Patent Publication Number: US-10775021-B2

Title: Artificial lighting system for simulating a natural lighting

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention is a 35 U.S.C. § 371 U.S. National Stage Application corresponding to PCT Application No. PCT/IB2013/060141, filed on Nov. 14, 2013, which claims priority to Italian Patent Application No. TO2012A000988, filed Nov. 14, 2012 and U.S. application Ser. No. 13/838,998, filed Mar. 15, 2013. The entire content of each of the aforementioned patent applications is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a system of artificial lighting. In particular, the invention relates to a system of artificial lighting that simulates natural lighting. Such lighting system may illuminate a room wherein it is inserted, with effects very similar to the effects that would occur in the same room if an aperture with sky and Sun beyond it was opened. 
     2. Background 
     It is known that, as of current date, artificial lighting systems for closed environments (“indoors”) are available which aim at improving the visual comfort experienced by users. In particular, lighting systems are known which simulate natural lighting, namely the type of lighting available in open-air environments (“outdoors”) The well-known characteristics of outdoor lighting depend on the interaction between light rays produced by the Sun and the Earth&#39;s atmosphere. 
     In pending European patent application EP2304480, filed by the present Applicant, a lighting system is described which comprises a light source, aimed at producing visible light, and a panel containing nanoparticles. When in use, the panel receives light rays coming from the source and acts as a so-called Rayleigh diffuser, namely it diffuses light rays similarly to the Earth&#39;s atmosphere in clear-sky conditions. 
     Additional details relative to the panel as in pending patent EP2304480 are described in pending European patent application EP2304478, filed by the present Applicant. Moreover, pending patent application EP2304480 describes various embodiments of the panel as well as various arrangements of the panel and the light source in respect to one another, aimed at simulating various conditions of natural lighting, such as for example lighting conditions that occur in nature in case of clear sky and i) Sun at Zenith or ii) Sun close to the horizon. 
     The lighting system described in patent application EP2304480 simulates natural lighting in that it generates, inside the surrounding environment, direct light with low Correlated Color Temperature (“CCT”), which mimics sunlight and generates shadows in presence of lit objects; also, the lighting system described in patent application EP2304480 simulates natural lighting in that it casts diffused light with high CCT, which mimics skylight and generates shadows with a blue tinge. Nonetheless, such lighting system does not flawlessly reproduce the perceptive effects an observer would experience in presence of a sky-facing window. In particular, such lighting system does not lead an observer to experience the visual perception of unlimited depth of field. 
     WO 2012/140579 discloses an optical element comprising a light transmitting cell, which comprises a light transmitting channel, a light input window, a light exit window and a wall. The light input window is arranged at a first side of the light transmitting channel and receives light from a light source. The light exit window emits light with the skylight appearance. At least a part of the light exit window is arranged at a second side of the light transmitting channel opposite to the first side. The wall is interposed between the light input window and the part of the light exit window; the wall encloses the light transmitting channel. At least a part of the wall is reflective and/or transmissive in a predefined spectral range to obtain a blue light emission at relatively large light emission angles with respect to a normal to the part of the light exit window. 
     U.S. Pat. No. 7,722,220 discloses a lighting device including a thermal conduction element, solid state light emitters and a reflective element. The conduction element defines an opening; the emitters and reflective element are mounted on a first side of the conduction element. 
     U.S. Pat. No. 4,747,028 discloses a luminaire employing means to provide an even, glare-free illumination. 
     U.S. Pat. No. 4,267,489 discloses a device including a diffusive transparent plate, a light homogenizing member, a fluorescent lamp and a reflection member. 
     BRIEF SUMMARY 
     Thus, the present invention aims at providing a lighting system capable of solving the known state-of-the-art limitations, at least in part. 
     The invention provides an artificial lighting system as set forth in the independent claims with advantageous possible implementations being subject of the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, embodiments are hereby described, purely as non-limitative examples and with reference to the enclosed drawings, wherein: 
         FIGS. 1, 2 and 7  show schematic cross-sections of embodiments of the present lighting system; 
         FIG. 3  shows a schematic cross-section of a possible lighting system different from the present lighting system; 
         FIGS. 4 a  and 4 b    schematically show perspective views of light sources; 
         FIGS. 5 a   ,  6  and  8  schematically show perspective views of portions of the present lighting system; 
         FIG. 5 b    shows a cross-section of a reflective element comprised in the lighting system&#39;s portion shown in  FIG. 5   a;    
         FIGS. 9 a    and  10  show perspective views of light sources; 
         FIG. 9 b    shows a cross-section of a portion of the light source shown in  FIG. 9   a;    
         FIG. 11  shows a cross-section of a portion of an embodiment of the present lighting system; and 
         FIGS. 12-15  show schematic cross-sections of portions of further embodiments of the present lighting system. 
     
    
    
     In general, the Applicant noticed that the capability of an observer to evaluate the distance of objects, and therefore the depth of field of the views that constitute a three-dimensional scenery, is based on multiple physiological and psychological mechanisms connected to focusing, binocular convergence, binocular parallax, movement parallax, luminance, size, contrast, aerial perspective, etc. Some mechanisms may gain significance compared to the others according to both the observing conditions (e.g., whether the observer is moving or still, watching with one or two eyes, etc.) as well as the characteristics of the scenery, these latter depending, for example, on whether objects with known size, distance or luminance are present, serving as a reference to evaluate how distant the observed element of the scenery is. 
     In particular, the Applicant noticed that an observer, who is watching a light projector through a window, loses the capability of estimating how far the projector is when such distance is higher than five meters (preferably, seven meters), provided that the background surrounding the projector is black and uniform. When such circumstances are met, the distance from the projector is undetermined by the observer. The capability of estimating distances is lost because i) precise focusing of a blinding light source is difficult, which prevents the observer from using the focusing mechanism to evaluate the object&#39;s distance, and ii) binocular convergence is scarcely efficient as an instrument for distance evaluation when the object is more than five meters away (preferably, seven meters); moreover, the capability to estimate is lost because the other psycho-physic mechanisms that are commonly valid and efficient in case of high distances fail, being inhibited by the absence of further points of reference. 
     The Applicant further noticed that, when a Rayleigh diffusion panel is interposed between the observer and the light projector, this latter being surrounded by a black, uniform background, the observer is induced to perceive the light projector virtually at infinite distance from him. More particularly, the effect of perception at infinite distance is obtained whenever the observer looks at the light projector through the Rayleigh diffusion panel, and this latter is thoroughly and uniformly lit by the projector, and the real projector-to-observer distance is five meters at least (preferably, seven meters). Such effect may be interpreted as a consequence of the so-called “aerial perspective”, a perception mechanism induced by the Rayleigh diffusion panel. As a matter of fact, the color and intensity of light scattered by the Rayleigh diffusion panel are virtually identical to the corresponding color and intensity of skylight, where intensity is evaluated as relative to the intensity of transmitted light. In particular, the so-called aerial perspective mechanism relates to the presence of an air layer interposed between any objects and the observer; the color and luminance of such air layer affect the estimation of the object-to-observer distance, the object being perceived by the observer as lying behind the air layer itself; such mechanism is dominant at high distances or, generally speaking, when the other psycho-physic mechanisms for distance evaluation are suppressed or scarcely efficient. 
     The Applicant further noticed that the observer is led to perceive light emitted by the Rayleigh diffusion panel as coming from a virtually infinite distance, provided that the spotlight is inside the observer&#39;s visual field. Such effect may be interpreted by considering that the Rayleigh diffusion panel acts as a secondary source of luminous radiation, and that the observer can hardly assess his distance from the emitting plane of such luminous radiation because of the high spatial uniformity of luminous radiation itself, which does not provide any visual point of reference to look upon. Thus, the presence of the light projector in the visual field at a (physical) distance of five meters (preferably, seven meters) affects the evaluation of the whole scenery&#39;s depth of field by “dragging” the estimated position of the Rayleigh diffusion panel beyond the threshold of distance perception by binocular convergence. Such effect is connected to the luminance of the light projector, and to the fact that, besides the Rayleigh diffusion panel, the light projector itself is the only spatially localized element perceived by the observer. Basically, when looking at the Rayleigh diffusion panel, the eyes of the observer are forced by the light projector to arrange themselves as if they were watching a very distant object. The mind is then pushed by such arrangement of the eyes to infer that the object in the middle of the visual field, that is light emitted by the Rayleigh diffusion panel, is very far compared to the real position of the panel itself. Also, the effect of perceiving a diffused-light source at great distance from the observer is favored by the fact that light scattered by the Rayleigh diffusion panel has the same color and luminance (compared to transmitted light) typical of skylight. Such effect, due to the aforementioned mechanism of aerial perspective, is particularly efficient, thereby causing the light projector to be perceived at virtually infinite distance. The Applicant also noticed that the described effect, that is the visual perception of an infinite depth of field (from now on called “breakthrough effect”), takes place irrespective of the direction of observation through the Rayleigh diffusion panel. 
     In addition, the Applicant noticed that the aerial perspective alone cannot perfectly assure the breakthrough effect, if the light projector is outside the visual field, because other psycho-physic mechanisms prevail, such as the focusing of scratches or borders of the Rayleigh diffusion panel. 
     Moreover, the Applicant noticed that the aforementioned breakthrough effect is reduced whenever the light projector is positioned next to the Rayleigh diffusion panel, e.g. without any mirrors or lenses to move afar its virtual image. In fact, the light projector&#39;s distance would be in this case easily estimated by the observer, which would limit the depth of field in the whole scenery despite the contribution of the aerial perspective. Similarly, the Applicant noticed that the aforementioned breakthrough effect is reduced whenever the light projector is not surrounded by a black, uniform background. In fact, an observer can determine the distance from a background other than a black and uniform background, thereby limiting the depth of field of the whole scenery, in spite of the contribution of the aerial perspective. 
     DETAILED DESCRIPTION 
     That having being stated,  FIG. 1  shows an artificial lighting system  1 , which will be from now on referred to concisely as lighting system  1 . 
     In detail, lighting system  1  comprises a first light source  2 , preferably directional, i.e. designed to emit light in an emission solid angle smaller than 4π sr. Moreover, the first light source  2  emits light in the visible region of the spectrum, that is having wavelengths between 400 nm and 700 nm. Moreover, the first light source  2  emits light (visible electromagnetic radiation) with spectral width Δλ preferably higher than 100 nm, more preferably higher than 170 nm. Spectral width Δλ may be defined as the standard deviation of the first light source&#39;s wavelength spectrum. 
     The lighting system  1  also includes a first diffuser panel  4 , which is for example shaped as a parallelepiped. In particular, the first diffuser panel  4  is delimited by a first surface S 1  and by a second surface S 2 , parallel to each other; preferably, the first diffuser panel  4  is thin, i.e. its thickness w, measured along a direction perpendicular to the first and the second surfaces S 1 , S 2 , has square value not higher than 5%, preferably 1%, of the area of the first and the second surfaces S 1 , S 2 . 
     More particularly, in the embodiment shown in  FIG. 1  the first diffuser panel  4  operates as a so-called Rayleigh diffuser, i.e. as a panel which substantially does not absorb light in the visible range and which diffuses more efficiently the short-wavelength in respect to the long-wavelength components of the impinging light, e.g. a panel which substantially does not absorb light in the visible range and which diffuses light rays of wavelength λ=450 nm (blue) at least 1.2 times, preferably at least 1.4 times, more preferably at least 1.6 times more efficiently than light rays of wavelength λ=650 nm (red), wherein diffusion efficiency is given by the ratio between the diffused light radiant power respect the impinging light radiant power. Optical properties and microscopic characteristic of Rayleigh like diffusers are also described in detail in the patent application EP2304478 of the same Applicant. A further insight on the microscopic features is also provided in what follows. 
     In the embodiment shown in  FIG. 1 , the first light source  2  is vertically aligned to the first diffuser panel  4 , i.e. it lies along an axis H, which is perpendicular to the first and second surfaces S 1 , S 2  and passes through the barycenter of these latter (in  FIG. 1 , the barycenter of the first surface S 1  is designated by O). In general, except where otherwise stated, in the present description the term “barycenter” is meant in its geometric acceptation, instead of its physical acceptation (center of mass), hence it is applicable also to plane surfaces, and anyway to objects having a substantially infinitesimal thickness. Therefore, the term “barycenter” has to be meant as “geometrical center” or “centroid” and it coincides, given an object (or surface) having an infinitesimal thickness, with the center of mass of this latter, calculated under the assumption that the object has a uniform density and, precisely, an infinitesimal thickness. Moreover, the first light source  2  illuminates entirely the first diffuser panel  4 . Embodiments wherein the first light source  2  is arranged off-axis in respect to the barycenter of the first and second surfaces S 1 , S 2  are, however, possible, as described in what follows. 
     The lighting system  1  is optically coupled to an environment, e.g. a room  6  shaped as a parallelepiped and delimited by a lower wall P 1 , an upper wall P 2  and four lateral walls P 1 . In particular, without losing generality, it is assumed that the upper wall P 2  has a cavity  8 , which has, seen from above, the same shape of the first diffuser panel  4  and is filled entirely by this latter. In any case, the present invention is not restricted to the shape and/or arrangement of the cavity  8 ; as an example, according to further embodiments (not shown), the cavity may be formed within a lateral wall. Furthermore, the present invention is not restricted to be used in indoor spaces; therefore, embodiments are possible in which the lighting device  1  is used as a system for outdoor day-like illumination during night time. Therefore, the lighting system  1  can be coupled to an outdoor environment, i.e. an environment equivalent to a room, the walls of which are either black or arranged at an infinitely large distance. 
     The lighting system  1  comprises a support element  10  that delimits, together with the first surface S 1  of the first diffuser panel  4 , an external volume V which is external to the room  6 ; the first light source  2  is placed inside the external volume V. Although not shown, embodiments are possible in which the support element  10  is mechanically coupled to the room  6  in a manner such that the external volume V is delimited at least in part also by a wall of the room  6 , as an example the upper wall P 2 . 
     The support element  10  is internally coated by an internal layer  12 , made of a material capable of absorbing incident luminous radiation; such material is, for example, a material with black color and coefficient of absorption higher than 70%, preferably higher than 90%, most preferably higher than 95%, even most preferably higher than 97% in the visible range. The internal layer  12  is aimed ad absorbing incident radiation that comes, for example, directly from the first light source  2 , or from reflection and/or scattering processes by the first diffuser panel  4 , or from the room  6  through the first diffuser panel  4 . Preferably, the volume V is internally coated by the internal layer  12  in its entirety, with the exception of the first surface S 1  of the first diffuser panel  4 . In other words, the support element  10  and the internal layer  12  define a sort of dark box (or chamber), wherein the term “dark” relates to a condition of little illumination and/or to the capability of absorbing the light, so as to make the box barely visible, as described hereinbelow; therefore, in what follows, reference will also be made to a dark box  10 . The light can enter/exit the dark box only through the first diffuser panel  4 . 
     Again referring to the first diffuser panel  4 , and assuming that a light beam generated by a CIE (International Commission on Illumination) D65 standard illuminant point-like source at large distance from the first diffuser panel  4  (a beam, thus, constituted by light rays parallel to one another) and directed perpendicularly the first surface S 1 , the first diffuser panel  4  separates such beam in four components, particularly in: 
     a transmitted component, formed by light rays that pass through the first diffuser panel  4  and do not experience significant deviations, i.e. by light rays experiencing a deviation smaller than 0.1°, with a luminous flux which is a fraction τ direct  of the overall luminous flux incident on the first diffuser panel  4 ; 
     a forward diffuse component, formed by light rays exiting the second surface S 2  along directions that are distributed around a direction perpendicular to the second surface S 2  (with the exception of such perpendicular direction and of directions differing from such perpendicular direction by an angle smaller than 0.1°), with a luminous flux which is a fraction τ scattered  of the overall luminous flux incident on the first diffuser panel  4 ; 
     a backward diffuse component, formed by light rays exiting the first surface S 1  along directions that are distributed around a direction perpendicular to the first surface S 1  (with the exception of such perpendicular direction and of directions differing from such perpendicular direction by an angle smaller than 0.1°), with a luminous flux which is a fraction ρ scattered  of the overall luminous flux incident on the first diffuser panel  4 ; and 
     a reflected component, formed by light rays exiting, or originating from, the first surface S 1  along a direction at a mirror angle (e.g. perpendicular, or differing from the perpendicular by an angle smaller than 0.1°, in the present case) to the first surface S 1 , with a luminous flux which is a fraction ρ direct  of the overall luminous flux incident on the first diffuser panel  4 . 
     That having being stated, the optical properties of the first diffuser panel  4  are such that: 
     τ scattered  is within the range 0.05-0.5, preferably 0.07-0.4, more preferably 0.1-0.3, still more preferably 0.15-0.25; 
     the average correlated color temperature (“CCT”) CCT_τ scattered  of the forward diffuse component is significantly higher than the average correlated color temperature CCT_τ direct  of the transmitted component, namely CCT_τ scattered &gt;h*CCT_τ direct  with h=1.2, preferably h=1.3, more preferably h=1.5; 
     the first diffuser panel  4  does not absorb significantly incident light, namely the sum τ direct +τ scattered +ρ direct +ρ scattered  is at least equal to 0.8, preferably 0.9, more preferably 0.95, still more preferably 0.97; 
     the first diffuser panel  4  scatters mostly forward, namely τ scattered &gt;η*ρ scattered , where η is at least equal to 1.1, preferably η=1.3, more preferably η=1.5, still more preferably η=2; and 
     the first diffuser panel  4  has low reflection, namely ρ direct &lt;0.09, preferably &lt;0.06, more preferably &lt;0.03, still more preferably &lt;0.02. 
     In greater detail, the first diffuser panel  4  comprises a solid matrix of a first material (e.g., a resin having excellent optical transparency, such as thermoplastic resins, thermosetting resins, photocurable resins, acrylic resins, epoxy resins, polyester resins, polystyrene resins, polyolefin resins, polyamide resins, polyimide resins, polyvinyl alcohol resins, butyral resins, fluorine-based resins, vinyl acetate resins, or plastics such as polycarbonate, liquid crystal polymers, polyphenylene ether, polysulfone, polyether sulfone, polyarylate, amorphous polyolefin, or mixtures or copolymers thereof), wherein nanoparticles of a second material (e.g. inorganic oxide such as ZnO, TiO 2 , ZrO 2 , SiO 2 , Al 2 O 3 ) are dispersed, this second material having a refractive index different from the first material&#39;s refractive index. Both the first and the second material basically do not absorb electromagnetic radiation in the visible wavelength range. 
     Moreover, in the embodiment shown in  FIG. 1 , the first diffuser panel  4  is uniform, in the sense that, given any point of the first diffuser panel  4 , the physical characteristics of the first diffuser panel  4  in that point do not depend on the point itself. Furthermore, the first diffuser panel  4  is monolithic, namely the solid matrix does not feature any discontinuity due to gluing or mechanical coupling. Such characteristics of the first diffuser panel  4  are not, however, necessary to the aims of the present invention, although they render the first diffuser panel  4  easier to be manufactured. 
     More particularly, the nanoparticles may be monodisperse. The nanoparticles may be spherically shaped or shaped otherwise. The effective diameter D of the nanoparticles (for a definition in the case of non-spherical shape, see below) falls within the range [5 nm-350 nm], preferably [10 nm-250 nm], more preferably [40 nm-180 nm], still more preferably [60 nm-150 nm], where the effective diameter D is given by the diameter of the nanoparticles times the first material&#39;s refractive index. 
     Moreover, nanoparticles are distributed inside the first diffuser panel  4  so that their areal density, namely the number N of nanoparticles per square meter, i.e. the number of nanoparticles within a volume element delimited by a portion of the first surface S 1  having an area of 1 m 2 , satisfies the condition N≥N min , where: 
               N   min     =     υ   ⁢         10     -   29         D   6       ·                m   2     +   2         m   2     -   1            2               
wherein ν is a dimensional constant equal to 1 meter 6 , N min  is expressed as a number/meter 2 , the effective diameter D is expressed in meters and wherein m is equal to the ratio of the second material&#39;s refractive index to the first material&#39;s refractive index.
 
     Preferably, the nanoparticles are distributed homogenously, at least as far as the areal density is concerned, i.e. the areal density is substantially uniform on the first diffuser panel  4 , but the nanoparticle distribution may vary across a direction perpendicular to the first and second surfaces S 1 , S 2 . The areal density varies, for example, by less than 5% of the mean areal density. The aerial density is here intended as a quantity defined over areas larger 0.25 mm 2 . 
     Alternatively, embodiments are possible wherein the areal density varies, so as to compensate illumination differences over the first diffuser panel  4 , as lit by the first light source  2 . For example, the areal density N(x,y) at point (x,y) within S 1  may be related to the illuminance I(x,y) produced by the first light source  2  at point (x,y) via the equation N(x,y)=N av *I av /I(x,y)±5%, where Nay and I av  are the averaged illuminance and areal density, these latter quantities being averaged over the first surface S 1 . In this case the luminance of the first diffuser panel  4  is equalized on the first diffuser panel  4 , in spite of the non-uniformity of the illuminance profile of the first light source  2  on the first diffuser panel  4 . To this regard, it is reminded that the luminance is the luminous flux of a beam emanating from a surface (or falling on a surface) in a given direction, per unit of projected area of the surface as viewed from the given direction, and per unit of solid angle, as reported, as an example, in the standard ASTM (American Society for Testing and Materials) E284-09a. 
     In the limit of small D and small volume fractions (i.e. thick panels) an areal density N≈N min  is expected to produce scattering efficiency of about 5%. As the number of nanoparticles per unit area gets higher, the scattering efficiency is expected to grow proportionally to N, until multiple scattering or interferences (in case of high volume fraction) occur, which might compromise color quality. The choice of the number of nanoparticles is thus biased by the search for a compromise between scattering efficiency and desired color, as described in detail in the patent application EP2304478. Furthermore, as the size of nanoparticles gets larger, the ratio η=τ scattered /ρ scattered  of the forward to backward luminous flux grows, such ratio being equal to one in the Rayleigh limit. Moreover, as η grows, the aperture of the forward scattering cone gets smaller. Therefore, the choice of η is biased by the search for a compromise between having light scattered at large angles and minimizing the flux of backward scattered light. However, in a per se known manner, an antireflection layer (not shown) may be deposited on the first and the second surface S 1  and S 2 , with the aim of minimizing ρ direct ; by doing so, the luminous efficiency of the lighting system  1  is raised and the visibility of the first diffuser panel  4  (as a physical element) from an observer in the room  6  is diminished. 
     Embodiments are however possible wherein nanoparticles do not have spherical shape; in such case, the effective diameter D can be defined as the effective diameter of the equivalent spherical particles, namely the effective diameter of spherical particles having the same volume as the aforementioned nanoparticles. 
     Furthermore, embodiments are possible wherein the nanoparticles are polydispersed, i.e. their effective diameters are characterized by a distribution N(D). Such distribution describes the number of nanoparticles per surface unit and unit interval of effective diameter in a neighborhood of the effective diameter D (that is, the number of particles per surface unit having an effective diameter between D 1  e D 2  is equal to 
                   N       D   2     -     D   1         =       ∫     D   1       D   2       ⁢       N   ⁡     (   D   )       ⁢           ⁢   dD         )     .         
These effective diameters may fall in the range [5 nm-350 nm], i.e. the distribution may be non-null within this range. In this case, considering that scattering efficiency grows approximately, i.e. in the limit of small particles, with the sixth power of the nanoparticle&#39;s diameter, the polydisperse distribution behaves approximately as a monodisperse distribution with a representative diameter D′ eff  defined as:
 
     
       
         
           
             
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     D′ eff  may by selected so as to lie within the range [5 nm-350 nm], preferably [10 nm-250 nm], more preferably [40 nm-180 nm], still more preferably [60 nm-150 nm]. 
     Moreover, the first diffuser panel  4  is at a distance d from the first light source  2 , measured along the axis H. Such distance d may be varied according to an expected position of the observer inside the room  6 , so that the distance between the expected position of the observer and the first light source  2  is equal to at least five meters, preferably seven meters. For example, in the case of a ceiling-type application, distance d may be equal to three meters. As a precaution, distance d may be equal to five meters, in case the observer is very close to the second surface S 2 . 
     According to a different embodiment shown in  FIG. 2 , the first light source  2  is once again arranged inside the external volume V, but it is arranged off axis, i.e. laterally with respect to the first diffuser panel  4 , i.e. it is not intercepted by any line that passes through the first diffuser panel  4  and is parallel to the axis H. Furthermore, the lighting system  1  includes a reflective system  20 , which includes a first mirror  22  and forms a light path that connects the first light source  2  to the first diffuser panel  4 ; in other words, the light rays generated by the first light source  2  are conveyed by the reflective system  20  onto the first surface S 1 . The first mirror  22  causes a last deviation (i.e. a last change of direction) of the light path before the first diffuser panel  4 . 
     In addition, the first light source  2  and the reflective system  20  are such that the first diffuser panel  4  is lit thoroughly by the light rays coming from the first light source  2 . Furthermore, for reasons that will be described more accurately later on, the first diffuser panel  4  and the reflective system  20  are arranged so that they satisfy the following geometric condition. There are no couples composed by a light ray RL 1  and a light ray RL 2  such that: 
     the light ray RL 1  passes through the first diffuser panel  4  (as an example, coming from the room  6 ) or originates from the first diffuser panel  4 ; and 
     the light ray RL 2  is the reflection of the light ray RL 1  by the reflective system  20  and it is directed so as to impinge again on the first surface S 1 . 
     The abovementioned geometric condition on light rays RL 1  and RL 2  is equivalent to stating that no light ray generated inside the room  6  and intersecting the first surface S 1  in a first point can be subsequently reflected by the reflective system  20  in a manner such that it hits again the first surface S 1  in a second point. Even alternatively, the reflective system  20  is arranged such that all inbound light rays emanating from the first surface S 1  and impinging onto the reflective system  20 , irrespective of the position within the first surface S 1  from which the inbound light rays emanate, are reflected onto the internal layer  12 . 
     The aforementioned geometric condition on light rays RL 1  and RL 2  leads to reducing the volume occupied by the lighting system  1 , mainly in terms of the volume occupied outside room  6 , without damaging the quality of lighting. In particular, the lighting system  1  features a reduction of the space occupied vertically, i.e. measured along the axis H. Given that a reduced vertical encumbrance is a prerequisite for a large number of applications, the abovementioned geometric condition allows to obtain the breakthrough effect in a great number of situations of practical interest. For the sake of brevity, from now on the reference to the vertical dimension of the occupied space will be generally omitted. 
     In greater detail, besides leading to a reduction of the occupied space, the reciprocal arrangement of the reflective system  20  and the first diffuser panel  4  prevent the occurrence of two phenomena which could spoil the natural quality of the lighting. 
     As shown in  FIG. 3 , should the aforementioned geometric condition on the light rays RL 1  and RL 2  be violated, the following would happen: 
     a light ray IR 1  generated by the first light source  2  hits the reflective system (here designated by  30 ) and is conveyed onto the first diffuser panel  4 , crosses the first diffuser panel  4  and reaches the observer; and 
     a light ray IR 2  generated by the first light source  2  hits the reflective system  30  a first time, is conveyed a first time onto the first diffuser panel  4 , is partially reflected due to Fresnel reflection by the first surface S 1 , hits the reflective system  30  a second time, is conveyed a second time onto the first diffuser panel  4 , crosses the first diffuser panel  4  and reaches the observer from a different direction with respect to the light ray IR 1 . 
     In this case, the observer would experience the vision of two different images of the first light source  2 , that are seen under different directions. The first image is the image formed by IR 1  and by all the light rays close to IR 1 , i.e. the light rays that have crossed only one time the first diffuser panel  4 . The second image is the image formed by the light ray IR 2  and by all the light rays close to IR 2 , i.e. the rays which, having been partially reflected by the first surface S 1 , are redirected by the reflective system  30  toward the observer. Since Fresnel reflection redirects only a portion (e.g. about 4% per each surface of the first diffuser panel  4 , for incidence nearly perpendicular and for PMMA material), the second image of the first light source  2  is weaker than the first one. Nevertheless, its luminance is still very high; therefore, an observer would perceive the difference with natural lighting, which is evidently characterized by the presence of only one image of the Sun. 
     In a similar way, should the aforementioned geometric condition on light rays RL 1  and RL 2  be missing, light rays coming from the room  6  and with any color could cross the first diffuser panel  4 , be reflected by the reflective system  30  and re-enter the room  6  after crossing again the first diffuser panel  4 . In such case, an observer would perceive the presence of luminous objects having colors different from the color of the first diffuser panel  4 , as if they were arranged beyond the first diffuser panel  4 . Furthermore, due to the so-called backscattering, the first diffuser panel  4  itself would be seen by the observer not only directly, but also trough the reflective system  30 ; in practice, the first diffuser panel  4  would generate a luminous spot, spatially limited by the mirror frame, which would spoil the uniformity of the background. In addition, an observer could notice the presence of the reflective system  30  because of swift change in luminance that could take place at mirror edges. All these effects would cause the lighting to appear unnatural, the image of the first diffuser panel  4  being different from that of natural sky. Moreover, the perception of objects or mirrors in the background of the first diffuser panel  4  would prevent from obtaining the breakthrough effect. 
     That having being stated, embodiments are possible where, as shown in  FIG. 2 , the first mirror  22  is plane and parallel to the first diffuser panel  4  (namely parallel to the first and second surfaces S 1 , S 2 ), so that the occupied volume is minimized. 
     Moreover, irrespective of the shape and the angle of inclination of the first mirror  22 , the first light source  2  and the first mirror  22  are arranged in such a way that, if the surface of the first mirror  22  is referred to as the reflecting surface S r , the barycenter O of the first surface S 1  and the barycenter O′ of the reflecting surface S r  can be connected by a line, having an angle of incidence AO with respect to the axis H which is between 40° and 65°, preferably between 42° and 50°, more preferably in a neighborhood of 45°. In such a way, a compromise is reached between minimizing the space occupied vertically by the lighting system  1 , which decreases as the angle of incidence AO gets larger, and minimizing the light losses due to the partial reflection which occurs at the first diffuser panel  4 , which grows as the angle of incidence AO gets larger, in the hypothesis that the first diffuser panel  4  has refractive index basically equal to 1.5 and that the angle of incidence AO is higher than 45°. It is worthwhile noting that the benefit of illuminating the first diffuser panel  4  at an angle of incidence AO substantially equal to 45° holds true for all possible embodiments, including those described later and those without any mirror. 
     The Applicant also verified that, when the first mirror  22  is plane, the vertical space occupied by the lighting system  1  is minimized for any angle of incidence AO, provided that the first mirror  2  is parallel to the first diffuser panel  4 . 
     Before moving on to describe, it is defined the “carrier ray” as the folded light path that connects the barycenter O″ of the emitting surface S f  (defined later on) of the first light source  2  to the barycenter O of the first surface S 1 , via the reflective system  20 , or the shortest among these light paths, if more than one light path exists; only one light path exists if the reflective system  20  is designed as an imaging-optic component. 
     Moreover, a Cartesian reference system is introduced, such a reference system having an origin in the barycenter O of the first surface S 1 , and including an x-axis and an y-axis lying in the plane defined by the first surface S 1  and arranged in such a way that the y-axis is perpendicular to the plane of incidence of the carrier ray onto the reflecting surface S r  of the first mirror  22  (i.e. the plane containing the two segments of the carrier ray that contact the first mirror  22 , as well as the line perpendicular to the reflecting surface S r  in the contact point). 
     Irrespective of the shape and the angle of inclination of the first mirror  22 , embodiments are possible wherein the first surface S 1  has a rectangular or at least an elongated shape, with its largest axis being coincident with the y-axis. The Applicant has verified that these embodiments allow to fulfill the geometric condition on the light rays RL 1  and RL 2  with a smaller height of the lighting system  1  along the axis H, with respect to the cases in which the first diffuser panel  4  is not elongated or is elongated along the x-axis, the area of the first diffuser panel  4  and the angle of incidence AO being the same. In other terms, these embodiments allow to maximize the area of the first diffuser panel  4  for a given height of the lighting system  1  and a given angle of incidence AO. In fact, the Applicant has noted that, for a given angle of incidence AO, the maximum width of the first diffuser panel  4  along the x-axis is proportional to the minimum height of the lighting system  1  along the axis H, the proportionality coefficient being close to 1 when the angle of incidence AO is close to 45°. 
     The Applicant also noticed that the natural quality of lighting is further improved if the first light source  2  features a circular ( FIG. 4 a   ) or elliptic ( FIG. 4 b   ) emitting surface S f . Given that the first light source  2  is directional, it is characterized by a main direction, which is the direction of the absolute maximum of the luminous intensity, and by a main plane, here defined as the plane, perpendicular to the main direction, in which the absolute maximum of the luminance occurs. That having being said, the emitting surface S f  is the portion of the main plane where the luminance along the main direction is higher than 10% of this absolute maximum of the luminous intensity. The emitting surface S f  is called circular or elliptic if there exists a circumference or ellipse enclosing it and having an area greater than the area of emitting surface S f , by no more than 30%, preferably 20%, more preferably 10%. 
     All that being said, the light rays hit the first surface S 1  in corresponding points of incidence and form corresponding angles of incidence with the lines perpendiculars to the first surface S and passing through the points of incidence. That having being said, the reflective system  20  and the arrangements of the first light source  2  and of the first diffuser panel  4  are such that, given: 
     a light ray RL 3  that connects, via the reflective system  20 , the barycenter O″ of the emitting surface S f  to the barycenter O of the first surface S 1 , and forms an angle θ 1  in respect to a line perpendicular to the first surface S 1  and passing through the barycenter O of this latter; and 
     a light ray RL 4  that connects, via the reflective system  20 , the barycenter O″ of emitting surface S f  to a point of the first surface S 1  which is spaced apart of a distance X from the barycenter O of this latter, and that forms an angle θ 2  in respect to a line perpendicular to the first surface S 1  and passing through this point; 
     it thus happens that:
 
|tan(θ 1 −θ 2 )|≤ X ·cos(θ 1 )/ L  
 
     where L is at least equal to three meters and, preferably, X&lt;&lt;L, e.g. X&lt;10 cm. Preferably, L is at least equal to four meters; still more preferably, L is at least equal to five meters. Note that such condition is satisfied also by the embodiment shown in  FIG. 1 , provided that the aforementioned distance d of the first diffuser panel  4  from the first light source  2  is equal to L. 
     In such a way, the light rays impinge on the first surface S 1  with almost parallel directions, in a way similar to what happens in nature. Furthermore, this condition can be met even when the first light source  2  is at a physical distance from the first diffuser panel  4  lower than L, provided that the reflective system  20  comprises converging mirrors, i.e. mirrors designed to form a virtual image of the first light source  2  at a distance greater than the physical distance. 
     The Applicant has further noticed that, in certain applications (e.g., in the case where the first diffuser panel  4  is set apart of a small distance from the observer), it is sufficient that L is at least equal to 50%, preferably 70%, even more preferably to 100%, of the maximum distance between any two points of the first surface S 1 . 
     Irrespective of the above details on the distance of the first diffuser panel  4  from the first light source  2 , embodiments are however possible, wherein the light source  2  is arranged so that the illuminance profile on the first surface S 1  varies between a minimum value ILLU min  and a maximum value ILLU max , wherein ILLU max ≤3*ILLU min , in order to limit the illuminance variations on the first diffuser panel  4 . Such a condition on the illuminance uniformity may be achieved by interposing a free-form optics between the first light source  2  and the first surface S 1  and/or by spacing the light source  2  apart from the first surface S 1  of a suitable distance. As an example, embodiments are possible, wherein the illuminance produced by the first light source  2  on the first surface S 1  is substantially uniform, owing to the fact that the following relationship holds true:
 
|tan(θ 1 −θ e )|≤0.5·cos(θ 1 )
 
     wherein θ e  is the angle at which a further light ray, which originates from the barycenter O″ of the emitting surface S f  of the first light source  2 , impinges on a point of the boundary of the first surface S 1 , this point being the point among the points of the boundary having maximum distance from the barycenter O of the first surface S 1 . The Applicant has further noticed that, also in this case, if the observer is set apart from the first diffuser panel  4  by a small distance, it is sufficient that L is at least equal to 50%, preferably 70%, even more preferably to 100%, of the maximum distance between any two points of the first surface S 1 . 
     The Applicant further noticed that the natural quality of lighting improves whenever the maximum luminance of the first light source  2  is greater than 10 6  cd/m 2 , preferably 0.1*10 6  cd/m 2 , more preferably 1*10 6  cd/m 2 , still more preferably 10*10 6  cd/m 2 . For such values, as a matter of fact, the first light source  2  generates enough glare for the source itself to be difficult to look at, thereby preventing the observer from evaluating the source&#39;s distance by means of the mechanism of eye focusing. These luminance values thus contribute to obtain the infinite breakthrough effect. Moreover, glare makes it difficult to detect possible non-uniformities in the luminance profile of the first light source  2 , thus making it difficult to detect differences between the image of the first light source  2  and the image of the Sun. 
     The Applicant also verified that the natural quality of lighting improves if the size and shape of the first light source  2  are such that, given a light ray that connects the perimeter of emitting surface S f  to the barycenter O of the first surface S 1 , the angle it forms with the abovementioned light ray RL 3  is lower than 4°, preferably 3°, more preferably 1.2°, and still more preferably 1.0°. As a matter of fact, the natural quality of lighting improves when the lower values of such angle are associated to the higher values of luminance, this condition allowing to obtain a more natural perception. 
     As shown in  FIGS. 5 a  and 5 b   , independently from the details of the first diffuser panel  4 , the first mirror  22  may be a concavely curved mirror, such as a concavely curved mirror having a parabolic curvature. In particular, as shown in  FIGS. 5 a  and 5 b   , the first mirror  22  may be shaped as a portion of a circular paraboloid, i.e. a surface obtained by rotating a generator parabola around its axis A, such that the intersection with any plane including the axis A defines the same generator parabola. In particular, the portion of circular paraboloid is obtained by splitting a portion of the circular paraboloid surface with a secant plane that crosses the axis A forming an angle different from 90°. For the sake of brevity, from now on, reference is made to the circular paraboloid, without mentioning explicitly that the mirror is formed by a portion of the circular paraboloid. 
     According to this embodiment, the first light source  2  is arranged in the focus of the circular paraboloid; more precisely, the barycenter O″ of the emitting surface S f  of the first light source  2  is arranged in the focus of the circular paraboloid, so that the light rays coming from that barycenter and reflected by the circular paraboloid hit the first surface S 1  with directions of propagation all parallel to the axis A. In this way, the observer perceives the first light source  2  as if it were arranged at a virtually infinite distance, similarly to what happens with the Sun, thereby enhancing the natural quality of lighting. In other words, the virtual image of the first light source  2  is at infinite distance from the observer. 
     Furthermore, the size of the first light source  2  perceived by the observer is given by the size of the image of the first light source  2  on the retina, and depends only on the physical dimension of the first light source  2  and on the magnification of the optical telescope system formed by the eye lens (the crystalline) and by the circular paraboloid; such optical telescope system has an image plane and an object plane which are arranged, respectively, in the focus of the eye lens and in the focus of the circular paraboloid. The abovementioned magnification is given by the ratio of the eye lens focal length to the circular paraboloid focal length; therefore, the size of the first light source  2  as perceived by the observer does not depend on the distance of the observer from lighting system  1 . Thus, this additional condition contributes in creating a natural lighting effect, because the Sun&#39;s perceived size do not depend on the observer&#39;s position. 
     The Applicant also noticed that, if the emitting surface S f  is shaped as a circle, the image of the first light source  2  perceived by the observer is still circle shaped, because the optical system shown in  FIG. 5 a    does not twist the image. 
     The embodiment shown in  FIG. 5 a    is characterized in that the vertical space occupied by the lighting system  1  is almost equal to the size of the first diffuser panel  4  along the x-axis, in case the light rays reflected by the circular paraboloid hit the first diffuser panel  4  at  450 , and the abovementioned geometric condition is met. 
     According to a variant, shown in  FIG. 5 a   , the first and second surfaces S 1  and S 2  of the first diffuser panel  4  have an elliptic shape, such a shape being encompassed by the projection of circular paraboloid  22  on the xy plane along the direction given by the axis A. Therefore, the first and second surfaces S 1  and S 2  can be circumscribed by the luminous spot SP formed by the circular paraboloid in the xy plane, thereby reducing light losses. Moreover, the first mirror  22  is cut so as to accept a light beam having circular divergence, i.e. is cut in a way that its projection on the plane orthogonal to the line connecting the barycenter O″ of the emitting surface S f  and the vertex of the circular paraboloid has a circular shape, or at least circumscribes a circle with good approximation. However, other shapes of the first mirror  22  are also possible, e.g. an elongated shape along the y direction. 
     The use of the circular paraboloid implies that the light entering the room  6  through the first diffuser panel  4  projects on the floor of the room  6  a light spot having the same shape and size as the first diffuser panel  4 , as it happens with sunlight through a window, thereby contributing to the natural lighting effect. Moreover, since the observer is capable of evaluating the distance of a generic light source based on the divergence of the light beam it generates, the lighting system  1  shown in  FIG. 5 a    creates an effect of large depth of field even if the first light source  2  is not directly in the observer&#39;s visual field. 
     As shown in  FIG. 6 , the first mirror  22  may be shaped as the portion of a paraboloid with cylindrical symmetry, i.e. as a portion of a parabolic cylinder, this portion being obtained by intersecting the parabolic cylinder with three secant planes. In detail, it is known that, given a generator parabola and a reference line R, the parabolic cylinder is the ruled surface formed by lines parallel to the reference line R and incident on the generator parabola; in other words, the parabolic cylinder is obtained by translation of the generator parabola along the reference line R. In what follows, the reference line R is also called cylindrical axis. 
     In the embodiment shown in  FIG. 6 , the parabolic cylinder is obtained by translation of the generator parabola in a direction parallel to the x-axis. Moreover, the generator parabola has its vertex in the xH plane and its axis A oriented along a line which is specular to the line that connects the barycenter O of the first surface S 1  and the barycenter O′ of the reflecting surface S r  of the first mirror  22 . In this embodiment, a plane tangent to the parabolic cylinder at the vertex of the generator parabola is parallel to the xy plane. Moreover, two out of three secant planes are, for example, parallel to yH plane, whereas the third plane is, for example, substantially parallel to plane xy. All that having being said, from now on, for the sake of brevity reference will be made to the parabolic cylinder, without mentioning explicitly that the mirror is formed by a portion of the parabolic cylinder. 
     In the embodiment shown in  FIG. 6 , the parabolic cylinder is laterally spaced apart along the x-axis, with respect to the axis H, so that the angle of incidence AO is substantially equal to 45°. 
     In greater detail, the barycenter O″ of emitting surface S f  is arranged in the xH plane, close to the line formed by the foci of the parabolae forming the parabolic cylinder, at the position which ensures the best collimation of the light rays directed toward the first diffuser panel  4 , for what concerns the ray propagation in the plane containing the y-axis and the barycenter of the parabolic cylinder, and more generally what concerns ray propagation in all the planes that intersect the first diffuser panel  4  along lines parallel to the y-axis. In the following, the average divergence in these latter planes is referred for simplicity as divergence along the y-axis direction. 
     The embodiment shown in  FIG. 6  allows to use a first diffuser panel  4  which is considerably longer along the y-axis than along the x-axis, thereby maximizing the area of the first diffuser panel  4  and so the angles under which the observer perceives the breakthrough effect. More precisely, such a large elongation of the first diffuser panel  4  is possible because the embodiment relies upon the usage of a mirror which features a large elongation in the y-axis direction, while preserving a limited (output) divergence in the same y-axis direction. More specifically, the Applicant has noticed that the perceived size of the first light source  2  along the y-axis direction, i.e. the diameter of the perceived emitting surface S f  along the y-axis direction, does not depend on the distance between the observer and the first light source  2 , or it depends from this distance very weakly. Regarding the size of the first light source  2  perceived by the observer along the x-axis direction, it depends on the position of the observer and decreases with distance. Therefore, with the aim of assuring that a circular shape of the first light source  2  is perceived, a light source with an elliptic emitting surface S f  can be adopted, wherein the eccentricity of the ellipse is fixed according to the expected point of observation inside the room  6 . 
     A further advantage given by using a parabolic cylinder is the fact that such a kind of mirrors are easy to be manufactured, since they can be obtained by a plane-mirror foil, e.g. an Aluminium mirror foil. Furthermore, referring to an observer standing in vertical position and looking at the first light source  2  from a central position, i.e. through the barycenter O of the first surface S 1 , and hence having the eyes aligned along the y-axis direction, he will perceive the first light source  2  at far distance, due to the fact that his eye-convergence operates only in planes containing both eyes (i.e. the y-axis direction), where high convergence occurs. That happens no matter the ray divergence in the orthogonal direction is. 
     In a different embodiment (not shown), the lighting system  1  is mounted in such a way that the first diffuser panel  4  is parallel to a vertical wall, instead of a ceiling, in such a way that the light beam from the first light source  2  enters the room  6  being parallel to floor and at an angle of about 45° with respect to the vertical wall. In this embodiment, the parabolic cylinder is obtained by translation of the generator parabola in a direction parallel to the y-axis, rather than the x-axis, this being the configuration that allows the largest depth perception for an observer, whose eyes are aligned in the x-axis. Also in this case, given a height of the lighting system  1  beyond the vertical wall, the largest area of the first diffuser panel  4  can be obtained by adopting a shape elongated in the y-axis direction. 
     In a different embodiment, the reflective system  20  may include a second mirror  24 , as shown for example in  FIG. 7 . That is, the aforementioned first mirror  22  may be one mirror of a plurality of mirrors of the reflective system  20 , which causes the last deviation of the light path along which the light rays generated by the first light source  2  are conveyed onto the first diffuser panel  4 . 
     The second mirror  24  is optically interposed between the first mirror  22  and the first light source  2 . In this case, the abovementioned geometric condition does not change, because this condition refers to the overall reflective system  20 . It is thus irrelevant if the light ray RL 2  is generated by reflection of the light ray RL 1  on just the first mirror  22 , or on the first and second mirrors  22 ,  24 . Similarly, the reflective system  20  may include additional reflective elements (not shown). 
     Any of the first and second mirrors  22 ,  24  may be plane or have different shape. In particular, as shown in  FIG. 8 , embodiments are possible wherein both the first and second mirrors  22 ,  24  are shaped as portions of two corresponding parabolic cylinders, that are generated by generator parabolae that lie in orthogonal planes, and that are translated along orthogonal directions and thus perform light collimation in orthogonal panes. In the present embodiment, for example, the first mirror  22  is similar to the parabolic cylinder shown in  FIG. 6 , whereas the second parabolic cylinder, which forms the second mirror  24 , is obtained by taking a second generator parabola in the xH plane and by translating it along the y-axis direction, thus obtaining a beam-divergence reduction in the xH plane. Moreover, the Applicant has verified that a good collimation in all the directions for the light rays reflected by the second mirror  24  towards the first mirror  22  is obtained when the second generator parabola has its axis oriented in a direction substantially parallel to the axis of the generator parabola of the first mirror  22 , provided that the two parabolic cylinders are arranged so that they share a common focus (or, more precisely, so that the generator parabolae share a common focus, where the position of the focus of the generator parabola of the first mirror  22  accounts for the reflection by mirror  24 ) and the first light source  2  is arranged substantially in such a common focus. 
     The embodiment shown in  FIG. 8  enables to use a diffuser panel having a length along the y-axis which is considerably larger than the length along the x-axis, and therefore enables to maximize the area of the first diffuser panel  4 , the vertical space occupied by this embodiment being equal to the one occupied in case of square panel. Moreover, this embodiment enable to generate a light beam that impinges on the first surface S 1  with a reduced divergence along both the x-axis and the y-axis (i.e. along planes that intersect the xy plane along lines that are parallel to the x-axis and the y-axis, respectively). Therefore, the transmitted light rays have a divergence similar to sunrays. This condition contributes in creating a large depth of field perception even when the first light source  2  is not in the observer&#39;s visual field. Moreover, since the first light source  2  is arranged close to the common focus, the size of the first light source  2  as perceived by the observer does not depend on the distance. Finally, the illumination of a diffuser panel elongated along the y-axis direction is here made possible by starting from a light beam which impinges on the second mirror  24  with substantially a same divergence both in the plane of incidence and in the orthogonal plane, i.e. by efficiently using a light source that generates a light beam having a square-like cross-section. This result, that is achieved by performing the reduction of the initial beam divergence in two separate steps in the two orthogonal directions, represents an advantage respect to the case of a single parabolic cylinder, for which asymmetric beams are required, as described below. 
     Irrespective of the details of the first diffuser panel  4  and the reflective system  20 , the first light source  2  may have, as explained before, an emitting surface S f  with a circular or elliptic shape. In particular, the emitting surface S f  may have an elliptic shape whenever the reflective system  20  includes at least one paraboloid with cylindrical symmetry, so that the different magnifications introduced along the x-axis and y-axis are compensated, thus allowing for the creation of a circle-shaped light spot on the observer&#39;s retina. 
     As shown in  FIGS. 9 a  and 9 b   , the first light source  2  may be formed by a set of emitting devices  50 . Each emitting device  50  is formed by a LED source  52  and a corresponding compound parabolic concentrator (“CPC”)  54  of rectangular type, which features an input aperture IN and an output aperture OUT; the input aperture IN and the output aperture OUT may be respectively shaped as a first and a second rectangle, parallel and aligned to each other, the first rectangle having an area smaller than the second rectangle. Moreover, the first rectangle has a different ratio between the lengths of its axes of symmetry than the second rectangle. For example, the first rectangle has a greater ratio, i.e. it is more elongated, than the second one. The LED source  52  may be formed by an array of LED emitters (not shown) and is arranged close to the corresponding input aperture IN, in such a way that radiation emitted by the LED source  52  is coupled to the CPC concentrator  54  through the input aperture IN, and exits the output aperture OUT. Other types of reflective concentrators are however possible; similarly, light emitting devices other than LEDs may be used. 
     The light beam generated by each emitting device  50  has a rectangular cross-section, its divergence being maximum in the plane containing the axis of the beam itself, i.e. containing the optical axis  56  of the pair formed by the concentrator  54  and the corresponding LED source  52 , and the greater of the symmetry axes of the rectangle defined by the output aperture OUT, designated by  57  in  FIG. 9 a   . In case of a different output aperture OUT, the plane of maximum divergence would be spanned by the elongation direction, i.e. the direction of maximum extension of the outlet OUT, and the optical axis  56 . 
     The amount of the beam divergence in each of the maximum-divergence plane and its orthogonal plane (also this latter containing the optical axis  56 ) scales with the ratio between the lengths of the corresponding side of the input rectangle, d IN , and of the corresponding side of the output rectangle, d OUT , and it is in particular equal a to twice the arcsine of this ratio, i.e. arcsin(d IN /d OUT ) In this regard, not only the areas, but also the shapes of the input and output apertures have to be different, in order to ensure different divergences in the two orthogonal planes. 
     The size of the input aperture IN should be chosen so that it encloses the LED source  52 . In the embodiment of  FIGS. 9 a  and 9 b   , each concentrator  54  has a funnel-like shape and is formed by four parabolic reflective surfaces, each of which is one-dimensionally curved and has a generator parabolas lying either in the maximum-divergence plane or in its orthogonal plane, all the generator parabolae having their focuses in the input plane in which the input aperture IN lies. Moreover, the four parabolic reflective surfaces have the same length along the direction of the optical axis  56 . 
     According to an embodiment, all the emitting devices  50  are equal, and the concentrators  54  are arranged so that the input apertures IN lie in the same input plane P_IN, and the output apertures OUT lie in the same output plane P_OUT. In particular, the concentrators  54  are arranged one next to the other, with output apertures OUT adjacent to each other, i.e. they are tightly packed, so that the maximum average luminance of the emitting surface S f  is assured; moreover, the number and arrangement of the concentrators  54  are such that the surface composed by the union of all the output apertures OUT approximates a circular surface, although embodiments are possible in which the composed surface approximates an elliptical shape. Finally, all the emitting devices  50  are arranged so as to have their axis  56  oriented in the same direction. In this circumstance, the first light source  2  has an own “plane of greater divergence”, which is the plane that contains the barycenter O″ of the emitting surface S f  and that is parallel to the planes of maximum divergence of the emitting devices  50 ; furthermore, the first light source  2  has an “axis of greater divergence”  58 , given by the intersection between the plane of greater divergence of the first light source  2  and the emitting surface S f  of the first light source  2 . Even if the axis of greater divergence has been introduced for the case of a plurality of rectangular concentrators  54 , it is evident that other shapes of funnel-like concentrators  54  having output apertures elongated along parallel axes  57  lead to a light source still having an axis of greater divergence, which is parallel to the axes  57 . 
     The first light source  2  shown in  FIGS. 9 a  and 9 b    allows to decouple the light beam characteristics, and in particular the shape of its cross-section and its divergence, from the shape of the emitting surface S f , without introducing any loss. In the present case, wherein the emitting devices  50  generate identical “unit light beams” having rectangular cross-section, the distances between the centers of output apertures OUT are small compared to the width of the composite light beam formed by the summation of all the unit light beams, this summation occurring because of the propagation of the composite beam and the divergence of each unit light beam. In practice, the unit light beams melt into one composite light beam that has the same rectangular cross-section and the same divergence as a single unit light beam. In other words, at distances which are great in respect to the diameter of the emitting surface S f , the composite light beam has the same shape and divergence of the beam generated by a single emitting device  50 , since it is formed by a plurality of identical unit light beams which are slightly shifted one respect to the other. Therefore, the embodiment shown in  FIGS. 9 a  and 9 b    allows for a composite beam to be generated, having a section, in a plane perpendicular to the axis of the composite beam itself and at a desired distance from the first light source  2 , which is a rectangle of desired area and shape. Furthermore, this embodiment enables to create a light source having an emitting surface S f  which can have any shape, e.g. a circular or an elliptical shape. In what follows, this light source is referred to as “rectangular-beam source”. It should be stressed that the result is not obtained by relying upon a knife-cut aperture and imaging optics as performed, e.g., for standard, theater-like, stage-light projectors, where the beam cut causes high transmission losses. Therefore, the rectangular-beam source allows to minimize the overall energy consumption. 
     Although not shown, a different embodiment is possible, in which the first light source comprises a plurality of emitting devices, each of which is formed by a LED source having square shape, and a corresponding compound parabolic concentrator of a square type, which features a square input aperture and a square output aperture. In such a way, each emitting device generates a square beam, which has the same divergence in the two orthogonal directions (i.e. in the two planes containing the concentrator axis and, respectively, the two axes of the output aperture that are parallel to the sides of the output aperture). In particular, the present embodiment allows to generate a square beam with a desired divergence, for an arbitrary shape of the emitting surface S f . In what follows, this first light source will be referred to as the “square-beam source”. 
     In a further different embodiment (not shown), the first light source comprises a plurality of emitting devices, each of which s formed by a LED source having a circular, and a corresponding compound parabolic concentrator of a circular type (not shown), which features a circular input aperture and a circular output aperture. In this case, the first light source generates a beam with a circular symmetry. Therefore, this first light source allows to generate a circular beam with a desired divergence, for an arbitrary shape of the emitting surface S f . In what follows, this first light source will be referred to as the “circular-beam source”. 
     In case the reflective system  20  is made of one or more plane mirrors, or in case the reflective system  20  includes a single mirror having the shape of a parabolic cylinder, the rectangular-beam source allows to obtain a luminous spot SP which is elongated along the y-axis, i.e. a luminous spot SP which circumscribes the first surface S 1  of the first diffuser panel  4 , the first surface S 1  having the shape of a rectangle elongated along the y-axis. In both cases, the rectangular-beam source is oriented so that its axis of greater divergence  58  is “mapped” by the reflective system  20  onto the y-axis, so as to reduce the complexity of the lay-out of the reflective system. In the context of the present invention, the reflector system is said to map the axis of greater divergence onto the y-axis if, given a narrow bundle of light rays including the carrier ray, originating at barycenter O″ of the emitting surface S f  and lying in the plane of greater divergence, the reflective system  20  causes the ray bundle to cross the first diffuser panel  4  along a line tangent to the y-axis. For example, if the reflective system  20  is such that the carrier ray is folded in a single plane, the rectangular-beam source is oriented with the axis of greater divergence  58  parallel to the y-axis. 
     In case the reflective system  20  comprises two mirrors having the shapes of parabolic cylinders having orthogonal cylinder axes, the use of the square-beam source is advantageous. In this case, in fact, it is possible to rely upon the fact that the initial divergence of a square beam is reduced at two different distances from the first light source  2 , for the purpose of obtaining a luminous spot SP elongated along the y-axis. This embodiment allows to achieve an optimal coupling between commercially available LED emitters, that are typically square-shaped, and the concentrators. 
     Furthermore, in case the reflective system  20  comprises a mirror having the shape of a circular paraboloid, the use of the circular-beam source is advantageous. In this case, however, the light source  2  may be made of a single circular CPC, which is coupled to a circular LED assembly, this solution allowing to obtain a circular emitting surface S f . 
       FIG. 10  shows an additional embodiment where the first light source  2  is again formed by identical CPC concentrators  54 , their input apertures IN and output apertures OUT being again, exemplarily, rectangular-shaped. In this case, however, a mask  60  is applied on the overall aperture formed by the output apertures OUT; the mask  60 , which lies in the output plane P_OUT, defines a mask aperture  62 , having a shape which is a rounded rectangle with an area larger than the area of a single output aperture OUT. In particular, the mask  60  may be formed by a layer of optically absorbing material, so that the radiation can cross the output plane P_OUT only through the mask aperture  62 . In this way, the first light source  2  is still perceived as having basically a circular emitting surface S f . The Applicant further noticed that the mask  60  does not substantially distort the shape of the luminous spot SP formed in the plane of first surface S 1 . 
     Independently from the number and the shape of the mirrors which form the reflective system  20 , the lighting system  1  may include a second light source, which comprises a diffused-light emitting layer, this layer being transparent, or at least partially transparent. In use, the additional light source emits diffused light from the emitting layer independently from being illuminated by the first light source  2 , while an observer that looks through the diffused-light emitting layer of the second light source can see the first light source  2  beyond this emitting layer. In the present description, the term “transparent” is used for indicating the so-called “see through” optical property, i.e. the property of an optical element of transmitting image-forming light. More specifically, considering a light beam generated by a point-like D65 standard illuminant source set at a great distance from the diffused-light emitting layer (a beam, thus, constituted by light rays parallel to one another) and directed perpendicularly to the diffused-light emitting layer, so that a portion of the diffused-light emitting layer is illuminated by a certain bundle of rays generated by the D65 standard illuminant, the diffused-light emitting layer is defined as partially transparent if at least 50%, preferably 70%, more preferably 85% of the light rays of the bundle are transmitted by the diffused-light emitting layer within a cone with a FWHM angular aperture not larger than 8°, preferably 4°, most preferably 2°. For the sake of completeness, it has further to be noted that also the first diffuser panel  4  is partially transparent. 
     From a practical point of view, given a standard illuminant (e.g. a D65 source) which emits light uniformly from a circular emitting surface, and given a standard observer who sees the emitting surface under a conical solid angle of 8°, preferably 4°, most preferably 2°, the luminance of the emitting surface as perceived by the observer when the partially transparent diffused-light emitting layer is interposed between the observer and the emitting surface is hence at least 50%, preferably at least 70%, more preferably at least 85% of the corresponding luminance perceived by the observer when the diffused-light emitting layer is absent. 
     All that having being said, as shown in FIG.  11 , the second light source (designated by  68 ) may be arranged parallel to the first diffuser panel  4 , e.g. above it, and for example in direct contact with it. 
     The second light source  68  may comprise a second diffuser panel  64  and an illuminator  66 , the second diffuser panel  64  being shaped as a light guide side-lit by the illuminator  66 , the illuminator  66  being formed, as an example, by a linear stripe of LEDs or a fluorescent tube lamp, so that light emitted by the illuminator  66  propagates in guided-mode inside the second diffuser panel  64 , which diffuses it homogeneously. The second diffuser panel  64  may be, for example, a commercial diffuser suitable for side-lighting as, e.g., “Acrylite® LED” or “Plexiglas® LED EndLighten”. Moreover, as shown in  FIG. 11 , the thickness along the axis H of the second diffuser panel  64  is negligible compared to thickness along a direction K perpendicular to the axis H. 
     In a particular configuration, the second diffuser panel  64  is formed by a third material (e.g., a material chosen from among the materials previously listed with reference to the first material), wherein microparticles of a fourth material (e.g., ZnO, TiO 2 , ZrO 2 , SiO 2 , Al 2 O 3 ) are dispersed; such third and fourth material do not absorb light with wavelengths in the visible range. In particular, the diameters of microparticles may range from 2 μm to 20 μm. 
     When in use, part of the radiation guided by the second diffuser panel  64  exits the second diffuser panel  64  while propagating along the second diffuser panel  64 , due to diffusion by microparticles of the fourth material. Since the second diffuser panel  64  has negligible thickness along the axis H compared to the direction K, the second diffuser panel  64  is basically transparent to radiation propagating along the axis H, but works as a diffuser for radiation propagating along the direction K. 
     Moreover, assuming that the second diffuser panel  64  is delimited on the upper and the lower side by a third and a fourth surface S 3 , S 4 , at least one out of such third and fourth surface S 3 , S 4  may be surface finished to introduce roughness. Such roughness contributes to the diffusion by the second diffuser panel  64  of the light generated by the illuminator  66 , the diffusion process being virtually homogeneous along any direction parallel to the direction K. In a per se known manner, roughness may be designed so that great part of the light generated by the illuminator  66  is scattered mainly through one between the third and the fourth surface S 3 , S 4 , and in particular towards the first diffuser panel  4 . In case at least one between the third and the fourth surface S 3 , S 4  features roughness, no microparticles need to be dispersed in the second diffuser panel  64 . In any case, roughness may be present on both the third and the fourth surface S 3 , S 4  of the second diffuser panel  64 . 
     In a different configuration, the second light source  68  includes a substantially transparent emitting surface, which is made of an OLED film. The OLED film is also capable to generate diffused light with controlled color and intensity, being at the same time transparent to the light that crosses it along a direction perpendicular to its surface. 
     The second light source  68  allows to change the color and intensity of the diffused-light component generated by lighting system  1 , basically without changing the color and intensity of the transmitted component. For this aim, it is possible to act on the color and intensity of the light emitted by the second light source  68 . 
     For example, aiming at reproducing the characteristics of late afternoon light, a lamp with low CCT, e.g. 2500 K, may be used as the first light source  2 ; in this way, the color of the transmitted component is similar to the color of sunlight before sunset. Without the second light source  68 , the color of the component scattered by just the first diffuser panel  4  would be different from the color of the corresponding natural component. As a matter of fact, what happens in nature is that the sky above the observer is lit by white sunlight, i.e. by sunlight that has not crossed the atmosphere yet, having a CCT which is equal approximately to 6000 K, a much higher value than the lamp&#39;s CCT. As a consequence, the CCT of the light scattered by the sky above the observer in the late afternoon hours is significantly higher than the CCT of the light scattered by the first diffuser panel  4 , in case the first light source  2  that illuminates this latter has low CCT. However, if the second light source  68  is used, and particularly if the second diffuser panel  64  is used together with the illuminator  66 , and this latter is made of an ensemble of red, green, blue (“RGB”) LED emitters, it is possible to adjust the luminous flux for each of such three elements; this allows for the second diffuser panel  64  to generate a scattered component having color and intensity which are such that the overall component that exits the first diffuser panel  4  and is scattered by the first and the second diffuser panel  4 ,  64 , has the desired color. In other words, the second source  68  allows for decoupling the color of the transmitted component from the color of the scattered component. Moreover, if a lamp with adjustable CCT is used as the first light source  2 , the variation of natural lighting at different times of the day can be reproduced. 
     Other embodiments are also possible where the second light source  68  is placed under the first diffuser panel  4 , in such a way that the light generated by the first light source  2  passes through the first diffuser panel  4  before passing through the second diffuser panel  64 . Moreover, additional embodiments are possible where the first and second diffuser panels  4 ,  64  are physically separated. 
     Embodiments are also possible where the second light source  68  is used in the absence of the first diffuser panel  4 , i.e. in the absence of the Rayleigh panel. In this case the axis H is a line perpendicular to the diffuse-light emitting layer and crossing the barycenter of the diffuse-light emitting layer. 
     In view of the above, all the disclosed embodiments refer to a system including a first light source, a diffused-light generator and a dark chamber, wherein the diffused-light generator is shaped as a layered component delimited by an inner surface (facing the dark chamber) and an outer surface (facing the room), and the first light source is configured to emit a visible-light beam, and the dark chamber is optically coupled to the room via the diffused-light generator. Moreover, the diffused-light generator is configured to receive the visible-light beam, and to be at least partially transparent to the visible-light beam, and to transmit at least part of the visible-light beam, and to emit visible diffused light from the outer surface, and to generate a transmitted light having CCT lower than the CCT of the visible diffused light. The diffused-light generator may be substantially free from chromatic absorption or reflection, i.e. from preferentially absorbing or reflecting a limited portion of the visible-light spectrum with respect to another portion. 
     More in particular, the CCT of the diffused light is higher than the CCT of the transmitted light; still more in particular, the CCT of the transmitted light is not greater than the CCT of the light beam generated by the first light source. Furthermore, as already said, in the context of the present invention, the light “transmitted” by an optical element is meant as the portion of the light rays impinging onto the optical element which cross the optical element without suffering significant angular deviation, e.g. being deviated by an angle smaller than 0.1°. Therefore, an optical component is said to “transmit at least a portion” of an impinging light beam whenever it produces a transmitted light component. 
     As explained above, the diffused-light generator may be formed by a Rayleigh diffusing layer, i.e. a layer which selectively diffuses the short-wavelength component of the luminous radiation coming from a main light source, this Rayleigh diffusing layer being shaped, e.g. as a flat panel (as in the case of the first diffuser panel  4 ), or as a curved panel (not shown). In addition, or alternatively, the diffused-light generator may be formed by a diffused-light source, i.e. a light source that emits diffused light from an extended layer orthogonal to the axis H, independently from the light received from the main light source. In the case of using just the diffused-light source, this source does not operate for correcting the color of the diffused light as produced by, e.g., the first diffuser panel  4 , but for generating the entire diffused component of the light emitted by the lighting system. In certain embodiments, the diffused-light generator may have an elongated shape, in the sense that a first circle inscribed into the inner surface has a diameter at least 1.5 times smaller, preferably two times smaller than a second circle circumscribed to the same inner surface. 
     Furthermore, the considerations regarding the presence of a Rayleigh diffusing layer and/or a light source emitting diffused light apply also to the variants which will be described in the following. 
     The advantages brought by the present lighting system are made evident by the previous description. 
     In detail, the present lighting system allows for an observer to perceive the existence of an unlimited space beyond the diffused-light generator, similarly to what happens in nature when the sky and the Sun illuminate a room through a window. Such result is due to the presence of the dark chamber, which is coupled to the room by means of the diffused-light generator. The dark chamber allows for perceiving an homogeneous black background for every direction along which the first and/or second diffuser panels are observed. Moreover, such effect is improved by adopting a suitable observer-to-source distance (and thus, a first and/or second panel-to-source distance), and/or by using a reflective system which reflects light rays in such a way that they feature a limited range of slopes. 
     Furthermore, some embodiments of the present invention give rise to the aforementioned breakthrough effect while limiting the space occupied by the lighting system. In particular, the embodiment shown in  FIG. 2  is an off-axis lighting system, namely a system wherein the light source and the first diffuser panel are not aligned, which allows to reducing the space occupied by the system itself, without spoiling the quality of the illumination. 
     Eventually, it is evident that modifications and variations can be made to the present lighting system without departing from the scope the present invention, as defined by the appended claims. 
     For example, the position of the light source with respect to the focus/focuses of the optical elements of the reflective system can be different than those described. Furthermore, instead of, or in addition to a converging mirror, the reflective system may comprise a diverging mirror. In addition, in order to achieve full divergence removal, at least along the y-axis direction, more complex shapes of the mirrors (e.g. free-form shapes) can be considered. 
     Furthermore, the form of the dark structure may be different from what has been previously shown. In fact, in order to provide a substantially uniform background, it is sufficient to form a dark structure the geometrical and/or light absorbing characteristics of which are such that, when the first light source  2  (and the illuminator  66 , if present) is on, a first structure condition applies, described hereinbelow, with reference to the  FIG. 12 . For simplicity, and without any loss of generality, in  FIG. 12  the first light source of the point-like type; furthermore, the dark structure is designated by  300  and is without corners, without that implying any loss of generality. The first structure condition described hereinbelow is in any case applicable also to the embodiments previously described, as an example by referring it to the characteristics of the support element  10  and the inner layer  12 . Furthermore, in  FIG. 12  angles are shown in a qualitative manner. 
     In detail, the abovementioned first structure condition provides that, given a direction sheaf (e.g., a conical sheaf)  200  with a top angle of at last 0.1 sterad and a sheaf axis  210 , at any first point  220  of at least a portion of the second surface S 2  having an area equal to at least 50%, preferably 80%, even more preferably 100%, of the area of the entire second surface S 2 , a first and a second luminances of the first point  220 , which hereinafter will be referred to as the first and second background luminances, differ from one another by no more than 50% of the first background luminance. In greater detail, the first and second background luminances are measured in a first and second observation directions  230 ,  240 , respectively, the first observation direction  230  being parallel to any of the directions of the direction sheaf  200  and being non-parallel to any of the local dazzling directions  250 , the second observation direction  240  being set apart from the first observation direction  230  by an angular distance in the range between 0.3° and  10  and being non-parallel to any of the local dazzling directions  250 , the local dazzling directions  250  being the directions which are set apart by less of  30  from any direction  260  under which any point of the first light source  2  is seen from the first point  220  (given the assumption of point-like source, only one direction  260  is present). In greater detail, each of the first and second background luminances is formed only by the light rays which have hit the dark structure and have never passed through the room  6  (not shown in  FIG. 12 ), hence which have never crossed the second surface S 2  coming from the room  6 . 
     As an example, referring to any of the first and second background luminances, it can be measured under the assumption of coupling the first diffuser panel  4  to a first anechoic chamber in the visible range, namely by assuming that the room  6  absorbs the 100% of the impinging light, and by carrying out the steps of: 
     after substituting the dark structure  300  with a second anechoic chamber in the visible range, measuring the luminance L 1  of the abovementioned first point  220 , in the first observation direction  230 ; and afterwards 
     removing the second anechoic chamber and providing the dark structure  300 ; and afterwards 
     measuring the luminance L 2  of the first point  220 , still in the first observation direction  230 ; and 
     computing the difference between the luminance L 2  and the luminance L 1 . 
     As shown in  FIG. 12 , the sheaf axis  210  may coincide with the direction  260  under which the first light source  2  is seen from the first point  220 . Furthermore, the direction sheaf  200  and the attitude thereof relative to the first diffuser panel  4  are invariant with respect to the position of the abovementioned first point  220  on the second surface S 2 . 
     As previously mentioned, the first structure condition may be fulfilled also by the other embodiments. Therefore, it is possible, as an example, that the support element  10  and the inner layer  12  are different from what has been shown, but anyway such as to fulfill the first structure condition. As an example, the inner layer  12  may coat only a portion of the support element  10 , which in turn may be formed in more than one piece. To this regard, at least part of the dark structure may be formed by a housing of the first light source  2 , or by one or more screens; similarly, the support element  10  may feature one or more optical apertures, as an example closed by corresponding elements which are matt in the visible range, or overlain by brickworks elements. 
     The dark structure  300  may further be configured to fulfill a second structure condition, and namely to prevent that, when the first light source  2  is on, the abovementioned first background luminance is greater than a luminance threshold value equal to 30% of the total luminance of the first point  220  in the first observation direction  230 , this total luminance being measured under the assumption of absence of light rays coming from the room  6 , and hence by means of the abovementioned first anechoic chamber. Furthermore, embodiments are possible, wherein the second structure condition is fulfilled, but the first structure condition no. Furthermore, the second structure condition may be fulfilled also by the other embodiments. Therefore, as an example it is possible that the support element  10  and the inner layer  12  are different from what has been shown, but such as to fulfill, in any way, the second structure condition. 
     A further example of dark structure, applicable to all the embodiments previously described, is shown in  FIG. 13 . In this example, only a portion of the support element  10  is coated by a corresponding portion of the inner layer, which is referred to as the absorbing patch  310 . The absorbing patch  310  has an absorbing coefficient in the visible range substantially uniform and/or an absorbing coefficient in the visible range at least equal to 70%, preferably 90%; furthermore, the absorbing patch  310  is preferably edge-free and has an area at least equal to 50%, preferably 80%, of the first surface S 1 . 
     A further embodiment is shown in  FIG. 14 , wherein, for simplicity, reference is made to the case in which the second diffuser panel  64  and the illuminator  66  are absent; furthermore, in  FIG. 14  the dark structure and the reflective system are not shown. In this embodiment, a visual reference element is arranged downstream the first diffuser panel  4 , such, as an example, a reflecting surface  320 , which is delimited by an edge and is arranged so that at least one portion of it is lit, together with a corresponding portion of the edge, by the light generated by the first light source  2  and transmitted by the first diffuser panel  4 . This portion of the reflecting surface  320  has an area at least equal to 50%, preferably 70%, even more preferably 100%, of the area of the entire reflecting surface  320 . Furthermore, this portion of the reflecting surface  320  is such that the shortest path among the optical paths connecting the first light source  2  to the reflecting surface  320  has a length equal to at least 50%, preferably 70%, even more preferably 100%, of the maximum distance between any two points of this portion of the reflecting surface  320 . 
     As shown in  FIG. 15 , the visual reference element may be formed by a diaphragm  350  between two rooms, which delimits a corresponding aperture, which puts the two rooms in optical communications. This aperture thus forms an immaterial surface (with the exception of the edge) and has a respective portion, delimited by a corresponding portion of the edge, which fulfills the requirements mentioned a little while ago with reference to the abovementioned portion of the reflecting surface  320 . 
     In practice, referring to the embodiments shown in  FIGS. 14 and 15 , it occurs that, the closer to one is the ratio of the speed at which the observer moves to the speed at which he sees the first light source  2  moving with respect to the visual reference (the edge of the abovementioned portions of the reflecting surface  320  and the aperture of the diaphragm  350 ), i.e. the closer to the visual reference is the observer, the greater is the depth effect induced by the so-called motion parallax. Furthermore, the abovementioned portions of the reflecting surface  320  and of the aperture of the diaphragm  350  may have narrow areas, as an example equal to 1/10, preferably 3/10, even more preferably ½, of the area of the second surface S 2 .