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 use of microstrip Blumlein pulse generators to produce nanosecond high voltage pulses is discussed: such generators can give higher output voltages than simple single-cable generators. Two versions of an inexpensive and easily-constructed circuit are described: the first uses and avalanche transistor as switch, producing fixed amplitude ( approximately 100 V) pulses of either polarity at kHz repetition rates: the second circuit uses a mercury-wetted reed relay as switch and gives 5 ns output pulses variable in amplitude from 0 and 4 kV, with stable amplitude, nanosecond rise and fall time, and very high reliability, with repetition rates up to 250 Hz.

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.

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.

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.

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 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.

The ladder network (transmission line) technique for measuring the specific contact resistance of ohmic contacts to semiconductors is analysed, and the results of the analysis compared with the common approximations made in analysing experimental data. It is shown that the method fails when the transfer length and the width of the potential-measuring contacts are comparable. A graph is presented which allows corrections to be made as the measurement limit is approached, and a table gives the limiting values of specific contact resistance that can be measured for specific ladder geometries and semiconductor sheet resistivities. The 'variable gap' structure is shown to be free of the limitations of the ladder network because it has no narrow, potential-measuring electrodes. Experimental data for gold-germanium contacts to gallium arsenide are given to illustrate the above results.

Dichroism is usually measured using two separate polarisation spectra. However, when the difference in the transmission of oppositely polarised light is very small, the superior method of modulating the polarisation of a single light beam should be used. The techniques involved in this method are reviewed.

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.