The vibrometer measurement can be classified, accordingto the involved overlap of certain laser beams, in a homodyneor heterodyne interferometer. Both instruments are basedon modified Mach–Zehnder interferometers. The homodyneinterferometers are used for larger displacements, which canbe determined by counting the interference maxima, which isalso known as the fringe counting method. The displacementis, thereby, only a function of the laser wavelength and thenumber of fringes. The limitation of the method is if thedisplacement comes in the order of λ/2, because this is theresolution limit of the fringe counting. For the lower limitfor a displacement of 400µm, one can obtain uncertainties of0.1% in the range from 10Hz to 1.5kHz