Optical brighteners – also known as ‘optical brightening agents’ (OBAs) or UV brighteners are white or colourless compounds that are added to the printing surfaces of many inkjet papers. Their purpose is to make the papers appear whiter and brighter than they actually are.
| They achieve this by absorbing ultraviolet (UV) radiation from the ambient lighting and re-emitting it through fluorescence, mainly in the blue portion of the visible spectrum. Consequently, they only work in lighting with a relatively high UV content, such as sunlight, fluorescent lights and halogen lamps.
Because the natural colour of the alpha cellulose and cotton linters from which paper is made is not pure white, all papers that claim to be white must have optical brighteners in their coatings. According to leading paper manufacturer, Hahnemuhle, it is impossible to produce pure white papers without adding optical brighteners because the natural whiteness of alpha cellulose tops out at 95%, while the natural whiteness of cotton linters is only 90%. Although many photographers prefer to print on bright white papers, there are both advantages and disadvantages in using papers that contain optical brighteners. But information on OBAs is not readily available. Many companies that distribute inkjet papers are reluctant to reveal which papers in their ranges are OBA-free or even which have low OBA levels. Photographers, however, need to be able to identify which papers contain OBAs and which are OBA-free so they can choose which type of paper to use for different applications. Making an inappropriate choice could be costly and time-consuming. One distinct advantage of papers containing OBAs is that a bright white paper base will allow the maximum colour gamut and black density of the printed image to be reproduced. However, on the negative side, the chemicals used in the OBAs can affect the integrity and longevity of a fine art print over time. OBA Stability As these chemicals break down, their ability to fluoresce deteriorates until, eventually, they will no longer do so. When this happens, the colour of the paper will revert to its normal creamy or yellowish hue. How long this process takes will depend on how much exposure the paper has to UV light and the ambient temperatures the prints are displayed in. According to independent media tester, Wilhelm Imaging Research, OBAs ‘may lose activity when subjected to high temperatures in accelerated thermal aging tests and, it may be assumed, in long-term storage in albums or other dark places under normal room temperature conditions’. The effect on image quality will depend on the amount of optical brighteners in the paper coating as well as the lighting the print is displayed in. Prints that are illuminated by direct or indirect daylight (which has a high UV component) will be affected sooner than prints displayed under incandescent tungsten illumination (where the UV component is low or negligible). As OBAs decompose they may also cause yellow stains to appear on prints. Post-printing protective coatings have been developed to reduce the effect of UV exposure of inkjet prints. However, their effectiveness has yet to be proven. Some can also change the appearance of the surface of prints. UV-blocking glass can also slow down the degradation of OBA additives but, at the same time, they will also reduce the level of fluorescence and, consequently the brightening effect of the OBAs. Metamerism You can test this phenomenon by making two prints of the same image, one on a paper with OBAs and one without. Take both prints outdoors and observe their appearance under normal daylight. The paper with the OBAs should look extremely white, while the paper without them will look creamy. Now, take the prints indoors where they are exposed to neither sunlight nor artificial lighting and observe the whiteness. You probably will not be able to see a difference between the two. Prints made on paper with and without OBAs will usually look the same under incandescent lighting, because of its low UV content. Since the OBAs can no longer fluoresce, the base colour of the paper will will show through and make the paper appear as creamy as the base colour of papers without OBAs. Metamerism has serious consequences for your digital printing because it adds an uncertainty to the printing process. When you profile a printing paper, the ‘white point’ of the paper substrate is an integral component of the profile’s accuracy. If the whiteness of your paper changes, so must your profile. The best way to avoid problems with metamerism is to install a viewing ‘box’ with standardised lighting and use it whenever you need to check print colours. (An alternative option is to view prints under the same type of lighting in which they will be displayed.) Your Choice If you like the look of the bright white paper and your prints are not being made for extended display periods, there’s no real reason not to use papers containing OBAs. All leading paper manufacturers have papers with and without OBAs in their portfolios and many manufacturers have products with very low OBA levels. In theory, it should be relatively easy to find the type of paper you want for different applications. However, professional wedding and portrait photographers should be wary of papers with OBAs if they expect to obtain orders for reprints several months after a wedding or portrait shoot. Because the original paper and profile will no longer produce a true match of the original print, making reprints that match the client’s original can be extremely difficult and costly. It is also wise to ask clients about how they plan to display enlargements before prints are made and take the type of lighting into consideration. Fluorescent lighting will maximise the effect of optical brighteners but may also have some metameric effects on certain colours. Incandescent and halogen lighting will do little to stimulate the OBAs and will usually give a ‘warm’ overall appearance that may even emphasise the colours on OBA-free papers. For photographers who make prints for exhibitions or for long-term archiving, it is probably best to avoid papers with OBAs or, at least use papers with low OBA levels, such as most Canson papers and Hahnemuehle Photo Rag. However, the best advice comes from Henry Wilhelm, who states: “When long-term image permanence is of critical importance with museum fine-art collections, for example papers with fluorescent brighteners should be avoided where possible.” Papers without OBAs: Papers with OBAs: Not Specified: |
