OBA means 'optical brightening agent'. 

These chemicals are often added in the coating (see RC papers for instance) or even in some cases, in the base paper itself. They help provide a higher whiteness to the paper. 
The visible spectrum spans from 380 nm (close to UV) to 780 nm (close to IR) for the human eye. The OBA can absorb the electromagnetic radiation between 300 and 400 nm and re-emit the energy into the visible (blue) part of the spectrum, between 400 and 500 nm, which is called fluorescence. 
The cellulose-base paper will absorb a part of the visible light and tend to get a yellow shade. As the blue is the complementary color of the yellow, adding an OBA ot the paper will therefore bring more whiteness to the paper. 

the OBA are unstable molecules and may turn yellow over time, which will lead to paper discoloration.

When considering the color management, the higher the OBA content in the paper, the more difficult is the control over the print result. 
Canson® pays the higher attention to the whitenes of its products and uses only natural pigments, so that the paper will keep its initial color.

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