Principle of the measurement

To evaluate the watertightness of the component, we measure the deformation obtained when the component is under compression (see figure 1).

If we note δ(x , y) the deformation of the component surface between two instants corresponding to two different loadings, the variation of optical phase produced by the surface displacement can be expressed:

Therefore, the interferometric signal is modified by the phase variation. After loading, we have:

The reader will note that since the speckle signal and its random phase are still present in the signal, the image F is also very noisy and observing is impossible.

Considering the expressions of E and F, the final interference pattern can be easily deduced from the initial pattern. Indeed, when the two patterns are identical and E=F. Within a complete cycle of variation of Δφ between and the two patterns progressively decorrelate and reach a maximum of decorrelation for .

Therefore, by comparing the two patterns it will be possible to recognize the identical regions, i.e. to visualize the lines or regions where the speckle grains have been modified on the same manner by the object distortion.

This is true at the condition that the speckle doesn't change too much when the object is being loaded. The speckle patterns obtained with the two loads must be correlated, which means that the displacement produced at the object surface must be smaller than the speckle grain correlation length .