On the Use of Optical Brighteners in Fine Art Prints and Photographs

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.

Figure 1.  Epson Exhibition Fiber paper (left) and Crane Museo Silver Rag paper (right).  The prints were photographed under 225 lux illuminance at the print surface which is also approximately 0.33 Watts per sq. meter of visible energy.  At this level of illumination, approximately 0.02 Watts per sq. meter UVA energy is also impinging on the print surface from the Solux lamps.  It is the UVA component that triggers the OBA fluorescence and causes the OBA containing Epson paper to appear more cool white than the non-OBA containing Crane paper.

Figure 2.  A Phillips F36T8BL8 blacklight providing 0.40 Watts/sq. meter of UVA energy incident at the paper surface causes heavy excitation of fluorescence in the blue visible region on the Epson paper.  The camera records the fluorescence easily, whereas the non OBA-containing Crane paper appears very dark because the visible energy now incident upon the Crane paper is not enough to properly expose the rest of the image (approximately 15 lux or 0.02 watts/sq. meter).  The scene reproduction appears very similar to the way the human observer sees blacklight induced OBA fluorescence when ordinary room lights are essentially turned off.

By combining the UVA energy output from the Philips blacklight with the reduced illumination level of the re-positioned Solux lamps I was essentially able to invert the UVA-to-VIS energy ratio. The large increase in the incident UVA energy combined with low incident visible light energy allows the human observer and a digital camera to detect exceptionally strong fluorescence of the OBAs as can be seen in figure 2. The reduced visible light energy also caused significant underexposure of the non fluorescing Crane paper because the camera shutter speed and aperture settings were left unchanged between image captures for figures 1 and 2. For photographers who may be interested, the second photo is about 4 stops underexposed compared to the first photo. Yet there was plenty of fluorescence from the Epson Exhibition Fiber print to record in the blue channel of the camera sensor.

OBAs are certainly not new. They have been incorporated in traditional photo papers and conservation mats and mount boards for many decades (See figure 3 ). Consumers have been conditioned to expect “whiter than white” clothing from the OBAs present in most laundry detergents. The same trend is readily apparent at office supply stores when one reads the scale-topping ISO or TAPPI brightness scores on many package labels of plain printing papers sold for everyday use in inkjet or laser copiers. The standardized scales were developed before manufacturers and consumers got so carried away with high brightness scores. It is thus understandable that the trend to whiter whites would also show up in inkjet papers. OBAs aren’t just for subtle assistance to paper whiteness anymore. They are now a key contributor to a paper’s visual appeal for artists who prefer a very bright and cool-white appearance over a more natural paper color.

Figure 3. A close-up view of the signed corner of a silver gelatin print on fiber-base photographic paper.  Printed circa 1978 by Ansel Adams, this 30 year old print contains optical brighteners.  The OBAs in the emulsion glow distinctly blue in color under examination with a blacklight.  Museum board is a composite construction, and paper core properties may differ from the outer laminated layers.  In this case, neither the mount board, window mat, nor the paper core material revealed by the bevel cut show any fluorescence. Although Adams chose a photographic paper containing OBAs for this print, the window mat and signed mount are free of OBAs.  Properly processed selenium-toned silver gelatin prints are among the most highly stable of all photographic processes, but the presence of optical brighteners in this valuable print invites caution.  It should be displayed under controlled lighting conditions and treated as if it were a fugitive color print in order to extend the functional life of the OBA and to guarantee that its subtle monochromatic tone and color remains in pristine condition.

Are OBAs in fine art papers good or bad? The answer depends on how one rationalizes the use of OBAs in fine art printing papers. On the one hand, OBAs help to achieve an initial paper color that cannot be obtained otherwise. The brilliant cool-white paper appearance can be very appealing for certain images. Cold toned black and white prints, color landscapes with dominant blues and greens, etc., can often appear visually more appealing on a cool white paper color than when printed on a warmer white paper. That’s the primary reason for an artist to choose an optically brightened paper, and it’s hard to argue with aesthetic preferences regarding initial print quality.

The caveats when selecting OBA brightened papers are three-fold. First, the paper color will only retain its brilliant white appearance in the presence of light sources and picture framing options that allow some UV radiation to strike the surface of the print (see figure 4). As noted previously, OBAs work by absorbing the UV energy and re-emitting a portion of this energy in the blue portion of the visible light spectrum. Hence, the paper becomes fluorescent only when UV energy is permitted to reach the print surface. Conservation framing techniques strive to filter out most UV radiation, and using best practices, most conservators or framing gallery owners will generally recommend to their customers glazing materials that block UV radiation. Thus, the bright white appearance of an optically brightened paper is at cross-purposes to modern conservation and framing practices that seek to eliminate the more harmful UV rays.

Figure 4.  Three different glazing materials were placed on top of the Epson Exhibition Fiber Paper and Crane Museo Silver Rag prints, and the prints were then subjected to the blacklight UV output.  From top to bottom: 1) standard acrylic picture frame glazing (Acrylite FF), 2) UV blocking acrylic (Acrylite OP3), and 3) ordinary soda lime window glass.  The standard acrylic allowed just 3.6% of the blacklight 370 ± 10nm radiation to reach the print paper.  The UV rated acrylic essentially blocked all blacklight UV output (less than 0.15% transmittance), and the glass allowed 86% transmittance of the blacklight energy through to paper surface.  The resultant levels of fluorescence correlate well with the measured amount of UV protection afforded by the different glazing choices.

Second, the whitening effect is not merely confined to paper white. Image colors, notably in the highlights and midtones of the image, will be influenced by the fluorescence of the OBAs. The “reach” of OBA fluorescence beyond paper white and into the highlights and midtone colors in the print can be verified in figure 2 by carefully observing the influence of OBA fluorescence on the various color patches of the printed target. Although not as blatantly obvious under more normal levels of UV content, the proportionate response of the OBA fluorescence is nonetheless present in color filled areas of the print not just the paper white areas. In other words, inkjet colorants are not opaque enough to hide the substrate reflectance and OBA blue light emission until print densities get quite high. Artists concerned with exacting standards of color and tone reproduction will balance their image colors to accommodate the interaction of the paper fluorescence with the image colorants, but this balancing is dependent on the choice of viewing illuminant and also the choice of picture frame glazing. Thus, OBAs can pose particular challenges to building ICC color profiles and generally cause color profiling inaccuracies when the UV/VIS energy ratio reaching the print surface differs appreciably from the UV/VIS ratio emitted by the color profiling instrumentation. Also, the presence of OBAs causes additional color constancy issues. Lack of color constancy is a metameric effect where the color appearance of the print can change in unexpected ways when viewed under different lighting conditions. Thus, different UV/VIS energy proportions in different light sources cause additional color constancy issues by activating the OBAs to different levels of fluorescence.

Third, and perhaps most importantly, OBAs are dyes, and like other dyes, they are more prone to fading than pigments. For artists who are concerned with print longevity and who want future generations to see faithfully what they saw when the print was made, OBAs may not be as stable as the other materials they have chosen. This situation is particularly true for artists working with pigmented inkjet systems. Some people discount the problem of OBA fading because they argue that the paper color will simply revert to a more “natural” color. No big deal. This argument implies that the paper merely reverts to the color it would have had if the paper chemistry did not include any OBAs. The reasoning further proceeds; having the desired bright white appearance for some portion of the print’s lifetime is better than not having it at all. Unfortunately, this rationale has some flaws. The artist is presumably concerned with all the colors in his or her image not just paper white! Because the OBA fluorescence influences the appearance of many image colors as noted previously, loss of brightener activity will alter more than just the paper white appearance. Because it is permanent and irreversible, the UV-induced degradation of OBAs poses a similar but worse consequence than the deactivation of brightener activity by UV filtered illumination.

We cannot foresee the “natural” paper color when purchasing an optically brightened paper. OBAs are not only used to achieve a certain level of initial color appearance. They are also sometimes used along with other dyes to “level” batch to batch color variations in paper manufacturing. If a manufacturer does not have tight control on the manufacturing process and uses OBAs and/or other additives to compensate, then the artist’s work will be subject to more variation in aged appearance over time. Eliminating OBAs from the materials list removes one more variable in the print aging equation.

My own personal preference is to avoid papers with OBAs whenever possible. However, almost every photographer has at least some images that will look better initially when printed on a bright white paper. It is good that we have papers available to us that fall into both OBA and non OBA containing categories. If you elect to use a paper with OBAs you will also probably need to frame the prints with what art conservators would view as less than “best practice” by selecting a cover glazing that allows transmission of most of the UVA content in the light source. If you sell your work unframed, it is helpful to let your buyer know that frame glazing options may affect the image color balance. That way, the buyer will be able to work with the framer to make more informed matting and framing choices. It is relatively easy to see how glazing affects the colors in the artwork. Just place a piece of glass, standard acrylic, UV blocking acrylic, or the glazing material of your choice over the print and observe how its color now looks under any chosen light source. Bear in mind that one challenge in most framing shops is that they seldom offer different light sources under which you can evaluate your choice. Whether the color quality you observe under your chosen illumination and glazing conditions reflects how the print may look if the OBA activity fades is a much larger leap of faith. It depends, of course, on what additional fading or discoloration is also occurring to the print and mounting materials over the course of time. Lastly, it is wise to remember that OBAs are dyes. If they are incorporated into print processes that are otherwise known for exceptional longevity (e.g., silver gelatin or pigmented inkjet on fiber based paper), prints which contain OBAs should probably be handled and displayed as if they belong to a class of more light-fugitive artwork.