Journal of Physics E: Scientific Instruments

The rigorous expression for the transmission of a thin absorbing film on a transparent substrate is manipulated to yield formulae in closed form for the refractive index and absorption coefficient. A procedure is presented to calculate the thickness to an accuracy of better than 1% with similar accuracies in the values of n. A method to correct for errors due to slit width is also given. Various formulae to calculate the absorption coefficient accurately over almost three orders of magnitude are discussed. Only data from the transmission spectrum are used and the procedure is simple, fast and very accurate. All formulae are in closed form and can be used on a programmable pocket calculator.

The technique of acoustic emission (AE) uses one or more sensors to 'listen' to a wide range of events that may take place inside a solid material. Depending on the source of this high frequency sound, there are broadly three application areas: structural testing and surveillance, process monitoring and control, and materials characterisation. In the first case the source is probably a defect which radiates elastic waves as it grows. Provided these waves are detectable, AE can be used in conjunction with other NDT techniques to assess structural integrity. Advances in deterministic and statistical analysis methods now enable data to be interpreted in greater detail and with more confidence than before. In the second area the acoustic signature of processes is monitored. In the third area, AE is used as an additional diagnostic technique for the study of, for instance, fracture, because it gives unique dynamic information on defect growth.

Presents a review of fluxgate magnetometry based on a survey of the literature and on the work of the author and colleagues. The theory is outlined and an evaluation of the merit of the different types of sensors is made based on the sensor geometry. The design of a second-harmonic feedback magnetometer is discussed with respect to sensor configuration, frequency response, noise, offset, temperature dependence and long-term stability. Examples of the parameters are compiled from the literature and some new results on the long-term stability are presented.

Describes the characteristics of three types of sensors currently in wide use: platinum resistance thermometers, thermistors and thermocouples. It then explains how the transfer characteristics of each sensor can be established using readily available temperature standards. The article then shows how these characteristics can be used to design signal conditioning circuits which convert the sensor output into a convenient voltage signal. Bridge circuits are necessary for the resistance thermometer and thermistor. It is possible to design a bridge with a nonlinear transfer characteristic which almost compensates for that of the thermistor. The design of an automatic reference junction circuit for a thermocouple is discussed. The article concludes by describing how a microcomputer can be incorporated into these systems as a signal processing element to improve accuracy. In the case of the thermocouple, the computer solves a quartic equation to arrive at an improved estimate of temperature.

A new calculation following traditional methods is proposed for deducing optical constants and thickness from the fringe pattern of the transmission spectrum of a thin transparent dielectric film surrounded by non-absorbing media. The particular interest of this method, apart from its easiness, is that it makes a directly programmable calculation possible; the accuracy is of the same order as for the iteration method.

A review of attempts which have been made to produce in vivo images of the spatial distribution of tissue resistivity is given. The collection systems for in vivo images and the methods for the reconstruction of such images are discussed. Some of the problems that remain to be solved for applied tomography are identified.

By reconsidering the basic principles of capacitance, and also clarifying some of the misconceptions about the principles of guarding, which were correctly described by Kelvin more than a century ago, it is shown how guarded multi terminal capacitor geometries can be designed, analytically precalculated and correctly measured using simplified transformer bridge systems with phase-sensitive detection. An overview is given of the archetypal multi terminal capacitor configurations as well as ways to modify them for practical sensor designs. Detrimental side-effects are discussed and rules for practical designs are given. The possibilities for combining bridge circuits with simultaneous and sequential measurements of complex sensors are described. Expectations for future developments, partly based on silicon microfabrication techniques, are discussed and a comparison of the major aspects of capacitive sensors with resistive strain gauges and inductive sensors is given.

The design and metrological applications of Fabry-Perot resonators in the microwave and millimetre regions of the electromagnetic spectrum are reviewed. The review opens with introductory discussions on open resonator principles and configurations, continues with an analysis of developments in theoretical and empirical resonator characterisation and concludes with a broad survey of recent microwave Fabry-Perot applications. These include dielectric, anisotropy, magnetic resonance, scattering and atmospheric measurements and gas refractometry and spectroscopy. Other applications discussed include quasi-optical filters and diplexers.

An electron-optical device has been constructed in which electrons originally emitted over 2 pi steradians from a region of small volume are formed into a beam of half-angle 2 degrees . The instrument makes use of a magnetic field that diverges from 1 to 10^-3 Tesla. The energies of the electrons parallelised in this way have been measured with a time-of-flight technique, giving energy resolutions as low as 15 meV. Electrons of energy 0-3 eV, formed in multiphoton ionisation, were used for these tests. The device can also act as an electron-image magnifier, giving a spatial resolution of a few mu m in the source plane. Detailed theoretical and computational results on the properties of the new apparatus are given.

A simple step-by-step procedure is described, which makes use of a He-Ne laser, a telescope and a conventional fluorescent lamp for the alignment of a Mach-Zehnder interferometer.

A new method of surface roughness measurement was developed for use in a production environment. This method employs a microcomputer-based vision system to analyse the pattern of scattered light from the surface to derive a roughness parameter. The roughness parameters were obtained for a number of tool-steel samples which were ground to different roughnesses. A correlation curve was established by plotting the roughness parameters against the corresponding average surface roughness readings obtained from a stylus instrument. Similar correlation curves were produced for different materials such as brass and copper. Surface roughness measurement was also performed for specimens immersed in oil, a condition similar to that of a production environment. Some observable trends were found. The proposed method provides a fast and accurate means for measuring surface roughness. Its repeatability and versatility compares favourably with other methods.

A model has been developed, and tested experimentally, to account for the apparent dependence of the effective cross-sectional area of pistons in gas-piston-gauge pressure standards on the particular gas with which the gauge is operated, in the absolute mode. The model treats a pressure drop or loss that depends on the pressure difference across the gauge (P₁-P₂), the fall rate of the piston, the density of the gas and the viscosity of the gas. The model was tested using helium, neon, argon, nitrogen and krypton, and several values of (P₁-P₂). An algorithm was developed for calculating the piston cross-sectional area. The experimental results confirmed the model and resulted in a reduction in measurement uncertainty arising from this effect by approximately one order of magnitude.

The feasibility of using silicon diodes as the temperature sensors for thermoelectric power measurements below room temperature has been established. The advantages and disadvantages of diodes as compared to thermocouples for this purpose are discussed.

The propagation of thermal waves in thin needle-shaped samples is described. The waves are generated by Joule power oscillation at one end of the sample. The amplitude and phase of the temperature oscillation are dependent upon thermal diffusivity. At low frequencies no phase shift appeared and only thermal conductivity was involved in the measurement. Specific heat was evaluated on the basis of thermal diffusivity and thermal conductivity.

A method is described for making LiF constructions involving LiF-LiF and LiF-metal seals which are vacuum tight over a wide temperature range. As an application, rectangular LiF (001) surfaces have been assembled to form a flat channel which is connected to a copper vacuum flange. LiF machining was done using an ultrasonic technique. Mixtures of LiF and PbF₂ were used as sealing agents throughout. The entire construction has been proved vacuum tight in the temperature range 78 K to 700 K.

The author reviews the basic characteristics and progress in transducer properties of the different types of piezoelectric ceramics. The application of these materials in practical ultrasonic transducers is also reported.

We review the imaging properties and electrical applications o f scanning optical microscopes. We show that the choice of a scanning approach allows the modification of the optical system to give differential phase contrast imaging and confocal imaging. The latter has unique properties which permit the high resolution imaging and metrology of thick device structures. We also discuss the optical beam-induced current method of device, VLSI circuit and material testing and consider the factors affecting its resolution.

Describes the development of a multi-element hot-wire anemometry system designed to measure the fluctuating velocity and temperature in non-isothermal boundary layer flows. This system utilised only standard constant-temperature anemometers and commercially-available sensors. Although the maximum frequency response for the present system was limited by the available signal digitisation rates the probe-anemometry combination itself was shown to be capable of resolving signals to approximately 50 kHz. The performance of the system was evaluated by comparing velocity and temperature statistics measured in an equilibrium turbulent boundary layer flow with similar results from other investigations.

Analyses the nonlinearity of linearised thermistor bridge thermometers, discussing the various approximations used.

The minimum zone, minimum circumscribing and maximum inscribing references (here grouped as 'bounding references') used in the measurement of roundness are investigated. Their behaviour is studied by formulating their definitions mathematically as optimisation problems. The advantages of limacon references, both for accuracy under practical measurement conditions and for computational convenience, are investigated and the relationships between these and circular references are discussed. The formulation leads to the development of efficient algorithms (which can be given simple geometric interpretations) for determining these references. The approach adopted is also believed to give a new insight into the interpretation of data derived from roundness measuring instruments with radius suppression.

Measurement of diffusion constants using NMR usually requires the generation of a large, known, linear magnetic field gradient. The design and construction of two sets of coils providing orthogonal gradients are outlined. Their measured performance is shown, and applications and limitations to 'pulsed' diffusion experiments are discussed.

Optimum sampling and smoothing conditions for digital recording of spectral lines are derived for a system consisting of a spectrometer with CR-filter output, a sampling instrument for measurement of the spectrometer output and a processor applying a quadratic smoothing polynomial on to the sampled signal. Relevant experimental parameters for smoothing of the noise process, for the integral square error and for the signal-to-noise ratio will be discussed, and a set of practical rules to obtain efficient signal enhancement is given.

The design and metrological applications of Fabry-Perot resonators in the microwave and millimetre regions of the electromagnetic spectrum are reviewed. The review opens with introductory discussions on open resonator principles and configurations, continues with an analysis of developments in theoretical and empirical resonator characterisation and concludes with a broad survey of recent microwave Fabry-Perot applications. These include dielectric, anisotropy, magnetic resonance, scattering and atmospheric measurements and gas refractometry and spectroscopy. Other applications discussed include quasi-optical filters and diplexers.

Speckle is the peculiar appearance acquired by a diffusion object when illuminated by a laser beam. A laser-illuminated surface gives rise to a randomly distributed amplitude in any plane parallel to its plane. The real and imaginary parts of the amplitude at any point of the plane under consideration have zero mean value and the contrast of the speckle pattern is maximum. The autocorrelation function of the intensity distribution is proportional to the Fourier transform of the aperture limiting the surface, and the average size of a speckle in the considered plane is given by the radius of its Fraunhofer diffraction pattern. The results are applied to the optical processing of information, to speckle photography and to speckle interferometry.

The measurement of longitudinal and normal velocity fluctuations by sensing the instantaneous temperature in the wake of a hot wire is investigated and design parameters for such probes are presented. Manufacturing methods are discussed and at present allow maximum friction velocities of 30 cm s^-1 to be measured in air-the use of such probes being equally suited to liquids and to gases. Fundamental concepts underlying the method, estimates of sensitivity to calibration error, and measurements of turbulence quantities in fully developed pipe flow are presented.

A new technique of multicolour radiation pyrometry is described. The directional emissivity of the surface whose temperature is being sought is estimated from measurement of the surface reflectance factor. Temperature measurements of a tungsten strip lamp in the range 1180-1680K using this technique agree within 0.3% with those obtained from its spectral radiance temperature and published emissivity data of tungsten.

The requirements of a capacitance measurement system for the assessment of deep levels in semiconductors are considered. It is concluded that a transformer ratio arm bridge used in the deflection mode operating at 1 MHz with a signal level of a few tens of mV is the most appropriate for high sensitivity measurements. A bridge has been designed which has a good noise performance and this has been used to measure capacitance transients resulting from charge exchange in deep levels where Delta C/C is as low as 10^-6. The performance is compared with other systems and it is apparent that the bridge provides more than an order of magnitude improvement in sensitivity compared to the most appropriate commercial instruments and a similar improvement on other techniques reported in the scientific literature.