Lentille optique | EPFL Graph Search

vignette|Une bougie se projetant sur une table par un presse-papier formant lentille. Une lentille optique est un composant fait d'un matériau généralement et transparent pour la lumière dans le domaine spectral d'intérêt. C'est le plus souvent un type de verre optique, ou des verres plus classiques, des plastiques, des matériaux organiques, voire des métalloïdes tels que le germanium. Les lentilles sont destinées à faire converger ou diverger la lumière. Leur utilisation implique que leur indice de réfraction soit différent de celui du milieu dans lequel elles sont plongées (air, huile, eau…). Les lentilles possèdent la plupart du temps un axe de symétrie confondu avec l’axe optique mais les techniques récentes et les besoins de l'industrie et de la recherche font qu'une part non négligeable des lentilles n'ont pas d'axe de symétrie. Il existe aussi des lentilles qui agissent sur d’autres types de rayonnement électromagnétique, au moyen d’un champ électrique et/ou d’un champ magnét

Methods for strong gravitational lens detection and analysis using machine learning and high performance computing

Christoph Ernst René Schäfer

The study of strong gravitational lenses is a relatively new scientific field in astronomy with many applications in cosmology. Its unique observables allow astronomers to trace dark matter, determine the expansion of the universe and study galaxy evolution. While strong lenses are relatively rare phenomena, recent and future improvements in observational instrumentation are expected to lead to a sharp increase in the number and quality of their observation. With that increase comes however an observational challenge from the sheer quantity of data the lenses are hidden in. Current classification and modelling techniques are simply not capable of handling the new data in a reasonable timeframe. As a consequence this thesis concerns itself with the development and improvement of numerical methods necessary to find and analyse these newly found lenses, borrowing from deep learning and high performance computing techniques. Deep learning, a field of machine learning, shows significant success in finding lenses in simulations. Performing significantly better than any other classification method, CNN-based lens finders reach almost the required classification efficiency and accuracy for a fully automated Euclid strong lensing pipeline. Its dependency on a varied and complete training set and the difficulty creating it make its immediate application to new surveys however more difficult. Incorporating high performance computing techniques and more concurrent algorithm design into lens analysis results in crucial speed-ups for strong lens analysis. Applied to Lenstool, a popular mass modelling tool for gravitational lenses, the resulting software can be run on modern supercomputers, using Graphics Processing Units (GPU) and CPUs simultaneously. Running it on state-of-the-art hardware makes it possible to reduce computation time to an acceptable level when mass modelling complex cluster lenses.

EPFL2020

Survey of Gravitationally lensed objects in HSC Imaging (SuGOHI). VIII. New galaxy-scale lenses from the HSC SSP

Hung-Hsu Chan

We conduct a search for galaxy-scale strong gravitational lens systems in Data Release 4 of the Hyper Suprime-Cam Subaru Strategic Program (HSC SSP), consisting of data taken up to the S21A semester. We select 103191 luminous red galaxies from the Baryon Oscillation Spectroscopic Survey (BOSS) sample that have deep multiband imaging from the HSC SSP and use the YattaLens algorithm to identify lens candidates with blue arc-like features automatically. The candidates are visually inspected and graded based on their likelihood of being a lens. We find eight definite lenses, 28 probable lenses, and 138 possible lenses. The new lens candidates generally have lens redshifts in the range 0.3 less than or similar to z(L) less than or similar to 0.9, a key intermediate redshift range to study the evolution of galaxy structure. Follow-up spectroscopy will confirm these new lenses and measure source redshifts to enable detailed lens modeling.

OXFORD UNIV PRESS2022

Optische Augenlängenmessung für klinische Anwendungen

Beate Möller

Cataract is the leading cause of blindness worldwide. The clouding of the human eye lens is called "Grauer Star" in German. In the industrialized countries this process of increased blindness is corrected by the surgical removal of the clouded lens and its replacement by a plastic lens (intraocular lens, IOL). Cataract surgery is the most frequent surgery worldwide. In Europe more than two million operations are performed annually. For the selection of an optimum replacement the refractive power of the lens is calculated from the radius of curvature of the cornea, the length of the eye and the anterior chamber depth. Therefore these data have to be measured before every surgery. Until now the length of the eye was measured exclusively by ultrasound in the clinical praxis. For this procedure the sound transducer has to be placed directly or via a water funnel onto the cornea. It was the aim of the work presented in this thesis to lay the framework for introducing into the clinical praxis a contact free optical length measurement of the eye. The basic parameters for the measurement were determined. Starting from the state of the art new solutions were found, implemented in a laboratory device, and tested clinically. A two-beam interferometer with light of short coherence length that had been developed at the University Vienna was analyzed and compared with the one-beam technique. Neglecting practical limitations due to the optical arrangement and assuming shot-noise limited detection the one-beam technique allows the measurement of reflected light that is a factor of two lower compared to the two-beam technique. From the point of view of detection both techniques are therefore equivalent. The advantage of the two-beam technique is its independence from the eye movement. Until now the two-beam technique utilizes for detection only 1 over 400 of the light reflected from the retina and 1 over 2500 to 1 over 4000 of the light reflected from the cornea. This causes the dynamic range to drop by 26 dB. Furthermore the noise of the electronics (excess noise) comes into play. In this work a novel concept for the optical configuration in the two-beam interferometer is presented. By using a diffractive lens cornea and retina are imaged simultaneously onto the detector and the losses of the light from the retina are minimized. The cornea is imaged with an efficiency of 5.6 per cent. In order to minimize the requirements on the patient the measuring time was shortened to 1 s. Therefore the scan speed was increased by a factor of four compared to the state of the art. In the laboratory device it fluctuated by 20 per cent. To keep the detection narrow band it was necessary to implement a signal processing that is largely independent of the scan speed. By using a synchronous demodulation of two 90 degrees phase-shifted channels that are added as vector quantities a filter with self-adjusting center frequency was created. Its bandwidth is 4 kHz with signal frequencies between 30 kHz and 90 kHz. This concept was realized as a compact slit-lamp head and tested clinically. A shot-noise limited detection with a dynamic range of 105 dB was achieved (input power 200 µW). On the healthy eye the signal-to-noise ratio was limited by the digitization range of 8 bit. On the eye with cataract the measuring was not limited by the degree of clouding any more. The optical lengths were determined along the line of sight. They agreed with the post operative results. In some cases refractive errors of more than 2 dpt would have been avoidable by determining the IOL from the measurement of the optical length of the eye. This technique allows a comfortable, contact free optical eye length measurement that works correctly in 95 per cent of all eyes with cataract. Additionally smaller post operative refractive errors can be achieved as compared with traditional techniques. These results were the basis for the development of the IOL-Master – a novel device for the cataract treatment – at the Carl Zeiss Jena GmbH. With this device all biometric data can be measured in a non-contact fashion and intraocular lenses can be calculated and selected.

EPFL2001