Giovanna Sansoni

Calibration and performance evaluation of a 3-D imaging sensor based on the projection of structured light

In this paper, the procedure developed to calibrate a whole-field optical profilometer and the evaluation of the measurement performance of the system are presented. The sensor is based on the projection of structured light and on active triangulation. The dependence of the measurements on the geometric parameters of the system is shown, as well as the criterion to calibrate the system. From the extensive set or experiments carried out to evaluate the measurement performance, good linearity has been observed, and an overall mean value of the measurement error equal to 40 /spl mu/m, with a variability of about /spl plusmn/35 /spl mu/m has been estimated.

A novel, adaptive system for 3-D optical profilometry using a liquid crystal light projector

A 3-D optical whole-field profilometer based on adaptive projection of structured light is presented. The system is based on the projection of gratings by means of an LCD unit. The gratings can be varied both in contrast and in period, to adapt to the shape of the object under measurement. A video camera acquires at a different angle the object-deformed pattern. Suitable pre-elaboration is performed, to decrease dependence on background illumination and nonuniform reflectivity of the surface. Adaptive demodulation of the pattern allows the object profile to be evaluated. The performance of the instrument has been evaluated by means of a system calibration against a traceable high-precision commercial Contact Measuring Machine (CMM). The overall accuracy of the system is equal to 0.15 mm with a precision of 0.2 mm. In this article, the theoretical aspects of the technique are discussed, and the description of the complete system is presented. Profile reconstruction, calibration, and certification of the system are also covered. The accuracy of the system is discussed, and experimental results are presented. >

3D imager for dimensional gauging of industrial workpieces: state of the art of the development of a robust and versatile system

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.

Adaptive whole-field optical profilometry: a study of the systematic errors

An analysis of the sources of systematic errors in an optical whole-field profilometer based on the projection of fringes is presented here. In the system, the determination of the object profile is performed by triangulation. Both the period of the fringes and the geometrical system parameters on which triangulation operates (i.e. the distance between the object and the acquisition/projection units, and the distance between the acquisition unit and the projection unit) represent the input parameters of the profile evaluation algorithm. The influence on the height error introduced by an inaccuracy in the determination of the projected fringe period, as well as of the geometrical parameters of the system, is investigated here. The distortion in reconstructing the object shape due to the finite distance illumination scheme used in the system is also studied. The results obtained from the analysis are used to increase the accuracy at the optical profilometer by means of a suitably developed correction algorithm. >

Comparative analysis of low-pass filters for the demodulation of projected gratings in 3-D adaptive profilometry

In 3-D adaptive profilometry based on structured light projection, the choice of the low-pass filter to he used in the deformed pattern demodulation is crucial. In this paper, we have studied the performance of a typical finite impulse response (FIR) and of an infinite impulse response (IIR) Butterworth low-pass filter. Adaptiveness of the filters to both coarse and small variations of the grating frequency has been investigated. The ability of the filters to adapt to coarse changes of the grating frequency has been quantified in terms of their speed of synthesis, while the ability of the filters to tolerate small variations of the grating frequency has been quantified by measuring the residual phase errors. The analysis shows that the IIR Butterworth filter performs better than the FIR filter both in the coarse and in the fine grating frequency variation cases. >

Light-induced transmission changes in liquid crystal displays for adaptive pattern projection

The changes in the transmission and contrast properties of a super-twisted nematic liquid crystal display panel, induced by the light impinging on the panel itself, have been studied. Upon illumination by laser radiation with power density close to that used in practice, the transmittance of the panel increases or decreases according to the brightness voltage that sets the transmittance level. The dynamics of the transmittance change are typically bi-exponential, with a shorter decay time on the order of a fraction of a second and a longer decay on the order of tens of seconds. The observed changes were interpreted and modeled by considering local temperature changes in the crystal because of the light impinging on it. The temperature changes produce a shift of the transmittance curve and a change of slope in its central region. The presence of the dip after the Frederickzs region accounts for the anomalous behavior of the transmittance in that portion of brightness voltages. >

Validation of the measurement performance of a three-dimensional vision sensor by means of a coordinate measuring machine

The results of the activity carried out to verify the measurement performance of an optical three-dimensional sensor based on the projection of structured light are presented in this paper. The followed approach is the substitution approach, in which repeated measurements are carried on a calibrated object. These measurements have been accomplished by using a coordinate measuring machine. The work carried out allows us to appreciate the performance of the optical system as well as to outline the critical aspects. These are mainly related to the quality of the master used for the calibration of the sensor, and to the efficiency of the alignment algorithm, whenever multi-view acquisition is required.

A special case of 3-D optical measurements and reverse engineering for automotive applications: the Ferrari 250 Mille Miglia

The complete process of 3D optical measurement, view alignment, mesh creation, production of CAD models, and rapid prototypation of a prestigious historic car is presented. The optical digitiser OPL-3D, developed in our laboratory, is the sophisticated instrument that allowed the accurate gauging of the car. The instrument has been used in different set ups, to optimise the acquisition speed while maintaining the resolution required for large areas as well as for fine details of the car body. The work highlights the impact that the novel 3D optical measurement instrumentation, based on incoherent light, whole field projection, in conjunction with current point cloud elaboration tools, with modern CAD environments, and with rapid prototyping systems, can have when applied to the automotive domain, for reverse design and engineering, for scaled model production, and for accurate 3D visualization of prestigious objects in virtual museums.

Whole-field optical profilometry: application of nonlinear processing algorithms to the enhancement of low-contrast images

A nonlinear filtering technique for the preprocessing of very low contrast images has been applied to optical profilometry, as an attempt to improve the accuracy of the measurement of objects in harsh conditions. The technique is based on the application of a nonlinear architecture composed of linear Laplacian filters followed by quadratic filters which detect correlated elements. The above sequence of operators results in efficient highpass filtering, keeping at the same time the signal-to-noise ratio within acceptable limits. When applied to highly transparent or weakly diffusive surfaces, the preelaboration technique has largely improved the accuracy of the profilometer. In this paper the preelaboration technique is presented. In particular, the influence of the nonlinear image elaboration on the overall system performance is discussed.

Optical interferometer using a high-birefringence optical fiber

A prototype of a new displacement measuring system based on an optical interferometer using a polarization-maintaining optical fiber is presented. The proposed configuration makes it possible to separate the laser head and the detectors from the actual interferometer (i.e., the measuring head). In this way, the interferometric head can be miniaturized and used to gauge targets which are difficult to access. The resolution provided by the electronic fringe counter is equal to lambda /8 ( approximately 80 nm), and the overall accuracy is 1 mu m over a measuring range of 20 mm. >