1,721,192 research outputs found
External cavity diode lasers and non-linear optical frequency conversion in spectroscopic applications
No full textSemiconductor diode lasers are successful tools in atomic spectroscopy. They are routinely used in frequency conversion applications to develop devices that access regions of the spectrum not directly available. This thesis describes the practical application of novel violet diode laser systems and their possible inclusion in spectroscopic systems.
The design, assembly and successful operation of a doubly resonant optical parametric oscillator is described. There is discussion of the spectral behaviour of the device and the potential for pumping with a violet diode laser. Methods to adapt the output from the solitary diode devices are demonstrated with the use of microlensed diode lasers and extended cavity configurations. Details of the current tuning, linewidth and
smooth tuning characteristics of a number of the lasers used are given. A commercial
violet diode laser is used within an extended cavity to measure the hyperfine structure
of atomic indium from a hollow cathode galvatron source at room temperature.
Stabilisation of the diode laser to a line from the indium spectrum is attempted.
The remainder of the thesis is concerned with the development of techniques to deliver
clearer and more precise spectral information about trace species. Microlensed
red and violet diode lasers are used to generate light at 254nm via sum frequency
generation for the direct detection and modulation spectroscopy of mercury vapour,
with microlensed lasers with modulation allowing more accurate discrimination between
spectral features than direct absorption measurements. In addition Raman
tweezers modulation spectroscopy is undertaken to investigate polymer microspheres
and biological cell samples where the use of the modulation technique demonstrated
improvements in the acquisition time and clarity of spectra through increased signal
to noise and rejection of background fluorescence effects. A comparison between the
direct and modulation techniques for all the systems indicates the greater sensitivity
of the modulation technique
Pressure Equilibration after Vacuum Impregnation of Apple Tissue Studied by Gas in Scattering Media Absorption Spectroscopy (GASMAS)
No full textVacuum impregnation (VI) is a unit operation in which a porous tissue is immersed in a solution. Vacuum is applied to the system provoking the flow out of the internal air trapped in the extracellular spaces of the tissue. Upon restoration of the atmospheric pressure, the external liquid flows into the pores replacing the air. In this paper, apple pieces were vacuum impregnated with isotonic sucrose solution (18 % w/v) for different times and reduced pressures (15, 30, 45 kPa (abs.)). Using GASMAS (Gas in Scattering Media Absorption Spectroscopy) it was possible to observe that apples, in which air has not been totally exhausted during the impregnation operation, keep an internal reduced pressure which rises slowly toward ambient over a time scale of hours. Both the residual vacuum and the timescale of pressure equilibration with ambient varied with applied vacuum level and apple variety. This phenomenon might be a direct consequence of the topology, geometry and hydrophobicity of the complex matrix of intercellular spaces in the tissue
Reconstruction of diffuse photon-density wave interference in turbid media from time-resolved transmittance measurements
Full text linkWe demonstrate an improved technique to precisely localize inhomogeneities in turbid media by means of reconstructing diffuse photon-density wave interference from time-resolved transmittance measurements applying the Fourier transform. This interference can also be obtained in the reverse mode, that is using a single source and combining the signals detected at several locations. This increases the collection efficiency and the possibility for postprocessing and allows one to evaluate the data from one measurement in different ways to make the analysis more robust. (C) 1996 American Institute of Physics
Diffuse optical techniques applied to wood characterisation
No full textWe propose an optical method for non-invasive characterisation of wood samples based on two optical techniques: time-resolved diffuse optical spectroscopy and gas in scattering media absorption spectroscopy. While the latter is sensitive to gases present inside wood pores, the former extracts information on the bulk material regarding light scattering and absorption. Measurements on spruce samples, cut along different wood fibre directions, are presented to show an example of the advantages of this combined approach: by applying these two non-destructive techniques together, in fact, relevant information on wood such as porosity, permeability and moisture content can be assessed. Furthermore, the chemical composition, internal structure and the anisotropy due to the wood fibres can be investigated
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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