In speckle pattern interferometry the starting phase is distributed randomly. Furthermore the background intensity and the modulation also are structured stochastically. This leads to a significant amount of noise in the interferogram analysis. Low modulated speckles are more difficult to evaluate than high modulated ones. We report on a method to reduce this problem. The principle is to use more than one wavelength at the same time and to separate the different wavelengths in the recording plane. With this method the precision can be improved by factor up to 2.3. Our first experiments show the high potential of this method.
The evaluation of the time dependent speckle modulation [2-3] due to optical path difference changes can be extended to vibration measurement. For a sinusoidal signal evaluation with varying frequency: from zero at its reversal point, to maximum frequency at its medium position with maximum speed. For both cases we assume a harmonic vibration. For evaluation, the distance of two adjacent zero frequency points is determined, giving half the mechanical time base, and the maximum frequency, giving the maximum mechanical speed, or, assuming a harmonic vibration, the mechanical amplitude.
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