Interferometry using a single wavelength delivers the surface topography and surface heights of optically polished surfaces. However, discrete steps and holes cannot be determined, the sensitivity is fixed, and the analysis of optically rough surfaces is not possible. Some of these limitations can be overcome by using two or more wavelengths. In wavelength scanning interferometry, the frequency of the modulation induced by the wavelength change is determined independently for each image pixel. The tuning range determines the resolution of measurements, while the tuning step limits the range of the measurements. Laser diodes can be tuned, but an external cavity is needed for a larger mode hop free wavelength variation. Polished and optically rough surfaces can be analyzed in the same manner. In a new development, the application of temporal evaluation of speckles for deformation and shape measurement will be discussed. It turns out that spectral and temporal phase analysis can be very useful for many applications in optical metrology. Experimental results will support the methods discussed.
Absolute interferometry can be a powerful tool for distance and shape measurement. Two and more wavelengths can be used to increase the range of unambiguity in interferometry. Furthermore it leads to the possibility to apply interferometric methods to measure optical rough surfaces. A continuously tunable laser in a two beam unequal path interferometer is used to overcome the ambiguity problem of classical interferometric methods. While the laser wavelength is tuned continuously, the variation of the interference intensity is recorded sequentially. At each image pixel the frequency of the signal modulation is analyzed, giving the absolute depth information for the corresponding object point. The wavelength tuning step governs the depth of measurement whereas the tuning range determines the depth resolution to be obtained. The method can be applied for optical as well as for technical surfaces. In the latter case, the intensity variation is observed independently in each speckle. However, the resolution of the measurement of optically rough surfaces is limited by the surface roughness. The continuous tuning of the wavelengths is performed with a diode laser with external cavity where a frequency variation of 30 nm can be obtained without mode hops within one second. Furthermore a novel method to measure the shape and steps heights by rotating the object and using temporal evaluation of the speckles' modulation is presented. Currently a temporal Fourier- transformation is used, similar to the evaluation method used in wavelength scanning interferometry.
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