From RGB camera to hyperspectral imaging: a breakthrough in Neolithic rock painting analysis

Abstract

Rock paintings undergo physical, chemical, biological and/or anthropic alterations that alter their visibility. Cameras and image enhancement tools (DStretch® plug-in, for example) are commonly used to help identify and record images that have become invisible to the naked eye. HyperSpectral Imaging (HSI) which is strongly developing in many research and application fields, is tested in this study to analyze Neolithic rock paintings. We particularly address the question of what kind of additional information can Visible Near InfraRed HSI instruments, coupled to mathematical transformations to reduce the dimensionality of the data, bring for rock paintings, compared to standard RGB cameras. From the analysis of a selection of panels painted on yellow-reddish altered sandstone walls and measured in Saharan shaded shelters, we show that HSI can reveal new figures by capitalizing both on its ability to extract the different pigment types with a greater contrast, and on the new discriminating information contained in the very near infrared part of the spectrum. Despite their much smaller image format, HSI can provide up to 5–7 contrasted images of the spatial distribution of the different types of pigments in the figures. It thus appears to be a promising non-invasive and efficient methodology to both reveal disappeared paintings and to study image juxtapositions and painted layer superimpositions.