A method for out-of-plane displacement detection is disclosed. The out-of-plane displacement is detected by analyzing all the fringe density indexes calculated using the frequency-domain information extracted from a series of interference images of the sample vibrating at different frequencies. The present invention further discloses a method and an apparatus for resonant frequency identification by detecting the peak value of all the fringe indexes calculated at different scanning frequencies. With the identified resonant frequency, the full-field vibratory surface profile of the sample in various resonance modes can be reconstructed.