TriENA is a hybrid system that combines three spectroscopic analytical techniques (LIBS, LED-IF, and DR) in one portable device. It is a deliverable of the CALLOS project and has been developed at the Institute of Electronic Structure and Laser of FORTH by the Photonics for Heritage Science Researchers, in collaboration with the conservators of EACA. The system is designed to meet the analytical challenges commonly encountered on Athenian monuments.

This hybrid mobile system combines three spectroscopic techniques: Laser-Induced Breakdown Spectroscopy (LIBS), Diffuse Reflectance Spectroscopy (DR), and LED-Induced Fluorescence (LED-IF), leading to an integrated characterization of the material under examination ‘in-situ’ with no sample removal or preparation restrictions. Moreover, the rapid data acquisition and the user-friendly software increase the system's applicability in archaeometry and art conservation.

LIBS offers a qualitative and semi-quantitative multi-elemental analysis, with its main advantage being the ability to perform stratigraphic analysis on a multi-layered object. The process involves focusing a compact Nd:YAG laser (1064 nm, pulse duration = 10 ns) on the object's surface using a lens, generating a micro-plasma. The emission from the micro-plasma is transmitted through a bifurcated optical fiber into a dual spectrometer unit (Avaspec-2048-2-USB2, Avantes) with a spectral range of 200 - 660 nm and a resolution of 0.2 - 0.3 nm. The analytical information is acquired within seconds.

Differential Reflectance (DR) and LED-Induced Fluorescence (LED-IF) provide information about the molecular composition of a material. DR is based on light absorption from the material, while LED-IF relies on fluorescence induced by a LED source. Both techniques are non-destructive, which makes them highly suitable for analyzing objects in cultural heritage and archaeology. A halogen tungsten lamp is used for the DR measurements, while for the LED-IF measurements, a LED source (375, 438, or 632 nm) excites the material. For DR measurements, a halogen tungsten lamp is used, while LED-IF measurements involve exciting the material with a LED source (375, 438, or 632 nm). To capture a wider range of spectra, the signals from DR and LED-IF are recorded using a low-resolution spectrometer (Avaspec-2048L-USB2, Avantes) with a spectral range of 200 - 1100 nm and a resolution of approximately 2.5 nm.

A miniature CCD camera provides a close-up view of the object's surface during analysis, allowing for precise aiming of the laser beam using a cross-hair indicator overlaid on the image. In addition, light sources, along with the required optics and a visualization camera, are integrated into a lightweight and compact optical probe head.

The LIBS microscope has the capacity to provide fast elemental mapping of flat surfaces, typically cross-sections of geological samples, marine shells, bones, teeth etc. The 2D-elemental maps of the scanned surface can be used to identify the distribution of mineral phases in rocks, to measure the variability of elemental proxies related to paleoenvironment in shell studies or to assess the diffusion of environmental pollutants into hard tissues. 

In the present micro-LIBS workstation a Q-switched Nd:YAG laser is used (λ = 1064 nm, pulse duration: 10 ns, pulse energy 5-20 mJ). The laser beam is focused on the sample through a laser objective lens down to a spot of 40 - 60 μm in diameter. The light emitted by the plasma is transferred via an optical fiber to the spectrometer which captures LIBS spectra for each one of the laser pulses that scans the surface. According to specific analysis requirements the spectral data is processed on-line or following completion of scanning. Samples are mounted on a motorized X–Y–Z micrometric stage and translated with respect to the laser beam that remains in a fixed position. The typical translation step is of the order of 100 μm. A CCD camera enables the user to have a clear view of the sample surface and to define the area that is to be mapped (any shape is acceptable). The measurement speed is about 0.9 s per point and each map could have 500 - 6000 points (i.e. an elemental map of 4000 points is obtained in about 1 hr). A dual spectrometer unit (AvaSpec-2048-2-USB2) records the LIBS signal (spectral range: 200 - 640 nm, resolution ~ 0.2 - 0.3 nm). For higher sensitivity, the signal can be recorded by a Czerny-Turner spectrometer (Jobin Yvon, TRIAX 320) with an ICCD camera (DH734–18F, Andor Technology) (spectral range ~ 45 nm).

Raman spectroscopy gives details about the molecular structure of samples on the basis of the characteristic vibrational modes of the molecule. It can be used to identify a variety of materials (minerals, pigments, organics etc.) and the fact that it is completely non-destructive makes it extremely attractive for the analysis of invaluable objects, such as artworks and archaeological objects.

Our mobile Raman microspectrometer (Exemplar Plus, ΒWTEK) uses a cw (continuous wave) diode laser working at 785 nm as excitation source. An optical probe head focusses the laser beam onto the sample surface by means of a set of objective lenses offering different levels of magnification. A white LED and a digital colour camera are included on the optical head, which allow visualization of the object’s surface and selection of the area (spot) to be analyzed. The spectrometer provides high spectral resolution (< 8 cm−1) and sensitivity, covering a spectral range between 100 – 3300 cm−1. The employed detector (Peltier-cooled) features high sensitivity with low dark counts.

IRIS-II, a portable MultiSpectral Imaging instrument, enables the compositional and structural study of multi-layered Cultural Heritage surfaces. It is fully portable enabling thus the examination of objects in situ (museums, conservation laboratories, archaeological areas etc.). This imaging system, provides detailed information related to the physical and chemical properties of materials, based on reflection and fluorescence spectroscopy.

The main elements of the multi-spectral imaging system include a camera, an imaging monochromator equipped with a filter wheel of 28 band-pass filters, the objective lens, electronics and a computer that controls all the components. The camera used on the system is a monochrome digital CMOS camera. The spatial resolution is 5MPixels, while the dynamic range applied is 8 bit. This sensor is sensitive from 350 nm up to 1200 nm. 

The whole system is designed to be portable and can be carried in a small case. 

Finally, custom-made software, entirely developed in LabView is employed. This software enables the control of the system and the data acquisition. Additional processing software for images normalization, calibration and analysis is also developed and used.

IESL-FORTH holds a number of laser systems with different wavelength, pulse duration and energy output characteristics available for laser cleaning investigations such as:

• Transportable Q-switched Nd:YAG lasers (Quantel Q-smart 850, LITRON TRLi, Spectron SL-805 modified, Quanta Palladio, BMI 5022 DNS 10) emitting both nano- and pico-second (EKSPLA SL 312) laser pulses at various wavelengths (such as 1064, 532, 355, 266 & 213 nm)
• Various excimer lasers emitting nano, pico and femto-second pulses in the UV
• A patented transportable ns Nd:YAG system with dual-wavelength beam output, developed for the laser cleaning project of the Athens Acropolis Monuments especially dedicated to remove pollution crust from stonework without any discoloration or damage
• A transportable LQS Nd:YAG system (ElEn, EOS1000) emitting IR pulses at longer pulse-widths
• An Er:YAG laser system (LITRON NANO L 200-20-Er) emitting at 2094 nm 
• A continuous CO2 laser system (Coherent Diamond C20) for the patented application related to the laser conservation of glazed objects.

Various workstations adaptable for different laser cleaning applications with the ability to integrate different optical and opto-mechanical components for the most appropriate beam delivery and control are available such as:

• Handheld units (using a articulated mirrored arm) 
• Automated beam scanning units for micrometer control and guidance of the laser beam to the sample (i.e. the painting surface).

The latter, a computer-driven mechanized component, can be adjusted on the basis of fluence values, spot size and pulse repetition rate enabling thus the homogeneous scanning of predefined areas.

Furthermore, a number of multi-modal diagnostic instruments for in-situ assessment of the cleaning result and monitoring of the laser ablation procedure are also available. These can be selected according to the specifications of each individual cleaning case and may be one or more of the following:

• Spectral Imaging to visualise the cleaning state
• Laser-Induced Fluorescence (LIF) to evaluate the thinning of varnish
• Vis-NIR Diffuse Reflectance spectroscopy to chemically characterise the irradiated surfaces

The DHSPI (Digital Holographic Speckle Pattern Interferometry) systems have been developed and continuously optimized at IESL-FORTH with the aim to investigate and monitor deformation, deterioration, and fracture mechanisms and thus to evaluate the structural condition of materials and systems as a result of ageing, mechanical alteration and materials’ failure.
DHSPI captures microscopic alterations of sub-surface topography on the basis of high-resolution interferometric imaging. Hidden defects are revealed as visible interference fringe patterns forming locally inhomogeneous intensity distribution patterns. The deformation data are extracted through the differential displacement of the surface under investigation and the deformation value is measured by multiples of half wavelength.

DHSPI-II, the most recent model, is a compact fully portable system with a built-in data acquisition and processing unit and dedicated user-friendly software, for the system control and data post-processing which enables real-time qualitative and quantitative structural diagnosis. DHSPI-II also allows control (via cable) from a remote pc (eg laptop) which provides extra flexibility for in-situ measurements. 

TECHNICAL INFO:  •Laser power: 300mW     •Coherence length: >30m     •CCD resolution: 5MP    •Spatial resolution: 144 lines/mm     •Displacement resolution: ≥ 266nm    •Sensor lens: C-Mount type exchangeable   •Beam Divergence: >40cm@1m (Gaussian Profile)