An important issue in every cleaning intervention on Cultural Heritage objects and monuments is the monitoring and follow-up of the cleaning process.
Any unnecessary or excessive actions may alter irreversibly the original surface and thus attention should be paid to ensure that no damage could occur to the object during cleaning.
Towards this aim multi spectral imaging techniques were considered & a new approach was investigated aiming to follow on-line the removal procedure & evaluate non-destructively the cleaning level.
The suggested methodology is based on the variation of light penetration in matter versus wavelength.
A custom made portable spectral imaging system (IRIS) was used to acquire images, obtained at various spectral bands, with a bandwidth of 25nm FWHM, during the cleaning intervention.
The images were then processed in order to calculate their differences and get a visual estimation of the cleaning depth.
The best visualization of the optimum cleaning level was achieved when the 950nm and 450nm spectral images were divided.
This is due to the fact that light at higher wavelengths (950nm) penetrates into the crust; being reflected from deeper layers, while at shorter wavelengths (450nm) light interacts mostly with the surface.
The suggested methodology was tested successfully on a series of different cleaning cases of encrusted stonework and thus the principle of operation was proved.
The applicability of this approach is not restricted to laser cleaned surfaces and may also apply for the monitoring of any cleaning technique (micro air-abrasive, etc).
A novel prototype instrument, to bring this methodology in everyday cleaning practice, is currently under development at IESL-FORTH aiming to ensure the safeguarding of the cleaned objects & monuments.
Figure:Visible (bottom) and intensity ratio I950/I450 (top) images of the tested marble surface showing cleaning tests at various stages.