Development of a low-cost and portable device for Reflectance Transformation Imaging

In this paper, a low-cost, automated and portable set-up for Reflectance Transformation Imaging (RTI) is presented. Thanks to its versatility, the technique is widely used in the field of Cultural Heritage to document and, in some cases, to decipher less visible details. Concerning the development of the methodology, it is possible to have a setup with fixed lights (e.g. light-dome) or a moving light (e.g. highlight-RTI). In both cases, the determination of light position during the acquisition of the RTI image is one of the most crucial aspects. To do this, it is desirable the employment of reflecting spheres that allow to capture all the information about the illumination angles during the acquisition. In this paper, a dedicated 3D-printed light dome has been used to analyze a sample object in order to study and define the optimal parameters to acquire the pictures. A fixed camera and variable angles of illumination are needed to build an RTI image. All the acquired images were processed with Polynomial Texture Mapping (PTM) and Hemispherical Harmonics (HSH) methods, calculating the luminance of each pixel in function of the light. RTI builder was the open-source software used for the creation of the RTI image, subsequently displayed with RTI viewer software. The results are promising since both methods (PTM and HSH) returned RTI images that can be easily manipulated in order to highlight details on the surface. In future, the dome will be equipped with different UV and IR LEDs in order to implement multispectral RTI.