Abstract: Radiation pulses are directed at a measuring spot on a rotating printing-image carrier in an offset printing press. The intensity of the radiation returned from the measuring spot outside of the angle of reflection of the pulse is measured for a predetermined wavelength. A pulse is directed at the measuring spot before printing and while the measuring spot is dry, and a signal representing the intensity of the returned radiation is stored in a working memory. During a subsequent printing operation, a dampening agent is applied to the printing-image carrier. In order to determine the amount of dampening agent on the printing-image carrier, radiation pulses are directed at the measuring spot as the printing-image carrier rotates during printing. The intensity of the returned radiation in the predetermined wavelength is converted into damp signals which represent the amount of dampening agent since the predetermined wavelength approximates or equals an absorption wavelength of the dampening agent.