Surface Profile Of Object Research Articles

High-Accuracy Surface Profile Measurement Based on the Vortex Phase-Shifting Interferometry

According to the principle of phase-shifting interferometry and spiral phase characteristics of the vortex beam, this article proposes a method for detecting the surface profile of a transparent object, in which the +1 order vortex beam is generated by a spatial light modulator and is taken as the reference light. The influence of the nonlinear phase modulation characteristics of the spatial light modulator on the measurement precision is studied. The results show that nonlinear phase modulation has a great impact on the measurement. Then, the vortex lights with initial phases of 0, π/2, π, and 3π/2 are used to measure the H-type thin film sample based on the Twyman-Green interference system after correcting the nonlinear phase modulation characteristics. The experimental results show that the measurement error of the surface profile to an object with the theoretical value of 20 nm is 1.146 nm, and the feasibility of the optical vortex phase-shifting technique used to measure the surface profile of an object is verified.

High-Spatial-Resolution Multimodal Imaging by Tapping-Mode Scanning Probe Electrospray Ionization with Feedback Control.

Direct extraction and ionization techniques using minute amounts of solvent can be employed for the rapid analysis of chemical components in a sample without any sample preparation steps. This type of approach is important for mass spectrometry imaging of samples with multiple chemical components that have different spatial distributions (i.e., biological tissues). To improve the spatial resolution of such imaging, it is necessary to reduce the solvent volume for extraction and deliver it to the sample surface. This report describes a feedback control system applied to tapping-mode scanning probe electrospray ionization. By combining the measurement technique of capillary probe vibration with the dynamic distance control system between the probe and the sample, the vibration amplitude of the probe is maintained while the probe scans over uneven samples. This method allows simultaneous high-resolution imaging of molecular distribution, surface topography, and amplitude/phase changes in the probe vibration. Such multimodal imaging is demonstrated on rhodamine B thin films in microwells and on a mouse brain tissue section. This technique can generally be applied to examine the multidimensional molecular distribution and the surface profiles of various objects.

Camera-free three-dimensional dual photography.

We report camera-free three-dimensional (3D) dual photography. Inspired by the linkage between fringe projection profilometry (FPP) and dual photography, we propose to implement coordinate mapping to simultaneously sense the direct component of the light transport matrix and the surface profiles of 3D objects. By exploiting Helmholtz reciprocity, dual photography and scene relighting can thus be performed on 3D images. To verify the proposed imaging method, we have developed a single-pixel imaging system based on two digital micromirror devices (DMDs). Binary cyclic S-matrix patterns and binary sinusoidal fringe patterns are loaded on each DMD for scene encoding and virtual fringe projection, respectively. Using this system, we have demonstrated viewing and relighting 3D images at user-selectable perspectives. Our work extends the conceptual scope and the imaging capability of dual photography.

A comparison of n-ary simple code and n-ary gray code phase unwrapping in high-speed fringe projection profilometry

Binary defocusing technique has been widely used in fringe projection profilometry for high-speed measurement. This technique projects designed binary patterns with a defocused projector to generate fringes for wrapped phase calculation, and n-ary simple codes (nSC) or n-ary gray codes (nGC) for phase unwrapping. The unwrapped phase reflecting the object surface profile is calculated by a phase-shifting algorithm with nSC/nGC phase unwrapping. However, it remains unclear which phase unwrapping method, nSC or nGC, is more appropriate for high-accuracy and high-speed measurement. This paper comprehensively compares nSC and nGC with the consideration of various factors, including the strategies for error removal, the base n, the step heights and invalid regions on the measured objects, defocusing level and noise level of the system. From simulations and experiments, we conclude that, (i) in general, nSC with a proposed identification and binary classification (IBC) method has a similar result as nGC; (ii) when accuracy is critical, binary simple code (nSC with n = 2) and binary gray code (nGC with n = 2) methods are recommended; (iii) when speed is critical, quaternary simple code (nSC with n = 4) method with continuity/geometry constraints is recommended.

Preliminary Study on Development of 3D Free-Form Surface Reconstruction System Using a Webcam Imaging Technique

Reverse engineering faces challenges when it comes to data extraction using indirect contact method. There are 3D scanners capable of extracting data from distance more than a few meters away without contacting the objects but yet limited to same space range only. Although synchronizing the 3D scanners from another place can enable long distance data extraction, synchronizing devices over long distance can lead to other issues such as lagging. Moreover, the uses of 3D scanners usually involve quite long consumption time regardless of same space range or long distance range application. To overcome these issues, this study introduces a 3D surface reconstruction system by including a webcam imaging technique to produce a 3D point cloud, which is the same as that produced by a 3D scanner, and thus, eliminate the use of a 3D scanner. Through webcam imaging, image data can be transferred over the Internet and can be shared instantly even in different locations. Hence, webcam imaging is an alternative to using a 3D scanner. This system will continuously generate 3D free-form surfaces to resemble the surface profile of physical objects. Acquiring data until the surface fitting process will take less time compared with using a combination of 3D scanner and CAD software. In addition, results produced from the webcam imaging technique are compared with conventional results from a 3D scanner.

On machine test of Inverse Projected-Fringe system for measurement of form deviation and surface profile of axisymmetric objects

An in-situ Inverse Projected-Fringe system for on-machine inspection of axisymmetric parts is developed. Parts considered for this study are specular with RMS surface roughness >1 μm and have large slopes with radii of about 50 mm. The in-situ system is developed based on the previously reported Inverse Projected-Fringe technique in order to determine the on-machine measurement repeatability and reproducibility for industrial applications. In order to measure the whole part, a stereo arrangement and a new data fusion technique are used to increase the field of view of the system. A measurement trace with the highest Signal to Noise Ratio (SNR) is selected to achieve the best accuracy. The repeatability and reproducibility tests of the system are reported. It is shown the in-situ Inverse Projected-Fringe system is capable of measuring the form deviation from a master-part with repeatability and reproducibility of ±1 to ±7 μm, equivalent to a dynamic range of one part in 25,000 to one part in 3,600, respectively for the areas with higher SNR to lower SNR. Simultaneously, the RMS surface roughness has been measured with an accuracy of ±1.2 μm or better.

Inverse projected-fringe technique for measurement of dimensions and surface profile of axisymmetric objects

An inverse projected-fringe technique based on simulating the system of a projector-part-camera is proposed for an on-machine inspection of axisymmetric parts. The parts considered for this study are relatively large, have high specular surfaces and have sharp slopes. A computer simulation-based method and the CAD model of the part was used to produce the inverse fringes as well as to find an appropriate trace passing through the symmetrical axis of the part that offers the best signal to noise ratio (SNR). This technique measures the deviation from a master-part and provides an accuracy of better than 10 microns for the part with a radial dimension of about 10 cm. It is shown that the proposed technique improves the SNR and the repeatability of the system compared to the standard fringe projection technique, in particular for the areas with a steep slope.

Disparity Surface Reconstruction Based on a Stereo Light Microscope and Laser Fringes.

Microscopic vision systems based on a stereo light microscope (SLM) are used in microscopic measuring fields. Conventional measuring methods output the disparity surface based on stereo matching methods; however, these methods require that stereo images contain sufficient distinguishing features. Moreover, matching results typically contain many mismatched points. This paper presents a novel method for disparity surface reconstruction by combining an SLM and laser measuring techniques. The surfaces of small objects are scanned by a laser fringe, and a stereo image sequence containing laser stripes is obtained. The central contours of the laser stripes are extracted, and central contours are derived for alignment. A disparity coordinate system is then defined and used to analyze the relationship between the motion direction and reference plane. Next, the method of aligning disparity contours is proposed. The results show that our method can achieve a precision of ±0.5 pixels and that the real and measured shapes described by the disparity surface are consistent based on our method. Our method is confirmed to perform much better than the conventional block-matching method. The disparity surface output obtained by our method can be used to measure the surface profiles of microscopic objects accurately.

Dual-frequency fringe projection for 3D shape measurement based on correction of gamma nonlinearity

In the fringe projection three-dimensional (3D) shape measurement, the gamma nonlinearity of the digital projector leads to measurement errors, a novel method based on the dual-frequency fringe projection using correction of gamma nonlinearity is proposed. In the proposed method, the pre-coded gamma values of computer generated fringe patterns in digital projector can guarantee the sinusoidal waveforms of the captured fringe pattern, and alleviate the measurement errors caused by the gamma nonlinearity. After the correction of gamma nonlinearity, the shape measurement results by dual-frequency grating projection are analyzed in detail. By combining the advantages of both low frequency and high frequency gratings, the object surface profile is obtained accurately. The simulation and experiment results are presented to validate the proposed method.

Laser-induced ultrasonic imaging for measurements of solid surfaces in optically opaque liquids [Invited

The paper describes a novel laser ultrasonic profilometry method which uses pulsed laser radiation for imaging of the surface profile of solid objects in optically opaque liquids by scattering of ultrasonic waves. Algorithms for the construction of laser ultrasonic images and for profile segmentation are presented. An experimental setup for profile measurements is described. It allows reconstructing of laser ultrasonic images with a frame rate of 10 Hz and performing an automated 3D scanning of samples. The results of the experimental testing of laser ultrasonic profilometry on duralumin samples are presented. The approximation error of duralumin cylinder surface profile measurements in water is 15 μm. The results are compared to those obtained by x-ray tomography.

Quantitative phase imaging by optimized asymmetric illumination.

We have presented a simple approach for quantitative phase imaging by optimizing asymmetric illumination of a conventional microscope. With this illumination, the light intensity modulation accompanying refraction at the surface profile of phase objects occurs, and "phase-gradient information" can be derived by detecting it. Two images with phase-gradient information on different axes are converted into the two-dimensional phase distribution of the specimen by introducing the phase-gradient transfer function, which is the intensity change due to refraction by the phase-gradient of a specimen. We experimentally confirm accurate and repeatable performance of our method and demonstrate phase imaging of live cells.

Method for observing phase objects without halos or directional shadows.

A new microscopy method for observing phase objects without halos or directional shadows is proposed. The method is based on transformation of the surface profile of phase objects into a light intensity pattern. The key optical element is an annular aperture at the front focal plane of a condenser. The light flux passing through the annular aperture is changed by the specimen's surface profile and then passes through an objective and contributes to image formation. Theory and experimental results are presented. Phase images of induced pluripotent stem (iPS) cells with clear outlines were obtained with this method.

Phase Error Caused by Speed Mismatch Analysis in the Line-Scan Defect Detection by Using Fourier Transform Technique

The phase error caused by the speed mismatch issue is researched in the line-scan images capturing 3D profile measurement. The experimental system is constructed by a line-scan CCD camera, an object moving device, a digital fringe pattern projector, and a personal computer. In the experiment procedure, the detected object is moving relative to the image capturing system by using a motorized translation stage in a stable velocity. The digital fringe pattern is projected onto the detected object, and then the deformed patterns are captured and recorded in the computer. The object surface profile can be calculated by the Fourier transform profilometry. However, the moving speed mismatch error will still exist in most of the engineering application occasion even after an image system calibration. When the moving speed of the detected object is faster than the expected value, the captured image will be compressed in the moving direction of the detected object. In order to overcome this kind of measurement error, an image recovering algorithm is proposed to reconstruct the original compressed image. Thus, the phase values can be extracted much more accurately by the reconstructed images. And then, the phase error distribution caused by the speed mismatch is analyzed by the simulation and experimental methods.

Single-camera microscopic stereo digital image correlation using a diffraction grating

A simple, cost-effective but practical microscopic 3D-DIC method using a single camera and a transmission diffraction grating is proposed for surface profile and deformation measurement of small-scale objects. By illuminating a test sample with quasi-monochromatic source, the transmission diffraction grating placed in front of the camera can produce two laterally spaced first-order diffraction views of the sample surface into the two halves of the camera target. The single image comprising negative and positive first-order diffraction views can be used to reconstruct the profile of the test sample, while the two single images acquired before and after deformation can be employed to determine the 3D displacements and strains of the sample surface. The basic principles and implementation procedures of the proposed technique for microscopic 3D profile and deformation measurement are described in detail. The effectiveness and accuracy of the presented microscopic 3D-DIC method is verified by measuring the profile and 3D displacements of a regular cylinder surface.

RGB representation of two-dimensional multi-spectral acoustic data for object surface profile imaging

Conventionally, acoustic imaging has been performed using a single frequency or a limited number of frequencies. However, the rich information on surface profiles, structures hidden under surfaces and material properties of objects may exhibit frequency dependence. In this study, acoustic imaging on object surface was conducted over a wide frequency range with a fine frequency step, and a method for displaying the acquired multi-spectral acoustic data was proposed. A complicated rigid surface with different profiles was illuminated by sound waves sweeping over the frequency range from 1 to 20 kHz with a 30 Hz step. The reflected sound was two-dimensionally recorded using a scanning microphone, and processed using a holographic reconstruction method. The two-dimensional distributions of obtained sound pressure at each frequency were defined as ‘multi-spectral acoustic imaging data’. Next, the multi-spectral acoustic data were transformed into a single RGB-based picture for easy understanding of the surface characteristics. The acoustic frequencies were allocated to red, green and blue using the RGB filter technique. The depths of the grooves were identified by their colours in the RGB image.

Optical frequency comb interference profilometry using compressive sensing

We describe a new optical system using an ultra-stable mode-locked frequency comb femtosecond laser and compressive sensing to measure an object's surface profile. The ultra-stable frequency comb laser was used to precisely measure an object with a large depth, over a wide dynamic range. The compressive sensing technique was able to obtain the spatial information of the object with two single-pixel fast photo-receivers, with no mechanical scanning and fewer measurements than the number of sampling points. An optical experiment was performed to verify the advantages of the proposed method.

A nanometre-scale resolution interference-based probe of interfacial phenomena between microscopic objects and surfaces

Interferometric techniques have proven useful to infer proximity and local surface profiles of microscopic objects near surfaces. But a critical trade-off emerges between accuracy and mathematical complexity when these methods are applied outside the vicinity of closest approach. Here we introduce a significant advancement that enables reflection interference contrast microscopy to provide nearly instantaneous reconstruction of an arbitrary convex object’s contour next to a bounding surface with nanometre resolution, making it possible to interrogate microparticle/surface interaction phenomena at radii of curvature 1,000 times smaller than those accessible by the conventional surface force apparatus. The unique view-from-below perspective of reflection interference contrast microscopy also reveals previously unseen deformations and allows the first direct observation of femtolitre-scale capillary condensation dynamics underneath micron-sized particles. Our implementation of reflection interference contrast microscopy provides a generally applicable nanometre-scale resolution tool that can be potentially exploited to dynamically probe ensembles of objects near surfaces so that statistical/probabilistic behaviour can be realistically captured.

Restoration of completely lost surface profile of stamped metal by the method of electrodeposition

The experimental results on the restoration of surface profile of stamped metal object, which was lost in the course of abrading and polishing, by the metal electrodeposition are presented.

Shape and deformation measurement system by combining fringe projection and digital image correlation

An optical measurement system, which is capable of simultaneously measuring surface profile and three-dimensional (3-D) deformation of small objects, has been developed by the combination of fringe projection and a two-dimensional digital image correlation (DIC) technique. In the proposed approach, it requires only one camera and one projector. The surface profile and out-of-plane deformation are determined by fringe projection with the phase-shifting method. The in-plane deformation is determined by comparing the surface texture of specimen before and after deformation with the DIC technique. A simple procedure is described which enables determination of texture image from phase-shifting images. As an application, the developed system was used to investigate the mechanical behavior of molded pulp material in micro-zone during tension testing.

Optical evaluation of the buckling behavior of stiffened composite laminates

Digital fringe projection profilometry (FPP) is suitable to inspect the surface profiles of objects with some apparent advantages such as being non-contact, making full-field measurement, using simple apparatus, being of high precision and enabling real-time profile automation. With our technique, four phase-shifted sinusoidal fringe patterns were circularly projected onto the surface of a composite laminate panel reinforced with crossed ribs. A phase shifting method was applied to inspect the surface profile variations during compression. The profile results revealed that different deflection directions of local buckling appeared at different crossed ribs, with the wave valleys located at the middle and the wave peaks occurring at the composite panel ends. From the details of the deflection analysis, it has been seen that the local buckling couple with the whole bending of the composite panel. The local buckling deflections agree well with finite element method (FEM) simulations.