Ulrich Droste

Microshape and rough-surface analysis by fringe projection

A fringe-projection system for microscopic applications, fringe-projecting microscopy, is developed and analyzed. Projection of the grating and imaging of the fringe system, modulated by the surface, are accomplished by the same high-aperture objective. The spectrum of the grating is spatially filtered and projected into the aperture with a lateral shift, which leads to a telecentric projection under oblique incidence and telecentric imaging. Topographies of specularly as well as diffusely reflecting surfaces can be obtained. The measurement of highly rough surfaces is described together with preprocessing steps. The resulting intensity distribution of the fringes is analyzed. Formulas for vertical and lateral resolution, measuring range, and dynamic range, based on noise considerations, are presented and verified by topographies of technical surfaces.

Topometry for locally changing materials.

Three-dimensional topometry is supplemented with ellipsometric measurements on the same pixel raster for calculation of the phase of the reflected waves and correction of the height fields. Lateral resolution is <1mum . The ellipsometric angles are determined by phase shifting and contrast evaluation. Three-dimensional fields of the ellipsometric angles, the real and the imaginary parts of the refractive index, and the corrected topography of the heights are presented.

Interferometry for Ellipso-Height-Topometry – Part 1: Coherence scanning on the base of spacial coherence

Summary A modified Linnik and a Mirau interferometer are introduced which can be used for Ellipso-Height-Topometry. With these, a set of topographies can be measured, where the measured height H(x, y), the ellipsometric angles Φ(x, y), Δ(x, y) and the degree of polarization P(x, y) all refer to the same pixels of the raster. This coherent set of topographies can be used to calculate topographies of further quantities, e. g. the complex refractive index N(x, y) = n(x, y) – k(x, y) i of bulk surfaces or parameters of thin films, even for discontinuous structures. Material maps which indicate the presence of specific materials and show their exact location in the frame can be generated. In part I, these interferometric configurations are described and results are presented. It is shown that for oblique incidence, which is obligatory for an ellipsometric detection, the envelope of the interferogram or correlogram can be narrowed for improved z-scan (vertical) discrimination, even in the case of the height detection. By a theoretical analysis, we prove that this is an effect of conventional spatial coherence as a function of the width of the source and can be utilized without any source modulation or source shaping even for small spectral bandwidth of the radiation. This is verified for tungsten incandescent lamps, and for LEDs. In part II algorithms for ellipsometric measurements, calibration procedures and first complete topography sets are presented.

Projection of structured light in object planes of varying depths for absolute 3D profiling in a triangulation setup

We are currently introducing a new scanning triangulation method for absolute 3D profiling of a macroscopic scene based on fringe projection technique. A scanning focal plane allows the phase to be determined for any desired depth range of the measurement volume. Furthermore, the limitations of the depth of focus occurring by projected light techniques will be overcome, allowing a large aperture and therefore better use of light. Two different systems based on this technique will be shown: System I uses both vertical and lateral translation of a Ronchi grating. System II uses an LCD element for generation of different fringes which has to be translated vertically, only. The basic principle of the new method is explained. First measurement results of both systems demonstrate the efficiency of the newly developed algorithms and the innovative measurement arrangements.

Reduktion von Überschwingern bei der 3D-Streifenprojektion durch „Inverse Filterung“ (Reduction of Overshooting in 3D Fringe Projection Measurements by Inverse Filtering)

Bei 3D-Messsystemen, die nach dem Prinzip der Streifentriangulation arbeiten, kommt es in Bereichen mit hohem Gradienten, zu Sprüngen in der Oberflächengeometrie sowie an Stellen mit abrupt wechselnder Reflektivität zu Messartefakten, welche sich in Form von Überschwingern äußern. Es gibt hierfür bereits unterschiedliche Erklärungsansätze, welche jedoch in unserem Fall nicht zu einer Lösung des Problems geführt haben. Daher präsentieren wir hier einen weiteren Ansatz, um diese Artefakte zumindest zu reduzieren. Der Ansatz basiert auf der Technik des “inverse filtering”. In dieser Arbeit werden zunächst eine Simulation und anschließend ein Messbeispiel gezeigt, die unseren Ansatz verifizieren. 3D fringe projection measurement systems often show artefacts at steep slopes, surface discontinuities, or at places with a sudden change in their reflection properties. These artefacts are known as overshooting. Several approaches reported so far to overcome these problems have not delivered a solution in the case discussed here. We introduce another approach, which is based on inverse filtering. In this work we will show a short calculation and an experimental example to verify our approach.

A new calibration scheme for the three-dimensional depth-scanning fringe projection measurement method

We present a measurement setup for the acquisition of topographic and 3-D point cloud data using the depth-scanning fringe projection technique (DSFP). We describe the signal generation, its processing using techniques known from short coherence interferometry and discuss a direct 3-D calibration method. Our measurement system delivers an absolute phase map of the scene under measurement. Calibration procedures for macroscopic measurement methods like fringe projection and / or photogrammetry consider the principal distance (that is to say the distance between the center of projection and the image plane) as a constant. This is feasible as long as no focusing and zooming are performed during measurement. Consequently the depth of the measurement volume is limited by the depth of sharpness of the imaging system. By focusing through the whole depth of the measurement volume, our system overcomes this problem, and offers a virtually unlimited measurement depth. However, we have to take the issue of focusing into consideration in order to calibrate our system. The well-known direct calibration method has been adapted to our DSFP setup in order to deal with the problem of geometrical aberrations and to provide a 3-D point cloud. It has been completed to a set of three polynomial transformations, which allow to include the depth-scanning principle in the calibration of the system.

Depth-scanning fringe projection technique (DSFP) with 3D calibration

We report on the depth-scanning fringe projection technique (DSFP) which is an innovative triangulation method for absolute 3-D profiling of macroscopic scenes. This measurement principle combines the confocal principle with the fringe projection and phase evaluation techniques known from white light interferometry. Scanning of the focal plane and additional lateral shifting allow the phase to be determined for any desired depth range of the measurement volume. Hence, the limitations of the depth of sharpness occurring with projected light techniques can be overcome. A 3-D calibration method has been implemented in order to deal with the problem of geometrical aberrations and to provide a 3-D point cloud. The well-known direct calibration method has been adapted to our DSFP setup. It has been completed to a set of three polynomial transformations which allow to include the depth-scanning principle in the calibration of the system.