2.1

Speckle analysis

As the object scatters light, we observe speckle patterns on interference images (Figure 2). Then the first object of study in this experiment is speckle.

Figure 2.

Experimental bench description

The CCD camera is linked to a computer, some numerical processing allows the students to adjust the experimental parameters to optimize the visualization of interference patterns.

2.2

Displacements measurement

To measure displacements on the plate surface, two methods can be performed. The principle is always to compute the numerical difference between two images, pixel by pixel:

In this two cases, the result is an image which measure the variations of phase, on the laser beam reflected by the object, due to the deformation.

2.3

Vibration analysis

The refresh frequency of the camera is 50Hz, therefore, when the plate vibrates at frequency near 10kHz, our device integrate vibration signal. However, numerical processing allows to have a measure of amplitude of vibration along the surface of the object. Indeed, it can be shown that two methods derived from the previous ones can give a map of the vibration mode as schown in figure 3.

Figure 3

Exemple of a speckle pattern

Figure 4

Vibration analysis of the PZT plate

2.4

Application

The vibration of the structure of an electrical motor causes accoustic noise. This experimental bench has been used to analyse the vibration modes of an electrical motor. Some students have performed theses measures in an experimental project.

All theses experiments give spectacular images and provide valuable teaching support.