When light interacts with matter, the radiation is absorbed, scattered, reflected, or propagated through it. Materials absorb, reflect and emit electromagnetic radiation in a manner that depends on their molecular composition and shape. Thus, by studying the interaction of light with matter, we can obtain information about the structure of matter and, in general, its quality and quantity.

Multispectral imaging (MSI) is a process of capturing images of an object by using different, but discrete wavelength bands of electromagnetic (E/M) spectrum, which include the visible region and extend beyond it into the ultraviolet and near-infrared regions.

Imaging in the ultraviolet region uses sources that emit in the ultraviolet region of the E/M spectrum (10nm to 400nm) and a detector that allows the capturing of radiation in the corresponding region. Typically, the sources used emit in the near UV (315nm-400nm, UVA) and mid-UV (280nm-315nm, UVB). The image taken by ultraviolet reflectance photography (UVR) is black and white and comes from the surface of the artwork, such as the varnish with which paintings are covered. Above (see Figure) is illustrated the different degree of penetration of radiation into the work of art depending on the wavelength. The top layer (blue color) corresponds to the varnish, the next layer (beige color) corresponds to the colors of the artwork, the next layer (gray color) corresponds to the draft, followed by the canvas (white color) and the support wood (brown color).

In addition to UV reflection imaging, UV fluorescence imaging is also used. The latter is based on property of certain pigments to fluoresce in the visible region of the spectrum after excitation with ultraviolet radiation with a wavelength between 300nm-400nm. Varnishes, used on the surface of artworks, whether older or more modern, have the property to fluoresce in contrast to any later interventions or surface damage, which appear as dark spots without any fluorescent property. Therefore, with this particular technique it is possible to examine and diagnose the state of conservation of the painted surface of an artwork [Subbarao, M. (1988). Parallel depth recovery by changing camera parameters. In 2nd International Conference on Computer Vision, pp. 149-155].

Imaging using radiation in the visible region of the spectrum gives us information about the artwork itself, since visible radiation penetrates the varnish and reaches a level below it (see Figure above). This information concerns the painter's technique or any subsequent aesthetic interventions and overpaintings. Depending on the way the sample is illuminated, two ways of observing the sample can be distinguished, which are the reflected and the transmitted observation. The first mode of observation is applied to opaque samples, while the second one to transparent or semi-transparent samples.

Finally, imaging using infrared radiation (also called infrared reflectography) uses the near infrared region (750nm to 2500nm). The radiation penetrates the first layer, which is usually the varnish or even the entire color layer and thus reaches the artist's draft [Fontana, R., Barucci, M., Dal Fovo, A., Pampaloni, E., Raffaelli, M. & Striova, J. (2018). Multispectral IR reflectography for painting analysis, in: Advanced Characterization Techniques, Diagnostic Tools and Evaluation Methods. Heritage Science, Springer, Cham, pp. 33–47. http://dx.doi.org/10.1007/978-3-319-75316-4 3]. Graphite, mainly used in an artist's draft, strongly absorbs infrared radiation.