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Aktuelle Forschungsnachrichten und Nachrichten aus dem Jahresbericht

Comparing form measurement data for aspheres and freeform surfaces is an important tool for ensuring the quality and functionality of the devices used to take such measurements and may also allow the underlying measurement methods to be evaluated. However, comparing the highly accurate form measurements of such complex surfaces is a demanding task. It is difficult to analyze measurement results whose accuracies are in the range of several tens of nanometers root-mean-square, especially when comparing data with different, and anisotropic distributions of the 3D measurement points on the surface under test. In this paper, we investigate eight different 3D measurement point distributions that are typical of highly accurate measurement systems currently in use and demonstrate the effects of these distributions on the comparison results by using virtually generated data and applying different evaluation strategies. The results show that, for the examples investigated, the different 3D measurement point distributions can yield different levels of accuracy for the comparison. Furthermore, an improved evaluation procedure is proposed and recommendations on how to significantly reduce the influence of the different 3D measurement point distributions on the comparison result are given. A method of employing virtually generated test data is presented that may be generalized in order to further improve and validate future comparison methods.

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An LED sphere radiator (LED-SR) was constructed to improve the accuracy in spectral radiance factor measurements performed with the robot-based gonioreflectometer at PTB. Its properties with respect to the spectral range and coverage, the temporal stability, and the homogeneity of the radiation field are presented. Two types of matte ceramic reflection standards were used for spectral radiance factor validation measurements comparing the standardly used halogen sphere radiator (Halogen-SR) and the LED-SR. Due to its designed spectral range at the border between the visible and the UV-A spectral range, the LED-SR is well suited for many applications in diffuse reflectometry. Its use for absolute radiance factor measurements and investigations of the fluorescence properties of diffuse reflecting samples is shown. Reliable polarization-resolved measurements at wavelengths below 430 nm could be carried out with PTB’s gonioreflectometer for the first time due to the beneficial signal-to-noise ratio of the LED-SR.

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The tilted-wave interferometer is an interferometrical measurement system for the accurate optical form measurement of optical aspheres and freeform surfaces. Its evaluation procedure comprises a high-dimensional inverse problem to reconstruct the form of the surface under test from measured data. Recent work has used a deep learning hybrid approach to solve the inverse problem successfully in a simulation environment. A quantification of the model uncertainty was incorporated using ensemble techniques. In this paper, we expand the application of the deep learning approach from simulations to measured data and show that it produces results similar to those of a state-of-the-art method in a real-world environment.

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Das Phasenrauschen hochfrequenter Signale wird in der Regel durch einen Vergleich des zu untersuchenden Signals mit einem rauscharmen Referenzsignal gleicher Frequenz gemessen. Bei kommerziellen Phasenrauschmessplätzen wird dieses Referenzsignal durch einen über einen weiten Frequenzbereich abstimmbaren Oszillator erzeugt. Dadurch wird die Messung zum einen durch den Frequenzbereich des Referenzoszillators, der bei typischen Geräten nur einige GHz beträgt, und zum anderen durch sein Rauschen beschränkt.
An der PTB wurde ein neuartiges System entwickelt, das es ermöglicht, zur Phasenrauschmessung Mikrowellensignale bis zu einer Frequenz von 100 GHz in den MHz-Bereich zu konvertieren.

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The EMPIR Project “Supporting smart specialisation and stakeholder linkage in Photometry and Radiometry (Smart PhoRa)” has started on September 1st, 2021, for an 18-month funding period.

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The gravitational wave (GW) observatories calibrate interferometer displacement using photon momentum, with laser power serving as the measurand. These observatories are traceable to the International System of Units through a primary standard maintained by the US’s National Metrology Institute (NMI), the National Institute of Standards and Technology (NIST). The bilateral degree of equivalence of laser power measurements for various NMIs indicated in the 2010 EUROMET.PR-S2 supplementary comparison reveals scale realization uncertainty unacceptably large for GW event parameterization. We offer here an analysis to identify the source of the discrepancy between the Physikalisch-Technische Bundesanstalt (PTB) and NIST results. Using an improved transfer standard in a bilateral comparison, with representatives of the Laser Interferometer Gravitational-Wave Observatory (LIGO) receiving results prior to their comparison, NIST and PTB demonstrated a degree of equivalence of −0.15% with an uncertainty of 0.95% (k = 2) for combined 100 mW and 300 mW comparison results.

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We apply an InGaAs quantum dot based single-photon source for the absolute detection efficiency calibration of a silicon single-photon avalanche diode operating in Geiger mode. The single-photon source delivers up to (2.55 ± 0.02) × 106 photons per second inside a multimode fiber at the wavelength of 929.8 nm for above-band pulsed excitation with a repetition rate of 80 MHz. The purity of the single photon emission, expressed by the value of the 2nd order correlation function g(2)(τ = 0), is between 0.14 and 0.24 depending on the excitation power applied to the quantum dot. The single-photon flux is sufficient to be measured with an analog low-noise reference detector, which is traceable to the national standard for optical radiant flux. The measured detection efficiency using the single-photon source remains constant within the measurement uncertainty for different photon fluxes. The corresponding weighted mean thus amounts to 0.3263 with a standard uncertainty of 0.0022.

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Im Rahmen des BMBF-Projektes SiM4diM entwickelt die PTB zusammen mit den Firmen JCMwave und Carl Zeiss IMT sowie der Hochschule Aalen neue Verfahren zur vollständigen Modellierung mikroskopisch-bildgebender Messsysteme auf der Grundlage rigoroser Beugungsrechnungen. Ziel ist es, mit diesem „virtuellen Mikroskop“ die bildbasierte industrielle Messtechnik signifikant zu verbessern und die Unsicherheit dimensionaler Metrologie auch im industriellen Umfeld um mehr als eine Größenordnung zu verringern.

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This contribution presents experimental and simulation results of a tiltable line scanning low coherence interferometer applied for form measurement of spherical and aspherical objects with a diameter of up to 300 mm.
The region of interest is sampled by multiple annular subapertures that are realigned employing stitching algorithms based on Cartesian- and Zernike polynomial fittings.
The paper addresses common challenges in the reduction and modeling of displacement errors associated with the motion of the interferometric sensor between subaperture measurements and compares the topography deviations of the experimental results with those simulated by a Monte Carlo based model.

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