## 1 Introduction  
Fluorescence is a photoinduced luminescence (PL) emission from a molecule's singlet excited state. The intensity, polarization, spectral features, and decay time (typically from \(10^{-9}\) to \(10^{-7}\) s [1]) of the fluorescence emission can be related to the emitting molecular species. UV-excited fluorescence has long been applied as a simple photographic technique for the non-invasive qualitative examination of artworks to assess the presence of intrinsically luminescent materials typically employed in cultural heritage (CH) [2; 3; 4; 5]. By exposing the object's surface to the light of a low-pressure mercury lamp, the distribution of coatings not perceivable to the naked eye--such as protective varnishes and restoration materials--can be revealed by the color difference of their fluorescence emissions [6]. In recent years, with the introduction of spectrally resolved laser-induced fluorescence (LIF) analysis [7; 8; 9], a broader number of luminescent organic and inorganic compounds have been investigated, including proteins, oils, waxes, resins, and pigments [8; 10; 11; 12]. Despite the widespread use of PL techniques, it is well recognized that the identification of fluorescent species based on their spectra alone can prove complex and misleading, confirming that the combination of complementary analytical techniques is often the best choice when using a non-invasive approach [13]. Difficulties in data interpretation are mainly due to intrinsic similarities in the emission spectra of different fluorophores, and to the fact that the detected signal is often generated by multiple species. Subtle spectral differences can be caused by the influence of the molecular environment, as well as by chemical modifications of the bulk material and competing optical phenomena, such as (self-) absorption and/or scattering [8; 14; 15]. Furthermore, the presence of non-fluorescent pigments dispersed in protein- and oil-based organic binders can strongly affect the emissions of the latter [14; 16; 17], sometimes obliterating them with optical absorption or quenching effects and, thus, producing false-negative results [12]. These factors may significantly hamper the application of fluorescence spectroscopy in CH studies, especially when the goal is the identification and mapping of coating materials. However, it has been demonstrated that the interpretation of the signal emitted by complex organic molecules--such as the fatty acids and proteins composing most binding and protective media [18; 19]--can be greatly facilitated by the analysis of the dynamic nature of their fluorescence emissions. Specifically, time-resolved photoluminescence (TRPL) spectroscopy and fluorescence lifetime (FL) spectroscopy and imaging are enable to enhance the specificity of fluorescence measurements by resolving the emission decay dynamics of fluorophores characterized by overlapping emission spectra but different fluorescence lifetimes. For example, FL imaging is particularly well suited for spatial mapping of compositional heterogeneities over an examined area, taking advantage of the fact that the emission lifetime is insensitive to changes in the concentration of the emitting material, as well as being independent of the fluorescence intensity [20]. Moreover, the effectiveness of the integration of time-resolved fluorescence with complementary analytical methods--such as Raman spectroscopy and X-ray fluorescence--has been proven by several applications on paintings [6; 21; 22], sculptures, and plastic-based objects [23], allowing in some cases for the detection of even the presence of superimposed organic materials [18]. The advantages offered by a synergic application of Raman and PL spectroscopy are well demonstrated by the recent development of hybrid setups based on a single pulsed laser source [24; 25; 22].