Method for the preparation of an aqueous composition, preferably of a water-based latex binder, by emulsion polymerization of at least one ethylenically unsaturated monomer in the presence of at least one water-soluble diaminostilbene disulphonic optical brightener of formula in which R 1 and R 6 signify independently from each other hydrogen, linear or branched C 1 to C 4 alkyl, linear or branched C 1 to C 4 hydroxyalkyl, CH 2 CH 2 CONH 2 or CH 2 CH 2 CN, R 2 and R 4 signify independently from each other hydrogen, linear or branched C 1 to C 4 alkyl, linear or branched C 1 to C 4 hydroxyalkyl, CH 2 CO 2 M, CH 2 CH 2 CONH 2 or CH 2 CH 2 CN, R 3 and R 5 signify independently from each other linear or branched C 1 to C 4 alkyl, linear or branched C 1 to C 4 hydroxyalkyl, CH 2 CO 2 M, CH(CO 2 M)CH 2 CO 2 M, CH(CO 2 M)CH 2 CH 2 CO 2 M, benzyl or R 2 and R 3 signify together with their neighbouring nitrogen atom a morpholino ring and/or R 4 and R 5 signify together with their neighbouring nitrogen atom a morpholino ring, R 7 and R 8 signify independently from each other hydrogen, linear or branched C 1 to C 4 alkyl X and Y signify independently from each other CO 2 M or SO 3 M, n and m signify independently from each other an integer selected from 0, 1 or 2, and M signifies at least one cation for balancing the anionic charge of the optical brightener and is selected from the group comprising or essentially consisting of hydrogen, an alkali metal cation, an alkaline earth metal cation, ammonium, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 to C 4 linear or branched alkyl radical, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 to C 4 linear or branched hydroxyalkyl radical, ammonium which is di-, tri-, or tetrasubstituted by a mixture of C 1 to C 4 linear or branched alkyl radical or C 1 to C 4 linear or branched hydroxyalkyl radical or mixtures of said cations wherein the emulsion polymerization is initiated by a water-soluble azo compound of formula and/or formula in which X and Y signify independently from each other O or NH, R and R' signify independently from each other hydrogen, or linear or branched C 1 -C 4 alkyl substituted with one or more hydroxyl, CO 3 M 1 or CO 3 M 2 groups, wherein M 1 and M 2 signify at least one cation for balancing the anionic charge of the azo compound of formula and are independently from each other selected from the group comprising or essentially consisting of hydrogen, an alkali metal cation, an alkaline earth metal cation, ammonium, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 to C 4 linear or branched alkyl radical, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 to C 4 linear or branched hydroxyalkyl radical, ammonium which is di-, tri-, or tetrasubstituted by a mixture of C 1 to C 4 linear or branched alkyl radical or C 1 to C 4 linear or branched hydroxyalkyl radical or mixtures of said cations. R 1 and R 6 signify independently from each other hydrogen, linear or branched C 1 to C 4 alkyl, linear or branched C 1 to C 4 hydroxyalkyl, CH 2 CH 2 CONH 2 or CH 2 CH 2 CN, R 2 and R 4 signify independently from each other hydrogen, linear or branched C 1 to C 4 alkyl, linear or branched C 1 to C 4 hydroxyalkyl, CH 2 CO 2 M, CH 2 CH 2 CONH 2 or CH 2 CH 2 CN, R 3 and R 5 signify independently from each other linear or branched C 1 to C 4 alkyl, linear or branched C 1 to C 4 hydroxyalkyl, CH 2 CO 2 M, CH(CO 2 M)CH 2 CO 2 M, CH(CO 2 M)CH 2 CH 2 CO 2 M, benzyl or R 2 and R 3 signify together with their neighbouring nitrogen atom a morpholino ring and/or R 4 and R 5 signify together with their neighbouring nitrogen atom a morpholino ring, R 7 and R 8 signify independently from each other hydrogen, linear or branched C 1 to C 4 alkyl X and Y signify independently from each other CO 2 M or SO 3 M, n and m signify independently from each other an integer selected from 0, 1 or 2, and M signifies at least one cation for balancing the anionic charge of the optical brightener and is selected from the group comprising or essentially consisting of hydrogen, an alkali metal cation, an alkaline earth metal cation, ammonium, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 to C 4 linear or branched alkyl radical, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 to C 4 linear or branched hydroxyalkyl radical, ammonium which is di-, tri-, or tetrasubstituted by a mixture of C 1 to C 4 linear or branched alkyl radical or C 1 to C 4 linear or branched hydroxyalkyl radical or mixtures of said cations wherein the emulsion polymerization is initiated by a water-soluble azo compound of formula and/or formula in which X and Y signify independently from each other O or NH, R and R' signify independently from each other hydrogen, or linear or branched C 1 -C 4 alkyl substituted with one or more hydroxyl, CO 3 M 1 or CO 3 M 2 groups, wherein M 1 and M 2 signify at least one cation for balancing the anionic charge of the azo compound of formula and are independently from each other selected from the group comprising or essentially consisting of hydrogen, an alkali metal cation, an alkaline earth metal cation, ammonium, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 to C 4 linear or branched alkyl radical, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 to C 4 linear or branched hydroxyalkyl radical, ammonium which is di-, tri-, or tetrasubstituted by a mixture of C 1 to C 4 linear or branched alkyl radical or C 1 to C 4 linear or branched hydroxyalkyl radical or mixtures of said cations.