Robin D. Alcock

Optical imaging of shock waves produced by a high-energy electromagnetic transducer.

An electromagnetic acoustic transducer (EMAT) and acoustic lens have been constructed to provide reproducible and well characterized acoustic shock waves in water. The peak pressure amplitude variation for the unfocused and focused shock waves was found to be within +/- 5%. The focal region has been characterized both optically and by conventional piezoelectric pressure measurements. Optical investigations centre around ruby laser illuminated Schlieren and focused shadow imaging. The temporal resolution of the optical technique was determined by the ruby laser pulse length of 20 ns, giving a spatial resolution of 30 microns for a shock front in water.

A compact, high numerical aperture imaging Fourier transform spectrometer and its application

This paper describes a compact imaging Fourier transform spectrometer with high numerical aperture. In comparison with other optical arrangements in which extended interferometer paths are required for the inclusion of dispersion compensation optics, this technique utilizes a rudimentary cubic beam splitter based Michelson interferometer with minimal optical path so that the numerical aperture of the system is maximized. Mathematical modelling is presented showing that the fringe distortions caused by the dispersion in the cubic beam splitter can be entirely removed without any loss of the spectral information. An illustration of the power of the technique is given classifying between different plant foliage performed using a Fisher discriminant function based optimal linear filtering.

Holographic Particle Image Velocimetry and its Application in Engine Development

This paper reviews Holographic Particle Image Velocimetry (HPIV) as a means to make three-component velocity measurements throughout a three-dimensional flow-field of interest. A simplified treatment of three-dimensional scalar wave propagation is outlined and subsequently used to illustrate the principles of complex correlation analysis. It is shown that this type of analysis provides the three-dimensional correlation of the propagating, monochromatic fields recorded by the hologram. A similar approach is used to analyse the Object Conjugate Reconstruction (OCR) technique to resolve directional ambiguity by introducing an artificial image shift to the reconstructed particle images. An example of how these methods are used together to measure the instantaneous flow fields within a motored Diesel engine is then described.

Remote detection of the pitch and orientation of a reflective diffraction grating: an optical strain gauge

Measurement of surface strain in hostile environments remains a significant metrology challenge. An elegantly simple non–contact method is to measure the change in pitch of a reflective diffraction grating bonded to the surface of interest. The grating is probed with a laser beam and the change in angular position of diffracted orders is detected as the pitch changes due to surface strain. Robust application of this technique has not been possible due to assumptions such as near–normal incidence, which were necessary in predicting the angular position of diffracted orders from the existing two–dimensional theory. This paper introduces a new three–dimensional theory of diffraction, which obviates the need for these assumptions. Importantly, the theory shows that the angular position of orders can be determined for arbitrary orientation of the grating relative to the incident probe beam. This latter development is critical for success in a practical application. It is now possible to micro–machine reflective gratings directly into a surface or surface coating and thus avoid problems with creep in the bonding material. Together with the new theory, this idea now has the potential to revolutionize how strain will be measured in the future.

An enhanced HPIV configuration for flow measurement through thick distorting windows

This paper reports on a new holographic particle image velocimetry configuration and analysis procedure that can be used to measure particle displacement through thick distorting windows. The technique builds upon the scanning fibre probe based object conjugate reconstruction geometry; however it avoids the requirement of using a holographic optical element to correct for window distortion of the beams. Removal of the distortion is instead accomplished by using a ray trace mapping between the wave vectors scattered by the particles at the time of each exposure and those measured by the interrogation system. The technique is ideally suited to the study of flow structure within the combustion chamber of a diesel engine, and preliminary experimental results that attempt to assess the accuracy of the technique in this situation are presented.

Rotationally invariant pattern recognition by use of linear and nonlinear cascaded filters

We discuss the merits of using single-layer (linear and nonlinear) and multiple-layer (nonlinear) filters for rotationally invariant and noise-tolerant pattern recognition. The capability of each approach is considered with reference to a two-class, rotation-invariant, character recognition problem. The minimum average correlation energy (MACE) filter is a linear filter that is generally accepted to be optimal for detecting signals that are free from noise. Here it is found that an optimized MACE filter cannot differentiate between the characters E and F in a rotation-invariant manner. We have found, however, that this task is possible when a single optimized linear filter is used to achieve the required response when a nonlinear threshold function is included after the filter. We show that this structure can be cascaded to form a multiple-layer, cascaded filter and that the capability of such a system is enhanced by its increased noise tolerance in the character recognition problem. Finally, we show the capability of a two-layer cascade as a means to detect different species of bacteria in images obtained from a phase-contrast microscope.

High temporal resolution of the acoustic transients associated with optical cavitation in water using a large area piezoelectric transducer

A PVDF piezoelectric transducer has been used to study the shock wave associated with a laser generated cavitation bubble in water. The measurements depend upon the accurate recording of the voltage developed in a large (3 mm) diameter but thin (9* m) piezoelectric film transducer placed close to a Nd YAG laser‐generated cavity in water. The spherical shock transients excite the film over the duration of the passage of the wave through the transducer, and an invertion algorithm of the voltage trace gives the pressure profile along a radius. Pressure profiles due to initial breakdown and first collapse are in excellent agreement with those obtained using full field high‐speed optical interferometry of the laser–liquid interaction.

Pre-detector processing of three-dimensional incoherent image fields

This paper describes a method to modify the point spread characteristics of an imaging system to perform convolution filtering of incoherent image fields prior to detection. The technique utilizes an aperture plane phase only optical element (kinoform) which is computer generated to optimize efficiency and is fabricated as a bleached hologram. In addition to providing a high speed alternative to digital enhancement of images, optical processing using this approach has several interesting properties. The most significant of these is the ability of the phase element to retain and process high spatial frequency image information from parts of the image which would otherwise be out of focus. As a result this technique allows an optical implementation of three- dimensional convolution filtering, a practical demonstration of which is given in this paper.

An optical transducer for the study of the pressure field around a collapsing cavitation bubble

An optical reflection transducer using a critical angle technique has been built to study the pressures developed around a single cavitation bubble in water. The bubble was generated at different distances from a planar glass surface, and total internal reflection at the glass water interface gives the pressure through the change in refractive index in both the water and the glass. A simple hemispherical pmma block system has been built to prove the feasibility of the system. In this case the change of the critical angle is not linear, but the use of a modified equilateral glass prism gives the pressure as a function of time around cavitation bubbles with maximum radii of approximately 1.0 mm and gamma values down to 0.3 with a temporal resolution only limited by the photodiode detection electronics (∼10 ns). The transducer shows the development of positive pressure as well as the migration of the bubble to the surface and is capable of recording the rarefaction waves due to free surface reflection.

Optical Diagnostics of Cavitation and Ultrasound in Materials

A capacitor discharge electromagnetic transducer and lens system has been built to generate high amplitude acoustic waves in water. Laser lit focused Shadowgraphy and schlieren photography shows the complex nature of the acoustic transients and the extensive cavitation associated with the event.