Wolfram Lyda

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.

Optical metrology: from the laboratory to the real world

Optical metrology has shown to be a versatile tool for the solution of many inspection problems. The main advantages of optical methods are the noncontact nature, the non-destructive and fieldwise working principle, the fast response, high sensitivity, resolution and accuracy. Consequently, optical principles are increasingly being considered in all steps of the evolution of modern products. However, the step out of the laboratory into the harsh environment of the factory floor was and is a big challenge for optical metrology. The advantages mentioned above must be paid often with strict requirements concerning the measurement conditions and the object under test. For instance, the request for interferometric precision in general needs an environment where high stability is guaranteed. If this cannot be satisfied to a great extent special measures have to be taken or compromises have to be accepted. But the rapid technological development of the components that are used for creating modern optical measurement systems, the unrestrained growth of the computing power and the implementation of new measurement and inspection strategies give cause for optimism and show that the high potential of optical metrology is far from being fully utilized. In this article current challenges to optical metrology are discussed and new technical improvements that help to overcome existing restrictions are treated. On example of selected applications the progress in bringing optical metrology to the real world is shown.

Optical Design of a Vertically Integrated Array-type Mirau-based OCT system

The presented paper shows the concept and optical design of an array-type Mirau-based OCT system for early diagnosis of skin cancer. The basic concept of the sensor is a full-field, full-range optical coherence tomography (OCT) sensor. The micro-optical interferometer array in Mirau configuration is a key element of the system allowing parallel imaging of multiple field of views (FOV). The optical design focuses on the imaging performance of a single channel of the interferometer array and the illumination design of the array. In addition a straylight analysis of this array sensor is given.

Genetic programming applied to automatic algorithm design in multi-scale inspection systems

In recent years image-processing has become a central part of optical inspection and measurement systems. Typically, after measuring the given specimen by utilizing a suitable sensor, image-processing algorithms are used to detect dedicated features such as surface defects. These algorithms are usually designed, optimized, and tested by an image-processing expert according to the task specifications. A methodology (based on genetic programming) is presented to automatically generate, optimize, and test such algorithms without the necessity of an image-processing expert. We also present several examples of inspection tasks to support the concept. For efficiency, an automated multi-scale multi-sensor inspection strategy is employed.

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.

Optical metrology for process control: modeling and simulation of sensors for a comparison of different measurement principles

To increase the quality of future products and decrease the manufacturing cost at the same time a systematic control of the fabricated objects is necessary. A promising approach for inline quality control of surface and form parameters is the use of optical measurement systems. This is due to the non-destructive nature of the optical measurement techniques. But in the production environment there are many challenges to overcome for optical sensors. Examples are temperature fluctuation, vibrations, fluids on the object surface and rough surfaces. Therefore, it is likely that not all optical measurement methods are suitable for that task. Hence, a classification of the different principles is necessary with the objective to identify the most appropriate measurement approach for a particular inspection task. In this contribution we start with a systematic approach for a review of sensors within production systems. Then we concentrate on the most robust class of optical sensors, the point sensors. In order to minimize the effect of mechanical vibrations it is desirable to employ measurement techniques that are able to measure the height of an object point in a very short time. Therefore, we focus in this work on chromatic-confocal microscopy and spectral interferometry. The aim is to compare these measurement methods for their ability to cope with the challenges given by the production environment in general. To this end we will develop simulation models for the mentioned techniques and compare two exemplarily sensors for their capability to be used for process control.

Automated surface positioning for a non-null test interferometer

An automatic method for the positioning of the test surface in a non-null interferometer is presented. A major task in the interferometric testing of surfaces is to avoid the introduction of surface aberrations due to an incorrect placement of the test object in the interferometer cavity. In the case of plane and spherical surfaces, adjustment errors can usually be distinguished from surface figure errors and therefore removed, but in the case of aspherical surfaces this task becomes nontrivial. In this work, the effect on the measured phase due to lateral and axial displacements of the aspherical surface is calculated, and an adjustment method for the positioning of the surface at a predefined measurement location presented. Experimental results showing the feasibility of the proposed procedure are shown.