Stefan Patzelt

Surface roughness measurement by means of polychromatic speckle elongation

A new approach for determining the roughness of engineering surfaces that is applicable to industrial in-process measurements is introduced. Laser speckle patterns, arising from light scattered from rough surfaces that are illuminated by polychromatic laser light, are detected in the far-field region. The incoherent superposition of these light intensities and the angular dispersion cause the effect of speckle elongation. This is characterized by increasing speckle widths and leads to a radial structure of the speckle patterns. With increasing surface roughness, the elongation is replaced more and more by the decorrelation of the monochromatic speckle patterns for the different wavelengths. Such effects were detected with the CCD technique and analyzed by local autocorrelation functions of intensity fluctuations that were calculated for different areas of the speckle patterns. The results of surface-roughness determination by means of the speckle elongation effect are presented.

Non-Destructive and Non-Contact Determination of Layer Thickness and Thermal Properties of PVD and Sol-Gel Layers by Photothermal Methods

Abstract Thin layers in combination with metallic substrates are often used for moulding tools. The layer properties depend on the coating parameters and technology (e. g., sol-gel, PVD, CVD). This paper presents the photothermal measurement of layer thickness and thermal properties of PVD and sol-gel layers which are needed for the development of moulding tools. The influence of optical parameters and surface roughness on the results is discussed.

Parametric optical surface roughness measurement by means of polychromatic speckle autocorrelation

A method for determining surface roughness of engineering surfaces that is applicable to in-process measurements under harsh circumstances of industrial production plants (e.g. vibrations, humidity) is introduced. The rough surface is illuminated with polychromatic laser light. The angular distribution of scattered light intensities, i.e. a polychromatic speckle pattern, is the result of an incoherent superposition of monochromatic speckle intensities. The angular dispersion leads to increasing speckle widths with an increasing distance to the optical axis an effect called speckle elongation. This gives rise to a radial structure of the speckle pattern. However, with increasing surface roughness the radial structure vanishes because of a decreasing similarity of the monochromatic speckle patterns of the different wavelengths. The markedness of this effect is analyzed by digital image processing algorithms, e.g. the procedure of polychromatic speckle autocorrelation. The latest approach to an in-process roughness measurement device was made by the use of singlemode fiber-pigtailed laser diodes in order to supply a trichromatic, temporally partially coherent laser beam. A brief introduction to the theoretical background is followed by the presentation of the experimental setup. The image processing algorithms for calculating an optical roughness measure from digitalized speckle patterns are explained, and first results of surface roughness determination are presented.

Precise Alignment of Workpieces Using Speckle Patterns as Optical Fingerprints

Abstract Engineering process chains sometimes require a precise repositioning method of workpieces within production machines and measuring devices. This causes difficulties, however, if the workpiece may not be marked permanently in order to set up a sample coordinate system. This paper presents a repositioning method based on the unique statistical properties of monochromatic speckle patterns emerging from a specific spot of a rough workpiece surface. The speckle pattern could be regarded as an individual “fingerprint” of this specific surface spot. The influence of the experimental parameters (laser wavelength, beam diameter, wavefront curvature) on the uncertainty of relocation is discussed.

Simulation of light scattering for surfaces with statistically distributed subwavelength cavities

This paper deals with an efficient computation method for scattered light intensity distributions, which occur, if a nanostructured surface is illuminated with a monochromatic laser beam of several millimeters in diameter. The minimization of the computational amount is an essential precondition in connection with the development of powerful design tools for laser optical surface measuring methods, which derive structure characterizing attributes from structure dependent scattering effects. The presented approach differs from concepts based on near-field solutions of the Maxwell equations (finite element methods (FEM), finite difference time domain methods (FDTD)) or approximation methods for the near-field (Discrete Dipole Approximation (DDA), Generalized Multipole Technique (GMT)) as the near-field is not computed. Instead, an electrically equivalent model based on pre-computed radiation sources like Huygens point sources, dipoles, quadrupoles, etc. is used, which for standard geometrical nanostructures (cylindrical holes, spheres and ellipsoids) leads to the same far-field distributions as the conventional methods. In order to simulate the scattered light by an arbitrary surface it is divided into subwavelength geometries, which can be substituted by electrically equivalent dipole radiation sources. The far-field is calculated with a numerical scalar method. The computational effort is much smaller compared to algorithms based on the solution of Maxwells equations.

Photothermal Investigation of Ti-Cu-N and Ti-Ni-N PVD Films

Thin titanium based PVD films on steel substrates are promising material combinations for high quality moulding tools required for the manufacturing of complex optical components. The chemical film composition and PVD process parameters determine the thermal and mechanical layer properties, which must remain stable at high temperatures during the moulding process. This paper describes the investigation of thermal and mechanical film properties with respect to film composition and thermal treatment at temperatures typical for moulding processes.

Acceleration of a finite-difference method with general purpose GPUs - Lesson learned

Modern massively parallel graphics cards (GPGPUs) offer a promise of dramatically reducing computation times of numerically-intensive data-parallel algorithms. As cards that are easily integrated into desktop PCs, they can bring computational power previously reserved for computer clusters to the office space. High performance rates make GPGPUs a very attractive target platform for scientific simulations. In this paper we present the lessons learned during the parallelization of a finite-difference time-domain method, an inherently data-parallel algorithm frequently used for numerical computations, on the state of the art graphics hardware.

Simulation of light scattering from surfaces containing spherical and elliptical nanoparticles

This paper presents a simulation approach for light scattering from surfaces containing spherical and elliptical nanoparticles. For this approach an electrically equivalent macro model is derived based on the analytical solutions of Maxwells equations (e.g. Mies solution of a sphere). These macro models do not necessarily fulfill the boundary conditions or give the correct near-field but they provide a suitable far-field solution. The benefit of this approach is an abstract model for the far-field computation that is much more efficient than known solutions like FEM. The radiation sources at the surface are reduced to a maximum like a single source for a whole particle, which gives the correct far-field but does not fulfill the boundary conditions. For the set of radiation sources used for the macro models the approach presented here reverts to the accurate computation of simple geometries. In this special case of spherical and elliptical particles the solution of the Mie theory can be used. In this paper it is shown that in the case of nanostructures the far-field of a sphere and an ellipse can be replaced by the radiation field from a set of dipoles. Based on these results it is possible to approximate an equivalent macro model of the surface containing spherical and elliptical elements. The presented macro model provides a very reasonable simulation approach with acceptable simulation times for large surface areas of several square millimeters.

Super Bright Light-Emitting Diode for Optical Roughness Characterization

Abstract A promising in-process roughness measuring technique is based on the detection of polychromatic scattered light Distributions and speckle correlation algorithms. The spectral light properties mainly influence the measuring range and the resolution. Further approaches use lasers, laser diodes, or a superluminescant diode (SLD), to produce light beams with a discrete or continuous spectrum and a sufficient temporal coherence to generate speckles. The presented approach investigates the suitability of a low-cost super bright light-emitting diode (SLED) for roughness characterization.

In-process-Charakterisierung von Mikrotopographien technischer Oberflächen durch polychromatische Speckleautokorrelation / In-process Characterization of Microtopographies of Engineering Surfaces Using Polychromatic Speckje Autocorrelation

Die ln-process-Charal<terisierung der Mil<rotopographien technischer Oberflächen ist eine meßtechnische Aufgabe, die bisher nicht oder nur unzureichend gelöst werden konnte. Seit längerem wird die Anwendung laseroptischer Streulicht-Meßsysteme für diesen Zweck untersucht. Durch konsequente Weiterentwicklung konnten Gerätekonzepte realisiert werden, die die notwendigen Voraussetzungen für den angestrebten Einsatz während der Produktion, d.h. an bewegten Oberflächen, erfüllen. Im vorliegenden Beitrag wird zunächst die Funktionsweise derartiger Speckle-Meßsysteme erläutert. Darauf aufbauend werden verschiedene Anwendungsbeispiele präsentiert, in denen sich die relevanten Meßeffekte zeigen, und anhand derer sich die Breite des Einsatzbereiches dieser Meßtechnik beurteilen läßt.