Table of contents

Diffraction gratings in which the grooves are formed by recording optical interference fringes are rapidly becoming established as alternatives to ruled gratings. In many respects they compare favourably with ruled gratings, but because of both the limitations and the novel features of the interference method of manufacture, a straightforward comparison is not always possible. Whether a ruled or interference grating is to be preferred will strongly depend upon the use to which it is to be put.

An ultrahigh vacuum apparatus designed for contamination-free quenching of foils is described. Using this system, a high density of dislocation loops and stacking fault tetrahedra has been produced in seven FCC metals including nickel, a metal in which widespread defect structures have not been reported previously. The quench rates attained were measured by two comparable methods and were found to depend principally on the thermal conductivity of the specimen material. The maximum rates range from 4000 degrees C s^-1 for Cu to 120 degrees C s^-1 for stainless steel.

A casting device which incorporates several novel features has been developed with a view to maximizing the cooling rate during the production of rapidly quenched metallic strips. The apparatus is described and it is shown, both experimentally and theoretically, that the attainment of the desired cooling behaviour required drawing during solidification.

A technique is described for welding uniform sized fine thermocouples, that are producible even by unskilled workers. Junctions welded using this technique showed a very uniform time response.

A water vapour laser has been used in humidity measurements as an intensive radiation source. The absorption coefficient of water was measured at lambda =27.97 mu m. Results are presented on measurements of water vapour in the outgoing smoke of a power plant.

An experimental method for obtaining wavelength modulation spectra using a small computer is discussed. The method has been applied to obtain the second derivative reflectance spectrum of anthracene crystal. The advantage and disadvantage of these wavelength derivative methods are compared with a mechanical derivative method which is commonly used.

There is an increasing demand for more accurate surface metrology instruments and for techniques which will allow complicated or large parts to be measured in situ. An analysis is presented of existing and new multi-orientation and multiprobe techniques and it is shown how they can be used to remove systematic and variable errors.

The application of error separation techniques to roundness measuring instruments is investigated experimentally, using digital computers. The accuracy of such techniques is substantiated by comparative measurements with two independent multi-orientation systems. Results obtained with these and other systems confirm that the errors of conventional roundness measuring instruments can be reduced by a factor of ten or more and meaningful polar graphs obtained at radial magnifications of up to one million. These advances in accuracy can be achieved in relatively poor working environments and with little extra skill on the part of the instrument operator.

From an original suggestion of Tolansky (1943) an interferometer system is described capable of high absolute accuracy in the measurement of small linear displacements. Because divergent light beams are used a great simplification may be made in the optical train and the use of a laser light source permits automatic fringe detection and recording against high background light intensities. An apparatus is described for the absolute measurement of the thermal expansion of copper to 1200K and it is pointed out that the apparatus is capable of measurements to still higher temperatures.

The design of a multi-manoeuvrable ultrahigh vacuum (UHV) crystal holder for a new soft X-ray spectrometer is described. The holder is capable of adjusting the position of the crystal by 80 mm in the z direction which is normal to the Rowland circumference of the spectrometer and 8 mm in the orthogonal x and y directions. Angular adjustment is allowed within a maximum range of 10 degrees . All adjustments are accurately reproducible and are made from outside the UHV system. The holder also makes provision for cooling of the crystal by a liquid or gas coolant.

A signal averaging system for fast signals appearing with a repetition rate up to 50 Hz is described. A television camera is used as a buffer memory to put these signals into a conventional, normal speed signal averager. Circuit diagrams of the most important electronic circuits are given and discussed in some detail. The performance of this low cost system was tested experimentally and found to meet expectations.

A laser Doppler velocimeter with a frequency preshift is used to measure low water velocities (usually 1 cm s^-1 or less). The use of a preshift removes uncertainty in the direct of movement of slow flows. Fluctuations in the preshift frequency normally vitiate accurate velocity determinations. In order to correct this a separate photodiode has been used to obtain an output at the preshift frequency, enabling an electronic mixer to eliminate this frequency from the frequency tracker input. In this way the main function of the method to register the difference in sign of velocities near zero is maintained. By reducing the effect of both drift and random fluctuation in the preshift, the accuracy of the equipment has been improved more than ten times.

Ion beam etching is applied to produce specimens less than 1 mu m thick with a free aperture of 4 mm in diameter. Plane specimens which have the same chemical composition and structure as the bulk material can be obtained. Infrared transmission spectra were obtained from fused silica samples Herasil and Suprasil W. The shapes of these transmission spectra are discussed. The absorption bands in the region 5-7 mu m have the same intensity for both glasses independent of the OH content of the silica. They are supposed to be overtones of the basic Si-O vibrations at 9.4 mu m and 12.4 mu m.

The authors have developed a technique to measure potential variations produced by space charge. The method consists in directing a beam of fast heavy ions through the region of interest and measuring the time of flight. The flight time is measured by intensity-modulating the beam and detecting the resulting phase shift between the collector and the ion source. Results of experiments performed using a simulator show that a direct reading can be obtained of potential changes of a few volts with a temporal resolution of about 10 mu s.

Modifications which have been made on the Rigaku-Denki low angle X-ray goniometer for studying weak scattering from solutions are described, and the positional stability of the goniometer during 2 theta stepping is discussed. A flow cell which permits solutions to be changed on stage and a variable temperature cell using cold air generated with a vortex tube are also described.

A circuit is described which varies the relative alignment of the mirrors of a repetitively scanned plane Fabry-Perot interferometer until the transmitted light intensity of the strongest spectral component is maximized. Using this scheme an interferometer can be completely stabilized against all drifts arising due to thermal and mechanical instabilities.

A null ellipsometer without a compensator is described that employs two mutually compensating reflections between a pair of non-parallel surfaces such that the planes of incidence for the two reflections are neither parallel nor perpendicular. The theory, design and operation of this proposed two-reflection null ellipsometer (TRNE) are discussed. The TRNE promises to be useful for spectroscopic applications, especially outside the visible spectrum, at wavelengths where compensators are either difficult or expensive to construct. When presumably identical surfaces are used, the TRNE executes measurements at two angles of incidence simultaneously.

A previously described (see ibid., vol.6, no.10, p.1045, 1973) oblique incidence interferometer has been used to measure the departure from flatness of a plane mirror 914 mm in diameter and 37 mm in thickness. The mirror was mounted on three different support systems and the change in figure of the mirror resulting from a change in the support system is shown to agree satisfactorily both with previous determinations for 300 mm diameter optical flats and with theory.

Describes experimental apparatus which enables the optical effects of thermal turbulence to be examined quantitatively. Preliminary results are presented for the RMS positional deviation in the detector plane of a laser beam propagating through liquid nitrogen in a known state of thermal turbulence, and are shown to be in general agreement with the predictions of an empirical theory. Continuing experiments examining the local structure of the turbulent medium are outlined.

The design, construction and operating characteristics of a versatile system for flash photolysis, photochemistry and laser pumping have been described. The system is of uncomplicated design and incorporates an elliptical optical pumping cavity, spark gap assembly and a charging and trigger circuit. The system allows both fast and slow flashlamp pulse operation with interchangeable lamps and near state of the art performance in short pulse ultraviolet flash operation.

A continuous mode device for generating mixtures of air and the vapour of a high boiling-point substance is described. Such a device is useful for calibration purposes in vapour trace detection work. Two streams of air, one containing the vapour and a second one diluting it, are mixed in an adjustable ratio so that low-level concentrations of the vapour in an air stream can be obtained. Concentrations can be kept constant over a wide range extending to the sub-PPB region. The vapour source has been used to generate the vapours of various explosives and pesticides; the results for ethylene glycol dinitrate, dinitrotoluene and fenitrothion are presented in this paper.

A technique is described for giving accurate measurements of cyclotron resonance and Faraday rotation in very high pulsed magnetic fields up to 130 T (1.3 MG). The megagauss fields were generated by the compression of magnetic flux with electromagnetic force in the theta -pinch configuration (Cnare (1966) effect). A tunable CO₂ laser was used for the infrared light source, and fast detection of the light was carried out with a high speed infrared detector. A simple method was developed for determining accurately the relative field sensitivity of pickup probes. Based on the calibration of the absolute field sensitivity of the probes by the Faraday rotation of CdS at 632.8 nm, the accuracy of the field intensity measurements reached +or-3%. The cyclotron resonance for electrons in Ge, GaAs and InSb was measured at room temperature.

A self-contained instrument for the measurement of surface tension is described. A ground glass rod is used to pull up a meniscus from a liquid and the adhesion tension is measured. The readout is given directly in mN m^-1 (or dyn cm^-1) with an accuracy of measurement of +or-0.1 mN m^-1.

An automatic control and recording system for use in low temperature calorimetry is described, in which all measurements are taken with a precision digital voltmeter. In addition to a general description of the control system and the computer program, some details are included of the design of programmable power supplies and a novel low level scanning switch. The system uses an 8k byte minicomputer which also handles the data analysis, but fully automatic operation could be realized with a much smaller minicomputer or microprocessor. Although the main purpose of the development was to automate a tedious experimental technique, small but significant improvements have resulted in the measurements from what has turned out to be a reasonably versatile system.