Christian F. UHLIR1 / Peter DORNINGER2
(1University of Salzburg, Research Group for Archaeometry and Cultural Heritage Computing (CHC) / 24D-IT GmbH, Pfaffstätten, Austria)

Keywords: 3D documentation, accuracy, 3D modelling, sculpture, crystalline rocks

A terrestrial laser scanner uses laser light to measure the distance to an object and results in a 3D scan of the object. The best achievable accuracy (in the range of a few millimetres) generally depends on the surface and material properties of the object. Common materials for historic monuments are marble or alabaster. These mono-mineral, crystalline rocks are translucent to some degree, therefore the laser light reflects not only at the surface but also at a deeper level within the object. Current laser scanner systems cannot differentiate between signals reflected at the surface and internal reflections. This results in a systemic measurement error of 1 to 4 cm beyond the real surface, depending on the material of the object and the scanning system used.
Scans of plates of marble with different grain-size and crystalline structure show undulating surfaces depending on individual structures. No standard procedure can be used to correct the measurement error.
We developed a coating technology as a solution to this problem. The coating reflects the laser at the surface, which completely eliminates the systemic error. The coating is non-destructive and completely removable. It fulfils the regulations for monument preservation. Various suspensions, composed of mineral pigments (size 1-5 micron) and water were tested. The best measurement results were achieved using talc.
A final test under real-world conditions was conducted in the park of Schönbrunn Palace, Austria. Opaque reference targets, which do not show any measurement errors themselves, were mounted on a sculpture. Scans without coating show discrepancies with up to 2 cm w.r.t. the reference targets. If object and targets are covered by talc, no such difference can be measured.