**2.2 Cartesian displacement device**

The painting dimensions required a large rail displacement system; in this case, XYZ Cartesian rails were developed with position accuracies of 25 microns on each axis. On board this rail, there were a platform that was moved by a computer system linked to the instruments. Onto this platform, the spectroradiometer with a CCD and a computer-controlled laser telemeter were installed. The laser telemeter was of higher importance because it permits to place the scientific instrument at a constant distance from the painting surface. As it was demonstrated after the painting surface has a topographical variation in the Z-axis making the surface non-flat, **Figure 6**. These deformations did not affect the measurement accuracy because the instrument was placed at a controlled distance all over the painting. The large dimension and weight of *Guernica* is part of this topographical issue that is not visually noted.

### **2.3 Color calibration**

To ensure the correct calibration of the system, it is necessary to check performance all over the measurement process. One of the most used charts for color calibration is the GretagMacbeth ColorChecker (X-RITE). In this measurement process, the 24 chart samples were measured. Each sample spectral reflectance factor and the CIELAB Lab data were collected. For this calculation, CIE 1931 standard observer and D50 CIE illuminant were used. The instrument deviation in relationship to manufactured data was of ΔE = 1.16 CIELAB color units.

## **2.4 Reflectance data**

Reflectance data from 2201 points were measured with this technique. The time for measuring that number of points was around 10 h working at night for 3 days. The measurement process is slow due to the robot movement and the stabilization period where the system needs to stop before a measurement is made [8].

**Figure 6.** *Guernica topography matrix, data in mm.*

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

These reflectance data along points were required by Museo Nacional Centro de Arte Reina Sofía in a specific format to have a valid reference for future inspection on the conservation state of the painting. As an example, **Figure 7** shows two of the selected points, one on white pigment and another one on black pigment. The circular black area in the zoomed images on the right (samples 7 and 1) is related to the measurement area as can be shown in **Figure 7**.

With the obtained data reflectance is calculated, **Figure 8** presents the spectral differences in two different samples. In this case, sample 1, which is mainly composed of a white pigment, has a higher reflectance than sample 7, which is measured over a black pigment.

**Figure 7.** *Sample measurement area and detail for samples 1 and 7.*

**Figure 8.** *Spectral reflectance for samples 1 and 7.*
