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Measurement of the SVBRDF of metallic surfaces to detect microscopic defects

Dipl.-Ing. Andreas Winkler

Quality assurance in production processes becomes increasingly important, especially in the automotive industry. The inspection of metallic surfaces for microscopically small defects like scratches and contaminants plays a key role in this context. Where conventional imaging and image processing techniques reach their limits, the concept of the SVBRDF, the "Spatially Varying Bidirectional Reflectance Distribution Function", represents a new approach.

The SVBRDF describes the reflectance of locations on a surface in terms of the directions of the incident light and the reflected light. Thus, it contains significantly more information about the surface and its possible defects than traditional photographs do. An observed point at a surface can be described by two coordinates and the directions of the incident and the reflected light may be parametrized by two angles each (the polar angle and the azimuth, Fig. 1). When the wave length of light is considered as an additional variable, the SVBRDF becomes a 7-dimensional function.

Fig. 1: Schematic of a surface element dA of a specimen which is illuminated by a LED and observed by a camera. The polar angles of the incident and the reflected light are measured to the vertical axis and the azimuths are measured in a plane perpendicular to the vertical axis.

A gonioreflectometer has been built in the lab to measure the SVBRDF. An important part of this device is the spherical wrist to move the specimen (Fig. 2). The spherical wrist consists of three rotation stages from the URS-series by Newport Inc. They are assembled such that the three axes of rotation meet in a single point. This allows to arbitrarily position the specimen along a virtual hemisphere. Differently colored high-power LEDs from the XQ-series by Cree, Inc. are used as light source. The LEDs have a quite small light emitting area and thus behave approximately like a point light source. Another advantage is that color LEDs typically have a quite narrow-band spectrum. A Prosilica GC camera by Allied Vision Technologies is used as the detector. It is rigidly mounted straight above the spherical wrist.

Fig. 2: The spherical wrist to move the specimen along a virtual hemisphere is the centerpiece of the gonioreflectometer.

The aim of this work is to detect microscopically small defects on metallic surfaces by comparing measured SVBRDFs with the SVBRDF of a reference surface. It is also investigated how dense the SVBRDF must be sampled in order to be able to detect defects of a certain minimal size. These insights are intended to enable this measuring method to be incorporated in real industrial production processes.

In order to minimize measurement errors, the poses of the camera and the light source relative to the specimen must be known and the inherent deviations of the optical system must be considered and calibrated. Moreover, radiometric calibration of the camera and the LEDs is required. Another challenge is to store and process the huge amount of measurement data, which has great potential with respect to the fact that computer systems are becoming more and more powerful.