Optical Brighteners and Paper: WhatÕs the Problem
By Ron Ellis
Nearly everyone in printing deals with the issue of optically brightened papers. Almost all papers produced for the commercial segment of the market have some level of optical brighteners included, with many papers being so bright that they have a slight blue cast to them. Even lower grades of paper are getting lighter and lighter, now taking on the brightness of more expensive grades of paper. There are of course numerous problems we face from using brighteners in papers, such as the fading of the brighteners and yellowing of the paper of time as well as the environmental impacts of using brighteners. There is another problem caused by optical brighteners as well, and this problem relates to standards and measurements.
Most coated papers right now have a b* measurement of -4 to -10. (The b* measurement is the blue-yellow axis of the L*A*B* measurement. The higher negative the measurement is, the bluer the paper is). For uncoated papers these measurements can be even further along the blue axis. This first started with commercial papers and has continued into pub papers. Optical brighteners are now even common in some newsprint.
Optical brighteners are not a static phenomenon – many papers seem to keep getting bluer and bluer. This contrasts with international standards such as ISO, and US print standards. Many of these print standards specify a paper with a neutral white. In many cases it can be difficult to get a paper that is neutral enough to adhere to these standards.
The problem isnÕt so much that the paper looks too white, it is that the ever brightening papers are hard to measure. To a spectrophotometer or densitometer the bright papers appear to be blue. To our eyes these same papers appear to be bright white. It probably doesnÕt seem like a big deal. It is very problematic however, especially as it related to modern print standards as well as color management. Modern print standards such as GRACoL 7 and SWOP 3 & 5 are based on spectrophotometers. These measurements work well when performed on neutral papers such as those specified by ISO 12647-2. When the paper uses optical brighteners the measurements are no longer accurate. Because these papers are bright white, they read blue when measured. When we view them they look white. What happens when we calibrate a paper using these bogus measurements? Typically we end up with a proof or press sheet that is less accurate – most often having an extreme yellow cast. If we are using a calibration method such as G7 using an optically brightened paper may skew our gray balance, giving us the impression that we have mathematically achieved gray balance. In reality the gray balance will not be correct, and depending on other papers may be even worse. On the proofing end it can be a problem because not only can the target data set be measuring incorrectly, but the paper can also contain brighters, this skewing the proof from itÕs intended position.
Why is this such a big deal? Why not just edit the proof to match? It is important because modern print standards are based on printing to the numbers to achieve a common visual appearance. With non-optically brightened papers readings are similar to each other, but with optically brightened papers the readings are very different depending on the amount of optical brighteners in the stock. If printing to the numbers does not take you to the same point then it means that printing to the numbers does not work – that you do not get to a common visual appearance based in printing to these numbers. At that point you have to begin to make visual alterations in order match the visual appearance of the standard, and the minute that you start making visual edits then it is no longer a standard. Think about it – every thing about a print specification such as G7, GRACoL or ISO is about printing to the numbers. If you canÕt trust the numbers then it brings into question the validity of these methods. In addition the constantly changing brightness of papers threatens to make printing a moving target.
How do you tell the amount of optical brighteners in a stock? In addition to reading the specifications from the paper manufacturer there are several methods. One method is the measure the L*A*B* values of the paper. A high L* reading (greater than about a b* reading of -3, for example a -6 b*) means that the paper has optical brighteners in it. Another method includes using a blacklight to view the amount of optical brighteners in a stock.
There are several options for dealing with optical brighteners. The first method is to ignore the optical brighteners. Using this method you measure with UV included and then perform the color match under controlled lighting conditions though the match will not look correct when not under controlled lighting conditions.
The second method would be to rewrite the print specifications, basing them on the current optically brightened paper. This however would be impractical because the paper and specification would be constantly changing – and constantly problematic.
The third method would be to advise users on how to compensate. If the problem is unsolvable due to brighteners then is it possible to calculate custom aim points for that paper. Of course doing that in technically breaks the standard, but it gives the user a realistic chance of using the paper and matching the proof. IDEAlliance, the organization that has developed GRACoL and SWOP is developing a custom aimpoint spreadsheet for use when calibrating papers with extreme optical brighteners. This approach is designed for users with unsolveable print problems. Most users will probably choose to use the current aim points.
Anyone who prints knows that day to day you will come across all types of papers and you have to make them work. Most plants have only one type of proof, and are tasked with matching a variety of papers to these common proofs. By moving solid ink densities up and down many printers can match the proof sufficiently on a variety of papers. Often when I calibrate I work to make sure that I am calibrating on a neutral paper with few optical brighteners so that the initial curves are accurate, and then tail in the customers normal and optically brightened stock to make sure we can match this as well. Dealing with optical brighteners is a major issue-facing printer. Knowing how it skews the measurements, and working within the limitations of the technology helps us to be able to avoid bogus measurements and understand how to best calibrate in these situations.
© 2009 Ron Ellis