Remote Laser Spectroscopy at Standoff Distances for Heritage Applications

Raman spectroscopy is a spectroscopic technique capable of identifying materials by measuring the spectrum of scattered laser light from the object. The shift of wavelength resulted from molecular vibrations allows highly specific identification of the material. It is non-destructive and non-contact. With the development of efficient laser sources and sensitive detectors in the last three decades, it has become one of the most widely used methods in the field of heritage science research.

Benefited from the advancement in laser optics and sensor technologies, laser-based analytical methods such as Raman spectroscopy, laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence (LIF), have allowed a wide range of applications in heritage science. Many of the modern spectroscopic methods involve lasers as light sources and are therefore called laser spectroscopy. It falls into several different types, depending on what kind of laser is used and which aspect of material's excited response is studied.

However, heritage samples are often immovable and fragile, sometimes at lofty heights or otherwise inaccessible, which limits the possibilities of sampling or contact measurements, and therefore necessitates in situ, non-destructive and even remote measurements. Remote laser spectroscopy detects light signals from a standoff distance (>5m), using a telescope and optical fibre for light collection. It overcomes the above limitations and significantly broadens the range of conceivable application scenarios.

LIBS is a type of atomic emission spectroscopy in which highly energetic laser pulses strike the surface of the sample and ablate a small amount of material, then create a plasma in high temperature. In the plasma, the excited ions will give characteristic emission lines. LIBS can detect the characteristic lines for each element and therefore evaluate the chemical compositions of the sample. As only a tiny amount of material (<1mm in diameter) is consumed in the process, LIBS is considered micro-destructive.

In this project, a combined remote LIBS-Raman instrument will be developed. This platform will be the first remote LIBS-Raman system dedicated to future cultural heritage research. The synergy between Raman spectroscopy and LIBS can provide complementary information about the sample.

Working at standoff distances (>5m), the system can provide both elemental and molecular information of objects at hard to reach places using both techniques in a single unified set-up. More importantly, when assisted with the existing PRISMS remote 3D spectral imaging system, the integrated system will have enhanced power to present a variety of information simultaneously. The resulting complementary information can be interpreted self-consistently for material identification in heritage science.

• Yu Li will present a poster and give a special oral presentation about a remote Raman system at standoff distances for wall paintings at the 12th conference on Lasers in the Conservation of Artworks (LACONA 12) to be held in Paris, France. (September 2018)
• Yu Li presented a poster about a remote Raman system at standoff distances for wall paintings at the Global Heritage Research Theme Showcase Event held at Nottingham Trent University, Nottingham. (18 May 2018)
• A preliminary investigation of a set of lace, pottery and alabaster samples at the Nottingham Castle Museum and Art Gallery was conducted in November 2017 identifying the dyes and pigments applied on these samples by our in-house developed mobile Raman spectroscopy system.

Academic Investigator: Professor Haida Liang

Co-supervisors

• Dr Quentin Hanley
• Dr Golnaz Shahtahmassebi

PhD Student

• Yu Li