Epitaph Wolf Huber (Passau).
Copyright: Universität Passau
The RTI (Reflectance Transformation Imaging) method is only suitable for (more or less) flat objects with a (more or less) deep relief structure - coins, for example, are exposed to it.
RTI is based on continuous shooting with a fixed camera position, but with variable incidence of light. The incidence of light, which changes from shot to shot, is stored digitally using auxiliary means: This can be a high-gloss sphere that is placed next to the object photographed, or a construction that encloses the object hemispherically with LEDs mounted on the inside (see below, semi-automated method). The surface is reconstructed using reflections and the inherent capture of texture.
RTI does not aim at a three-dimensional representation of the object, but at an optically exaggerated reproduction of the surface texture, which is why it is sometimes referred to as a 2.5-dimensional digitization technique. By means of RTI, details can sometimes become visible during digitalization that a conventional photographic capture could only reproduce inadequately. The result is appealing digital representations with a high information value.
There are different application options for RTI, the choice of method depends on the type of object:
1.) Fully manual method: In a studio situation, a sphere is attached to an object; the camera operated by an employee is fixed on a tripod, a second employee illuminates the object from different perspectives with a flash.
2.) Semi-manual method: The object and the sphere are placed on a rotating plate construction ("RTI-arm"), which has a device for fixing an LED light; an employee can use this fixation to take the photos on his own.
3.) Semi-automated method: The object is placed in the centre of the circle at the bottom of a dome-shaped construction, the RTI Dome, at the top of which an inwardly pointing industrial camera is attached. Remotely triggerable LEDs are also mounted on the inside. The lighting pattern manifesting itself in the photo series serves as the calculation basis for the digital representation.