Saturday, March 7, 2009

The issues of Optical Brightening Agents in paper and ink

As ICC color managed workflows become more prevalent in the graphic arts, so do the difficulties caused by OBAs (Optical Brightening Agents) that are encountered in the pressroom, prepress, and their customers. OBAs are used to increase the apparent brightness and whiteness of papers and their use is becoming more prevalent in paper manufacturing. They increase brightness and whiteness by absorbing energy in the ultra violet and emitting (fluoresce) the energy in the blue area of the visible spectrum. Because, to the eye, blue/white looks "whiter" than yellow/white OBAs are not really whiteners, but bluing agents. OBAs are also used in ink to expand gamut or brighten 4/C image printed on poor substrates - e.g. newsprint.
When fluorescence is present, the light coming from the sample is the combination of the light that is reflected and the light that is fluoresced. ISO 12647-2 printing paper grades specifies low OBA content, however, there is no specification as to the amount of OBA content. And, although they typically use light sources with little or no UV radiance, there is no specification describing the UV content of light sources in measurement instruments such as spectrophotometers. The addition of fluorescence to either the inks or the substrate greatly increases the level of uncertainty in instrument readings of the optical properties of printed images. This, in turn, may lead to a significant lack of reproducibility between two imaging centers that attempt to apply color management principles to their individual measurements of the same image printed on various substrates.

While it is not practical for printers to quantitively measure the OBA content of the materials that they use, it is quite an easy matter to qualitatively see the OBA content. All it takes is an inexpensive (less than $15 USD) "black light" such as the one illustrated below (do not bother with incandescent black lights):For example, with the black light it is easy to see that the paper used for the Pantone Goe system swatch book (on the left in the image below) contains more OBAs than the conventional Pantone spot color swatchbook on the right. Also, it's clear that the uncoated paper section in the Pantone spot color swatchbook contains more OBAs than the coated section.The bottom line – the significance of which will be more apparent in the next parts of the blog on this topic – is that, although you may not be able to do anything about it, just being able to be aware of OBA content can help solve issues related to their use.

OBAs are sometimes use as additives in ink. In the example below it has been used in the yellow ink in a process set – while the paper itself contains little if any OBAs.Using OBAs in the yellow ink is a common strategy with newspaper printers as a way to add brightness to imagery and compensate for the poor whiteness of newspaper stock. The fact that the OBAs fade and cause a color shift over time is typically not a concern in that market. It is important to be aware of the OBA content of your process ink set particularly if the print specifier is concerned with the longevity of their printed materials - especially if they will be exposed to sunlight.
OBAs are also used in so called Hi-Fi inks, notably those used in the Pantone Hexachrome process, to add vibrancy and expand the gamut beyond conventional four color process. However the OBAs can make the inks more problematic in the pressroom as well as result in presswork that does not have a long shelf life (due to fading and color shift).

The inks that are typically used in four color process printing block, to varying degrees, the fluorescence in papers containing OBAs. Black and magenta block the greatest amount, yellow a lesser amount, and cyan ink least of all. What this means is that when an image is printed using a halftone screen, lighter/pastel tones allow more more of the brightening and color shift of OBAs (towards blue) than the shadows. Color is effectively skewed towards the blue from shadows to highlights – but only when the paper being printed on has a high OBA content.In daily presswork this disconnect usually appears in midtones and pastels – sometimes the color matches the proof in those areas and at other times the color doesn't – depending on the OBA content of the paper being run. FM screenining will lessen this effect. The effect on an AM screen is emphasized in the image below to illustrate the issue.
The use of OBAs in paper has a significant impact on the reliability of proofing and alignment of presswork to the proof. This is an issue where the use of a black light really "shines". When the proofing paper contains OBAs the hue of pastel colors can shift depending on the amount of UV being emitted by the viewing light source as illustrated below:Although they cannot control the light under which their customers evaluate proofs, many printshops will use UV blocking filters to cover the D50 bulbs in their viewing booths. The notion is that, most of the time, the proof will be looked at by the customer under lighting with little UV content. The UV block filter helps the press operator to "ignore" the presence of OBAs in the proof/presswork.
Another strategy is for the printshop to try and align the OBA content of their proofing paper and press sheets. A black light can provide a qualitative measure of the OBA content of the media as illustrated here using a vendor's swatchbook of their proofing media:Selecting pairs of press and proofing papers according to their OBA content helps in the alignment of presswork and proofs and thereby enhances the printer's ability to set expectations correctly with their customers.

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