Patent Publication Number: US-2023152498-A1

Title: Chromatic effect light reflective unit

Description:
TECHNICAL FIELD 
     The present invention relates in general terms to chromatic effect light reflective units and in particular to light reflective units having a nanostructured reflective surface in order to interact with an incident light such as to generate chromatic effects in the reflected light and thus to offer to the observer a particular visual perception thereof. Specifically, the present invention relates to a light reflective unit particularly suitable for use in interior or exterior wall coating structures of buildings. 
     BACKGROUND 
     In the field of wall coating, it is known to use panel structures of various types and materials depending on the particular aesthetic effect that is to be given internally or externally to the building or the particular technical result to be achieved. In exemplary terms, buildings can be coated internally or externally with insulating panels to improve the characteristics of energy consumption, photovoltaic panels for energy recovery through the conversion of solar energy into electricity, shading panels to shield from sunlight and so on. It is also known to coat buildings with coloured panels and/or structures capable of offering a specific chromatic effect, for example to give the facades of the buildings particular aesthetic characteristics. 
     A highly appreciated aesthetic effect for the coating of building walls is achieved through the use of reflective units of the type described in international application no. PCT/EP2015/001454 of the same Applicant. These reflective units comprise at least one layer of material loaded with nanoparticles that covers the reflective surface so as to interact with the incident light, reproducing the typical chromatic characteristics of the sky and the sun. In particular, the interaction of the incident light with the material loaded with nanoparticles leads to a reflective behaviour that varies as a function of the wavelength, presenting a regular spectral reflectance (hereinafter simply regular reflectance) that is greater in red than in blue and, vice versa, a diffuse spectral reflectance (hereinafter simply diffuse reflectance) that is higher in blue than in red. In the context of this description and the subsequent claims, the terms “regular reflectance” and “diffuse reflectance” refer to the definitions provided in the E284 standard relating to the terminology describing the appearance of materials and light sources (ASTM E284 -09a, Standard Terminology of Appearance, ASTM International, West Conshohocken, PA, 2009). Furthermore, the term “spectral” refers to the regular reflectance and diffuse reflectance evaluated as a function of the wavelengths of light. 
     Another type of facade coating unit with chromatic effect is known from the international application no. PCT/EP2016/066995 of the same Applicant. This unit provides a support structure and a chromatic reflective layer formed on the support structure. The chromatic reflective layer comprises a reflective layer and a chromatic diffusing layer with characteristics similar to those described above. Otherwise, the chromatic facade unit comprises an absorbent medium provided in or on the chromatic diffusing layer and/or on the reflective layer, which is configured to absorb electromagnetic radiations with wavelength in the infrared. 
     This reflective behaviour, and in particular the dependence of the regular and diffuse reflection of the wavelength of the incident light, generates a light blue colouring of the illuminated panel, observed outside the regularly reflected beam of light. This bluish colouring is given to the panel by the light reflected diffusedly, or subsequently simply diffused. In contrast, the regularly reflected light is characterized by a correlated colour temperature (CCT) lower than the CCT of incident light, as regular reflectance is greater for wavelengths in the red than for wavelengths in the blue region. 
     Specifically, the Applicant has found that the aesthetic effect obtained through the reflective units described in international applications no. PCT/EP2015/001454 and no. PCT/EP2016/066995 is characterized by:
     a regularly reflected beam having chromatic coordinates comprised in a region of the colour plane CIE 1976 u′-v′ with coordinates u′&gt; 0.210 and v′&gt; 0.470 and a maximum Cartesian distance in this colour plane less than 0.1 from the Planck curve referred to the light source which illuminates the reflective unit, where such light source is a standard illuminator CIE E; and   a diffused reflected beam having chromatic coordinates comprised in a region of the colour plane with coordinates u′&lt;0.210 and v′&lt;0.430.   

     The realization of nanoparticle reflective units of the known type requires the coating layer in a material loaded with nanoparticles to be applied in an extremely uniform manner in order to preserve the appearance homogeneity of the illuminated unit. A non-uniformity, for example, in the thickness of the coating layer results in a non-homogeneous colouring of the reflective unit when illuminated. However, the deposition of strictly uniform layers requires the use of expensive techniques, mostly resulting in a high percentage of waste. In addition, it is highly complex to obtain a thickness uniformity in case the reflective units have to form panels with a non-flat conformation - for example stepped, embossed, microstructured or ashlar. In fact, at the folds, the coating layer undergoes thickening and/or thinning which modify the appearance of the panel at that point, when illuminated by a light source. 
     The Applicant has therefore strongly perceived the need to realize a chromatic effect light reflective unit particularly suitable for use in building wall coating structures which on the one hand can be made using simple and inexpensive techniques, and on the other hand, is able to offer a uniform chromatic effect. 
     In particular, the Applicant has identified the need to realize chromatic effect light reflective units which are particularly suitable for use in coating structures for building facades. Within the scope of this object, the Applicant has recognized the need to realize a chromatic effect light reflective unit which is capable of guaranteeing a homogeneous chromatic effect even in the case of surface conformations that are not strictly planar. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the present invention is directed to a chromatic effect light reflective unit. The unit comprises:
     a reflective layer having at least one reflective surface, and   a chromatic diffusion layer having a first surface proximal to the reflective surface and a second surface, opposite and substantially parallel to the first, configured to be illuminated by incident light.   

     Advantageously, the chromatic diffusion layer comprises a nano-pillar or nano-pore structure in a first material having a first refractive index, immersed in a second material having a second refractive index other than the first, in which the first and the second materials are substantially non-absorbing or transparent to electromagnetic radiations with wavelength included in the visible spectrum, that is, substantially comprised in the range 380 nm ≤ λ ≤ 740 nm. 
     According to the present invention, the ratio between a higher refractive index and a lower refractive index chosen between the first and second refractive indexes is comprised between 1.05 and 3. Furthermore, the nano-pillars or nano-pores have a development along a main direction not parallel to the first surface and the second surface of the chromatic diffusion layer. In other words, the nano-pillars or nano-pores are not coplanar or parallel to the surfaces of the chromatic diffusion layer, i.e. they extend between them. 
     Furthermore, the nano-pillar or nano-pore structure is characterized by a plurality of geometric parameters which comprises a nano-pillar diameter or nano-pore diameter d p , a nano-pillar or nano-pore length l p  along said main development direction, a surface density of nano-pillars or nano-pores D p  and/or a structure porosity P p , wherein the pillar diameter or pore diameter (d p ) is comprised between 40 nm and 300 nm, the length (l p ) along the main development direction is comprised between 300 nm and 40 µm (300 nm &lt; l p  &lt; 40 µm) and at least one between the surface density of nano-pillars or nano-pores (D p ) and the structure ( 30 , 70 ) porosity (P p ) is configured to provide the unit with a higher regular reflectance for wavelengths of incident light comprised in the range of red with respect to wavelengths of the incident light comprised in the range of blue and a higher diffuse reflectance for wavelengths of incident light comprised in the range of blue than wavelengths of incident light comprised in the range of red. 
     By “range of red” it is meant a range of wavelengths comprised between 600 nm and 740 nm. 
     By “range in blue ” it is meant in a broad sense a range of wavelengths comprised between 380 nm and 500 nm, thus also comprising the wavelengths that conventionally range from violet to cyan. 
     Advantageously, the nano-pore or nano-pillar layer allows obtaining chromatic effects similar to those obtained through a layer of material loaded with nanoparticles of the type described in the international patent application no. PCT/EP2015/001454, when illuminated by a collimated beam of incident light, wherein by collimated beam it is meant a beam of light having a main direction of propagation and an angular divergence around said direction of propagation less than 45°, preferably less than 10°, even more preferably less than 2°. In addition, the nano-pore or nano-pillar layer is particularly resistant and offers a high degree of uniformity. In particular, thanks to the solution according to the present invention it is possible to obtain a chromatic diffusion layer of constant thickness even when the light reflective unit comprises concavity and convexity. In other words, it is possible to obtain uniform diffuse reflectance and regular reflectance coefficients along the surface of the unit - i.e. the regular reflectance and the diffuse reflectance do not depend on the specific local conformation of the surface - even for light reflective units with non-flat conformation, for example stepped, embossed, microstructured or ashlar. 
     The unit according to the present invention can comprise one or more of the following additional characteristics, which can also be combined together at will in order to satisfy specific requirements defined by a corresponding application purpose. 
     In a variant of the invention, the unit can have a regular reflectance in blue R(450 nm) - measured at the wavelength equal to 450 nm by way of reference - which is comprised in the range from 0.05 to 0.95, preferably from 0.1 to 0.9. In some examples, the regular reflectance in blue R(450 nm) is comprised between 0.2 and 0.8. In variants that want to simulate the presence of a clear blue sky, the regular reflectance in blue R(450 nm) can be comprised in the range from 0.4 to 0.95, preferably from 0.5 to 0.9, preferably between 0.6 and 0.8. In variants that want to reduce/minimize the contribution of the reflected scene, the regular reflectance in blue R(450 nm) can be comprised in the range from 0.05 to 0.6, preferably from 0.1 to 0.5, preferably from 0.2 up to 0.4. 
     In a variant of the invention, the regular reflectance in red R(630 nm), measured by way of reference at the wavelength equal to 630 nm, is at least 1.05 times, preferably1.2 times, even more preferably 1.6 times greater than the regular reflectance in blue R(450 nm). 
     In a variant of the invention, the diffuse reflectance in blue R(450 nm) is at least 1.2 times, preferably at least 1.4 times, more preferably at least 1.6 times greater than the diffuse reflectance in the red R(630 nm). 
     In a variant of the invention, the regularly reflected beam has a CCT of at least 10% less, preferably at least 15%, more preferably of at least 20% than the CCT of the incident beam. 
     In a variant of the invention, the diffusely reflected beam has a CCT of at least 20% higher, preferably at least 30%, more preferably of at least 50% than the CCT of the incident beam. 
     To quantify the chromatic separation it is also possible to define a variation in the CCT of the regularly reflected beam with respect to the CCT of the incident beam. The reduction indicated above is characteristic of a shift of the CCT of the regularly reflected beam towards red and at the same time a shift of the CCT of the diffusedly reflected beam towards blue, since the chromatic diffusion layer is made in a first and a second material that are both substantially non-absorbing, or transparent to electromagnetic radiations with wavelength included in the visible spectrum. 
     In a variant of the invention, the Euclidean distance on the chromaticity diagram CIE 1976 u′-v′ between the colour point of the regularly reflected beam (u′ R , v′ R ) with respect to the white colour point (u′ B ,v′ B ) - where u′ B  = 0.210 and v′ B  = 0.474 for the standard illuminator defined below - is equal to at least 0.01, preferably 0.015, more preferably 0.02 with u′ R  &gt; u′ B  and v′ R  &gt; v′ B . To quantify the chromatic separation it is also possible to calculate a shift of the colour point on the chromaticity diagram CIE 1976 u′-v′ between the position of the colour point of the incident beam (white point) and the position of the colour point of the regularly reflected beam. As seen above with reference to the CCT, in the unit according to the invention a shift in the direction of the red of the regularly reflected beam necessarily implies a shift in the direction of the blue of the colour point associable with the diffused light (diffusedly reflected light), thus being index of the phenomenon of chromatic separation. 
     In the context of the present description and the subsequent claims, for the quantification of CCT values, in general and for those indicated above, reference is made to an incident beam produced by a standard illuminator CIE D65. Otherwise, for the quantification of the values u′-v′, in general and for those indicated above, reference is made to an incident illumination coming from a white light source, for example a standard illuminator CIE E, which within the visible spectrum radiates equal energy and has a constant spectral power distribution (SPD). Although this is a theoretical reference, the standard illuminator CIE E is particularly suitable in the event of diffusion variability as a function of the wavelengths, as it has a uniform spectral weight with respect to all wavelengths. 
     According to an embodiment, it is possible to associate to the set of single developments of the nano-pillars or nano-pores with respect to the main direction, an order parameter S defined as S = 2&lt;cos 2 ϑ&gt; - 1 comprised between 0.7 and 1, more preferably between 0.9 and 1, wherein ϑ is the angle comprised between the main development direction identified in a section plane transversal to the surfaces of the chromatic diffusion layer and an axis associable with each nano-pillar or nano-pore of a plurality of nano-pillars or nano-pores lying in said section plane. The definition of the order parameter S is defined on the basis of the actual experimental measurement methods adopted by the Applicant and better described below. 
     Thanks to a high order degree along the axis identified by the directrix, there is greater control over chromatic variability for samples that exhibit it. 
     In a variant of the invention, the diameter d p  is comprised between 70 nm and 200 nm, preferably comprised between 80 nm and 160 nm. 
     According to one embodiment, the length along the main direction of the nano-pillars or nano-pores is comprised between 500 nm and 40 µm (500 nm &lt; l p  &lt; 20 µm), preferably comprised between 500 nm and 20 µm (500 nm &lt; l p  &lt; 20 µm). 
     In another variant of the invention, the surface density D p  is such as to define an inter-pore or inter-pillar distance I p  less than 2.8 times the diameter d p , preferably less than 2.6 times the diameter d p , more preferably less than 2.4 times the diameter d p . 
     In the present description and in the attached claims, by inter-pore or inter-pillar distance I p  it is meant a distance measured starting from one or more images obtained by scanning electron microscopy or SEM showing the second surface of the chromatic diffusion layer, i.e. the distal surface from the reflective layer. In other words, this quantity is measured at the free end of the nano-pillars or nano-pores. 
     According to an embodiment, the porosity P p  of the structure is comprised between 20% and 80%, preferably between 25% and 75%. 
     Through tests carried out by the Applicant, the ranges of the geometric parameters have been identified and which allow to establish a chromatic effect in the regular reflection and in the diffuse reflection (or simply diffusion) as a function of the angle of incidence, which is expressed, among other things, in the variation of the CCT of a regularly reflected light beam and/or of the CCT of a light beam reflected diffusedly (or simply diffused) by the unit, with respect to the CCT of the incident light beam. This effect occurs in an invariable way or in a static way (slightly variable) - that is, in conditions whereby, being the unit illuminated by a beam collimated along a direction at a certain angle of incidence with respect to the local normal of the surface on which the beam strikes, the CCT of the regularly reflected light beam and/or that of the diffusedly reflected light beam do not entirely depend or only weakly depend on this angle of incidence. Alternatively, this effect manifests itself in a variable way - i.e. in conditions so that the CCT of the regularly reflected light beam and/or that of the diffusedly reflected light beam depend substantially on the angle of incidence of the beam that illuminates the unit. 
     According to a different embodiment, the diameter d p  is greater than a diameter threshold value d p_threshold  and/or the length l p  is greater than a length threshold value l p_threshold  such as to provide a variability in the correlated colour temperature of a luminous flux reflected by the unit by regular reflection, as a function of an angle of incidence, preferably comprised between 0° and 60°, of a corresponding luminous flux incident on the unit with wavelength comprised between 380 nm and 740 nm. In particular, the correlated colour temperature of a luminous flux reflected by the unit by regular reflection decreases as the angle of incidence increases. Furthermore, a maximum Euclidean distance ΔRmax(u′,v′) between pairs of colour points of a regularly reflected beam that belong to a plurality of colour points of the regularly reflected beam and identified at different angles of incidence is greater than 0.02. 
     In particular, in the present description and in the subsequent claims by the terms ‘light’, ‘light beam’, ‘light ray’ or ‘luminous flux’ it is meant one or more electromagnetic radiations with wavelength included in the visible spectrum (i.e., substantially 380 nm ≤ λ ≤ 740 nm). Furthermore, in the present description and in the subsequent claims, by the expression ‘collimated beam of light’ or ‘collimated light beam’ it is meant a light beam having a main direction of propagation and an angular divergence around this direction of propagation less than 45°, preferably less than 10°, even more preferably less than 2°. 
     Preferably, when the ratio n M /n m  between the higher refractive index n M  and the lower refractive index n m  is comprised between 1.7 e 1.9, for example when the first material is aluminium oxide (n 1 =1.78) and the second material is air (n 2 =1), the diameter threshold value d p_threshold  is comprised between 50 nm and 120 nm, more preferably between 60 nm and 100 nm, even more preferably it is equal to about 80 nm. 
     Preferably, when the ratio n M /n m  between the higher refractive index n M  and the lower refractive index n m  is comprised between 1.7 e 1.9, for example when the first material is aluminium oxide (n 1 =1.78) and the second material is air (n 2 =1), the length threshold value l p   _   threshold  is comprised between 800 nm and 5 µm, more preferably between 1 µm and 4 µm, even more preferably it is equal to about 3 µm. 
     Preferably, when the ratio n M /n m  between the higher refractive index n m  and the lower refractive index n m  is comprised between 1.1 e 1.3, for example when the first material is aluminium oxide (n 1 =1.78) and the second material has a second refractive index  n   2  comprised between 1.4 e 1.6, the diameter threshold value d p_threshold  is comprised between 150 nm and 220 nm, more preferably between 160 nm and 200 nm, even more preferably it is equal to about 180 nm. 
     Preferably, when the ratio n M /n m  between the higher refractive index n m  and the lower refractive index n m  is comprised between 1.1 and 1.3, for example when the first material is aluminium oxide (n 1 =1.78) and the second material has a second refractive index  n   2  comprised between 1.4 and 1.6, the length threshold value l p_threshold  is comprised between 6 µm and 12 µm, more preferably between 8 µm and 10 µm, even more preferably is about 9 µm. 
     Thanks to this solution it is possible to obtain units capable of changing the correlated colour temperature of both a regularly reflected luminous flux and a diffusedly reflected luminous flux as a function of the angle of incidence of the beam that illuminates the unit. In particular, the parameters make it possible to obtain surfaces capable of varying chromatic tones in a similar way to the earth’s atmosphere based on the position of the sun with respect to the horizon. 
     According to a different embodiment, the diameter d p  is greater than a second diameter threshold value d p_threshold_2  and/or the length l p  is greater than a second length threshold value l p_threshold_2  such as to provide a dichroic reflectance ratio r = R(λ r , θ)/ R(λ b , θ of the electromagnetic radiation reflectances at the wavelengths of λ b  = 450 nm and λ r  = 630 nm of a luminous flux reflected by the unit by regular reflection, increasing as the angle of incidence of a corresponding luminous flux incident on the unit increases and exhibiting a variation of the dichroic reflectance value higher than 5%, preferably higher than 10%, more preferably 15% of the dichroic reflectance value (r) of a luminous flux reflected by the unit by regular reflection in the case of a luminous flux incident on the unit at an angle of incidence of about 10°. 
     Preferably, when the ratio n M /n m  between the higher refractive index n M  and the lower refractive index n m  is comprised between 1.7 and 1.9, for example when the first material is aluminium oxide (n 1 =1.78) and the second material is air (n 2 =1), the second diameter threshold value d p_threshold_2  is comprised between 40 nm and 100 nm, preferably between 60 nm and 80 nm, even more preferably it is equal to about 70 nm. 
     Preferably, when the ratio n M /n m  between the higher refractive index n M  and the lower refractive index n m  is comprised between 1.7 and 1.9, for example when the first material is aluminium oxide (n 1 =1.78) and the second material is air (n 2 =1), the second length threshold value (l p_threshold_2 ) is comprised between 300 nm and 2 µm, preferably between 1 µm and 1.7 µm, more preferably it is equal to about 1.4 µm. 
     Preferably, when the ratio n M /n m  between the higher refractive index n M  and the lower refractive index n m  is comprised between 1.1 and 1.3, for example when the first material is aluminium oxide (n 1 =1.78) and the second material has a second refractive index  n   2  comprised between 1.4 and 1.6, the diameter threshold value (d p_threshold ) is comprised between 150 nm and 190 nm, more preferably between 160 nm and 180 nm, more preferably it is equal to about 170 nm. 
     Preferably, when the ratio n M /n m  between the higher refractive index n M  and the lower refractive index n m  is comprised between 1.1 and 1.3, for example when the first material is aluminium oxide (n 1 = 1.78) and the second material has a second refractive index  n   2  comprised between 1.4 and 1.6, the second length threshold value (l p_threshold_2 ) is comprised between 4 µm and 8 µm, preferably between 5 µm and 7 µm, more preferably it is equal to about 6 µm. 
     According to one embodiment, the first material is a metal oxide. 
     This choice of the first material allows to easily realise a robust and resistant chromatic diffusion layer. In fact, the nano-pillar or nano-pore structure in metal oxide can be obtained in a simple and economical way starting from known oxidation processes - for example, as described in Runge, Jude Mary, “The Metallurgy of Anodizing Aluminum Connecting Science to Practice”, 2018, Springer International Publishing - which stimulate the growth of oxide on the metal. This growth takes place in a uniform manner, allowing to obtain layers of substantially any size, characterized by a substantially uniform thickness and therefore able to offer homogeneous chromatic effects. 
     Furthermore, the metal structure on which the nano-pillar structure is grown can easily assume conformations other than the flat one, without compromising the uniformity of the nano-pillar layer. 
     Preferably, this metal oxide is aluminium oxide (alumina), titanium oxide (titania) or zinc oxide. 
     According to one embodiment, the second material is air. Alternatively, the second material is a polymer, a resin, a silicone, a different oxide (for example deposited by sol-gel) that are transparent or substantially non-absorbent at least to electromagnetic radiations with wavelength included in the visible light spectrum, preferably with refractive index comprised between 1.3 and 1.55, even more preferably between 1.41 and 1.52, for example polyvinyl chloride (PVC), polymethyl methacrylate (PMMA), polyfluorides (e.g. PVDF) or transparent polyacrylates. 
     By selecting the material in which the nano-pillars or nano-pores structure is immersed, it is possible to further vary the chromatic variability presented by the unit even once the parameters of the nano-pillars or nano-pores have been set. 
     According to an embodiment, the nano-pillars or nano-pores can have a distribution with respect to the second surface of the chromatic diffusion layer divided into coherence areas extending less than 100 µm 2 , more preferably less than 10 µm 2 , even more preferably less than 1 µm 2 , wherein each nano-pillar or nano-pore within one of said coherence area of the second surface is substantially equidistant from adjacent nano-pillars or adjacent nano-pores, present in the same coherence area. 
     Within the scope of the present description and in the subsequent claims with “each nano-pillar or nano-pore within a coherence area is equidistant” it is meant that the nano-pillars or nano-pores within this coherence area have the same distance between adjacent pores, unless deviations less than 10% with respect to an average distance value calculated on the basis of the values of distances between adjacent nano-pillars or adjacent nano-pores measured within this area. 
     The Applicant has found that thanks to this characteristic it is possible to avoid the occurrence of interference phenomena due to the Bragg grating diffraction and the presence of iridescence in the reflected or diffused light with the consequent manifestation of colours, such as for example colours in the region of green or fuchsia unrelated to the colour of the natural light of the sky and the sun. Furthermore, the Applicant has observed that a greater randomness of distribution of the nano-pores or nano-pillars inside the structure favours the establishment of the desired chromatic effect. 
     According to one embodiment, the unit further comprises an intermediate layer. This intermediate layer is interposed between the chromatic diffusion layer and the reflective layer. Preferably, the intermediate layer being at least partially non-absorbent or transparent to electromagnetic radiations with wavelength included in the visible spectrum. 
     Thanks to this solution it is possible to couple together a chromatic diffusion layer and a reflective layer that cannot be directly coupled to each other. In addition, by selecting a reflection coefficient and/or a capability of filtering one or more electromagnetic radiations with a predetermined wavelength in order to obtain particular chromatic yields in the light reflected by the unit. 
     According to another embodiment, the unit further comprises a coating layer, placed at the second surface of the chromatic diffusion layer. Preferably, the coating layer is at least partially non-absorbent or transparent to electromagnetic radiations with wavelength included in the visible spectrum. 
     Thanks to this solution it is possible to realise chromatic effect light reflective units that are particularly resistant to the wearing action of atmospheric agents. 
     According to a variant of the invention, the reflective layer comprises a rear surface opposite to its own reflective surface and the unit comprises at least one stiffening composite layer placed at the rear surface of the reflective layer and comprising a shimming panel and a coating sheet, wherein the shimming panel has a specific weight at least 5 times less than the specific weight of the coating sheet, preferably at least 10 times less than the specific weight of the coating sheet. Furthermore, the shimming panel has a thickness at least 2 times higher than the thickness of the coating sheet, preferably at least 5 times higher than the thickness of the coating sheet. 
     Preferably, the shimming panel is made of a non-combustible material, such as fiberglass, expanded glass granulate, rock fibre, cellular glass, ceramic fibre, carbon fibre, vermiculite (expanded or not), expanded clay or perlite (expanded or not). 
     Preferably, the shimming panel is made in the form of a grating, such as for example a honeycomb grating with axis of the cells that is orthogonal to the reflective layer, or has a wavy profile according to a section orthogonal to the reflective layer. 
     Preferably, the coating sheet is made of aluminium and has a thickness comprised between 0.2 mm to 1 mm, preferably equal to about 0.5 mm. 
     Thanks to the composite stiffening layer, it is possible to realise a lightweight, but at the same time robust unit, while also giving it characteristics of non-flammability and thermal and/or acoustic insulation. 
     A different aspect in accordance with the present invention proposes a coating element comprising:
     at least one chromatic effect light reflective unit according to one of the embodiments described above;   a support structure, said support structure being configured to mechanically support the at least one unit so that the second surface of the chromatic diffusion layer faces the external environment, and   coupling means, configured to allow a mechanical coupling of the support structure to a bearing element.   

     The coating element makes it possible to coat objects, in particular buildings, in a uniform way, creating surfaces capable of replicating the colouring of the earth’s atmosphere when hit by solar radiation or artificially illuminated with white light. 
     A further aspect in accordance with the present invention relates to an illumination system comprising at least one chromatic effect light reflective unit according to one of the embodiments described above and at least one illuminator to illuminate the at least chromatic effect light reflective unit, the illuminator being configured to emit or project a cone of light which at least partially strikes on the second surface of the chromatic diffusion layer configured to be illuminated by incident light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and form part of the description, illustrate exemplary embodiments of the present invention and, together with the description, are intended to illustrate the principles of the present invention. 
       In the drawings: 
         FIG.  1    is a sectional perspective view of a light reflective unit according to an embodiment of the present invention; 
         FIGS.  2  -  5    are schematic cut-away axonometric views of a portion of the light reflective unit according to embodiments of the present invention; 
         FIG.  6    reports SEM images showing the surface of six different chromatic diffusion layers of as many light reflective units according to different embodiments of the present invention; 
         FIGS.  7  and  8    are schematic side views of a portion of a light reflective unit according to an embodiment of the present invention; 
         FIGS.  9  -  11    are SEM images showing sectional side views of three different light reflective units according to embodiments of the present invention; 
         FIG.  12    corresponds to the view of  FIG.  1    in which a chromatic variability effect of the light reflective unit is schematically illustrated according to an embodiment of the present invention; 
         FIG.  13    is a graph of the course of the regular reflectance of the light reflective unit according to embodiments of the present invention as a function of the wavelength of an electromagnetic radiation as the angle of incidence of a light beam on the unit varies; 
         FIG.  14    is a graph of the course of the ratio between the reflectance of the unit according to an embodiment of the present invention at two different wavelengths as a function of the angle of incidence of a light beam on the unit; 
         FIGS.  15   a  and  15   b    schematically illustrate test arrangements for assessing the chromatic properties of the unit; 
         FIG.  16    is a representation of the colour plane in which the desired regions of the colour points of the beams reflected regularly and diffusedly by the unit are highlighted; 
         FIG.  17    schematically illustrates three different coherence areas of the nano-pore structure included in a light reflective unit according to an embodiment of the present invention; 
         FIG.  18    is a flow chart of a procedure for growing a chromatic diffusion layer comprising a nano-pore structure according to an embodiment of the present invention; 
         FIGS.  19  and  20    are perspective views in side section of two coating elements according to as many embodiments of the present invention; 
         FIG.  21    is a sectional perspective view of a light reflective unit according to a different embodiment of the present invention; 
         FIG.  22    is a schematic cut-away axonometric view of a portion of light reflective unit according to an alternative embodiment of the present invention; 
         FIG.  23    is a sectional perspective view of a light reflective unit according to a further embodiment of the present invention; 
         FIG.  24    is a sectional perspective view of a light reflective unit according to another embodiment of the present invention; 
         FIG.  25    is a schematic top view of a portion of light reflective unit according to a different embodiment of the present invention; 
         FIG.  26    is a schematic view of an illumination system according to a first variant using a light reflective unit according to the present invention; 
         FIGS.  27  and  27   a    are respectively a schematic view of an illumination system in accordance with a second variant using a light reflective unit according to the present invention and an enlarged detail of the light source used therein; 
         FIGS.  28  and  28   a    are respectively a schematic view of an illumination system in accordance with a second variant using a light reflective unit according to the present invention and an enlarged detail of the light source used therein; 
         FIG.  29    is a schematic side view of an illumination system in accordance with a fourth variant using a light reflective unit according to the present invention; and 
         FIGS.  30 - 33    are schematic representations of preferred embodiments of a grid illumination system using a light reflective unit according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following is a detailed description of exemplary embodiments of the present invention. The exemplary embodiments described herein and illustrated in the drawings are intended to convey the principles of the present invention, allowing the person skilled in the art to implement and use the present invention in numerous different situations and applications. Therefore, the exemplary embodiments are not intended, nor should they be considered, to limit the scope of patent protection. Rather, the scope of patent protection is defined by the attached claims. 
     In the following description, for the illustration of the figures, identical numbers or reference symbols are used to indicate construction elements with the same function. Further, for illustration clarity, some references may not be repeated in all the figures. 
     The use of “for example”, “etc.”, “or” indicates non-exclusive alternatives without limitation, unless otherwise indicated. The use of “comprises” and “includes” means “comprises or includes, but not limited to”, unless otherwise indicated. 
     Furthermore, the use of measures, values, shapes and geometric references (such as perpendicular and parallel) associated with terms such as “approximately”, “almost”, “substantially” or similar, is to be understood as “without measurement errors” or “unless inaccuracies due to manufacturing tolerances” and in any case “less than a slight divergence from the values, measures, shapes or geometric references” with which the term is associated. 
     Finally, terms such as “first”, “second”, “upper”, “lower”, “main” and “secondary” are generally used to distinguish components belonging to the same type, not necessarily implying an order or a priority of relationship or position. 
     Chromatic Effect Light Reflective Unit 
     With reference to  FIG.  1    it schematically illustrates a chromatic effect light reflective unit, hereinafter referred to as ‘unit’ for brevity’s sake, according to an embodiment of the present invention. The unit  1 , 1   a - 1   g  comprises a reflective layer  10  and a chromatic diffusion layer  20  coupled together. 
     In detail, the reflective layer  10  comprises at least one surface  11  configured to regularly reflect an incident light beam comprising one or more electromagnetic radiations having wavelengths included at least in the visible spectrum (i.e., 380 nm ≤ λ ≤ 740 nm), also indicated with the terms ‘light beam’, ‘light’ ray, ‘luminous flux’ or ‘light’ in the following. For example, the reflective layer has a regular reflectance of at least 50%, preferably at least 75%, more preferably at least 90% is made of a metallic material, such as aluminium (Al), titanium (Ti), silver (Ag), zinc (Zn), etc. or an alloy, such as stainless steel, comprising such materials. Optionally, the reflective surface  11  of the reflective layer  10  can be subjected to a polishing process (mechanical or chemical). 
     The chromatic diffusion layer  20  comprises a first surface  21  proximal to the reflective surface  11  and a second surface  23 , opposite and substantially parallel to the first surface  21 , separated by a thickness t. In the embodiment considered, the first surface  21  of the chromatic diffusion layer  20  is coupled to the reflective surface  11  of the reflective layer  10 , while the second surface  23  faces the external environment. In particular, the second surface  23  is configured to be illuminated by incident light. 
     Advantageously, the chromatic diffusion layer  20  comprises a nano-pore  30  structure (illustrated in  FIGS.  2  -  5   ) or a nano-pillar  70  structure (illustrated in  FIG.  22   ). This nano-pore  30  or nano-pillar  70  structure is formed in a first material having a first refractive index  n   1  and is immersed in a second material having a second refractive index  n   2 . For example, the first material that constitutes the nano-pore  30  structure is aluminium oxide, or alumina (Al 2 O 3 ), preferably anodic aluminium oxide or AAO (acronym for the expression ‘Anodic Aluminum Oxide’). 
     Otherwise, the second material which fills the nano-pore  30  structure or in which the nano-pillar  70  structure is immersed is air. Alternatively, the second material which fills the nano-pore  30  structure or in which the nano-pillar  70  structure is immersed is a polymer, a resin, a silicone, a different oxide (for example deposited by sol-gel) that are transparent or substantially non-absorbent at least to electromagnetic radiations with wavelength included in the visible light spectrum, with refractive index  n   2  comprised between 1.3 and 1.55, preferably between 1.41 and 1.52, for example polyvinyl chloride (PVC), polymethyl methacrylate (PMMA), polyfluorides (eg PVDF) or transparent polyacrylates. 
     Preferably, the second material which fills the nano-pore  30  structure or in which the nano-pillar  70  structure is immersed is a resin based on soluble fluoropolymers, in particular a polyurethane resin with a high fluorocarbon content, for example the known resin on the market under the trade name Lumiflon®. In particular, the fluoropolymer-based resin is selected with a refractive index  n   2  comprised between 1.45 and 1.50, more preferably equal to 1.48. 
     Nano-Pore Structure 
     The nano-pore  30  structure comprises a plurality of nano-pores  31  (as schematically illustrated in  FIGS.  2  -  5   ) formed in the first material (for example aluminium oxide), having a distribution that in the specific example of  FIGS.  2 - 5    has a substantially hexagonal conformation with respect to a plane II (illustrated in  FIG.  2   ) substantially parallel to the first and second surfaces  21  and  23 ; for example, the plane in which the second surface  23  of the chromatic diffusion layer  20  lies. 
     Each nano-pore  31  comprises an opening facing the second surface  23  of the chromatic diffusion layer  20  and extends in the chromatic diffusion layer  20  towards the first surface  21  of this layer  20 . As will be evident to the skilled person, the nano-pores  31  have, in general, a non-regular shape as evident in  FIG.  6    - which shows six top views a) - f) obtained by scanning electron microscopy (acronym SEM) of as many real nano-pore structures - instead of a regular circular shape as illustrated for simplicity’s sake in the schematic examples of  FIGS.  2  -  5   . 
     Advantageously, the surface dimension of each nano-pore  31  is defined by a diameter d p  corresponding to a circumference that inscribes the pore  31  in the plane Π. In other words, the diameter d p  is preferably determined at the second surface  23  and is, substantially, a measure of the maximum distance between two points on the edge of the nano-pore  30  structure which delimits a corresponding opening of the nano-pore  31 . 
     Furthermore, each nano-pore  31  develops from the first surface  21  towards the second surface  23  defining a length dimension l p . Although  FIGS.  2  -  5    illustrate - for simplicity’s sake - pores  31  parallel to each other and orthogonal to the surfaces  21  and  23  of the chromatic diffusion layer  20 , the nano-pores  31  extend, in general, for the length dimension l p  along respective non-parallel directions (i.e. transversal) with respect to the first surface  21  and the second surface  23  of the layer  20  and in any case not strictly parallel to each other - as evident in  FIGS.  7 - 11   , which show side views of real nano-pore  30  structures sectioned along a plane (section plane) substantially transversal to the surfaces  21  and  23  of the chromatic diffusion layer  20  obtained by SEM. In the case of ordered nano-pore  30  structures, it is possible to identify in the section plane a main (group) development direction n̂ for the nano-pores  31  - as illustrated in  FIGS.  7 - 11    - and characterize the nano-pore  30  structure through a directional order parameter S (two-dimensional), measured with respect to the main development direction n̂ in the section plane transversal to the surfaces  21  and  23  of the chromatic diffusion layer  20 , and calculated as: 
     
       
         
           
             S 
             = 
             2 
             &lt; 
             
               
                 cos 
               
               2 
             
             ϑ 
             &gt; 
             − 
             1 
             , 
           
         
       
     
      wherein ϑ is the angle comprised between the main development direction n̂ and an axis associable with each nano-pore  31  of a plurality of nano-pillars or nano-pores lying in the section plane. In detail, as illustrated schematically in  FIG.  7   , the main development direction n̂ is defined as the direction identified by the average value &lt;α&gt; of the angles α defined between the intersection straight line between the section plane and the first surface  21 - corresponding to the reflective surface  11  of the substrate  10  in the example considered - and each nano-pore  31  along a plane transversal to the surfaces  21  and  23  of the chromatic diffusion layer  20 , where the transversal plane coincides with the section plane. As will be evident to the person skilled in the art, in the case of total disorder the average value of &lt;cos 2 ϑ&gt; is ½, hence S = 0, while for a perfectly ordered system (axes of nano-pores  31  aligned to the directrix) we have &lt;cos 2 ϑ&gt; = 1, hence S = 1. 
     In the context of the present description and subsequent claims, the term “ordered nano-pore structure” means a nano-pore  30  structure of the chromatic diffusion layer  20  of the unit  1 , 1   a - 1   g  characterized by a directional order parameter S comprised between 0.7 and 1 (i.e. 0.7 ≤ S ≤ 1) or, more preferably between 0.9 and 1 (i.e. 0.9 ≤ S ≤ 1) for at least one section plane. 
     The Applicant has identified that it is possible to determine the order parameter in the following way. Initially, an image of a cross section of the chromatic diffusion layer  20  is collected through scanning electron microscope (SEM) for which it is reasonable to identify the first surface  21  with a substantially straight line. Next, the image is analysed to identify a statistically significant number - for example, equal to or greater than 50 distinct elements - of 31 nano-pores with an aspect ratio between height (i.e., length dimension l p ) and width (i.e. diameter d p ) of the nano-pore  31  at least equal to 10 - which can reasonably be approximated to a segment. In particular, if a nano-pore  31  defines one or more bifurcations - as visible in  FIG.  9    - each bifurcation is considered as a distinct nano-pore  31 , where each of such distinct nano-pores  31  shares a common portion. If it is not possible to identify a statistically significant number of nano-pores  31  with this aspect ratio, the image is discarded and a new image is acquired. Subsequently, for each identified nano-pore  31  a development axis is defined, by joining the ends of the nano-pore  31 . For each development axis thus defined, an angle α is measured between this axis and the intersection straight line between the section plane and the first surface  21  - in other words, an angle α is measured for each nano-pore  31  with the desired aspect factor, identified in the image. The angles α are then averaged to obtain an average angle &lt;α&gt; along which the main directrix n̂ is oriented with respect to the first surface. The deviation angle ϑ with respect to the main directrix n̂ of the axes of each nano-pore  31  previously considered is therefore measured. Finally, these deviation angles are used for calculating the order parameter S according to the formula (1) above. 
     The nano-pore  30  structure is also characterized by the ratio n M /n m  between a higher refractive index n M  a lower refractive index n m  of the refractive indexes  n   1 , n   2  that characterize the first material of which the nano-pores  31  are made and the second nano-pore filling material  31 . 
     In the considered embodiment, the nano-pores  31  are filled with air. Therefore the walls of the pores  31  define an interface surface between the materials characterized by different refractive indexes. Alternatively, other filling materials can be used to fill the nano-pores  31  and obtain different desired refractive index ratios as described below. For example, alternative filling materials comprise, in a non-limiting way, a polymer, a resin, a silicone, a different oxide (for example deposited by sol-gel) that are substantially transparent at least to electromagnetic radiations with wavelength included in the light visible spectrum. In other words, the nano-pore  30  structure is immersed in the selected filling material. 
     The nano-pore  30  structure is also characterized by a periodicity of the arrangement of the nano-pillars or nano-pores limited to coherence areas A C1 , A C2  and A C3 , schematically illustrated in  FIG.  17   , extending less than 100 µm 2 , more preferably 10 µm 2 , even more preferably less than 1 µm 2 , where the coherence areas are sub-portions of the second surface  23 . In each of these coherence areas, each nano-pore  31  inside is equidistant from the adjacent nano-pores  31  within the same coherence area. In the present description, the term ‘adjacent’ is intended to indicate the nano-pores  31  placed at a minimum distance (substantially corresponding to the inter-pore distance Ip) from a reference nano-pore  31  along any direction that lies in the reference plane -for example the plane II - and passes through said reference nano-pore  31 . The periodicity of the arrangement of the nano-pillars or nano-pores is determined starting from one or more images obtained by scanning electron microscopy or SEM of the second surface  23 , of the type illustrated in  FIG.  6   . 
     Furthermore, it is possible to define a surface density D p  in terms of number of nano-pores  31  per unit area of the second surface  23  of the chromatic diffusion layer  20  which can be measured as the number of nano-pores per square micron or in terms of (average) distance between adjacent pores, or inter-pores distance Ip, and a porosity P p  of the structure  30  defined as the percentage of area occupied by the material having a lower refractive index n m  (for example air) with respect to the area of the second surface  23 . 
     In general terms, therefore, a nano-pore  30  structure according to the invention is of the ordered type, has a limited periodicity and can be characterized through a series of geometric parameters including in particular:
     the diameter d p  of the pores  31 ;   the length dimension l p  of the pores  31 ;   the surface density D p  of the pores  31 ;   the porosity P p , and   the ratio n M /n m  between the refractive indexes of the materials making up the structure  30 .   

     The Applicant has determined that, in the case of ordered and limited periodic nano-pore  30  structures, thanks to the combined effect of the chromatic diffusion layer  20  and the reflective layer  10 , the control of the aforementioned geometric parameters allows to control the establishment of a chromatic reflective and diffusion effect of the incident light, i.e. a dependence of the regular reflectance and the diffuse reflectance of the unit  1 , 1   a - 1   g  on the wavelength, which, again as a function of these parameters, can be of an invariable type, that is independent of the illumination direction of the unit  1 , 1   a - 1   g  with respect to the normal to its surface, static, that is, weakly dependent on the illumination direction of the unit  1 , 1   a - 1   g  with respect to the normal to its surface, or of variable type, i.e. of a type substantially dependent on this angle of illumination, resulting in distinct chromatic effects of the unit  1 , 1   a - 1   g  perceived by an observer. 
     The chromatic effects, indicated respectively as chromatically invariable and static, are due to the interaction of a light beam incident on the unit  1 , 1   a - 1   g  with the nano-pore  31  structure so that the unit  1 , 1   a - 1   g  has a higher regular reflectance for wavelengths of incident light comprised in the range of red with respect to wavelengths of incident light comprised in the range of blue. Otherwise, the nano-pore  31  structure affects the diffuse reflectance of the unit  1 , 1   a - 1   g , making it greater for wavelengths of incident light comprised in the range of blue with respect to wavelengths of the incident light comprised in the range of red. Consequently, when a light beam hits the unit  1 , 1   a - 1   g , the electromagnetic radiations with wavelengths comprised in the blue (380 nm ≤ λ ≤ 500 nm) of the light beam preferentially undergo a diffusion - also referred to as scattering - with respect to the wavelengths comprised in the range of red (600 nm ≤ λ ≤ 720 nm). 
     For example, the chromatic effect light reflective unit  1 , 1   a - 1   g  does not substantially absorb light in the visible range and diffuses light at the wavelength of 450 nm (blue) at least 1.2 times, for example at least 1.4 times, as well as at least 1.6 times more efficiently than the light at the wavelength of about 630 nm (red). In other words, at a wavelength of 450 nm (blue) the diffuse reflectance of the unit  1 , 1   a - 1   g  is at least 1.2 times, for example at least 1.4 times, as well as at least 1.6 times greater than the diffuse reflectance at 630 nm (red). 
     Similarly, the chromatic effect light reflective unit  1 , 1   a - 1   g  regularly reflects light at the wavelength of 630 nm (red) at least 1.05 times, for example at least 1.2 times, as well as at least 1.6 times, more efficiently than the light at the wavelength of about 450 nm (blue). In other words, at the wavelength of 630 nm (red) the regular reflectance of the unit  1 , 1   a - 1   g  is at least 1.05 times, for example at least 1.2 times, as well as at least 1.6 times greater than the regular reflectance at 450 nm (blue). 
     Consequently, the unit  1 , 1   a - 1   g  assumes a substantially light blue colour - due to the diffuse reflection - when hit by a substantially directional (collimated) beam of white light, for example a beam of white light that strikes on the surface of the unit from a direction which forms an angle θ with respect to the normal of said surface and having divergence less than 45°, preferably less than 10°, even more preferably less than 2° - for example, solar radiation - if observed from any direction substantially other than the specular direction with respect to the illumination direction, i.e. from a direction such that the observer does not see the specular reflection of the source, for example from a direction forming an angle with the specular direction with respect to the direction of the incident beam greater than semi-divergence of said incident light beam. At the same time the unit, when hit by a directional light beam of white light, assumes a warm colour, for example a yellow colour, or preferably orange, or even more preferably reddish, if observed in the specular direction with respect to the illumination direction, i.e. from a direction such that the observer sees the specular reflection of the source. This first chromatic effect does not vary as the angle of incidence θ varies and is therefore indicated as chromatically static. 
     A further chromatic effect, indicated as chromatically variable, occurs at the onset of a dependence of the regular reflectance and/or diffuse reflectance of the unit  1 , 1   a - 1   g  not only on the wavelength, but also on the direction of illumination or of incidence θ. 
     In other words, the colour whereby an observer sees the unit  1 , 1   a - 1   g  from a direction of observation in proximity to the direction of specular reflection, and possibly, but not necessarily, also the colour whereby an observer sees the unit  1 , 1   a - 1   g  from an observation direction far from the direction of specular reflection, depends on the angle of incidence θ of the light beam incident on the unit  1 , 1   a - 1   g . 
     In fact, the correlated colour temperature (acronym CCT) of the regularly reflected beam appears to depend on the angle of incidence θ of the corresponding incident light beam with respect to the normal to the unit or to the reflective surface  11  of the reflective layer  10 . In particular, in the examples considered, the correlated colour temperature of the regularly reflected light beam decreases as the angle of incidence θ of the light beam incident on the unit  1 , 1   a - 1   g  increases. For example, as schematically illustrated in  FIG.  12   , when a first light beam I 1  having a correlated colour temperature CCT 1 , strikes on the unit  1 , 1   a - 1   g  with an angle θ α  with respect to the normal to the reflective surface  11  of the reflective layer  10 , a corresponding first regularly reflected light beam R 1  will be obtained having a first correlated colour temperature CCT 2  other than a second correlated colour temperature CCT 3  of a regularly reflected light beam R 2  generated by the regular reflection of a second light beam I 2  having spectral content and CCT identical to those of the light beam I 1 , but incident on the unit  1 , 1   a - 1   g  with an angle θ β  with respect to the normal to the reflective surface  11  of the reflective layer  10 , other than the angle θ α . In particular, the first correlated colour temperature CCT 2  of the first reflected beam is greater than the second correlated colour temperature CCT 3  of the second reflected beam, when the angle θ β  is greater than the angle θ α . 
     In other words, the chromatic behaviour of the unit  1 , 1   a - 1   g  is dependent on the angle with which a light beam strikes on the unit  1 , 1   a - 1   g  itself. In particular, the regular reflectance R turns out to be a function of the incident wavelength λ and of the angle of incidence θ of a corresponding incident light beam, R(λ, θ), as illustrated in  FIG.  13   , where the dependence of the regular reflectance R(λ, θ) is traced as a function of the wavelength, normalized to the maximum value of this regular reflectance in the wavelength range and for the angle considered, for different angles of incidence θ 1  = 10°, θ 2  = 20°, θ 3  = 30°, θ 4  = 40° and θ 5  = 50° of a corresponding incident light beam. As is evident from  FIG.  13   , the decrease in the value of the regular spectral reflectance as the wavelength decreases is greater the greater the angle of incidence θ. It results (i) from the increase in diffuse reflectance as the angle θ increases (i.e. the luminance of the unit  1 , 1   a - 1   g  observed for directions far from that of specular reflection increases as the illumination angle θ increases), and (ii) from the fact that the diffuse reflectance is greater for wavelengths in the range of blue than in that of red. 
     The Applicant has determined that it is possible to characterize the chromatic properties of the unit  1 , 1   a - 1   g  in terms of the variation of a ratio between the regular reflectances evaluated at two different wavelengths for different angles of incidence θ. Preferably, the dichroic reflectance ratio r = R(λ r , θ/R(λ b , θ) of the electromagnetic radiation reflectances at the wavelengths of λ b  450 nm and λ r  630 nm, is considered, as shown in  FIG.  14   . In the total absence of chromatic variation, this ratio r remains almost constant as θ varies. On the contrary, if there is a chromatic variation, like in the example of  FIG.  14   , this ratio increases as the angle θ increases. In particular, in the presence of a variation of the dichroic reflectance value r less than 5%, preferably less than 10%, more preferably less than 15% with respect to the dichroic reflectance value r in the case of a luminous flux incident on unit  1 , 1   a - 1   g  at an angle of incidence of about 10°, then the unit  1 , 1   a - 1   g  is considered chromatically invariable. Above these variation values, the unit  1 , 1   a - 1   g  is considered to be chromatically weakly variable (static) or variable. 
     On the basis of the foregoing, the Applicant has determined that it is possible to establish the effect of chromatic reflection and diffusion, whether it is invariable, i.e. independent of the angle of incidence, or static, i.e. weakly dependent on the angle of incidence, or, again, variable, i.e. substantially dependent on the angle of incidence, as defined above, by acting on one or more of the following parameters characterizing the ordered nano-pore  30  structures of the chromatic diffusion layer  20 :
     the length l p  of the nano-pores  31 ;   the surface density D p  of the nano-pores  31  (i.e., the inter-pore distance Ip);   the diameter d p  of the nano-pores  31 ,   the porosity P p  of the nano-pore  30  structure, and   the ratio n M /n m  between the higher refractive index n M  and the lower refractive index n m  between the refractive index  n   2  of the pore filling material - for example, air - and the refractive index  n   1  of the material of the nano-pore  30  structure - i.e., aluminium oxide in the example considered.   

     Tests carried out by the Applicant have made it possible to highlight how the variation of parameters such as the ratio of indexes n M /n m  of the materials constituting the nano-pore  30  structure, the length of the nano-pores l p , the diameter of the nano-pores d p , the surface density D p  of the nano-pores and the porosity P p  of the nano-pore  30  structure allow to establish an invariable, static or variable chromatic reflection and diffusion effect as the angle of incidence of a corresponding incident light beam of white light varies. 
     In particular, the Applicant has determined that, in some embodiments, due to the establishment of chromatic reflection and diffusion effects, the ratio n M /n m  between the higher refractive index n M  and the lower refractive index n m  between the refractive index  n   1  of the first material and the refractive index  n   2  of the second material must be comprised between 1.05 and 3, wherein, the refractive indexes  n   1  and  n   2  are calculated according to standard refractive index measurements measured with wavelength equal to 589.29 nm. 
     In other embodiments, the Applicant has found that for the establishment of chromatic reflection and diffusion effects, the ratio n M /n m  must be preferably comprised between 1.10 and 1.8, more preferably between 1.15 and 1.4 or between 1.6 and 1.78. 
     In other embodiments, the Applicant has found that for the establishment of chromatic reflection and diffusion effects, the ratio n M /n m  must be preferably comprised between 1.7 and 2.7, more preferably between 1.7 and 2.05 or between 2.45 and 2.65. 
     In further embodiments, the Applicant has found that for the establishment of chromatic reflection and diffusion effects, the ratio n M /n m  must be preferably comprised between 1.4 and 2.1, more preferably between 1.45 and 1.7 or between 1.95 and 2.05. 
     Furthermore, the Applicant has highlighted how, in some embodiments, the chromatic reflection and diffusion effects of the incident light occur due to:
     diameters d p  of the nano-pores  31  comprised between 40 nm and 300 nm, resulting particularly intense for diameters d p  of the nano-pores or nano-pillars  31  comprised between 70 nm and 200 nm, and   lengths l p  of the nano-pores  31  comprised between 300 nm and 200 µm (300 nm &lt; l p  &lt; 200 µm), preferably comprised between 300 nm and 100 µm (300 nm &lt; l p  &lt; 100 µm), more preferably comprised between 300 nm and 40 µm (300 nm &lt; l p  &lt; 40 µm).   

     The Applicant has also observed how, in some embodiments, the chromatic reflection and diffusion effects of the incident light occur for surface densities D p  such as to define an inter-pore distance I p  less than 2.8 times the diameter d p , preferably less 2.6 times the diameter d p , more preferably less than 2.4 times the diameter d p  and/or porosity P p  comprised between 20% and 80%, preferably between 25% and 75%. 
     By way of example,  FIG.  3    schematically illustrates a unit 1a which comprises a nano-pore  30  structure characterized by dimensional parameters falling within the ranges indicated above. 
     The Applicant has in particular observed that, in some embodiments, there is an interdependence between the diameters d p  of the nano-pores  31  and lengths l p  of the nano-pores  31  such that in the case of diameters d p  of the nano-pores  31  greater than 70 nm (d p  &gt; 70 nm) a chromatic reflection and diffusion effect is established already for lengths l p  of the nano-pores  31  comprised between 300 nm and 40 µm (300 nm &lt; l p  &lt; 40 µm), allowing to shorten the production of the chromatic diffusion layer  20 . 
     The Applicant has also identified, in some embodiments, that in the presence of nano-pore  30  structures in which the length l p  of the nano-pores  31  is greater than a length threshold value l p_threshold  and in any case less than 200 µm, preferably less than 100 µm, the chromatic diffusion effect of the incident light is of a variable type. In particular, in the case where the first material is aluminium oxide and the second material is air, the length threshold value l p_threshold  is generally comprised between 800 nm and 5 µm, preferably between 1 µm and 4 µm, even more preferably it is equal to about 3 µm. Furthermore, in the case where the first material is aluminium oxide and the second material is characterized by a second refractive index  n   2  comprised between 1.4 and 1.6, the length threshold value is lp_threshold comprised between 6 µm and 12 µm, more preferably between 8 µm and 10 µm, even more preferably it is equal to about 9 µm. 
     The Applicant has also found, in some embodiments, that in the presence of nano-pore  30  structures in which the length l p  of the nano-pores  31  is greater than the length threshold value l p_threshold , the chromatic effect is of a variable type, once the diameter d p  of the nano-pores  31  exceeds a diameter threshold value d p_threshold . In particular, in the case where the first material is aluminium oxide and the second material is air, the diameter threshold value d p_threshold  is generally comprised between 50 nm and 120 nm, preferably between 60 nm and 100 nm, even more preferably it is equal to about 80 nm. 
     Furthermore, in the case where the first material is aluminium oxide and the second material is characterized by a second refractive index  n   2  comprised between 1.4 and 1.6, the diameter threshold value d p_threshold  is generally comprised between 150 nm and 220 nm, preferably between 160 nm and 200 nm, even more preferably it is equal to about 180 nm. 
     The Applicant has also found that as the ratio n M /n m  between the higher refractive index n M  and the lower refractive index n m  of the refractive indexes  n   1 , n   2  of the first and second materials decreases, the threshold values of length l p_threshold , and of diameter d p_threshold  increase. 
     For example,  FIGS.  4  and  5    schematically show units 1b and 1c which respectively comprise a nano-pore  30  structure characterized by dimensional parameters falling within the ranges that lead to a variable chromatic diffusion effect, wherein  FIG.  4    illustrates a unit 1b with a nano-pore  30  structure characterized by a surface density D p  of the nano-pores  31  greater than the surface density D p  of the nano-pores  31  of the nano-pore  30  structure of the unit 1c of  FIG.  5   . 
     As regards the measurement of the dependence of the regular reflectance on the wavelength, one can proceed as illustrated in  FIG.  15   a   . The unit  1 , 1   a - 1   g  is oriented such that its normal N (indicated by a dashed line in  FIG.  15   a   ) forms an angle δ with the incident ray R I  emitted by a light source S with white light, for example a source having the spectrum of the standard illuminator CIE D65, and where the spectrum of the light reflected at the specular angle is measured by a detector (a spectrophotometer) RIV. This spectrum is thus normalized with respect to the emission spectrum of the source S, acquired for example by positioning the detector RIV on the path of the beam R I  in the absence of the unit  1 , 1   a - 1   g . In this way the dependence of the spectral reflectance of the unit  1 , 1   a - 1   g  on the wavelength is obtained without having to take into account the spectral characteristics of the source. Finally, a colour point is associated with the spectral profile of regular reflectance in the chromaticity diagram 1976 u′-v′. This point corresponds to the chromatic coordinate that would be obtained by measuring the regular reflective component if the unit  1 , 1   a - 1   g  were illuminated by a light source having the spectral characteristics of a standard illuminator CIE E. The measurement can be repeated for different angles δ comprised between 10° and 90°. 
     To evaluate the colour point associated with a direction of observation far from the direction of specular reflection, one can proceed as illustrated in  FIG.  15   b   . The unit  1 , 1   a - 1   g  is oriented such that its normal N forms an angle δ with the incident ray R I  emitted by a light source S with white light; through a detector (a spectrophotometer) RIV placed at an angle β with respect to the incident ray R I  the spectrum of diffused light R D  is recorded, to which a colour point is associated in the chromaticity diagram 1976 u′-v′ after normalization of the revealed spectrum, analogously to what is described with reference to the measurement of regular reflectance. The angle β is chosen outside the light cone of the light reflected by the sample (e.g. β = 150° for a sample comprising a nano-pore structure with pore directrix perpendicular to the reflective surface). 
     More generally, the spectrum of the light diffused by the unit  1 , 1   a - 1   g  is detected by positioning the detector outside the beam of light regularly reflected by the unit  1 , a - 1   g , and a first set of measurements is collected by fixing the inclination of the sample with respect to the direction of the incident beam R I  and by detecting the spectrum of the diffused light at various angles β at which the detector is placed. In particular, the acquired measurements are used to identify the pair of angles (8, β) that determines the point of maximum distance from the white point (having coordinates (u′ B  = 0.210; v′ B  = 0.474) in the example of  FIG.  16   ). 
     On the basis of the colour points identified as described above, the nano-pore  30  structure of the unit  1 , 1   a - 1   g  is considered in accordance with one of the embodiments of the present invention if the following properties of the colour points derived by the spectral analysis of the beams that are regularly and diffusedly reflected by the unit  1 , 1   a - 1   g  considered are verified. In particular, it is verified whether for a standard observer CIE 1931 (2°) the spectrum of the regularly reflected beam corresponds to colour points on the chromaticity diagram CIE 1976 u′-v′ with chromaticity coordinates comprised in a region of acceptability of the corresponding colour point to the regular spectral reflectance R of the chromaticity diagram having coordinates u′ &gt; 0.210 and v′ &gt; 0.470 (illustrated in  FIG.  16   ). Furthermore, it is verified whether the chromatic coordinates of the colour points of the regularly reflected beam are at a maximum Euclidean distance of less than 0.1 from the defined colour points and the curve defined by the colour points associated with the emission spectrum of a black body, or Planckian locus P, preferably 0.05, even more preferably 0.03. The maximum Euclidean distance Δ R   max (u′,v′) between pairs of colour points of the regularly reflected beam among the plurality of colour points of the reflected beam regularly identified at different angles δ is determined. The maximum Euclidean distance Δ R   max (u′,v′) is compared with a threshold value Δ R   threshold (u′,v′), preferably Δ R   threshold (u′,v′) = 0.02, to discriminate between an invariable or static chromatic diffusion characteristic of the unit  1 , 1   a - 1   g , and a chromatically variable diffusion characteristic. In detail:
     a. if Δ R   max (u′,v′) ≥ Δ R   threshold (u′,v′) the unit  1 , 1   a - 1   g  is chromatically variable;   b. if Δ R   max (u′,v′) &lt; Δ R   threshold (u′,v′) the unit  1 , 1   a - 1   g  is chromatically invariable or static.   
 Furthermore, in the case where Δ R   max (u′,v′) ≥ Δ R   threshold (u′,v′), then the point of maximum blue (defined as the colour point of the diffused light located at maximum distance from the previously defined white point) in the chromaticity diagram CIE 1976 u′-v′ relative to the spectrum of diffused light is comprised in the portion of the plane having coordinates u′ &lt;0.220 and v′ &lt;0.480, indicated as the first region of acceptability D1 (illustrated in  FIG.  16   ). Otherwise, in the case where Δ R   max (u′,v′) &lt; Δ R   threshold (u′,v′), then the point of maximum blue is comprised in the portion of the plane having coordinates u′ &lt;0.210 and v′&lt;0.430, defined as the second region of acceptability D2 (illustrated in  FIG.  16   ). Furthermore, the minimum Euclidean distance Δ RD   min  in the chromaticity diagram CIE 1976 u′-v′ between the colour point of maximum blue associated with the diffused light spectrum and the colour point closest thereto among the colour points associated with the reflected light spectrum must be greater than or equal to 0.02, more preferably greater than or equal to 0.03, even more preferably greater than or equal to 0.04. Therefore, it is not possible to obtain a sample such that the colour point of maximum blue associated with the diffused light spectrum and the colour points associated with the reflected light spectrum are within the overlap region between the region of acceptability of the reflection R and the first region of acceptability D1.
     The Applicant has also identified, in some embodiments, that in the presence of nano-pore  30  structures in which the length l p  of the nano-pores  31  is greater than a second length threshold value l p_threshold_2  and in any case less than 200 µm, preferably less than 100 µm, the chromatic diffusion effect of the incident light is static or variable. 
     In particular, in the case where the first material is aluminium oxide and the second material is air, the second length threshold value l p_threshold_2  is generally comprised between 300 nm and 2 µm, preferably between 1 µm and 1.7 µm, even more preferably it is equal to about 1.4 µm. 
     Furthermore, in the case where the first material is aluminium oxide and the second material is characterized by a second refractive index  n   2  comprised between 1.4 and 1.6, the length threshold value is l p_threshold_2  is comprised between 4 µm and 8 µm, more preferably between 5 µm and 7 µm, even more preferably it is equal to about 6 µm. 
     The Applicant has also found, in some embodiments, that in the presence of nano-pore  30  structures in which the length l p  of the nano-pores  31  is greater than the second length threshold value l p_threshold_2 , the chromatic effect is static or variable, once the diameter d p  of the nano-pores  31  exceeds a second diameter threshold value d p_threshold_2 . In particular, in the case where the first material is aluminium oxide and the second material is air, the second diameter threshold value d p_threshold_2  is generally comprised between 40 nm and 100 nm, preferably between 60 nm and 80 nm, even more preferably it is equal to about 70 nm. 
     Furthermore, in the case where the first material is aluminium oxide and the second material is characterized by a second refractive index  n   2  comprised between 1.4 and 1.6, the second diameter threshold value d p_threshold_2  is generally comprised between 150 nm and 190 nm, preferably between 160 nm and 180 nm, even more preferably it is equal to about 170 nm. 
     The Applicant has also found that as the ratio n M /n m  between the higher refractive index n M  and the lower refractive index n m  of the refractive indexes  n   1 , n   2  of the first and second materials decreases, the second threshold values of length l p_threshold_2 , and of diameter d p_threshold_2  increase. 
     Below is a series of exemplary examples relating to various samples of nano-pore structures analysed. 
     Example 1 According to the Invention - Static Chromatic Diffusion Sample 
     Sample A with nano-pore structure obtained by anodizing an aluminium substrate (alloy 1050) in 0.1 M phosphoric acid at 20° C. at a potential of 80 V for an anodizing time equal to  60  minutes. The nano-pore structure has a length l p  of 1.5 µm, the pores have a diameter d p  of  85  nm and an inter-pore distance I p  of 185 nm thus equal to 2.2 times the diameter d p . Sample A therefore has a porosity of about 21%. The analysis of sample A allows determining the coordinates of the colour points shown in the following Table 1. 
     
       
         
          TABLE 1
           
               
               
               
               
             
               
                 salient colour points for sample A 
               
               
                   
                 u′ 
                 v′ 
                 Planckian distance 
               
             
            
               
                 10° 
                 0.2227 
                 0.5012 
                 0.0015 
               
               
                 50° 
                 0.2339 
                 0.5063 
                 0.0028 
               
               
                 Blue 50-150 
                 0.1772 
                 0.3849 
                 0.0066 
               
            
           
         
       
     
      The maximum Euclidean distance Δ R   max (u′,v′) is equal to 0.012 (Δ R   max (u′,v′) = 0.012), less than the threshold value Δ R   threshold (u′,v′). Consequently, the sample A considered is chromatically static. The point of best blue is within the second region of acceptability D2 and the minimum distance between the point of best blue and the points relative to the spectrum of regularly reflected light is Δ RD   min  = 0.125 (acceptable). The dichroic reflectance ratio r evaluated by tilting the sample by an inclination angle δ first equal to 10° and then equal to 50° according to the configuration shown in  FIG.  15   a   , shows an increase by about 50%. In other words, the sample A is representative of a unit according to the present invention characterized by a diffuse reflectance of the static type. 
     Example 2 According to the Invention - Sample With Variable Chromatic Diffusion 
     Sample B with nano-pore structure obtained by anodizing an aluminium substrate (alloy 1050) in 0.1 M phosphoric acid at 40° C. at a potential of 80 V for an anodizing time equal to 60 minutes. The nano-pore structure has a length l p  of 8.5 µm, the pores have a diameter d p  of 160 nm and an inter-pore distance I p  of 190 nm thus equal to 1.2 times the diameter d p . Sample B therefore has a porosity of about 50%. The analysis of sample B allows determining the coordinates of the colour points shown in the following Table 2. 
     
       
         
          TABLE 2
           
               
               
               
               
             
               
                 salient colour points for sample B 
               
               
                   
                 u′ 
                 v′ 
                 Planckian distance 
               
             
            
               
                 10° 
                 0.2436 
                 0.5212 
                 0.0035 
               
               
                 50° 
                 0.2932 
                 0.5391 
                 0.0024 
               
               
                 Blue 50-150 
                 0.1916 
                 0.449 
                 0.0015 
               
            
           
         
       
     
      The maximum Euclidean distance Δ R   max (u′,v′) is equal to 0.053 (Δ R   max (u′,v′) = 0.053), greater than the threshold value Δ R   threshold (u′,v′). The colour points of the reflection are in the area of acceptability R and the point of best blue is within the first region of acceptability D1 and the minimum distance between the point of best blue and the points relative to the spectrum of regularly reflected light is Δ RD   min  = 0.089 (acceptable). Consequently, the sample B considered is chromatically variable. In other words, the sample B is representative of a unit according to the present invention characterized by a regular/diffuse reflectance of variable type. 
     Example 3 According to the Invention - Static Chromatic Diffusion Sample 
     Sample B 2  with nano-pore structure obtained by anodizing an aluminium substrate (purity 99.99%) in 0.8 M phosphoric acid at 30° C. at a potential of 80 V for an anodizing time equal to 30 minutes. The nano-pore structure has a length l p  of 8.6 µm, the pores have a diameter d p  comprised between 140-160 nm and an inter-pore distance I p  comprised between 160-190 nm thus equal to 1.2 times the diameter d p . Sample B 2  therefore has a porosity of about 50%. The nano-pore structure of sample B 2  is completely immersed in a resin based on soluble fluoropolymers having a refractive index equal to n 2 =1.48. The analysis of sample B 2  allows determining the coordinates of the colour points shown in the following Table 2bis. 
     
       
         
          TABLE 2
           
               
               
               
               
             
               
                 salient colour points for sample B 2 
 
               
               
                   
                 u′ 
                 v′ 
                 Planckian distance 
               
             
            
               
                 10° 
                 0.2296 
                 0.5074 
                 0.0091 
               
               
                 50° 
                 0.2367 
                 0.5172 
                 0.0135 
               
               
                 Blue 50-150 
                 0.1814 
                 0.4082 
                 0.0023 
               
            
           
         
       
     
      The maximum Euclidean distance Δ R   max (u′,v′) is equal to 0.0092 (Δ R   max (u′,v′) = 0.0092), less than the threshold value Δ R   threshold (u′,v′). Consequently, the sample A considered is chromatically static. The point of best blue is within the second region of acceptability D2 and the minimum distance between the point of best blue and the points relative to the spectrum of regularly reflected light is Δ RD   min  = 0.1165 (acceptable). The dichroic reflectance ratio r evaluated by tilting the sample by an inclination angle δ first equal to 10° and then equal to 50° according to the configuration shown in  FIG.  15   a   , shows an increase by about 29%. In other words, the sample B 2  is representative of a unit according to the present invention characterized by a diffuse reflectance of the static type. It is highlighted how the chromatically static sample B 2  has a geometry substantially comparable to that of the chromatically variable sample B according to example 2. The different behaviour of the two samples is therefore attributable to the different (lower) ratio between the first refractive index  n   1  of the nano-pore structure (higher refractive index n M ) and the second refractive index  n   2  of the material that fills the nano-pore structure (lower refractive index n m ), with the consequent raising of the threshold values of diameter d p_   threshold  and of length l p _ threshold  beyond which the sample is characterized by a diffuse reflectance of the variable type. 
     Comparative Example 1 - Sample With Nano-pores With Insufficient Diameter 
     Sample C with nano-pore structure in an aluminium oxide layer grown on aluminium. The nano-pore structure has a length l p  of 30 µm, the pores have a diameter d p  of 25 nm and an inter-pore distance I p  of 65 nm thus equal to 2.6 times the diameter d p . Sample C therefore has a porosity of about 14%. The analysis of sample C allows determining the coordinates of the colour points shown in the following Table 3 (where the value(s) marked with an asterisk symbol identify an unacceptable parameter). 
     
       
         
          TABLE 3
           
               
               
               
               
             
               
                 salient colour points for sample C 
               
               
                   
                 u′ 
                 v′ 
                 Planckian distance 
               
             
            
               
                 10° 
                 0.2125 
                 0.4781 
                 0.0087 
               
               
                 50° 
                 0.2142 
                 0.4791 
                 0.0092 
               
               
                 Blue 50-150 
                 0.1965 
                 0.4344 (*) 
                 0.011 
               
            
           
         
       
     
      The maximum Euclidean distance Δ R   max (u′,v′) is equal to 0.002 (Δ R   max (u′,v′) = 0.002), less than the threshold value Δ R   threshold (u′,v′). Consequently, the sample C considered is not characterized by chromatic variability. Furthermore, the point of best blue is outside the second region of acceptability D2. In other words, sample C is not representative of a unit according to the present invention, since the diameter d p  of the nano-pores of the nano-pore structure does not allow to obtain the desired diffuse reflectance characteristics. 
     The comparison of the samples A and B of the examples 1 and 2 according to the invention with the sample C described in the comparative example 1, shows how the variation of the diameter d p  of the nano-pores (therefore also of the porosity P p  of the structure) allows controlling the chromatic characteristics of the unit. 
     Comparative Example 2 - Inadequate Pore Density and Porosity 
     Sample D with nano-pore structure in an aluminium oxide layer grown on aluminium. The nano-pore structure of sample D has the following characteristics: pore diameter d p  40 nm, length l p  30 µm and inter-pore distance I p  of 125 nm, thus equal to 3.1 times the diameter d p . Sample D therefore has a porosity of about 10%. The analysis of sample D allows determining the coordinates of the colour points shown in the following Table 4 (where the value(s) marked with an asterisk symbol identify an unacceptable parameter). 
     
       
         
          TABLE 4
           
               
               
               
               
             
               
                 salient colour points for sample D 
               
               
                 125-40-30 
                 u′ 
                 v′ 
                 Planckian distance 
               
             
            
               
                 10° 
                 0.219 
                 0.493 
                 0.0035 
               
               
                 50° 
                 0.215 
                 0.484 
                 0.0059 
               
               
                 Blue 50-150 
                 0.195 
                 0.435 (*) 
                 0.0044 
               
            
           
         
       
     
      The maximum Euclidean distance A R   max (u′ ,v′) is equal to 0.009 (Δ R   max (u′,v′) = 0.009), less than the threshold value Δ R t hreshold (u′,v′); consequently sample D is chromatically static. Furthermore, the point of maximum blue for sample D is outside the second region of acceptability D2. Therefore, sample D does not represent a unit according to the present invention since the density of the nano-pores of the nano-pore structure is higher than a maximum density which allows obtaining the desired diffuse reflectance characteristics. 
     Comparative Example 3 - Insufficient Nano-Pore Length 
     Sample E with nano-pore structure obtained by anodizing an aluminium substrate (alloy 1050) in 0.1 M phosphoric acid at room temperature at a potential of 80 V for an anodizing time of 60 seconds. The nano-pore structure of sample E has the following characteristics: pore diameter d p  80 nm, length l p  150 nm and inter-pore distance I p  185 nm, thus equal to 2.3 times the diameter d p . Sample E therefore has a porosity of about 18%. The analysis of sample E allows determining the coordinates of the colour points shown in the following Table 5 (where the value(s) marked with an asterisk symbol identify an unacceptable parameter). 
     
       
         
          TABLE 5
           
               
               
               
               
             
               
                 salient colour points for sample E 
               
               
                   
                 u′ 
                 V′ 
                 Planckian distance 
               
             
            
               
                 10° 
                 0.206 (*) 
                 0.469 (*) 
                 0.0072 
               
               
                 50° 
                 0.207 (*) 
                 0.469 (*) 
                 0.0079 
               
               
                 Blue 50-150 
                 0.223 (*) 
                 0.476 (*) 
                 0.0154 
               
            
           
         
       
     
      The maximum Euclidean distance Δ R   max (u′,v′) is equal to 0.001 (Δ R   max (u′,v′) = 0.001), less than the threshold value Δ R   threshold (u′,v′). Consequently, the sample E considered is chromatically static. Furthermore, the point of best blue is outside the second region of acceptability D2. In other words, sample E does not represent a unit according to the present invention, since the length of the nano-pores of the nano-pore structure is less than a minimum length which allows obtaining the desired diffuse reflectance characteristics. 
     By comparing the samples A and B of the examples according to the invention with the samples D and E of the comparative examples, it is clear that the variation of the length l p  of the nano-pores, of the density D p  of the nano-pores  31  (therefore also of the porosity P p ) of the structure allows controlling the chromatic properties of the unit  1 , 1   a - 1   g  according to the present invention. 
     The Applicant has also found that by varying two or more of these parameters and the diameter d p  of the nano-pores, a synergistic effect is obtained which determines the variation of the correlated colour temperature of a beam of light reflected by the unit  1 , 1   a - 1   g  as the angle of incidence of the light beam incident on it varies. Consequently, it is possible to determine different combinations of values of the dimension of the diameter d p , of the length l p  and of the density D p  of the nano-pores  31 , as well as of P p  of the structure in order to obtain the same desired chromatic effect, in terms of correlated colour temperature of regularly reflected and diffused light. 
     Furthermore, the Applicant has determined that, by selecting different materials in which to immerse the nano-pore  30  structure, it is possible to obtain a ratio between the refractive indexes  n   2  and  n   1  (comprised between 1.05 and 3) suitable for influencing the diffuse reflectance and the regular reflectance of the chromatic diffusion layer  20  and, therefore, the correlated colour temperature of a beam of light regularly reflected by the unit  1 , 1   a - 1   g . 
     Nano-Structure Growth Process 
     The Applicant has identified a growth process  100 , schematically illustrated in  FIG.  18   , which allows controlling the parameters of the nano-structure  30  included in the chromatic diffusion layer  20  in a particularly effective way. 
     Initially, a substrate is selected on which to grow the chromatic diffusion layer (block  101 ). In the example considered, an aluminium alloy plate  1050  is selected as the growth substrate for the chromatic diffusion layer. Advantageously, although not limiting, this substrate can be used as a reflective layer  10  of the unit  1 , 1   a - 1   g . 
     The substrate is then subjected to brightening or polishing, for example electropolishing, in order to eliminate a layer of native aluminium oxide that covers the substrate and, possibly, reduce a surface roughness of the substrate (block  103 ). For example, electropolishing is performed by immersing the substrate in a mixture of ethanol (CH 3 CH 2 OH) and perchloric acid (HC1O 4 ) in a 4:1 ratio and then by applying an electrical potential difference ΔV p  comprised between 5 V and 30 V between the growth substrate and a cathode made of graphite or aluminium for a time interval Δt p  comprised between 1 and 60 minutes. 
     In one embodiment of the present invention, electropolishing is performed so that the surface of the growth substrate is substantially reflective - i.e. a ‘mirror’ polishing is obtained -eliminating the texture inherited from the production processes and the growth substrate can be used as the reflective layer  10  of the unit  1 , 1   a - 1   g . 
     After electropolishing, the substrate is subjected to anodization (block  105 ). For example, the substrate is immersed in an electrolyte substantially consisting of a solution of phosphoric acid with molarity 0.1 M, and a voltage is applied by applying an electric potential difference ΔV a  comprised between 70 V and 110 V, preferably comprised between 80 V and 100 V, between the growth substrate and a cathode made of graphite or aluminium for a time interval Δt a  comprised between 30 minutes and 120 minutes, preferably 60 minutes. Furthermore, during the anodization a temperature T a  comprised between -10° C. and 50° C., preferably, selected between 20° C. and 40° C., is maintained. 
     The Applicant has identified that it is possible to control an average diameter of the nano-pores  31  by adjusting the values of electric potential ΔV a  and temperature T a . In particular, as the values of electric potential ΔV a  and temperature T a  increase, it is possible to increase an average pore diameter while maintaining the anodization time interval Δt a  constant as indicated in the Table 6 shown below: 
     
       
         
          TABLE 6
           
               
               
               
             
               
                 Electric potentialΔV a  (V) 
                 Temperature T a  (°C) 
                 Average diameter of the nano-pores (nm) 
               
             
            
               
                 80 
                 20 
                 76-86 
               
               
                 80 
                 30 
                 95-105 
               
               
                 80 
                 40 
                 153-163 
               
               
                 90 
                 20 
                 80-90 
               
               
                 90 
                 30 
                 95-125 
               
               
                 90 
                 40 
                 175-185 
               
            
           
         
       
     
     Furthermore, the Applicant has observed that it is possible to control the thickness of the chromatic diffusion layer  20  for the same anodization time interval Δt a  by adjusting the temperature T a ; in particular, the thickness of the chromatic diffusion layer  20  increases as the temperature T a  increases, maintaining the anodization time interval Δt a  constant. 
     Last but not least, the Applicant has identified that it is possible to control the inter-pore distance Ip through a preventive patterning step of the substrate on which to grow the chromatic diffusion layer. This preventive step provides a growth imprint for the pore position of the nano-pore  30  structure. By controlling the diameter d p  and the inter-pore distance Ip it is also possible to set the porosity P p  of the structure  30 . 
     At the end of the anodization, on the substrate there is a chromatic diffusion layer  20  comprising a nano-pore  30  (or a nano-pillar  70 ) structure with the desired characteristics. Subsequently, the substrate with the chromatic diffusion layer  20  is washed and dried - for example, in a convection oven - in order to remove any foreign bodies present in the nano-pores  31  of the nano-pore  30  structure (block  107 ). 
     Optionally, the nano-pore  30  structure (or the nano-pillar  70  structure) is immersed in a resin. To this end, a resin layer is initially applied to the substrate with the nanostructure  30 , 70 . The nanostructured substrate with the applied resin can be treated in an environment where vacuum is created; in this way it is guaranteed that the resin penetrates inside the structure  30 , 70  before solidifying. The nanostructured substrate with the applied resin is then treated according to the appropriate polymerization procedures provided for the specific resin. 
     Optionally, the chromatic diffusion layer  20  is separated from the substrate (block  109 ) to be coupled with a desired reflective layer  10  (block  111 ). 
     Coating Element 
     According to embodiments of the present invention, illustrated in  FIGS.  19  and  20   , the unit  1 , 1   a - 1   g  described is used for the production of a surface coating element, referred to below as element 2 for the sake of brevity. In particular, the element 2 is suitable for coating building surfaces, for example the external facades of buildings. 
     The element 2 comprises a support structure  40 , one or more coupling means  50  and at least one unit  1 , 1   a - 1   g .In particular, the support structure  40  is configured to mechanically support the unit  1 , 1   a - 1   g , so that the second surface  23  of the chromatic diffusion layer  20  faces the external environment, when the support is mounted on a surface to be coated. 
     In the example of  FIG.  19   , the support structure  40  is configured to surround the perimeter of the unit  1 , 1   a - 1   g  by means of a frame 1. However, in other embodiments (not shown) the support structure  40  may be frameless or comprise a partial frame - for example, with frame edges configured to approach opposite or adjacent sides of the unit  1 , 1   a - 1   g . 
     As illustrated in the example of  FIG.  20   , the support structure  40  can be integrated or made as a single piece with the reflective layer  10 . In this way, the element 2 is particularly compact and more economical to make. Furthermore, the absence of a frame allows several elements 2 to be approached so as to create a chromatic effect light reflective surface of desired width and substantially without interruption. 
     The coupling means  50  develop from the support structure  40  in the opposite direction with respect to the unit  1 , 1   a - 1   g  and are configured to allow a mechanical coupling of the support structure  40  to the surface to be coated. For example, the coupling means  50  comprise one or more brackets provided with holes for receiving fixing elements such as screws and/or bolts which are fixed to the surface to be coated or to a bearing structure - such as a structure comprising one or more uprights - arranged in proximity or in contact with the surface to be coated. In addition or alternatively, the coupling means  50  comprise fixing elements by interlocking or by interference suitable for coupling mechanically to corresponding fixing elements provided on the surface to be coated or on the afore-mentioned bearing structure. As will be evident to the skilled person, the coupling means  50  can be made in a single piece with the support structure  40  or can be coupled thereto at a later time, in a removable or non-removable way. 
     The invention thus conceived is susceptible to several modifications and variations, all falling within the scope of the inventive concept. For example, in an alternative embodiment -illustrated in  FIG.  21    - the unit  1   d  comprises an intermediate layer  60  interposed between the reflective layer  10  and the chromatic diffusion layer  20 . In particular, the intermediate layer  60  is at least partially non-absorbent or transparent to electromagnetic radiations with wavelength included in the visible spectrum - for example, the intermediate layer  60  can be made of a material such as silicon oxide, borosilicate glass, etc. 
     In embodiments of the present invention - of which an example is shown in  FIG.  22    - the chromatic diffusion substrate  20  of the unit 1e comprises a nano-pillar  70  structure instead of the nano-pore  30  structure described above. In this case, the nano-pillar  70  structure has characteristics similar to the characteristics of the nano-pore  30  structure described above. In particular, the nano-pillars  71  are characterized by length l p ′, diameter d p ′, directional order parameter S′, surface density D p ′, porosity P p ′ and periodicity substantially corresponding to what is indicated above for nano-pores  31 . 
     Similarly to what has been described above, the nano-pillar  70  structure can be immersed in a material selected to control the ratio between the refractive index  n   2  of the material surrounding the nano-pillars  71  - for example, air - and the refractive index  n   1  of the material of the nano-pillar  70  structure - for example, aluminium oxide. 
     The Applicant has found that for the nano-pillar structures  70  it is possible to observe relations similar to those described with reference to the nano-pore  30  structures which link the single geometric parameters to the chromatic effects of the static type and of the variable type described above. 
     In an alternative embodiment illustrated in  FIG.  23   , the unit 1f additionally comprises a coating layer  90 , placed at the second surface  23  of the chromatic diffusion layer  20 . In particular, the coating layer  90  is at least partially non-absorbent or transparent to electromagnetic radiations with wavelength included in the visible spectrum - for example, the coating layer  90  can be made of a material such as silicon oxide, borosilicate glass, etc. 
     In case the coating layer  90  fills at least partially the nano-pore  30  structure or the nano-pillar  70  structure is at least partially immersed in the coating layer  90 , this layer  90  is preferably made with a polymer, a resin, a silicone, a different oxide (for example deposited by sol-gel) transparent or substantially non-absorbent at least to electromagnetic radiations with wavelength included in the visible light spectrum, with a third refractive index  n   3  comprised between 1.3 and 1.55, preferably between 1.41 and 1.52, for example polyvinyl chloride (PVC), polymethyl methacrylate (PMMA), polyfluorides (such as, PVDF) or transparent polyacrylates. In particular, the coating layer  90  is made with a resin based on soluble fluoropolymers, in particular a polyurethane resin with a high fluorocarbon content, for example the resin known on the market under the trade name Lumiflon®. In particular, the fluoropolymer-based resin is selected with a refractive index  n   2  comprised between 1.45 and 1.50, more preferably equal to 1.48. Also with reference to the coating layer  90 , the ratio (n 1 /n3) between the first ( n   1 ) and the third ( n   3 ) refractive index is comprised between 1.05 and 3. 
     In an alternative embodiment illustrated in  FIG.  24   , the reflective layer  10  of the unit 1 g comprises a rear surface  12  opposite its reflective surface  11  to which a stiffening composite layer  120  is coupled. The stiffening composite layer  120  comprises a shimming panel  121  and a coating sheet  122 . In particular, the shimming panel  121  has a specific weight at least 5 times less than the specific weight of the coating sheet  122 , preferably at least  10  times less than the specific weight of the coating sheet  122 . Furthermore, the shimming panel  121  has a thickness at least 2 times higher than the thickness of the coating sheet  122 , preferably at least 5 times higher than the thickness of the coating sheet  122 . In particular, the coating sheet  122  is made of aluminium and has a thickness comprised between 0.2 mm to 1 mm, preferably equal to about 0.5 mm. 
     In preferred embodiments, the shimming panel  121  is made of a non-combustible material, such as fiberglass, expanded glass granulate, rock fibre, cellular glass, ceramic fibre, carbon fibre, vermiculite (expanded or not), expanded clay or perlite (expanded or not). 
     In alternative embodiments, the shimming panel  121  is made in the form of a grating, such as for example a honeycomb grating with axis of the cells that is orthogonal to the reflective layer, or has a wavy profile according to a section orthogonal to the reflective layer. 
     Furthermore, it is possible to realize a nano-pore  30  structure with nano-pores configured such to delimit a portion of the structure  80  which inscribes a circumference with diameter comprised between 30 nm and 300 nm, as schematically illustrated in  FIG.  25   . In this case, the nano-pore structure has a behaviour corresponding to a nano-pillar structure of corresponding diameter. In a dual way, it is possible to realize a nano-pillar structure with nano-pillars configured in such a way as to delimit an interspace that inscribes a circumference with diameter comprised between 30 nm and 300 nm. In this case, the nano-pore structure has a behaviour corresponding to a nano-pore structure of corresponding diameter. 
     In alternative embodiments (not shown), the chromatic effect light reflective unit may comprise a nano-pore or nano-pillar structure having a distribution other than the hexagonal distribution, such as for example a square, rectangular, octagonal distribution and so on. 
     In alternative embodiments (not shown), the chromatic effect light reflective unit can define a curved surface and/or define edges, convexity and/or concavity. Correspondingly, the coating element which comprises such a chromatic effect light unit has corresponding curved surfaces and/or edges, convexity and/or concavity. This coating element is therefore suitable for coating non-flat surfaces, corner elements - for example of a building - and, more generally, it can be used to define non-flat surfaces in order to obtain a desired aesthetic effect. 
     Furthermore, there is nothing to prevent the provision of coupling elements at perimeter edges of the coating unit so as to allow a mechanical coupling among a plurality of coating elements. 
     Finally, the materials used, as well as the contingent shapes and sizes, can be whatever according to the requirements without for this reason departing from the scope of protection of the following claims. 
     In particular, alternative embodiments of the chromatic effect light reflective unit provide a chromatic diffusion layer in a material other than aluminium oxide, preferably non-absorbent or transparent to electromagnetic radiations with wavelength included in the visible spectrum in a similar way to aluminium oxide. 
     In fact, other types of metal oxides can be used to make the chromatic diffusion layer. For example, in alternative embodiments of the present invention, the nano-pore or nano-pillar structure of the layer is made of titanium oxide, or titania (TiO2), preferably anodic titanium oxide (acronym ATO). Alternatively, the nano-pore structure or, more preferably, the nano-pillar structure can be made of zinc oxide (ZnO). 
     Furthermore, there is nothing to prevent the definition of the diameter d p  of each nano-pore  31  or nano-pillar  71  as an average value of the diameters of the circumferences that inscribe the nano-pore  31  or nano-pillar  71 , calculated at a plurality of predefined distances from the first surface  21  of the chromatic diffusion layer  20  along the development direction of the nano-pore  31  or nano-pillar  71  considered. 
     Furthermore, a three-dimensional order parameter can be calculated to characterize the main development direction of the nano-pores  31  or nano-pillars  71 . 
     In an alternative embodiment illustrated in  FIG.  26   , the chromatic effect reflective unit  1 , 1   a - 1   g  is used in an illumination system  200  comprising an illuminator  210  to illuminate the reflective unit  1 , 1   a - 1   g . The illuminator  210  is, for example, a light source of white light. In a particular embodiment, the white light source has CCT&gt; 5000 K, preferably CCT&gt;5500 K, more preferably CCT&gt; 6000 K. The illuminator  210  of  FIG.  26    emits or projects light on the unit  1 , 1   a - 1   g .In  FIG.  26    there is shown by way of example a light cone  211  of the light emitted by the illuminator  210  which completely covers and substantially corresponds to the extension of the unit  1 , 1   a - 1   g . In the embodiment of  FIG.  26   , the chromatic effect reflective unit  1 , 1   a - 1   g  has a substantially planar surface, specifically the chromatic effect reflective unit  1 , 1   a - 1   g  has a surface conforming to a rectangle having a first side greater than a second side, for example a first side 2 times, preferably 3 times, more preferably 4 times greater than a second side. 
     In a particular embodiment (not shown), the illuminator  210  is a linear illuminator comprising a plurality of LED sources and a plurality of collimators, each collimator of the plurality of collimators being coupled to each LED source of the plurality of LED sources, such collimator being arranged along a direction parallel to the first side of the rectangle-shaped unit  1 , 1   a - 1   g . 
     In an alternative embodiment illustrated in  FIG.  27   , a different illumination system  300  is shown which differs from the illumination system of  FIG.  26    in that it comprises a reflective unit  1 , 1   a - 1   g  which has substantially conformation of a parabolic cylindrical reflector, hereinafter also “parabolic cylindrical unit”, and a linear illuminator  310  arranged along a direction parallel to a focal axis of the parabolic cylindrical unit  1 , 1   a - 1   g . In a particular configuration, the linear illuminator  310  is arranged in a position proximal to the focal axis of the parabolic cylindrical unit  1 , 1   a - 1   g .In a different configuration, the linear illuminator  310  is arranged in a position such that the light produced by the illuminator  310  illuminates the parabolic cylindrical unit  1 , 1   a - 1   g  as if the rays produced by the illuminator  310  emerged from a region of the space proximal to the focal axis of the parabolic cylindrical unit  1 , 1   a - 1   g . The illuminator  310  in turn comprises a plurality of light sources  303 , for example LED sources. In a particular embodiment, the LED sources emit white light having CCT&gt;5000 K, preferably CCT&gt;5500 K, more preferably CCT &gt;6000 K. The illuminator  310  of  FIG.  27   a    also comprises, optionally, a cylindrical collimator  304 , for example an extruded cylindrical lens, capable of collimating the light produced by the light sources  303  in the plane orthogonal to the focal axis of the parabolic cylindrical unit  1 , 1   a - 1   g , giving it a first angular luminance profile (a) such that the parabolic cylindrical unit  1 , 1   a - 1   g  is substantially all illuminated, for example it is uniformly illuminated, and/or (b) having a peak with a first width at half maximum (HWHM) defining a semi-divergence  305  of the first angular luminance profile or a first semi-divergence  305 . 
     The linear illuminator  310  further preferably comprises a plurality of source collimators  306 , wherein each source collimator  306  is coupled to a light source  303 . The source collimators  306  are for example of the radially symmetrical type, or astigmatic or cylindrical and are positioned and configured to give each light source  303  of the plurality of light sources a second angular luminance profile in a plane  307  containing the focal axis of the parabolic cylindrical unit  1 , 1   a - 1   g  and passing through the centre line axis that divides the parabolic cylindrical unit  1 , 1   a - 1   g  into two parabolic cylindrical sectors of substantially equal area. The second angular luminance profile is characterized by a maximum value for a peak direction  308  substantially common to all light sources  303  and having a peak with a second width at half maximum (HWHM) defining a semi-divergence  309  of the second angular luminance profile or second half-divergence  309 . In a particular embodiment of the present invention, the second half-divergence is significantly less than the first half-divergence, for example 3 times, preferably 6 times, more preferably 10 times lower, or the second half-divergence is equal for example to no more than 15°, preferably no more than 10°, more preferably no more than 5°. 
     In a different embodiment of the present invention, the source collimators  306  are configured to produce a second half-divergence  309  substantially equal to the value of the half-divergence associated with the angular luminance profile of the beam reflected by the parabolic cylindrical unit  1 , 1   a - 1   g  and measured in the plane orthogonal to the focal axis, where this divergence depends, among other things, on the dimensions of the linear illuminator  310  and on its distance from the focal axis. 
     In a further configuration of the present invention, the illumination system  300  comprises a redirection system (not shown) of the light produced by the linear illuminator, for example an electro-mechanical type device, capable of acting on the second angular luminance profile and of modifying a peak direction  308  in the plane  307 , for example of modifying it in a neighbourhood of the direction perpendicular to the focal axis. In a particular embodiment, the redirection system operates by translating along the direction of the focal axis the position of the centres of the LED sources  303  with respect to those of the centres of the source collimators  306 . 
     In a different configuration (not shown), the linear illuminator  310  comprises a plurality of mini-reflectors, each mini-reflector being coupled to each light source  303  of the illuminator  310 , where the mini-reflectors are for example rotated along an axis perpendicular to a plane containing the focal axis and the peak direction  308 . 
     The light reflected by the parabolic cylindrical unit  1 , 1   a - 1   g  has a third angular luminance profile in a plane perpendicular to the focal axis, this profile being substantially independent, net of edge effects, of the position along the direction of the focal axis. The third angular luminance profile has a peak which identifies a direction  301  in a plane orthogonal to the focal axis. The light reflected by the parabolic cylindrical unit  1 , 1   a - 1   g  finally has a fourth angular luminance profile in a plane parallel to the focal axis that contains the direction  309 , where the fourth angular luminance profile has a peak that defines a direction  302  in that plane. As the peak direction  308  of the second angular profile varies as a result of the redirection system, the peak direction  302  of the fourth angular luminance profile varies correspondingly. In fact, the peak directions of the second  308  and the fourth  302  angular luminance profile are necessarily characterized by having the same projection in the plane orthogonal to the direction  301 . 
     Assuming that the observer is positioned with the right-left axis directed parallel to the focal axis, and having the cardinal direction of WEST to his right, the image of the sun will run through the skylight passing from EAST, to SOUTH up to WEST as the direction  308  varies. Thanks to the fact that the luminance profile of the reflected light, and in particular the third and fourth luminance profile, do not depend on the position of the observer, since these profiles are spatially uniform, he perceives the image of the sun at an infinite distance. In fact, if the observer walks along the direction of the focal axis, the image of the sun follows him precisely. 
     If the parabolic cylindrical unit  1 , 1   a - 1   g  is a chromatic diffusion unit of a variable type, the variation in the position of the sun as perceived on the horizon will also be combined with a variation in the colour of the light of the sun itself, this colour being colder when the direction of the light emitted by the linear illuminator  310  forms the minimum angle with respect to the normal to the surface of the parabolic cylindrical unit  1 , 1   a - 1   g , i.e. when the light reflected by the parabolic cylindrical unit  1 , 1   a - 1   g  has direction perpendicular to the focal axis, and therefore the observer perceives the sun at the maximum height above the horizon, and specifically in the SOUTH direction, and being warmer when the direction of the light emitted by the linear illuminator  310  forms the maximum angle with respect to the normal to the surface of the parabolic cylindrical unit  1 , 1   a - 1   g , i.e. when the light reflected by the parabolic cylindrical unit  1 , 1   a - 1   g  deviates maximally from the normal to the focal axis, and therefore the observer perceives the sun at the maximum minimum height above the horizon, and specifically in the EAST direction or in the WEST direction. 
     As illustrated, the illumination system  300  is capable of producing a variation in the colour of the sunlight with the height of the sun above the horizon similar to that produced in nature. In nature, the effect is due to the variation in the length of the optical path of the sun rays in the atmosphere associated with the variation of the angle of incidence of the rays on the layer of air that constitutes the sky. In the case of the present invention, it is likewise due to the variation of the angle of incidence of the light produced by the linear illuminator  310  with respect to the direction along which the nano pores or nano pillars are oriented inside the parabolic cylindrical unit  1 , 1   a - 1   g . 
     In an alternative embodiment, illustrated in  FIGS.  28  and  28   a   , the chromatic effect light reflective unit  1 , 1   a - 1   g  is used in an illumination system  400  comprising an illuminator  410  to illuminate the reflective unit  1 , 1   a - 1   g , such as a light source of white light. In a particular embodiment, the white light source has CCT&gt; 5000 K, preferably CCT&gt;5500 K, more preferably CCT&gt; 6000 K. The illuminator  410  of  FIG.  28    comprises an emissive surface  412  from which it emits or projects light  411  onto the unit  1 , 1   a - 1   g , preferably substantially completely covering the extension of the unit  1 , 1   a - 1   g .In particular, as illustrated in the schematic detail of  FIG.  28   a   , the reflective layer  10  and the chromatic diffusion layer  20  of the light reflective unit  1 , 1   a - 1   g  define a substantially convex surface facing towards the light source  410 . 
     According to an alternative variant, illustrated in  FIG.  29   , the reflective layer  10  and the chromatic diffusion layer  20  of the light reflective unit  1 , 1   a - 1   g  define a surface positioned and configured so as to comprise at least two non-coplanar illuminated portions and mutually oriented such that the projection of the normals  413  in the centres of the two portions on a plane passing through the centres of the two portions and through a point belonging to an emissive surface of the illuminator  412  defines two mutually diverging directions. 
     In an alternative embodiment illustrated in  FIG.  30   , the illumination system  700  comprises a support grid  701  configured such as to define a support plane  710  for a plurality of light sources  702 . In the illustrated embodiment, the grid  701  has a square pitch, but in a completely equivalent way it is possible to make a grid with a hexagonal pitch or other regular pitch. 
     The light sources  702  are arranged on the support plane  710  defined by the support grid  701  in a manner substantially equidistant from each other at a source distance ds. In the illustrated embodiment, the light sources  702  are arranged on the vertices of the grid  701 . 
     The chromatic effect light reflective unit  1 ;  1   a - 1   g , which can be made as a single panel or as a plurality of side-by-side and co-planar panels, is arranged co-planar to a reflection plane  802 , for example parallel to the resting surface  710 . The light sources of the plurality of light sources  702  are positioned and configured to substantially uniformly illuminate the at least one chromatic effect light reflective unit  1 ;  1   a - 1   g . Further, each light source  702  of the plurality of light sources is arranged and configured to generate a beam of light  704  with an angular source luminance profile having a peak along a main direction  705  and an angular half width at half maximum of the peak θs _ HWHM , where the main direction  705  and the angular half width of source  θS_HWHM  are common to all the light sources of the plurality of light sources  702 , and the main direction  705  is inclined with respect to the normal to the reflection plane  802  by an angle comprised between 0° and 80°, preferably between 0° and 70°, more preferably between 0° and 60°. Furthermore, the minimum distance Dmin between each light source  702  and the chromatic effect light reflective unit  1 ;  1   a - 1   g  measured along the main direction  705  satisfies the relationship: Dmin &gt; 0.5 ds tan(θs_H WHM ), preferably Dmin &gt; ds tan(θs_H WHM ), more preferably Dmin &gt; 2 ds tan(θs_ HWHM ). 
     In a variant of the invention the chromatic effect light reflective unit  1 ;  1   a - 1   g  is configured to produce a reflected light having an angular luminance profile characterized by a peak in a neighbourhood of the specular reflection direction with angular half width at half maximum  θRF_HWHM  when illuminated by a substantially unidirectional light, for example with HWHM divergence less than 0.5°, and a monochromatic light, for example with HWHM spectral width less than 2 nm, and with wavelength of about 632 nm incident at an angle of 15° with respect to the normal on a surface of the same  1 ;  1   a - 1   g . In particular, the angular peak half width of the light reflected θ RF_HWHM  by the chromatic effect light reflective unit  1 ;  1   a - 1   g  satisfies the following relationship with respect to the angular peak half width of the beam of light  704  generated by each light source  702 : θ RF_HWHM  &gt; θ S_HWHM , preferably θ RF_HWHM  &gt; 2 θ S_HWHM , more preferably θ RF_HWHM  &gt; 3 θ S_HWHM . 
     Advantageously, the Applicant has noted that the chromatic effect light reflective unit  1 ;  1   a - 1   g  developed by him, without prejudice to the capability of originating a chromatic diffusion process similar to the Rayleigh scattering process, otherwise maintains the optical properties of the substrate, and in particular the optical properties of the substrate to reflect the component of the incident light complementary to that diffused by the nano-structure in accordance with the surface characteristics of the substrate itself. Specifically, if the substrate used is of the glossy type, that is, it is able, in the case of a flat surface, to reflect an image without distorting it, even the anodized sample will maintain the same capability of reflecting the images, the difference being in that the images reflected, if brighter than the rest of the scene, will have a different colour here from the original images, since the reflection is deprived of the diffuse component at small wavelengths. Conversely, if the substrate has a surface roughness capable, for example, of giving it the ability to blur a reflected image in a controlled manner, or rather to operate, as far as the reflection is concerned, as a low-angle white light diffuser or “ frosted” diffuser, then also the anodized sample will maintain the same property, where the blurred reflected image will also have a different colour from the original image, since the reflection is deprived of the diffuse component at small wavelengths. The ability of a flat reflective surface to blur an image can be quantified by referring to the de-focusing angle, that is the angle that subtends the image of the reflection of a punctiform object. In the absence of any surface roughness, this angle is in fact substantially equal to zero, and the surface is of the glossy type. As the roughness increases, the de-focusing angle increases up to the limit of giving the surface the characteristic of substantially isotropic diffusion of the incident light. Operationally, in the context of the present invention, the de-focusing angle of a reflective surface is conventionally defined by the half width at half maximum or HWHM of the angular distribution of reflected light, in the case of incident light at 15° with respect to the normal to the surface, with a substantially unidirectional incident light, i.e. having half width at half maximum or HWHM of the angular distribution less than 1°, and wavelength of about 632 nm, i.e. the light produced by a HeNe laser. The choice of using a red light for the definition of the de-focusing angle is herein motivated by the need to minimize the effect of the diffusion produced by the nanostructure, which acts mainly at wavelengths in the opposite region of the visible spectrum, i.e. in the region of blue. As defined herein, the de-focusing angle coincides with the half width at half maximum or HWHM of the angular luminance profile θ RF-HWHM  of the reflected light when the chromatic effect light reflective unit  1 ;  1   a - 1   g  is illuminated by a substantially unidirectional and monochromatic light with wavelength of about 632 nm incident at 15° with respect to the normal to a surface thereof  1 ; 1   a - 1   g . 
     Specifically, what the Applicant has observed is that the chromatic effect light reflective unit  1 ;  1   a - 1   g  developed by him substantially does not alter, or alters in a very small way, the value of the de-focusing angle of the anodized sample with respect to that of the substrate, for example for de-focusing angles of the order of ten degrees it alters it by less than 30%, preferably by less than 20%, more preferably by less than 10%. This easily allows to realize chromatic reflectors having the desired value of the de-focusing angle by selecting the appropriate substrate, as different substrates with different values of the de-focusing angle are easily obtainable on the market, such as for example in the case of reflective aluminium substrates. 
     In a particular configuration, a reflective unit  1 , 1   a - 1   g  according to the present invention will have a de-focusing angle lower than 4°, preferably lower than 3°, preferably lower than 2°. Advantageously, this material can be used for devices aimed at producing the maximum chromatic contrast between reflected light and diffused light, i.e. to reproduce natural light effects and in particular the chromatic contrast between direct sunlight and diffused skylight, being the sunlight characterized by an angular luminance profile with half width at half maximum or HWHM of only 0.25°. 
     In a different configuration, a reflective unit  1 , 1   a - 1   g  according to the present invention will have a de-focusing angle comprised between 4° and 20°, preferably comprised between 5° and 15°, more preferably comprised between 6° and 12°. Advantageously, this material can be used for devices where it is intended to illuminate the material so that it, by diffusing the blue component of the light and reflecting the complementary one, produces the image of the sky, and in particular the image of a clear and serene sky. In this case, the property of the material to reflect, in addition to the light that illuminates it, also the image of the surrounding environment, significantly reduces the effectiveness of the reconstruction, especially if the surrounding environment is very bright. This drawback is particularly serious in the case, commercially of particular interest, of the reconstruction of large artificial skylights. While in fact for small skylights the luminance of the diffused light component, i.e. the luminance of the artificial sky, can be made very large, and in particular much greater than the luminance of the reflected environment, in the case of large skylights this is obviously impossible, either for reasons related to energy consumption, and because as the size of the skylight increases, the brightness of the environment it illuminates also increases. In such circumstances, having a material with a large de-focusing angle is certainly helpful since, despite the lower sharpness of the shadow produced, the image of the environment reflected in the reflector remains blurred and therefore less recognizable. In particular, if the scene comprises points or zones of small area and high brightness, the de-focusing process significantly reduces the brightness of these points in the reflection. 
     In a preferred embodiment, the coating layer  90  is configured in such a way that the Fresnel reflection of the first surface produces a reflected light having an angular luminance profile characterized by a peak in a neighbourhood of the direction of specular reflection, with angular half width at half maximum (θ COVER_HWHM ) when illuminated by a substantially unidirectional and monochromatic light and with wavelength of about 632 nm incident at 15° with respect to the normal to a surface of the same  1 ;  1   a - 1   g , where the angular half width at half maximum (θ COVER_HWHM ) of the peak of the angular luminance profile of the coating layer  90  is equal to at least 2°, preferably to at least 3°, more preferably to at least 5°, or it is substantially equal to the de-focusing angle of the reflective unit  1 , 1   a - 1   g . 
     Further advantageously, a material with a large de-focusing angle can be used to produce a reflected image of a single light source, i.e. a reflected image of a single sun, even when the material is illuminated by a plurality of sources separated from each other, provided that all the sources visible to the observer are subtended by an angle lower than the de-focusing angle of the material, for example at an angle 1.5 times lower, preferably at an angle 2 times lower, more preferably at an angle 3 times lower. Therefore, the availability of a material with a high de-focusing angle allows realizing devices adapted to reproduce the light and the image of the sky and the sun by using a plurality of light sources spaced between them for the simulation of the sun, for example light sources spaced between them by a distance greater than their size, i.e. a distance greater than the diameter of the circle that circumscribes the projection of each source of the plurality of sources on a plane orthogonal to the main direction of emission, i.e. the direction along which the angular luminance profile of the source exhibits its maximum value, where by source it is meant herein the light emitter comprising its own collimation optics. 
     In a variant of the invention illustrated in  FIG.  31   , the illumination system  700  comprises a masking structure  707  positioned and configured so as to prevent the view of the light sources  702  from the observer of the chromatic effect light reflective unit  1 ;  1   a - 1   g  through the support grid  701 . In particular, the masking structure  707  is a pergola comprising a distribution  708  of live or artificial plants. The illumination system  700  can further comprise a substantially transparent containment net (not shown) arranged between the masking structure and the chromatic effect light reflective unit. The containment net is in particular positioned and configured to prevent the growing plants from interfering between the sources and the chromatic effect light reflective unit  1 ;  1   a - 1   g . 
     More specifically, the present invention makes it possible to produce devices adapted to reproduce the light and the image of the sky and the sun by using for the simulation of the sun a plurality of light sources spaced from each other by a distance equal to at least twice, preferably 3 times, more preferably 4 times their size. This means making it possible for the observer to effectively look at the skylight, and in particular both at the sky and at the sun reflected in it, having the plurality of sources in a position interposed between his own eye and the skylight itself, the area obscured from the view being less, for example, at ¼, preferably ⅑, more preferably 1/16 of the area that the observer would see in the absence of any obstruction from the view produced by the sources. 
     Advantageously, the proponent has observed that for such reduced values of the percentage of obscured area the observer perceives the plane where the sources are positioned and the plane behind which the image of the sun corresponds to as two distinct planes, without the occurrence of any conflict as regards the visual perception, or “visual cue” of the relative distances or depth of field. Advantageously, the image of the sun that is produced in this case is perceived at an infinite distance. In fact the observer, moving along any direction, for example along a direction parallel to a plane containing the sources, will always see the image of the sun under the same angle, or in other words he will see the sun following it moving, with respect to the sources, at the same speed with which he moves, a fact that involves the perception of the sun object at an infinite distance. Advantageously, the effect described is not limited by the size of the device, i.e. it can be obtained for an arbitrarily wide distribution of sources, and therefore for arbitrarily large artificial skylights, i.e. it allows the development of modular artificial skylights which can be arranged side by side in order to produce an artificial skylight of arbitrary size. 
     In a variant of the invention illustrated in  FIG.  32   , the illumination system  700  comprises a containment screen  803  arranged in proximity to the outer edges of the chromatic effect light reflective unit  1 ;  1   a - 1   g  and configured in such a way as to prevent the light emitted by the light sources  702  from illuminating regions external and distant from the chromatic effect light reflective unit  1 ;  1   a - 1   g , and/or diffuse and/or reflect at least in part a light incident on them or on at least a portion of them. In particular, at least a portion of the containment screen  803  absorbs at least in part a light incident thereon. 
     In a variant of the invention illustrated in  FIG.  33   , the illumination system  700  comprises a strip of absorbent material  805  which at least partially surrounds an external perimeter of the chromatic effect light reflective unit  1 ;  1   a - 1   g , where the strip  805  is configured so as to absorb a light that reaches it coming from the plurality of light sources  702 .