Aberration chromatique | EPFL Graph Search

Une aberration chromatique est une aberration optique qui produit différentes mises au point en fonction de la longueur d'onde. On observe alors une image floue et aux contours irisés. Elle résulte de la décomposition de la lumière blanche en plusieurs bandes de couleurs. Historique Les aberrations chromatiques ont été constatées dès les premières lunettes astronomiques et considérées comme gênantes. Isaac Newton, qui crée son propre télescope pourvu d'un miroir et donc dépourvu d'aberrations de ce type, clame, dans un premier temps, l'impossibilité physique de la correction de ces dernières. Leonhard Euler postule, pour contrer cette affirmation, le caractère sans aberrations chromatiques de l'œil humain, fait dont le caractère erroné ne sera démontré que plus tard. La primauté de la fabrication des premières lunettes corrigeant ces aberrations revient à Chester Moore Hall, barrister et opticien amateur, qui réalise le premier prototype dès 1733. Il utilise pour cela de

Electron beam dynamics in an ultrafast transmission electron microscope with Wehnelt electrode

High temporal resolution transmission electron microscopy techniques have shown significant progress in recent years. Using photoelectron pulses induced by ultrashort laser pulses on the cathode, these methods can probe ultrafast materials processes and have revealed numerous dynamic phenomena at the nanoscale. Most recently, the technique has been implemented in standard thermionic electron microscopes that provide a flexible platform for studying material's dynamics over a wide range of spatial and temporal scales. In this study, the electron pulses in such an ultrafast transmission electron microscope are characterized in detail. The microscope is based on a thermionic gun with a Wehnelt electrode and is operated in a stroboscopic photoelectron mode. It is shown that the Wehnelt bias has a decisive influence on the temporal and energy spread of the picosecond electron pulses. Depending on the shape of the cathode and the cathode-Wehnelt distance, different emission patterns with different pulse parameters are obtained. The energy spread of the pulses is determined by space charge and Boersch effects, given by the number of electrons in a pulse. However, filtering effects due to the chromatic aberrations of the Wehnelt electrode allow the extraction of pulses with narrow energy spreads. The temporal spread is governed by electron trajectories of different length and in different electrostatic potentials. High temporal resolution is obtained by excluding shank emission from the cathode and aberration-induced halos in the emission pattern. By varying the cathode-Wehnelt gap, the Wehnelt bias, and the number of photoelectrons in a pulse, tradeoffs between energy and temporal resolution as well as beam intensity can be made as needed for experiments. Based on the characterization of the electron pulses, the optimal conditions for the operation of ultrafast TEMs with thermionic gun assembly are elaborated. (C) 2016 Elsevier B.V. All rights reserved.

Elsevier Science Bv2016

Lens aberration compensation in interference microscopy

Yves Bellouard, Jakub Drs, Mingyu Liu

Emergence of products that feature functional surfaces with complex geometries, such as freeform optics in consumer electronics and augmented reality and virtual reality, requires high-accuracy non-contact surface measurement. However, large discrepancies are often observed between the measurement results of optical methods and contact stylus methods, especially for complex surfaces. For interference microscopy, such as coherence scanning interferometry, the three-dimensional surface transfer function provides information about the instrument spatial frequency passband and about lens aberrations that can result in measurement errors. Characterisation and phase inversion of the instrument's three-dimensional surface transfer function yields an inverse filter that can be applied directly to the three-dimensional fringe data. The inverse filtering is shown to reduce measurement errors without using any data processing or requiring any a priori knowledge of the surface. We present an experimental verification of the characterisation and correction process for measurements of several freeform surfaces and an additive manufactured surface. Corrected coherence scanning interferometry measurements agree with traceable contact stylus measurements to the order of 10 nm.

2020

Curved Transflective Holographic Screens for Head-Mounted Display

Jürgen Brugger, Victor Javier Cadarso Busto, Jonas Grossenbacher, Arnaud Mader, Christophe Moser, Eric Tremblay, Seyedpayam Vahdati

A volume hologram recorded with a lens array is proposed as a transflective screen for Head Worn Display (HWD) systems. Design, fabrication as well as proof of concept are reported. Light from a projection system, with similar properties to one mounted on the side of an eyewear, is efficiently diffracted towards the eye with an angular spread given by the numerical aperture of the lenses forming the lens array. Using a dual-focus contact lens, high-resolution images can be added to the HWD user's normal vision, as light from the surrounding environment is transmitted through the screen with low aberrations. This screen offers the possibility for small footprint and large field of view HWD's.

Spie-Int Soc Optical Engineering2013

Electron beam dynamics in an ultrafast transmission electron microscope with Wehnelt electrode

Elsevier Science Bv2016