Optical brighteners and proofing

The goal of the proof is to represent the final destination. But what can you do if your proof keeps changing colour when it is viewed under various light sources? Not everyone has a 5000K viewing booth to evaluate the colour of their inkjet proofs, so they end up examining their proofs under daylight, fluorescent, or tungsten light sources. If you are serious about getting stable and accurate colour representation on your proof then you need to look into optical brightening agents (OBAs) and how they are creating chaos in the printing industry.

Why are Optical brighteners used?

The purpose of optical brighteners in inkjet papers and the printing industry is to remove the yellowish appearance of the raw materials used in the manufacturing of paper. Optical brighteners in paper increase the whiteness by converting Ultraviolet light energy to blue light—they’re a cheap way to brighten paper. Bright white paper sells better than a dull, yellow paper and it is for this reason we see a lot of OBA in inkjet papers. Paper manufacturers are being pushed to produce paper at the lowest possible cost and sell it at the lowest possible price. This means that they use more optical brighteners instead of bleaching, which is more costly.

How do I detect high levels of Optical Brighteners?

Their are three methods to detect if your paper has a high level of OBAs.

1) Examine your paper under a fluorescent, blue light—remember the one that made your Black Sabbath posters glow? A paper with high OBA will glow under blacklight. This test is good for making comparisons but not great for determining exact values.

2) Measure the paper using a device showing Lab values. If the b value exceeds -3 there are OBAs. A value of -2 to -3 is acceptable but if the values are higher than -4, I would seriously discount the paper for proofing. I have seen inkjet papers with values of -5 and -6 used in high-end printing facilities.

3) Visually compare your standard house stock used on press to your inkjet proofing paper under 5000K lighting. Do the papers match, or is your inkjet paper bluer?

Issues with Optical Brighteners Agents (OBA):

1) Increased colour shift when viewed under various lighting conditions.

2) Yellowing of paper over time and reduced longevity of print quality.

3) Colour measurement devices are wrongly influenced by OBAs—this problem is most apparent with inkjet inks because of their transparent nature.

4) OBAs are sensitive to environmental influences, particularly UV light. This means that high-OBA proofing media viewed under UV light (sunlight) will look more yellow than media with low OBA levels.

5) When creating a profile, the “white point” of your substrate is an integral component of the profile’s accuracy and high OBAs can trick measurement devices. The spectrophotometer used to create the profile sees the blueness of the paper so it adds more yellow to the profile. Our eyes do not see the optical brighteners as the spectrophotometer does, and the resulting profile has too much yellow. Some spectrophotometers offer a UV filter to combat this issue, but the best solution is to choose a proofing paper with low optical brightener levels.

A true test would be to profile two media: one with high degrees of OBA and another with low levels. Viewing both in natural daylight will reveal the high-OBA paper as more yellow. If you view the two proofs under non-daylight conditions, the proofs will look the same.

Many low-cost inkjet media will have higher degrees of OBA. Many of the “photo” papers also have more OBA. True proofing media often costs two or three times more because it is more expensive to whiten a paper using bleaching and dyes.

I’ll leave you with this interesting piece of information I came across in my research—apparently OBAs aren’t just for papers!

Optical brighteners are used in some washing detergents to make the whites whiter. Care should be taken that military uniforms are not washed with optical brighteners in your detergent, as these will make them more visible through Night Vision Devices or under low light conditions.

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