The emission decay depends on both radiative and non-radiative processes, because both of them influence the excited state population, leading to a strong dependence of the lifetime on the microenvironment of the fluorophore. In other terms, any interaction of fluorochromes with the microenvironment provides a specific emission quenching that is reflected by the emission lifetime. Therefore, this parameter carries clues about chemical changes (e.g., bond breaking) affecting the emitters due to oxidation, aging and other modifications of organic molecules. This represents one of the main advantages provided by the measurement of the fluorescence lifetime. Moreover, some emitters (e.g., fluorescent pigments) show specific fluorescence lifetimes, which can be considered characteristic.

Intensity measurements, especially those which are spectrally resolved, are affected by several drawbacks: first the presence of absorbers can severely extinguish the fluorescence signal and distort recorded spectral features. This is mainly true when dealing with paintings, because colors in the painted layer can strongly modify the emission from organic binders, which are often the main subject of the scientific investigation. Further, intensity measurements are affected by the spatial distribution of the excitation light, which is typically uneven, and by ambient light, which can seldom be avoided, unless measurements are done in complete darkness in the laboratory. For these reasons, fluorescence images may often be misinterpreted because areas with comparable chemical features appear different because of artifacts.

In contrast the relaxation dynamics of the fluorescence or phosphoresce emission, like the tone of a sound, are almost insensitive to the intensity of the signal, provided that they can be reliably measured. Finally, time-resolved measurements are, at least on the first order, insensitive to ambient light since any continuous wave light, being uncorrelated with excitation pulses, gives a negligible contribution to the signal in the very low duty cycle measurement gates, provided that the repetition rate used is below few kilohertz.1.3. Overview of the Lifetime of Luminescent Materials in Cultural HeritageDifferent organic materials can be found on cultural heritage objects, which include protein and oil-based binders, varnishes, restoration treatments, adhesives and glues.

These materials are often luminescent due to the presence of delocalized electrons in molecules containing AV-951 multiple aromatic rings or long-chains of conjugated double bonds. The related decay kinetics associated with these molecules is on the order of picoseconds or nanoseconds [1] and is highly affected by a number of factors, which include pH, temperature, solvent polarity and molecular flexibility.