which has not yet been de inked is determined at measurement location mo which is preferably arranged between the coarse sorting and pre flotation stages a degree of whiteness soft sensor compensates for the influencing factors of material density fine and loading content and is able thereby to supply the degree of whiteness of a test sheet made from material which has not yet been de inked the measurement location m 1 between the pre flotation and disperger bleaching stages permits still finer differentiation to be made at a measurement location m 1 a in the accepted stock at the pre flotation stage and a measurement location m 1 b following the thickening operation here the degree of whiteness of a test sheet is determined with the aid of sensors a further measurement location m 2 is arranged between the process stage p 2 and the process stage p 3 in other words following a preferably oxidizing disperger bleaching operation and preferably in the in feed to the post flotation activity analogously to the mla and m 1 b measurement locations sensors at the measurement locations m 3 a and m 3 b or measurement location m 3 acquire the degree of whiteness of the material following the flotation processing for example a transmitter at measurement location m 4 inside the bleaching pipe measures the degree of whiteness of the de inked finished material the control module r 1 or r 3 of a flotation stage.