Table of contents

Process tomography (PT) refers to a methodology by which the internal characteristics of process vessel reaction or pipeline flows are acquired from measurements on or outside the domain of interest in a non-invasive fashion. As a generic 'tool' PT is extremely useful in improving, for example, the modelling and design of many complex processes, in understanding the dynamic mechanisms of flowing and mixing of colloidal dispersions, and in multiphase flow phenomena, hydraulic transport and process control. Over two decades of research worldwide, PT has become a routine research tool in many research laboratories and is being accepted for process measurement and control in some industrial applications.

This is the fourth special feature on process tomography after previous publications in this journal in 1996, 2001 and 2002. In this issue, recent developments in sensors, measurements and algorithms with new features for specific distinctive applications are addressed, such as the high temporal resolutions of 1000 frames/s and beyond obtained by both x-ray and impedance tomography for flow measurement and fast process reaction; interferometric tomography combining the Mach–Zehnder interferometer and tomography to utilize the phase difference in propagation for visualization of particular features in a process and new three-dimensional image reconstruction algorithms in process applications. The important aspect of this issue is that it demonstrates current developments focusing on the improvement of performance at the temporal resolution, phase information and 3D algorithms for specific application.

Looking back over two decades of research, we can see that the process tomography technique is maturing and its applications in industrial manufacture are being deployed as a result of the determined efforts of researchers worldwide.

As Guest Editor of this special feature, I would like to thank my colleagues at the Virtual Centre for Industrial Process Tomography (VCIPT) for their efforts in the organization of the original articles. In particular, I would like to thank the authors who have risen to the challenge and made this feature possible, the referees who have devoted their precious time and rich knowledge and the staff at IOP for their dedication in meeting the short timescale and their scientific approach in making the publication of such a high standard.

During the last decade there has been a rapidly growing interest in integrated optical (IO) sensors, especially because many of them principally allow for sensitive, real-time, label-free on-site measurements of the concentration of (bio-) chemical species. This review aims at giving an overview of the most relevant developments in this area. After a general introduction into the field of IO sensors for the chemical domain, relevant aspects of integrated optics and chemical sensing are presented in short. A large variety of IO sensing platforms are introduced and discussed: interferometers, resonators, coupling-based devices such as grating couplers and surface plasmon resonance based sensors and finally a new class of sensors based on chemically induced field profile changes. Strong and weak points of principle and of configurations based on these principles are indicated and the main performance data of the IO sensing platforms, especially the obtained resolution, are indicated. Best resolutions of the chemically induced refractive indices on the order of magnitude 10⁻⁶–10⁻⁸ RIU can be obtained, corresponding to a resolution of 10⁻³–10⁻⁵ nm in the chemically induced growth of layer thickness of chemo-optical transducer materials. Depending on the analyte and the type of transduction layer chemical concentrations down to some ppb or some pg ml⁻¹ can be determined. Several IO sensing systems are commercially available. Extension of individual sensors to sensor arrays is treated and finally an outlook for the future is given.

We investigated the imaging capability of a fast linearly scanned electron beam x-ray tomography approach with respect to the phase structure recovery for two-phase flows in a cylindrical pipe. As a consequence of the suggested linear electron beam deflection pattern we need to solve an inverse problem of the limited-angle type which introduces some artefacts in the reconstructed images. To reduce these artefacts we have devised a modified iterative image reconstruction algorithm denoted as binary ART including a level-set based image smoothing operation. To assess the achievable quality of spatial phase structure recovery from the limited-angle data we performed a simulation study on three-dimensional flow data sets obtained with a fast and high-resolution conductivity wire-mesh sensor under real two-phase flow conditions. The simulations revealed that the reconstruction error remains below 2% for up to 1% of Gaussian noise in the projection data and even for up to 5% noise in the case of bubble diameters below 3 mm.

Three-dimensional flow phenomena with unsteady shock waves and vortices have been observed in a shock tube experiment by using an interferometric CT (computed tomography) technique. A model with small ducts, which has a pair of cylindrical nozzles, is introduced in the test section of the shock tube. The model can be rotated around its central axis to change the observation angle. The projection image of density distribution for each observation angle is obtained by using a fixed Mach–Zehnder interferometer. The three-dimensional density distribution is reconstructed from these projection images. The shock Mach numbers are fixed to 2.3 at the exits of the nozzles in nitrogen gas of 19.4 kPa initial pressure. The resultant 3D density flow fields are illustrated by several visualizing methods to clarify the 3D features of shock waves, vortices and their mutual interactions.

Temperature monitoring and control of a convex molten polymer sheet and surrounding cooling air flows are demanded in order to improve both the productivity and the quality of the polymer sheet products. The accurate measurement of temperature is desired when molten polymer changes its phases. An intrusive thermometer such as a thermocouple has difficulty because a molten polymer blown out from a ring die is extremely thin. Also temperature measurement with an infrared camera is not applicable since the polymer film is lightly transparent for the infrared radiation and the intensity of infrared radiation from the polymer film is too weak. In this research, we proposed a novel method for non-intrusive temperature measurement with the quantitative interferometer in conjunction with direct cylindrical computer tomography, and discussed its feasibility for measuring the temperature distribution on a convex surface.

An electrical impedance tomography (EIT) sensor's model for analyses of the behaviour of the coupling circuits and electrode–electrolyte interface is presented. With transient time analyses of the model, a novel switching scheme, termed the over-zero switching (OZS) scheme, was designed to eliminate both the dc offset potential and the charged residual potential of measurements. The effects of the transient time in conventional and OZS coupling circuits were simulated. Both analyses and simulations revealed that the coupling time can be dramatically reduced by the employment of the OZS scheme. The technique has been used in a fast impedance tomography system to achieve a speed of more than 1000 dual-frames per second.

In many industrial applications the aim is to obtain information on three-dimensional (3D) material distribution within the process vessels. With standard two-dimensional (2D) techniques only vague cross-sectional information can be obtained. It could be possible to carry out several 2D reconstructions on different layers and in this way to obtain 3D information. However, in this approach errors are induced since no real 3D information is utilized in the image reconstruction. In this paper we describe an approach to measure, reconstruct and visualize three-dimensional electrical impedance tomography images in real time. The reconstruction is based on a difference imaging scheme. An efficient current injection and voltage measurement protocol is used in order to increase the sensitivity and reduce the data collection time. The proposed approach can produce and visualize up to 15 3D EIT images per second when 80 measurement electrodes are used. Imaging results from a stirred vessel and a flow loop will be shown. The reconstructions show, for example, that 3D air/liquid distribution in the stirred vessel can reliably be visualized in real time and material flow can be monitored in a 3D section of the flow loop. Reconstructions can be visualized and analysed in many different ways in order to produce essential information on the behaviour of the processes.

This paper describes a multi-plane implementation of a current-pulse electrical resistance tomography (ERT) data capture system. This is achieved by extending a single plane system, with 16 electrodes and 16 parallel measurement channels, to a one capable of acquiring data in a specified sequence across multiple planes (up to eight) by inserting multiplexer modules in parallel between the instrument and the electrode array. This approach allows high-speed capture systems to be configured for applications such as dual plane cross-correlation velocity measurements or more complex current injection and measurement sequences yielding 3D data sets. The measurement timing and multiplexer measurement sequences are implemented by an embedded processor. Both the executable code and the measurement sequence tables are downloaded to the instrument at start-up. This allows flexibility in specifying the data acquisition sequences and timing required for specific applications without modification of the hardware or embedded code. The effect of measurement noise on the estimated conductivity is quantified and spatial resolution discussed for the case of a 2D online imaging algorithm. Example reconstructions from recorded data sets are presented which verify the operation of the instrument.

A combined multilayer feed-forward neural network (MLFF-NN) and analogue Hopfield network is developed for nonlinear image reconstruction of electrical capacitance tomography (ECT). The (nonlinear) forward problem in ECT is solved using the MLFF-NN trained with a set of capacitance data from measurements based on a back-propagation training algorithm with regularization. The inverse problem is solved using an analogue Hopfield network based on a neural-network multi-criteria optimization image reconstruction technique (HN-MOIRT). The nonlinear image reconstruction based on this combined MLFF-NN + HN-MOIRT approach is tested on measured capacitance data not used in training to reconstruct the permittivity distribution. The performance of the technique is compared against commonly used linear Landweber and semi-linear image reconstruction techniques, showing superiority in terms of both stability and quality of reconstructed images.

A new reconstruction method called generalized vector sampled pattern matching (GVSPM) was applied to the image reconstruction of an electrical capacitance computed tomography in freely falling particles in a vertical pipe. This new method is able to achieve stable convergence without the use of an empirical value. Experiments were carried out using three particle types with various electric charges and four particle flow rates to measure the capacitance of a pipe cross section. The three particle types were polyethylene pellets (PP), silica particles (SP) and polyvinyl chloride (PVC). Four particle flow rate settings were used resulting in a volume flow rate ranging from 3.08 × 10⁻⁵ to 1.04 × 10⁻³ m³ s⁻¹. The GVSPM method is compared with conventional methods in terms of volume fraction, residual capacitance and correlation capacitance. Overall, the GVSPM method proved superior to conventional methods in the case of polyethylene pellets with relatively low electric charge. GVSPM achieves accurate reconstruction by using an objective function that is calculated as the inner product calculation between the experimental capacitance and the reconstructed image capacitance.

Tomographic techniques have been widely accepted as a valuable tool for process control and monitoring. The classic tomographic approach is to reconstruct a 2D image of a process cross section. However, most processes take place in 3D space. Effective imaging in 3D process space can be achieved using 3D image reconstruction in two ways. The first (called 2.5D by the authors) is to use a few independent 2D images and to interpolate them into a 3D image. This method has been widely used in medical applications of tomography for many years already. The second method and the subject of this paper is 'real' three-dimensional reconstruction, where sensors provide three-dimensional measurements and a 3D image is directly obtained during the reconstruction process. The latter method has evolved from classic 2D cross-sectional definition to real and direct 3D imaging. The paper presents the authors' work on a 3D capacitance tomography system including issues such as sensor layout, measurement protocol, data simulation, reconstruction algorithm and 3D visualization.

The paper describes a miniature electrical capacitance tomography system. This is based on a custom CMOS silicon integrated circuit comprising eight channels of signal conditioning electronics to source drive signals and measure voltages. Electrodes are deposited around a hole that is fabricated, using ultrasonic drilling, through a ceramic substrate and has an average diameter of 0.75 mm. The custom chip is interfaced to a host computer via a bespoke data acquisition system based on a microcontroller, field programmable logic device and wide shift register. This provides fast capture of up to 750 frames of data prior to uploading to the host computer. Data capture rates of about 6000 frames per second have been achieved for the eight-electrode sensor. This rate could be increased but at the expense of signal to noise. Captured data are uploaded to a PC, via a RS232 interface, for off-line imaging. Initial tests are reported for the static case involving 200 µm diameter rods that are placed in the sensor and for the dynamic case using the dose from an inhaler.

This paper describes a novel planar electromagnetic tomography system for the detection of conductivity inhomogeneity on a metallic plate. The proposed system differs from traditional electromagnetic inductance tomography (EMT) systems in its spatial arrangements of coils. Sensor coils are distributed to form a circular array with their axes not parallel but perpendicular to the plate under inspection. The forward solution for the sensor array next to a homogeneous conductive plate is based on the analytical solution provided by Cheng. The sensitivity matrix for a prototype sensor was computed by numerical evaluation of the analytical solution. For the inverse solution, a modified Newton–Raphson method was used to adjust the conductivity distribution to fit a set of inductances measured from the sensor array in a least-squared sense. Frequency- dependent sensitivity analysis was performed to find an optimum testing frequency. The far-field and near-field effects in electrical tomography are discussed. Good estimates for the conductivity distribution were obtained at the optimum frequency. Experimental tests were performed by taking the difference in mutual inductance of the coil pairs when placed next to a homogeneous reference conductor and next to a conductor with faults. Inverse results based on experimental data verified this method.

A cylindrical cell containing GaInSn alloy and aqueous solution of KOH, a highly simplified aluminium reduction cell, is constructed. A direct current is applied to the cylindrical cell to generate the magnetic field. In a range of amplitude and frequency of vertical vibration, a stable, non-axisymmetric wave pattern is produced, and the displacement of the oscillating interface is measured by a digital camcorder. The perturbation of the magnetic field caused by the non-axisymmetric interface is measured by fluxgate sensors and processed by fast Fourier transforms. The measurements are consistent with forward calculations, and have been exploited to reconstruct the deformed interface by solving an inverse problem so as to develop magnetic field tomography to reconstruct an unknown interface between two electrically conducting fluids.

We have developed, in conjunction with Solartron ISA, an electromagnetic cavity resonator based sensor for multiphase flow measurement through an oil pipeline. This sensor is non-intrusive and transmits low power (10 mW) radio frequencies (RF) in the range of 100–350 MHz and detects the pipeline contents using resonant peaks captured instantaneously. The multiple resonances from each captured RF spectrum are analysed to determine the phase fractions in the pipeline. An industrial version of the sensor for a 102 mm (4 inch) diameter pipe has been constructed and results from this sensor are compared to those given by simulations performed using the electromagnetic high frequency structure simulator software package HFSS.

Piezoelectric transformers have a high potential for use in power supplies due to large power density and flexibility in design. A lot of work has been done in piezoelectric transformer design and analysis of their operation in different regimes and in different electronic circuits. In this paper an attempt has been made to investigate the influence of temperature, mechanical and electrical field stress, properties of piezoceramic material and regimes of piezoelectric transformer operation on the value of its power density. The results of the presented calculations of piezoelectric transformer parameters show a good correlation with measured data on experimental transformers. A substantial improvement of piezoelectric transformer power density seems feasible, making them more attractive for a wider range of applications in power supplies. In addition, useful guidelines for piezoelectric transformer design and selection criteria for piezoelectric materials for transformers have been established.

For the quantitative measurement in an optically dense spray, the intensity of the attenuated signal should be corrected. Therefore, the optical line patternator was applied to get the original distribution of the dense spray injected from a swirl injector at high ambient pressure up to 4.0 MPa. The optical line patternator is a combined technique of laser extinction measurement and image processing for the spray characterization. The spray was scanned with the laser beam and the line image of Mie scattering was captured simultaneously in the path of each laser beam by using a CCD camera. A photo-diode was used to obtain the transmission data that was the amount of the incident laser beam passing through the spray region. The distribution of the attenuation coefficients in the spray was obtained by processing the transmission data and Mie-scattering distribution data by an algebraic reconstruction technique. From the distribution of attenuation coefficients, we can obtain the accurate surface distribution from the Mie-scattering signal. Because the optical line patternator uses a laser beam instead of a laser sheet to scan the spray, the effect of multiple scattering, due to the increased number density of droplets in a high pressure environment is reduced significantly. The optical line patternator is suitable for investigating the characteristics of a relatively large spray under high pressure environments such as liquid rocket engines.

Phase change materials (PCM) are able to store thermal energy in small temperature intervals very efficiently due to their high latent heat. Accurate knowledge of the enthalpy as a function of temperature, or the storage capacity at each temperature, is the key to design any application. Conventional methods for thermal analysis however often lack sufficient accuracy or sample size to be applied to PCM. The T-history method is a simple method to determine the storage capacity of PCM and allows the use of large sample sizes. The experimental setup and methods of data analysis have been significantly improved in recent years. In this paper, a proper methodology to verify the correct setup and data analysis method of a T-history installation using standard materials with known properties is described and tested. The implementation of the T-history method has been done at the ZAE-Bayern. Three standard materials, gallium, water and hexadecane, were measured, as well as two commercial PCM, RT27 and sodium acetate trihydrate graphite compound (SAT+G). The obtained results confirm that the T-history installation can be used to analyse different PCM.

Two-dimensional, two-component micron resolution particle image velocimetry (micro-PIV) allows measurement of the in-plane velocity components across a single plane within a micro-scale device. The technique has become well established over recent years for the study of micro-scale flows. Stereoscopic micro-PIV uses two cameras and a stereomicroscope to capture all three components of the velocity field. Recently, preliminary results have been published from stereoscopic micro-PIV systems. Here, a complete validation of a stereoscopic micro-PIV system is carried out by comparing results for flow over a micro step with a computational fluid dynamics (CFD) solution. It is found that the accuracy of the correlation-based PIV technique is limited by the degree of overlap of the two focal planes in the stereomicroscope. This prompts the adoption of a 'super-resolution' particle tracking velocimetry (PTV) algorithm, which does not require exact alignment of the two focal planes. The PTV is more accurate than the PIV and systematic deviations near to surfaces are greatly reduced. In addition, since the PTV algorithm tracks the three-dimensional coordinates of individual particles the measured velocities are no longer automatically averaged onto the focal plane of the measurement. Thus the PTV data represent a truly three-dimensional, three-component mapping of the flow field, whose resolution is independent of the microscope lens parameters. To demonstrate this, data are presented at a resolution of 10 × 10 × 10 µm across a field of view of 900 × 720 µm, corresponding to the 10× lens used in the experiment. To obtain this out-of-plane resolution with a correlation-based algorithm we would require a numerical aperture more typical of a 20× lens, with a correspondingly smaller field of view. The data agree with the CFD within the experimental uncertainties and it is proposed that the method could be important even in flows where only the in-plane velocity components are of interest. With the current system, a single measurement can determine the in-plane velocity field throughout a 900 × 720 × 45 µm volume, large enough to cover a significant section of a micro-device. The out-of-plane resolution is limited only by the number of images taken and, ultimately, the size of the particles.

Five Pt/Pd thermocouples, constructed and calibrated at IMGC at fixed points in the temperature range from 0 °C to the Ag point, were calibrated by comparison with the local primary standard radiation thermometer with the aim of replacing the presently used Pt/Pt–Rh alloy thermocouples as secondary reference standards up to 1500 °C. To fully exploit accuracy of Pt/Pd thermocouples, high-level calibration techniques need to be adopted. For this purpose, a new high-temperature three-zone furnace was arranged and characterized in order to obtain the best axial uniformity and a specially designed blackbody cavity was used as a transfer source for calibrating the thermocouples in the temperature range from 962 °C up to 1500 °C. At the end of the comparison measurements, additional calibrations at the Ag fixed point were made, in order to check the stability of the thermocouples' signals. A comparison between experimental results and the reference function is presented and an extrapolation of the fixed-point calibration data is analysed.

This paper examines the problem of systematic measurement errors in optical triangulation when some light sheets that illuminate the measured surface exhibit non-negligible bending (curvature). The problem is demonstrated experimentally by triangulation measurement of two reference bodies whose geometry reveals systematic measurement errors due to light sheet curvature. To correct these errors a triangulation model is developed which assumes parabolic light sheet shape and allows exact solution of system equations. Test measurements show that the model successfully compensates for systematic measurement errors originating from the curvature of light sheets.

When homodyne quadrature techniques are used for primary vibration calibration at high frequencies the problem of low displacement amplitudes arises, which inhibits the accurate measurement of acceleration amplitude and initial phase. By exploiting the effect of disturbing motion caused e.g. by hum of the power amplifier or environmental vibrations, it is possible to still recover the displacement time history by phase unwrapping. However, it is then necessary to deal with the superimposed drift-like motion components. This contribution proposes two techniques to either include these components in the sine approximation or to apply a wavelet-based detrending technique. The performance and consistency of both methods are demonstrated by simulation tests, taking into account all significant influences known from real acceleration measurements, and by comparison of accordingly performed real calibrations with the respective calibration results with validated heterodyne techniques.

The production of Fabry–Perot-based optical fibre sensors has long been an iterative and labour-intensive process. This paper demonstrates the production of Fabry–Perot-based optical fibre strain sensors using chemical etching techniques. Utilizing hydrofluoric acid and single-mode optical fibres, a preferential etching mechanism was observed around the core portion of the fibres. These etched fibre ends were then spliced together successfully to form enclosed Fabry–Perot cavities between 18 and 60 µm in length. These sensors have then been deployed for strain monitoring and have been subjected to strains of up to 1400 με on tensile test specimens. Etched Fabry–Perot cavity lengths were monitored using a white light interferometry system based on a CCD spectrometer and an 850 nm super luminescent diode. A linear and repeatable response to these strain tests has been shown with negligible sensitivity to temperature.

An electron-stimulated desorption (ESD) system was assembled in order to study ESD ions desorbed from the cold surface of materials. We have investigated electronically stimulated ions desorbed from physisorbed hydrogen on Cu and rare-gas solid. The desorption yield showed a dependence on incident electron energy. Desorbed H⁺ and H⁺₂ ions of exposed H₂ on the surface of rare-gas solid were much higher than H₂ exposed directly to the surface of Cu. The kinetic energy of desorbed H⁺ and H⁺₂ ions depends on incident electron energy. When the incident electron energy was 200 eV, the kinetic energies of H⁺ and H⁺₂ were 5.0044 eV and 0.8405 eV, respectively.

The influence of dynamics on a propagating fatigue crack has not been studied experimentally yet mainly due to quasi-static loading from traditional fatigue-testing machines. To overcome this serious drawback, a novel base-excited fatigue-testing device was designed and built to allow measurement of the dynamic responses of a single-edge-notch beam (SENB) under a growing fatigue crack. In this paper, the details of the novel test rig including initial development, modification and instrumentation are given. The experimental time histories obtained for harmonic and chaotic excitations have shown that the fatigue rig is capable of generating a wide range of loading patterns. Moreover, the experimental crack growth curves and features of the fracture surface have confirmed that the rig is capable of inducing proper fatigue cracks.

We have successfully proved the feasibility of an optical salinity meter for marine applications in a two-week measurement campaign, carried out for the realization of in situ salinity measurements in seawater. An optical instrument (optode), in which the main element is a fibre-optic refractive-index sensor based on surface plasmon resonance (SPR), has been developed for that purpose, and has been especially designed to be able to operate in realistic conditions. The performance of the optode has been evaluated on an oceanographic ship in the Baltic Sea, close to the Vistula estuarine area. The obtained results (in different tests, such as depth-profiling, towing and stationary measurements) show good correlation with the data provided by a commercial probe. Although the device is currently a part of a more complex measuring platform and uses an axial spectrograph as detector, the output power measurement used and the simplicity of its conception allow us to conceive a closed, extremely compact set-up which can be in principle commercially competitive with existing sensors.

Wafer warpage can affect device performance, reliability and linewidth control in various processing steps in microelectronics manufacturing. Early detection will minimize cost and processing time. We have previously demonstrated an on-line approach for detecting wafer warpage and the profile of the warped wafer. The proposed approach demonstrates that the profile of the wafer can be computed during thermal processing steps in the lithography sequence. However, the approach is computationally intensive and information is made available at the end of the thermal processing step. Any attempts at real-time correction of the wafer temperature are thus not possible. In this paper, we proposed an in situ approach to detect wafer warpage and its profile midway through the thermal process. Based on first principles thermal modelling, we are able to detect and estimate the profile of a warped wafer from available temperature measurements. The proposed approach can be implemented on conventional thermal processing systems. Experimental results demonstrate the feasibility and repeatability of the approach. A 75% improvement in computational time is achieved with the proposed approach.

A one-port coaxial/cylindrical transition line is considered for the broadband complex permittivity measurement of civil engineering materials. Cylindrical samples of heterogeneous material with large aggregate dimensions (up to 25 mm) can be measured over a frequency range from 50 MHz to 1.6 GHz. The choice of this line technology results in the simplification of the sample machining and enhancement in the high frequency limit, in comparison to the classical coaxial line technology. From a mode-matching technique, the relation between the material complex permittivity and the reflection coefficient at the coaxial/cylindrical transition is obtained including axisymmetric higher order modes excited at the transition. Once the line is calibrated using a specific calibration kit, complex permittivities are retrieved from an iterative optimization procedure. Preliminary results obtained for a set of bituminous concrete samples with different porosities and natures of rock aggregates are shown.

The construction and evaluation of a novel instrument for measuring air permeability characteristics of fabrics at differential pressure levels up to 3 bar is described. Fabrics requiring such testing have reduced porosity, leading to low air permeability, and as a result, the use of standard air permeability testers is impractical. A pressure-related measuring technique is therefore employed in the new instrument, which also enables air permeability characteristics to be measured over a range of pressure levels. The instrument is also equipped for the measurement of distension of the test specimens, so that any variation of fabric air permeability with pressure can be investigated. It was demonstrated that the technique can still be applied, in conjunction with a low pressure transducer, for the measurement of fabrics having normal levels of air permeability.

Imaging techniques based on the principle of nuclear magnetic resonance (NMR) rank with the most advanced methods for studying chemical and biological properties of substances. Their universality makes them particularly suitable for use in a wide range of scientific branches. NMR has found significant application in medicine. The diffusion effects of water molecules in tissues reduce the magnitude of spin echo in NMR measuring methods. Based on the amplitude change in the image, the diffusion coefficients and their distribution in the specimen being measured can be calculated. MR images of an object weighted by diffusion coefficients require a defined sequence of gradient pulses and accurate knowledge of their time behaviour. Methods for measuring the diffusion coefficients require knowledge of the minimum length of leading and trailing edges, the defined magnitude of the magnetic field gradient being excited and also the symmetry of positive and negative gradient pulses (the zero integral of pulses of the same magnitude with opposite polarity). To determine the above characteristics of the time behaviour of gradient pulses of both polarities a simple measuring method was developed and experimentally tested on a tomograph with 4.7 T induction of the basic magnetic field. This method is based on the principle of measuring the instantaneous frequency of the MR signal in the presence of a gradient pulse subsequent to exciting a thin defined layer of the specimen under examination outside the gradient field centre.

The resonant properties of an alternating gradient field magnetometer (AGFM) of the Flanders design have been investigated as a function of temperature and sample magnetic moment. Measurements reveal that room temperature operation of the AGFM is prone to long-term stability problems, which are attributable to variations in ambient temperature, but as long as these variations in temperature are not severe, it is possible to work away from resonance and still reconstruct the m–H loop for a sample. Other variations of the resonant properties of the AGFM, observed when recording m–H loops, have been attributed to the twisting of the AGFM probe when measuring samples with permanent magnetic moments lying in the plane perpendicular to the applied field.

The paper describes an innovative idea for an ionizing particle detector. The principle is based on a latchup effect that is common in to-date CMOS technologies working in a radiation environment. In principle the detector can operate at room temperature, does not require a high voltage power supply and is intrinsically more tolerant to radiation effects than the common solid-state detectors. A latchup-based detector can be constructed using state-of-the-art technologies and could be applied for beam monitoring or as a heavy-ion selector. A prototype made up of discrete components is described and its rough sensitivity is exploited. Tests with daylight, electrons, via a current pulse generator and with a laser beam have proved that charge sensitivity of the order of 1 pC can be easily achieved.

We have built a versatile environmental simulation chamber capable of reproducing atmospheric compositions and surface temperatures for most of the planetary objects. It has been especially developed to make feasible in situ irradiation and characterization of the sample under study. The total pressure in the chamber can range from 5 to 5 × 10⁻⁹ mbar. The required atmospheric composition is regulated via a residual gas analyser with ca ppm precision. Temperatures can be set from 4 K to 325 K. The sample under study can be irradiated with ion and electron sources, a deuterium ultraviolet (UV) lamp and a noble-gas discharge UV lamp. One of the main technological challenges of this device is to provide the user the possibility of performing ion and electron irradiation at a total pressure of 0.5 mbar. This is attained by means of an efficient differential pumping system. The in situ analysis techniques implemented are UV spectroscopy and infrared spectroscopy (IR). This machine is especially suitable for following the chemical changes induced in a particular sample by irradiation in a controlled environment. Therefore, it can be used in different disciplines such as planetary geology, astrobiology, environmental chemistry, materials science and for instrumentation testing.

Moisture and soluble salts are the main causes of degradation of mural paintings, in particular, frescoes. Water is the 'driving force' of damage such as the detachment of the painted layer and the whitening of the painting due to the crystallization of salts (efflorescence). Indeed, the appearance of efflorescence itself is related to the alterations caused by moisture in the process of the evaporation of water through the surface of the wall. Early detection of the presence of moisture under the wall surface is therefore essential for avoiding such kind of damage. In this paper a non-invasive microwave system is described which allows the measurement of the moisture content and the detection of salts in frescoes and mural paintings. The system performs a sub-surface measurement with an investigated depth up to about 2 cm. The measurement system consists of an evanescent-field resonant sensor, a network analyser and a numerical code. The method has been validated by measurements performed on some reference materials with known dielectric characteristics. Several tests on moistened plaster samples, some of them containing calcium nitrate at different concentrations, have been performed in order to verify the effectiveness in quantifying the moisture and salt content. In situ investigations have been carried out by measuring both the moisture content and salt content on frescoes in several museums and churches. The preliminary results prove the usefulness of the method as a diagnostic tool for investigating the health status of frescoes.

Dielectric behaviour of bound water in grain was investigated through measurement of the dielectric properties of hard red winter wheat at microwave frequencies over a wide temperature range between –80 °C and +10 °C. Measurements were performed in free space between 2 and 18 GHz on two wheat samples of 15.7% and 23.6% moisture content, respectively. Cole–Cole plots reveal the existence of different distributions of relaxation times at different temperatures for the lower moisture sample. In contrast, the Cole–Cole plot of data corresponding to the higher moisture sample indicates the existence of a single distribution of relaxation times for all temperatures. Analysis of data obtained at a single frequency for each wheat sample shows that for both samples ε' and ε'' increase linearly with temperature with a characteristic slope change at around –20 °C, which might be interpreted as the freezing temperature of bound water in wheat.

The dielectric properties (components of the complex permittivity relative to free space) of ground hard red winter wheat of 11–25% moisture content were determined by dielectric spectroscopy measurements with an open-ended coaxial-line probe and impedance analyser over the frequency range from 10 MHz to 1.8 GHz at temperatures from 5 to 95 °C. Both the dielectric constant and dielectric loss factor, over the stated range of variables, decreased with increasing frequency and increased with increasing moisture content and temperature. Plots of the dielectric constant and loss factor in the complex permittivity plane revealed a linear relationship between the two permittivity components at frequencies above 1 GHz, but they showed nonlinearity at lower frequencies due to the increasing influence of ionic conduction as confirmed by Cole–Cole plots of the permittivity data.

Optical surface flatness evaluated by the discrete wavelet transform and grey level co-occurrence matrix (GLCM) is presented. A PC-based measurement system can be used to detect the interference fringe of an optical reflective surface captured from a Twyman–Green interferometer. Wavelet analysis and GLCM process associated with the entropy criterion appears to be a good method for recognizing automatically the flatness of the optical surfaces. Three-dimensional plots of the GLCM and surface deformation contour for various captured interference patterns of glass substrates have been compared and discussed. The parameter of entropy has been calculated from the GLCM and then can be used as an indicator for optical surface characteristics.

A multipoint diffraction strain sensor has been developed using a moiré interferometer with the novel added feature of whole-field strain determination. This unique feature has been implemented by simultaneous tracking of sampled wavefront diffracted from the component under test. In this sensor a high-frequency diffraction grating is bonded on the specimen, which is illuminated by two symmetric collimated laser beams, as in a typical moiré interferometer. The first orders of diffracted beams impinge on a CCD camera, via a microlens array. The lens array serves a dual purpose—to sample the diffracted wavefront and to focus the wavefront to a number of spots on the CCD. The deviation of the individual spots generated by both the beams is directly proportional to the normal strain and a component of the shear strain. Simultaneous strain measurement at more than a thousand points can be readily obtained and is demonstrated in this paper. This novel technique is expected to be very valuable in numerous industrial metrology applications.

A novel fibre optic sensor system has been developed for linear location of acoustic emission (AE). The demonstration of linear location of acoustic emission was conducted using a pair of serial multiplexed fibre coupler-based acoustic emission sensors in conjunction with a single light source. The simulated AE source, via a pencil lead-break test, was located to within ±5 mm.

For a long time the complete measurement of parts with complex structure and features has been a bottleneck of 3D measurement in reverse engineering. This paper presents a laser sensor that is mainly composed of three CCD cameras and four laser line projectors. With the principle of projecting laser planes from three directions and receiving the reflected stripes from the other three directions, the sensor can measure complicated parts from multi views. A concise approach for calibrating the sensor is proposed. The orientation of each laser plane is determined first. Then according to the orientation calibration points in the laser plane are generated as an array by controlling the CMM to let the laser plane intersect with a single-tooth target. The 2D coordinate system in the laser plane is established with the axes parallel to the line and row of the array. Thus the extrinsic calibration of the sensor is avoided. The 2D data in a laser plane can be obtained from a CCD image after the relationship between the laser plane and the CCD image is solved using the calibration points. Finally the 2D data in three laser planes are merged by transforming them into a world coordinate system. Experimental studies show that the developed sensor possesses good accuracy, and a complicated part can be measured as long as it can be accessed from at least one view.

3D spherical particle positioning is found by using a defocusing method that consists in measuring the central spot size of an experimental image from a particle and comparing its nearest value with a numerically calculated central spot size matrix array. The numerical calculations were carried out using the generalized Lorenz–Mie and the Huygens–Fresnel diffraction theories. On comparison, the experimental results are in good agreement with the predictions made with the numerical calculations and an error analysis is presented.

In order to increase the storage capacity and the density of near-field optical disk drives, a flying pickup-head has to allow a slider to fly at a stable height above the disk surface with the use of near-field optics. Since both the precision of the track pitch and the flying height are of a nanometre scale, it is necessary to increase the motion accuracy of the pickup-head. In this study, a piezoelectric bender is used as an actuator of the pickup-head, and two quadrant photodetectors are used to sense the pickup-head displacement and the rotating disk deformation. Based on an optical lever method that magnifies a small displacement, the flying height variation of the pickup-head above the disk is measured. Further results show that using the proposed measurement method in the real-time control of flying height is feasible.

This paper addresses a relatively simple method of measuring Young's modulus of electroplated nickel using an atomic force microscope. The thin layer of nickel to be measured is electroplated onto the tip side of an AFM silicon cantilever, whose Young's modulus and geometric dimensions are well defined from the manufacturer. The resonant frequency and quality factor of the electroplated AFM cantilever are measured by the tapping mode of the AFM and its spring constant is calculated using Sader's method. The spring constant of the electroplated cantilever is also calculated by using the laminar composite beam theory. Comparing two spring constants, Young's modulus of electroplated nickel is determined. The measured Young's modulus of nickel at the end of each plating step ranged between 148.04 GPa and 159.90 GPa with a relative uncertainty of 4.21%.

A mathematical model of a cavity ring-down (CRD) fibre amplified loop gas sensing system is initially proposed. A digital least mean square (LMS) adaptive filter is designed and demonstrated the reduction of the detection error due to an amplified spontaneous emission (ASE) noise which is induced by an erbium doped fibre amplifier (EDFA) in the CRD loop. The simulation results show that the detection error of 16 ppm is obtained.

Based on the analysis of the empirical mode decomposition (EMD) method in this paper, an improved arithmetic for local mean is presented in detail. In the new arithmetic, the local mean is obtained by connecting all window means of the signals between two successive extrema with the cubic spline interpolation. The new arithmetic is more time saving than that used in the conventional empirical mode decomposition method. The improvement of the new arithmetic is validated by decomposing simulating signals. Moreover, the improved empirical mode decomposition method is successfully applied in decomposing vibration signals of ball bearings and provides a new method to diagnose defects of ball bearings. In contrast to the traditional envelope analysis, the empirical mode decomposition method could decompose the signal into different frequency bands adaptively, and the central frequency and bandwidth of band-pass filter, which was used to extract the resonance vibration, should no longer be decided. The results show that the EMD method is powerful for defect diagnosis of ball bearings.

In the evaluation of novel scintillators, it is important to ensure that the spectrum of the light emitted by the scintillator is well matched to the response of the photomultiplier. In attempting to measure this spectrum using radioactive sources, it is found that so few photons are emitted per scintillation event that conventional spectroscopic techniques cannot easily be used. A simple photon counting technique is presented, using two photomultipliers operated in coincidence, the one viewing the scintillator directly, while the other views it through a monochromator. This system allows the spectrum to be measured without using specially cooled photomultipliers, intense radioactive sources or particle beams.

Precise measurement of luminous transmittance and haze of transparent media is increasingly important to the LCD industry. Currently there are at least three documentary standards for measuring transmission haze. Unfortunately, none of those standard methods by itself can obtain the precise values for the diffuse transmittance (DT), total transmittance (TT) and haze. This note presents a new apparatus capable of precisely measuring all three variables simultaneously. Compared with current structures, the proposed design contains one more compensatory port. For optimal design, the light trap absorbs the beam completely, light scattered by the instrument is zero and the interior surface of the integrating sphere, baffle, as well as the reflectance standard, are of equal characteristic. The accurate values of the TT, DT and haze can be obtained using the new apparatus. Even if the design is not optimal, the measurement errors of the new apparatus are smaller than those of other methods especially for high sphere reflectance. Therefore, the sphere can be made of a high reflectance material for the new apparatus to increase the signal-to-noise ratio.

A compact, low-cost, ultra fast, low distortion and low noise preamplifier suitable for data-acquisition applications utilization micro channel plate (MCP) detectors has been built with a new monolithic integrated circuit amplifier in two stages. The gain, bandwidth and noise are optimized to the MCP performance in order to achieve the best rise time, response and stability with minimal jitter. The use of commercial components makes this instrument very low in cost and easy to build.