We describe the construction of a bio-aerosol monitor designed to capture and record intrinsic fluorescence spectra from individual aerosol particles carried in a sample airflow and to simultaneously capture data relating to the spatial distribution of elastically scattered light from each particle. The spectral fluorescence data recorded by this PFAS (Particle Fluorescence and Shape) monitor contains information relating to the particle material content and specifically to possible biological fluorophores. The spatial scattering data from PFAS yields information relating to particle size and shape. The combination of these data can provide a means of aiding the discrimination of bio-aerosols from background or interferent aerosol particles which may have similar fluorescence properties but exhibit shapes and/or sizes not normally associated with biological particles. The radiation used both to excite particle fluorescence and generate the necessary spatially scattered light flux is provided by a novel compact UV fiber laser operating at 266nm wavelength. Particles drawn from the ambient environment traverse the laser beam in single file. Intrinsic particle fluorescence in the range 300-570nm is collected via an ellipsoidal concentrator into a concave grating spectrometer, the spectral data being recorded using a 16-anode linear array photomultiplier detector. Simultaneously, the spatial radiation pattern scattered by the particle over 5°-30° scattering angle and 360° of azimuth is recorded using a custom designed 31-pixel radial hybrid photodiode array. Data from up to ~5,000 particles per second may be acquired for analysis, usually performed by artificial neural network classification.
The paper describes the design, construction and testing of a fibre optic pressure sensor based on a reflecting Fabry -Perot etalon.The etalon comprised one fixed mirror and a second mirror designed to flex under the action of the pressure being monitored.A single multimode fibre was used to connect the passive, remote sensor to the transmitter /receiver section, and dual wavelength referencing was used to eliminate the effects of bending-induced attenuation in the fibre.
A Q-switched laser beam with a rise time of 10 ns was directed through a glass plate and absorbed by an opaque layer of silver paint to generate a compressive stress wave of comparable duration, with a peak stress of up to 1 GN m−2, measured using a new technique involving a `Sandia quartz gauge'. On reflection from the free surface, the stress wave becomes tensile and can damage the surface and cause spallation. There is a well-defined stress level for the onset of spallation which depends upon the size and density of the initial surface microcracks. A single hertzian cone crack, 240 μm long, required a peak stress of 0·05-0·07 GN m−2, in agreement with theoretical predictions on the basis of a quasistatic fracture model. In contrast, the threshold stresses for abraded surfaces were one or two orders of magnitude smaller than the predicted values, suggesting a fundamental difference between the response of single- and multiple-cracked surfaces to short-duration stress waves. A second higher threshold is observed at which cracks spread inwards from the surface and concentrate in a planar region parallel to the surface.
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