Tracking LILIS (Light Induced Low-Intensity Staining)
LILIS is caused by the light induced degradation of optical brighteners (OBAs) incorporated in modern media, and it also correlates with the combined presence of both OBAs and titanium dioxide (TiO2). The concentration of OBAs and their location(s) within the media coatings are also important factors. LILIS manifests as noticeable yellowing of the media white margins and image midtones and highlights beyond what can be attributed merely to the loss of fluorescence as the OBAs degrade upon exposure to light.
The predicted yellowing effect of light induced low‑intensity staining (LILIS) in an image printed on a popular microporous resin coated (RC) inkjet photo paper is shown in the before/after illustration above. This particular image was chosen because it contains a large amount of image area with low chroma highlight colors including pastel blues that are very sensitive to yellow color balance shifts.
The yellowing appearance is initiated by the light induced loss of fluorescence of the optical brighteners but further compounded by additional post exposure discoloration taking place under low intensity illumination or dark storage environments. Its effect on this image has been accurately estimated by applying the colorimetric data for ID# 319 in the Aardenburg Light Fade Database measured at 50 Megalux hours of accumulated light exposure combined with approximately three years of additional dark storage under typical indoor ambient temperature and humidity conditions. ID#319 is representative of today’s popular resin coated (RC) inkjet photo papers when printed with high stability pigmented ink sets. No noticeable fading of the colorants has occurred at 50 Mluxhrs nor does thermal aging play a significant role yet. Hence, the easily noticeable color balance shift in this image is all due to OBA fading and subsequent yellow stain formation (LILIS) resulting from the accumulated light exposure plus some additional time in dark storage. Again, note the impact of this yellowing on the pastel blue sky areas and other parts of the scene with low chroma and nearly neutral highlights.
The manufacturer-sponsored lightfastness display rating for this printer/ink/and media combination when framed under glass converts to a megalux hour exposure dose of approximately 270 megalux hours “before noticeable fading and/or discoloration occurs“. Yet easily noticeable changes are going to occur well before 270 Mluxhrs according to Aardenburg test results for this printer/ink/media combination. To be clear, there are key technical differences between the Aardenburg Extended Light Fade Test Protocol and tests conducted by other labs. Aardenburg Imaging & Archives is the only testing lab in the world currently evaluating and reporting the adverse effects of LILIS on modern digital prints. When those effects are ignored or overlooked, the testing outcome is generally much more favorable to any print processes that exhibit the LILIS problem, especially when the image forming colorants otherwise exhibit high light fastness properties. The media becomes the weakest link!
Some images will always show a greater appearance change than others due to the interaction of the media discoloration with the hiding power of the image structure which forms the colors and tones present in the printed image. Some folks will also object to changes in image appearance more quickly than others depending on their personal print quality standards, as well as in consideration of the perceived purpose of the photograph such as fine art, commercial display, consumer snapshot, etc. Nevertheless, for any prints with historic, artistic, and/or sentimental value, curators and collectors should seek to avoid noticeable light induced fading and/or discoloration by implementing display policies that take into account recommendations like the Aardenburg Conservation Display ratings rather than relying on the marketing claims of manufacturers. Artists would also be wise to choose OBA-free media whenever possible because it eliminates the LILIS problem altogether!
Our goal for 2021-2024 calendar years is to once again use the Aardenburg extended testing protocol to evaluate popular non RC gloss/luster media, i.e., the so‑called “Baryta” and baryta-like traditional fiber fine art media. Careful comparative choices in this media category should help us to better understand the relationship between OBAs and TiO2 with respect to LILIS because this category of media contains OBA-free, OBA with TiO2, OBA‑free with TiO2, and OBA without TiO2 formulations. This category of papers has also undergone some supply chain necessitated reformulations of various popular brands, thus another reason for a new round testing.
Mark McCormick-Goodhart, Director
Aardenburg Imaging & Archives