a Robust Technology for High Resolution Optical 3D Surface Metrology using Python

Focus Variation – a Robust Technology for High Resolution Optical 3D Surface Metrology INTRODUCTION The 3D measurement of technical surfaces is a crucial part in checking and controlling the properties and the function of materials or engineering parts. Traditionally, 3D measurements have been performed merely by tactile devices, which can be divided into two main categories. Among the first are contact stylus systems for the measurement of small scale surface features such as surface roughness. These systems typically operate with a stylus tip, which is traced along a profile over the specimen surface in order to deliver roughness parameters such as Ra, Rq, and Rz. Among the second category are (micro) coordinate measurement machines (CMMs) where a stylus tip, usually a synthetic ruby ball, is moved to (few) different positions on the specimen in order to measure large scale features such as different form parameters (e.g. a sphere radius, the cylinder diameter, etc.). A good overview of surface metrology systems in general and tactile devices in particular can be found in [1]. Tactile systems have a long tradition in surface measurement and are well understood and accepted in science and industry. Moreover, a lot of international standards on tactile systems exist, which describe the basis structure of a tactile system [2] and standards how to calibrate it [3]. Nevertheless, optical measurement devices have become increasingly popular in the last decade as described by Jiang [4] in his historical overview of surface metrology. This is above all due to their ability to perform area based measurements which are a prerequisite for many powerful surface texture parameters [5]. Although tactile systems nowadays are also able to perform area based measurements, such measurements usually last very long. Apart from the measurement time, there is a range of additional advantages of optical instruments. Firstly, they operate in a non contact way and therefore, do not damage the surface. Secondly, they usually do not require as much maintenance as a tactile instrument since there are typically none or only very few parts that have to be regularly replaced. Moreover, they do not suffer from several limitations of tactile systems such as a “smoothing effect” of surface profiles due to the radius of the contact stylus tip. In the field of optical measurement many technologies have become increasingly popular recently. Among them are methods based on white light interferometry, phase shifting interferometry, confocal microscopy, chromatic probe microscopy, structured light techniques, atomic force microscopy and scanning electron microscopy [6]. However, also many optical techniques have their limitations when measuring certain surfaces. Optical techniques that have been typically used, such as white light interferometry are very good for measuring smooth surfaces, but have restrictions in terms of complex geometry measurement, large Z heights, high slope angles and high aspect ratio measurements. White light interferometry for example, has been found to produce erroneous results for roughness measurements of periodic standards with Ra values between 50 and 300 nm [7]. Another report shows jumps or spikes of half the mean wavelength, which is reported more frequently as the surface gradient increases or when there is a step discontinuity [8].