Red-Green-Blue wavelength interferometry and TV holography for surface metrology

Abstract

Surface discontinuities greater than half a wavelength can not be measured unambiguously using single wavelength interferometry. In TV holography, it is difficult to quantify the data under relatively large loading conditions due to the overcrowding of fringes with a single wavelength. Further, single wavelength does not reveal the contour of rough surfaces with TV holography. Multiple wavelengths measurements using red, green, and blue wavelengths can overcome these problems. In this paper a multiple wavelength interferometric system with colour CCD is demonstrated for such measurements. The interference patterns are recorded by a single chip colour CCD camera which makes the data acquisition as simple as in single wavelength case. The interference pattern is decomposed into its R G B components to evaluate the phase at individual wavelengths. An eight step phase shifting algorithm is used to simplify the application of phase shifting technique in the multiple wavelength system. Experimental results on an etched silicon sample for the measurement of large discontinuities and on a MEMS pressure sensor for the measurement of shape and relatively large deflection are presented.