System for surface inspection arranged to detect relative displacement and/or vibration characteristics of a plurality of points of a plurality of elements forming part of a mechanical structure, said system comprising: a light source arranged to generate at least one light beam; a position sensitive detector arranged to receive the light beam when reflected off the mechanical structure and to produce at least one output signal in response to receipt of said light beam; an electronic control system; scan means for relative displacement of said light beam with respect to the mechanical structure so as to scan said mechanical structure with the light beam, following instructions from the electronic control system; characterised in that said electronic control system is arranged to control the scan means so as to displace the light beam along the mechanical structure along a first trajectory, so as to detect a plurality of subsequent reference positions, each in one element, along said first trajectory, wherein said electronic control system is operatively associated with said position sensitive detector so as to determine said reference positions as a result of an analysis of at least one output signal from said position sensitive detector; wherein said electronic control system is further arranged to control the scan means so as to displace the light beam along the mechanical structure along a plurality of second trajectories, each of said second trajectories being associated with one of said reference positions; said electronic control system further being arranged to obtain, during displacement of the light beam along each of said second trajectories, a plurality of position signal outputs from said position sensitive detector. a light source arranged to generate at least one light beam; a position sensitive detector arranged to receive the light beam when reflected off the mechanical structure and to produce at least one output signal in response to receipt of said light beam; an electronic control system; scan means for relative displacement of said light beam with respect to the mechanical structure so as to scan said mechanical structure with the light beam, following instructions from the electronic control system; characterised in that said electronic control system is arranged to control the scan means so as to displace the light beam along the mechanical structure along a first trajectory, so as to detect a plurality of subsequent reference positions, each in one element, along said first trajectory, wherein said electronic control system is operatively associated with said position sensitive detector so as to determine said reference positions as a result of an analysis of at least one output signal from said position sensitive detector; wherein said electronic control system is further arranged to control the scan means so as to displace the light beam along the mechanical structure along a plurality of second trajectories, each of said second trajectories being associated with one of said reference positions; said electronic control system further being arranged to obtain, during displacement of the light beam along each of said second trajectories, a plurality of position signal outputs from said position sensitive detector.