A system based on digital holographic interferometry in combination with an endoscope is described. A Q-switched pulsed laser is used. Two digital holograms of the test object, corresponding to the two laser pulse, are captured at separate video frames of the CCD-camera, transferred in a frame grabber and further processed in a PC. If during the interval between the two laser pulses the object undergoes a deformation, a fringe pattern will result from the difference between the two holograms. This fringe pattern has the information needed to quantitatively evaluate the amount of the deformation. A compact system has been developed to be used for many applications, both mechanical and biological, where measurements need to be performed at 'hidden' surfaces or inside more or less closed objects. The quality of the result obtained by using mechanical objects is usually better, this can be easily explained by the fact that a biological surface is much more complex, in particular some parts reflect well the light and some other absorb it. A comparison of different endoscopes as object image carrier in pulsed digital holography is presented.
Accurate measurement of the refractive index of solids is of great importance in various field of industrial processes. We present and discuss a method based on digital holographic interferometry for measuring refractive indices of transparent materials. Two recorded digital holograms are added producing a digital double exposure hologram. The numerical reconstruction of the interferogram intensity enables to determine the index of refraction of the sample under investigation. Advantages and limitations of the prosed approach are discussed.
The replacement of the standard replication technique by gluing the pre-embossed foil grating for the use in moire interferometry is the concern of the article. The manufacturing of the pre-embossed foil diffraction grating is explained. Relations of the influence of the specimen surface profile to the zero interference field are derived. Some measurements of the profile of glued grating are shown including calculated extraneous interference field. Example of moire interferogram is shown using glued foil grating.
We have developed a prototype for in-line detection of surface defects in metallic wires, specially for scratches. A simple geometrical relationship between surface topography and conical reflection, permits to correlate the defects with intensity patterns in a simple way. The presented apparatus consists basically in a grating-divided laser beam incident on angular equidistant points. A CCD and an associated optics capture the information of the whole wire perimeter at once. Analytic rudiments are provided in agreement with the experimental results.
Within the recent years, 3D measurement of shape has gained importance due to increasing tolerance requirements in quality assurance. Optical methods are well suited for this task since they provide 3D-shape of surfaces with high density of information while working contactless and fast. In many cases, the surfaces of the inspected workpieces are reflecting and may not be modified for measurement purposes. If so, common methods such as fringe-projection-systems cannot be used since they require a non-reflecting surface. In this paper, an optical measurement technique is presented that unlike known optical methods allows the direct and unambiguous determination of 3D-coordinates of reflecting surfaces based on the observation of a separate reference grid structure and its image reflected by the inspected surface. For the calculation of 3D-coordinates from the recorded images of the reference grid, photogrammetric methods and phase shifting techniques are applied. Finally, application examples are given, depicting the potential of the presented measuring method.
Conventional laser interferometers offers a nanometer resolution but their result are ambiguous if distance variations of more than half a wavelength occur between two measured points. This a rather strong limitation for surface profilometry on surfaces with steps larger than this value. By using multiple wavelengths the accessible range of unambiguousness can be extended to half the result in synthetic wavelength. With three laser diodes emitting in the near IR synthetic wavelengths of approximately 15 micrometers and 290 micrometers could be achieved. This allows calculating the phase of the optical wavelength unequivocal within 145 micrometers . A nanometer resolution was reached with a phase interpolation of 1/100 of the optical wavelength. The laser beams are coupled into an interferometer through a single monomode fiber, and all interference signals are measured by one photo diode simultaneously. This leads to an easy alignment of the optical set-up and avoids the use of polarization optics and retardation plates. The injection currents of the laser diodes are modulated with different frequencies around 1MHz. Using lock-in amplifiers the three interference signals are separated electronically. The high modulation frequencies allow a fast measuring rate of up to 10 kHz. The sample surface as one mirror of the interferometer is scanned by moving the sample with mechanical translation stages in x- and y-direction. These mechanical stages exhibit unwanted vertical movement of up to 250 nm on a travel of several millimeter. By combining the mechanical stages with a piezo driven stage this vertical movement can be corrected resulting in a nanometer resolution in z-direction over a lateral range of several centimeters.
The basic methodologies used in animation are presented and their most significant problems connected with combining real and virtual worlds are recognized. The optical method of shape and movement determination is proposed for fast virtual object generation. A fringe pattern observed on the object surface is analyzed by spatial carrier phase shifting method. The analysis of fiducial points position during the measurements provides an information about object shifts within the measurement volume. Combined information about actual object shape and its displacement during the measurement enables to generate a virtual model of the object together with the description of its movement. The concept described above is experimentally tested and the exemplary result together with a short error analysis are presented. The further works to implement this technique are discussed.
The Research Center for Automatic control of Nancy has initiated a project to create on automatic 3D digitizing system. Our works try to solve the problem of automatic measurement of mechanical pieces. So we propose a method for automatic free form digitization based on a CAD model. This method uses visibility concept. This paper shows algorithms used to generate laser sensor trajectories. First, we explain our visibility calculation system and then, how we process the sensor path from the visibility calculation results. The visibility calculation algorithms uses a discretized model of the piece we want to scan and a discretized sphere to represent all the directions of visibility. The sensor path-processing algorithm uses the field of view width.
A new non-contact tool 3D geometry measurement system based on machine vision is described. In this system, analytical and optimization methods are used respectively to achieve system calibration, which can determine the rotation center of the drill. The data merging method is fully studied which can translate the scattered different groups of raw data in sensor coordinates into drill coordinates and get 3-D topography of the drill body. Corresponding data processing methods for drill geometry are also studied. Statistical methods are used to remove the outliers. Laplacian of Gaussian operator are used to detect the boundary on drill cross-section and drill tip profile. The arithmetic method for calculating the parameters is introduced. The initial measurement results are presented. The cross-section profile, drill tips geometry are shown. Pictures of drill wear on drill tip are given. Parameters extracted from the cross-section are listed. Compared with the measurement results using CMM, the difference between this drill geometry measurement system and CMM is, Radius of drill: 0.020mm, Helix angle: 1.310, Web thickness: 0.034mm.
In the paper the sequential steps of reverse engineering based on the data gathered by full-field optical system are discussed. The compete conversion process of a cloud of point coordinates to CAD/CAM is presented. The triangulation algorithm, which automatically creates the triangle mesh from the input cloud of points is described. Each block of this algorithm is explained din details with special attention paid to the parameters controlling the quality of the data conversion process. The adaptive process of reducing the number of the triangles on the base of second derivative of local curvature of objects' surface is explained. The error analysis is discussed at each step of the cloud data processing in dependency of the algorithm initial parameters.
A simple experimental test bench is described for the measurement of eccentricity of fiber optic connectors for telecommunications applications. The measuring principle is based on the detection of the track of the light beam exiting from the fiber optic tip by means of a 2D position sensing detector. Displacement due to fiber eccentricity, which is in the order of a few micrometers can be optically amplified by lenses in order to improve eccentricity measurement uncertainty. Optomechanical and electrical solutions which have been used in order to reduce the global measurement uncertainty and the effect of many interfering quantities has been deeply studied form both a theoretical and experimental point of view. Finally some operating consideration is also discussed.
In this paper, the author reports a photoelastic modulator based linear birefringence measurement system (BMS) using a near IR laser . This instrument determines both the magnitude and angle of a birefringent sample without rotating the sample. It records birefringence images by scanning a sample that is mounted on a computer-controlled X-Y translation stage. The accuracy, repeatability and other key performance test for the NIR-BMS are provided in this report. Selected samples, including silicon optical components, a silicon wafer and waveplates commonly used at visible wavelengths, are studied using the NIR-BMS.
It is shown that when a part of a wave-front bears a sharp change in its phase, the Fresnel diffraction becomes noticeable. To change the phase sharply, one can reflect the wave-front from a step or transmit it through a transparent medium having a sharp change in its thickness or refractive index. The visibility of the corresponding diffraction fringes depends on the amount of phase change and can be varied from zero to one. Since the phase change can be accomplished by various means, the effect renders to measure phase change, refractive index change, displacement, and so on. Here, the change of visibility is the measurement criterion, therefore the fluctuations of the source intensity do not affect the measurement precision. In this paper Fresnel diffraction from one dimensional step, circular step, and single strip are studied, and some of its applications are briefly discussed.
The objective of this work was to determine the feasibility and reliability of using the moire interference phenomenon as a means to detect human intrusion within a monitored zone. We applied moire interference principle for use in low-cost, safety-critical industrial monitoring applications. Moire interference is usually applied in the context of industrial applications for shape measurements. In this framework, we show how we can apply this concept to build a new safety product that detects human intrusion into dangerous areas on the factory floor. We demonstrate that a solution based on moire interference offers the potential for detection true 3D objects while preventing false alarms due to lighting variations or shadows and simplifying the image processing software. In addition, our prosed approach is advantageous in the product certification process because it is an active detection method.
The possibilities of numerical simulation of speckle phenomena and visualization of optical arrangements have been studied. Simulation results of objective and subjective speckle patterns in various optical setups are presented. Based on the speckle simulation program a virtual optical laboratory can be generated as an effective visualization tool.
An improved method for measurement of continuous displacement and deformations with digital speckle pattern interferometry is presented. The initial random phase of the speckle pattern is evaluated by having many phase-shifting steps before the deformation. By this way the accuracy of the initial phase estimation can be increased and the handling of the image noise is improved. This makes it also possible to use the phase stepped speckle patterns as references for comparison with the speckle patterns of the deformed object, thereby increasing the reliability and accuracy of the phase estimations of the deformed patterns. The technique can be used for measuring deformations such as transients and other dynamic events, heat expansion as well as other phenomena where it is difficult to accomplish phase shifting during deformation.
The Ritchey-Common test is a well-known method for large flat measurements. This paper describes a straightforward implementation ofthe formulas, to allow accurate surface height calculation using relatively few separate measurements. Both Ritchey-Common test and direct measurement results are presented. In comparison of the two methods, the RitcheyCommon test is in good agreement with the direct measurement.