1. A method of measuring a suspension which contains wood fibers, the method comprising directing optical radiation at pulp and measuring optical radiation emitted by the pulp, the method comprising measuring at least one of the following two properties: kappa number and brightness; and the method comprises changing the pulp consistency in a desired consistency range extending from a desired initial consistency to a desired final consistency; measuring the pulp consistency; measuring the strength of a desired wavelength from the optical radiation at different consistencies in the desired consistency range; modeling measurement points with a function of the desired type by fitting the parameters of the function according to the measurement points; and determining at least one of the following two properties of the pulp by means of the modeling function: kappa number and brightness.