Aplicabilidad del uso de un panel de PTFE (politetrafluroetileno) como blanco de referencia

El teflón (politetrafluoroetileno. PTFE) es un material inerte, impermeable y flexible que no se altera por la acción de la luz. Su utilidad como panel de referencia ya se ha demostrado. Sus características y precio hacen que sea una alternativa cuando se necesitan paneles de referencia de ciertas dimensiones. En este trabajo se presentan ventajas e inconvenientes de usar un panel de teflón de 3 mm de grosor como panel de referencia. A partir de medidas de reflectividad realizadas con un espectro-radiómetro Ocean Optics 2000, en un rango de 400-900 nm, se determina el comportamiento espectral y la sensibilidad del panel de teflón en tres escenarios diferentes, sobre superficie blanca, lisa y homogénea (Escenario 1), superficie negra, lisa y homogénea (Escenario 2); y superficie blanca, rugosa y heterogénea (Escenario 3). Se constata que el espesor del mismo es insuficiente confiriéndole propiedades translúcidas y una flexibilidad que supone una pérdida de homogeneidad cuando la superficie base no es lisa. Pese a ello, el panel es una buena alternativa siempre y cuando se utilice en condiciones óptimas en términos de geometría de iluminación y observación. Los resultados obtenidos indican que la reflectividad del panel de teflón sobre fondo blanco es aproximadamente entre un 10% y 18% menor que la del Spectralon, dándose las mayores diferencias en el infrarrojo cercano.PTFE is an inert, impermeable and flexible material, not altered by the sunlight. Its usefullness as reference panel has already been tested. Due to its properties and low cost it is a good alternative when a large size panel is needed. This research shows the advantages and disadvantages of using PTFE as reference panel. Reflectance measures have been taken using an Ocean Optics 2000 spectroradiometer in the 400-900 nm range. Three different scenes (1: white, smooth and homogenous surface; 2: black, smooth and homogenous surface; and 3: white, rough and heterogeneous surface) have been used for proving the spectral behaviour and the sensitivity of the PTFE panel. It has been shown that the thickness (3 mm) is not enough and the flexibility causes a loss of homogeneity when the surface is not plain. However, the PTFE is a good option when used under optimal illumination and observation geometric conditions. The results show that reflectance of the PTFE on a white surface is around 10-18% lower than the Spectralon, especially in the near infrared region

Generation of a flat-top laser beam for gravitational wave detectors by means of a nonspherical Fabry-Perot resonator

We have tested a new kind of Fabry-Perot long-baseline optical resonator proposed to reduce the thermal noise sensitivity of gravitational wave interferometric detectors--the "mesa beam" cavity--whose flat top beam shape is achieved by means of an aspherical end mirror. We present the fundamental mode intensity pattern for this cavity and its distortion due to surface imperfections and tilt misalignments, and contrast the higher order mode patterns to the Gauss-Laguerre modes of a spherical mirror cavity. We discuss the effects of mirror tilts on cavity alignment and locking and present measurements of the mesa beam tilt sensitivity

Assessing the potential of the dart model to discrete return lidar simulation—application to fuel type mapping

Fuel type is one of the key factors for analyzing the potential of fire ignition and propaga-tion in agricultural and forest environments. The increase of three-dimensional datasets provided by active sensors, such as LiDAR (Light Detection and Ranging), has improved the classification of fuel types through empirical modelling. Empirical methods are site and sensor specific while Radiative Transfer Models (RTM) approaches provide broader universality. The aim of this work is to analyze the suitability of Discrete Anisotropic Radiative Transfer (DART) model to replicate low density small-footprint Airborne Laser Scanning (ALS) measurements and subsequent fuel type classification. Field data measured in 104 plots are used as ground truth to simulate LiDAR response based on the sensor and flight characteristics of low-density ALS data captured by the Spanish National Plan for Aerial Orthophotography (PNOA) in two different dates (2011 and 2016). The accuracy assessment of the DART simulations is performed using Spearman rank correlation coefficients between the simulated metrics and the ALS-PNOA ones. The results show that 32% of the computed metrics overpassed a correlation value of 0.80 between simulated and ALS-PNOA metrics in 2011 and 28% in 2016. The highest correlations were related to high height percentiles, canopy variability metrics as for example standard deviation and Rumple diversity index, reaching correlation values over 0.94. Two metric selection approaches and Support Vector Machine classification method with variants were compared to classify fuel types. The best-fitted classification model, trained with the DART simulated sample and validated with ALS-PNOA data, was obtained using Support Vector Machine method with radial kernel. The overall accuracy of the classification after validation was 88% and 91% for the 2011 and 2016 years, respectively. The use of DART demonstrates its value for simulating generalizable 3D data for fuel type classification providing relevant information for forest managers in fire prevention and extinction

I.B.S. coatings on large substrates: Towards an improvement of the mechanical and optical performances

présenté par A. RemillieuxLarge mirrors (350 mm), having extremely low optical loss (absorption, scattering, wavefront) were coated for the VIRGO interferometer. The new mirror generation needs better wavefront and lower mechanical loss. The first results are discussed

Original optical metrologies of large components

présentée par A. RemillieuxThe coating deposition on large optical components (diameter 350 mm) has required the development of new metrology tools at 1064 nm. To give realistic values of the optical performances, the whole surface of the component needs to be scanned. Our scatterometer (commercial system) has been upgraded to support large and heavy samples. The other metrology tools are prototypes we have developed. We can mention the absorption (photothermal effect) and birefringence bench, a control interferometer equipped with an original stitching option, the optical profilometer (RMS roughness and small defect measurements). A detailed description of these metrology benches will be exposed. Their sensitivity, accuracy and capability to map the optical properties of substrates or mirrors will be discussed. We will describe the recent developments: the stitching option adapted to the Micromap profilometer to measure the RMS roughness on larger area (exploration of a new spatial frequency domain), the accurate bulk absorption calibration

Surface-enhanced Raman spectroscopy of tears: Toward a diagnostic tool for neurodegenerative disease identification

Significance: A noninvasive method based on surface-enhanced Raman spectroscopy (SERS) of tears was proposed as a support for diagnosing neurodegenerative pathologies, including different forms of dementia and Alzheimer's disease (AD). In this field, timely and reliable discrimination and diagnosis are critical aspects for choosing a valid medical therapy, and new methods are highly required. Aim: The aim is to evince spectral differences in SERS response of human tears from AD affected, mild cognitive impaired (MCI), and healthy control (Ctr) subjects. Approach: Human tears were characterized by SERS coupled with multivariate data analysis. Thirty-one informed subjects (Ctr, MCI, and AD) were considered. Results: Average SERS spectra from Ctr, MCI, and AD subjects evidenced differences related to lactoferrin and lysozyme protein components. Quantitative changes were also observed by determining the intensity ratio between selected bands. We also constructed a classification model that discriminated among AD, MCI, and Ctr subjects. The model was built using the scores obtained by performing principal component analysis on specific spectral regions (i-PCA). Conclusions: The results are very encouraging with interesting perspectives for medical applications as support of clinical diagnosis and discrimination of AD from other forms of dementia

Measurements of a low temperature mechanical dissipation peak in a single layer of Ta2O5 doped with TiO2

Thermal noise arising from mechanical dissipation in oxide coatings is a major limitation to many precision measurement systems, including optical frequency standards, high resolution optical spectroscopy and interferometric gravity wave detectors. Presented here are measurements of dissipation as a function of temperature between 7 K and 290 K in ion-beam sputtered Ta2O5 doped with TiO2, showing a loss peak at 20 K. Analysis of the peak provides the first evidence of the source of dissipation in doped Ta2O5 coatings, leading to possibilities for the reduction of thermal noise effects

Intermediate snowpack melt-out dates guarantee the highest seasonal grasslands greening in the Pyrenees

In mountain areas, the phenology and productivity of grassland are closely related to snow dynamics. However, the influence that snow melt timing has on grassland growing still needs further attention for a full understanding, particularly at high spatial resolution. Aiming to reduce this knowledge gap, this work exploits 1 m resolution snow depth and Normalized Difference Vegetation Index observations acquired with an Unmanned Aerial Vehicle at a sub-alpine site in the Pyrenees. During two snow seasons (2019–2020 and 2020–2021), 14 NDVI and 17 snow depth distributions were acquired over 48 ha. Despite the snow dynamics being different in the two seasons, the response of grasslands greening to snow melt-out exhibited a very similar pattern in both. The NDVI temporal evolution in areas with distinct melt-out dates reveals that sectors where the melt-out date occurs in late April or early May (optimum melt-out) reach the maximum vegetation productivity. Zones with an earlier or a later melt-out rarely reach peak NDVI values. The results obtained in this study area, suggest that knowledge about snow depth distribution is not needed to understand NDVI grassland dynamics. The analysis did not reveal a clear link between the spatial variability in snow duration and the diversity and richness of grassland communities within the study area

The Virgo data acquisition system

International audienc