by Patrick Herold © CHROMiX, from CHROMiX ColorNews Issue #36
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Optical brighteners are additives that paper manufacturer's put into paper in order to help a paper look "whiter." They are also called optical brightening agents (OBA), or sometimes "artificial whiteners."
In order to make paper appear brighter, it is common for most paper manufacturers to add certain chemicals to the paper which can take invisible ultraviolet light and cause it to re-emit in the blue spectrum - or "fluoresce" - at a point that is just barely within our ability to see. While our eyes see this as a brighter, blue-ish white - a light measuring instrument will only see this as a different form of blue. That is why printer profiles made with paper using a lot of optical brighteners can end up printing out images that have a yellow tint to them. The profile is trying to correct for what it sees as too much blue in the paper.
Before you say "Well I use a really good quality paper - I'm sure they don't use this stuff," you should know that just about every paper that has a nice, satisfying "white" to it has some amount of optical brightener. In fact, you usually have to hunt around a bit to find paper that specifically has no OB in it. It's usually labeled "natural white" or something similar and is noticeably less bright with a small tint of yellow to it.
Examples of paper with OBA in it are common office bond paper, Epson Premium Matte, Luster, Glossy, etc., and most every other brand of commercial inkjet paper. Paper without OBA (or very little) include "silver halide" RA-4 process photographic paper, certain press proofing papers, fine art papers and other specific paper types that are marketed by paper manufacturers as having no OBA.
Before you panic and think this is the printing equivalent of hormone-injected beef, consider that this is a very reasonable way to get more white onto a paper. Without it, many of our papers would be rather drab looking by comparison, given the natural color of wood pulp and cotton. Consider that the chemicals required for bleaching a paper white might be worse in many ways than the OBA. You should be aware of some issues with this paper though.
The chemical agents in paper causing this fluorescence will 'run out of juice' over time (sort of like the pocket warmer in the bottom off my sleeping bag on our last camping trip. It was nice while it lasted.) That means that over the course of several years or even months, the apparent brightness of your paper will decrease; it won't be "glowing" with the artificial white that it does now. This is part of the reason why some people choose to print with "natural" papers.
How can you tell if your paper has OBA's? The quickest way is to shine a black light on the paper. A black light lamp will cause the OBA's to glow in a big way. You've probably seen this effect with white clothing under a black light. Paper (or clothes) without artificial brighteners will not react to the black light at all. Black lights that screw in to normal household sockets are available, but sometimes hard to find. Your kid's "invisible ink pen" he got at the toy store probably has a black light lamp on it to illuminate the OBA's in the invisible ink.
Now that I've found out that my paper has optical brighteners, what do I do about it?
The industry has a long history of instruments that make use of some kind of UV-filtering. The SpectroLino/SpectroScan (now discontinued) had interchangeable filters that would connect to the end of its measuring head. One of these was a UV filter. The X-rite DTP-70 (now discontinued) came with a UV filter that would mechanically move in and out of the measurement path as needed by the user. The X-rite DTP-41 was available from the factory in either a UV-filtered model, or a non-filtered model. The same is true for the i1 Pro Spectrophotometer: You decide at purchase if you want it non-filtered, or UV-filtering built in.
The iSis chart reader is X-rite's replacement for the DTP-70. It can measure with UV light included and excluded, on the same instrument, even during the same measurement of a page. Its function is a little different than the others. The iSis has two different light sources:
- One utilizes normal, visible light (which is used to create the non - UV measurement file.)
- One utilizes UV light only, and this measurement is mathematically combined with the other to create the UV-included measurement.
This new instrument, handling its filtering differently, has led to some semantic confusion around this whole topic of UV filtering. Since the iSis does not have an actual "filter" inside it that filters the UV, it's not proper to call this measurement UV-filtered. X-rite has chosen to follow a naming convention similar to their previous instruments, but it is arcane, and can lead to confusion. You can have "UV-cut" or "no filter" At CHROMiX, we have chosen a different naming scheme. We identify these measurements as what they are: "UV included" or "UV excluded" (UVi; UVx). We think these two terms can describe all such measurement conditions regardless of whether the instrument has a filter or not.
Also, to answer a question we often hear, the optional UV filter on the i1 Pro instrument does NOT affect monitor measurement and calibration. The UV filter is for the light source, not the reflected (or emitted) light that's gathered by the instrument. So UV and non-UV versions of the i1 will calibrate and profile displays identically.
A final not about filtering. While fluorescence discussions usually center around papers, pigments may might be fluorescent too.
In addition, the GretagMacbeth (now Xrite) ProfileMaker software has a software algorithm that can automatically detect optical brighteners and corrects their effects in the profile. The i1Match software also applies this software fix when making printer profiles.
So far, these hardware and software solutions I've mentioned don't take into account how *much* optical brightener is being used in the different papers, or how the OBA's react in different viewing environments. At CHROMiX, we are looking into making use of a combination of UV included and UV excluded measurements to provide a more precise correction as part of our new ColorValet Pro service (mentioned earlier in this newsletter.)
In our next issue of ColorNews, we'll take a look at X-rite's Optical Brightener Correction (OBC) module. This can be a useful solution to some of these problems experienced by those who work in a press room or other color-critical environment where a light booth is used.
Thanks for reading,
Patrick Herold
CHROMiX Tech Support / Lab Operations
Chromix news articles can be found at
http://www.chromix.com/ColorNews/
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Last updated: February 26, 2021
