Fringe-Contrast Interferometry

Deflectometry with structured light for high-precision measurement of optical specular surfaces has attracted considerable attention. However, conventional deflectometry schemes often encounter challenges when they deal with lens-like transparent objects due to parasitic reflections arising from the rear side of objects. To address this issue, we propose a novel stereo deflectometry method that replaces traditional phase-shift patterns with Fourier-based patterns inspired by single-pixel imaging. This method enables the acquisition of Fourier spectra of light response coefficients for each camera pixel. Specifically, through inverse discrete Fourier transform and the Fourier slice theorem, we obtain the light response coefficients of the light source to the camera pixel. Subsequently, a geometry optical analysis method is proposed to separate the light response coefficients components associated with the front surface given the lack of epipolar constraint between the camera and LCD screen. An iterative algorithm integrated with Particle swarm optimization is proposed to accelerate the reconstruction process and obtain the surface normal field of the upper surface of the lens-like transparent object. The wavefront reconstruction algorithm for gradient integration is used on the normal vector field to enables high-precision reconstruction of the object's surface shape. Experiments on actual optical components demonstrate that the proposed method can accurately reconstruct the front surface of transparent lens by eliminating the interference from parasitic reflection, and an error of less than 200 nm is obtained.

This paper considers the segmentation of range image measurements into surface patches which are either plane or curved and which are described formally by a function. After a formal description of the segmentation, we present and compare three methods suited for plane and curved patch segmentation and show the results of experiments conducted for testing their practical behaviour. The two first methods use the classical approach of region growing whereas the third method is based on a relaxation process. This original and last method exhibits simplicity and low computational complexity. Thanks to its parallel nature, it can be considered as a good candidate for range image segmentation in real-time applications.

Stereo DIC allows to measure with a good accuracy the displacement field and the shape changes of the surface of objects during deformation processes. Moreover, using multiple cameras makes possible to measure the deformation of the whole surface of the object in 360°. Such boundary information can be exploited to compute the internal displacement of a solid object using interpolation functions, especially if the deformation is large. This can be applied, for instance, to investigate the strain localization that occurs in metal specimen subjected to plastic deformation, and to study the necking initiation and the fracture development. In this paper, an interpolation method based on Bezier curves is developed and tested using simulated experiments on specimen with different shape. In particular, a flat and a round specimens are studied, looking at the deformation in the necking zone. The accuracy of the interpolation algorithm is evaluated at different level of deformation using both isotropic and anisotropic materials. Examples on real measurement are also discussed.

This paper examines the consequences of the inversion of refelcted waves from an object based on the Helmholtz wave equation model; the primary wave is a multi-frequency source and the object/transmitter/receiver geometry is fixed. We show that a parametric reconstruction in this inverse problem is not possible; this is true for both the Born and Rytov inversions. However, the available reflected data can be used to accurately reconstruct the surface (shape) of objects. Reconstruction results for penetrable and non-penetrable objects are presented.KeywordsAttenuation CoefficientSpatial DomainTest ObjectSource FunctionScattered WaveThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

BackgroundThe shading of an object provides an important cue for recognition, especially for determining its 3D shape. However, neuronal mechanisms that allow the recovery of 3D shape from shading are poorly understood. The aim of our study was to determine the neuronal basis of 3D shape from shading coding in area V4 of the awake macaque monkey.ResultsWe recorded the responses of V4 cells to stimuli presented parafoveally while the monkeys fixated a central spot. We used a set of stimuli made of 8 different 3D shapes illuminated from 4 directions (from above, the left, the right and below) and different 2D controls for each stimulus. The results show that V4 neurons present a broad selectivity to 3D shape and illumination direction, but without a preference for a unique illumination direction. However, 3D shape and illumination direction selectivities are correlated suggesting that V4 neurons can use the direction of illumination present in complex patterns of shading present on the surface of objects. In addition, a vast majority of V4 neurons (78%) have statistically different responses to the 3D and 2D versions of the stimuli, while responses to 3D are not systematically stronger than those to 2D controls. However, a hierarchical cluster analysis showed that the different classes of stimuli (3D, 2D controls) are clustered in the V4 cells response space suggesting a coding of 3D stimuli based on the population response. The different illumination directions also tend to be clustered in this space.ConclusionTogether, these results show that area V4 participates, at the population level, in the coding of complex shape from the shading patterns coming from the illumination of the surface of corrugated objects. Hence V4 provides important information for one of the steps of cortical processing of the 3D aspect of objects in natural light environment.

The simulation procedures based on the exact theoretical description of the interference-image formation in the real-time or double-exposure holographic interferometry are discussed. The theory takes into account not only the displacement and strain distributions on the object surface but also reconfiguration of measuring-system parameters between two recorded states of the whole set-up. These state changes influence the form of fringe pattern, with a possible carrier frequency of fringes, as well as the fringe-contrast distribution in theoretical description of the interference image. The program makes it possible to create interference images of the form like that obtained in a real measuring system. The algorithm calculating the displacement and strain fields for a theoretical model of the loaded object is the exchangeable part of the program. The program can be applied in designing real measuring systems, in verification of theoretical models of loaded objects, in analysis of computer-aided systems for quantitative interpretation of holographic interferograms, as well as in eduction in the field of photomechanics.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The precise matching of the optical system and the detector has attracted more and more attention to satisfy requirements of future infrared (IR) imaging systems, both for miniaturization and long-distance detection. Considering the trade-off between the spatial resolution and the sensitivity, this paper proposes a general optimization method, which can optimize parameters of the IR imaging system automatically by modifying λF/d. Firstly, on the basis of the trade-off between the spatial transfer characteristics and the spatial sampling characteristics, we establish a more accurate minimum resolvable temperature difference (MRTD) model, and analyze the coupling relationship between the optical system and the detector under the optimum MRTD. Secondly, we analyze the relationship between parameters of the IR imaging system and MRTD model, and propose a trade-off optimization approach of the IR imaging system. Finally, this paper verifies the rationality of the optimization method by optimizing CA-295 system. The trade-off optimization method proposed in this paper can not only satisfy the design requirement, but also reduce the size, weight, power consumption and cost (SWaP$) of IR imaging systems.

An analytical relationship between the contrast in polychromatic image speckle patterns and the positions of the diffuse object's surface normal, the illumination, and the axis of the imaging system has been obtained. When the axis coincides with the direction in which the mirror reflects the illumination about the normal, the contrast in the image speckle pattern is the greatest. Some qualitative experimental results are also given.

We describe a novel device that can be used as a 2.5D “scanner” for acquiring surface texture and shape. The device consists of a slab of clear elastomer covered with a reflective skin. When an object presses on the skin, the skin distorts to take on the shape of the object's surface. When viewed from behind (through the elastomer slab), the skin appears as a relief replica of the surface. A camera records an image of this relief, using illumination from red, green, and blue light sources at three different positions. A photometric stereo algorithm that is tailored to the device is then used to reconstruct the surface. There is no problem dealing with transparent or specular materials because the skin supplies its own BRDF. Complete information is recorded in a single frame; therefore we can record video of the changing deformation of the skin, and then generate an animation of the changing surface. Our sensor has no moving parts (other than the elastomer slab), uses inexpensive materials, and can be made into a portable device that can be used “in the field” to record surface shape and texture.

We present a miniaturized microscopic imaging system to achieve multi-contrast label-free imaging. In our imaging system, a highly integrated optical system using a miniaturization lens with fixed focal length replaces the complex optical path of traditional microscope systems, significantly reducing the size of the microscope to 14*16.5*20 cm³ . A programmable full-color light emitting diode (LED), which is controlled by independently designed operating software, is used to illuminate the sample in different illumination patterns. In addition, we developed a QT-based operating software to implement the synchronous control of the hardware system to achieve various label-free imaging approaches, including bright field, dark field, rainbow dark field, Rheinberg, differential phase contrast, and quantitative phase imaging. All the microscopic imaging approaches and system parameters can be controlled by the software without any hardware modification. In addition, some functions of the cell analysis are also added to our system, which can realize cell counter, 3D information measurement. We demonstrated our miniaturized system and its imaging results through experiments, which showed that our system is simple and fast to operate, providing more visual imaging results and high-resolution 3D information for samples. The experiment on living cell demonstrated that our miniaturized system can be built into an incubator for imaging and analysis of living cells in an environment suitable for cell growth.

The relevance and nature of a new technology for measurement of vibrational displacement of a material point through normal toward the object plane are stated in the article. This technology provides registration and processing of images of a round mark or a matrix of round marks, which are applied to the surface of a control object. A measuring signal here is the module of radius increment of the round mark image at vibrational blurring of this image. The method for calculation of the given error of measurements, as a function of a number of pixels of the round mark image, has been developed and proven in the present research. The results of pilot studies are given. Linearity of transformation of the measured size into a measuring signal has been proven. The conditions of a technical compromise between the field of view area of a recording device during distribution measurement of vibrational displacements along the surface of a control object, and the accuracy of this measurement are determined. The results are illustrated with numerical examples of calculations of the given error of measurements in the set field of view and the one at the given maximum set error of measurements.

An imaging model provides a mathematical description of correspondence between points in a scene and in an image. The dominant imaging model, perspective projection, has long been used to describe traditional cameras as well as the human eye. We propose an imaging model which is flexible enough to represent an arbitrary imaging system. For example using this model we can describe systems using fisheye lenses or compound insect eyes, which violate the assumptions of perspective projection. By relaxing the requirements of perspective projection, we give imaging system designers greater freedom to explore systems which meet other requirements such as compact size and wide field of view. We formulate our model by noting that all imaging systems perform a mapping from incoming scene rays to photosensitive elements on the image detector. This mapping can be conveniently described using a set of virtual sensing elements called raxels. Raxels include geometric, radiometric and optical properties. We present a novel ray based calibration method that uses structured light patterns to extract the raxel parameters of an arbitrary imaging system. Experimental results for perspective as well as non-perspective imaging systems are included.

Today, with quality becoming increasingly important, each product requires three-dimensional in-line quality control. On the other hand, the 3D reconstruction of transparent objects is a very difficult problem in computer vision due to transparency and specularity of the surface. This paper proposes a new method, called Scanning From Heating (SFH), to determine the surface shape of transparent objects using laser surface heating and thermal imaging. Furthermore, the application to transparent glass is discussed and results on different surface shapes are presented.

The measurement of transparent objects has always been regarded as one of the difficult problems in 3D reconstruction technology. For transparent objects conforming to the birefringence model, we propose a method for measuring the parameters of transparent objects based on multi-wavelength incident light. This paper analyzes the transparent objects where light enters the birefringence model. Because the transparent objects refract light of different wavelengths differently, when the lights with different wavelengths emitted from the same point and same direction passes through the transparent objects, the position and direction of light with different wavelength will come out differently. The position and direction of these outgoing rays are related to the refractive index and surface topography of the transparent objects. By obtaining the equations of these outgoing rays, we can derive the equations of the refractive index and surface topography of the transparent objects. In this paper, the incident light obtained by computer simulation is used as the initial data, and the simulated annealing algorithm is used to verify the model. The final results show that within the range of random error values given by the computer, the surface shape, curvature, thickness and refractive index of transparent objects with small error values can be obtained. At the same time, this paper introduces the Ceres Solver C++ nonlinear optimization library into the simulated annealing algorithm, which improves the speed and accuracy of the algorithm and greatly enhances the robustness of the algorithm.

Mask Projection Microstereolithography is capable for fabricating true three-dimensional microparts and hence, holds promise as a potential micro-fabrication process for micro-machine components. In this paper, the Mask Projection Micro-Stereolithography (MPµSLA) system developed at the Rapid Prototyping and Manufacturing Institute at Georgia Institute of Technology is presented. The dimensional accuracy of the system is improved by reducing its process planning errors. To this effect, the MPµSLA process is mathematically modeled. In this paper, the irradiance received by the resin surface is modeled as a function of the imaging system parameters and the pattern displayed on the dynamic mask. The resin used in the system is characterized to experimentally determine its working curve. This work enables us to compute the dimensions of a single layer cured using our system. The analytical model is validated by curing test layers on the system. The model computes layer dimensions within 5% error.