Imaging Spectroscopy for Cultural Heritage (ArtIS)

Fluorescence is an appreciated diagnostic tool in medicine and in other research fields, like the conservation of Cultural Heritages. Very often it is important to map the spatial distribution of the different fluorochromes that contribute to the light emission.

To this purpose, the research group at the Physic Department of Politecnico di Milano developed two Fluorescence Lifetime Imaging Systems, which differ from each other for time resolution and other specific features.

The first system has a time resolution within one nanosecond and can be easily transported to the field. The second system is characterized by a high time resolution of few tens of picoseconds, but it is mainly for laboratory use.

Both systems include:

• A pulsed laser source to excite the fluorescence emission. Different laser sources can be used to generate short pulses from ultraviolet to infrared, with a low (tens of Hz) or high (MHz) repetition rate.
• A low noise CCD camera coupled to a gated light intensifier with typical gate width in the range of nanoseconds of picoseconds.
• A precision delay generator with a low jitter device to synchronise the acquisition gate with the laser pulses.
• A control software to manage both image acquisition and processing.

Fluorescence Lifetime Imaging systems are very suited to discriminate weak fluorescence signals on the basis of the different decay time. Moreover, they give other important advantages with respect to CW devices. The following features are worth mentioning:

1. Effective removal of the scattered laser light without requiring sharp spectral filters. In fact, when the acquisition gate is operated the laser light is already off.
2. Easy operation in normal ambient light even when faint fluorescence signals are measured. In fact, the short detection gates are synchronised with a very low duty cycle excitation light. This feature is very important when the systems are employed in hospitals for medical diagnosis or in archaeological sites to analyse the conservation of Cultural Heritages.