Digital Speckle Pattern Interferometry (DSPI)

When a rough surface is illuminated with coherent light, speckles will appear. In speckle pattern interferometry, a speckled object wave and a reference wave are combined to obtain interference. In this way, one can obtain the out-of-plane displacement with a very high precision (sub wavelength resolution, up to 20 nm) in a full-field and non-contact measurement. Other setups are also possible:

• Shearography: measurement of the derivative of displacements
• In-plane displacement: by using a different laser illumination
• Vibration analysis

Figure: Example for beak deformation
Left: Top view of the beak with white light to show setup
Right: the obtained out-of-plane displacement for this beak during  a 3.5N tip loading (colorbar shows the displacements in 10^-5 m)