With state-of-the-art 3D measurement systems, short-wave structures such as tool marks cannot be resolved directly inside a machine tool chamber. Up to now, measurements had to be performed outside the machine tool. We present an interferometric sensor that carries out such measurements inside the machine tool, which saves time-consuming and expensive setup procedures. Our sensor HoloCut uses digital holography as measurement principle. By the use of multiple wavelengths, we get a large unambiguous axial measurement range of up to 2 mm and achieve micron repeatability, even in the presence of laser speckles. With a lateral resolution of 7 μm across the entire 20 x 20 mm2 field of view, both macro- and microstructures (such as tool marks) are measured with an axial resolution of 1 μm. Consequently, this qualifies HoloCut for in-situ measurements and integration in a machine tool. In this paper, the boundary conditions of integrating interferometers inside a machine tool are evaluated. Occurring vibrations and limited available space are particularly challenging constraints: The optical and mechanical design of HoloCut is introduced along with numerical correction algorithms: A piezo-stage setup is used to induce known displacements. Using these algorithms, measurements even with a closed-loop control of the machine tool head activated are demonstrated on a coin measurement. The use of HoloCut is motivated on the base of the daily operation of a 5-axis machine tool: We present an evaluation of an exemplary ISO 25178 parameter Sq using HoloCut measurements and compare those with reference, yet not inline-capable systems.
Component surfaces feature more and more complex functional properties and deterministic geometric structures. The result is that an areal characterization of surfaces is more often necessary. The increasing incidence of areal surface topography measuring instruments in geometrical product specification enables the acquisition of more information about a surface topography. However, also more complex calibration procedures are required as an increasing number of metrological characteristics need to be verified. This verification is achieved with areal material measures which are described in the standard ISO 25178-70. State of the art is a manufacturing of the proposed geometries with many different principles because there is a broad range of geometries whose structure size is usually in the micrometer-range. Typically applied principles include lithography, etching and ultra-precision cutting. The application of an ultra-precise 3D-printing technology, two-photon laser lithography alias direct laser writing, exhibits enormous potential for areal material measures as arbitrary 3D-freeform surfaces can be manufactured with a high repeatability in the nanometer-range.
Hence, a feasibility study of the application of direct laser writing for the manufacturing of areal material measures is conducted. In doing so, different standardized material measures are manufactured and the resulting surface topographies are compared to their target geometries in order to qualify the manufacturing process. The measurements are performed with different surface topography measuring instruments in order to examine the overall suitability of the principle for the manufacturing of areal material measures. The standardized measurands of the ISO 25178-70 serve as evaluation criteria just as recently defined new parameters for the verification of surface topography measuring instruments. As a new resolution criterion, for example the small scale fidelity limit is evaluated. The enhancement of the resolution of the manufacturing process with stimulated emission depletion is examined and the resolution limits of the manufacturing and the measuring processes are compared.
The samples that are manufactured with direct laser writing are further examined regarding their practical abilities. An important property of material measures is their stable provision of constant evaluation parameters. In order to examine this relevant characteristic of the samples, different studies which describe the aging behavior of varying coating materials are conducted. Based on the results, a suitable coating material with suitable optical characteristics can be chosen and the time-dependent behavior of the geometries can be evaluated. Because optical surface topography measuring instruments which are calibrated with the proposed material measures may feature varying magnifications and fields of view, in another study scaling effects are examined and material measures with different structure sizes are manufactured in order to evaluate the scalability of the different types of material measures.
It can be concluded that almost any standardized areal material measure can be manufactured reliably with direct laser writing. Due to the scalability of the structures, a calibration of optical surface topography measuring instruments with varying fields of view can be ensured.
Two-photon laser lithography has become one of the most promising additive manufacturing techniques on the micron scale and is applied, e.g., in fields of micro-optics and -robotics as well as optical and mechanical metamaterials. Here, we report on the feasibility, limits and general benefits of this method to fabricate material measures for the calibration of industrial optical topography measuring devices. Since calibration procedures are essential in the scientific and industrial application of those measuring instruments, appropriate material measures are highly required. In contrast to traditional manufacturing technologies, we show that two-photon laser lithography allows a highly resolved fabrication of multiple, almost arbitrary standardized calibration geometries on a micron length scale. Hereby, all structures are fabricated on only one single substrate, therefore enabling a mapping of a broad range of metrological characteristics for topography characterization. The most required calibration geometries are manufactured and analyzed regarding their aging behavior, their quality improvement by a post-UV development and the resolution limits within the manufacturing as well as the calibration process. Thus, the general industrial and scientific relevance of manufacturing material measures with two-photon laser lithography is demonstrated.
In nowadays industry, complex surfaces with material contrasts or surface coatings are present and represent a challenge
for optical topography measuring instruments. The reason is that varying optical properties lead to phase jumps and to
topography deviations when the surface height is evaluated. Thus, Ellipso-Height-Topometry as a measurement
technique which can measure both topography and material properties of technical surfaces was proposed in order to
achieve a correction of the occurring topographic artefacts. The height correction value can be obtained for the
compensation of material-induced height deviations and the thickness of surface layers can be evaluated. Currently, it is
possible to calculate the surface characteristics from ellipsometric parameters for at most two layers. However, the
described height corrections are only possible when well-defined and realistic models of surface layers can be utilized,
e.g. a given set of homogeneous oxide layers. Oxidation effects however describe statistical processes which can be
predicted with underlying material distribution models. This leads to an uncertainty in ellipsometry, which is considered
with a new approach that will be discussed in this publication. Therefore, an extended multi-layer approach which is
capable of handling additional layers based on a parallelized algorithm using graphic processing units and the commonly
known CUDA technology is proposed. This algorithm can also be used to consider material proportions which result
from oxidation effects in z direction. The new approach for the Ellipso-Height-topometry measurement technique is
compared with the current procedures which often neglect the existence of an oxide layer for the basic material. To
experimentally verify the approach and according algorithm, it is applied for the evaluation of actual surfaces with
multiple plane layers and different materials. Test samples with different materials are used in order to evaluate the
complex refractive index, the distribution of identified materials and the layer thicknesses with actual Ellipso-Height-
Topometry measurements. The results of the measurements are compared to the predicted theoretical results.
The ISO standards which are related to the calibration of areal surface topography measuring instruments are the ISO 25178-6xx series which defines the relevant metrological characteristics for the calibration of different measuring principles and the ISO 25178-7xx series which defines the actual calibration procedures.
As the field of areal measurement is however not yet fully standardized, there are still open questions to be addressed which are subject to current research. Based on this, selected research results of the authors in this area are presented. This includes the design and fabrication of areal material measures. For this topic, two examples are presented with the direct laser writing of a stepless material measure for the calibration of the height axis which is based on the Abbott- Curve and the manufacturing of a Siemens star for the determination of the lateral resolution limit.
Based on these results, as well a new definition for the resolution criterion, the small scale fidelity, which is still under discussion, is presented. Additionally, a software solution for automated calibration procedures is outlined.
The calibration of the height axis of optical topography measurement instruments is essential for reliable topography measurements. A state of the art technology for the calibration of the linearity and amplification of the z-axis is the use of step height artefacts. However, a proper calibration requires numerous step heights at different positions within the measurement range. The procedure is extensive and uses artificial surface structures that are not related to real measurement tasks.
Concerning these limitations, approaches should to be developed that work for arbitrary topography measurement devices and require little effort. Hence, we propose calibration artefacts which are based on the 3D-Abbott-Curve and image desired surface characteristics. Further, real geometric structures are used as an initial point of the calibration artefact.
Based on these considerations, an algorithm is introduced which transforms an arbitrary measured surface into a measurement artefact for the z-axis linearity. The method works both for profiles and topographies. For considering effects of manufacturing, measuring, and evaluation an iterative approach is chosen. The mathematical impact of these processes can be calculated with morphological signal processing.
The artefact is manufactured with 3D laser lithography and characterized with different optical measurement devices. An introduced calibration routine can calibrate the entire z-axis-range within one measurement and minimizes the required effort. With the results it is possible to locate potential linearity deviations and to adjust the z-axis. Results of different optical measurement principles are compared in order to evaluate the capabilities of the new artefact.
Confocal microscopy is a state of the art optical principle to measure the topography of technical surfaces. The output of
the measuring process is an intensity curve for each scanned point on the topography. The maximum of the intensity
curve correlates to the point height. However, the intensity function is influenced by the geometrical properties of the
surface topography and its material. Simple peak picking of the intensity curve leads to insufficient results when
calculating the point height. Therefore, the centre-of-gravity or fitting algorithms are preferred. Both have an integral
behaviour and are able to suppress unwanted signal components. Today, the centre-of-gravity is often the state of the art method. Disadvantages are e.g.: the method is sensitive against vibrations of the instrument during the measuring process. In contrast to the centre-of-gravity calculation, fitting algorithms are numerically inefficient and slow. Moreover, the fitting process needs a priori information about the curve of the intensity function. We propose an alternative algorithm for the robust evaluation of intensity curves. The amplitude spectra of each intensity curve of a
measured reference data set are analysed. The applied technique is based on the Cramér-Rao bound and leads to a
threshold operator for calculating the centre-of-gravity in the frequency domain. A possible phase distortion (an
asymmetrically shape of the intensity function) caused by diffraction or optical aberrations will also be significantly
suppressed. The performance of the algorithm is shown and we compare the algorithm to the popular centre-of-gravity.
Coherence scanning interferometry CSI with a broadband light source (short known as white light interferometry) is,
beside the confocal technique, one of the most popular optical principles to measure surface topography. Compared to
coherent interferometry, the broadband light source leads, theoretically, to an unambiguous phase information.
The paper describes the properties of the correlogram in the spatial and in the frequency domain. All deviations from the
ideal correlogram are expressed by an addition phase term. The uncertainty of height information is discussed for both,
the frequency domain analyse (FDA) proposed by de Groot and the Hilbert transform. For the frequency domain
analyse, the uncertainty is quantified by the Cramér-Rao bound.
The second part of the paper deals with the phase evaluation of the correlogram, which is necessary to achieve a high
vertical resolution. Because the envelope function is often distorted, phase jumps lead to ambiguous height informations.
In particular, this effect can be observed measuring rough surfaces.
In this paper the concept of a so-called assistance system for interferometric and confocal sensor systems is
presented. Goal of the research described here is the development of a software-based user friendly tool for
three-dimensional optical topography measurements. With this assistance system the user will be enabled to use
his measurement system in an optimal way for his special task. Additionally a reliable usage of these systems in
the production environment is provided.
This assistance system will be developed in the project "OptAssyst", where five research institutes are collaborating
with commercial partners from the automobile industry and their suppliers as "users" of the considered
systems. Furthermore several manufacturers of optical measurement systems are involved.
The extraction of 3D shape and roughness by optical measurement techniques become more and more import in
industrial applications. Optical systems are measuring fast with high accuracy and give reliable information about the
workpiece form or surface roughness. The classical systems based on triangulation, white light, confocal, shadow or
fringe projection techniques and are applied with a great success in recent years. In future there will be a growing interest
in robust inline measurement techniques to monitor the manufacturing process. E. g. some automotive manufactures are
using confocal systems to characterize the surface of cylinder liners inline. But there is another robust and powerful
technique suitable for inline measurement purposes: scattered light sensors. In this paper, a special type of a scattered
light sensor based on the former Rodenstock RM 400 sensor is considered. The sensor enables the user to measure form
and roughness in a robust manner. The properties of the sensor are analyzed system-theoretically.
A new, fast and easy process for nanostructuring of hard surfaces is currently being developed: explosive embossing.
The Institute for measurement and control engineering (IMR) of the Leibniz-Universität Hannover and
the Fraunhofer-Institute for Chemical Technology (ICT) are currently presiding over the project1 which deals
with the practicability of explosive embossing for nanostructures such as holographic structures. Within this
project the IMR is concerned with the digital creation of holographic data and the numerical simulation and the
evaluation of the transfer characteristics of the explosive-embossing-process.
The mathematical fundamentals of some black box calibration procedures for fringe projection system are introduced. These calibration procedures are based upon a direct mathematical transformation between the measuring volume and the image data obtained with a camera. Aided by a mathematical model of a fringe projection system various calibration procedures are compared to each other in numerical simulations. The numerical simulations facilitate statements about the attainable measuring error depending on the calibration procedure and system parameters.