1,720,974 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Using liquid crystals to improve the performance of luminescent solar concentrators
No full textThe luminescent solar concentrator (LSC) has been proposed as a cost-effective solution to bring electricity generation from sunlight into the built environment. The basic LSC design consists of a plastic plate containing or topped by a thin layer of fluorescent dyes. The dyes absorb sunlight and re-emit it at a longer wavelength. A fraction of this light is trapped by total internal reflection and becomes concentrated along the edges of the waveguide, where one can place small photovoltaic cells to convert the emitted light into electrical current. With its flexibility in color, shape, and size, it would appear ideally suited for exploitation by the architect for use in façades and other structures in the urban surroundings. However, limitations in performance have hindered widespread adoption of the devices. In this work I present modifications to the standard LSC using liquid crystals to assist in collecting, directing, and controlling both the incoming and emitted light in the device, as well as describing additional features to allow on-demand adjustment of transparency
Light management in luminescent solar concentrators : aligned organic dyes and organic wavelength selective reflectors
Get PDFDwindling oil reserves have turned world governments towards the use of renewable energy sources. In particular, buildings, which account for 40% of all energy usage in well-developed countries, have attracted special attention for the possibility of implementing energy generating devices from renewable energy sources. For buildings, the devices must be adaptable to the demands of the architect and should not detract from the appearance of the structure, while maintaining a low costs for the generated energy (~€0.06/kWh). An energy source readily available for each building is the sun. The implementation of standard photovoltaic cells in these constructions has been difficult, since the cost of the energy generated is still too high and the adaptability to the demands of the architect is limited. An alternative to standard photovoltaic cells is a luminescent solar concentrator (LSC) which holds promise for low cost and of better meeting the demands of the architects. LSCs are simple devices that consist of an optically transparent plastic or glass plate acting as waveguide. A luminophore is embedded in or as a thin film on top of the waveguide and absorbs the incoming sunlight, re-emitting this light as photons with a longer wavelength. Part of this emitted light is waveguided in the plate due to total internal reflection and transported to the edges of the plate where small photovoltaic cells are placed which convert the photons into electricity. The efficiency of LSCs has been limited due to several loss mechanisms. One of the most important losses is the photons lost through the surfaces of the plate, a result of a limited trapping efficiency of emitted photons combined with sequential re-absorption and re-emission of photons in the waveguide mode (called ‘photon recycling’). In this work, I present a simple model that predicts the spatial distribution of emitted photons from dichroic dye ensembles in an isotropic or liquid crystalline host. The model predicts the emission from dichroic dyes in an isotropic host is non-isotropic when illuminated with a collimated light source. The model furthermore predicts that by changing the alignment of the dichroic dye molecules, the spatial distribution of the emitted photons may be altered. The model is tested by experiments in which the molecular order of dichroic dyes in a planar aligned liquid crystal is changed. LSC waveguide with the dye molecules aligned parallel to the top and bottom surfaces and parallel to two edges (planar) were made, and I measure the light energy emitted from the edge parallel to the alignment direction can be 60% higher than from the edge perpendicular to the alignment direction, supporting the calculated result. The amount of surface loss from LSCs with dichroic fluorescent dye molecules randomly distributed in the LSC is calculated and determined experimentally. Calculations show that the trapping efficiency is 74.3%, meaning 25.7% of the photons are lost through the surfaces of the LSC waveguide. Experiments show that the photons lost though the surface of LSCs made from polycarbonate is actually ~ 50% of all emitted photons for LSCs containing BASF Lumogen F Red 305, the standard dye used in LSCs. This large surface loss is caused by photon recycling events of waveguided photons and the consequent change in propagation path. Aligning dichroic dye molecules can be used to reduce surface loss of LSCs. Calculations and experiments both show aligning dye molecules in a homeotropic fashion result in a reduction of surface loss to less than 10%, but with concurrent reduction in the incident light absorbed. Tilting the director of the dye ensembles at an intermediate angle can combine the advantages of planar (increased absorption) and homeotropic (decreased surface loss) dye ensembles. Prediction of the optimal tilt angle and order of the dye ensemble with the model presented in this thesis was not possible. I suggest this model needs implementation in simulation software so the influence of photon recycling is better represented. A second method to reduce the surface loss in LSCs is the application of organic wavelength selective reflectors transmitting the incoming sunlight and reflecting the light emitted by the luminophore. Calculations show approximately 90% of all surface emitted photons from luminophore BASF Lumogen Red 305 may be reflected back into the LSC waveguide, leading to an increase in LSC efficiency of 66% if a 400 nm bandwidth reflector is used, or by 45% with a 175 nm reflector. Experimentally, the increase in energy leaving the edge of an LSC containing Red 305 with a peak absorbance of 1.0 is only about 5%. Reducing the number of photon recycling events in an experimental system with a 175 nm bandwidth reflector led to enhancement in LSC efficiency of around 20%, showing it is photon recycling that limits the effectiveness of the wavelength selective reflectors. The organic wavelength selective reflectors show angular dependency: the reflection band shifts to shorter wavelengths with increasing angle of incidence, possibly leading to the reflection of absorbable incident sunlight at these larger angles. Calculations show that for non-direct sunlight or direct sunlight at oblique incidence angles there is a decrease in effectiveness of the reflectors and potential decrease in device efficiency. Simulations show that special dispersion cholesteric reflectors (SDC) have a less pronounced angular dependency. Using SDC reflectors will enhance the LSC efficiency in any solar condition and only incident light at angles larger than 70° are reflected away by the reflector. Both luminophore alignment and organic selective reflectors presented in this thesis reduce the surface loss of the LSC, and increase the device efficiency. Experimentally, the increase is not as large as expected from the calculations. To increase the LSC efficiency the methods presented in this thesis need to be employed using a luminophore with a larger Stokes shift, so photon recycling is limited. In addition, the amount of sunlight that can be absorbed by the luminophore molecules (e.g. the spectral coverage) needs to be increased for the LSC to become viable in the marketplace, as this is still the single largest loss in LSCs
Variations on the Author
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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