The total appearance of a painting is defined by its spatially varying spectral reflectance factor, surface macrostructure (depth or surface normal), and surface microstructure (bi-directional reflectance distribution function, BRDF). For paintings with uniform BRDF (e.g., varnished), their total appearance can be measured using equipment commonly found in a photographic studio. Such a system was built and tested for several acrylicdispersion paintings. The system consisted of three strobes affixed with triacetate film linear polarizers and a Dual-RGB camera also affixed with a linear polarizer in order to achieve cross polarization. Using the principles of photometric stereo, images of each light source taken sequentially from 30° from the normal and 120° apart annularly were used to measure surface normal. A learning-based algorithm was used to measure colorimetry and spectral reflectance factor. Software, Artviewer, was written to render images for specific geometries and for studio lighting. The system produced images that approximated, but not equaled, conventional studio photography. Because diffuse data were collected, these images are useful for the long-term evaluation of color changes. Evaluation of the surface normal provide new information for the technical examination of artwork.
Roy S. Berns, Tongbo Chen, "Practical Total Appearance Imaging of Paintings" in Proc. IS&T Archiving 2012, 2012, pp 162 - 167, https://doi.org/10.2352/issn.2168-3204.2012.9.1.art00036