Generation of Speckles
If a rough surface is illuminated by laser light the light scattered back from multiple surface points is generating interference effects. If the surface is observed by a camera or by eye a speckled image is observed. Similar to a finger print, these speckles represent the microstructure of the surface and can be used for high precision surface measurements.
The figure shows a schematic of the ESPI set-up for the measurement of the out-of-plane component of the displacement. The laser beam is split into two illuminating beams. The object beam is illuminating the measuement surface. From there the light is backscattered to the camera. The reference beam is comined with the object beam through the beam splitter and directed to the camera chip. There they form an interference pattern - a speckle pattern - which is recorded by the CCD camera. Before the object is deformed, the phase difference between the object and the reference beam is φ. After the object is deformed, the phase difference becomes (φ + Δ) and a phase change Δ results from the deformation. The subtraction of the image before (reference image) and after deformation reveals directly the deformation.
To measure the in-plane component, a symmetric dual-beam illumination is commonly used. In this case the speckle pattern is generated by the interferences of the back scattered light waves from the two illuminations. Therefore, the measuring direction lies in the plane orthogonal to the viewing direction of the camera. Therefore only in-plane deformations within the plane generated by the illumination sources and the observation direction cause a phase change Δ. If the in-plane component vertical to the the plane of this display should be measured, a second pair of illuminations is required.
A 3D measurement is carried out by combination of both methods. In order to record the complete in-plane displacements two orthogonal pairs of illumination are required. One of these illuminations is coupled with an internal reference beam to receive the out-of-plane component of deformation. Those different illumination directions are automatically switched and the respective speckle images recorded. The result is a set of displacements or deformations of the complete surface under observation.
- out-of-plane ca. 30nm
- in-plane ca. 100 ... 300 nm
- Measuring area up to ca. 200 x 300 mm2
- Measuring speed:
- 3D measurement ca. 2 sec / exposure
- 1D measurement ca. 0.5 sec / exposure
- Serial measurement for large deformations