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The most established way to inspect machined surfaces is measuring with profilometers. Roughness measurements by these methods take plenty of time and an on-line integration in production facilities is not feasible due to different reasons. Over two decades ago, Goodman and Parry discovered the correlation between laser speckles of different light wavelengths and surface roughness. Since then several authors have been working on this topic. Nevertheless, industrial sensors based on the speckle correlation phenomenon measuring a roughness value are not available yet. In this article a new method is described, which is suitable for measuring the roughness of surfaces online under production conditions. The surface which is to be inspected is illuminated by a dichromatic laser beam. The scattered light is converted by lenses, and the far field speckles are detected in the Fourier plane of the lenses by a separate CCD-array for each wavelength. The CCD-data are captured by a frame grabber and stored for evaluation. A digital computer processes the data by calculating 2D-cross correlation functions of the two related speckle patterns for the different light wavelengths. The maximum values of the cross correlation function represents the correlation coefficient, which can then be translated into the root mean square of the surface heights of the workpiece. The main benefit of the method described is the feasibility of measuring roughness during processing. This is achieved by a time synchronous detection of speckle patterns of two different wavelengths, which are used to determine the roughness of a surface. The basic measuring time is determined by the shutter speed of the cameras, which is in the pilot project 1/10000 s. Therefore, only vibration frequencies above 1 kHz disturb the measuring results. A further reduction of the measuring time will be no major problem. Thus any realtime roughness measuring problem can be solved this way. This article describes the state of the art and the state of the experimental investigations measuring roughness by means of this new measuring process.
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