Vibration Analysis by Speckle Counting

A method for the simultaneous determination of the amplitude and frequency of in-plane vibrations is described, based on a velocity measurement within small time intervals made by analysing the moving speckle pattern. This is done by counting the intersections between the actual speckle intensity and a threshold given by the mean intensity, which is evaluated by spatial integration of the speckle intensity with the aid of an aperture of appropriate diameter, thus avoiding the use of any time constant. A special control measurement enables the inversion point of the vibration to be detected precisely, which enables the frequency to be determined, even if shot noise is not negligible.