Philips blacklight emits virtually all of its output in a narrow band of the UV spectrum from about 360-380 nanometers (nm) with a sharp peak at 370nm. Its output is sufficiently intense and in an ideal spectral band to dramatically activate the optical brighteners.