Fluorescence can occur when a molecule (fluorophore) resonantly absorbs light that promotes the molecule to an excited electronic state. Subsequent radiative relaxation of the excited states results in emission of light, where some of the light is emitted at longer wavelengths compared to the wavelength that was absorbed (Stokes shift).
Thus, when a painting is illuminated by UV radiation, the latter is partially reflected and partially absorbed by the painting surface.

Part of the absorbed radiation is then re-emitted in the visible spectrum (i.e. the 380-780 nm range as fluorescence radiation, and the painting UV fluorescence image can be seen and acquired with an imaging system.

NB.-The fluorescence phenomena can be induced in different spectral bands, and consequently be revealed in different spectral ranges. Here, we will only analyze UV fluorescence, and therefore, for simplicity, we will use the word "fluorescence" as synonymous of "UV fluorescence".

Fluorescence emission depends on the contribution of the artwork outer layers, namely on the external (semi-)transparent varnishes, and on the painted layers, generally composed by binding media and pigments, and on their chemical interactions.