**3. Applications in cultural heritage**

The first points of the interferograms are missing due to the penetration depth of mirror coat‐ ings, and according to Fourier transform theory, they correspond to the cosine contributions having the longest period in the spectra calculation. By inserting a bandpass filter in the opti‐ cal setup and using the information that the spectrum has to be zero in certain regions of the electromagnetic spectrum, it is possible to find the amplitude of the missing points of the interferogram and reconstitute the original spectrum by applying the discrete Fourier transform (DFT) [10]. In **Figure 4**, we present the spectrum obtained from the DFT applied to the calibrated interferogram in **Figure 3** using the Hanning apodization function. The DFT spectrum is expressed in the frequency domain, and the spectrum of interest is in the band 400–720 nm (416–750 THz) according to the bandpass filter. The peak of the LED is at about 472 THz, corresponding to 635 nm, and since the base of the FPI is the Airy function, and not the cosine as in the Michelson interferometer, DFT creates the harmonics of the peak at 472 THz that decease as *R*n/*n*, where *R* is the reflectance of the mirrors and *n* is the order of the harmonic [11]. The interferogram in **Figure 3** is obtained with a maximal OPD of about 30 µm that corresponds to a spectral resolution of about 10 THz. The frequency interval in the spec‐ trum, as visible in the inset of **Figure 4**, is decreased below the spectral resolution by using the zero padding method [10]. A phase correction has been applied to the spectrum calculation in order to take into account the phase dispersion of the mirror coatings [11]. In the right side of spectrum are evident the aliases of the LED peak, artefacts of the DFT. The effect of aliases on the original spectrum is decreased by increasing the number of points per fringe and decreas‐ ing the reflectivity of the mirrors. Since the aliases of the LED peak have a phase dispersion

**Figure 3.** The calibrated, resampled and rescaled interferogram from **Figure 2**.

218 Fourier Transforms - High-tech Application and Current Trends

Once a spectrum for each pixel is calculated, all the spectral information are stored in a hyper‐ spectral cube, a three dimensional array with the spatial information of the scene on the *x*

that is not corrected they have deformed peaks.

In the field of cultural heritage, the study of the artwork intended as physical object is very important for knowing constitutive materials, state of preservation and degradation phenom‐ ena. It is surely helpful for taking measurements suitable for the artwork conservation during the exposition, that is, in museums or art galleries, but also during possible conservation treatment. Moreover, it may provide precious information about the object's history and pro‐ venience, sometimes affecting its economic value.

This study normally implies the development of a diagnostic campaign through different analytical techniques, from the so‐called *imaging* techniques to the *punctual*—noninvasive or micro‐invasive, destructive or not destructive—chemical analyses. The main aims are to map and chemically identify in the artwork both the original materials, with connected alteration and degradation phenomena, and the substances possibly superimposed during past inter‐ ventions, such as pictorial retouching or inpainting.

Since the artwork is a unique piece, it has to be fully preserved and therefore the use of non‐ invasive diagnostic techniques is always preferred: on the contrary, the sample taken for analysis has to be limited because it is inevitably an irreversible operation. Luckily, from the interaction between artwork and radiation, many data about materials can be collected in a completely noninvasive manner.

Going beyond the traditional multispectral analyses, where few radiation bands are used for investigating materials, the HSI techniques represent the innovation so that they are making inroads as diagnostic tools [12, 13]: the possibility of combining the painting's image with the spectral information of each pixel has evident advantages in comparison with punctual analy‐ ses. The punctual analysis corresponding to the HSI is the fibre optic reflectance spectroscopy (FORS) [14–18]: for this reason, in the next case studies and applications, we will report the comparison between the spectra extracted from the hyperspectral cube to the ones coming from the spectrophotometer. In both kinds of spectra, it is possible to reveal characteristic peaks, shoulders or absorption bands [19]; the performance of the two instruments in material identification deeply varies depending on the wavelength range used [20, 21].

In the examples below, concerning artworks studied at CCR, FORS analyses were carried out in the 350–1000 nm band with 0.5 nm spectral resolution by means of an Ocean Optics HR2000+ES spectrophotometer, bounded by optical fibres of 400 µm in diameter to an Ocean Optics HL2000 halogen lamp. Spectralon© 99% was used as white reference. Measures were acquired with a probe in 45°x/0° geometry, so following the standard illuminating/view‐ ing geometry defined by CIE (*Commission Internationale de l'Eclairage*) [22], on areas of fixed dimension (approximately 3 mm in diameter).

## **3.1. Study of the spatial resolution of HSI**

As for other imaging techniques, the application of the HSI requires to choose the right dis‐ tance between the camera and the object, an important parameter that inevitably determines the quality of the result.

In order to evaluate the spatial resolution performance of the HSI device here discussed [23], a small Flemish painting on copper (34 cm wide and 49 cm high, **Figure 5a**: "Passaggio del Mar Rosso", inv. 299, *Musei Reali di Torino: Galleria Sabauda*), characterized by crowded and analytic composition with brilliant colours and very subtle brushes typical of the Flemish School was chosen for the test.

Interesting points/pixel areas of the painting were thus selected from the HSI cube, even in correspondence of very small paintbrushes, for extracting the interferogram and calculating the reflectance spectra; FORS analyses on the same points were carried out and used as com‐ parison means for validating the results.

Placing the painting 120 cm distant from the HSI camera and illuminating the scene at 45° angle on the right by means of a halogen lamp, as for the FORS analyses, we acquired videos framing about 10 cm x 10 cm painting areas. The image size is about 1000 pixel x 1000 pixel due to the aperture of the FPI.

and chemically identify in the artwork both the original materials, with connected alteration and degradation phenomena, and the substances possibly superimposed during past inter‐

Since the artwork is a unique piece, it has to be fully preserved and therefore the use of non‐ invasive diagnostic techniques is always preferred: on the contrary, the sample taken for analysis has to be limited because it is inevitably an irreversible operation. Luckily, from the interaction between artwork and radiation, many data about materials can be collected in a

Going beyond the traditional multispectral analyses, where few radiation bands are used for investigating materials, the HSI techniques represent the innovation so that they are making inroads as diagnostic tools [12, 13]: the possibility of combining the painting's image with the spectral information of each pixel has evident advantages in comparison with punctual analy‐ ses. The punctual analysis corresponding to the HSI is the fibre optic reflectance spectroscopy (FORS) [14–18]: for this reason, in the next case studies and applications, we will report the comparison between the spectra extracted from the hyperspectral cube to the ones coming from the spectrophotometer. In both kinds of spectra, it is possible to reveal characteristic peaks, shoulders or absorption bands [19]; the performance of the two instruments in material

In the examples below, concerning artworks studied at CCR, FORS analyses were carried out in the 350–1000 nm band with 0.5 nm spectral resolution by means of an Ocean Optics HR2000+ES spectrophotometer, bounded by optical fibres of 400 µm in diameter to an Ocean Optics HL2000 halogen lamp. Spectralon© 99% was used as white reference. Measures were acquired with a probe in 45°x/0° geometry, so following the standard illuminating/view‐ ing geometry defined by CIE (*Commission Internationale de l'Eclairage*) [22], on areas of fixed

As for other imaging techniques, the application of the HSI requires to choose the right dis‐ tance between the camera and the object, an important parameter that inevitably determines

In order to evaluate the spatial resolution performance of the HSI device here discussed [23], a small Flemish painting on copper (34 cm wide and 49 cm high, **Figure 5a**: "Passaggio del Mar Rosso", inv. 299, *Musei Reali di Torino: Galleria Sabauda*), characterized by crowded and analytic composition with brilliant colours and very subtle brushes typical of the Flemish

Interesting points/pixel areas of the painting were thus selected from the HSI cube, even in correspondence of very small paintbrushes, for extracting the interferogram and calculating the reflectance spectra; FORS analyses on the same points were carried out and used as com‐

Placing the painting 120 cm distant from the HSI camera and illuminating the scene at 45° angle on the right by means of a halogen lamp, as for the FORS analyses, we acquired videos

identification deeply varies depending on the wavelength range used [20, 21].

ventions, such as pictorial retouching or inpainting.

220 Fourier Transforms - High-tech Application and Current Trends

dimension (approximately 3 mm in diameter).

**3.1. Study of the spatial resolution of HSI**

the quality of the result.

School was chosen for the test.

parison means for validating the results.

completely noninvasive manner.

**Figure 5.** (a) Picture of the set during a hyperspectral video acquisition. (b) Main scene (numbers refer to the FORS analyses' measurement points); details of the painting "Passaggio del Mar Rosso" (inv. 299), *Musei Reali di Torino: Galleria Sabauda*.

Without changing the camera‐object distance, we chose various scenes of the painting por‐ trayed in different views in order to analyse paintbrushes of different size: for example, the main central scene displaying Moses (**Figure 5b**) has bigger homogeneous areas of paint. The crowed scene on the left is portrayed in the background and so it presents very small brushes (**Figure 6a**); the lateral scene on the right, portrayed in the foreground, has medium or small size details (**Figure 6b**). Reflectance spectra were extracted from the hyperspectral cube, attempting to select as most as possible homogeneous coloured pixel areas or paintbrushes. The examples in **Table 1**, reported as blue, green, yellow, orange, red and purple, called p1, p2, etc., are from 25 to 279 pixel areas. The smallest pixel area corresponds to a paintbrush that is about 0.5 mm x 0.5 mm. These videos took 180 s each, spatial resolution is about 100 µm corresponding to about 250 ppi, spectral resolution is 10 THz.

FORS analyses were performed on the same selected areas (p1, p2, etc.) to verify the reliability of the results of the Fabry‐Perot hyperspectral device when used as diagnostic tool. **Figures 7**–**9** report some examples: the correspondence among calculated HSI and FORS spectra is evi‐ dent. In red areas of paint (p1 and p32), it is possible to recognize the flex at around 585 nm ascribable to the red pigment *cinnabar* (**Figure 7**). In the green area (p24), spectra suggest the presence of a green *copper‐based* pigment (**Figure 8**); in purple area (p34), spectra show the double absorption band at around 530 and 560 nm ascribable to the use of *red lake* (**Figure 8**), probably coming from *cochineal* [24, 25]. In blue areas (**Figure 9**), spectra acquired in two bands report the double absorption band at around 595 and 650 nm (p26) suggesting the pres‐ ence of the smalt pigment [26, 27].

Actually, the spectrum of the green area p24 comes from the smallest pixel area, a paintbrush that on the painting is 0.5 mm × 0.5 mm, and so it defines the spatial resolution performance of the HSI system: it was possible to use this HSI instrument as diagnostic tool for pigment identification selecting areas of just 25 pixels.

**Figure 6.** (a) Lateral scene on the left; (b) lateral scene on the right (numbers refer to the FORS analyses' measurement points); details of the painting "Passaggio del Mar Rosso" (inv. 299), *Musei Reali di Torino: Galleria Sabauda*.


**Table 1.** Description of the areas of paint selected in the details of **Figures 5a**, **6a** and **6b**.

#### Fourier Transform Hyperspectral Imaging for Cultural Heritage http://dx.doi.org/10.5772/66107 223

**Figure 7.** Spectral reflectance factor of the areas p1, p32 acquired by FORS (blue curves) compared to the ones calculated from the hyperspectral cube (red curves).

**Figure 8.** Spectral reflectance factor of the areas p24, p34 acquired by FORS (blue curves) compared to the ones calculated from the hyperspectral cube (red curves).

This feature will be helpful in the diagnostic investigation on Flemish, Divisionist or Pointillist paintings that present very small paintbrushes or glazing, but also on three‐dimensional sur‐ faces that may be difficult to reach with punctual contact instruments.

### **3.2. Study of the pigment recognition**

bands report the double absorption band at around 595 and 650 nm (p26) suggesting the pres‐

Actually, the spectrum of the green area p24 comes from the smallest pixel area, a paintbrush that on the painting is 0.5 mm × 0.5 mm, and so it defines the spatial resolution performance of the HSI system: it was possible to use this HSI instrument as diagnostic tool for pigment

**Figure 6.** (a) Lateral scene on the left; (b) lateral scene on the right (numbers refer to the FORS analyses' measurement

**X Y [pixel] [mm x mm]**

Blue (p2) 122–130 990−1020 279 0.9 x 3.1 Yellow (p10) 574–580 999−1006 56 0.7 x 0.8

Purple (p34) 1245–1249 835–870 180 0.5 x 3.6 Green (p35) 1065–1080 588–591 64 1.6 x 0.4

Orange (p8) 317–321 288–297 50 0.5 x 1 Red (p21) 294–301 406–415 80 0.8 x 1 Green (p24) 156–160 314–318 25 0.5 x 0.5 Graysh blue (p26) 258–264 94–103 70 0.7 x 1

**Painting detail Selected areas Image coordinates Area size Area size**

Main central scene (**Figure 5b**) Red (p1) 325–335 775−790 176 1.1 x 1.6

Lateral scene on the left (**Figure 6a**) Red (p32) 1131–1139 715–719 45 0.9 x 0.5

**Table 1.** Description of the areas of paint selected in the details of **Figures 5a**, **6a** and **6b**.

points); details of the painting "Passaggio del Mar Rosso" (inv. 299), *Musei Reali di Torino: Galleria Sabauda*.

ence of the smalt pigment [26, 27].

Lateral scene on the right

(**Figure 6b**)

identification selecting areas of just 25 pixels.

222 Fourier Transforms - High-tech Application and Current Trends

Here we report two important case studies where the use of HSI for the comprehension about materials was fundamental. The first one refers to the noninvasive study of the "Croce astile" (*Museo Poldi Pezzoli*, Milan), attributed to Raffaello Sanzio (1483–1520). The second one refers to some Egyptian wooden coffins (*Museo Egizio*, Turin), dating back to the Third Intermediate Period (1070–712 BC approximately).

Concerning the wooden cross, it presents polychrome areas and gold leaf gilding. Figures of the saints are portrayed within rounded medals, one per each extreme of the cross on the front and on the back; mouldings of the wooden support are along the perimeter and all around the medals (**Figure 10**).

**Figure 9.** Spectral reflectance factor of the area p26 acquired by FORS (blue curve) compared to the ones calculated from the HS cube (red curve) in two bands (400–720 nm and 600–1000 nm) then combined together.

Because of the discontinuities of the support, the pigment identification by FORS, in particular on some lateral details nearby the moulding, would have been complicated since this technique normally requires putting the probe (usually of few centimetres in diameter) in contact with the artwork surface. Therefore, the hyperspectral imaging investigation was decisive.

Reflectance spectra extracted and calculated from the HSI cubes showed to be very helpful for pigment identification. The comparison and the integration with other complementary analyses, as required by an efficient diagnostic investigation, allowed the identification of the pictorial palette used for the cross. **Figure 11** reports the example showing the use of azurite in the blue backgrounds (blue curves), red lake in the pink stoles (red curves), Sienna earth in the brown hair (orange curves), green copper‐based pigments in the green gowns (green curves), and ochre‐based mixtures in the representation of the skin (grey curve).

Concerning the Egyptian coffins, the hyperspectral investigation allowed the rapid identifica‐ tion of the palette and the total pigment mapping over the surface thanks to the implementation of an algorithm built for recognizing similar reflectance spectra within the hyperspectral cube.

As known, paints of the same hue can have different saturation levels due to various factors, such as different amount of white pigment in the mixture or differences in the state of preserva‐ tion. Therefore, the resulting reflectance spectra may change even if the main "coloured" pig‐ ment is the same. In fact, the possibility of identifying the pigment lies in the fact that the trend of the reflectance curve is recognizable, since it maintains the position of the absorption bands fixed in the spectrum, while the percent reflectance values over the spectrum can vary more or less uniformly. **Figure 12** shows the example of two red ochre‐based mixtures containing 50% and 90% in weight of white pigment. Converted in colorimetric coordinates expressed in the CIELAB 1976 colour space [22], L\* (lightness) increases when the paint is clearer, whereas a\* and b\* (hue) components maintain approximately constant values. From 50% to 90% of white, L\* passes from 33.6 to 38.9, a\* from 34.4 to 34.1 and b\* from 32.7 to 31.3 (measures obtained by a Konica Minolta CM700d colorimeter in D65, SCE and d/8° condition).

and on the back; mouldings of the wooden support are along the perimeter and all around

Because of the discontinuities of the support, the pigment identification by FORS, in particular on some lateral details nearby the moulding, would have been complicated since this technique normally requires putting the probe (usually of few centimetres in diameter) in contact with the artwork surface. Therefore, the hyperspectral imaging investigation was

**Figure 9.** Spectral reflectance factor of the area p26 acquired by FORS (blue curve) compared to the ones calculated from

Reflectance spectra extracted and calculated from the HSI cubes showed to be very helpful for pigment identification. The comparison and the integration with other complementary analyses, as required by an efficient diagnostic investigation, allowed the identification of the pictorial palette used for the cross. **Figure 11** reports the example showing the use of azurite in the blue backgrounds (blue curves), red lake in the pink stoles (red curves), Sienna earth in the brown hair (orange curves), green copper‐based pigments in the green gowns (green curves),

Concerning the Egyptian coffins, the hyperspectral investigation allowed the rapid identifica‐ tion of the palette and the total pigment mapping over the surface thanks to the implementation of an algorithm built for recognizing similar reflectance spectra within the hyperspectral cube. As known, paints of the same hue can have different saturation levels due to various factors, such as different amount of white pigment in the mixture or differences in the state of preserva‐ tion. Therefore, the resulting reflectance spectra may change even if the main "coloured" pig‐ ment is the same. In fact, the possibility of identifying the pigment lies in the fact that the trend of the reflectance curve is recognizable, since it maintains the position of the absorption bands

and ochre‐based mixtures in the representation of the skin (grey curve).

the HS cube (red curve) in two bands (400–720 nm and 600–1000 nm) then combined together.

the medals (**Figure 10**).

224 Fourier Transforms - High-tech Application and Current Trends

decisive.

**Figure 10.** Hyperspectral imaging session on the wooden cross "Croce astile" attributed to Raffaello Sanzio (*Museo Poldi Pezzoli, Milan*).

In imaging acquisition as in HSI, moreover, the intensity of the light source may be not so uni‐ form over the artwork, so that the reflectance spectra have a vertical scaling. Aiming at obtain‐ ing the pigment clustering and recognition from the spectrum analysis, independently of the light intensity, a metric has to be defined in order to measure the similarity of two spectra, represented as vectors in the vector space formed by the spectral components. A vertical scal‐ ing of the spectrum would change the length of the vectors without changing the direction in the vector space. We consider the metric, and the closeness of two vectors, as the angle between the two vectors using the spectral angle mapper (SAM) technique [28]. **Figure 13** shows the false colours elaboration coming from hyperspectral cube acquired on the lid of an anthropoid coffin (Museo Egizio, S. 05239): red and yellow ochres/earths, Egyptian blue and copper green, commonly used in Ancient Egypt, have been identified and their distribution over the coffin surface is highlighted by the false colours. The lower part of the figure shows the red, green, blue (RGB) image composed of two HSI cubes and rendered for a standard illuminant (D65).

**Figure 11.** Details of the wooden cross "Croce astile" attributed to Raffaello Sanzio (*Museo Poldi Pezzoli, Milan*). Hyperspectral images calculated for D65 illuminant and 10° standard observer, with reflectance spectrum extracted from the HS cubes.

### **3.3. Study of the colour rendering**

Another important advantage of the hyperspectral imaging is the possibility of using the HSI data for colour rendering operations. This means that the RGB image of the artwork may be calculated starting from the reflectance spectrum of each pixel, as shown previously.

If necessary, by correcting the spatial non‐uniformity (in intensity) of the illumination, it is possible to obtain "hyperspectral images" of the artworks where each spectrum depends only

on the pictorial material's characteristics, and it is not affected by the light source (neither the quality, nor the intensity) used during the acquisition.

false colours elaboration coming from hyperspectral cube acquired on the lid of an anthropoid coffin (Museo Egizio, S. 05239): red and yellow ochres/earths, Egyptian blue and copper green, commonly used in Ancient Egypt, have been identified and their distribution over the coffin surface is highlighted by the false colours. The lower part of the figure shows the red, green, blue (RGB) image composed of two HSI cubes and rendered for a standard illuminant (D65).

226 Fourier Transforms - High-tech Application and Current Trends

Another important advantage of the hyperspectral imaging is the possibility of using the HSI data for colour rendering operations. This means that the RGB image of the artwork may be

**Figure 11.** Details of the wooden cross "Croce astile" attributed to Raffaello Sanzio (*Museo Poldi Pezzoli, Milan*). Hyperspectral images calculated for D65 illuminant and 10° standard observer, with reflectance spectrum extracted

If necessary, by correcting the spatial non‐uniformity (in intensity) of the illumination, it is possible to obtain "hyperspectral images" of the artworks where each spectrum depends only

calculated starting from the reflectance spectrum of each pixel, as shown previously.

**3.3. Study of the colour rendering**

from the HS cubes.

**Figure 12.** Reflectance spectra of red ochre‐based mixtures acquired by FORS, containing 50% in weight (blue line) and 90% in weight (red line) of white pigment (calcium carbonate).

In this way, it is possible to calculate and render the hyperspectral (HS) image for the lamp you prefer, simply by choosing the emission spectrum for the illumination you want to simu‐ late (i.e. CIE standard illuminants, or commercial light sources, depending on the purpose).

In the conservation field, the possibility of studying how the artwork—or more generally, a pictorial material—would appear under different illumination conditions represents a real advantage not only for the aspects linked to the exhibition lightening but also for the choice of pigments for conservation. In fact, the pictorial retouching and the inpainting made in labora‐ tory shall appear different outside it, losing their harmony with the original painting.

The case study of the painting "Gesù tra i dottori" attributed to Giovanni Battista Beinaschi (1636–1688), belonging to the *Castle of Racconigi* (province of Cuneo, Italy), is particularly sig‐ nificant for this aspect. The surface presents many lacunae, at present filled with stucco, that have to be inpainted at CCR. In **Figure 14**, we present two different details from the painting calculated from the HSI cube, the detail on the left is acquired using a xenon lamp and rendered with a D65 illuminant, with standard CIE 10° observer, on the right a particular acquired using a halogen lamp and rendered with an LED4000K illuminant, with standard CIE 10° observer.

It is not so easy to find pigments "suitable" for the inpainting treatment. Light plays an impor‐ tant role, obviously, also on the in‐painting's colour appearance, and often we know very few about the lighting system of the museum.

Based on some selected pigments and light sources, we exploited the HSI data for giving an evaluation of the colour appearance variability. Just to make an example, **Figure 15** shows the HSI images of two mockups, the one on the left made of copper blue (Kremer Pigmente GmbH & Co. KG, n° 45364), the one on the right made of Zirconium Cerulean Blue (n° 45400). Images are rendered with one halogen lamp (3000 K) and with some different, from "warm" to "neutral" white, LED sources (LED 3000K, typical light source in art galleries or museums simulating halogen lamp; LED 2700 K and LED 4000 K).

**Figure 13.** False colours hyperspectral image, portion of lid of an anthropoid coffin (*Museo Egizio*, Turin, inv. S. 05239), picture of the lid (up); combination of two portions of the lid, HS image rendered for a standard illuminant (D65).

the HSI images of two mockups, the one on the left made of copper blue (Kremer Pigmente GmbH & Co. KG, n° 45364), the one on the right made of Zirconium Cerulean Blue (n° 45400). Images are rendered with one halogen lamp (3000 K) and with some different, from "warm" to "neutral" white, LED sources (LED 3000K, typical light source in art galleries or museums

**Figure 13.** False colours hyperspectral image, portion of lid of an anthropoid coffin (*Museo Egizio*, Turin, inv. S. 05239), picture of the lid (up); combination of two portions of the lid, HS image rendered for a standard illuminant (D65).

simulating halogen lamp; LED 2700 K and LED 4000 K).

228 Fourier Transforms - High-tech Application and Current Trends

**Figure 14.** Detail of the painting "Gesù tra i dottori" attributed to Giovanni Battista Beinaschi (1636–1688), belonging to the *Castle of Racconigi* (province of Cuneo, Italy). (a) Image acquired using a xenon lamp and rendered with a D65 illuminant, with standard CIE 10° observer, (b) image acquired using a halogen lamp and rendered with a LED 4000K illuminant, with standard CIE 10° observer.

**Figure 15.** Hyperspectral image of two mockups: copper blue in polyvinyl acetate (PVAc, left column) and in linseed oil (right column); zirconium cerulean blue in PVAc (left column) and in linseed oil (right column). (a) Images are rendered for the halogen 3000 K lamp, (b) the 3000K LED, (c) the 2700K LED, and (d) the 4000 K LED..

Beyond the visual comparison that would be surely affected by many factors (first of all by the computer screen), the HSI images allows to extract the colorimetric values of the mockups for each lamp and to find which one has the minimum chromatic variability. In this case, the zirconium cerulean blue presents a slightly lower variability with respect to the copper blue and relatively to the four lamps tested (**Table 2**).


**Table 2.** Colorimetric coordinates (CIELAB 1976 colour space) of mockups reported in **Figure 15**.

Obviously, only the precise knowledge of the museum lighting system, that will host the artwork, would be the key for the right consideration about materials, to prefer in the conser‐ vation treatment. We think this example may show the system potentiality and its precious contribution in the choice of pigments and binders for restoration and in promoting interven‐ tions that might consider not only the chemical properties of the materials but also the colour appearance.
