Dr. Jens Illemann Profile

Dr. Jens Illemann

at Physikalisch-Technische Bundesanstalt

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Proceedings Article | 10 September 2004 Paper

Absolute deflectometric measurement of topography: influence of systematic deviations

Jens Illemann, Andreas Just

Proceedings Volume 5457, (2004) https://doi.org/10.1117/12.545666

KEYWORDS: Calibration, Sensors, Autocollimators, Mirrors, Reflectors, Prisms, Head, Mercury, Reconstruction algorithms, Zerodur

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For industry and research there is an interest in increasing large master surfaces and other unique optics. Even for meter-scale objects, an uncertainty of form measurement in the range of nanometers has to be fulfilled. The deviation in the radius of curvature should reach 1000 kilometers and more. We present a robust optical profiler that allows the measurement of absolute topographic profiles of flat and slightly curved surfaces up to one meter in length. It is based on simultaneous multiple angle measurement, actually carried out with a common electronic autocollimator and a specially designed front aperture. By implementation of the Extended Shear Angle Difference (ESAD) algorithm - developed at the PTB - the complete angular topography can be reconstructed from two data sets of angle difference. At last, integration leads to the height topography. The uncertainty of measurement of the parabolic form contribution is close to the one claimed above and is referenced to a smaller diameter flatness transfer standard. All other form components are measured absolutely. Measurements on an excellent optical flat standard of 60 cm in length show a reproducibility of about 0.2 nm rms. The systematic errors contributing to the uncertainty budget are listed and investigated experimentally. Starting from this, suggestions will be made how advancement can be reached, and it is proposed, that the limit is clearly over 1000 kilometer in radius of curvature.

Proceedings Article | 20 November 2003 Paper

High-accuracy form measurement of large optical surfaces

Michael Schulz, Ralf Geckeler, Jens Illemann

Proceedings Volume 5190, (2003) https://doi.org/10.1117/12.503700

KEYWORDS: Calibration, Sensors, Autocollimators, Interferometers, Prisms, Zerodur, Optical testing, Deflectometry, Spherical lenses, Time metrology

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Surfaces as needed in optical systems, ranging from the visible even into the EUV region, become larger and often have a length or diameter of 500 mm and more. The form of these surfaces, describing the surface spatial frequency content with components below 1 mm-1, has to be characterized on the nanometer and sometimes even on the sub-nanometer scale. The extendibility of the measuring systems accuracy to large specimen dimensions basically depends on the method of the measurement and the scaling of different systematic uncertainty components with lateral coordinate values. This is analyzed for flatness and sphericity measuring systems, with a focus on the systems for Extended Shear Angle Difference (ESAD) and Large Area Curvature Scanning (LACS) used at PTB. Both are scanning methods working absolute and with a good natured scalability to large dimensions. For the measurement of optical flats the dominant uncertainty of topography is in the quadratic or spherical contribution of the surface in terms of a polynomial description. For calibration flats, as used for large interferometers, this often cannot be measured absolutely with sufficient accuracy. The potential of ESAD and other methods is analyzed with respect to this uncertainty component. Uncertainty considerations and measurement results for large flats are presented. For the form measuremetn of largely extended convex or concave surfaces, where classical interferometric set-ups are not possible due to the lack of a master surface or the extrme costs incurred for large optical components, the potential of LACS is presented.

Proceedings Article | 4 November 2003 Paper

Absolute high-accuracy deflectometric measurement of topography

Jens Illemann

Proceedings Volume 5188, (2003) https://doi.org/10.1117/12.507867

KEYWORDS: Calibration, Sensors, Mirrors, Autocollimators, Prisms, Fourier transforms, Geometrical optics, Manufacturing, Wafer inspection, Measurement devices

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The absolute measurement of the surface figure of large, slightly curved specimens with nanometer accuracy is a demanding task and has not yet been satisfactorily solved. Deflectometric methods offer the possibility of measuring specimens of arbitrary large size and arbitrary figure without a reference surface being required. Here the slope is measured and from this the topography is obtained by integration. The method needs a calibrated (multi-) angle measuring tool (e.g. a commercial autocollimator) and well-defined lateral positioning. Recently, two systems have been described that eliminate the influences of scanning guide and other errors by the use of angle differences only (Extended Shear Angle Difference - ESAD). With the first one of these systems, sub-nm uncertainty has already been achieved for optical flats 150 mm in diameter. In this paper we present recent experimental results obtained with the second system and scan lengths up to 88 cm. The contribution of experimental uncertainties to the uncertainty of topography will be discussed. The behavior of ESAD systems with respect to their spatial frequency response will be discussed. It is shown that the uncertainty of the parabolic part (i.e. curvature) of the figure in particular has to be treated separately and dominates the topography. Applications might reach from wafer inspection to straightness stamdards to flat glass production to free-form synchrotron mirror correction.

Proceedings Article | 24 December 2002 Paper

Topography measurement of nanometer synchrotron optics

Jens Illemann, Ralf Geckeler, Ingolf Weingartner, Carsten Schlewitt, Bernd Grubert, Olaf Schnabel

Proceedings Volume 4782, (2002) https://doi.org/10.1117/12.450982

KEYWORDS: Calibration, Head, Autocollimators, Prisms, Zerodur, Mirrors, Optical testing, Synchrotrons, Synchrotron radiation, Optical components

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The technological limit on the beam quality when modern synchrotron radiation sources are used is determined by the geometrical accuracy of the optical components. This in turn is limited by the accuracy of the measuring technique which is within the range of up to 0.05 arcsec rms for one meter mirror length, which corresponds (in the absence of waviness) to an uncertainty of the topography of 5 nm rms. If the topography can be measured with higher accuracy, modern methods of ion beam processing allow the surface to be postprocessed with a high resolution of depth. We will present first tests with a novel measuring device which allows deflectometric measurements by the ESAD principle (Extended Shear Angle Difference) to be carried out. The basic item of this device is a commercial electronic autocollimator (AC) whose exit aperture is tripartite. By suitable evaluation one is in consequence able to simultaneously determine the angle information belonging to three surface points situated next to one another. According to the ESAD method, the angular topography can be completely reconstructed from two sets of angular difference data. The uncertainty of measurements of angular difference is transferred with a factor close to 1 to that of the set of reconstructed angle. First measurements show a reproducibility of about 25 milli-arcsec rms at a time of integration of 0.4 seconds per point. With this set-up, in the first order, no guide errors, vibrations or air turbulences enter.

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