This picture shows an example of one of the modern technologies used to examine and restore The Last Supper, the infrared reflectoscope. Imperfections such as hidden, microscopic damage, and potential inscriptions can be detected using this process because infrared penetration causes the additional paint layers to slowly become transparent. This method allows for the identification and chemical properties of inorganic pigments found within the painting.
Source: Karagiannis, G., et al. “Nondestructive Multispectral Reflectoscopy between 800 and 1900nm: An Instrument for the Investigation of the Stratigraphy in Paintings.” Review of Scientific Instruments, vol. 78, no. 6, 2007, p. 065112., doi:10.1063/1.2744236.
Image Source: Karagiannis, G., et al. “Nondestructive Multispectral Reflectoscopy between 800 and 1900nm: An Instrument for the Investigation of the Stratigraphy in Paintings.” Review of Scientific Instruments, vol. 78, no. 6, 2007, p. 065112., doi:10.1063/1.2744236.