Table of contents

The principal characteristics of displays are discussed with reference to various technologies. Commonalities and contrasts in the ways in which different devices achieve colour or can be addressed are emphasised. Factors limiting the resolution of devices are touched on. The main part of the paper gives a review of liquid-crystal technology. Basic aspects are covered under the headings of 'phases', 'textures' and 'anisotropy'. Applications to display are treated under 'effects' and 'addressing'; the latter topic including optical addressing which leads into direct 'imaging' into liquid-crystal devices using thermo-optic effects or separate light-sensitive sub-components. Devices capable of image-wise modulation by optical addressing are discussed under the heading 'spatial light modulators'.

A method has been developed of distinguishing the spectral peaks detected as local maxima on a measured curve from the noise-induced artefacts when peak positions are not known beforehand. The method is based on a derivation of the distribution function for local maxima of the random signal component in the dependence on their relative heights. The resulting probability of a 'noise peak' being formed within a given number of experimental points determines the mean statistical authenticity of any detected maximum. Examples are given for white Gaussian noise, for this noise filtered by the Savitzky-Golay algorithm and also an application to Auger electron spectra.

The method and first results are presented for an experimental examination of the statistics of secondary electron emission detected within narrow intervals of energy and solid angle. It is shown that, at least down to 120 electrons per signal sample, the process is governed by the statistics of an ideal electron gas, i.e. the electron count behaves as a random variable with the properties of white Gaussian noise whose variance is approximately equal to the square root of the count.

A computer code for a comprehensive design of a magnetically focused long electron gun is presented. An optimal design of the magnetic-field generating coils for vessels within practical design constraints was investigated. Three such designs are presented. The three-dimensional model is verified experimentally on a scaled down model.

A simple pilot balloon theodolite was designed using an assembly of a camera mounted on a tripod and connected with a Apple II computer for fast and accurate data acquisition and analysis of wind velocities. An accuracy of 0.3 degrees for its two axes of rotation could be achieved and is comparable with that of a commercially available theodolite. This system provides continuous tracking up to a height of 8 km and is useful for the determination of low-level wind profiles.

A thermoelectric device has been used in the design of a microcomputer-based system for the simultaneous measurement of thermoluminescence and thermally stimulated conductivity in wide band-gap semiconductors.

The transient temperature response at the centre of a glass sphere was measured using a copper-constantan thermocouple. The error introduced by the high conductivity copper wire was virtually eliminated by winding the copper wire around the constantan. The theoretical and measured response were in good agreement.

A detailed design is given for an oscillator dissipating less than 10^-9 W in a tank circuit within a cryostat, which achieves a frequency stability of a few p.p.m. without the need to control the external circuit temperature. It is suitable for either susceptibility measurements or thermometry in the millikelvin region.

The development of a high-speed spark gap for grid-cathode protection of pulsed travelling wave tubes is described. Protection requirements are discussed, together with the interaction of the gap with the associated circuitry. From this it is possible to define the required characteristics of the protection gap. A description is given of how the physical mechanisms of gas discharges are utilised to obtain the desired characteristics.

A simple, inexpensive circuit for the accurate logarithmic compression of LIDAR signals is presented. The logarithmic amplifier features an automatic bias control that removes the effect of varying background signals produced by different sky brightnesses, thus allowing the backscattered LIDAR signal to be digitised more effectively. The bandwidth varies with the input current, and increases from 2.5 MHz for currents of 30 mu A to 6 MHz for currents greater than 500 mu A.

A torque magnetometer is described which is designed to operate in the 25 mm diameter, variable temperature (4.2-400 K), sample tube of a superconducting solenoid able to provide a field of up to 13 T. The existing instrument is able to measure torques up to 0.3 N m with a resolution of 10^-6 N m, and is controlled by a BBC microcomputer. Examples of torque curves measured on samples of sintered and powder Nd₂Fe₁₄B are given.

A high-Q confocal Fabry-Perot microwave resonator for precision spectroscopy of atoms, ions and molecules is described, giving convenient access to the overlap region of interacting species, microwave field and light. Microwave power is coupled with almost 100% efficiency into the interferometer by means of a small copper wire dipole antenna. The concept of an uncomplicated and inexpensive frequency stabilisation of microwave sources through coupling to a Fabry-Perot reference resonator is discussed.

A new pulse programmer designed and built for an NMR spectrometer is described. Constructed from TTL circuits with 2 kbyte of on-board memory, the pulse programmer provides 100 ns pulse width resolution with a 10 MHz clock. It is specially designed to be interfaced to any computer with parallel I/O lines. Output consists of four independent channels, four trigger lines and a dedicated acquisition line. Operation is independent of the host computer to ensure precise timing. In conjunction with the hardware, a complete software system, with a number of innovations, has been written in FORTRAN to control the pulse programmer and manage the NMR experiment.

A method is proposed for measuring waveforms of a time-varying field gradient used for high-speed NMR imaging. It utilises a self-encoding gradient which calibrates the gradient modulation to be measured. Results are obtained in the form of a time integral of the gradient waveform, which can be used directly for Tropper's image reconstruction. Spectroscopic one-dimensional imaging experiments have been performed using rectangular and cosine gradient modulationsl. It has been demonstrated that, in both cases, significant improvement can be achieved by making use of actually measured gradient modulations for the reconstruction. It has also been shown that in addition to Tropper's reconstruction, appropriate deconvolution is necessary to obtain images with less distortion in the case of cosine modulation.

A thermometric phase transition detector with several novel features is described. High differential sensitivity, in situ measurement of temperature, the small mass of mixture required ( approximately 1 mg) and its coupling to a polarising microscope make it very useful for the determination and identification of very weak enthalpy changes such as those found in the high-order transitions of liquid crystals.

A simple fixed-frequency microwave reflectometer installed on the JET tokamak device is described. Data from the reflectometer are combined with data from other JET diagnostics to obtain the radial distribution of the electron density during the rise and decay phases of the plasma. The profiles are compared with those obtained using a conventional multichord transmission interferometer, and good agreement is obtained. The reflectometer is used to study movements of particular density layers due to MHD and sawtooth activity during the equilibrium phase of the plasma. The limitations of the device are discussed and some possible future developments outlined.

A high-sensitivity differential interferometer has been developed to detect small density fluctuations over an optical path length of the order of the boundary layer thickness near transition. Two experimental configurations have been used to evaluate the performance of the interferometer: an open shear-layer configuration and a wind-tunnel turbulent spot configuration. In each experiment small temperature fluctuations were introduced as the signal source. Simultaneous cold-wire measurements have been compared with the interferometer data. The comparison shows that the interferometer is sensitive to very weak phase variations of the order of 10^-3 of the laser wavelength.

A hydrogen sensor has been developed in which the sensing element is a palladium wire, mechanically attached to a monomode optical fibre. In the presence of hydrogen, the palladium is converted to palladium hydride, with consequent changes in the physical dimensions of the wire. In turn, these dimensional changes produce a longitudinal strain in the optical fibre. The strain is transduced to a phase retardance in a light beam guided by the fibre, and which is recovered interferometrically. The sensor has been demonstrated in a nitrogen atmosphere at laboratory temperature. The phase retardance is proportional to the square root of the hydrogen partial pressure, and has been demonstrated in the partial pressure range 0-100 Pa, with an accuracy of +or-2 Pa.

A flammable gas sensor is demonstrated, in which the sensing element is a 100 mm monomode fibre, coated with platinum metal. The rate of the exothermic reaction of the hydrocarbon gases with oxygen is increased in the presence of a platinum catalyst. The resultant heat of the chemical reaction is transduced to a phase retardance in a fibre-guided light beam, and which is recovered interferometrically. The experiments have been performed in an oxygen (or air) atmosphere at laboratory temperature.

One of the major barriers to the application of medium-energy ion scattering (MEIS) to surface analysis has been the requirement for very high voltage, programmable power supplies to deliver the electrode potentials. This problem and associated size considerations have put the area of MEIS outside most researchers' resources. The development of a new form of electrostatic energy analyser is reported which allows substantial reductions in the size and voltage requirements for this form of analyser. These improvements will encourage the further application of MEIS to surface analysis.

An electron spectrometer is described for use in threshold photoionisation studies. The spectrometer incorporates a penetrating field extractor stage, a 127 degrees cylindrical deflection analyser and an electrostatic lens system to match the two together. The high collection efficiency, resolution, transmission and sensitivity of the spectrometer are demonstrated with threshold photoionisation measurements in argon and helium. The spectrometer is also able to handle non-threshold photoelectrons. The high performance and ease of operation of the spectrometer suggest that it could be used as a general diagnostic tool on synchrotron VUV and soft X-ray beam lines.

Optical properties, such as focal length, and defects of a symmetrical and parallel three-slit plane electrode lens are presented in diagrammatic form with aperture gap widths and applied potential differences as parameters. In calculating these properties as potential distributions on the optical axis, the analytical expression determined by G.D. Archard (1954) was applied. Calculated results show that the aperture gaps have considerable influence on the focal length, F, and spherical aberration coefficient, C_S, values, but little effect on the chromatic aberration coefficient, C_C. For example, typical values of C_SF^-1 and C_CF^-1 are of the order 4.2*10² and 2.5 respectively, for F=10 and infinite magnification, when the aperture gaps of the three electrodes are each equal to half the spacing between the outer electrodes.

The authors describe the design and performance of an optical instrument for neutrons of wavelengths larger than 1.2 nm, containing cylindrical neutron lenses and multislits. The achromatic optics resemble a slide projector with a magnification of one, and will be used in a neutron beam deflection experiment aimed at reducing the upper limit of a possible electric charge of the free neutron.

A curved neutron monochromator composed of 145 small Cu platelets is described. Focusing in the horizontal and vertical directions may be achieved. Compared with a flat monochromator an intensity gain of more than one order of magnitude is observed. A monochromator of this type is mainly used for diffuse scattering work.

A self-maintained vibrating reed chopper for molecular beam modulation in ultra-high vacuum is described. Taking the vibrating reed as one plate of a capacitor, the periodically changing capacity results in a voltage signal coherent with the modulation frequency. After amplification this signal is led as the driving voltage to a second capacitor, also having the reed as one plate. Thus the reed is maintained in resonant vibration. The device consists only of clean metal plates and some insulating mica. With a special material and the shape of the reed, amplitudes of 2-3 mm at a frequency of 200 Hz have been obtained.

A corona preionisation technique has been developed. The novel preioniser consists of copper-clad epoxy board with a slit aperture in the centre. The slit width is equal to the main electrode gap so that a rapid transverse gas flow can circulate sequentially through corona preioniser and main electrodes. A TEA CO₂ laser using this preionising board has been operated successfully with an output energy of 480 mJ per pulse. Repetition rates up to 100 Hz could be achieved.

A technique for measuring the 2D energy distribution of laser beams of a wide variety of sizes is described. A thermal imager is used to view the temperature distribution produced on a target when irradiated by a laser. It has many advantages over existing methods and in the case of low energy density expanded beams is the only practical way of profiling. The technique is applicable to the measurement of pulsed and CW lasers over a wide range of wavelengths.

The implementation of a new scheme for a thermistor anemometer for the measurement of both velocity and temperature in a two-dimensional horizontal flow is described. The anemometer is composed of a cluster of four bead thermistors, consisting of a central heater bead and three surrounding sensor beads. Empirical relationships for the dependence of flow direction on sensor/heater spacing, fluid flow speed, and fluid, heater and sensor temperatures are presented which permit in situ calibration of large numbers of thermistor anemometers. Fabrication details, calibration procedures and results of measurements from a large number of anemometers under actual experimental conditions in a thermally stratified, rotating fluid flow are included.

Describes a novel insert for a helium storage Dewar which allows temperatures of 1.25 K to be attained quickly and economically. The system is ideal for the rapid testing of small samples.

A simple inexpensive technique is outlined for minimising the periodic tracking errors common in small telescope drives. Details of the drive corrector and its operation, resulting in an order of magnitude improvement in the performance of the drive, are presented.

The authors report a new type of instrument, the Edinburgh Instruments Model 2997 differential fluorometer, for the measurement of the decay of fluorescence anisotropy. The instrument incorporates three multiplexed detection channels for simultaneous measurement of two polarised fluorescence decays and the excitation pulse, using time-correlated single-photon counting. This arrangement corrects automatically for temporal and intensity fluctuations of the spark source. A study of the probe 1,6-diphenyl-1,3,5-hexatriene (DPH) in a white oil reveals that the instrument can measure rotational correlation times with subnanosecond time resolution in a third of the time of conventional methods.