Development of an algorithm for corneal reshaping with a scanning laser beam

The corneal-ablation rate, the beam-intensity distribution, and the initial and the desired corneal topographies are used to calculate a spatial distribution map of laser pulses. The optimal values of the parameters are determined with a computer model, for a system that produces 213-nm radiation with a Gaussian beam-intensity distribution and a peak radiant exposure of 400 mJ/cm(2). The model shows that with a beam diameter of 0.5 mm, an overlap of 80%, and a 5-mm treatment zone, the roughness is less than 6% of the central ablation depth, the refractive error after correction is less than 0.1 D for corrections of myopia of 1, 3, and 6 D and less than 0.4 D for a correction of myopia of 10 D, and the number of pulses per diopter of correction is 2500 when the beam-intensity distribution is Gaussian and 580 when it is flat.     

Competence via the web

With the rapid growth and technical development of the internet, the interest in distributing education over the web has increased greatly during recent years. Educational systems and industry see the possibilities for more flexible and cost efficient solutions. At the same time, the internet industry is sensing a new market. The challenge lies in producing competence for the individual student through good teaching, while at the same time using technical solutions to create added value via the net. This article gives a short background to the new possibilities in distributed education over the internet, and discusses various factors for success in competence development efforts. The discussion focuses on the different aspects of educational design of web-based educational material.     

Improved algorithm for simulations of divergent-light halos

Divergent-light halos are produced when light from nearby light sources is scattered by ice crystals in the atmosphere. We present a theory of divergent-light halos leading to an improved algorithm for the simulation of such halos. Contrary to the algorithm that we presented earlier for simulating such halos, the new algorithm includes a mathematically rigorous weighting of the events. The computer implementation is very compact, and the whole procedure is elegant and conceptually easy to understand. We also present a new simulation atlas showing halos produced by crystals of different shapes and orientations for a set of elevations of the light source.     

Novel iron oxide–silica coreshell powders compacted by using pulsed electric current sintering: Optical and magnetic properties

The properties of the bulk materials consolidated of silica coreshell powders with iron oxide core have been studied. Iron oxide nanoparticles smaller than 20 nm in size were synthesized by a reverse co-precipitation process in ambient atmosphere. Coreshell structures with various amounts of iron oxide were prepared via a modified Stöber method. The powders were compacted by using pulsed electric current sintering (PECS) at 1373 K. The morphologies, microstructures, phases, optical, and magnetic properties of the samples were studied by using transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD), UV–visible spectroscopy (UV–Vis), and vibrating sample magnetometer (VSM). Transmittance values in the 250–800 nm range varied with the amount of iron oxide. Sample with the lower content was transparent while the sample with the highest content was opaque with microporosity. The compact with the highest iron oxide content showed the ferromagnetic behaviour at 300 K. The phase transformations in the coreshell powders during the sintering process are discussed.     

Tracing enteric viruses in the European berry fruit supply chain

In recent years, numerous foodborne outbreaks due to consumption of berry fruit contaminated by human enteric viruses have been reported. This European multinational study investigated possible contamination routes by monitoring the entire food chain for a panel of human and animal enteric viruses.     

Cloning of two loci for synapse protein Snap25 in zebrafish: comparison of paralogous linkage groups suggests loss of one locus in the mammalian lineage

Recreational and subsistence hunters and anglers consume a wide range of species, including birds, mammals, fish and shellfish, some of which represent significant exposure pathways for environmental toxic agents. This study focuses on the Department of Energy's (DOE's) Savannah River Site (SRS), a former nuclear weapons production facility in South Carolina. The potential risk of contaminant intake from consuming mourning doves (Zenaida macroura), the most popular United States game bird, was examined under various risk scenarios. For all of these scenarios we used the mean tissue concentration of six metals (lead, mercury, cadmium, selenium, chromium, manganese) and radiocesium, in doves collected on and near SRS. We also estimated risk to a child consuming doves that had the maximum contaminant level. We used the cancer slope factor for radiocesium, the Environmental Protection Agencies Uptake/Biokinetic model for lead, and published reference doses for the other metals. As a result of our risk assessments we recommend management of water levels in contaminated reservoirs so that lake bed sediments are not exposed to use by gamebirds and other terrestrial wildlife. Particularly, measures should be taken to insure that the hunting public does not have access to such a site. Our data also indicate that doves on popular hunting areas are exposed to excess lead, suggesting that banning lead shot for doves, as has been done for waterfowl, is desirable.     

On the growth of intercrystalline wedge-cracks during creep-deformation

From a study of the crack length distribution and of the increase in crack density versus strain, an expression has been derived for the growth of intercrystalline wedge-cracks in a 20 Cr-35 Ni-steel during creep-deformation. The square root of the crack length is found to increase proportionately with the strain. The growth rate versus strain at 750°C is found to change only slightly over the stress interval studied, 13 to 22 kp/mm², and is highest at the lowest stress. By combining the expression for the crack growth with the rupture criterion, known earlier, an expression is obtained for the rupture strain.     

A method of predicting the image noise in paediatric multi-slice computed tomography images

The purpose of this study was to develop an equation with which to determine the tube current to be used in order to obtain a certain image noise level for differently sized children undergoing multi-slice computed tomography examination. The relationship between image noise and detector dose for different examination protocols was established for a LightSpeed Ultra, an eight slice CT from GEMS, using homogeneous water phantoms of different sizes. Three different anatomical areas (head, thorax and abdomen) were studied in 111 patients between 0 and 17 y of age. The mean ratio between the calculated and the measured noise in patient images was established for the different areas. Head examinations showed the best correlation (measured-to-calculated noise ratio = 1.01). In the thorax, the calculated noise was generally higher than the measured noise (ratio = 0.74), and in the abdomen, the opposite result was found (ratio = 1.20).     

Friction models for sliding dry, boundary and mixed lubricated contacts

Friction, lubrication, and wear have a strong influence on the performance and behavior of mechanical systems. This paper deals with different friction models for sliding contacts running under different conditions. The models presented are suited to different situations, depending on the type of contact, running conditions, and the behavior of interest. The models will be discussed from simulation and tribological points of view. The different types of friction models considered are:

• friction models for transient sliding under dry, boundary and mixed lubrication conditions,

• friction models for micro-displacements of engineering surfaces subjected to transient sliding,

• friction models often used in the simulation and control of technical systems,

• combined friction models that represent physical behaviors fairly well but are also suitable for use in simulating systems,

• friction models that take into account the stochastic nature of interacting surface asperities.