This PDF file contains the front matter associated with SPIE Proceedings Volume 10819 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Phase unwrapping is an important step for the phase shifting profilometry. The dual-frequency phase unwrapping method can unwrap the object with discontinues when the object is static by employing more fringe patterns. However, errors will occur when moving object is reconstructed. In this paper, a new phase unwrapping method with dual-frequency phase unwrapping method for the moving object measurement is proposed. The fringe pattern with low fringe pattern and high frequency are projected onto the moving object surface. Then, the phase values are retrieved for the two frequencies respectively. The relationship between the movement and phase value is analyzed and the phase variations caused by the movement is compensated. At last, the phase value is unwrapped by the traditional dual-frequency phase unwrapping method. The effectiveness of the proposed method is verified by simulations.

It is believable that a diode pumped alkali laser (DPAL) will generate a continue-wave (CW) high-powered output in the near future. In this paper, we report the first experimental demonstration of modulating a laser-pumped rubidium-cesium vapor laser system with two wavelengths. Being different from the conventional dual-wavelength solid-state lasers in which stimulated emissions with two wavelengths often interfere with each other, the rubidium-cesium vapor laser with two wavelengths, i.e. 794.736 nm for rubidium and 894.335 nm for cesium, has a prominent advantage of employing different alkali metal vapors as laser media in the same oscillator without any disturbance in the lasing processes for dual wavelengths. In the study, we also modulated one pump source and kept the other pump source unchanged in time domain. The experimental results reveal that such a rubidium-cesium vapor laser may provide a new light source for the applications in the fields of laser ranging, laser radar, and laser surface depiction.

Piezoelectric transducer (PZT) is often used in the field of precision measurement to realize the micro-positioning function, such as Fizeau interferometer. But due to its inherent severe hysteresis and nonlinear characteristics, the use of piezoelectric transducer is affected. A piezoelectric transducer control instrument was designed to measure the displacement characteristic curve of piezoelectric transducer. The instrument included the control system and data received system. The control system produces a triangular wave signal with adjustable amplitude and frequency, then calculated from received signal to drive the piezoelectric transducer. The output displacement information of piezoelectric transducer is collected by the signal acquisition circuit composed of displacement sensor and ADC analog-digital converter. The purpose of the single chip is collecting and processing displacement data. According to the experimental results, we can get the characteristics curve of the piezoelectric transducer, which support the basis analysis and available for the future experiment.

In this paper, direct phase determination method which is based on the phase-locked loop technique and the null method is proposed for heterodyne-displacement-measuring interferometers. The requirement of the proposed method is to detect high-speed phase changes in the phase shift between two sinusoidal input signals, leading to measuring high-speed displacements in industrial applications. The method is estimated for two situations that include the use of (a) two pure sinusoidal signals from a function generator and (b) two interferometric signals from a heterodyne interferometer, which is integrated to an electro-optic modulator for phase modulation. The experimental results of the method provide the phase-variation-measuring speed with a modulation frequency of 20kHz in the 2.7MHz-input sinusoid. The principle, signal-processing program, experiments and results of the method are presented in the paper.

In recent years, high-speed inspection using non-contact measurement has been increasingly desired in the field of manufacturing. Inner diameter measurement of a cylindrical part is also the same. One of the promising methods for the non-contact inner diameter measurement is the light sectioning method using a ring beam device which consists of a conical mirror and a laser diode. However, further investigation is needed when it comes to practical use at the manufacturing site, especially for the requirement of measuring accuracy of micron order. We have been developing an inner diameter measuring device using the ring beam sectioning method with accuracy on the order of micrometers, aiming at the practical use at the production site. One of the features of our system is the robustness against the relative positional deviation between the workpiece and the sensor probe. Robustness is very important for the use at the production site. In this paper, we report on its evaluation results. We designed and manufactured the ring beam device so that the ring beam is uniform and exits horizontally from the conical mirror for the sake of achieving high accuracy and robustness. We created a simulation model for robustness evaluation by using the measured characteristics of the ring beam. Against the misalignment of central axis between the workpiece and the sensor probe, measurement dependency of less than 0.2 um/mm was obtained. And less than 0.1 um/min was obtained against the angular misalignment. We are also conducting experimental verification, and discuss the difference between the simulation and the experiment.

The NA of immersion lithography has reached 1.35, in which case polarization effect must be taken into account. The performance of the projection lens should be characterized by polarization aberration. We proposed a polarization aberration measurement theory and method, using the binary grating structure as the mask pattern, with intensity distribution signal as the measuring signal. Pauli Zernike polynomials are adopted to characterizing the polarization aberration, and a linear relationship between intensity signal and Pauli Zernike coefficients was derived. Simulation results show that using the proposed method, the polarization aberration can be reconstructed with relative error of refactoring to 10^-2.

As nanotechnology have been developed rapidly, it is strongly needed to measure a displacement accurately and precisely. A sinusoidal frequency modulation (SFM) and a sinusoidal phase modulation (SPM) applied interferometers could be good candidates. In the SFM, a frequency of a light source is modulated sinusoidal way. In the SPM, a phase is modulated. Characteristics of these interferometers are defined by a modulation index. The modulation index is a function of both a frequency excursion and an initial optical path difference in the SFM, and is an amplitude of phase in the SPM. For displacement measurement, the displacement is obtained by a phase of an interference signal. A phase determination method using a phase-locked loop (PLL) is reported and a few pico-meters resolution is achieved for displacement measurement with a restriction of a specific modulation index. For precise and high resolution interferometer, a stabilizing the frequency of the light source is necessary. Because the modulation index is an essential parameter in the frequency stabilization, performing phase determination with the PLL in the arbitral modulation index must be developed. The nonlinear relationship between the phase and the displacement is a problem in arbitral modulation index. We have proposed a PLL for arbitral modulation index which successively changes the relationship from the nonlinear to the linear. PC-based calculations with a theoretical and an actual interference signals have been conducted to check the feasibility of proposed PLL. In this report, displacement measurements using PC-based calculation with actual interference signals are described.

Two kinds of 3D label free Bio-Transfer-Standards (BTS) have been further developed at the University of Helsinki (UH). The first one, NanoRuler, is a staircase BTS featuring eight fatty acid bilayers which allows vertical calibration in the range of 5 to 40 nm. The second one, NanoStar, is a V-shaped BTS featuring two 5 nm tall bilayers that overlap at 10° angle. This standard enables the determination of the Instrument Transfer Function (ITF). A stability test was conducted on the BTSs, during which the standards were stored in laboratory conditions, and were profiled each week. Profiling was done using a custom-built Scanning White Light Interferometer (SWLI). The stability of NanoStar was ± 0.3 nm, and of NanoRuler ± 0.5 nm to ± 2.5 nm. The BTSs maintained their specified properties for at least six months and therefore allow vertical calibration and ITF determination. In addition, changes in surface morphology of one NanoRuler subjected to water immersion are presented. This paper reports intermediate findings during an ongoing stability test that will run for 24 months.

The optical geometry characterization of wrought hot components can help to quantify material distortion effects during air-cooling. The component's shrinkage behavior is affected by inhomogeneous heat dissipation due to the object's complex geometry and - in case of hybrid materials - differing thermal expansion coefficients. As optical triangulation techniques rely on the rectilinear expansion of light, the hot component's heat input into the surrounding medium air influences the reachable accuracy of optical geometry measurements due to an inhomogeneous refractive index field around the hot component. In previous work, the authors identified low pressure measurements in air as a possible approach to reduce the magnitude and expansion of the inhomogeneous refractive index field above cylindrical high-temperature objects and thereby allow precise geometry acquisition. We now present experimental data of the 2D refractive index field above a hot cylinder in different pressure scenarios using the well-known background oriented schlieren (BOS) method in order to illustrate the decrease in refractive index variations dependent on the pressure state. For this purpose, a ceramic rod is placed in a vacuum chamber and heated up to temperatures of about 1000°C. Using a monochromatic camera, a wavelet background and an optical ow algorithm, the developing 2D refractive index field for a low pressure scenario is compared to ambient pressure conditions. The experimental data illustrates a reduction in the convective heat flow above the hot heating rod at lower pressure values and therefore a homogenization of the density-coupled refractive index in air, validating former simulation results.

In this paper, we propose a high-speed 3-D shape measurement technique based on composite structured-light patterns and a multi-view system. Benefiting from the multi-view system, stereo phase unwrapping, as a novel method for the phase unwrapping algorithm, can eliminate the phase ambiguities and obtain absolute phase map without projecting any additional patterns. However, in order to ensure the stability of phase unwrapping, the period of fringe is generally around 20, which leads to the limited accuracy of 3D measurement. To solve the precision-limited problem reasonably, we mathematically developed an optimized design method of the composite pattern. By skillfully embedding speckles without compromising the phase measurement accuracy, we can realize phase unwrapping with high-frequency fringes. In addition, a computational framework will be provided to further achieve the robust and high-performance 3D acquisition. It is demonstrated by several experiments that our method can achieve a high-speed, dense, and accurate 3-D shape measurement with 64-period fringe patterns at 5000 frames per second.

3D foot digital models have great potential for application in ergonomics design and online virtual shoes try-on. Traditional techniques usually take 10 seconds to several minutes to acquire dense data, which is a critical limitation to large scale data collection. To enhance data collection efficiency, a novel approach which combines simulated laser speckle projection stereo with 3D silhouette is proposed. Laser speckle has more statistical advantages so that it can deal with lacking of texture of human skin and make stereo matching easier. With dark field lighting that strengthen foot contour, 3D silhouette can remove border noise caused by our active stereo reconstruction. Besides, all light sources in our work are infrared to avoid ambient light inference. In our design, five active stereo rigs are installed around 4π solid angle centered at the foot position to capture whole foot’s surface mesh data. Composed of a pair of stereo IP-cameras with visible cut filters, an infrared simulated laser speckle mini film projector and a cluster of infrared LEDs surrounding camera lens, each active stereo rig takes charge of obtaining 3D information of corresponding foot part. The five rigs are controlled by an MCU controller to successively capture one pair of speckle pattern images for stereo reconstruction and one pair of edge enhanced dark field lighting images for silhouette. The system design resolution is less than 0.3 mm per pixel. Data capture could be performed in less than 1 second for each foot and more than 500 thousand valid points are acquired as dense point cloud model. Finally, foot mesh model is generated using Poisson reconstruction algorithm.

Scanning electron microscopes (SEM) allow a detailed surface analysis of a wide variety of specimen. However, SEM image data does not provide depth information about a captured scene. This limitation can be overcome by recovering the hidden third dimension of the acquired SEM micrographs, for instance to fully characterize a particle agglomerate’s morphology. In this paper, we present a method that allows the three-dimensional (3D) reconstruction of investigated particle agglomerates using an uncalibrated stereo vision approach that is applied to multiple stereo-pair images. The reconstruction scheme starts with a feature detection and subsequent matching in each pair of stereo images. Based on these correspondences, a robust estimate of the epipolar geometry is determined. A following rectification allows a reduction of the dense correspondence problem to a one-dimensional search along conjugate epipolar lines. So the disparity maps can be obtained using a dense stereo matching algorithm. To remove outliers while preserving edges and individual structures, a disparity refinement is executed using suitable image filtering techniques. The investigated specimen’s qualitative depth’s information can be directly calculated from the determined disparity maps. In a final step the resulting point clouds are registered. State-of-the-art algorithms for 3D reconstruction of SEM micrographs mainly focus on structures whose image pairs contain hardly or even none-occluded areas. The acquisition of multiple stereo-pair images from different perspectives makes it possible to combine the obtained point clouds in order to overcome occurring occlusions. The presented approach thereby enables the 3D illustration of the investigated particle agglomerates.

Semiconductor wafer is elementary unit in semiconductor industry. In the fabrication of semiconductor wafer, surface defects such as dirties, scratches, burrs, chippings and holes may be generated which severely affect the quality of downstream production. Typical inspection of these defects mainly depends on human experts inspecting system which is time-consuming and low efficiency. With the fast development of digital imaging and processing technique, Computer vision automatic inspection method has shown vast potential for product quality test. Due to low contrast and weak context characteristics of wafer surface defects, the existing methods have difficulty to extract whole defect patterns. A novel algorithm for defect contour extraction is proposed based on multi-frame differential image summation. For each side of semiconductor wafer surfaces, multiple images are captured by high resolution digital camera. For each image the gradient is calculated using common used differential mask, and then the gradient is thinned based on edge extraction using Canny operator and smoothed using Gaussian smooth filter. All refined gradient images are added up to enhance defect features and smooth defect-free regions furtherly. Finally, the Canny operator is applied again to extract whole defect’s contour from gradient summation image. Experiments using real semiconductor wafers illustrate that the proposed algorithm can detect most of defects correctly and effectively.

Dimensional measurement of internal profile is conventionally performed mechanically with instruments like vernier calipers. Here, we cope with the measurement by using an optical caliper. The caliper consists of a semiconductor laser, a right conical mirror, a lens system, and an image sensor. The concept of the measurement approach is intersecting the object with a disk light sheet, collecting the cross-sectional profile image with an image sensor, and extracting dimensions of the profile. Example of practical measurement is presented.

Because of their high efficiency and power weight ratio, triple-junction GaInP/InGaAs/Ge solar cells have become the main energy source for space on-orbit applications. Calibration of space solar cells under AM0 conditions is extremely important for satellite power system design, and accurate prediction of them is critical to solar array sizing. However, it’s not easy to conduct accurate measurement for multi-junction solar cells, especially in laboratory on earth. In this paper, by employing a highly AM0 spectrum-matched light source, which combined a Xenon lamp and a Halogen lamp with special filters, a method to measure the AM0 performance for triple-junction GaInP/InGaAs/Ge solar cells will be presented. The calibrated values of reference solar cells, including two component solar cells, were come from a national standard facility based on DSR (Differential Spectral Responsivity) method. After calibrating the compound light source by using the reference solar cells, key parameters of AM0 performance of triple-junction GaInP/InGaAs/Ge solar cells would be measured out, such as short-circuit current, open-circuit voltage and maximum power, etc. Spectral mismatch and other main influencing factors are also considered. It will provide a reliable route for multi-junction space solar cells’ photoelectric property measurement in laboratories on earth.

A compact lateral shearing interferometer (LSI) based on circular Modified Hartmann Mask (cMHM) is proposed for the measurement of wavefront aberrations. A cMHM grating consists of a circular apertures amplitude grating and a phase chessboard grating. By choosing the radius of the circular aperture of the amplitude grating to be the first positive root of Bessel function, residual diffraction orders are suppressed. As a result, the diffraction field of cMHM is close to that of the ideal quadriwave lateral shearing interference which only contains ±1 orders in two orthogonal directions. An interferometer adopting cMHM as the diffraction element exhibits a diminished Talbot effect on the detection plane as those adopting the conventional Modified Hartmann Mask (MHM) grating or the improved Randomly Encoded Hybrid Grating (REHG). Compared with the REHG, the cMHM requires non-strict manufacturing process. Numerical simulations shows a better diffraction efficiency compared with that using the conventional MHM grating. In the experiments, the interferograms captured by the cMHM-LSI exhibit the same level of contrast as those by MHM-LSI.

Advanced signal processing is required to make exact measurements with nanometer order accuracy. A complex-valued interference signal of a white-light scanning interferometer (WLSI) obtained from the detected real-valued interference signal through Fourier transform provides an accurate position of an object surface with an error less than 4 nm. Moreover, the sampling points of the interference signal of the WLSI detected with a camera are corrected with the measured scanning positions which are obtained from an interference signal detected by using an optical band-pass filter. This correction method provides more accurate surface profiles with an error less than 2 nm. In experiments a surface profile with a step shape of 3 μm-width is measured accurately.

Scanning electron microscope (SEM) and its variations, such as critical dimension SEM (CD-SEM) and electron beam inspection (EBI-SEM), are getting increasingly critical in process control in very large-scale integration circuits (VLSI) manufacturing. For rapid capturing of image, the patterned wafer surface needs to be maintained in a range smaller than the depth of focus (DoF) of the electron beam. A triangulation optical system is used to sense the wafer surface for the rapid leveling of the wafer height in real time. An optical grating illuminated by a LED light source is projected onto the wafer and further imaged to a camera. The wafer height is calculated from the displacement of the optical grating image. However, pattern variation under the measurement area of the wafer may affect the transmission ratio of the light to the sensor and further the measurement accuracy. To achieve a high precision measurement result, an automatic light intensity adjustment system is developed. A computer records the optical grating image and the image intensity is analyzed. If the grating image intensity in the bright area is not in the desired range, a control software will convert the delta into the LED driving current increment or decrement, and the LED light output will be adjusted accordingly so that the image intensity is brought back to the expected range and keep there for the entire process. The LED feedback control experiment shows that the system remains at the target grey level without noticeable variation when the system transmission ratio varies by a factor of 7.5 times.

In order to meet the experimental requirement of in-situ measurement for spectral reflectance of advanced thermal control coatings, a high accuracy in-situ measurement system for spectral reflectance of thermal control coatings of spacecraft is developed based on dual-beam spectrophotometry. The measurement wavelength range is 200 to 2500 nm, and the measurement accuracy is better than 0.5%. In the space ultraviolet radiation environmental effect test, it can realize the integrated test process of sample delivery, sampling, separation, in-situ measurement of spectral reflectance in vacuum.

To solve problems including the surface distortion and profile error cannot be considered in robotic off-line programming, and the object parts are often texture-less or even without contour, we propose a new visual servo control method directly using the phase map as the visual features. The high accuracy of phase map ensures the high accuracy of positioning or target tracking tasks. Every entry of the phase map is used as visual features so no complex image processing and feature extracting and matching are needed. The corresponding interaction matrix is deduced and control law is constructed. The validity of the proposed method is proved by simulation.

A method for extracting properties of individual components of a retader-linear diattenuator- retarder system is proposed. Since the evaluation performs in noncontact and nondestructive way, this method is applicable to inspect as-produced devices and optics. Example for its application to inspection of the polarizing beamsplitter is presented.

A key problem when using the derotator is to ensure the coaxial alignment between the optical axis of the derotator and the rotation axis of the measured object. Moreover, accurate measurement of the rotation axis is the prerequisite of achieving the coaxial alignment. In this paper, we propose a strategy for estimation of the rotation axis with the assistant of an auxiliary laser and a binocular stereo vision sensor. When a laser source is temporarily attached to the measured object or the rotor shaft, the rotation of the laser will generate a ruled surface. The binocular sensor can accurately measure the coordinates of laser dots in three-dimensional (3D) space. The pose vector of the laser in each certain rotation angle can be estimated with measured 3D coordinates of the laser dots. Finally, the rotation axis of the object can be estimated with multiple pose vectors considering the constraint of the fixed axis rotation. The validity and precision of this proposed strategy are demonstrated by means of experiments.

This paper presents a novel visual tracking approach that combines the NMI metric and the traditional SSD metric within a gradient-based optimization frame, which can be used for direct visual odometry and SLAM. We firstly derivate the closed form expression for first- and second-order analytical NMI derivatives under the assumption of rigid-body transformations, which then can be used by subsequent Newton-like optimization methods. Then we develop a robust tracking scheme that utilizes the robustness of NMI metric while keeping the optimization characteristics of SSD-based Lucas-Kanade (LK) tracking methods. To validate the robustness and accuracy of the proposed approach, several experiments are performed on synthetic datasets as well as real image datasets. The experimental results demonstrate that our approach can provide fast, accurate pose estimation and obtain better tracking performance over standard SSD-based methods in most cases.

Defocusing binary patterns to generate digital sinusoidal fringe patterns with a digital-light-processing projector has been pivotal in phase measurement profilometry with fringe projection techniques. However, despite all its merits, squared binary defocusing (SBD) has been borne with limitations: (1) limited defocusing range; (2) difficulty in quantifying the amount of defocus together with non-negligible residual high-frequency harmonic phase errors. Recently, three-dimensional profilometry with exponential fringe projection has been successfully demonstrated to be robust to high-order harmonics and related phase errors. In this paper, we compare the potential errors for digital sinusoidal fringe generation with both binary-exponential defocusing (BED) and squared binary defocusing (SBD) induced by varying the degree of defocusing specifically at low levels of defocus or extended defocusing range. Results show that in most scenarios, the error for the BED method is smaller than that of the SBD method especially at extended defocusing range. Therefore, generating a sinusoidal fringe image using a BED method seems to be appealing and promising for three-dimensional profilometry with projector defocusing at an extended defocusing or projection range.

Due to recent developments in sheet-bulk metal forming processes, holistic measuring systems are required which provide reliable results for the measurement of all relevant formed part features. The presented multi scale multisensor approach, which combines several fringe projection sensors of different measuring resolutions and ranges, accomplishes the requirements of the new production technology. Calibration of the single measurement systems to a global coordinate system is realized with a specially developed calibration object. A high-precision hexapod positions the calibration object and the workpiece within the measuring range. The result is a holistic dimensional measurement for characteristics whose size exceeds the measuring range of a single sensor. It is also shown that the distance of the measurement sensor to the suface of the measurement object is relevant for the uncertainty of the obtained point cloud. By positioning the workpiece in the focus of camera of the fringe projection system, the standard deviation of the measured point cloud can be significantly reduced.

Aircraft maintenance, repair and overhaul (MRO) faces many challenges, for example the line maintenance performed between flights is always time-critical. Furthermore, due to the size and shape of the aircraft, some areas are hard to reach and to be inspected, such as the crown or the vertical stabilizer. To improve the inspection process, a camera can be used to capture the aircraft surface and identify the damage. However, an RGB camera can only help to pick up the damage based on the colour difference and the results might be affected by the aircraft livery. Hyperspectral imaging (HSI) looks at reflectance to distinguish between different materials or chemicals despite being apparently the same colour. It has been widely applied in industries such as food safety, agriculture, pharmaceutics, etc. However, it has rarely been considered in the aerospace industry. This paper introduces a case study of inspecting damage on aircraft using an HSI camera. An HSI camera covering the range 400 - 1000 nm is used for this case study on metal and composite parts of aircraft. The aircraft parts, which came from decommissioned aircraft, are hit by simulated lightning strikes to recreate damage on aircraft. The damage can be identified by using HSI camera with range of 400 - 1000 nm. In the future, other ranges of wavelength may be used to study the damage since there might be more significant features of the reflectance spectrum to be used as markers. Thus, the damage ought to be easier identified since there will be more spectral information provided.

Airplanes are regularly inspected for any external damage between flights and during maintenance, especially when aircrew report possible lightning strike. Even today, the inspection is mainly done visually by authorized ground staff to look for evidence of possible damage, such as cracks and burn marks, etc. The process is not only inefficient and with poor traceability, but also troublesome when there is a need to inspect upper parts of the airframe. Approaches are available to automate the image acquisition, such as mounting a camera onto a moving robot and take multiple shots to cover the whole airplane. However, the acquired images still need to be screened thoroughly by technicians, which becomes an obstacle to automate the visual inspection process. The main reason for needing human intervention is the large number of distractions in the form of other features on the aircraft, not to mention the clutter produced by aircraft company livery. In this paper, novel methods to analyze the two-dimensional (2D) images and identify evidence of possible damage are presented. The methods are based on autocorrelation function (ACF) which is mostly used for fabric analysis. A pre-processing is firstly applied on the airframe image to remove background and enhance its quality. ACF is then implemented to look for abrupt changes which might be indications of damages. Lastly, a post-processing step is taken to filter out possible distractions. The proposed methods can work efficiently in various scenarios, which enables the possibility of automating the aircraft visual inspection process.

Aiming at achieving quantitative detection of internal defect in materials, this work employs phase-shifting dualobservation digital shearography for determining embedding depth of internal defects in a nondestructive way. Firstly, a mechanical model of internal defect is established based on the principles of elastic mechanics. An analytical solution about embedding depth is hereby obtained. Then, dual-observation digital shearography is used for measuring the firstorder derivative of out-of-plane displacement at the region with defect. Consequently, the embedding depth of internal defect is determined by combining theoretical formula and experimental results obtained from shearogram. Finally, the measurement accuracy of embedding depth using proposed method is validated by comparison with another two commonly used methods. The measurement error of embedding depth is less than 5%, which completely confirms higher accuracy of the proposed method.

A method for measuring the damping coefficient of vibrating target based on Fourier transform of self-mixing interference signal is presented. Theoretical researches indicate, the dominant harmonic order of the laser self-mixing signal is proportional to the amplitude of vibration. Based on this theory, damping efficient can be extracted from the ratio between the amplitudes of the vibration target at different moments. The error of the damping coefficient is about 4%.This method characterized by simple structure, non-contact measurement and high accuracy, can be extensively applied in measurement of industrial harmonic damping vibration.

Cylindrical surface is widely used for modern industry. However, cylindrical surface measurement has become a quite difficult problem. In this paper, we will present coherent technique to measure cylindrical surface, including cylindrical surface and cylindrical ring. Inner surfaces measurement of cylindrical ring can be achieved without map stitching, by a Fizeau interferometer with a 90°conical mirror. The alignment of this arrangement, however, is very crucial to the accomplishment. Any small misplacement of 90° cone or hollow cylinder from their ideal settings may result in large measurement errors. These errors are not intuitive and hard to be removed if their origins are not well understood. In other words, it is very important to know how these measurement errors are generated from the optical misalignment in order to eliminate them. Finally, we have aligned our experimental setup and gotten some results which were so closed with our theoretical analysis.

In recent years, the measurement of specular aspheric surface has attracted intensive attention in precision engineering. Phase measuring deflectometry is a powerful measuring technique, which could accurately measure specular surfaces. The software configurable optical test system and a four step phase shifting approach are applied to obtain the normal vectors of the measured surface. The geometric parameters are recalculated by optimization to improve the calibration accuracy. Then the surface is reconstructed using a optimization algorithm. The configuration parameters should be set according to specific surface shapes and measuring conditions. Numerical experiments demonstrate that good performance can be achieved using this method.

Subwavelength grating polarizers were designed to generate particular cylindrical vector beams. Based on the rigorous vector diffraction theory, the mechanism of the subwavelength gratings converting circularly polarized beam into cylindrical vector beams was analyzed. To obtain the particular type of cylindrical vector beam, a method of design of subwavelength grating polarizers was proposed and the designing procedures has been discussed. The basic function of the polarizer designed in this work is to convert the circularly polarized beam into radially/azimuthally polarized beam. By further improvement, the polarizer could generate complex cylindrical vector beams. Using the Richards-Wolf vectorial diffraction integral, the intensity distributions of the output focused beams from the polarizer were calculated. A phenomenon of divided focal spot was found in the vicinity of the geometrical focus point.

In this study, we propose and experimentally demonstrate a picosecond pulse laser at 850 nm. To generate picosecond laser pulse, we operate a vertical cavity surface emitting laser under a gain-switched pulsed mode, which is realized by driving it with our home-made drive circuit based on field programmable gate array and radio frequency devices. The obtained laser pulses are with the pulse width of less than 675 ps, and with repetition rate from single shot to megahertz. On the other hand, based on our gain-switched pulsed laser, we design and realize a cost-effective optical time domain reflectometry prototype equipment with photon counting technology for monitoring the healthy condition of aeronautical fiber. Our prototype equipment achieves a spatial resolution of less than 9 cm, and a dynamic range of around 18 dB above the noise floor. Such prototype equipment has already been employed to monitor an optical cable with 32 fiber channels on plane.

By introducing polarized light detection and polarization indices analyzing, our research has indicated that some certain polarization indices can identify the morphology and absorption of suspended particulates specifically. In this article, we will present the experimental results of polarization indices from three kinds of pollutants discharged from three different sources with respective shape and absorption. A series of simulation results on different shape and absorption will also be given to show the correlation of polarization indices and particulates’ parameters. At last, the particulates from a certain source will be modeled, and the agreement between the preliminary simulations of pollutant sources with the experiment results in a field test will be shown.

A CMM with multiple probing systems have the power to deliver tremendous benefits to most notably manufacturing, and have the advantage of high automation, high integration and high precision. These probing system combinations must be tested to check their compliance with the specifications and to trace back the measurement results. In this paper, we present a novel multi-ring artifact and appropriate test procedures for probing system combinations similar to the well-known test procedures described in the ISO standard 10360-9. The characteristic of multi-rings artifact is keeping topology geometric relationships among 2D rings. Then, a series of representative experiments were carried out on a commercial combined probing system equipped with an imaging probe and a line laser scanner, and results have proved such multi-ring artifact as a fast way for performance test.

The SI derived unit lumen is a unit of total luminous flux, describing a total quantity of visible light emitted from a light source. The realization of lumen unit is implemented by the facility called goniophotometer in National Institute of Metrology China (NIM). The goniophotometer is a facility measuring the luminous intensity of a light source at various angles, and the total luminous flux is calculated by integrating the spatial distribution of luminous intensity. Because of sampling characteristic of the goniophotometer, the spatial distribution of luminous intensity is a discrete curve. In our previous method, the goniophotometer measure the luminous intensity value at polar angle step size of 1°, 2° or 5°. For measurement of LED-light sources, which are likely oriented light sources, the spatial distribution of light is not homogeneous. Carefulness must be taken because the choice of angle sampling interval is going to be a considerable uncertainty component. Recently, a timeline-based sampling method is developed and applied in the primary standard goniophotometer in NIM, to realize the lumen unit, as well as measuring the LED light sources. The control computer is not to record the luminous intensity at exact integer angle, but to record the luminous intensity and its related time, and to record the angle and its related time. These two record threads are running respectively and simultaneously during the whole testing process. An interpolation is used to calculate out the luminous intensity and the angle at the same timeline. Thus, the spatial distribution of luminous intensity curve has much more effective data than that from the previous method. It is an improvement to help to get a more accurate result. The repetitive fluctuation is evaluated to be 0.025%, a much lower level than that of the previous method.

Pose calibration is widely used in the fields of 3D (three-dimensional) shape reconstruction, vision-based robot navigation, augmented reality, and so on. However, if two cameras have the non-overlapped FOV (Field Of View), existing methods cannot calibrate their relative position because the calibration target is invisible to one camera. This paper presents a novel calibration method for two cameras with non-overlapped FOV by using a calibration target and a flat mirror. For the camera invisible to the calibration target, the camera can capture the virtual image via the flat mirror. PNP (Perspective-N-Point) problem is solved via the captured virtual image, and then the relative orientation between the virtual calibration target and the camera is obtained. Based on the reflection theory, moving the mirror to several positions, the orthogonal constraint equations are established. This orthogonal constraint is satisfied by all mirror positions and the reference points on target. Therefore, one can obtain the external orientation between the calibration target and the camera. For the camera visible to the calibration target, the traditional calibration method is used to obtain the external orientation between the target and the camera. After obtaining the relative orientation of the two cameras to the calibration target, an ICP (Iterative Closest Point) algorithm is used to compensate binocular pose matrix. The calibration results are analyzed by using an error model. The experimental results show that the proposed calibration method can obtain the relative orientations between two cameras effectively and accurately.

Publisher’s Note: This manuscript, originally published on 2 November 2018, has been withdrawn by the publisher for editorial reasons.

Phase measuring deflectometry (PMD) is used to measure the shape of the highly reflective hyperparaboloid mirror. The potical model of PMD was constructed by the aid of Zemax software, and the shape was reconstructed according to both the distorted fringes and the optical parameters, such as the distance between the text mirror and the LCD screen or the CCD camera, the off-axis distance of specular surfaces and the relative position between the screen and the camera. The slope of the text surface was obtained with the four-step phase-shifting method, phase unwrapping and camera calibration, and the shape of the hyperparaboloid mirror was restored by integrating the slope data, which provided a technical reference for the next actual measurement model.

With the development of precision optical engineering, higher manufacturing qualities are demanded for advanced optical systems. The characterization of the surface topographies of optical elements is required to be more specific and more comprehensive. In this paper, the contourlet transform is adopted to extract the topological features of optical elements. The performance of the contourlet transform(CT) is analyzed carefully. The multiscale analysis techniques based on contourlet transform for peak/pit extraction, tool trace identification and sharp edge detection on non-smooth microstructured optical surfaces were shown. The experimental examples are given to demonstrate the validity of the proposed method.

This study presents an algorithm for automatic selection of a frequency domain filter for the Fourier transform method of interference fringe analysis in a pulse-train interferometer. In this approach, the equality between the interference-fringe envelope peak point and the phase-crossing point of different frequency components is used to distinguish between the signal and noise in the frequency domain. The efficacy of the proposed technique is demonstrated by numerical experiments.

Background oriented schlieren(BOS) technology is an efficient measurement method for quantitative diagnostics of fluid field in recent decades, and it has a broad application prospect in flow field measurement. It not only has high spatial and temporal resolution, as well as can be employed for quantitative measurement of the density gradient distributions of convection fields. In this paper, a new method for reconstructing density distributions is proposed. Initially, we obtained the point displacement image according to the basic principle of BOS. Secondly we used the local basis function method to discretize the volume to obtain the coefficient matrix. We had to choose an appropriate finite support basis function to ensure the coefficient matrix was sparse. Finally we obtained the density field by using algebraic iteration method and other methods.Numerical simulation experiments are presented to verify our method. The experimental results of refractive index and density field distribution of flow field are obtained after the simulation experiment, which indicates that the method of CTBOS technology can obtain the quantitative refractive index and density distribution of flow field, while in the real condition, there is great application value.

Borescopes are widely used for civil aviation engine inspection. In this paper, a novel scaled shape from shading (SFS) approach is proposed to measure the civil aviation engine chambers with 3D information from the borescope images. In this work, a more accurate point light source model is firstly investigated. By introducing the matrix of camera, the relationship between the CCD grayscale image and the surface depth map is developed, which enables the new method can recover the 3D information of the surface with only one image. Under the constraints of brightness, smoothness and integrability, the depth map of surface is calculated through iteration. To evaluate our method, three synthetic images were simulated. By using the ratio of RMSE as criteria, the simulation results show a perfect match with the errors of 1.19%, 1.01% and 1.16%, respectively. In the experiment, the nozzle inner wall with clear features of Turbomeca Turmo IV C, a helicopter engine of Safran, is measured and its 3D shape is successfully recovered. The results prove the effectiveness of this method and show great potential of applications in civil aviation engine inspection.

In nuclear power plants, it is a common procedure to use video inspection when reloading fuel assemblies regularly. Because of the turbulence generated by residual heat of nuclear fuel assemblies, a video sequence suffers from severe geometric deformations and is hard to be used to position the location hole of fuel assemblies. The paper proposes a novel algorithm to recover a geometrically correct image of nuclear fuel assemblies or scene from a video sequence distorted by turbulence underwater and achieve the precise position of the location hole. At the first, an average image is utilized to compare with image sequences to estimate nonrigid registration based on B-splines. By the estimated nonrigid registration, new image sequences are obtained and are used to get a new average image. After multiple iterations, the better image sequence can be shown. Then the better image sequence are divided into image patches. The more blurry and severely distorted image patches are removed in image patch sequences. Finally, the selected image patches are synthesized together. Based on restored images, template matching is utilized to quickly find the initial position of the location hole. And then the sub-pixel centroid method is used to achieve the sub-pixel position in the image. The calibrated camera parameters are utilized to calculate the position of the location hole of the fuel assemblies. Experiments verify that the algorithm can online locate the center of location holes on recovered images underwater, and has high measurement precision.

When underwater camera is used to carry out the visual inspection after fuel reloading in nuclear power plants, heat exchange between fuel assemblies and water can generate underwater turbulence, which causes imaging distortion. Turbulence severely affects core verification of nuclear fuel assemblies, serial number of which should be identified. With the aim to recover the images from a video sequence severely distorted by turbulence, an image enhancement method is proposed. At first, an image quality assessment metric FSIM is used to select the better quality frames. Next an iterative robust registration algorithm is used to eliminate most geometric deformations and recover the water surface. The temporal mean of the sequence is utilized to overcome the structured turbulence of the waves through the algorithm. Finally, the sparse errors are extracted from the sequence through rank minimization to remove unstructured sparse noise. After image processing, optical character recognition is performed by KNN and CNN, obtaining high recognition rates of 99.33%, 100% respectively. The experimental results show that the suggested method significantly performs better in distorted image restoration and image text recognition on the task of visual inspection of nuclear fuel assemblies.

Fourier transform profilometry method has great value in high-speed three-dimensional shape measurement. In the method of Fourier transform profilometry, it is necessary to obtain the phase of the deformation fringe containing the height information of the object through Fourier transform, frequency domain filtering and inverse Fourier transform. Filtering in frequency domain is a very important and essential process. Filtering window is usually selected manually, which is inefficient and subjective. Too large filtering window can not filter useless information, and too small filtering window will lose the height information of the object. In this paper, an adaptive spectrum extraction method is used. In order to be more convenient and simple, this paper presents a method of frequency domain filtering based on convolution neural network.Convolution neural network can realize image recognition and image feature extraction. The proposed method uses convolution neural network to identify the carrier frequency components carrying the details of the object in the spectrum image. This paper introduces the theoretical analysis and the training process of convolution neural network. The adaptive spectrum extraction method and the convolution neural network method are compared. The method of spectrum extraction based on convolution neural network is feasible.

Background Oriented Schlieren (BOS) technique can be applied for quantitative flow field diagnosis with simple experimental configurations. One of the crucial steps of BOS techniques is the extraction of image displacement vectors. The cross-correlation algorithm widely used in PIV techniques have been introduced to address the BOS extraction. However, the cross-correlation algorithm depends on interrogation windows, which usually results in low spatial resolving or unstable results. This paper proposes an improved BOS approach based on three step phase-shifting algorithm with the usage of a colored-fringe pattern as background. RGB coded carrier-fringe image is composed of three phase-shifted images. Displacement vectors are extracted by comparing the different phases between the corresponding points in three separated images. This technique avoids the problem of selecting the interrogation window. Only one image is required in this approach. An experimental setup on the measurement of hot air gun was carried out by use of our proposed method. The results demonstrate that this technique can be used for quantitative measurement in flow field.

The localisation accuracy of axial peaks is an important factor for height determination in a confocal microscope. Several algorithms have been proposed for height extraction in surface topography measurements. However, some algorithms ignore the influence of error and discrete sampling on the accuracy. This paper analyzes the localisation accuracy of some common algorithms under different aberrations and random errors, and discusses the effect of axial scanning interval on the accuracy of each algorithm. Finally, we get the application scope of each algorithm. Our results offer a reference for selecting algorithms for confocal metrology.

The monochromator has been widely used in the field of optical precision measurement. It can effectively separate the monochromatic light of the specific wavelength required for the experiment from a complex spectrum and a continuous spectrum light source. The wavelength accuracy of a monochromator is an important indicator of its performance，and the research of wavelength accuracy calibration methods to improve measurement accuracy is a hot theme for researchers around the world.When the monochromator is utilized for calibrating the wavelength in the ultraviolet, visible, and near-infrared region, low-pressure discharged lamps, such as mercury lamps and neon lamps, are usually used to calibrate on the well-known limited atomic emission lines of lamp to obtain the wavelength deviation value at the wavelength points of these lines.For some high-precision requirements, for example, when measuring the spectral responsivity of a reference solar cell using a tunable laser as light source, it is difficult for these discrete and finite lines to fully satisfy the demand.To solve this problem, a new method based on combination of continuous spectrum light source and fourier transform spectroradiometer was used. A calibrated experimental optical path was successfully built, and the wavelength deviation values of 322 wavelength points from 400 nm to 2000 nm were obtained. The optimal measurement repeatability reached 0.3 pm, which met the need for high-precision measurement requirement. As a comparison, a low-pressure discharged lamp method was also used. Using a mercury lamp and a neon lamp, a wavelength calibration experiment was performed on the same monochromator in the same wavelength range, and only 20 wavelength deviation values were obtained at its atomic emission lines wavelength point. The number of wavelength deviation values is less than 6.5% of that of the new method.The new method proposed in this paper,which not only can significantly improve the quality of the monochromator calibration wavelength deviation values, but in which the obtained values are able to establish traceability to the international SI unit system, is an ideal wavelength accuracy calibration method.

UV laser wavelength standard plays important role in UV wavelength meter and spectrograph calibrating, and it is also widely used in laser induced fluorescence, molecules phosphorescence, and alkali metals gas mixtures. Usually, the continuous wave UV laser sources are obtained by two-step second harmonic generation (SHG) effects from YAG:Nd lasers, or one-step SHG with the tunable dye lasers or diode lasers as fundamental beams. However, these laser systems are expensive and complicated. In this paper, we report a low costs and simple experimental arrangement to emit a fixed wavelength UV laser by a He-Ne laser. It takes the 632.8 nm He-Ne laser as the fundamental beam to generate the second harmonic in the nonlinear optical crystal of Beta barium borate (BBO), and obtains a 316.4 nm UV laser radiation with a power of 50 μW. Benefiting from the wavelength uncertainty of the He-Ne laser, the UV laser takes the same wavelength uncertainty of 5×10^-6. It means that no extra stabilization techniques are needed for the some special applications with the lower wavelength accuracy requirement. The UV laser takes an intracavity SHG configuration with two concave reflectors of short focal length to form a folded cavity. The internal power of the fundamental beam is more than 100 mW when the BBO crystal is installed. The length of the bore tube is about 30 cm, and the cavity length is no more than 50 cm. To our knowledge this is the shortest cavity in the He-Ne laser intracavity SHG ever demonstrated.

Axicon is widely used in optical alignment and Bessel–Gauss beam generation. There are rigorous requirements for a highly accurate surface metrology. In this paper, a polarization phase-shifting interferometer measurement method using a concave axicon mirror is proposed to obtain the surface of axicon lens. The measuring beam produced by a polarization phase-shifting interferometer is incident on the flat surface of an axicon lens under test perpendicularly and it is reflected along the original optical path by a concave axicon mirror, which is easy to be manufactured by ultra-precision diamond-turning machine. The reflected beam by the concave axicon mirror interferes with the reflected reference beam by transmission flat (TF) in the interferometer. Consequently, the surface of the axicon lens can be obtained. The measurement method is simple, timesaving and easy to achieve surface metrology of axicon lens of any cone angle. In experiments, the evaluation parameters of the axicon surface profile errors are given by the peak-to-valley(PV) error and Root-Mean-Square(RMS) error. The measurement results verify that the measurement for axicon can be achieved by the proposed method which plays a crucial role in evaluating the manufacturing and image qualities of axicon.

Seed morphological characteristics and weight are important evaluation indicators of seed quality, and they are closely related to seed germination rate and crop yield. In order to detect the quality of melon seeds and improve the efficiency of melon production, a melon seed morphological feature extraction and weight detection system was developed. The light source unit with a ring structure consisted of high power LED lamps, providing stable light for the system; The image acquisition unit collected color images of melon seeds; The weighing unit weighed the weight of melon seeds in real time; The control processing unit processed seed images real-time, triggered the image acquisition unit to collect image and control the collection speed of seeds images, controlled weighing unit to work, and saved seeds image, weight and processing results. The system can extract the number, area, perimeter, long diameter, short diameter and weight of melon seeds. After testing of the system, the detection accuracy of melon seeds number was 100%, detection relative errors of seeds weight, seed perimeter, length and width were less than 5%, and the detection relative error of melon seed area was less than 10%. The results show that the developed melon seed morphological feature extraction and weight detection system can meet the actual needs of melon seed production.

This article presents an algorithm to avoid camera overexposure or underexposure by calculating and analyzing the camera response function. First, the range of intensity value and time is set to ensure that the intensity value of each image pixel is within the measurement range when the camera's exposure time is within the time range. Then, the reference image which has the maximum number of pixel intensity values within the intensity value range is selected. Third, the camera response function will be calculated based on the variation of the reference point intensity value with the exposure time. Finally, the whole exposure times will be recovered based on the computed response function, and used in the process of structured light measurement. The experimental results verify the performance and feasibility of the proposed method.

Conformal coating is a thin film used for protecting printed circuit boards (PCBs) from harsh environmental conditions, which reduces the failure rate of PCBs. The thickness of conformal coating is one of the key factors determining the protection efficacy on PCB. Therefore, the thickness measurement is highly desired to qualify the conformal coating. In this study, we propose to employ high-resolution spectral-domain optical coherence tomography (SD-OCT) for measuring the conformal coating thickness. An SD-OCT with axial resolution of 1.72 μm is developed. The system can provide cross-sectional imaging of the conformal coating layer. Then a boundary detection algorithm is developed to identify the coating layer from the OCT image and eventually calculate the thickness of the coating layer. Our proposed method is evaluated through comparing with metallographic slicing method, which cuts PCB into cross-section and measure conformal coating thickness under a microscope. The results demonstrate that our method produces a very consistent measurement results as compared to metallographic slicing method. In addition to the good accuracy, our algorithm’s computation load is low (about one hundred milliseconds per B-scan), indicating the potential to achieve on-line inspection of coating thickness.

Three-dimensional measurement based on structured light has been widely used in many fields. Since center locations are used for calculating 3D coordinates, it is important for measurement accuracy. However, affected by the occlusion, shape or color of the measurement object, the angle between object and measurement system and so on, the gray distribution of stripe is degenerated from symmetric to asymmetric. Stripe center locating accuracy is decreased by asymmetric, and some measurement data with big error which decreasing the measurement accuracy seriously appears. In order to recognize those large error data, a new method is proposed by evaluating the quality of stripe gray distribution. The asymmetrical degree of stripe gray distribution is evaluated by the skewness coefficient of stripe gray distribution. The skewness coefficient is defined by the third-order central moment. Then the relationship between the skewness coefficient of stripe gray distribution and the stripe center locating error is analyzed and established by statistical methods. Based on the relationship, the threshold of skewness coefficient is set according to the requirement of measurement accuracy. The asymmetry of gray distribution is estimated by calculating the coefficients. According to the skewness of stripe gray distribution and threshold large error data with low reliability are identified. Higher measuring accuracy is achieved by rejecting the identified data. The validity and reliability of the method have been proved by experiments.

Considering actual industrial production, precise positioning for irregularly shaped workpiece is required. If the workpiece is located by the method of machine vision, the critical step is to get the position of workpiece contour in image. However the edge information quality in image can be affected by workpiece shape, material, lighting method and other factors. Especially for the complex edge information, the traditional edge detection algorithm is usually hard to eliminate the noise points near the true edge, these noise points will be misjudged as true edge points, which will reduce the accuracy of the contour positioning results. In this paper, a precise contour positioning method for workpiece with irregular shape was proposed. Firstly, based on the initial results of template matching, edge detection region with variable size according to the edge normal direction was created, then a set of edge points can be obtained. Secondly, according to the correlation between true edge points, the position deviation of each point was calculated, and the edge point evaluation function was defined by combining gradient amplitude and position deviation. Finally, removing the points with lower defined scores to obtain final set of edge points, which determines the position of workpiece contour in image. The experiments show that this method can effectively exclude noise points in edge point set, obtain the true contour of workpiece with any shape, and overcome the shortcomings that the traditional edge detection algorithm is greatly influenced by edge noise. The method has high accuracy, stability and strong practicality.

Testing wavefront distortions at the design wavelength is critical for optical system qualification. The wavefront aberrations is usually expressed in the Zernike polynomials form. As the available wavelength of the laser is limited, wavefronts of an optical system at only a few wavelengths can be test precisely. We consider the change in wavefront with wavelength is caused by the change in the refractive index of the optical material. In this paper we put forward a method for calibrating transmitted wavefront at any wavelength within certain limits. Transmitted wavefronts can be estimated at any wavelength utilizing the relationship between transmitted wavefront and wavelength. We study the relationship between transmitted wavefront Zernike coefficients and wavelength, and choose the Conrady formula to express the function of Zernike coefficients and wavelengths. Zernike coefficients at any wavelength in a certain range can be calculated by the Conrady formula at three other wavelengths. The transmitted wavefront at a specific wavelength can be fitted. Finally, we verify different kinds of optical systems by this method. The result shows the method is effective for the monochromatic system and the achromatic system, while the apochromatism system is form with special glass. The relationship between of the transmitted wavefront Zernike coefficients and wavelength is more complex.

Robust direct vision-based pose tracking using normalized mutual information (Addendum)