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3D-Lasermeter with refractive index compensation

Interferometric length measurement under difficult ambient conditions

PTB-News 3.2017
Especially interesting for

the aviation industry and other manufacturers of large-scale components (wind power industry, etc.)

manufacturers and operators of coordinate measuring machines

providers of calibration services

Within the scope of an international project, PTB has developed the prototype of a 3D-capable interferometer that compensates for the optical refractive index of air intrinsically. This so-called 3D-Lasermeter has been successfully tested in two series of tests under difficult industrial ambient conditions.

Verification of the 3D-Lasermeter during measurements at the Polish metrology institute (GUM). A constant distance of 19 m was measured with the wavelengths 532 nm and 1064 nm of the 3D-Lasermeter manufactured by SIOS GmbH (small picture). When using a network of temperature sensors to compensate for the refractive index, the distance is allegedly reduced by up to 30 μm (red curve) during the warm-up phase. Contrary to this, the distance from the intrinsically determined correction of the refractive index (blue curve) remains stable (apart from the slightly higher noise).

The requirements placed on the dimensional accuracy of large-scale components (with dimensions of several meters) are becoming more and more demanding in industry and fundamental research. In the aviation industry, for instance, drill holes in wing segments must be positioned with an accuracy on the order of a few tens of micrometers. Such accuracies can be attained with various optical measurement procedures – in particular interferometry-based ones. For correct measurements, the refractive index of the ambient air must, however, be known with sufficient precision. This, in turn, requires the correct detection of the following ambient parameters: pressure, temperature, relative humidity and CO2 concentration. If a length is to be determined with micrometer accuracy over a length of 10 m, for example, then the temperature inside the beam must be known with 0.1 K accuracy. For measurements performed outside well-air-conditioned laboratories, this can only be achieved approximatively by means of a very dense and elaborate network of sensors to pick up the ambient parameters.

Within the scope of the international LUMINAR (Large Volume Metrology in Industry) project, an alternative approach has been pursued: the refractive index is determined parallel to the actual length measurement by dispersive refraction compensation. Hereby, the geometrical length is measured by means of two interferometers with optical frequencies (or wavelengths) that are different from each other but very accurately known. By combining the information gained about the path length in this way, it is possible to determine the refractive index – and thus the exact geometrical length – without additional i n forma t i on and with an accuracy of up to 1 · 10–7.

Within the scope of the LUMINAR project, PTB, together with SIOS Meßtechnik GmbH, has developed the prototype of an auto-tracking interferometer which intrinsically compensates for the refractive index. The probe can automat ica l ly follow a measuring reflector in space. To this end, two wavelengths of two NdYAG lasers, which are stabilized against each other, are used.

The 3D-Lasermeter was tested within the scope of the same project at the 50 m comparator section of the Polish metrology institute (GUM) under difficult controlled ambient conditions. Moreover, it was also successfully tested under real industrial conditions in an Airbus test hangar in Filton, UK. Thereby, it was possible to demonstrate that the measurement uncertainty of the intrinsically optical refractive index compensation is in the micrometer range.

Two patents have been granted with regard to the procedure. By means of established multilateration procedures, the system can be used to determine the position of points in space – i.e. in the future, it could be used to calibrate large-scale coordinate measuring machines (such as those used to measure the components of wind turbines).


Florian Pollinger
Department 5.4
Interferometry on Material Measures
Phone: +49 (0)531 592-5420
Opens window for sending emailflorian.pollinger(at)ptb.de

Scientific publication

K. Meiners-Hagen, T. Meyer, G. Prellinger, W. Pöschel, D. Dontsov, F. Pollinger: Overcoming the refractivity limit in manufacturing environment, Opt. Express 24, 25092 (2016)