Table of contents

During the last decade, significant advances in the field of remote detection of ultrasound have taken place. Optical systems with increasing sensitivity for monitoring ultrasound on samples with optically rough surfaces have been developed, so that some designs are now moving from the research laboratory into industrial environments. In this review, a range of optical techniques is presented. These techniques are analysed regarding their use for the measurement of ultrasonic waves. They include the use of optical self-mixing, phase modulators and photorefraction-based interferometers. For the case of established instruments such as two-beam interferometers and Fabry-Pérot interferometers, various newly developed configurations with associated ultrasonic transfer functions are described. Many interferometers have broadband frequency responses that extend up to several hundred megahertz. Some systems offer in-plane ultrasonic measurement, whilst most offer out-of-plane measurements. Emphasis is placed on their potential for industrial applications, taking into account the likely sample-surface roughness and environmental vibration.

A theory combining both the pyroelectric and the piezoelectric photoacoustic methods is used to determine thermal diffussivity of a bulk superconducting YBCO sample at room temperature. We use polyvinylidene difluoride (PVDF) as the detector for the thermal waves. As PVDF has both piezoelectric and pyroelectric properties we find that the signal from PVDF detected using the lock-in amplifier is a combination of the piezoelectric and pyroelectric signals. Although both the piezoelectric and pyroelectric properties are found in the resultant signal at all the frequency ranges considered, we find that when the samples are thermally thick the piezoelectric contribution to the detected signal is more than the pyroelectric contribution, and vice versa when the sample is thermally thin. This behaviour of the combined signal is made use of in determining the thermal diffussivity of a thermally thick superconductor at low temperatures. The thermal diffussivity curve shows a dip at the resistivity transition onset temperature and a cusp at the offset temperature where the electrical resistance disappears. Thus, we find that the PVDF transducer is very effective in determining the normal-to-superconductor transition and also for the measurement of the thermal parameters of the superconducting samples at low temperatures such as thermal diffussivity, thermal conductivity and the specific heat capacity.

A centro-symmetrical, monolithic three-dimensional fine-motion stage was investigated in terms of its static and dynamic characteristics. The outer frame with the size of a 100 mm cube includes differently sized, concentric X/Y/Z linear motion stages. The yaw, pitch and roll of each stage are reduced to less than 0.014 arcsec µm^-1 by adjusting the position of the axial load. From three-axis laser-interferometer measurements, a uniaxial load led to linear displacement with the deviation along the other axes of less than 0.06% of 10 µm travel. A simple frequency spectrum for the impulse response without any resonance crosstalk other than the expected resonance peak was obtained for each axis.

We developed a methodology based on acoustic resonance for determination of the speed of sound in various liquids. A frequency sweep of an annular piezoelectric transducer performed using an impedance analyser reveals the structural resonance frequency. Upon immersing the tube in a liquid the impedance curve obtained from the frequency sweep has an additional resonance peak. The peak is the result of the acoustic resonance in the tube that feeds back to the piezoelectric structure. By using the fundamental acoustic mode in pipes and accounting for end corrections and for the compliance of the piezoelectric tube, a formula for the intrinsic speed of sound in a liquid was determined. Several liquids were tested and the speed of sound was obtained. The comparison of the values obtained with literature data showed that the difference was less than 3%.

A photoacoustic sensor system for automatic detection of low concentrations of water vapour is described in this paper. A Littman-configuration external-cavity diode laser operating at 1125 nm was used as a light source in combination with a high-sensitivity measuring photoacoustic cell, a reference photoacoustic cell and PC-controlled electronics. The system was calibrated with synthetic gas samples and a detection limit of 13 µmol per mol of water vapour was determined. Adsorption/desorption phenomena at the walls of the measuring photoacoustic cell were found to be an important limiting factor for the sensitivity of the system.

Two techniques are evaluated for the accurate measurement of the microwave permittivity of polycrystalline yttrium iron garnet (YIG) at frequencies between 5.5 and 12.5 GHz: split post dielectric resonator (SPDR) and ferrite disc resonator (Courtney). Both techniques separate YIG permittivity from that of YIG permeability by applying a magnetic induction bias to the YIG sample under test. The SPDR method needs no special sample preparation in the case of YIG substrates, whereas the Courtney method requires the grinding of rods from bulk YIG. The Courtney measurements of the YIG real permittivity are found to be higher on average than SPDR measurements. Agreement between the two techniques improves with increasing magnetic induction bias.

Establishment of appropriate vibration criteria is essential when designing vibration-sensitive metrology laboratories. Boundary values that are too severe may lead to unnecessarily high construction costs, whereas limits that are too broad may result in degradation of the performance of measurement equipment. The Norwegian Metrology and Accreditation Service (Justervesenet) inaugurated a new facility early in 1997. The facility will allow measurements of mass, density, dimensional, force, volume, optical, pressure, temperature and electrical quantities. Vibration control is of concern in most of the laboratories. Vibration criteria have been defined in terms of frequency-dependent peak values. In this paper, these criteria are described and the most conservative criterion is compared with other known vibration criteria for standard laboratories and high-technology facilities. The vibration criteria considered have different formulations and cannot be compared directly. They are therefore compared with regard to three different kinds of idealized vibration excitation, that is, transient, harmonic-motion and broad-band noise. The comparison shows that the most conservative Justervesenet vibration criterion is stricter with respect to high-frequency vibrations than are the others, but it is less strict for low-frequency vibrations.

The problem of the closest packing of Nequal nonoverlapping circles on a sphere has been of interest in geometry, chemistry, biology, engineering and optimization. The problem of packing Nequal nonoverlapping circles on a hemisphere is a different problem of the same type and is also of interest in various fields such as detecting signals in a multisource neighbourhood and measuring solar radiation. The paper introduces a method for locating Nequal nonoverlapping circles on a hemisphere arranged along parallel rings. The method provides information on the elevation and orientation angles of the circles and circle size; and calculates the surface coverage by the circles. The circles may represent the view angles of detectors arranged on a hemisphere. The solution is given for 2-40 detectors.

This paper introduces a new coordinate measuring machine (CMM) comprising a parallel kinematic mechanism with three spatial degrees of freedom and describes its structure, measuring theory and characteristics. Compared with the conventional CMM, this kind of CMM has a simple structure, a flexible probe posture, low moving errors, high stiffness and minimal deformations etc. In this paper, the measuring model of the new parallel CMM is established according to the theory of the spatial mechanics and verified by computer simulation. This research offers a theoretical basis for developing new CMMs.

In this paper we describe a new design for an optically pumped tandem magnetometer situated at the GeoForschungsZentrum Potsdam. A tandem magnetometer combines the fast response of a self-oscillating vapour magnetometer with the accuracy of a narrow line M_z-type magnetometer. A newly patented method of coupling the two sensors avoids any stray magnetic fields and so allows a compact design of the instrument itself, as well as facilitating its operation in close proximity to other magnetometers. A prototype Cs-K tandem magnetometer for use in magnetic observatories is described. We then show typical results of a long-term comparison with both a second type of optically pumped magnetometer and an Overhauser proton magnetometer. Finally, a resuméis given of four years of continuous operation of this new type of magnetometer with respect to the data quality produced and its operational reliability.

This work demonstrates the feasibility of detection of air bubbles in ducts using PVDF transducers. The transducers are easy to realize and the matching of acoustic impedance between the PVDF and the tube can be achieved with simple means. The modelling of the acoustic part is in good agreement with the experimental results and can be used to design sensors of the same type but using different materials for the support and tube.

We assembled an experimental arrangement for the measurement of the velocity field of glycerol suspensions, in the diametric plane, inside glass capillaries with internal diameter of the order of 200 µm. Glass spheres of mean diameter of 10 µm were used to seed the flow. The velocity fields were determined by the particle image velocimetry (PIV) method, whereby, in contrast to the usual implementations of this method based on light-sheet imaging, a forwards-scattering technique with the entire volume flow illuminated was used and the plane of interest was determined by the objective lens of a microscope. Statistical analysis of acquired experimental results was performed and possible limitations of the proposed technique were investigated. The method, with its present implementation, was used to measure velocities up to 4 mm s^-1 and seems to be promising for similar measurements in glass capillaries or in microcirculation.

The detection of inclusions in hot melts is of great importance to the manufacturing process as every aspect of quality is affected by the presence of second phases. However, the major dilemma is how to quantitatively determine the level of inclusions. In this paper, we present a theoretical model and preliminary experimental results of a Lorentz-force-based detection system to monitor small inclusions of micron size. The developed technique potentially has a better resolution performance than other on-line methods. The application area includes hot melts of metals and other highly conductive non-transparent fluids. Low-temperature modelling with liquid gallium was used to prove the measurement concept.

We present a magneto-optical method using Brown's relaxation in ferrofluids. This method permits one to make some viscosimetric measurements by putting a drop of miscible ferrofluid into a test liquid, glycerol or diethylene glycol (DEG). To test this method we have compared our results obtained for glycerol with those obtained from a more conventional method and we observed good agreement. After this validation, we were able to study the viscosity of DEG between -20 and +90 °C by using our magneto-optical method. The comparison of these results with those obtained in standard ways (in the temperature range from -10 to +20 °C) revealed very slight deviations.

We have constructed a low-temperature helium pressure device that incorporates optical-reflectivity detection and is useful for the study of thermochromic materials. We have tested the device up to 1600 bar (0.16 GPa) and down to 10 K. The performance of the device is illustrated by the results of constant-pressure and constant-temperature runs for the spin-transition solid [Fe(btr)₂(NCS)₂]·H₂O. The data establish a consistent pressure-temperature phase diagram, which is discussed.

Electrical capacitance tomography (ECT) is a so-called `soft-field' tomography technique. The linear back-projection (LBP) method is used widely for image reconstruction in ECT systems. It is numerically simple and computationally fast because it involves only a single matrix-vector multiplication. However, the images produced by the LBP algorithm are generally qualitative rather than quantitative. This paper presents an image-reconstruction algorithm based on a modified Landweber iteration method that can greatly enhance the quality of the image when two distinct phases are present. In this algorithm a simple constraint is used as a regularization for computing a stabilized solution, with a better immunity to noise and faster convergence. Experimental results are presented.

A simple technique to characterize the response of a SEM deflection system is described. By measuring the delay in the SEM video signal as the primary beam is deflected and swept over a knife-edge, the transient deflection response can be mapped out. The transfer function of the beam deflection is then approximated by exponential functions and appropriate analogue filters are designed to pre-emphasise the deflection coil drive signal in order to increase the deflection speed. This technique is used to decrease the deflection response time from typically ~6 µs to ~0.5 µs, thereby allowing the implementation of non-raster scanning schemes that require fast point-to-point deflection.

An automated instrument based on the beam-addition technique has been developed for measurement of the linearity of photodetectors. The system is designed for absolute characterization of a transfer standard photodetector, against which the linearity of other detectors can be measured. The measurement set-up has been made as simple as possible. A diode laser at 633 nm is used as the light source due to the good stability and high power output available with the device. Two measurement beams of 0.9 mm diameter are aligned to intercept on the photodetector. As an example, a silicon photodiode of the type S1227 has been studied and found to be linear within 3 × 10^-5 up to 7 mW of optical power.

This paper describes an investigation into the optimum design of optical fibre sensing arrays to be incorporated in an optical tomographic measurement system for on-line monitoring of particles and droplets. Two approaches are considered to cover opaque and transparent materials; optical path length and optical attenuation. Four flow models are investigated: single-pixel flow representing a single particle or droplet, two-pixel flow as a simple check on aliasing in the reconstructed image, half flow representing half the sensing cross section filled with material and full flow, where the whole sensing cross section is full of material. Six projection geometries of the fibre sensors are considered. For tomographic imaging, the forward problem, which assumes particles are placed in specific places in the measurement cross section and calculates voltage outputs for the individual sensors, is modelled. The solutions from the forward problem are used to solve the inverse problem, which uses actual sensor voltage readings to estimate the spatial distribution of the material in the measurement cross section. The solution of the inverse problem is used to derive the linear back projection (LBP) and filtered LBP algorithms. In order to improve image quality, a hybrid reconstruction algorithm is implemented. This algorithm first checks if any sensors read zero and sets (locks for this estimation) all pixels associated with them to zero (no material). The algorithm then proceeds as for the LBP.

The distribution of virtual current in an electromagnetic flow meter is an important aspect in the theory of electromagnetic-induction flow measurement. In this paper, the virtual current in the two-dimensional domain with a number of bubbles is obtained by a semi-analytical method. The bubbles can be of any number, any radius and any position within the domain as long as they are circular and do not overlap. An example is given for the three-bubble case. The calculation method and the numerical accuracy are shown. The extension of the method to the three-dimensional problem is also discussed.

A double whole-field filtering technique to measure pure twist fringe distribution of a deformed object is proposed. In this method, the slope fringes which are contained in the pattern of twist fringes can be completely eliminated from the twist fringes. Thus a pattern of pure twist fringes with a good contrast can be obtained. Theoretical analysis and experimental results are given in this paper.

We report an elegant small-signal capacitance measurement system for capacitive immunosensors, prototyped around a computer-controlled two-phase lock-in amplifier. Capacitive immunosensors require low-ionic-strength electrolytes for effective antibody-antigen interaction and low dielectric thickness for high sensitivity to phenomena occurring at the insulator-electrolyte interface. The problem of measuring a high intrinsic sensor capacitance in the presence of a modest parasitic cell resistance is overcome by using a potential-sensing electrode close to the electrolyte-dielectric interface for measuring and controlling the stimulus to the intrinsic device. The current phasor through the capacitive sensor is measured after effecting potential control via a software feedback loop and the sensor capacitance calculated. Capacitance can be measured in the range 100 pF to 0.1 µF with an accuracy of ±0.5% at 1 kHz. The technique obviates the need for expensive high-speed potentiostats or impedance analysers for such measurements and can also be used for small-signal admittance measurements in other electrolyte-insulator-semiconductor or electrolyte-insulator-metal systems. The suitability of the proposed measurement system for immunodetection is shown by capacitance measurements on fibronectin-sensitive capacitive immunosensors.

The customary model, that is, bsin = m, for the measurement of the sizes of slots and thin wires by optical diffraction has been used widely for a long time. In this design note, the model is analysed theoretically and experimentally and improved to the new model, btan = m. Two calibrating slots and thin wires (thin cylinders) are measured by means of optical diffraction. The results show that the new model btan = m is more suitable for describing optical diffraction phenomena and more accurate for measuring a thin wire's diameter.

The design and performance of the Astrid-2 magnetometer are described. The magnetometer uses mathematical routines, implemented by software for commercially available digital signal processors, to determine the magnetic field from the fluxgate sensor. The sensor is from the latest generation of amorphous metal sensors developed by the Department of Automation at the Technical University of Denmark.

For the last five years, Measurement Science and Technology has awarded a Best Paper prize. The Editorial Board of the journal believes that awarding such a prize is an opportunity to say `thank you' to authors for submitting their best material, and serves as a focus for their on-going quality review. I am delighted to announce that the 1998 award is presented for the article:

D T Read (National Institute of Standards and Technology, Boulder, CO, USA) Young's modulus of thin films by speckle interferometry9 676 - 85 (published April 1998).

The award, comprising a cash honorarium and certificate, will be presented to the author at a suitable venue in the near future. In making its recommendation, the Award Committee gave the following endorsement:

`This paper used a novel mode of electronic speckle pattern interferometry to examine in-plane elastic displacements in thin film samples. Miniature samples were composed of four different types of material, such as copper film. The samples were less than 1 mm long, typically about 1.0 µm thick, and 250 µm wide. In the paper, the author began by explaining speckle interferometry and its use for displacement measurements. He described phase shifting techniques and difficulties with measurements in such small samples.

After producing speckle images by photomicroscopy, he provided a clear account of difficulties encountered when attempting to create displacement fringes by the classic ESPI technique of subtracting speckle images. Such a pattern showed faint fringes, but they were unusable for measurement, being overwhelmed by the underlying speckle. Instead, the author adopted a novel process of monitoring the behaviour of an individual intense speckle when a force is continuously increased in the sample, within the elastic limit.

The paper went on to provide a clear exposition of data reduction and experimental uncertainties before reporting on some specific values of Young's modulus in thin films. He has, therefore, developed a measurement technique to provide valuable information on the effect of texture and other microstructural features within thin films.

The Committee considered that a non-contact optical technique was well suited to this type of problem. The paper was well balanced, putting the new contribution into perspective with previous work. Good references were made to previous work, and proper consideration was given to experimental uncertainties.'

Yet again the Committee found its task particularly difficult due to the large number of high quality papers published in the journal. We therefore felt that two further papers should be individually commended:

C Lopez et al (Vigo University, Spain) Fibre optic reflectometric technique for the automatic detection and measurement of surface cracks9 1413-31 (published September 1998)

F Winquist et al (Linkoping University, Sweden) Monitoring of freshness of milk by an electronic tongue on the basis of voltammetry9 1937-46 (published December 1998).

Nominations for the 1999 Best Paper Award are currently being accepted. If, as a reader, you feel particular papers are worthy of an award please let us know. If you are an author, the Editorial Board would like to encourage you to think of Measurement Science and Technology as the home for your best submissions.

Cam Tropea Honorary Editor August 1999