Vibration analysis of plates by a time-averaged projection-moiré method

A time-averaged moiré principle is described that is appropriate for vibration analysis of plates. If the projected grating, deformed by the vibrating object, is superposed upon a reference grating in the area of the screen of the projector and observed on a ground-glass plate for more than one period, moiré patterns can be seen without the interfering gratings, depicting the nodal lines and the slope contours in the extreme deflected positions of the plates.