The invention provides a method for determining vibration-related information by projecting an aerial image at an image position in a projection plane, mapping an intensity of the aerial image into an image map, the image map arranged for comprising values of coordinates of sampling locations and of the intensity sampled at each sampling location, and measuring intensity of the aerial image received through a slot pattern. The method further includes determining from the image map a detection position of a slope portion of the image map, at the detection position of the slope portion, measuring of a temporal intensity of the aerial image and measuring of relative positions of the slot pattern and the image position, the relative positions of the slot being measured as position-related data of the slot pattern and determining from the temporal intensity of the aerial image vibration-related information for said aerial image.