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Tensile structures, like cable-nets and textile membranes are, because of their lightness and technological features, in recent years very often used for roof covering of permanent and temporary wide-span buildings (stadiums, exhibition and sport halls...). Since are these structures exclusively tensile resistant, they are characterized by self-forming process, where the form of the structure will be result of boundary conditions and prestressing forces. This is the reason why the form finding of these structures appears as the special engineering problem, where satisfying conditions of equilibrium and structural safety, as well as the architects intentions, shape of the structure is to be determined. This procedure usually involve numerical solving of highly nonlinear equations of equilibrium which requires starting approximation. Applying linearised equations, the force-density method is the most convenient for approximation of the shape. Dealing with first stage of tensile structures design process, architects usually use simple physical models, which always satisfy equilibrium conditions and offer excelled visualization. Possibility to measure the 3-D geometry of the model, getting digital results suitable as approximation for the further numerical analysis would significantly increase the efficiency of the design process. The main goal of this work is to develop such procedure of experimental shape approximation, and to evaluate results achieved by this method. For this purpose, highly automatised digital topometric equipment ATOS has been used to determine shape of convenient, stretch-fabric architectural model built for the purpose of investigation. The method is tested on simple boundary condition task of minimal net (such configuration where the whole cable length is minimal). Results achieved by this method are compared with the results of traditional force-density approximation method, and idealized, numerically calculated minimal net knot coordinates. There is shown that quality of experimental approximation is equal and in some cases even better than the numerical one.

Different concepts of correspondence findings in contactless optical three-dimensional (3-D) measurement systems using fringe projection are analyzed concerning the accuracy of the 3-D point calculation. These concepts are different concerning the kind of performance of the triangulation procedure in order to calculate the resulting 3-D points and the use of geometric constraints versus second projection sequence. Triangulation may be alternatively performed between camera pixels and the phase origin of the projection, between one camera pixel in the prior camera and the image of the corresponding measured phase value in the second camera, or between the image points of certain raster phase values in the two observation cameras. Additionally, triangulation procedures can be distinguished concerning the use of two perpendicular projection directions of the fringes versus the use of geometric constraints, i.e., epipolar geometry instead of the second projection direction. Advantages and disadvantages of the different techniques are discussed. In addition, a theoretical analysis of the application of synthetic data has been simulated as well as experiments performed on real measurement data. Both simulations and real data experiments confirm the theoretical assumptions concerning the magnitudes of the random errors occurring in 3-D point determination.

A two-dimensional small-rotation-angle measurement system based on fringe projection is proposed and demonstrated. Simple and effective signal processing is described and applied to the angular calculation. Several measurements indicate a measurement accuracy of ≈ 0.4 arcsec, which is comparable to that of a high-priced autocollimator. The two-dimensional two-pitch grating used in our prototype system provides a wide measurement range, which is expected to reach 1300 arcsec for both x - and y -direction rotations.

A two-grating interferometer for measurement of profiles of polished surfaces is proposed. The +/-first-order beams diffracted by the first grating produce parallel fringes of about 50-µm spacing on an object surface. By using the second grating, we eliminate the parallel fringes and extract a phase distribution produced by the surface profile of the object. Moreover, the vibration of the second grating of a small size enables us to use the sinusoidal phase-modulating interferometry. Measurement resolution is higher than 0.3 µm at the equivalent wavelength of 35 µm.

The inspection of capital goods with complex geometries is a challenging task due to the limited maneuvering and measuring space available. We developed a set of optical measurement systems for the inline inspection of such machines and capital goods. At first we introduce a borescopic fringe projection system based on of-the-shelf components. It is capable of detecting geometric variances in hard to reach areas, e.g. inside machines or in between parts with highly complex geometries like blade integrated discs (blisks). Single parts like compressor blades are measured with an inverse fringe projection system, which uses fully adaptable fringe patterns. The adaptable patterns lead to a high sensitivity and high speed. Results can be achieved with only one fringe pattern per measurement. In order to perform microscopic measurements and surface characterizations, we use a michelson interferometer with advanced 3D reconstruction algorithms to detect microscopic variances of the objects surface. These newly developed algorithms lead to higher sensitivity and improved results. Together these three inspection systems enable us to detect and to quantify geometric defects or variances of different industrial parts. Based on this information the prediction of the reliability of a part can be improved and the lifetime of an industrial part can be extended leading to a reduction of maintenance costs.

Wavelet transform analysis of projected fringe pattern for phase recovery in 3-D shape measurement of objects is investigated. The present communication specifically outlines and evaluates the errors that creep in to the reconstructed profiles when fringe images do not satisfy periodicity. Three specific cases that give raise to non-periodicity of fringe image are simulated and leakage effects caused by each one of them are analyzed with continuous complex Morlet wavelet transform. Same images are analyzed with FFT method to make a comparison of the reconstructed profiles with both methods. Simulation results revealed a significant advantage of wavelet transform profilometry (WTP), that the distortions that arise due to leakage are confined to the locations of discontinuity and do not spread out over the entire projection as in the case of Fourier transform profilometry (FTP).

In our laboratory, the performance of a 3D imaging system, based on structured light projection, has been improved by means of the development of three procedures, which are based on (i) the Gray Code projection Method (GCM), (ii) the Phase Shift Method (PSM), and (iii) the combination of GCM and PSM. In this paper, all the procedures are described and the results of the measurement performed on target objects are presented. The analysis of the systematic height error due to the crossed axes optical geometry of the system is performed, and the procedure for its compensation is presented.

A three-dimensional (3-D) imaging system based on Gray-code projection is described; it is thought to be used as an integration to the already developed profilometer based on the projection of multifrequency gratings. The Gray-code method allows us to evaluate the 3-D profile of objects that present even marked discontinuities of the surface, thus increasing the flexibility of the measuring system as to the topology of the objects that can be measured. The basic aspects of Gray-code projection for 3-D imaging and profiling are discussed, with particular emphasis devoted to the study of the resolution of the method and to the analysis of the systematic errors. The results of this study allow us to determine the optimal setting of the parameters of the measurement and to develop a suitable calibration procedure. The procedures for implementing the Gray-code method are presented, and some interesting experimental results are reported. Calibration of the system reveals an accuracy of 0.2 mm, corresponding to 0.1% of the field of view.

The effective and non-invasive diagnosis of skin cancer is a hot topic in biophotonics since the current gold standard, biopsy followed by histological examination, is a slow and costly procedure for the healthcare system. Therefore, authors have put their efforts in characterizing skin cancer quantitatively through optical and photonic techniques such as 3D topography and multispectral imaging. Skin relief is an important biophysical feature that can be difficult to appreciate by touch, but can be precisely characterized with 3D imaging techniques, such as fringe projection. Color and spectral features given by skin chromophores, which are routinely analyzed by the naked eye and through dermoscopy, can also be quantified by means of multispectral imaging systems. In this study, the outcomes of these two imaging modalities were combined in a machine learning process to enhance classification of melanomas and nevi obtained from the two systems when operating isolately. The results suggest that the combination of 3D and multispectral data is relevant for the medical diagnosis of skin cancer.

For three-dimensional (3D) shape measurement using fringe projection techniques, the information about the 3D shape of an object is encoded in the phase of a recorded fringe pattern. The paper proposes a high-order instantaneous moments based method to estimate phase from a single fringe pattern in fringe projection. The proposed method works by approximating the phase as a piece-wise polynomial and subsequently determining the polynomial coefficients using high-order instantaneous moments to construct the polynomial phase. Simulation results are presented to show the method's potential.