Foyer (optique)

Résumé

En optique géométrique, un foyer est un point vers lequel convergent les rayons lumineux issus d'un point après leur passage dans un système optique. Son nom provient de l'extension du sens mathématique. Ce terme concorde avec son étymologie puisqu'il est possible d'allumer un feu à l'aide d'une lentille convergente en concentrant les rayons du Soleil en un seul point : le foyer (technique employée selon la légende par Archimède). Le concept de foyer nécessite d'avoir un système stigmatique (au moins de manière approchée ou dans l'approximation de Gauss). En effet, dans le cas contraire, les rayons issus d'un même point ne convergent pas nécessairement vers un point unique après le système optique. Foyers principaux En optique géométrique, dans le cas d'un système centré (c'est-à-dire possédant un axe de symétrie de révolution), il existe deux foyers principaux appelés foyer objet (ou foyer principal objet) et foyer image (ou foyer principal image

Source officielle

https://fr.wikipedia.org/wiki/Foyer_(optique)

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Partial discharge localization in power transformers using acoustic time reversal

Florent Quentin Aviolat, Mohammad Azadifar, Hamidreza Karami, Marcos Rubinstein

The localization of partial discharges (PD) is important to monitor the conditioning of high voltage insulator materials. One of the key components in power grids is the high power transformer. Localization of PDs in these pieces of equipment can increase the reliability of power networks. One of the most commonly used methods to localize PDs is the time difference of arrival (TDoA) technique. Recently, time reversal (TR) techniques in the electromagnetic regime have been proposed to localize PD sources with a lower number of needed sensors compared to the TDoA technique. Contrary to TDoA, TR-based techniques do not need line-of-sight wave propagation from PD sources to the sensor(s). In this study, we extend the TR technique to localize PD sources in the acoustic regime for three-dimensional structures. Similar to the electromagnetic TR methods, a typical time reversal process includes three steps: 1) record the acoustic fields caused by a PD source using at least one sensor, 2) time reverse and back inject into the medium the recorded acoustic signal(s), 3) use a proper criterion to obtain the focusing point associated with the location of the PD source. The acoustic TR technique presented in this paper uses only one sensor. Numerical simulations are used to evaluate the performance of the acoustic TR technique for three-dimensional power transformer models. To do this, an open source MATLAB toolbox is used to solve the acoustic wave equation. The numerical results show that the TR technique can accurately localize the PD sources in three-dimensional power transformer models. Our analysis shows that the presence of windings inside the transformer tank does not degrade the efficiency of the acoustic TR technique.

Elsevier2022

Chargement

High speed laser Doppler perfusion imaging system e.g. blood flow imaging system, detects laser Doppler signal and/or laser speckle signal from light reflected from selected blood vessel area, to obtain flow data

Theo Lasser

An instrument for high-speed laser perfusion imaging comprises a laser source, a detector, a signal-processing unit, data memory, and a screen to display the results. A section of the sample surface is illuminated with laser light; the reemitted light from the irradiated surface is collected by the focusing optics on a 2D array of integrating photo detectors. The elements of the 2D array can be accessed individually or in a pre-defined selection of pixels at high speed. This 2D array of random-pixel-access integrating photo detectors (for example an integrating CMOS image senor) is utilized to measure the intensity variations at each individual pixel. The average amplitude and the mean frequency of the measured signal contain information about concentration and speed of moving blood cells. For real-time imaging, the exposure time is used as a parameter to measure relative perfusion changes. These data are stored in a memory and processed with a signal-processing unit. The instrument delivers 2D flow maps of the illuminated sample section. In parallel a conventional image of the sample can be obtained with the same 2D array of photo detectors allowing a simple overlay between a conventional image and processed flow maps. The instrument enables objective high-speed tissue perfusion imaging and real-time perfusion monitoring.

2006

Chargement

An instrument for high-speed laser perfusion imaging comprises a laser source, a detector, a signal-processing unit, data memory, and a screen to display the results. A section of the sample surface is illuminated with laser light; the reemitted light from the irradiated surface is collected by the focusing optics on a 2D array of integrating photo detectors. The elements of the 2D array can be accessed individually or in a pre-defined selection of pixels at high speed. This 2D array of random-pixel- access integrating photo detectors (for example an integrating CMOS image senor) is utilized to measure the intensity variations at each individual pixel. The average amplitude and the mean frequency of the measured signal contain information about concentration and speed of moving blood cells. For real-time imaging, the exposure time is used as a parameter to measure relative perfusion changes. These data are stored in a memory and processed with a signal-processing unit. The instrument delivers 2D flow maps of the illuminated sample section. In parallel a conventional image of the sample can be obtained with the same 2D array of photo detectors allowing a simple overlay between a conventional image and processed flow maps. The instrument enables objective high-speed tissue perfusion imaging and real-time perfusion monitoring.

2008

Chargement