Do I have to measure the optical brightener (OBA) in paper

Optical brightener will influence the color measurement. The more UV-radiation is in the illumination, the whiter the substrate appears. If the color of the product is measured without UV-radiation, it will never correlate to the visual assessment at daylight illumination. If the sample is measured with an unfiltered xenon flash-lamp, the UV-radiation is much too high, the sample will be measured whiter (=bluer) than it looks in reality.

When a sample includes OBA in a quantity that can be visualized in a light booth with UV-radiation, it is absolutely necessary to measure the OBA separately. This is the only way to have a reliable color measurement which corresponds to the visual assessment and with the measurements of the customers.