## 3 Photonic Technologies
### Interferometric Techniques
#### 3.2.2 Electronic Speckle Pattern Interferometry (ESPI)  
Electronic Speckle Pattern interferometry (ESPI) is a whole-field optical interferometric technique to determine static and dynamic strains/stresses and displacements of objects with optically rough surfaces. This technique is based on interference of the scattered light from the specimen with a reference speckle pattern, and the resultant pattern shows also a randomly-distributed intensity [54]. The changes in the object state, like deformation and displacement, are proportional to the phase changes within each speckle. To visualize this effect, the image and reference beams are combined on a video camera and recorded. When the object has been displaced/deformed, the new image is subtracted point by point from the first image. The resulting image is a speckle pattern with black fringes. ESPI have four configurations: out-of-plane displacement measurement; out-of-plane vibration measurement; in-plane measurement; and in-plane displacement gradient measurement [55; 56]. 3D-Electronic-Speckle-Pattern-Interferometry (3D-ESPI) was used to assess the impact of different restorative materials on the deformation of teeth. The deformation pattern of restored teeth is material-specific but the extent of deformation is primarily limited by the remaining tooth substance [57]. This technique was also used to develop a non-destructive test for characterizing the modulus of resin-based filling materials, and as it is a non-destructive test, time-dependent effects of composites can be determined using the same specimens leading to substantial time and material savings [58]. Dimensional changes of dental composite resins, due to the influence of post-polymerization reactions, water absorption connected with the elution of species from the material, and changes in the temperature inside the oral cavity, were investigated by means of ESPI [59]. A similar study was carried out by [PERSON] [59] to investigate the relationship between post-polymerization of dental composites, with water uptake and thermal fluctuations, by measuring the deformations in the three directions. [PERSON] _et al._[60] have used ESPI to study the intrinsic reaction of the tooth crown to load and verified that it is complemented by the structures supporting the tooth. This was further verified in another study. [PERSON] _et al._[61] used electronic speckle pattern-correlation interferometry (ESPI) combined with a mechanical compression apparatus to measure the strain and Young's modulus of root dentin, compressed under water. In another study, [PERSON] _et al._[62] used the same technique to compare the mechanical behavior of the natural DEJ and the dentin ceramic junction (DCJ) manufactured with a CAD-CAM system. They verified that a smooth transition occurs between dentin and enamel and in the modeled prosthetic, the same kind of accommodation effects also occur, but with a steeper transition slope between dentin and ceramic. This technique was also applied in the study of tooth structure and to characterize cortical bone [63,64]. This experimental technique was also used in orthodontic studies to evaluate the displacement patterns, induced in a pig jaw, by two orthopedic and orthodontic devices (Figure 3) [65]. The authors verified that the use of orthodontic or orthopedic devices induces displacements in the entire skull and they provoked a rotation for vestibular position of posterior teeth and alveolar processes. Figure 3: Displacement field induced by two orthodontic devices: Disjunctor and Quad-helix. Left: phase map; Right: displacement field calculated [65]. Digital image correlation (DIC) was also used to study the shrinkage of resin-based dental composites by determining the in-plane displacement field by matching different zones of two characterized pictures. The DIC method facilitates a full-field measurement of shrinkage profile [66, 67, 68, 69] and its kinetics [70].