17 research outputs found
Lifetime Prediction of Current-and Temperature-Induced Degradation in Silicone-Encapsulated 365 nm High-Power Light-Emitting Diodes
No full textWe report on the degradation mechanisms and dynamics of silicone encapsulated ultraviolet A (UV-A) high-power light-emitting diodes (LEDs), with a peak wavelength of 365nm. The stress tests were carried out for a period of 8665 hours with forward currents between 350mA and 700mA and junction temperatures up to 132°C. Depending on stress condition, a significant decrease in optical power could be observed, being accelerated with higher operating conditions. Devices stressed at a case temperature of 55 °C indicate a decrease in radiant flux between 10-40% varying with measurement current, whereas samples stressed at higher case temperatures exhibit crack formation in the silicone encapsulant accompanied by electromigration shorting the active region. The analyzed current and temperature dependency of the degradation mechanisms allows to propose a degradation model to determine the device lifetime at different operating parameters. Additional stress test data collected at different aging conditions is used to validate the model's lifetime predictions.</p
Step-by-step instructions for retina recordings with perforated multi electrode arrays.
No full textMulti-electrode arrays are a state-of-the-art tool in electrophysiology, also in retina research. The output cells of the retina, the retinal ganglion cells, form a monolayer in many species and are well accessible due to their proximity to the inner retinal surface. This structure has allowed the use of multi-electrode arrays for high-throughput, parallel recordings of retinal responses to presented visual stimuli, and has led to significant new insights into retinal organization and function. However, using conventional arrays where electrodes are embedded into a glass or ceramic plate can be associated with three main problems: (1) low signal-to-noise ratio due to poor contact between electrodes and tissue, especially in the case of strongly curved retinas from small animals, e.g. rodents; (2) insufficient oxygen and nutrient supply to cells located on the bottom of the recording chamber; and (3) displacement of the tissue during recordings. Perforated multi-electrode arrays (pMEAs) have been found to alleviate all three issues in brain slice recordings. Over the last years, we have been using such perforated arrays to study light evoked activity in the retinas of various species including mouse, pig, and human. In this article, we provide detailed step-by-step instructions for the use of perforated MEAs to record visual responses from the retina, including spike recordings from retinal ganglion cells and in vitro electroretinograms (ERG). In addition, we provide in-depth technical and methodological troubleshooting information, and show example recordings of good quality as well as examples for the various problems which might be encountered. While our description is based on the specific equipment we use in our own lab, it may also prove useful when establishing retinal MEA recordings with other equipment
Long-Term Temperature-Dependent Degradation of 175 W Chip-on-Board LED Modules
No full textWe report on the degradation dynamics and mechanisms of the commercially available chip-on-board (COB) high-power light-emitting diode (LED) modules with an electrical power of 175 W. Due to the associated thermal load, the temperature dependence of the aging processes is additionally analyzed within the scope of this work. The aging tests were performed for a period of 6000 h at four different case temperatures between 55 °C and 120 °C. The results of the accelerated stress tests indicate a temperature-Activated aging process, which severely limits the lifetime of the modules. In addition, the following key findings can be reported: 1) a significant decrease in optical power occurs within 6000 h of operation; 2) depending on the stress test condition the accompanying color shifts exceed a limit of ; and 3) the limiting degradation mechanism can be attributed to the package of the device and can be accelerated with temperature, current, and chemicals. Reported findings can be manifested by additional optical material inspections, allowing to use the results for optimizations of future module generations.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Electronic Components, Technology and Material
Optogenetik als mögliche Therapie bei degenerativen Netzhauterkrankungen
No full textZusammenfassung
Bei neurodegenerativen Erkrankungen der Netzhaut sind die lichtempfindlichen Zellen, die Photorezeptoren, oft als Erstes betroffen. Die Optogenetik ist ein vielversprechender Ansatz, die Netzhaut wieder lichtempfindlich zu machen und dadurch das Sehvermögen wiederherzustellen. Bei der Optogenetik werden lichtempfindliche Proteine über gentechnische Methoden in die Netzhaut eingebracht; die Aktivität der Zielzellen wird durch diese Behandlung durch Licht beeinflussbar. Dieser Einfluss kann die direkte lichtinduzierte Änderung des Membranpotenzials sein (sowohl hemmend als auch erregend) oder die lichtinduzierte Aktivierung intrazellulärer Signalkaskaden. Dies hat zur Folge, dass das Zielgewebe, die Netzhaut, wieder auf Licht reagiert. Diese Übersicht beschreibt die Prinzipien der Optogenetik und den gegenwärtigen Stand im Hinblick auf ihre Anwendung zur Behandlung von Blindheit.</jats:p
Characterizing Visual Performance in Mice: An Objective and Automated System Based on the Optokinetic Reflex
No full text
Advancements in Spectral Power Distribution Modeling of Light-Emitting Diodes
No full textThe unique radiative, photometric and colorimetric characteristic of a light-emitting diode is derived from its spectral power distribution. Modeling such characteristics with respect to the forward current, temperature or operating time has been subject of various studies. Deriving a simple analytical model, however, is not trivial due to the unique emission pattern varying with different types and technologies of light emitting diodes. For this purpose, curve fitting multiple superimposed Gaussian probability density functions to the spectral power distribution is a common approach. Despite excellent R2 goodness of fit results, significant deviations within the photometric and colorimetric parameters, such as luminous flux or chromaticity coordinates, are observed. In addition, most studies were conducted on a small sample set of very few different spectral power distributions. This work provides a comprehensive comparison and evaluation of 19 different (superimposed) probability density function based models provided by the literature tested on a total of 15 different spectral power distributions of monochromatic blue, green and red light-emitting diode as well as phosphor-converted spectra of lime, purple and white samples with different correlated color temperatures. All models were evaluated by means of their coefficient of determination, radiant flux, chromaticity coordinate deviation and Bayesian Information Criterion. This study shows that a superimposed (split) Pearson VII model is able to outperform the commonly used Gaussian model approach by far. In addition, an application example in regard of forward current dependence is given to prove the proposed approach.Electronic Components, Technology and Material
Recording stability.
No full text<p><b>A)</b> Responses of one ganglion cell to a step in contrast over 6 hours. A two second light decrement step has been shown >120 times over a period of 6 hours. Each dot in the raster plot represents one spike produced by the ganglion cell. The ganglion cell stably responded to the stimulus during the whole recording time. Changes in latency and number of spikes are due to different mean brightness levels used during the experiment. <b>B)</b> Receptive field of one ganglion cell calculated from checkerboard stimuli. 15×15 checkers out of 40×40 shown here. The stimulus has been repeated approximately every 90 minutes. Time above each receptive field map: presentation time of the checkerboard stimulus (0 min = beginning of experiment). The receptive field location and shape was stable during the whole 8 hours, indicating that the retina did not move significantly.</p
Experimental procedure Step 1: Filling of MEA chamber.
No full text<p><b>Step 1a)</b> Placing the MEA chamber on the baseplate. <b>Step</b><b>1b)</b> Preparation of perfusion and vacuum. <b>Step</b><b>1c)</b> Filling the MEA. Detailed description is given in the text.</p
Experimental procedure Step 4: Transfer of retina to MEA chamber and setup.
No full text<p><b>Step 4a)</b> Placing the retina on the electrodes. <b>Step 4a) iii:</b> Top: Good MEA preparation. All electrodes are clearly visible; the retina looks homogeneous, flat, and without tears or holes. The retina and filter paper are nicely centered over the middle of the electrode array. Bottom: Bad MEA preparation with air bubble (blue arrow) and holes due to excessive negative pressure (gray arrow). Further, the filter paper is shifted towards the upper left corner. Orange arrow: optic nerve head. <b>Step 4b)</b> Transfer of MEA amplifier to setup. <b>Step 4c)</b> Installation of upper perfusion loop. Details are given in the text.</p
