Abstract: The invention is directed at an arrangement for determining cantilever deflection in a scanning probe microscopy system. The system includes a scan head supporting a probe, including the cantilever and a probe tip, comprising a specular reflective surface. The arrangement comprises an optical source for providing an optical beam. The optical beam is impinged onto the specular reflective surface. An optical sensor receives the reflected beam from the specular reflected surface, forming a light spot on the sensor. The optical sensor provides a sensor signal from which location information of the light spot on the sensor is obtainable. The optical sensor comprises an array of photo diode elements. Each photo diode element is configured for providing a photo diode signal to be included in the sensor signal, and comprises a photo sensitive surface having an effective area dimension in a plane transverse to the beam direction which is smaller than the cross sectional area of the reflective beam.