By digitizing the UFM signal without using a lock-in amplifier, substantially all of the information initially contained in the UFM output signal may be maintained and may then be used for further data processing. Consequently, any type of model or evaluation algorithm may be used without being restricted to a very narrow bandwidth, as is the case in lock-in based techniques. The digitizing is performed on a real-time basis, wherein a complete UFM curve is digitized and stored for each scan position. In this way, quantitative meaningful values for specific surface-related characteristics with a nanometer resolution may be obtained.