**4. Lighting**

Cultural heritage objects must be exhibited for their appreciation, but inadequate temperature and lighting conditions can cause their deterioration [14,15,26]. Generating yellowing, discoloration or colour variation, corrosion, alteration and corruption, increase in surface temperature, and acceleration of deterioration on works of art [16]. This section reviews the main studies related to the lighting of artworks.

The damage caused by the emission of UV and IR radiation is controlled by filters or LED sources, but the radiation emitted by the visible range of the electromagnetic spectrum is necessary for the appreciation of the artwork and at the same time could produce irreparable damage to it [15]. The International Commission on Illumination (CIE) differentiates between the damage caused by the photochemical effect on different pigments and the damage due to the thermal action of light [26]. Photochemical damage is produced when a photon is absorbed by a pigment, causing the chemical properties of this pigment to change, manifested in artworks as changes in the mechanical properties of the pigments or in their colour [14]. The damage caused by thermal action, according to the CIE, has been more ignored in museums, since the damage has not been as visible as in the case of damage by photochemical action. Elena Lucchi (2016) evaluated the energy and environmental quality of museums, based on 50 European institutions. The result was that light is the most important environmental parameter in museums because it is directly related to the preventive conservation of works of art and to the comfort of viewers [27]. As the control of photochemical damage in museums has gained importance, so has the control of damage by the thermal action of light, taking into account that an increase in temperature favours chemical action at the molecular level in the different pigments [26,28–30]. Recently, a process to minimize the risk of deterioration in multifuncional historic buildings has been developed. It puts forward the profitability associated with the lighting parameters optimization in order to avoid damage to the cultural heritage shown in the exhibition [31].

The CIE made recommendation 157: 2004 to regulate the damage caused by photochemical action and the effects of thermal radiation of light on the different artworks exhibited in museums. Knowing that not all materials respond in the same way to the harmful effects of visible radiation, the CIE established, in its regulations, maximum irradiance limits and a defined exposure time to illuminate artworks, depending on the sensitivity to light radiation of the materials that constitute the artwork [26,28]. Mayorga et al. (2015) introduced the concept of Global Risk Factor (GRF), presenting a system for quantifying the damage produced by natural light, so that it could be used as the main or secondary source, minimizing the intrinsic risks it carries and using its advantages in lighting such as its ability to save energy, non-polluting, renewable source, psychological comfort, circadian cycle, and colour reproduction [32].

In addition to taking into account the damage due to radiation in museum exposures, it is important that the source used does not produce distortions in the perception of the artwork. For this, the colour rendering index (CRI) is usually used, which defines the colour reproduction capacity of a light source [26].

Even so, not all pigments used in an artwork respond in the same way to the effects of radiation, and not in a linear way [15]. This section reviews the main techniques developed for lighting in museums, which, in addition to protecting artworks from the effects of light, allow us to correctly appreciate them.

Historically, incandescent or fluorescent lighting was responsible for lighting in museums [15,33], however, in recent years, LED lighting has become very important for this application. LEDs offer great advantages, such as compactness, long useful life, adjustable intensity [33] and absence of radiation in the ultraviolet range and infrared range [33,34]. In addition, the radiation in the visible spectrum is reduced compared to continuous spectrum emission [34], their surface temperature rarely exceeds 50 ◦C, allowing them to be placed near wooden objects [14]. In recent years, LED technology has gained higher luminous efficiency, and by combining it with phosphors, it is easy to optimize the spectrum for the desired application [33].

Delgado et al. (2010) presented an optimization strategy applied to improve the luminous efficiency of a light source to illuminate artworks, excluding the part of the visible spectrum that does not contribute to the adequate perception of the illuminated object [35].

Berns et al. (2011) optimized a simulated triband light source made up of three LEDs with the fundamental objective of illuminating without causing damage to the artwork and producing the same colour rendering as the D65 illuminant, which is usually used as a reference in simulations due to its similarity to daylight. To this end, they performed four simulations using the emission of LEDs with bandwidths of 25 nm, 50 nm, 75 nm and 100 nm. The LEDs with bandwidths of 50, 75 and 100 nm provided emissions almost identical to D65, and the LED with a bandwidth of 25 nm had its efficiency improved by combining it with LEDs of greater bandwidth, obtaining very favourable results of 4*E*, colour reproduction and luminous efficacy, these being 1.2 4*E* ∗ <sup>94</sup>, 92 CRI, and 325 lm/W, respectively. The 1.2 4*E* ∗ <sup>94</sup> is imperceptible to the human eye in lighting conditions between 50 and 200 lux [34].

Furthermore, Berns (2011) developed an optimization algorithm capable of increasing or decreasing chroma depending on the demands of the artwork. On the one hand, an increase in chroma would make it possible to compensate for the lack of colour, respecting in any case the artist's intention. On the other hand, works exhibited in caves or churches must maintain their low chroma status since they were conceived that way [34].

Using an optimized white LED illumination obtained through the combination of different LEDs, Vienot et al. (2011) recreated the appearance of a bird specimen. They were able to intensify the saturation of faded colours by taking advantage of the distortions caused by narrow-band LED lighting [33].

Durmus and Davis (2015a) developed a lighting system with an optimized emission spectrum in order to produce energy savings without altering the appearance of the illuminated object. To do this, they started from the idea of dispensing with the emissions at those wavelengths absorbed by the object, designing a lighting system tuned by detectors with the reflectance of the objects, emitting fundamentally at the wavelengths reflected by the object and perceptible by the human eye. In this way, they managed to develop a lighting system through which an energy savings of 44% can be obtained and without appreciable changes for the human eye in the appearance of the object, with low values of 4*E* ∗ *ab* calculated in the CIE 1976 colour space L \* a \* b \* [36].

Durmus and Davis (2015b) further perfected that technique by implementing added bandwidths of the order of 10 nm to the peaks of the selected emission spectrum, obtaining energy savings of between 55% and 71% maintaining low values of 4*E* ∗ *ab* [37].

Mayorga et al. (2016) analysed how lighting affects 23 different pigments in order to develop a system to measure the spectral response of exposed materials. This system allowed estimating the time and the maximum irradiance with which a artwork can be illuminated without noticeable changes in its colour [38].

Stenger (2015) based the lighting of Mark Rothko's murals on the limitation established by the CIE of an illuminance of less than 50 lux, restricting UV and IR radiation [26], since, in addition, one of the artist's requirements was that the artwork not be too illuminated [24].

Taking into account the need to develop a lighting system capable of displaying artworks without causing damage to them, the Zeus project emerged. It has been mainly promoted by scientists from the Complutense University of Madrid. This project also includes institutions involved in the conservation and dissemination of cultural heritage objecds and researchers in Communication and Audiovisual Technology. Among other strategies related to the analysis of artworks for their correct conservation and visualization, the Zeus project developed a selective lighting system that calculates the appropriate light distribution point by point based on the characteristics of the artwork [30].

Durmus et al. (2018) designed an optimized lighting system by which the reflectance of the artworks was obtained in order to be able to project onto them an illumination capable of reducing photochemical damage due to spectral absorption of radiation in the pigments used based on the reflectance spectrum of the pigments. In this development, it was essential not to affect the appearance of the objects so as not to alter the intention of the artist. To this end, they used multi-objective genetic optimization algorithms (MOGA). Genetic algorithms (GAs) are based on the theory of evolution, where, starting from a wide range of species, only those suitable survive, the rest facing extinction. The MOGAs, unlike the GA, allow a greater number of solutions by defining an aptitude function that evaluates the solutions at each step, thus generating more precise solutions. Considering that when using a visible spectrum light source, visibility and damage conflict, they used MOGA optimization algorithms since by themselves they can produce more than one result that does not allow the optimal solution. Therefore, it was essential to use Pareto's optimal solution technique, which was responsible for selecting alternatives when there is no single optimal solution, allowing obtaining an optimized lighting source to minimize absorption damage without causing a notable damage to the colour of the artwork [29].

Carla Balocco and Giulia Volante set out a methodology for sustainable cultural heritage lighting that not only complied with the regulations for the correct conservation and protection of works of art, but also provided visual and perception comfort from the point of view of observer reducing the power consumption of the light source. Thus, they carried out a study to illuminate those areas of interest of the work of art by analyzing the eye movements of a series of subjects when viewing it [39].

Muñoz De Luna, in his doctoral thesis published in 2017, developed a point-by-point spectral reflectance measurement system using non-contact spectrophotometers in order to subsequently carry out a colourmetric analysis of the artworks that allows a temporal monitoring of these in an objective. This system was applied to study the following works of Pablo Picasso: *Guernica* and *Woman in Blue*. Using Michiel Sweert's *Boy with a Turban and Corsage*, he carried out a comparative study of the colourimetric values of the artwork, illuminating it using the illuminants that the CIE defines as standard illuminant, finding in certain areas of the work, appreciable colour differences for human perception. Finally, using the reflectance measurement technique previously described and the knowledge of how the different illuminants affect the conservation and appreciation of an artwork, there was studied the lighting used for the cave paintings of the cave of *El Castillo* in Puente Viesgo (Cantabria). To this end, an LED lighting system was developed using a minimum optimization algorithm that allowed the emission of an optimal spectral distribution in terms of minimal deterioration, maximum contrast between the colour stimulus and the background, and minimum chromatic difference between the observed painting under the light used by the artist and observed with the developed illuminant [28].

The lighting system developed by Vázquez et al. (2020) for projecting onto artworks was optimized to minimize radiation damage, while also producing a restored appearance of the art [21]. With the information from the CIELab coordinates and reflectance information, they developed a merit function based on Fernandez-Balbuena et al. [40] to optimize the 4*E* ∗ <sup>00</sup> between the D65 reference and test illuminant. A second merit function was developed in order to minimize the radiation damage through the concept of GRF, developed by Mayorga et al. [32,38]. Using the optimization algorithm, based on Durmus and Davis [36,37] and Muñoz De Luna [28] described above, and the lighting system described by the Zeus Project [30], they calculated the optimal SPD and the intensity of each point in the image [21].

Rui Dang et al. proposed a method of quantifying damage taking the type of paint, the level of illuminance and the exposure time as three variable units, thus allowing to optimize lighting systems in a much more precise way. They found that some works of art allow higher levels of illuminance than those set by the regulations, thus achieving an improved visualization by the viewer [41].
