Proceedings Article | 17 September 1997
KEYWORDS: Speckle, Fringe analysis, Radium, Mirrors, Semiconductor lasers, CCD cameras, Surface roughness, Demodulation, High power lasers, Brain-machine interfaces
There are many optical and non-optical methods to investigate surface structures and morphologies. Among the optical techniques, the speckle interferometric methods have become more and more interesting during the last 25years, and this paper shall be restricted to interferometric techniques based on the speckle effect only. The introduction of phase-shifting methods rendered the possibility to automate and to analyze measured interferograms more easily. And it was quite early when the angular (ASC) and spectral (SSC) speckle contouring methods have been proposed to measure surface shapes. As demonstrated in some earlier papers, the method to summarize phase interferogram images incrementally enlarged the measurement range of angular speckle contouring. Furthermore it has become possible now not only to measure macroscopic surface shapes by ASC, but also to map microscopic surface topographies and to determine the according surface parameters as roughness, for example. In a first investigation, the shape of a plastic cylinder is measured by ASC and SSC using the same interferometer set-up. As a practical application, the cylindricity of the object is determined by both methods. For all measurements a laser-diode is used as laser source, and in the SSC technique, the injection current is changed to realize different hopping modes yielding different laser wavelengths to be used for the contouring approach. It is the first time that phase images corresponding to different synthetic wavelengths are summarized modulo 2t in the spectral speckle contouring technique. The measurement results in form of phase maps, bias images, 3D-plots, and iD-profiles of the cylinder object are compared between the SSC and ASC method, and the results are shown to be in good agreement. In a second investigation, the 3D-structure of a reshaped composite hemisphere is measured using the ASC and the SSC method. The results are referenced again, compared to each other qualitatively and quantitatively, and they are also in good agreement. As an interesting parameter, the curvature radius of the reshaped composite hemisphere is determined by both methods. Finally the new method to measure surface roughness parameters by the ASC technique improved by the addition of incremental phase images modulo 2t is illustrated. Measurements have been carried out on side-milled Rugo test surfaces, and the angular speckle contouring results are compared with classical mechanical stylus profilometric reference data. A concrete application is further demonstrated for the evaluation of the surface roughness of painted composite aircraft structures. The surfaces of these carbon fiber-reinforced specimens with copper mesh have been treated by high-power laser beams for the periodically necessary removal of the paint. The intention of the investigation of the laser-treated surface by the speckle contouring method was to determine the surface morphology, and to find out whether there is a correlation between the roughness and the removed amount of paint. The interesting experimental result is that the roughness of the laser-treated surface is a strictly monotonous function of the number of high-power laser pulses at constant energy density. Reference measurements have been carried out by mechanical stylus profilometry and agree well with the speckle results. Of course all measurements have been carried out without the usual fine, white powder for contrast enhancement and without the usual vibration isolation, too. Keywords: Speckle Interferometry, Angular Speckle Contouring, Spectral Speckle Contouring, Shape, Roughness, Micromeasurement
