Raman spectroscopy is a non-destructive analysis technique that allows the direct identification of painting materials without sampling. It is based on focus a laser beam on the area to analyze and collect and detect the scattered light. We obtain a frequency spectrum, called Raman spectrum, which is as a characteristic fingerprint of the material illuminated by the laser. The obtained Raman spectrum is compared with all we have in our pictorial materials database. This comparison allows the identification of the pictorial material corresponding to the obtained Raman spectrum (Figure 1).
Figure 1. Identification of an unknown pigment starting from the comparison of its Raman spectrum with one stored in our database of pictorial materials.
Figure 2. Raman set-up used in the analysis of artworks.
The optical-fiber Raman system we use is a Jobin Yvon InduRam equipment (Horiba Group) .
Figure 2 shows the system setup used for the analysis of the artworks. It incorporates three interchangeable lasers (helium-neon at 632.8 nm, argon at 514.4 nm and tunable semiconductor at 785 nm) whose output light is guided by an optical fiber to the optical head. This one focuses the laser light onto the area to analyze and collects the scattered light (Raman signal). Another optical fiber guides the scattered light to the monochromator where it is spatially separated (as a prism) and sent to the CCD detector. It performs the optical-electrical conversion and transmits the information to the computer which shows the Raman spectrum and it also allows the control of the system.
It is important to note that the optical head is placed on a motorized Newport translation system capable of motion along two axes perpendicular to the direction of the incident radiation. So, we can point the laser spot onto the artwork with an accuracy of 100 nm. The focusing is manually controlled by a micro stage.