**Abstract**

Cultural heritage is a valuable and characteristic symbol of every country. It should be handled with care and it must be exhaustively investigated and measured with non-destructive techniques. In this chapter, we will talk about different reflectance measurement techniques to obtain the conservation state of the artwork. With this reflectance characterization, conservators, and curators could soon determine the best maintenance procedures for restoration purposes. Also, a new technique for lighting will be discussed, where the artwork can be also photonically restored illuminating with the correct light in the desired area of the artwork using a spectrally selective projection system.

**Keywords:** reflectance measurements, non-destructive, characterization, virtual photonic restoration, artwork, cultural heritage

#### **1. Introduction**

The color of an object depends on the reflectance properties of the object that are dependent on its physical and chemical composition. But also, color depends on the light source and the human being that views the object [1]. In this case, the light source and the observer who sees the artwork can vary but the reflectance of the artwork is inherent to it. Digitalizing artworks or paintings for color-accurate measurements is widely studied in the literature; usually, CIE color spaces must be considered to match reflectance and perceived color [2].

Spectral reflectance defines how a material reflects light when it hits the surface. The reflectance is a function of the incidence angle and the material itself. It can be quantified with different methods. The Commission Internationale de l'Eclariage (CIE) has a recommendation that comments on the practical methods for the measurement of reflectance and transmittance [3]. The best method to measure reflectance is the use of an integrating sphere. Classical colorimeters use this optical technique and are widely used in the industry, for example, in the automotive sector. The use of an integrating sphere needs contact with the surface, so it is not recommended in cultural heritage because of possible damage to the artwork. In this case, following the CIE recommendations and the geometric conditions, it is possible to use a light source and a spectrophotometer, **Figure 1**. This setup is

**Figure 1.** *Schematic setup for non-contact spectral reflectance measurements.*

non-destructive for the artwork and requires a white diffuse reflectance standard, a stabilized light source, and a spectrophotometer, both of which should be positioned carefully at 45° or the desired angle to measure the reflectance.

The measurement process involves the use of a calibrated white diffuse standard that will be measured in the same position as the artwork. The spectral reflectance will follow the formula:

$$\rho\left(\mathcal{X}\right) = \frac{\phi\left(\mathcal{X}\right)\_{\text{out}}}{\phi\left(\mathcal{X}\right)\_{\text{in}}},\tag{1}$$

where *F*out is the reflected flux and *F*in is the incident flux that must be obtained using the reference white that has a known spectral reflectance, so:

$$\left(\phi\left(\boldsymbol{\lambda}\right)\_{in} = \frac{\phi\left(\boldsymbol{\lambda}\right)\_{out}}{\rho\left(\boldsymbol{\lambda}\right)\_{standard}}.\tag{2}$$

Using this technique, the spectral reflectance of an artwork can be measured point by point, but a spectrophotometer measuring in this way has a circular relative measurement area of around 5 mm in diameter that depends on the used object lens and the focusing distance, as in the example of the spectral characterization in Picasso's painting *Guernica* [4]. Therefore, the utility of the measurement as a global characterization is low because part of the area of the painting is not covered by the measurements.

This measurement procedure can also be applied to sculptures, but there is a higher difficulty in establishing the correct illumination and detection angle than usually is recommended like angle 45*°* incidence and 0*°* detection. **Figure 2** shows a spectroradiometer being calibrated with a white standard. The system comprises a lighting ring and a portable spectroradiometer. The system is designed to light in a 3D annular way with 45*°* over the selected surface and receive light at a 0*°* angle for

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

**Figure 2.** *Spectroradiometer with standard white at Pórtico de la Gloria.*

**Figure 3.** *Sculpture head with cleaned area in Pórtico de la Gloria. Santiago de Compostela.*

spectroradiometric detection. The white diffuse reflectance standard had a reflectance of 0.75 over the visible spectrum, and if the same distance as in the sample is maintained, the absolute spectral reflectance is properly measured.

The system was used by the authors of this chapter to measure the Pórtico de la Gloria at Santiago de Compostela's Cathedral in Spain. One of the most complex problems in measuring spectral reflectance on sculptures is that the system must be not only aligned with every *x, y* point surface normal but also at the defined calibration distance. This setup was used to evaluate pigment cleaning over laser and chemical cleaning procedures. **Figure 3** shows the cleaning process that was done by Fundación Barrié, which was the leader and the funder of this project.

To solve the problem of measuring only a small area, it is possible to measure multi- or hyperspectral data with high spatial resolution and also integrate spectral data into a 3D coordinate system [5]. This method uses a multi- or hyperspectral camera usually called a 2D spectroradiometer that needs to be calibrated to obtain good acquisition data [6].

Multispectral or hyperspectral imaging is based on a calibrated CCD that has a set of bandpass filters to obtain the hyperspectral data cube. These bandpass filters are usually installed in a rotational optomechanical wheel.

In this chapter, we will discuss the use of 0–45*°* reflectance measurement system applied to two Picasso paintings, one for control of the painting condition and the other one to determine the effects on restoration of eliminating a varnish layer that was added after the painting and was drawn by the artist. In the next section, we will introduce a novel design that has a matrix of CCDs with a bandpass filter in front of each one. This will be used for spectral reflectance measurements on Dali's painting *Dos Figuras,* which gives a better resolution on the painting spectral data. Finally, conclusions will be presented.

### **2. Reflectance measurements on Picasso's paintings** *Guernica* **and** *woman in blue*

As commented in the introduction, spectral measurements can report and are part of the preventive conservation of cultural heritage. If the artwork has been measured during time pass you got a time photograph of conservation and deterioration with regard to color. But non-invasive techniques are required in order not to damage the artwork. In the first part of this section, we will describe a spectral measuring system and a lighting source that are designed for reflectance measurements in a large artwork painting (dimensions 7.77 m × 3.49 m) [4] with non-contact and non-invasive techniques.

Guernica was painted by the famous Spanish artist Pablo Picasso in the year 1937. The artist painted it to attract the world's focus to the Guernica Basque country town located in northern Spain. This village was bombed by the German and Italian army on April 26, 1937. The painting was charged to Picasso by the Republican government for the 1937 Paris' World Exhibit forum. This painting is related with the sacrifice of war and is considered by experts as a peace and anti-war symbol.

We will show a detailed matrix data of Picasso's *Guernica* paint by measuring reflectance factors in selected points located in rows and columns over the painting. After the measurement process, a database containing spectral and colorimetric data has been obtained. This database is georeferenced on the painting and could be useful in the future to track the conservative status development of this painting. This database also could be useful to optimize lighting when exhibited.

#### **2.1 Measurement system configuration**

A non-contact spectrometer, a lighting source designed for this task, and several additional instruments conform to the measurement system. Spectral *Reflectance Measurements on Cultural Heritage DOI: http://dx.doi.org/10.5772/intechopen.100288*

#### **Figure 4.**

*Annular ring source for 35° lighting with a hole in the central area for spectroradiometric measurement.*

measurements were taken between 400 and 780 nm every 4 nm with a Photo Research PR-655 spectroradiometer equipped with MS-75 accessory optical lens [7].

Geometrical conditions are determinant on the measurement repeatability, and in this case, the instrument was placed 570 mm away from the painting surface. The relationship with instrument optical lenses makes that the measured area reach 23.7 mm2 . The lighting source was placed around the spectroradiometer in a ring-shaped form with an angular incidence of 35° with the normal vector of the measured area, **Figure 4**.

It must be noted that reflectance data are dependent on angular conditions; thus, obtained data for lighting at 35° could be different when lighting is at 45°. Therefore, system configuration should be maintained in the case of new measurements along time.

The source was made of four halogen light bulbs at 3500 K and eight white LEDs at 6500 K. The halogen light bulbs were selected to fill the spectra in the wavelengths that the LED has less flux to improve the spectral response of the system. The lighting system was powered by a current power supply to improve LED stability over time, and anyway during the measurement process lighting condition will be checked using the standard patron. **Figure 5** shows the spectral emission of the designed source used to light the measurement area.

**Figure 5.** *Source spectral radiance (halogen+LED).*

#### *Heritage - New Paradigm*

The use of a LabSphere white diffuse reflectance standard during the measurement permits us to obtain the reflectance factor at the desired x, y location. In this way, temporal changes on the source flux due to aging, current variation, or any other are considered in the reflectance calculation.
