Effects of fiber-optic probe design and probe-to-target distance on diffuse reflectance measurements of turbid media: an experimental and computational study at 337 nm

Fiber-optic probes are widely used in optical spectroscopy of biological tissues and other turbid media. Only limited information exists, however, on the ways in which the illumination-collection geometry and the overall probe design influence the interrogation of media. We have investigated both experimentally and computationally the effect of probe-to-target distance (PTD) on the diffuse reflectance collected from an isotropically (Lambertian) scattering target and an agar-based tissue phantom. Studies were conducted with three probes characterized by either common (single-fiber) or separate (two bifurcated multifiber probes) illumination and collection channels. This study demonstrates that PTD, probe design, and tissue scattering anisotropy influence the extent of the transport of light into the medium, the light-collection efficiency, and the sampling volume of collected light. The findings can be applied toward optimization of fiber-optic probe designs for quantitative optical spectroscopy of turbid media including biological tissues.     

Electrical effects of clustered defects in heteroepitaxial Si films

Our work indicates that the behavior of injected majority and minority carriers in representative Si/spinel samples is dominated by deep-level defects that are agglomerated into defect clusters rather than being uniformly distributed in the volume of the Si layer. Since these defect clusters will be surrounded by space-charge regions, the resulting trap occupancy-dependent potential barriers result in slow, nonexponential majority carrier trapping effects but subnanosecond minority carrier lifetimes. Accurate numerical calculations of the electrical properties of a sheet of clustered defects are shown to agree with detailed measurements at 90 and 300°K of the transient conductance decay in P- and N-type films following pulsed carrier injection from electron bombardment. A significant conclusion of the theoretical calculations is that the electrical behavior of a defect cluster is insensitive to any of the model parameters chosen as long as there are a sufficient number of defects to completely deplete the material in the vicinity of the defect plane. Our results show that the observed short minority carrier lifetimes, long-term majority carrier trapping effects and mobility temperature dependences can all be explained by the same cluster defect model and cannot be explained by uniformly distributed point defects. Our calculations also imply that minority carrier lifetime in these films should be strongly injection level dependent. Thus, lifetime measured by the MOS transient capacitance technique will not be the lifetime needed to evaluate the material for bipolar device applications.     

Microchip-based purification of DNA from biological samples

A microchip solid-phase extraction method for purification of DNA from biological samples, such as blood, is demonstrated. Silica beads were packed into glass microchips and the beads immobilized with sol-gel to provide a stable and reproducible solid phase onto which DNA could be adsorbed. Optimization of the DNA loading conditions established a higher DNA recovery at pH 6.1 than 7.6. This lower pH also allowed for the flow rate to be increased, resulting in a decrease in extraction time from 25 min to less than 15 min. Using this procedure, template genomic DNA from human whole blood was purified on the microchip platform with the only sample preparation being mixing of the blood with load buffer prior to loading on the microchip device. Comparison between the microchip SPE (microchipSPE) procedure and a commercial microcentrifuge method showed comparable amounts of PCR-amplifiable DNA could be isolated from cultures of Salmonella typhimurium. The greatest potential of the microchipSPE device was illustrated by purifying DNA from spores from the vaccine strain of Bacillus anthracis, where eventual integration of SPE, PCR, and separation on a single microdevice could potentially enable complete detection of the infectious agent in less than 30 min.     

The value of the VaNTH Engineering Research Center

This article summaries some of the project-level assessment and evaluation activities that have been used to assess the value added for innovations that incorporate learning science principles and learning technology underlying the HPL model proposed by Bransford et al. [1]. Based on a counterfactual model of causal analysis, VaNTH investigators have been encouraged to use experimental and quasi-experimental research designs to estimate the "value added" for their innovations. Although some studies have been undertaken with little difficulty, practical constraints have resulted in the need to allow quasi-experimental designs to emerge over time and institutions. By applying the logic, principles, and criteria of a counterfactual causal model, as opposed to a "cookbook" application of designs and statistical procedures, VaNTH investigators have begun to develop a firm knowledge base about the relative effectiveness of their HPL-inspired innovations. The overall effects and differences among studies are being investigated through the use of a statistical technique known as meta-analysis. This article demonstrates that it is possible to assess and evaluate, in a quantitative way, the relative effects of educational innovations in engineering courses. It also demonstrates, albeit in a preliminary fashion, that a broader assessment of the HPL model underlying VaNTH can be undertaken by systematically looking across studies within VaNTH. By implication, the knowledge gained about engineering education from A&E efforts within VaNTH should be much greater than the sum of its parts.     

On the problem of modelling for parameter identification in distributed structures

Structures are often characterized by parameters, such as mass and stiffness, that are spatially distributed. Parameter identification of distributed structures is subject to many of the difficulties involved in the modelling problem, and the choice of the model can greatly affect the results of the parameter identification process. Analogously to control spillover in the control of distributed-parameter systems, identification spillover is shown to exist as well and its effect is to degrade the parameter estimates. Moreover, as in modelling by the Rayleigh–Ritz method, it is shown that, for a Rayleigh–Ritz type identification algorithm, an inclusion principle exists in the identification of distributed-parameter systems as well, so that the identified natural frequencies approach the actual natural frequencies monotonically from above.     

The return of impact assessment: Assessing the impact of Regional Shopping Centre proposals in the United Kingdom

The impact of Regional Shopping Centres upon the retail hierarchy is a major consideration for all those with vested interests in the future of the town centre. However, at a time when the planning and policy system is weak, impact techniques are also ill-equipped to forecast how these new forms of retail development will interact with and impact upon the high street. A review of over fifty impact studies forwarded at the present round of inquiries has highlighted the weaknesses in the methodologies and revealed the need for greater theoretical rigour.