I believe.

Clariant has a significant range of colorants, optical brightening agents and chemicals, providing powerful, cost-effective and consistent solutions to a multitude of problems which can arise in paper production.

http://www.chemicals-technology.com/contractors/polymeraddit...

At the request of many customers Hahnemühle offers bright white papers, where optical brighteners are needed in production. The whiteness of the base materials may alter slightly over a period of between 20 to 200 years. Some users regard this as a “natural patina” of fine art prints, while others find this not to their liking.
Hahnemühle FineArt offers its customers three categories of white papers:
1. Natural white papers without optical brighteners, such as William Turner, Albrecht Dürer, Museum Etching, Natural Art Duo and Photo Art. This range will be expanded in the beginning of 2007 with a new product – Photo Rag® Pearl.
2. White papers with a minimum of brightener (< 0.1%) such as Photo Rag® or German Etching.
3. Bright white papers with 0.1 to 0.8% optical brightener such as Photo Rag® Bright White, Torchon or FineArt Pearl.

http://www.hahnemuehle.com/site/us/1595/hahnemuehle-fineart-...


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Note added at 42 mins (2008-01-12 12:43:01 GMT)
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Was sind Optische Aufheller?
Optical brightener [engl.] - Eclaircisseur [frz.]
http://www.austropapier.at/index.php?id=113