PhoHS group has developed an innovative, transportable ns Nd:YAG system with dual-wavelength (2λ) beam output.

The 2λ prototype, is capable of operating at two wavelengths simultaneously (infrared at 1064nm and ultraviolet at 355nm) and is able to remove thick pollution accumulations in a controlled and safe way for both the object and the operator. The combination of the two wavelengths ensures that no discoloration or damaging phenomena occur on the original substrate while revealing its unique ancient surface. The system is being used on the Athenian Acropolis Sculptures since 2000 till nowadays.

 The two-wavelength laser cleaning methodology was suggested and developed in 2001 aiming to address a number of conservation challenges and side-effects; yellowing discoloration of stone surfaces being the most characteristic. The methodology allows the regulation of different laser material ablation regimes and thus can be adapted to different cleaning issues with emphasis to cases in which conventional laser cleaning methodologies (i.e. using IR wavelengths) are not effective or successful. As a general rule for the combination of the 1064nm and 355nm their relative ratio is determined on the basis of the composition and morphology of the material to be removed. In order to remove relatively thick and inhomogeneous crusts the contribution of the IR beam (which is highly absorbed by the bulk of the crust) must be dominant, while for thinner soiling layers UV favoured ablation is recommended. Further research and fine-tuning, of the 2λ methodology on different cleaning challenges provided encouraging results as for example the combination of 1064nm and 532 nm, which has been found particularly promising for the removal of biological encrustation from stonework.

Ιn 2012 the International Institute for Conservation of Historic and Artistic Works (IIC) appreciated the collaborative efforts of the Acropolis Museum and IESL-FORTH to remove controllably dark pollution crusts and reveal the authentic marble sculptures on the basis of this prototype laser system which was operating openly (but safely) at the Museum. The 2012 Keck award was jointly given to the two organisations highlighting the “Laser rejuvenation of Caryatids opens to the public at the Acropolis Museum: A link between ancient and modern Greece”.