Centre for Sensors, Instruments and Systems Development

Universitat Politècnica de Catalunya

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Design and implementation of a multispectral system in the ultraviolet, visible and infrared range. Application to study and conservation of artworks

On March 25, Jorge Alexis Herrera defended his doctoral thesis titled "Design and implementation of a multispectral system in the ultraviolet, visible and infrared range. Application to study and conservation of artworks" The thesis has been co-directed by Jaume Pujol and Meritxell Vilaseca, both researchers at CD6. The thesis has been developed within the project "New developments in visual optics, vision and hyperspectral systems: Clinical and industrial applications" funded through the National Plan for R&D of the Spanish Ministry of Science and Innovation.


Multispectral systems have several applications and there are different possible configurations in which they can be implemented. Different characteristics make them useful, but the basic one is the access to spectral information of a scene or sample with high spatial resolution. In this thesis, the main objective has been the design and implementation of a multispectral system that covers part of the UV, the visible, and part of the IR ranges of the electromagnetic spectrum to be applied to artwork studies.

The main part of the thesis is dedicated to the design, characterization and application of a multispectral system based on multiplexed illumination using light emitting diodes (LED). This system comprises two modules: Module 1 UV-Vis, with a CCD camera sensitive between 370nm and 930nm, coupled to a source of LEDs with 16 different channels, i.e. 16 wavelengths of emission; and Module 2 IR with an InGaAs camera with sensitivity between 930nm and 1650nm coupled to an LED source with 7 emission wavelengths. Thus, the complete system covers from 370nm to 1650nm with a total of 23 channels obtained with LED illumination.

The system elements were characterized and simulations were performed to assess their performance in the reconstruction of spectral reflectance under ideal conditions, and also under conditions of quantization noise and additive noise. Its performance was evaluated using the formula CIEDE2000 color difference, the mean square error (RMSE) and the goodness-of-fit coefficient (GFC). The simulation results showed a good overall system performance, but with better results for module 1 UV -Vis due to the increased amount of LED channels in its spectral range.

Computer programs with their respective graphical interfaces to control the hardware and processing the information provided by the system were implemented. For the spectral reconstruction we employed the method based on direct interpolation using splines, and the methods based on training set of samples with known digital system responses and spectral reflectances: the undetermined pseudo-inverse (PSE -I) and simple pseudo-inverse (PSE). The equipment was evaluated over real samples of the Color Checker Chart CCCR and a series of frescoes patches painted with pigments used in artworks. The results of the metrics CIEDE2000, RMSE and GFC showed that the methods of the PSE-I and PSE have similar performance, with slightly better results for the second one. The interpolation method presented a slightly lower performance, but it has the practical value of not needing training. The results for the PSE method were similar to those obtained through simulation, and proved again that the module 2 IR has lower performance. It was concluded that overall system performance was good and that the system offers a new cost effective and versatile tool for several applications in spectral imaging.
The developed system was applied in the museum of Saint Mary of Pedralbes Monastery, Barcelona, and the churches of Sant Pere in Terrassa. Different images of murals of San Miguel's chapel in the Monastery of Pedralbes were captured. The assessment of the system performance for this museum application agreed with the reported one in laboratory. We also captured a painting of large format, oil on wood named: La Virgen de la Leche. For this artwork the modular design and easy movement of the system was used to generate a complete picture by composition from several smaller images. At the churches of Sant Pere, we explored wall paintings dating from the Visigoth (VI -VII) and Romance times (XII -XIII) to assess whether there were features in the paintings that were not evident in the visible range, but in other spectral ranges. Enhancement algorithms were implemented for this task. The results obtained in this thesis demonstrate the potential of the developed multispecral system for obtaining spectral information in the ultraviolet, visible and infrared regions.
CD6 Centre for Sensors, Instruments and Systems Development
Rambla de Sant Nebridi, 10  ·  08222  ·  Terrassa (Barcelona)