Signal processing for optical sound field measurement and visualization

Conditions of the digital recording of the fringe pattern determine the phase reconstruction procedure, which in turn directly shapes the final accuracy and throughput of the full-field (non-scanning) optical measurement technique and defines the system capabilities. In this way, the fringe pattern analysis plays a crucial role in the ubiquitous optical measurements and thus is under constant development focused on high temporal/spatial resolution. It is especially valuable in the quantitative phase imaging technology, which emerged in the high-contrast label-free biomedical microscopy. In this paper, I apply recently blossomed two-frame phase-shifting techniques to the QPI and merge them with advanced adaptive interferogram pre-filtering algorithms. Next, I comprehensively test such frameworks against classical and adaptive single-shot methods applied for phase reconstruction in dynamic QPI enabling highest phase time-space-bandwidth product. The presented study systematically tackles important question: what is the gain, if any, in QPI realized by recording two phase-shifted interferograms? Counterintuitively, the results show that single-shot demodulation exhibited higher phase reconstruction accuracy than two-frame phase-shifting methods in low and medium interferogram signal-to-noise ratio regimes. Thus, the single-shot approach is promoted due to not only high temporal resolution but also larger phase-information throughput. Additionally, in the majority of scenarios, the best option is to shift the paradigm and employ two-frame pre-filtering rather than two-frame phase retrieval. Experimental fringe analysis in QPI of LSEC/RWPE cell lines successfully corroborated all novel numerical findings. Hence, the presented numerical-experimental research advances the important field of fringe analysis solutions for optical full-field measurement methods with widespread bio-engineering applications.

Imaging of sound aids the understanding of the acoustical phenomena such as propagation, reflection, and diffraction, which is strongly required for various acoustical applications. The imaging of sound is commonly done by using a microphone array, whereas optical methods have recently been interested due to its contactless nature. The optical measurement of sound utilizes the phase modulation of light caused by sound. Since light propagated through a sound field changes its phase as proportional to the sound pressure, optical phase measurement technique can be used for the sound measurement. Several methods including laser Doppler vibrometry and Schlieren method have been proposed for that purpose. However, the sensitivities of the methods become lower as a frequency of sound decreases. In contrast, since the sensitivities of the phase-shifting technique do not depend on the frequencies of sounds, that technique is suitable for the imaging of sounds in the low-frequency range. The principle of imaging of sound using parallel phase-shifting interferometry was reported by the authors (K. Ishikawa et al., Optics Express, 2016). The measurement system consists of a high-speed polarization camera made by Photron Ltd., and a polarization interferometer. This paper reviews the principle briefly and demonstrates the high-speed imaging of acoustical phenomena. The results suggest that the proposed system can be applied to various industrial problems in acoustical engineering.

In this doctoral thesis, the basic understanding of low-temperature combustion processes and alternative tailor-made fuels is improved with optical measurement methods. Therefore, different optical measurement techniques have been applied using different fuels in a high-pressure injection chamber. In the high-speed measurements different techniques are working simultaneously to detect characteristic values of mixture formation and combustion like liquid and gaseous penetration length as well as flame lift-off length: Mie scattering, shadowgraphy and OH*-radiation. With the help of fundamental spectroscopic laser measurement techniques based on laser induced fluorescence, nitrogen oxide can be quantitatively detected in the jet central region for the first time. Several steps are required for the quantification: correction of the absorption through the flame, correction of the different temperatures in the flame, correction of fluctuations in laser energy and determination of an absolute calibration factor. Quantitative nitrogen oxide concentrations were measured at different times and at two different distances from the injector nozzle hole. The measurement of the quantitative nitrogen oxide concentrations is also possible early in the flame and can be conducted directly next to the lift-off length. It was also used with two alternative fuels. The quantitative nitric oxide measurement shows non-negligible nitrogen oxide concentrations in the middle of the burning jet, in contrast to the prevailing classical diesel combustion model where nitrogen oxides are only formed at the edge of the diffusion zone of the flame. Furthermore, it is shown that due to the different flow states in the combustion nitrogen oxides are affected. To support the measurement, the application of a 0-d constant pressure simulation with a detailed flame chemistry and a prompt-NOx mechanism was performed. As a result, the detection of prompt-NO formation in the flame is possible. To compare with the experiments a 3-d-CFD-LES-simulation was conducted.

Optical window freeform surfaces have emerged as a critical research focus in advanced optical engineering owing to their extensive surface degrees of freedom. These surfaces enable the simultaneous correction of on-axis and off-axis aberrations while satisfying stringent requirements for high-performance, lightweight, and compact optical systems. In the initial metrological characterization of these surfaces, coordinate measuring machines (CMMs) are conventionally employed for target point cloud acquisition. However, the achievable measurement accuracy (>2 μm) inherently constrained by CMM precision imposes fundamental limitations for subsequent optical inspections requiring sub-micron to nanometer-level resolution. Meanwhile, although optical measurement methods can result in higher measurement accuracy, they also lead to an increase in costs and testing difficulties. To overcome these limitations, we propose an accelerated point cloud registration methodology based on point-to-triangulation distance estimation. In simulation, using optimal coordinate transformation enabled good capabilities for exceptional surface characterization: peak-to-valley (PV) surface error of 10−6 nm, residual error of 5 nm, and registration accuracy of log10 (mm/°). Further, in the experiment, the PV surface error was reduced from 27.3 μm to 6.9 μm, equivalent to a reduction of 3.95 times. These results confirm that the point-to-triangulation distance approximation maintains sufficient fidelity to the nominal point-to-surface distance, thereby empirically validating the efficacy of our proposed methodology. Notably, compared with conventional 3D alignment methods, our novel 2D estimation registration approach with point-to-triangulation surface normal vectors demonstrates significant advantages in computational complexity, which achieved a 78% reduction from O(n3) to O(n) while maintaining sub-millisecond alignment times. We believe that the method has potential for use as a low-cost optical precision measurement in manufacturing technology.

Since mechanical properties of parts in mechanical engineering are influenced in many ways by their surface structure, detailed knowledge of the surface structure is essential for predicting and describing these properties. Tactile and optical measurement methods are well established but cannot always reach every area of a part due to geometric constraints. To enable measurements in these areas, a method utilizing impressions and a laser scanning microscope is proposed. It is easy to use and able to correctly reproduce surface structures of technical surfaces, which is proven by a comparison of original parts and impressions of three surface areas on an example specimen. Two application examples from the domain of fatigue strength tests are shown. The surface structure is measured directly in the notch radius of shaft shoulders and inside hollow shafts made in different manufacturing processes. Utilizing modern optical measurement instruments, the proposed method enables accurate measurements of surface structure in previously inaccessible areas.

Bioreactors are used for the industrial-scale culture of cells to obtain valuable products such as pharmaceuticals, enzymes, and biofuels; however, monitoring the growth conditions within the vessels is challenging and is often dependent on single-point ex-situ measurements. Further, spatial heterogeneities are known to exist within these environments, thereby creating regions of low growth or incomplete reactions, reducing yield, and importantly, reducing the applicability of single-point measurement methods. Optical imaging is an attractive method for remote spatially-resolved measurement platforms; however, the strong optical scattering within cell cultures makes imaging almost impossible. Here, we utilise this parasitic scattering effect and present a spatially resolved optical method for monitoring cell density within a bioreactor, using optical measurement of local scattering parameters as a proxy measurement for cell density. Our method is non-invasive and does not require the removal of any cell material from within the vessel. We propose that our optical measurement method can be incorporated into process-control feedback systems, providing insightful information on cell growth that can be used to deliver higher spatial homogeneity and increased yields.

Due to its low complexity and acceptable accuracy, phase retrieval technique has been proposed as an alternative to solve the classic optical surface measurement task. However, to capture the overall wave field, phase retrieval based optical surface measurement (PROSM) system has to moderate the CCD position during the multiple-sampling procedure. The mechanical modules of CCD movement may bring about unexpectable deviation to the final results. To overcome this drawback, we propose a new PROSM method based on spatial light modulator (SLM). The mechanical CCD movement can be replaced by an electrical moderation of SLM patterns; thus the deviation can be significantly suppressed in the new PROSM method. In addition, to further improve the performance, we propose a new iterative threshold phase retrieval algorithm with sparsity-constraint to effectively reconstruct the phase of wave field. Experimental results show that the new method provides a more simple and robust solution for the optical surface measurement than the traditional techniques and achieves higher accuracy.

With reference to the IEC 60793-1-140 international standard of optical fibre measurement methods and test procedures in attenuation, we studied the optical fibre attenuation measurement by cut-back method using tuneable lasers source. By using a power stabilised laser source, we measured the fibre attenuation in the wavelength range from 1270 nm to 1350nm and from 1520 nm to 1620 nm using ‘cut-back’ technique. The power measurement before and after cut-back have better repeatability. Besides, the evaluation of the splicing losses before and after cut-back as well as the evaluation of effective refractive index (N_eff) will improve the accuracy in calculating the fibre attenuation. Our method will improve accuracy and reduce uncertainties in the measurement and thus enable us to establish our own optical fibre spectral attenuation standard.

A novel method for optical beam collimation measurement is presented. The collimating lens is utilized in four parts of quadrants with the beam aligned onto the first quadrant and configured to pass the subsequent quadrants. This allows the test beam to pass the collimating lens for four times. Subsequently, the test beam is reversed to achieve a total number of eight passes. Hence, for a defocus introduced, the collimation state of the test beam can be evaluated at the amplification of eight. The evaluation of the test beam is performed based on the approach of collimation testing using lateral shearing interferometer. The proposed technique provides a differential collimation sensitivity for accurate setting of a highly collimated beam.

ABSTRACTIn this paper, a method for optical length measurement that employs a multi-passband microwave photonic filter (MPF) is analysed and demonstrated experimentally. The MPF consists of a sliced broadband light source, a phase modulator, a dual-parallel dispersion medium and a photodetector (PD). By cascading two Mach–Zehnder interferometers (MZIs) with different arm length difference, the MPF with eight passbands has been obtained. The variation of the length difference of MZI will lead the change of the free spectral range (FSR) of the interference spectrum, as well as the frequency of the passbands. By changing the arm length of the MZI, a high length measuring sensitivity of −2.420 GHz/mm has been achieved. The MPF has broad application prospects in the field of multi-parameter sensing. Moreover, the sensor reveals the advantages of highly sensitive, easy to implement and is able to overcome the influence of the environmental perturbation, for example temperature.

A new method for optical pH and redox measurements with a commercially available water dispersion of polyaniline (PANI) nanoparticles (mean particle size, 46 nm) is presented. The pH measurements are based on the acid-base equilibrium of PANI and were carried out either by combining both the automated sequential injection analysis (SIA) and UV-visible spectrophotometric techniques or with a fiber-optic light guide. In the former case, the detection was done in continuous mode at lambda = 800 nm by using the SIA technique for transporting the sample to a flow-through cell, which was placed in the light path of the photometer. With the fiber-optic light guide, the detection was done in batch mode at lambda = 400 and 580 nm. In both methods, fresh pH reagent (PANI) solution was used in each measurement, thus overcoming the problem with hysteresis (memory effect), which is usually observed with PANI films. The PANI nanoparticles were characterized with UV-visible spectroscopy in pH buffer solutions between pH 2-12 and a protonation constant of logK(0.5H,L)(H(0.5)L) = 4.4 was calculated from these data. Fast pH measurements can be done between pH 6 and 10.5 depending on the measuring technique. It is possible to determine pH with an accuracy of 0.1 pH unit between pH 8 and 10.5 (RSD, 0.5-2%). Redox transitions typical for PANI films were also observed for water solutions of PANI nanoparticles in the presence of the hexacyanoferrate(II/III) and the iron(II/III) oxalate redox couples. The absorbance at lambda = 875 nm is linearly dependent on the logarithm of the concentration ratio (0.1-10) of the iron oxalate redox couple.

Coded light measurement is an important method of non-contact optical measurement. Because of the camera view limitations, it was hard to complete the measurement of the whole surface at a time. Then it needs registration of 3D point clouds. The registration of point clouds can reflect the 3D surface of the object to be measured, which can achieve the full range of requirements. According to the encoded optical 3D measurement system has been built in laboratory, exploring a method which is high accurate and high speed to reach a full range measurement to industrial parts. This paper describes the color target cube. According to the existing method of coordinate transformation (the Nearest Point Iterative Method), Evaluating the registration accuracy. The registration method based on auxiliary three-dimensional targets was been validated, which has its own rationality.

Liquid composite molding (LCM) is growing in importance as an alternative to traditional prepreg-autoclave methods for manufacture high-performance composites. The most significant roadblock to industry’s implementation of LCM is the usually higher void content compared with prepreg processing. One tool for reducing void levels in LCM involves optimization of flow velocity, which requires models to be developed to describe void formation at a given velocity. To help solve this problem, the following research illustrates the first known method for optical void measurement in situ during infusion in a carbon fiber reinforcement. Similar to previous studies on glass fiber, this work utilizes fluorescent dye and a digital camera to produce sufficient contrast and resolution for image analysis. Visible bubbles are photographed against the opaque carbon fiber background. An automated method of image analysis is outlined, which was used to analyze 230 images for three different flow orientations of a single fabric, producing the highest amount of experimental data seen so far on in situ void measurement. The resulting data identifies a minimum velocity threshold for minimal macro-void formation. The resultant void characterization framework will better enable optimization of LCM processing for high-performance composites based on carbon reinforcements.

The experimental investigation is described of the reinforced concrete construction pro blems mentioned in the title, making use of the Laser-Speckle-Photography optical measurement method- Proceeding from the material-conditional particularities and the high demands on the accuracy of the measurement values, a semi-automatically working device. was developed for analysing the Laser-Speckle photographs. The method of working with this device, the statistical routines applied in the process and the attainable accuracy are reported on. Moreover, reference is also made to the strains in a reinforced concrete member and the crack. formation. In the last section, the results of measurements of the joint rotation on the inner support of a two-span beam of reinforced micro-concrete, as well as the shear bearing behavior observed in this context, are discribed.

There is no denying that 360-degree shape measurement technology plays an important role in the field of threedimensional optical metrology. Traditional optical 360-degree shape measurement methods are mainly two kinds: the first kind, by placing multiple scanners to achieve 360-degree measurements; the second kind, through the high-precision rotating device to get 360-degree shape model. The former increases the number of scanners and costly, while the latter using rotating devices lead to time consuming. This paper presents a low cost and fast optical 360-degree shape measurement method, which possesses the advantages of full static, fast and low cost. The measuring system consists of two mirrors with a certain angle, a laser projection system, a stereoscopic calibration block, and two cameras. And most of all, laser MEMS scanner can achieve precise movement of laser stripes without any movement mechanism, improving the measurement accuracy and efficiency. What's more, a novel stereo calibration technology presented in this paper can achieve point clouds data registration, and then get the 360-degree model of objects. A stereoscopic calibration block with special coded patterns on six sides is used in this novel stereo calibration method. Through this novel stereo calibration technology we can quickly get the 360-degree models of objects.