Confocal microscopy is a state of the art optical principle to measure the topography of technical surfaces. The output of the measuring process is an intensity curve for each scanned point on the topography. The maximum of the intensity curve correlates to the point height. However, the intensity function is influenced by the geometrical properties of the surface topography and its material. Simple peak picking of the intensity curve leads to insufficient results when calculating the point height. Therefore, the centre-of-gravity or fitting algorithms are preferred. Both have an integral behaviour and are able to suppress unwanted signal components. Today, the centre-of-gravity is often the state of the art method. Disadvantages are e.g.: the method is sensitive against vibrations of the instrument during the measuring process. In contrast to the centre-of-gravity calculation, fitting algorithms are numerically inefficient and slow. Moreover, the fitting process needs a priori information about the curve of the intensity function. We propose an alternative algorithm for the robust evaluation of intensity curves. The amplitude spectra of each intensity curve of a measured reference data set are analysed. The applied technique is based on the Cramér-Rao bound and leads to a threshold operator for calculating the centre-of-gravity in the frequency domain. A possible phase distortion (an asymmetrically shape of the intensity function) caused by diffraction or optical aberrations will also be significantly suppressed. The performance of the algorithm is shown and we compare the algorithm to the popular centre-of-gravity.

Date Published: 13 May 2013
PDF: 13 pages
Proc. SPIE 8788, Optical Measurement Systems for Industrial Inspection VIII, 87880T (13 May 2013); doi: 10.1117/12.2020551

Published in SPIE Proceedings Vol. 8788:
Optical Measurement Systems for Industrial Inspection VIII
Peter H. Lehmann; Wolfgang Osten; Armando Albertazzi, Editor(s)