PRODUCTION PROCESS Limits for interferometric measurements on rough surfaces in streaming inhomogeneous media Christoph Gerhard • Andreas Stephen • Frank Vollertsen Received: 8 February 2010 / Accepted: 17 March 2010 / Published online: 30 March 2010 Ó German Academic Society for Production Engineering (WGP) 2010 Abstract In this paper, we present the determination of critical angles for interferometric measurements on rough metallic surfaces and the inﬂuence of streaming inhomo- geneous media on interferometric measurements. For this purpose, measurements using different materials with dif- ferent surface roughness were performed in air. Further, the inﬂuence of both streaming and index of refraction-ﬂuc- tuant water on the measurability of the interferograms from rough surfaces was investigated. As a result, schlieren and streams in water do not signiﬁcantly affect the observed interferograms and critical angles, when using small beam waist diameters for measuring. Keywords Interferometry  Optical metrology  Laser material processing 1 Introduction Optical interferometry is a well-established and widespread technique for measuring distances, surface proﬁles and roughness. It has become one of the most important testing methods in manufacturing of optical and mechanical components. However, detecting steep ﬂanges interfero- metrically is limited by several parameters such as the angle of incidence (AOI) of the incoming light and surface roughness of the sample [1, 2]. Free-form surfaces, which feature such ﬂanges, can be measured in different ways. As an example, white-light interferometry is often used for three-dimensional proﬁlometry by scanning the surface topology [3]. The work presented in this paper is based on the detection of backscattered light by a classical Michelson interferometer. On reﬂective rough surfaces, light is backscattered by diffuse reﬂection. As shown in Fig. 1, the intensity of backscattered light for a non-Lambertian sur- face depends on the angle of incidence of the incoming beam and on its wavelength. An increasing angle of incidence leads to a decrease in the intensity of the backscattered light until a critical angle is reached at which the contrast of the interferogram is unsufﬁcient for an analysis of the topography of the sam- ple. The critical angle depends on the material and the surface roughness of the measured sample [1]. In this work, samples with different surface roughness, made of Stellite Ò 21 were used for the interferometric measurements. Stellite Ò 21, a cobalt base alloy, shows a very low friction coefﬁcient and a high resistance to cor- rosion and mechanical wear [4, 5]. Therefore, it is often used in demanding ﬁelds of application, for example for the producing of implants or equipment in food industry. Due to its mechanical properties, it is also a well-suitable basic material for long-living tools such as deep-drawing dies. For comparison, the experiments were also performed using brass and stainless steel samples. In some production processes such as stereo lithography [6] or laser chemical manufacturing [7, 8], work pieces are processed in liquid media. Due to the work piece’s surface proﬁle and ﬂuid ﬂow caused by the injection of liquids, turbulent streams can occur during the manufacturing process. In addition, the punctual heat injection on the workpiece’s surface leads to punctual changes of the index of refraction n in the used liquid. Hence, the inﬂuence of such turbulent streams and ﬂuctuant temperature ﬁelds on C. Gerhard (&)  A. Stephen  F. Vollertsen BIAS—Bremer Institut fu ¨r angewandte Strahltechnik GmbH, Klagenfurter Str. 2, 28359 Bremen, Germany e-mail: gerhard@bias.de URL: www.bias.de 123 Prod. Eng. Res. Devel. (2010) 4:141–146 DOI 10.1007/s11740-010-0224-7