Fluorescence spectroscopy with a laser or LED (light emitting diode) excitation source is based on the phenomenon of fluorescence, in which fluorescent molecules/materials, when irradiated with light of a suitable wavelength, are excited to a higher electron state, and during de-excitation (with a typical lifetime of a few ns to µs) emit characteristic radiation of a longer wavelength (and therefore lower energy) than that of the excited radiation. The fluorescence spectrum may exhibit characteristic peaks which are directly related to the fluorescent material on the surface under study. The sources predominantly used in Fluorescence Spectroscopy have a central emission length in the ultraviolet (350-400 nm) or low in the visible, in the blue region (400-450 nm).

Fluorescence spectroscopy is widely used for the characterisation of materials (organic, biomaterials, inorganic) in cultural heritage objects. In the case of paintings, fluorescence spectroscopy can help to identify varnishes or to differentiate between them in cases where a painting has more than one protective layer of varnish. It can also help to identify pigments if they are fluorescent.

Fluorescence spectroscopy can be applied both by using optical fibres and by using a hyperspectral camera, which allows mapping of materials that have the property of fluorescence. In this case too, spectral cubes are produced from which fluorescence spectra can be extracted for the regions of interest.

Key features

● Excitation radiation in the ultraviolet (~ 375 nm).
● Fluorescence emission in the visible.
● Collection of the emitted fluorescence radiation via an infrared camera.
● Spectral analysis of fluorescence.
● Fluorescence emission spectrum is associated with specific materials (fluorescent chromophores).

Application

Varnishes

• Localization. The fluorescence spectrum shall be compared with standard spectra of known varnishes in order to identify the standard spectrum that matches the spectrum extracted from the measurement. It requires the existence of a database of standard spectra, which can be created by combining standard spectra found in the literature together with measurements of standard varnishes in the laboratory.
• Differentiation. A comparison is made between the extracted fluorescence spectra from different points/areas of the painting. Any variation in fluorescence spectra corresponds to the use of different varnishes for these areas.

Organic pigments

Identification. The fluorescence spectrum shall be compared with standard spectra of known organic dyes in order to identify the standard spectrum that matches the spectrum extracted from the measurement. It requires the existence of a database of standard spectra which can be generated by combining standard spectra found in the literature together with measurements of standard organic dyes in the laboratory.