Interferometric measurements determine the change of a wave front due to interaction with an object. Phase shifting can be used to reveal the shape of the object [3]. The deformation of an object can be found by interferometric deformation measurements. In this case the phase maps of the initial and the excited state of the object are acquired and subtracted.

Classical Laser Interferometry (LI) is based on plane waves in object and reference arm. The basic configuration is the Michelson interferometer shown in the upper image in figure 1. Displacements of the object change the optical path length difference (OPD) in the interferometer and the deformation can be extracted from the interference pattern. Using a camera and an imaging system the spatial distribution of the deformation can be measured.

Electronic Speckle Pattern Interferometry (ESPI) [10] is applied for non-specular objects. The reflected speckle field from the object is used to record the state of the object in the initial and the excited state. LI and ESPI techniques are suited for the measurement of deformation and the detection of resonance frequencies [11].

Digital Holography (DH) is similar to classical holography [12], except that a CCD sensor is used as the recording medium instead of a holographic film plate. As opposed to ESPI, where a lens is used to focus the object surface onto the camera target, the interference pattern of the object and the reference wave is allowed to fall directly onto the CCD sensor. The reconstruction is performed numerically in the computer by applying diffraction theory. Digital Holography yields direct information about the phase of the wavefield, whereas in ESPI the phase needs to be calculated from the intensity.

Low Coherence Interferometry (LCI) is used for shape and topography measurements and for the investigation of the internal structure of the object. Classical LCI is also referred to as white-light interferometry and utilises the temporal coherence of a broad band light source for high accuracy detection of position of the sample surface [13]. Stroboscopic measurements allow dynamic characterisation [14]. LCI is able to detect the deformation of interfaces inside objects. This requires dispersion compensation [15]. The lower image in figure 1 shows the basic setup of a low coherence interferometer. The two main differences from laser interferometry are the coherence length of the light source and the necessity to control the zero-OPD between the object and the reference arm in the interferometer. The position of the reference mirror selects a corresponding layer in the object. Interference will only occur for object light having an OPD within the coherence length of the source.

Optical Coherence Tomography (OCT) [16] is well established for structural imaging based on depth-resolved interferometric measurements. OCT utilises sources with a temporal coherence layer between 1 and 10µm to enable high depth resolution.

Low Coherence Speckle Interferometry (LCSI) is used for the characterisation of changes of internal interfaces in transparent and semi-transparent multi-layered materials or structures. LCSI is based on a speckled object wave where the deformation measurement algorithms of ESPI are applied.

Application examples:

• Testing and calibration of micro elements (i.e. MEMS and MOEMS) - SMARTIEHS.
• Characterisation of adhesion
• Optical non-destructive material testing