Robert Windecker

Three-dimensional topometry with stereo microscopes

A stereo microscope arrangement used for visual quality in- spection is extended to a fast 3-D topometer using the fringe projection technique. With specially designed binary gratings, an ideal sinusoidal intensity distribution can be achieved for an optimized illumination. Using a two-grating technique, the unambiguity range can be increased signifi- cantly. Thus, various applications can be solved with this method as is discussed. The resolution that can be achieved depends on the field of view, which at a large magnification is below 0.2mm.

Optical roughness measurements using extended white-light interferometry

Roughness measurements of engineering surfaces are an important and interesting application for optical sensors. The increased speed, the ability to measure 3-D profiles, and the noncontact mode of optical sensors are of great interest for practical uses. A most challeng- ing task is achieving comparability to the established methods, such as tactile stylus sensors, which are the most commonly used instruments in industry. We show that white-light interferometry is a useful method to measure in a microscopic range as well as with an extended field of view. The determination of the fringe contrast as a function of the focus position enables highly resolved data acquisition on engineering sur- faces with a mean roughness of the order of a few nanometers to several micrometers. The measurements show that amplitude roughness pa- rameters of engineering surfaces are not strongly affected by the spatial resolution. A good agreement between the results of a stylus sensor and the results of our optical method are demonstrated on different types of surfaces.

Fast algorithms for data reduction in modern optical three-dimensional profile measurement systems with MMX technology

Confocal microscopy and white-light interferometry are two promising methods for the three-dimensional microstructure analysis of technical and biologic specimens. For both methods the specimen is scanned through the focus position by means of an actuator. A large series of intensity frames is acquired. These data are used for the final calculation of the topography. We demonstrate that the multimedia extended (MMX) instruction set, which is implemented in modern Intel microprocessors, can be used for effective real-time preprocessing and for fast evaluation algorithms. So this new technique enables the implementation of more-complex algorithms with acceptable run times even on standard computer technology. The possibilities of the MMX instruction set are discussed for confocal microscopy and for white-light interferometry.

Optical roughness measurements with fringe projection

The characterization of roughness of engineering surfaces over an area is an important task for different applications as well as for manufacturing processes. The surface roughness is in particular an important factor in determining the performance of a workpiece. We demonstrate that the fringe projection technique allows very fast three-dimensional surface inspections. The inspection time for an entire measurement is reduced to less than 5 s with standard hardware. Based on a zoom stereo microscope setup, we demonstrate a modular measuring instrument. The magnification-dependent vertical resolution can be as high as 0.1 microm. The special properties for roughness measurements are demonstrated, especially the comparability with a tactile sensor and with other optical sensors, which is discussed in connection with amplitude parameters.

Testing micro devices with fringe projection and white-light interferometry

Optical sensors are very suitable for the analysis of microscopic structures and micro devices. We compare two very promising methods: the white-light interferometry and the fringe projection technique for the application to this task. The fringe projection is very useful for fast measurement of objects with vertical dimensions of some μm. White-light interferometry is especially useful for highly resolved 3-D measurements. Furthermore, we present a new technique, the scanning fringe projection (SFP), which enables absolute 3-D measurements with one single grating period.

Theoretical limits of scanning white-light interferometry signal evaluation algorithms

Scanning white-light interferometry is widely used for the microstructure analysis of technical and biological specimens. For each pixel in the focal plane of the apparatus a white-light interferogram is acquired and evaluated by means of an algorithm. We discuss some properties of mathematically optimal evaluation methods and the best possible achievable resolution derived therefrom depending on the setup parameters. A comparison of the results to one of the algorithms described in the literature is given.

TOPOMETRY OF TECHNICAL AND BIOLOGICAL OBJECTS BY FRINGE PROJECTION

Fringe projection is a fast and accurate technique for obtaining the topometry of a wide range of surfaces. Here some features of the principle are described, together with the possibilities of adapting this technique for the measurement of vaulted surfaces. We discuss various methods of phase evaluation and compare them with simulated computer data to obtain the resolution limits. Under certain restrictions a semispatial algorithm, called the modified Fourier analysis algorithm, gives the best results. One special subject of interest is the application of fringe projection for the measurement of the three-dimensional surface of the cornea. First results of in vivo measurements are presented.

One-grating projection for absolute three-dimensional profiling

A new method for absolute 3-D profiling, based on the fringe projection technique using a vertical scan mode, is described. With a combined measurement of the modulation and the phase it is possible to acquire the topography without any ambiguity problems by using one grating period only. Consequently, the vertical measurement range is limited only by the free working distance and the resolution is comparable to standard fringe projection techniques. We present a microscopic solution, based on a zoom stereo microscope, which enables interesting applications in an extended microscopic range, such as the measurement of microcomponents and microshapes, for example.

Two methods for fast coherence tomography and topometry

Abstract Coherence tomo- and topography have great potential in the building of optical measuring tools for a lot of different technical and biological applications. One major limiting property of scanning coherence methods is their data acquisition time. In this paper we describe two methods for increasing the measuring speed; one for an analogue and one for a digital preprocessing sensor. In particular, we present a novel scanning device for an analogue 1-D sensor which enables measurements with a very fast vertical scanning rate to be made. A periodical change of the optical path difference of up to 3 mm and frequencies of 1200 Hz and more can be achieved by using a resonant scanner. We describe an optical fibre setup using this novel scanner in combination with a homemade low cost light source which is most appropriate for optical fibre coherence application.

Fast Coherence Scanning Interferometry for Measuring Smooth, Rough and Spherical Surfaces

Abstract Coherence scanning interferometry is a useful instrument for measuring the geometry of a wide range of surfaces with high resolution. By introducing two objectives with a high numerical aperture in both arms of a Michelson interferometer it is possible to investigate spherical and aspherical surfaces. In addition to classical interferometry, the method can be applied to smooth, rough or separated surfaces.