## 2 Materials and Methods
### Test Stand for Experimental Verification of FEM Results  
A schematic drawing of the ESPI interferometer system is shown in Figure 2. The DPSS laser (1) beam is split by a plate beam splitter (2) into two coherent beams. The transmitted beam is diverged by diverging lens (3) and, after passing through polarizers (4), illuminates reference surface of diffuser made of ground glass (5) and passes through cube beam combiner (6). The beam split by beam splitter (2) is reflected by mirrors (7 and 8) and diverged by diverging lens (9). The diverged beam illuminates the object surface (10). Speckle patterns are generated by light-diffusive surfaces. After passing through cube beam combiner (6) and digital camera (11), the speckle patterns meet and interfere with each other on the image plane of the CMOS sensor. The resulting image is displayed on a computer monitor and transferred to the hard disc of the computer (12). As a result of subtraction of the current speckle image from the initial speckle image, a stationary correlation fringe pattern is obtained for a static deformation of surface. The ESPI interferometer used for the measurements under discussion was developed as part of research work described by [PERSON] [20]. Figure 2: Schematic drawing of the ESPI interferometer: 1—laser, 2—plate beam splitter, 3 and 9—diverging lenses (100\(\times\) microscope objective lens), 4—polarizers, 5—diffuser made of ground glass with a reference surface, 6—beam combiner, 7 and 8—mirrors, 10—object surface, 11—digital camera (CMOS), 12—computer. A test stand for the measurement of orthopaedic braces was built and is presented in Figure 3. The stand consists of: optical bench (3), instrument for brace fastening (7) along with a pressure sensor (8), additional load application system (9) in the form of a tension member running over a rotary pulley with a 10 g weight, DPSS laser (1) emitting a beam with power of 50 mW and wavelength \(\lambda\) = 532 nm, optical set (2), digital camera (4), beam combiner (5) and computer with software (10). Together, components (1), (2), (4) and (5) form the electronic speckle pattern interferometer (ESPI). The divergence imposed on the brace to cover the whole object area (6) is 11.4\({}^{\circ}\). The brace fastening instrument (7) allows for the application of preliminary load, stabilising the brace from the interior side of the brace, measured by the pressure sensor (8). Preliminary force A (Figure 5b) is applied by a support positioned inside the brace, as a result of the tensioning of the straps of the orthosis. The values of the brace displacements caused by this initial force exceed the measuring range of the interferometer and are not measured. Next, an additional force \(D\) of 0.1 N (Figure 5b), causing brace displacements within the measuring range of the interferometer, is applied close to the place of application of the preliminary force and implemented using system (9) consisting of a tension member, a 10 g weight and a pulley. Displacements of the brace as a result of loading with the additional force are registered and processed by the computer with software (10) [21]. The test stand is characterised by the simplification of the number and directions of the forces acting on the brace for the purpose of reflecting actual boundary conditions in the numerical model as accurately as possible. Through the application of the ESPI interferometer, information on the distribution of displacement fields \(\Delta z\) is determined for every point in the field of view of the interferometer's camera (Equation (1))  
\[\Delta z=n\times\frac{\lambda}{2} \tag{1}\]  
where: \(\Delta z\)--displacement value in nm, \(n\)--order of correlation fringe, \(\lambda\)--wavelength of emitted light (in this case 532 nm).