Color calibration of an RGB camera mounted in front of a microscope with strong color distortion

International audienceThis paper aims at showing that performing color calibration of an RGB camera can be achieved even in the case where the optical system before the camera introduces strong color distortion. In the present case, the optical system is a microscope containing a halogen lamp, with a nonuniform irradiance on the viewed surface. The calibration method proposed in this work is based on an existing method, but it is preceded by a three-step preprocessing of the RGB images aiming at extracting relevant color information from the strongly distorted images, taking especially into account the nonuniform irradiance map and the perturbing texture due to the surface topology of the standard color calibration charts when observed at micrometric scale. The proposed color calibration process consists first in computing the average color of the color-chart patches viewed under the microscope; then computing white balance, gamma correction, and saturation enhancement; and finally applying a third-order polynomial regression color calibration transform. Despite the nonusual conditions for color calibration, fairly good performance is achieved from a 48 patch Lambertian color chart, since an average CIE-94 color difference on the color-chart colors lower than 2.5 units is obtained

Nanomoulding of transparent zinc oxide electrodes for efficient light trapping in solar cells

Nanopatterning has gained tremendous importance in the field of photovoltaics, as absorption of sunlight in solar cells can be enhanced drastically by proper engineering of photonic nanostructures1– 8. However, despite intensive efforts, neither the ideal surface morphology nor the ideal scattering characteristics foroptimumlight trapping have been identified. Experimentally, a method capable of implementing arbitrarily designed surface morphologies directly into functional devices is desirable. Here, we establish a nanomoulding process that provides exactly such a platform, enabling precise, large-area, nanoscale patterning of functional zinc oxide films at low cost. We illustrate the application of nanomoulded zinc oxide films as transparent front electrodes in amorphous silicon solar cells, demonstrating excellent initial conversion efficiencies of 10.1%. In the quest to find the most efficient light-harvesting scheme, we anticipate that nanomoulding will catalyse the development and integration of exciting new nanophotonic structures

Light trapping in solar cells: can periodic beat random?

Theory predicts that periodic photonic nanostructures should outperform their random counterparts in trapping light in solar cells. However, the current certified world-record conversion efficiency for amorphous silicon thin-film solar cells, which strongly rely on light trapping, was achieved on the random pyramidal morphology of transparent zinc oxide electrodes. Based on insights from waveguide theory, we develop tailored periodic arrays of nanocavities on glass fabricated by nanosphere lithography, which enable a cell with a remarkable short-circuit current density of 17.1 mA/cm(2) and a high initial efficiency of 10.9%. A direct comparison with a cell deposited on the random pyramidal morphology of state-of-the-art zinc oxide electrodes, replicated onto glass using nanoimprint lithography, demonstrates unambiguously that periodic structures rival random textures

Advanced nanostructured materials for pushing light trapping towards the Yablonovitch limit

We give an overview on recent progress in the synthesis, fabrication and integration of advanced nanostructured materials for efficient light trapping in high-efficiency thin-film silicon solar cells

Real-time recognition of surgical tasks in eye surgery videos.

International audienceNowadays, many surgeries, including eye surgeries, are video-monitored. We present in this paper an automatic video analysis system able to recognize surgical tasks in real-time. The proposed system relies on the Content-Based Video Retrieval (CBVR) paradigm. It characterizes short subsequences in the video stream and searches for video subsequences with similar structures in a video archive. Fixed-length feature vectors are built for each subsequence: the feature vectors are unchanged by variations in duration and temporal structure among the target surgical tasks. Therefore, it is possible to perform fast nearest neighbor searches in the video archive. The retrieved video subsequences are used to recognize the current surgical task by analogy reasoning. The system can be trained to recognize any surgical task using weak annotations only. It was applied to a dataset of 23 epiretinal membrane surgeries and a dataset of 100 cataract surgeries. Three surgical tasks were annotated in the first dataset. Nine surgical tasks were annotated in the second dataset. To assess its generality, the system was also applied to a dataset of 1,707 movie clips in which 12 human actions were annotated. High task recognition scores were measured in all three datasets. Real-time task recognition will be used in future works to communicate with surgeons (trainees in particular) or with surgical devices

Optimization of thin film silicon solar cells on highly textured substrates

Doped layers made of nanostructured silicon phases embedded in a silicon oxide matrix were implemented in thin film silicon solar cells. Their combination with optimized deposition processes for the silicon intrinsic layers is shown to allow for an increased resilience of the cell design to the substrate texture, with high electrical properties conserved on rough substrates. The presented optimizations thus permit turning the efficient light trapping provided by highly textured front electrodes into increased cell efficiencies, as reported for single junction cells and for amorphous silicon (a-Si)/microcrystalline silicon tandem cells. Initial and stabilized efficiencies of 12.7 and 11.3%, respectively, are reported for such tandem configuration implementing a 1.1 mu m thick microcrystalline silicon bottom cell

Low-conductivity doped layers for improved performance of thin film silicon solar cells on highly textured substrate

The performances of thin film silicon solar cells are reported for the use of increasingly rough zinc-oxide (ZnO) front electrodes. Experimental results show that textured substrates favor increased light trapping but also the local creation of undesired current drains, degrading the electrical performance of the cells. While an appropriate plasma treatment of the ZnO surface and a high silicon material quality are required to decrease the density of such current drains, doped nano-crystalline silicon oxide layers are proposed to limit the impact of these local non-uniformities onto the performance of both hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) junctions. The possible physical roles of such low-conductivity doped layers are discussed. Conversion efficiency relative increase of 7 % for a-Si:H single junction and of 4 % to 20 % for respectively low (0.3 nm/s) and high deposition rate (1 nm/s) i-layer in μc-Si:H junctions deposited on highly textured substrates are shown. Micromorph solar cells integrating such doped silicon oxide layers could be developed on rough ZnO front electrodes with a high light trapping potential with up to 13.7 % and 11.5 % initial and stabilized efficiency, respectively

Angular reflectance model for ridged specular surfaces, with comprehensive calculation of inter-reflections and polarization

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