Abstract: The invention refers to a method for determination brightness of paper pulp by measuring fluorescence from UV. The measurement is carried out directly in the pipe in which the paper pulp, which shall constitute the paper, is fed forward, by an in-line brightness meter, which also is capable to measure fluorescent substances in the paper pulp, a direct control of the partial stages in the paper pulp process where fluorescent substances can exist or a determination at the end of the process can be accomplished.