We all know instinctively that we like whiter and brighter papers. They give us a clean feeling, print more vibrant colours and catch our eye more than dull newsprint. This demand for brighter papers has been recognized by paper manufacturers, but the way they accomplish this causes problems for printers trying to achieve accurate colour.
What are Optical Brightening Agents?
Paper mills add chemical additives called optical brightening agents (OBAs) into their mix that artificially brighten the paper. Paper is naturally yellow as a result of the pulp it’s made from. We see this as objectionable and dirty for high-quality colour reproduction. We offset this yellowness by either removing it, usually by bleaching or other expensive chemical processes, or by adding blue. Optical brighteners do the latter by absorbing invisible ultraviolet light and emitting blue light through a fluorescent chemical reaction. By evening out the yellowness and blueness of the paper, the vibrancy of colours that the paper is capable of reproducing is increased.
Problems Caused by Optical Brighteners
The effect of OBAs can cause colour casts if left uncorrected, depending on how much UV-light is present in the light source the print is being viewed under. If the light source includes UV, such as daylight, the OBAs are stimulated and will add a blue cast to everything, primarily the whites and lighter colours. If there is very little UV in the light source, such as an incandescent bulb, the brighteners are not stimulated and the effect is not present. Even standardized lighting conditions, like D50 or D65, do not emit the same amount of UV-light from bulb to bulb and can change the print’s appearance. Colour management already has to take the intended visible lighting condition into consideration, but now the amount of OBAs in the paper and the invisible lighting condition are also important.
Proofing a press-sheet that is brightened with OBAs is a problem as well. It wasn’t an issue when a proof could be made on the same paper that the job would be printed on (by using a proofer like the Kodak Approval). However, inkjet printers using proofing papers are now the norm for colour proofing. Inkjet proofers have to simulate not only the printed colours, but the paper and the effects of the brighteners as well. This simulation can only be achieved for one light source at a time since in the simulation, the effect of OBAs is created by ink, not the media it’s printed on.
Adjusting for this mystical agent that messes with colour (depending on how you look at it) is a significant challenge, made even more difficult because the effect of optical brighteners varies over time. It is a chemical reaction and, therefore, is used up over time. How the print is kept will determine how long the OBAs will continue to react. Temperature and exposure to light have a lot to do with the shelf life of the OBAs in paper. This begs the question: when do I colour manage the print for? Today? Or down the road when the effect is exhausted?
Dealing with Optical Brighteners
The common practice when setting up a colour management system for a press and proofing that includes brightened paper is to filter out UV-light when taking measurements so that the OBAs are not stimulated and the effect is not present. This basically ignores the problem, in hopes that no one notices. Some places will try and make adjustments by hand, by altering tonal curves, taking colour management back from being a science to an art, as it once was.
There is a new commercial product that is designed to compensate for the effects of OBAs for specific light sources. The product adjusts colour management profiles that can be used for proofing and press outputs. Many of the advanced proofing systems also attempt to adjust for the effects in their own way with some success, but their common practice still uses measurements that do not stimulate the OBAs.
Optical brightening agents work with and against you. They help to create a bright paper capable of producing vibrant colours under the right light, but can be a troublesome foe to be reckoned with when striving for accurate colour reproduction.