Asphärische Optiken: Physikalische Grundlagen

Zusammenfassung: Hersteller von intraokularen Linsen (IOLs) und Firmen in der refraktiven Hornhautchirurgie verfolgen vermehrt Konzepte, die optische Qualität des menschlichen Auges durch asphärische Optiken zu verbessern. Unter einer asphärischen Optik versteht man eine Optik mit mindestens einer Oberfläche (z.B. brechende Hornhautoberfläche), die von der Kugelform abweicht. Eine asphärische (nicht kugelförmige) Oberfläche ermöglicht die Korrektur der Abbildungsfehler, insbesondere der sphärischen Aberration, da man die Oberfläche weitgehend frei formen kann. Die optischen Aberrationen des menschlichen Auges lassen sich theoretisch durch den Einsatz asphärischer Optiken minimieren, jedoch ist das Ergebnis immer durch die optischen Eigenschaften von Hornhaut und Linse geprägt. Asphärische Intraokularlinsen erlauben es, die postoperative sphärische Aberration des Patientenauges zu minimieren. Eine optimale individualisierte Anpassung kann allerdings nur unter Berücksichtigung der Hornhautform (Asphärizität) erfolgen. Ebenso ist die ideale korneale Asphärizität nach einem refraktiven Eingriff für ein individuelles Auge nicht ohne die Kenntnis der optischen Eigenschaften der intraokularen Strukturen zu definieren. Theoretische Betrachtungen zu asphärischen Optiken in der refraktiven Chirurgie zeigen, dass sich mit asphärischen Ansätzen eine hohe optische Qualität für das menschliche Auge erzielen lässt. Diese theoretischen Vorteile müssen sich jedoch im klinischen Alltag bewähre

Erste erlebte qualitative Stolpersteine: Ein kritisch-selbstrefleiver Rückblick

Da kein Abstract des Artikels vorhanden ist, finden Sie hier den Beginn des Artikels: In diesem Beitrag möchte ich eine zu einem früheren Zeitpunkt des Studiums verfasste Arbeit aus dem Blickwinkel meines gegenwärtigen Wissenskontexts kritisch betrachten. In meinen Rückblick mit selbstreflexivem Anspruch sollen allfällige Schwächen und Probleme meines damaligen, forschenden Vorgehens und Schlussfolgerns aufgezeigt werden und diese in eine methodenkritische Reflexion einbezogen werden. Im Zentrum steht dabei das rückblickende und selbstkritische Überdenken der früheren Forschersituation. Im Rahmen einer selbstreflexiven Auseinandersetzung gilt es in einem weiteren Schritt neben den Schwierigkeiten im Feld auch jene am Schreibtisch, sowohl im Vorfeld der Untersuchung als auch während des Forschungsprozesses, zu thematisieren

Stratospheric influence on ECMWF sub‐seasonal forecast skill for energy‐industry‐relevant surface weather in European countries

Meteorologists in the energy industry increasingly draw upon the potential for enhanced sub‐seasonal predictability of European surface weather following anomalous states of the winter stratospheric polar vortex (SPV). How the link between the SPV and the large‐scale tropospheric flow translates into forecast skill for surface weather in individual countries – a spatial scale that is particularly relevant for the energy industry – remains an open question. Here we quantify the effect of anomalously strong and weak SPV states at forecast initial time on the probabilistic extended‐range reforecast skill of the European Centre for Medium‐Range Weather Forecasts (ECMWF) in predicting country‐ and month‐ahead‐averaged anomalies of 2 m temperature, 10 m wind speed, and precipitation. After anomalous SPV states, specific surface weather anomalies emerge, which resemble the opposing phases of the North Atlantic Oscillation. We find that forecast skill is, to first order, only enhanced for countries that are entirely affected by these anomalies. However, the model has a flow‐dependent bias for 2 m temperature (T2M): it predicts the warm conditions in Western, Central and Southern Europe following strong SPV states well, but is overconfident with respect to the warm anomaly in Scandinavia. Vice versa, it predicts the cold anomaly in Scandinavia following weak SPV states well, but struggles to capture the strongly varying extent of the cold air masses into Central and Southern Europe. This tends to reduce skill (in some cases significantly) for Scandinavian countries following strong SPV states, and most pronounced, for many Central, Southern European, and Balkan countries following weak SPV states. As most of the weak SPV states are associated with sudden stratospheric warmings (SSWs), our study thus advices particular caution when interpreting sub‐seasonal regional T2M forecasts following SSWs. In contrast, it suggests that the model benefits from enhanced predictability for a considerable part of Europe following strong SPV states

Year-round sub-seasonal forecast skill for Atlantic-European weather regimes

Weather regime forecasts are a prominent use case of sub‐seasonal prediction in the midlatitudes. A systematic evaluation and understanding of year‐round sub‐seasonal regime forecast performance is still missing, however. Here we evaluate the representation of and forecast skill for seven year‐round Atlantic–European weather regimes in sub‐seasonal reforecasts from the European Centre for Medium‐Range Weather Forecasts. Forecast calibration improves regime frequency biases and forecast skill most strongly in summer, but scarcely in winter, due to considerable large‐scale flow biases in summer. The average regime skill horizon in winter is about 5 days longer than in summer and spring, and 3 days longer than in autumn. The Zonal Regime and Greenland Blocking tend to have the longest year‐round skill horizon, which is driven by their high persistence in winter. The year‐round skill is lowest for the European Blocking, which is common for all seasons but most pronounced in winter and spring. For the related, more northern Scandinavian Blocking, the skill is similarly low in winter and spring but higher in summer and autumn. We further show that the winter average regime skill horizon tends to be enhanced following a strong stratospheric polar vortex (SPV), but reduced following a weak SPV. Likewise, the year‐round average regime skill horizon tends to be enhanced following phases 4 and 7 of the Madden–Julian Oscillation (MJO) but reduced following phase 2, driven by winter but also autumn and spring. Our study thus reveals promising potential for year‐round sub‐seasonal regime predictions. Further model improvements can be achieved by reduction of the considerable large‐scale flow biases in summer, better understanding and modeling of blocking in the European region, and better exploitation of the potential predictability provided by weak SPV states and specific MJO phases in winter and the transition seasons.The overall sub‐seasonal forecast performance (biases and skill) for predicting seven year‐round Atlantic–European weather regimes is highest in winter and lowest in summer. The year‐round skill horizon is shortest for the European Blocking and longest for the Zonal Regime and Greenland Blocking (see figure). Furthermore, the winter skill horizon tends to be enhanced following a strong stratospheric polar vortex but reduced following a weak one. Madden–Julian Oscillation phases 4 and 7 tend to increase and phase 2 to decrease the year‐round skill horizon.Helmholtz‐Gemeinschaft http://dx.doi.org/10.13039/50110000165

Faster Geometric Algorithms via Dynamic Determinant Computation

The computation of determinants or their signs is the core procedure in many important geometric algorithms, such as convex hull, volume and point location. As the dimension of the computation space grows, a higher percentage of the total computation time is consumed by these computations. In this paper we study the sequences of determinants that appear in geometric algorithms. The computation of a single determinant is accelerated by using the information from the previous computations in that sequence. We propose two dynamic determinant algorithms with quadratic arithmetic complexity when employed in convex hull and volume computations, and with linear arithmetic complexity when used in point location problems. We implement the proposed algorithms and perform an extensive experimental analysis. On one hand, our analysis serves as a performance study of state-of-the-art determinant algorithms and implementations. On the other hand, we demonstrate the supremacy of our methods over state-of-the-art implementations of determinant and geometric algorithms. Our experimental results include a 20 and 78 times speed-up in volume and point location computations in dimension 6 and 11 respectively.Comment: 29 pages, 8 figures, 3 table

Highly Efficient Year-Round Energy and Comfort Optimization of HVAC Systems in Electric City Buses

In this paper, we present a novel approach to perform highly efficient numerical simulations of the heating, ventilation, and air-conditioning (HVAC) system of an electric city bus. The models for this simulation are based on the assumption of a steady-state operation. We show two approaches to obtain the minimum energy requirement for a certain thermal comfort criterion under specific ambient conditions. Due to the computationally efficient approach developed, we can evaluate the model on a large dataset of 7500 scenarios in various ambient conditions to estimate the year-round performance of the system subject to different comfort requirements. Compared to a heating strategy based on positive temperature coefficient (PTC) elements, we can thus show that a heat pump (HP) can reduce the annual mean power consumption by up to 60%. Ceiling-mounted radiant heating elements complementing a PTC heating system can reduce the annual mean power consumption by up to 10%, while they cannot improve the energy efficiency when used in conjunction with a HP. Finally, a broad sensitivity study reveals the fact that improving the HP's heating-mode coefficient of performance (COP) manifests the largest leverage in terms of mean annual power consumption. Moreover, the annual energy expenditure for cooling are around eight times smaller than those for heating. The case study considered thus reveals that the advantages of improving the COP of the cooling mode are significantly lower.Comment: This work has been accepted to IFAC for publication under a Creative Commons Licence CC-BY-NC-N

Optimized multi-echo gradient-echo magnetic resonance imaging for gray and white matter segmentation in the lumbosacral cord at 3 T

Atrophy in the spinal cord (SC), gray (GM) and white matter (WM) is typically measured in-vivo by image segmentation on multi-echo gradient-echo magnetic resonance images. The aim of this study was to establish an acquisition and analysis protocol for optimal SC and GM segmentation in the lumbosacral cord at 3 T. Ten healthy volunteers underwent imaging of the lumbosacral cord using a 3D spoiled multi-echo gradient-echo sequence (Siemens FLASH, with 5 echoes and 8 repetitions) on a Siemens Prisma 3 T scanner. Optimal numbers of successive echoes and signal averages were investigated comparing signal-to-noise (SNR) and contrast-to-noise ratio (CNR) values as well as qualitative ratings for segmentability by experts. The combination of 5 successive echoes yielded the highest CNR between WM and cerebrospinal fluid and the highest rating for SC segmentability. The combination of 3 and 4 successive echoes yielded the highest CNR between GM and WM and the highest rating for GM segmentability in the lumbosacral enlargement and conus medullaris, respectively. For segmenting the SC and GM in the same image, we suggest combining 3 successive echoes. For SC or GM segmentation only, we recommend combining 5 or 3 successive echoes, respectively. Six signal averages yielded good contrast for reliable SC and GM segmentation in all subjects. Clinical applications could benefit from these recommendations as they allow for accurate SC and GM segmentation in the lumbosacral cord