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High-quality aspherical and freeform surfaces are in high demand, and the high-accuracy form measurement of such surfaces is a challenging task. To explore the current status of form measurement systems for complex surfaces such as aspheres and freeforms, interlaboratory comparison measurements are performed. In a new publication, the pseudonymized results obtained using three different surfaces (metal asphere, glass asphere, toroidal surface) in a total of six different round robins are presented. These results were taken from a total of 13 different measurement instruments based on 9 different measurement principles and operated at 12 different laboratories. They were analyzed using a sophisticated procedure that was first developed in 2018 and then refined and tested on simulated data in 2022 to address the challenges of such a comparison at this level of accuracy. In the current study, we applied these refined methods to data acquired from tactile and optical point measurements as well as from optical areal measurements. As there are no absolutely measured and very well characterized reference standard aspherical and freeform surfaces available at the accuracy level of a few tens of nanometers root-mean-square, the approximated true forms of the surfaces were derived from the measurements and indicate the manufacturing accuracy of the surface forms. Then, the measurement's differences to the approximated true forms were analyzed, which directly indicate the systematic measurement errors of the instruments. By also comparing the approximated true forms from the two different round robins for each surface, additional insights into the reliability and stability of these so-called virtual reference topographies were gained.

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Accurate and flexible form measurements for aspherical and freeform surfaces are in high demand, and non-null-test interferometric methods such as tilted-wave interferometry have gained attention as a promising response to this need. Interferometric methods, however, display ambiguities between the measurement of certain form errors and the misalignment of the measured specimen. Therefore, improved knowledge of the absolute measurement position of the specimen in relation to the interferometer setup may improve the form measurement result. In a new publication, we propose a concept that uses a white light interferometer to measure the absolute distance between a transparent specimen's surface and the interferometer's objective and present preparatory data to qualify the white light interferometer for the improvement of tilted-wave interferometer measurements.

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The tilted-wave interferometer is a promising technique for the development of a reference measurement system for the highly accurate form measurement of aspheres and freeform surfaces. The technique combines interferometric measurements, acquired with a special setup, and sophisticated mathematical evaluation procedures. To determine the form of the surface under test, a computational model is required that closely mimics the measurement process of the physical measurement instruments. The parameters of the computational model, comprising the surface under test sought, are then tuned by solving an inverse problem. Due to this embedded structure of the real experiment and computational model and the overall complexity, a thorough uncertainty evaluation is challenging. In a new publication, a Bayesian approach is proposed to tackle the inverse problem, based on a statistical model derived from the computational model of the tilted-wave interferometer. Such a procedure naturally allows for uncertainty quantification to be made. We present an approximate inference scheme to efficiently sample quantities of the posterior using Monte Carlo sampling involving the statistical model. In particular, the methodology derived is applied to the tilted-wave interferometer to obtain an estimate and corresponding uncertainty of the pixel-by-pixel form of the surface under test for two typical surfaces taking into account a number of key influencing factors. A statistical analysis using experimental design is employed to identify main influencing factors and a subsequent analysis confirms the efficacy of the method derived.

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For observation of aerosol properties, such as the essential climate variable aerosol optical depth (AOD), several measurement networks, such as AERONET, GAW-PFR and SKYNET, are established. The temperature coefficients of sun photometers from AERONET (AErosol Robotic NETwork) network are required to correct for varying ambient temperatures within the framework of the EMPIR project 19ENV04 MAPP “Metrology for aerosol optical properties”. Spectrally resolved measurement of the temperature dependency of the photometers were performed and are presented in this paper.

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Here we present a compact LED-based source designed within the framework of the EMPIR project 19 ENV04 MAPP “Metrology for aerosol optical properties” for monitoring responsivities of the AERONET Europe radiometers. The aim of the task was to design an LED source flangeable to an integrating sphere that would allow to track responsivity changes of the network instruments. For this purpose the spectral flux of the source must cover the bands of the Cimel sun photometers used in this network and be stable and reproducible at a level of ~0,1%. The design includes 9 LEDs in surface-mount device (SMD) package, one for each band of the sun photometer, soldered on a printed circuit board (PCB). The temperature of the LEDs is accurately controlled by a Peltier device attached to the other side of the PCB. The LEDs are connected in series and operated in stabilized current (CC) mode. Several such LED boards have been built. The devices have been investigated with respect to the spectral properties, stability, and reproducibility. First measurements showed the spectral flux of the LEDs coupled to an integrating sphere to be sufficiently high for being detected by the sun photometers. This report shows the validation of the stability of the flux of the LEDs and gives an outlook about the implementation of the LED source.

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Triply positive charged ions of the isotope Th-229 (Th3+) are of particular interest as the basis for a high-precision optical clock. As the isotope is radioactive, it is only available in very small quantities and conventional methods for realising an ion source (for example by vaporisation) are not applicable in this case. PTB has now developed an apparatus for generating, cooling and storing Th3+ as recoil ions from the radioactive decay of U-233. A thin film of U-233 emits the desired ions into the vacuum, but with a high initial energy of more than 80 keV. By decelerating in high-purity helium gas, it is possible to collect the ions and transfer them to an ion trap. There they are stored together with Sr+ ions, which are cooled to a temperature in the millikelvin range using laser light. The coupling with the Sr ions also reduces the energy of the Th3+ to a value that is about 11 orders of magnitude below the starting energy after the U-233 decay. The stored Th3+ ions are now available at low energy for precision measurements of their resonance frequencies. This provides a basis for studying the properties and structure of this unusual isotope.

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Optical properties of atmospheric aerosols, such as aerosol optical depth (AOD), are determined from solar radiation measurements using filter radiometers, known in such measurement applications as sun photometers. Here we present calibration of filter radiometers from the European branches of aerosol monitoring networks AERONET, GAW-PFR, and SKYNET with respect to their spectral irradiance responsivities. The calibrations were carried out at PTB within the framework of the EMPIR project 19ENV04 MAPP “Metrology for aerosol optical properties” using the tunable laser-based setup, TUable Lasers In Photometry (TULIP). The realization of the spectral responsivity measurements of the filter radiometers at the TULIP setup, results, uncertainty contributions and implications are discussed in this conference paper.

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A new EURAMET Traceability Project Opens external link in new window“Traceability of the GAW-PFR reference Precision Filter Radiometers to the SI” was started in collaboration between PTB, Working Group 4.11 “Spectroradiometry” and PMOD/WRC. PMOD/WRC is designated by the World Meteorological Organisation (WMO) to operate the World aerosol Optical depth Research and Calibration Center (WORCC). WORCC is responsible for hosting the world reference for Aerosol Optical Depth (AOD), formed as a set of Precision Filter Radiometers (PFR). Furthermore, it operates the AOD monitoring network established within the frame of the Global Atmospheric Watch (GAW) program (GAW-PFR network). The WORCC mandate also encompasses to provide AOD traceability from its GAWPFR reference to other global and regional AOD monitoring networks. This is achieved through the Filter Radiometer comparison campaign held a PMOD/WRC every five years, and by collocating PFR travel standards at other network calibration sites (e.g. ACTRIS CARS). The collaboration between PTB and PMOD/WRC with respect to the SI-traceability of spectral solar irradiance of the PFR started in 2019 and continued in the framework of the EMPIR project Opens external link in new window19ENV04 MAPP, “Metrology for aerosol optical properties”. The proposed project aims at providing SI-traceability to the global AOD monitoring community through comparisons with the GAWPFR reference, which is made up of a set of PFR instruments, of which one or more instruments will be annually calibrated at PTB.

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Die Arbeitsgruppe 4.53 Solarmodule beteiligt sich im Rahmen eines DFG-Forschungsprojekts an der Untersuchung, wie die flächendeckende Installation von Photovoltaik (PV)-Anlagen das städtische Mikroklima im Freien und in Innenräumen hinsichtlich der thermischen Behaglichkeit der Einwohner und der Luftqualität beeinflusst. Projektleiter ist die Leibniz Universität Hannover und weiterer Projektpartner die Technische Universität Dresden.

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