**3. Reflectance measurement on Dali's painting** *dos Figuras*

The project Photonic Restoration Applied to Cultural Heritage on Dali's painting *Dos figuras* (1926) is a competitive project subsidized by the Spanish Ministry of Science, Innovation and Universities. The project aim is to restore virtually with light projection the Dali painting "Dos figuras". This restoration will require and will be based on spectral measurement data.

This painting is in a bad state of conservation with some irreversible damage that prevents the painting from being appreciated in its original state, **Figure 9**. The Reina Sofía Museum has consolidated the canvas and stabilized the pigments to avoid further damage. But to maintain the painting, it was decided not to intervene anymore in a physical way after consolidation was done. Thus, this project opens the possibility to restore this painting virtually without physical intervention, just with light.

The project was divided into three phases: The first one is the study of the artwork and the process that has led to the actual conservation state (**Figure 10**); the second one is the characterization of the reflectance of the painting and the obtainment of 3D geometric data; the final phase consists in designing an algorithm to virtually restore the painting without physical intervention.

To characterize the painting, it is possible to use any of the reflectance methods commented in previous paragraphs. The example of the characterization of *Guernica* is a low-resolution method, therefore the UCM team (www.ucm.es/lightingandcolor) decided to improve the characterization resolution. In this way, a new design of a hyperspectral camera [9] with higher resolution and an interchangeable filter possibility was necessary.

#### **3.1 Hyperspectral matrix camera**

A hyperspectral camera is a system that can measure a hyperspectral cube of spectral data [10]. Typical scientific hyperspectral cameras have resolutions of around 1.4 MP. In the case of this painting, which is 2 m long, it implies approximately 1 × 1 mm square measurement resolution in the painting. This could be improved with higher sensor resolution. The designed CCD has 12 cameras, **Figure 11**, with a 12.3 MP

**Figure 9.** *Original state showing the painting and Dalí, year 1926.*

*Reflectance Measurements on Cultural Heritage DOI: http://dx.doi.org/10.5772/intechopen.100288*

**Figure 10.** *Painting dos Figuras (1926), Salvador Dalí, year 2021.*

**Figure 11.** *Optical filter detail for hyperspectral camera.*

sensor that provides a 0.3 × 0.3 mm resolution on the painting for spectral reflectance data. The hyperspectral camera can be suited with 12 narrowband filters selected depending on the measurement needs.

This camera is currently being calibrated on the group laboratory, **Figure 12**, using a calibrated reflectance standard and a stabilized LED light source. Also, for spectral reflectance measurements, a white background was previously measured

and accordingly calibrated in spectral reflectance. This white background was made of a uniform rectangular piece of foamed PVC and once calibrated, it will be used to calculate spectral reflectance all over the picture area.

Hyperspectral system calibration is essential to obtain reliable measures and needs to be executed before any measure is made. In **Figure 13a**, comparison is made between a commercial system that measures each 4-nm band but only in a

**Figure 12.** *Hyperspectral CCD calibration procedure.*

**Figure 13.**

*Reflectance comparison between commercial PR655 spectroradiometer and custom made 12 filters hyperspectral system.*

5-mm area and the hyperspectral system that measures each 30-nm band but the measurement area is 0.3 mm.
