Marc Gronle

Improved signal model for confocal sensors accounting for object depending artifacts

The conventional signal model of confocal sensors is well established and has proven to be exceptionally robust especially when measuring rough surfaces. Its physical derivation however is explicitly based on plane surfaces or point like objects, respectively. Here we show experimental results of a confocal point sensor measurement of a surface standard. The results illustrate the rise of severe artifacts when measuring curved surfaces. On this basis, we present a systematic extension of the conventional signal model that is proven to be capable of qualitatively explaining these artifacts.

Advantages of chromatic-confocal spectral interferometry in comparison to chromatic confocal microscopy

Chromatic confocal microscopy (CCM) and spectral interferometry (SI) are established and robust sensor principles. CCM is a focus-based measurement principle, whose lateral and axial resolutions depend on the sensors numerical aperture (NA), while the measurement range is given by the spectral bandwidth and the chromatic dispersion in the axial direction. Although CCM is a robust principle, its accuracy can be reduced by self-imaging effects or asymmetric illumination of the sensor pupil. Interferometric principles based on the evaluation of the optical path difference, e.g., SI, have proven to be robust against self-imaging. The disadvantage of SI is its measurement range, which is limited by the depth of focus. Hence, the usable NA and the lateral resolution are restricted. Chromatic-confocal spectral interferometry (CCSI) is a combination of SI and CCM, which overcomes these restrictions. The increase of robustness of CCSI compared to CCM due to the interferometric gain has been demonstrated before. In this contribution the advantages of CCSI in comparison to CCM concerning self-imaging artifacts will be demonstrated. Therefore, a new phase-evaluation algorithm with higher resolution concerning classical SI-based evaluation algorithms is presented. For the comparison of different sensor systems, a chirp comparison standard is used.

itom: an open source metrology, automation, and data evaluation software

Modern optical sensors and measurement systems usually are a powerful combination of optical elements, active hardware components like actuators or sensing devices as well as a sophisticated control software and data evaluation algorithms. In order to develop and operate such systems, it is necessary to have a flexible, intuitive, and fast underlying software framework that also allows for rapid prototyping of a sensor in a dynamic lab environment. This software must be able to control and communicate with all necessary hardware devices and has to provide all the highly performant evaluation, data, and image processing algorithms required. In this publication, we want to present the open source measurement and data evaluation software suite itom, which has been designed considering the denoted requirements and whose development began in 2011.

Open-source graphics processing unit-accelerated ray tracer for optical simulation

Abstract. Ray tracing still is the workhorse in optical design and simulation. Its basic principle, propagating light as a set of mutually independent rays, implies a linear dependency of the computational effort and the number of rays involved in the problem. At the same time, the mutual independence of the light rays bears a huge potential for parallelization of the computational load. This potential has recently been recognized in the visualization community, where graphics processing unit (GPU)-accelerated ray tracing is used to render photorealistic images. However, precision requirements in optical simulation are substantially higher than in visualization, and therefore performance results known from visualization cannot be expected to transfer to optical simulation one-to-one. In this contribution, we present an open-source implementation of a GPU-accelerated ray tracer, based on nVidias acceleration engine OptiX, that traces in double precision and exploits the massively parallel architecture of modern graphics cards. We compare its performance to a CPU-based tracer that has been developed in parallel.

Laterally chromatically dispersed, spectrally encoded interferometer

We present a single-shot line sensor based on spectral interferometry. Light of a broadband laser source is chromatically dispersed by a grating and focused onto a line on the surface such that each focal point on this line is formed by another wavelength. The entire height profile is obtained by applying a phase evaluation algorithm to the registered interference signal, followed by a model-based approach. The sensor concept is finally verified by experimental results.

Model-based, active inspection of three-dimensional objects using a multi-sensor measurement system

Considering modern manufacturing processes, there is an increasing demand for flexible, fast and precise inspection systems. Usually, the holistic inspection of technical components with a complex three-dimensional surface, like gears, needs to be separated into inspection steps. Different areas on the object need to be verified with respect to varying characteristic specifications, e.g. related to defects or roughness properties. Such manifold inspection tasks can for instance be realized using a multi-sensor measurement system which is also equipped with a multi-axis system to optimally move and rotate each sensor with respect to any desired position at the object’s surface. In order to generate an automatic inspection system, the entire process is defined with respect to a polygonal model of the measurement specimen, such that different sub-regions are connected with different specifications and parameterizations that this region must meet and hence needs to be verified by the inspection system. However, the data acquisition with respect to sub-regions on the model’s surface and the integration of obtained datasets in the model’s coordinate system is only feasible if the transformation of the real object to the model is determined before. Consequently, this needs to be determined in the initialization phase of the overall inspection process.

View and sensor planning for multi-sensor surface inspection

Modern manufacturing processes enable the precise fabrication of high-value parts with high precision and performance. At the same time, the demand for flexible on-demand production of individual objects is continuously increasing. These requirements can only be met if inspection systems provide appropriate answers. One solution is the use of flexible, multi-sensor setups where multiple optical sensors with different fields of application are combined in one system. However, the challenge is then to assist the user in planning the inspection for individual parts. A manual planning requires an expert knowledge of the performance and functionality of every sensor. Therefore, software assistant systems help the user to objectively select the right sensors for a given inspection task. The planning step becomes still more difficult if the manufactured part has a complex form. The implication is that a sensors position must also be part of the planning process since it significantly influences the quality of the inspection. This paper describes a view and sensor planning approach for a multi-sensor surface inspection system in the context of optical topography measurements in the micro- and meso-scale range. In order to realize an online processing of the assistant system, a significant part of the calculations are done on the graphics processing unit (GPU).

Multi-layer topography measurement using a new hybrid single-shot technique: Chromatic Confocal Coherence Tomography (CCCT)

Often measurement tasks occur, where specimens consist of multiple layers or topography shall be examined through contaminations. Especially for unknown layer materials, it is important to measure the layers refractive index to compensate for the errors induced on the measurement of underlying surfaces. Chromatic Confocal Coherence Tomography is proposed as a new hybrid single-shot scheme for a simultaneous measurement of thickness and refractive index of semitransparent layers, combining chromatic confocal and interferometric information. As a proof of concept, first measurements are presented along with a short discussion about their uncertainties, where minimal layer thickness and resolution are dominated by the confocal part of the signal, that is mainly influenced by the chosen microscope objective.

Realistic simulation of camera images of local surface defects in the context of multi-sensor inspection systems

Industrial automation has developed rapidly in the past decades. Customized fabrications and short production time require flexible and high speed inspection systems. Based on these requirements, optical surface inspection systems (OSIS) as efficient and cheap systems for detecting surface defects and none-defects becomes more and more important. To achieve a high recognition rate, huge amounts of image data of defects need to be stored. We introduce a virtual surface defect rendering method to obtain large amount of defect images. In this paper, the ray tracing methods are applied to realistically simulate camera images in OSIS. We used three different bidirectional reflectance distribution function (BRDF) rendering models to describe the scattering between collimated white light and aluminum materials.

Extrinsic calibration of a fringe projection sensor based on a zoom stereo microscope in an automatic multiscale measurement system

Multi scale systems offer the opportunity to balance the conflict between execution time, measurement volume and resolution for the inspection of highly complex surface profiles. An example of such a task is the inspection of gears. At first, the coarse position and form of the specimen is registered by a sensor measuring with comparatively low resolution but a large field of view. Possible defects near to the resolution limit are indicated and new regions of interest for higher resolved measurements are identified. As prerequisite for a successful multi-scale inspection, every sampled data set, acquired in different scales and at varying positions, must be registered in one global data model. This is only possible if the extrinsic coordinate transform from the sensors internal coordinate system to the common, global coordinate system of the inspected object and its uncertainties are known. In this paper, we present an approach for the extrinsic calibration using the example of a multi-zoom fringe projection sensor mounted on a multi-axes measurement system. Finally we show the measurement result of a gear, where several sampled patches are merged together into one point cloud with the aid of the presented calibration.