Table of contents

Interest in the characterization of non-spherical and/or non-homogeneous particles has increased dramatically in recent years from a wide variety of application areas. For characterization by optical means, several essential elements are identified and briefly discussed, with some indication of their respective state-of-the-art. This article serves as an introduction to the special cluster of articles which follows in this issue, and attempts to put these contributions into perspective with respect to the overall problem.

The manner in which a particle scatters incident light is a function of the particle's shape, its size, its structure and its orientation with respect to the illumination direction, as well as to the wavelength and polarization state of the light. The spatial intensity distribution of scattered light thus contains information by which the particle may often be classified or even identified. This paper briefly reviews the development of spatial light scattering analysis as an analytical and monitoring tool now finding application in areas as diverse as environmental monitoring, aerosol and hydrosol research, microbiology and fine-powder characterization. The paper concludes with a practical example of the use of spatial scattering analysis to achieve real-time discrimination of a specific particle type, namely airborne asbestos fibres.

The boundary matching method, a mathematical procedure for determining the electromagnetic fields associated with the interaction of monochromatic light with irregularly shaped particles, is used to calculate single-particle far-field scattering efficiencies and nondimensionalized far-field scattering intensity distributions for plane wave illuminated particles of various geometries and of both homogeneous and layered compositions. Particle geometries considered include prolate spheroid, oblate spheroid, and axisymmetric `corrugated' particles with radial shape functions of , where n = 2, 4, 6 and 10. Results are presented for an equivalent sphere volume size parameter of 20. The calculations demonstrate that even relatively modest deviations from spherical geometry (e.g. for the n = 10 corrugated particle) can result in significant differences in far-field scattering, particularly in the backscatter direction.

The phase-Doppler technique, allowing one to measure velocities and sizes of individual spherical particles in moving flows, is a very popular technique. Nevertheless, the bulkiness of the device and the need for accurate adjustments still impose restrictions in use in a large number of industrial situations. We present here a miniature phase-Doppler probe in which the emitting and collecting units are assembled in a compact way. The performance of the device is numerically and experimentally demonstrated.

We discuss several approximate methods for computing electromagnetic scattering by objects of complex shape. Depending on the relative size of the scatterer compared to the incident wavelength, different techniques have to be designed. We review some of them which are often used in practice, and present numerical results of scattering by complex objects.

Multi-angle azimuthal measurements of light scattering by individual submicrometre particles have been made with the DAWN-A multi-angle light scattering (MALS) instrument. Variability in azimuthal scattering at a polar angle of has been used to determine spherical and nonspherical fractions of laboratory-produced and atmospheric aerosols classified at mobility diameters of 0.2-m (size parameters of 1.3-5.1). Parameters used for shape fractionation were derived empirically by comparing scattering signatures from crystalline sodium chloride cubes and spherical dioctyl sebacate (DOS) droplets and then applied to test aerosols and atmospheric aerosols sampled at low (3-10%) and high (50-80%) values of relative humidity. DOS and NaCl contrast was great enough to permit their distinction with less than 5% uncertainty for 0.4-m, but contrast was observed to decrease as the size parameter approached unity. Crystalline ammonium sulphate test aerosol appeared mildly aspherical, with nonspherical fraction values in the range 15-40%. Nonspherical fractions of atmospheric aerosols measured during the Southeastern Aerosol and Visibility Study (SEAVS), conducted during July and August of 1995, were generally small (<10%), providing confidence in our application of Mie theory for determination of aerosol optical properties. DAWN-A shape data have been compared with aerosol chemistry and hygroscopicity measured during SEAVS, revealing an overall strong correlation of nonspherical fractions to soil dust content and `less hygroscopic' fractions.

A method to obtain the diameter of inhomogeneous droplets by using phase-Doppler anemometry (PDA) is described. Inhomogeneities included in a fluid droplet disturb light scattered by the droplet. Consequently the phase difference measured between two Doppler bursts deviates from the value calculated by geometrical optics theory for a droplet of diameter d. Because evaluation of the measured phase differences is based on a linear relationship between phase difference and particle diameter the accuracy of PDA is deteriorated. The difference between the real and the measured diameters of such droplets can be characterized by convolution with a Gaussian probability function. For monodisperse droplets and for a given type of inhomogeneity the standard deviation of the Gaussian function increases linearly with increasing mass concentration of these inhomogeneity. Applying infrared laser-PDA (IR-PDA) with a wavelength of 1312 nm, the measuring range to investigate droplets of, for instance, water-latex suspensions containing 450 nm polystyrene spheres can be extended to suspensions with six times the measurable concentration standard PDA using a He-Ne laser at nm. The relative improvement obtained by applying IR-PDA (in comparison with a standard PDA system equipped with a 632.8 nm laser) can be estimated by the decrease of the extinction efficiency of the inclusions with laser wavelength. The linear relation between extinction E and standard deviation can be used by a blind deconvolution algorithm. Starting from the measured diameter distribution of the inhomogeneous droplets the real diameter distribution and the standard deviation of each size class of the particles can be calculated by the algorithm. From this standard deviation it is possible to determine the extinction E and the mass concentration of the inhomogeneities.

A new light scattering instrument is described for the measurement of airborne mineral fibres. The measuring principle is based on the evaluation of the azimuthal light scattering pattern generated by single particles which are illuminated by circularly polarized laser light. The instrument uses eight different optical detectors placed at a polar angle of and in an azimuthal angular regime from to . The variability in the azimuthal light scattering pattern can be used to measure the fibre length whereas the fibre diameter can be derived from the intensity averaged over the corresponding values of the eight detectors. Detector responses are calculated on the basis of the boundary value method (van de Hulst solution) in order to show the theoretical resolving power of the instrument with respect to size and length. Measured data from polystyrene latex spheres (PSL), sodium chloride particles, quartz particles and glass fibres are presented. The differently shaped particle types were used to demonstrate the ability of the instrument in distinguishing between fibrous and nonfibrous particles. The evaluation of fibre diameter and length from measured values using calculated calibration data is shown for several samples.

This paper reports size measurement of droplets with optically inhomogeneous media by shadow Doppler velocimetry (SDV), which can provide spatially and temporally precise in situ readings of the size and velocity of a single particle with irregular shape and with arbitrary optical properties of the particle medium. In this work, water, instant coffee solution and water-based paint with various solid contents were measured to evaluate the capability and limitations of the measurement. The experiment with instant coffee solutions of 2 and 5% (wt:wt), which contained m discrete particles, atomized by a standard paint spray gun, demonstrated that the accuracy of sizing was not affected by the optical properties of the medium. Insensitivity to the optical properties is one of the primary advantages of SDV over the other optical, single-particle sizing methods. As a further demonstration, paint samples atomized by the same gun containing solid flakes of nominal diameter m were also measured. The results revealed a spatially uniform arithmetic mean diameter of m and suggested that the atomization characteristics were influenced in the highest flake volume fraction case (red paint containing mica, 1.5%) with the result that the mean diameter was 20% larger than that of a similar paint with a smaller flake volume fraction (green paint containing aluminium, 0.4%). It was also found that the measurable number density is limited to no more than 1000 droplets in the case of droplets with an average size of m.

A regression analysis method is applied for data transforming and error evaluation in the sine-approximation method (SAM) for the calibration of vibration pick-ups by laser interferometry. A new simple algorithm for phase unwrapping in the SAM is also proposed. To make a more valuable comparison of this modified SAM with other techniques, i.e. the fringe counting method (FCM) and the minimum point method (MPM), the resolution of the measuring system is increased by using a double optical path difference homodyne interferometer. Additionally the uncertainty of the FCM is suppressed by implementing the procedure of fringe phase analysis in the turning points of the vibration cycle. This paper presents a theoretical analysis of the above mentioned problems.

The regression analysis method applied for data transforming and error evaluation in the sine-approximation method (SAM) for the calibration of vibration pick-ups is experimentally verified. The new algorithm used for phase unwrapping in the SAM has also been examined. Detailed procedures for the analysis of data obtained from the SAM, the modified fringe counting method (FCM) and the minimum point method (MPM) are presented and a comparison of measurements taken has been carried out. The uncertainty of the FCM, suppressed by implementing fringe phase analysis in a single vibration cycle, is evaluated experimentally. The influence of distortions of the modulation phase signal is shown. The error budgets for all applied methods are given and compared. Expanded measurement uncertainties of about 0.2% for acceleration measurements and about 0.5% for sensitivity measurements (with a 99% confidence level) have been obtained.

A scanning focus-sensing (SFS) method has been developed for measuring the surface shape of a fish scale. This method uses an acousto-optic deflector as a diffractive scanner to allow the laser beam to scan an objective lens, and then uses the resulting focused spot to scan the sample surface. The focus information contained in the laser beam scattered from the sample surface is then transformed into the movement of the spot on a position sensor for detection. The measurement error caused by variations in the shape or intensity of the spot can therefore be reduced. We successfully applied the method to measure the surface shape of a fish scale.

The dependence of the reflectance of standard aluminium mirrors on the polarization status of the incident light has been calculated and measured for an angle of incidence close to normal , so that the systematic uncertainty introduced by not controlling the light polarization status in spectrophotometric measurements of specular reflectance can be known and then eliminated or minimized.

The development of a fibre optic sensor for embedding in cementitious composites and the measurement of displacements associated with the opening of microcracks are described. The sensor can be employed as a transducer for the measurement of crack-tip opening displacements during fracture tests. A white light fibre optic interferometer was used to sense strain in a fibre embedded in a specimen concrete beam. A direct relationship between the fibre deformation and the optical path variation of the fibre sensor has been obtained. A calibration procedure was developed by which the optical signals were converted into displacements. The sensor was employed in a series of fracture tests and the experiments involved embedment of the optical fibre in concrete beams. Specimens were centre-edge-notched and the fibre optic sensor of the gauge length L was embedded at the tip of the notch. This arrangement allowed for the direct measurement of displacements associated with the opening of microcracks at the crack tip. Experimental results are presented and crack-tip opening displacement (CTOD) results are compared with crack-opening displacements (CODs) measured by conventional transducers at the crack mouth.

A novel laser-based technique for the measurement of strain rate and vorticity in transparent fluid flows is presented. The technique is based on the collection and direct heterodyning of coherent light scattered from individual seed particles in two adjacent locations and, thereby, allowing the determination of a component of the strain rate tensor. The beat frequency of the heterodyned light, which is proportional to the velocity difference of the two points, is analysed using a conventional LDA processor. The angle between the laser beam and the direction of the scattered light determines the measured component of velocity. Therefore, a component of the vorticity vector can be measured by using two sets of transmitting and collecting optics, focused at a single location, along with two LDA processors. The basic principle of the technique is verified through measurements on a thin rotating wire and in a laminar flow between two concentric cylinders with excellent results. In the latter experiment, the measured velocity differences between two radial locations in the flow agree very well with the exact solution.

We describe a system for the measurement of the surface resistance of high- superconductors (HTCSs) at 48 GHz as a function of the temperature, the external magnetic field and its orientation with respect to the crystallographic axes of the sample. The set-up is based upon a resonant cavity. Thin films, bulk ceramics and single crystals can be accommodated in the cavity. A geometry in which there is no c axis microwave current flow, or c axis microwave field penetration, is chosen, so that unwanted effects depending on the anisotropy of HTCSs are ruled out. Geometrical factors for samples smaller than the cavity endplate are calculated. Some measurements on HTCS thin films and single crystals are presented, as well as some of the items of physical information obtainable from the measurements.

Optimal receivers are widely used in many applications and especially in military radars. In this paper, a correlation receiver connected to a pseudo-random sequence coded radar waveform is presented. This system is an original solution dedicated to a collision avoidance radar system for road vehicles. Experiments have been carried out at 10 GHz to show the efficiency of this method. The advantages of the system are its low cost, good resolution and low emitted peak power.

The design and operation of a simple electronic circuit called a TTL pulse generator (TPG) has been described in the present design note.

1998 February 25-26 5th International Symposium on Gas Analysis by Tunable Diode Lasers, Freiburg, Germany Details: VDI/VDE-GMA, PO Box 10 11 39, D-40002 Dusseldorf, Germany. Fax: +49 211 6214 215, WWW: http://www.vdi.de/pr/text/597/16697.htm March 31-2 April Second Scientific Instrument Association Conference and Exhibition, London, UK Details: SiA, PO Box 192, Watford, Herts WD1 8GF. WWW: http://www/interlab.co.uk/interlab 31-2 April Fifth International Conference on Adhesion and Surface Analysis, Loughborough, UK Details: D M Brewis, Institute of Surface Science and Technology, Loughborough University, Loughborough LE11 3TU, UK. Fax: +44 (0)1509 223986, e-mail: d.m.brewis@lboro.ac.uk April 6-8 Modeling and Simulation of Microsystems, Semiconductors, Sensors and Actuators, Santa Clara, CA, USA Details: MSM98 Conference Secretariat, 2394 Alsace Court, Livermore, CA 94550, USA. e-mail:msm@dmtsun.epfl.ch, WWW: http://www.iamcm.org/MSM98 (USA), WWW: http://dmtwww.epfl.ch/MSM98 (EUROPE) 16-17 Optical Methods and Data Processing in Heat and Fluid Flow, London, UK Details: Mrs U Otuonye, Conference and Events Department, Institution of Mechanical Engineers, 1 Birdcage Walk, London SW1H 9JJ. Telephone: +44 (0)171 973 1304, fax: +44 (0)171 222 9881, e-mail: u_otuonye@imeche.org.uk May 4-5 Metrology for Electronic Packaging and Interconnection, NIST, Gaithersburg, MD, USA Details: Michael Schen, NIST, Building 224, Room B320, Gaithersburg, MD 20899-0001, USA. Telephone: +1 301 975 6741, fax: +1 301 975 3239, e-mail: michael.schen@nist.gov June 14-17 8th European Symposium on Semiconductor Detectors, Schloss Elmau, Germany Details: http://www.hll.mpe-garching.mpg.de/sds98, e-mail: pxh@mpe-garching.mpg.de 14-19 9th CIMTEC Forum on New Materials: Solid State Chemical and Biochemical Sensors, Florence, Italy Details: 9th CIMTEC Forum on New Materials, PO Box 165, 48018 Faenza, Italy. Telephone: +39 546 22461, fax: +39 546 664138 16-19 3rd International Conference on Vibration Measurements by Laser Techiques and Short Cource on Laser Vibrometry, Ancona, Italy Details: Professor E P Tomasini, Dipartimento di Meccanica, Università di Ancona, Via Brecce Bianche, I-60131 Ancona, Italy. Telephone: +39 (0)71 2204489, fax: +39 (0)71 2204813, e-mail: aivela@mehpl.cineca.it, WWW: http://www.unian.it/html/congress/aivela/aivela.htm 24-26 Fifth International Conference on Structures Under Shock and Impact, Southampton, UK Details: Liz Kerr, Wessex Institute of Technology, Ashurst Lodge, Ashurst, Southampton SO40 7AA. Telephone: +44 (0)1703 293 223, fax: +44 (0)1703 292 853, e-mail: wit@wessex.ac.uk, WWW: http://www.wessex.ac.uk July 7-9 International Microscopy Conference and Exhibition, London, UK Details: Royal Microscopical Society, 37/38 St Clements, Oxford OX4 1AJ, UK. Telephone: +44 (0)1865 248768, Fax: +44 (0)1865 791237, e-mail: meetings@rms.org.uk (for conference), info@rms.org.uk (for exhibition), WWW: http://www.rms.org.uk 13-15 2nd European Conference on Magnetic Sensors and Actuators, Sheffield, UK Details: Conferences, The Institute of Physics, 76 Portland Place, London W1N 3DH. Telephone: +44 (0)171 470 4800, fax: +44 (0)171 470 4900, WWW: http://www.shef.ac.uk/~phys/emsa98.html http://www.iop.org/Confs/EMSA 13-16 9th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal Details: M V Heitor, Instituta Superior Técnico, Mechanical Engineering Department, 1096 Lisboa Codex, Portugal. Fax: +351 1 849 61 56 August 24-28 11th International Conference on Experimental Mechanics, Oxford, UK Details: BSSM, Surrey Technology Centre, Surrey Research Park, 40 Occam Road, Guildford GU2 5YG, UK. Telephone: +44 (0)1483 295 715, fax: +44 (0)1483 573 704 September 29-1 October 4th Optical Fibre Measurement Conference, Teddington, UK Details: Mrs Jane Tsakalou, Conference Office, National Physical Laboratory, Teddington TW11 0LW, UK. Telephone: +44 (0)181 943 5400, WWW: http://www.npl.co.uk/npl/news/ofmc97.html October 11-16 October 6th International Conference on Nuclear Microprobe Technology and Applications, Cape Town, South Africa Details: Mrs G Stone, National Accelerator Centre, PO Box 72, Faure 7131, South Africa. Telephone: +27 21 843 3820, Fax: +27 21 843 3543, e-mail: icnmta98@nac.ac.za, WWW: http://www.nac.ac.za/icnmta98 December 8-11 2nd Asian Meeting on Ferroelectrics, International (AMF-2), Singapore Details: Weiguang Zhu, fax: +65 792 0415, e-mail: ewzhu@ntuvax.ntu.ac.sg 1999 May 17-21 3rd CCM Conference on Pressure Metrology from Ultra High Vacuum to Very High Pressures (10-7 Pa to 109 Pa), Torino, Italy Details: M Bergoglio, Istituto di Metrologia `G Colonnetti', Strada delle Cacce 73, 10135 Torino, Italy. E-mail: bergoglio@imc.to.cnr.it