WO2019224688A1

WO2019224688A1 - Chromatically diffusing multi-layer film structure for sun-sky-imitating lighting systems

Info

Publication number: WO2019224688A1
Application number: PCT/IB2019/054134
Authority: WO
Inventors: Vittorio Fossati; Simone BONANOMI; Sandro USSEGLIO NANOT
Original assignee: Coelux S.R.L.
Priority date: 2018-05-23
Filing date: 2019-05-20
Publication date: 2019-11-28
Also published as: JP3235338U; CN214927806U; IT201800005634A1

Abstract

A process (200) for producing a chromatically diffusing multi-layer film structure (10", 10"') for generating a sun-sky-imitating effect in lighting systems (40,60) comprising the steps of coating (220) at least one first flexible substrate film layer (101,102) with at least a scattering system (103) thereby creating a multi-layer film structure (101,102,103) comprising a first flexible substrate layer (101,102) and at least one coating layer of scattering system (103) having substantially uniform thickness, wherein the scattering system (103) comprises a dispersion of scattering elements configured to preferentially scatter short-wavelength components of impinging light with respect to long-wavelength components of impinging light thereby implementing a light scattering in Rayleigh-like regime, wherein the scattering elements are dispersed in a polymeric transparent matrix; and wherein a number N of scattering elements per unit area of the scattering system coating layer (103) in dependence of an effective particle diameter D = d n_h ( D given in [ meters ] ), is within the range defined by N ≥ N _min = Formula (I), and N ≥ N _max= Formula (II); or within the range defined by = N ≥ N _min = Formula (III), and N ≥ N _max= Formula (IV) laminating (230) the at least one first flexible substrate film (101,102) coated with the layer of scattering system (103) with a second flexible substrate film (102,101) so as to sandwich the layer of scattering system (103) between the pair of flexible substrate films (101,102); and subjecting (240) the multi-layer film structure (101,102,103) to solidification so as to obtain a chromatically diffusing film structure (10', 10"') at least comprising a first and a second flexible substrate layers (11,16) and a flexible chromatic diffusive layer (12) coupled to and sandwiched between the first and the second flexible substrate layers (11,16) and obtained through solidification of the coating layer of scattering system (103).

Description

CHROMATICALLY DIFFUSING MULTI-LAYER FILM STRUCTURE FOR

SUN-SKY-IMITATING LIGHTING SYSTEMS

Technical Field

[01] The present disclosure relates generally to lighting systems, in particular to lighting systems for optically providing a widened perception/impression of the ambient space and in particular for imitating natural sunlight illumination. Moreover, the present disclosure generally relates to a multi-layer structure with chromatic features used in such lighting systems as such or as part of chromatically diffusing panels.

Background

[02] Several applications of the applicants such as EP 2 304 478 Al, EP 2 304 480 Al, and WO 2014/076656 Al disclose lighting systems that use a light source producing visible light, and a panel containing nanoparticles used in transmission, i.e. the light source and the illuminated area are positioned on opposing sides of the panel.

[03] Further applications of the applicants such as WO 2015/172821 and WO

2016/134733 disclose lighting systems that use a light source producing visible light, and a chromatic mirror panel structure containing nanoparticles used in reflection.

[04] During operation of those lighting systems, the diffusing panel receives the light from the light source and acts in transmission as a so-called Rayleigh diffuser, namely it diffuses light rays similarly to the earth atmosphere in clear-sky conditions. Accordingly, the lighting systems generate directional light with lower correlated color temperature (CCT), which corresponds to sunlight and generates shadows in presence of lit objects, and diffuse light with larger CCT, which corresponds to the light of the blue sky and, in principal, can generate shadows with a blue tinge.

[05] Generally, it is known to obtain Rayleigh diffusers by using nano-sized particles able to diffuse light by Rayleigh-like scattering. By way of example, application WO

2017/085079 of the applicants describes a stratified panel structure containing nanoparticles. The stratified panel structure of WO 2017/085079 is made of two cover panels - at least one of which is a transparent panel such as a glass sheet - wherein the two cover panels are bound together by an adhesive transparent polymeric layer and to at least one of the inner faces, a nanoparticle-based Rayleigh-like diffusing paint is applied.

[06] Even if the known stratified panel structures provide very satisfying results with regard to its Rayleigh-like diffusing properties, the applicants recognized that they are not free of drawbacks. In particular, the resulting panel structure is a quite bulky and heavy object linked to high transportation and logistics costs. Moreover, the panel structure is also rigid and can thus be used in a limited number of applications.

[07] Moreover, from patent application EP 2 304 478 A 1 cited above, it is known to mix together PMMA pellets and small quantities of nanoparticles using a double screw extruder in order to produce new pellets containing the nanoparticles. The nanocomposite pellet material is then shaped in the desired way through moulding. Even though this technique allows obtaining an extruded panel which distinguishes from the stratified panel structure by its lower weight, it revealed to be not suitable for providing the effect required for sun-sky- imitation.

[08] In fact, the extruded polymeric panel is negatively affected by strong variations in the nanoparticle distribution within the PMMA. The non-uniformity of the nanoparticle distribution leads, on its turn, to a non-uniform coloring of the panel when lit by the impinging light. However, the uniformity in the sky colour is a feature of paramount importance in order to provide a natural and realistic imitating effect.

[09] Accordingly, the present disclosure is directed to improving or overcoming one or more aspects of prior art systems.

Summary of the Disclosure

[10] Applicant contemplated the problem of finding a process for obtaining a

chromatically diffusing film characterized by a simpler implementation and a higher manageability, and at the same time offering a valid alternative to the chromatically diffusing panel structures nowadays used in lighting systems generating a sun-sky-imitating effect, namely a chromatically diffusing film capable of providing a comparable degree of colour uniformity of the sky.

[11] Within the scope of the above problem, the Applicant considered the objective of producing an independent structural unit such as a chromatically diffusing film structure, configured to be used alone or to be joined to a cover panel by lamination, thereby offering the possibility of easily and quickly producing chromatically diffusing panels, or turning already existing standard panels into chromatically diffusing panels.

[12] Therefore, the technical problem underlying the present invention consists in devising a process for producing a chromatically diffusing film structure with Rayleigh-like diffusing properties.

[13] Accordingly, a first aspect of the present invention relates to a process for producing a chromatically diffusing multi-layer film structure for generating a sun-sky-imitating effect in lighting systems comprising the steps of:

- coating at least one first flexible substrate film layer with at least a scattering system, thereby creating a multi-layer film structure comprising a first flexible substrate layer and at least one coating layer of scattering system having substantially uniform thickness, wherein the scattering system comprises a dispersion of scattering elements configured to preferentially scatter short-wavelength components of impinging light with respect to long- wavelength components of impinging light thereby implementing a light scattering in Rayleigh-like regime, wherein the scattering elements are dispersed in a polymeric transparent matrix, and wherein a number N of scattering elements per unit area of the scattering system coating layer in dependence of an effective particle diameter D = d _¾

( D given in [ meters ] ), is within the range defined by d

or within the range defined by

- laminating the at least one first flexible substrate film coated with the layer of scattering system with a second flexible substrate film so as to sandwich the layer of scattering system between the pair of flexible substrate films; and

- subjecting the multi-layer film structure to solidification so as to obtain a chromatically diffusing film structure at least comprising a first and a second flexible substrate layers and a flexible chromatic diffusive layer coupled to and sandwiched between the first and the second flexible substrate layers, wherein the flexible chromatic diffusive layer is obtained through solidification of the coating layer of scattering system.

[14] Within the scope of the present description and appended claims, with the term “solidification” the process of passing from a liquid and/or viscous state to another state characterized by a reduced mobility of molecules with respect to the previous state is meant. The final state can result in a flexible or rigid structure depending on the chemico-physical properties of the material subject to solidification.

[15] Within the scope of the present description and appended claims with the expression “polymeric transparent matrix” a polymeric matrix is meant, characterized by a light transmittance in the visible wavelength range of at least 50%, preferably of at least 70% and more preferably, of at least 90%. More in general, within the scope of the present description and appended claims with the expression“transparent” generally refers to the feature of light transmittance in the visible wavelength range of at least 50%, preferably of at least 70% and more preferably, of at least 90%.

[16] Herein, as defined in the Standard Terminology of Appearance, ASTM international, E 284 - 09a, the transmittance is in general the ratio of the transmitted flux to the incident flux in the given conditions. The regular transmittance T(l) is the transmittance under the undiffused angle, i.e. the angle of incidence. In the context of the present disclosure, for a given wavelength and a given position on the chromatic diffusing layer, the regular transmittance is intended for non-polarized incident light with an incident angle

corresponding to the main light beam propagation.

[17] The applicants realized that by implementing the Rayleigh-like scattering by means of a layer provided onto a flexible substrate, may allow to obtain at the same time a very manageable chromatically diffusing film structure, as well as an increase in quality of the homogeneity of the Rayleigh-like scattering properties.

[18] In fact, differently than extrusion, by coating a scattering system on a flexible substrate, wherein the scattering system comprises scattering elements uniformly dispersed within a polymeric transparent matrix, a flexible film structure characterized by a uniform areal distribution of the scattering elements is obtained. The coating step allows obtaining both, a uniform spatial distribution of the scattering elements within the coated layer and a highly uniform layer thickness.

[19] Moreover, the solidification step assures that the uniform areal distribution, particularly the highly uniform layer thickness, is kept over time. This leads to a

simplification of a possible subsequent panel production, in that the multi-layer structure can be easily applied to a cover sheet by lamination without the risk of thickness

inhomogeneities .

[20] Besides, the so-obtained multi-layer film structure is light and flexible and can be wound into reels in order to optimize the storing space and the logistics and transportation costs.

[21] Not least, the number N of scattering elements per unit area identified by the Applicant assures that in lighting systems operating in transmission or in reflection, respectively, the amount of scattering is in the range where a chromatic effect takes place (Rayleigh-like regime), namely the scattering efficiency depends from the wavelength of the impinging light, and can be perceived. In other words, the number N of scattering elements per unit area identified by the Applicant assures both, that a bluish appearance is generated when white light impinges on the scattering system and that the layer of scattering system sandwiched between the two flexible substrate layers does not substantially modify or only marginally modifies the haze of the ensemble of the two flexible substrate layers. In detail, the layer of scattering system characterized by the above defined number N of scattering elements when inserted between the two flexible substrate layers which collectively have a haze of less than 1%, confers to the multi-layer film structure a haze which is in any case less than 45%, preferably less than 40%. Haze values are determined in accordance with Standard ASTM D1003 (Procedure A).

[22] A second aspect of the present invention relates to a chromatically diffusing multi layer film structure for generating a sun-sky-imitating effect in lighting systems comprising:

- at least one first and at least one second flexible substrate layers; and

- at least one flexible chromatic diffusive layer coupled to and sandwiched between the at least one first and at least one second flexible substrate layers, the chromatic diffusive layer comprising a solidified scattering system comprising a dispersion of scattering elements configured to preferentially scatter short-wavelength components of impinging light with respect to long-wavelength components of impinging light thereby implementing a light scattering in Rayleigh-like regime, wherein the scattering elements are dispersed in a polymeric transparent matrix and wherein a number N of scattering elements per unit area of the scattering system coating layer in dependence of an effective particle diameter D = d _¾

( D given in [ meters ] ), is within the range defined by d

or within the range defined by d

[23] In a variant of the invention, the chromatically diffusing multi-layer film structure is obtained through the production process of the invention.

[24] Advantageously, the chromatically diffusing film structure of the invention achieves the technical effects described above with regard to the production process.

[25] A third aspect of the present invention relates to a chromatically diffusing panel structure comprising a first panel element bonded to a chromatically diffusing film structure according to the invention.

[26] Advantageously, the chromatically diffusing panel structure of the invention achieves the technical effects described above with regard to the chromatically diffusing film structure.

[27] A fourth aspect of the present invention relates to a lighting system comprising:

a light source configured to generate a visible light beam;

a chromatically diffusing film structure or a chromatically diffusing panel structure as described above, illuminated by the light source, wherein

a portion of the light beam forms an illuminating light beam by passing through the chromatically diffusing film or panel structure essentially unscattered, and a portion of the light beam is Rayleigh-like scattered by the scattering elements within the chromatically diffusing film or panel structure.

[28] Advantageously, the lighting system of the invention achieves the technical effects described above with regard to the chromatically diffusing film and panel structure.

[29] The present invention in at least one of the above aspects may have at least one of the following preferred features; the latter may in particular be combined with each other as desired to meet specific implementation purposes.

[30] In a variant of the invention, the Rayleigh-like scattering relates to light having a wavelength spectrum extending in the visible spectrum, for example, over at least 150 nm.

[31] In a variant of the invention, the scattering elements of the scattering system comprise organic and/or inorganic nanoparticles which are transparent and/or do not substantially absorb light in the visible range and have an average size smaller than 350 nm, preferably having a peak at particle sizes below 350 nm in the particle size distribution. [32] In a further variant of the invention, the scattering elements of the scattering system have a diameter size comprised between 5 nm and 350 nm, preferably between 10 nm-250 nm, more preferably between 40 nm-l80 nm, even more preferably between 60 nm-l50 nm.

[33] In a further variant of the invention, the scattering elements of the scattering system have refractive index n_p and the matrix of the scattering system has refractive index n i, , wherein the ratio m between the particle and host medium refractive indexes (with

is in the range of 0.5 < m < 2.5 , preferably in the range of 0.7 < m < 2.1 , more preferably in the range of 0.7 < m < 1.9 .

[34] Advantageously, the specific sizes of scattering elements identified by the applicants together with the refractive index mismatch (and the scattering elements areal density) allows achieving the Rayleigh-like scattering phenomenon.

[35] In a variant of the invention, the number N of scattering elements acting as Rayleigh- like scatters per unit area of the chromatic diffusive layer in dependence of an effective particle diameter D = d _¾ ( D given in [ meters ] ), is within the range defined by

7.13 XKT²⁹ m² + 2

N ³ N min [meters ²], and N £

D⁶ m²— 1

2.03x 10 -27 m² + 2

N max [meters ²] ;

D⁶ m 2—1

for example, for embodiments aiming at simulating the presence of a pure

8.15 x 10 -28 m² + 2 clear sky, [meters ²], and N £ N =

D m —1

[meters ²] such

7.l lx l0 ²⁸ m² + 2

N max [meters ²], more specifically

D⁶ m² - 1 [meters ]

and

for example, for embodiments aiming at simulating a Nordic sky,

8.15x10 -28 m² +2 2.03x10 -27 m² + 2

N³ N, [meters²], and N £ L/ = [meters

D m 2 -1 D m 2 -1

²] such

8.15x10 -28 m² +2

[meters more specifically N³ A, [meters ²] and N £

D m 2 -1

-27

1.28x10 m² + 2

A_ = [meters²] .

D m² -1

[36] Advantageously, the number A of scattering elements acting as Rayleigh-like scatters per unit area identified by the Applicant allows achieving a pure clear sky and a Nordic sky, respectively, in lighting systems operating in transmission.

[37] In an alternative embodiment, the number N of scattering elements acting as

Rayleigh-like scatters per unit area of the chromatic diffusive layer in dependence of an effective particle diameter D = d _¾ ( D given in [ meters ] ), is within the range defined by

3.47x10 -29 m² +2 1.03x10 -27 m² +2

N³ —N min = [meters ²], and N < A,

D m 2 -1 2 [meters

D^b m -1

2

];

for example, for embodiments aiming at simulating the presence of a pure clear sky,

3.47x10 -29 m² + 2

N³ —N min = eters ² ters

D m 2 [m [me

-1

²] such

[meters more specifically N ³ N =

[meters ²] and

for example, for embodiments aiming at simulating a Nordic sky,

[meters ²], more specifically

6.37x10 m² + 2

A = [meters ²] .

D m ² - 1

[38] Advantageously, the number N of scattering elements acting as Rayleigh-like scatters per unit area identified by the Applicant allows achieving a pure clear sky and a Nordic sky, respectively, in lighting systems operating in reflection.

[39] In a further variant of the invention, the scattering elements of the scattering system comprise inorganic nanoparticles with its external surface having been functionalized with an organic coating.

[40] In a variant of the invention, the organic coating used for functionalizing the external surface of the inorganic nanoparticles are surfactant molecules.

[41] Advantageously, the functionalized inorganic nanoparticles prevent the formation of large aggregate/agglomerates and inhomogeneous distribution of nanoparticles, thereby improving the scattering properties of the diffusive layer.

[42] In a variant of the invention, the coating step comprises at least one between roll-to- roll coating processes such as slot die coating (e.g. extrusion coating, curtain coating), roller coating, gravure coating, spray coating, knife coating and metering rod (Meyer bar) coating. [43] The selected coating techniques are all suitable for creating a very uniform coating layer of the flexible substrate film(s). Accordingly, all the selected coating techniques allow achieving a diffusive layer having a highly uniform thickness.

[44] In a variant of the invention, the diffusive layer has a thickness comprised within 5 pm to 500 pm.

[45] Advantageously, the selected layer thickness allows achieving an areal density of scattering elements suitable for achieving the Rayleigh-like scattering phenomenon.

[46] In a variant of the invention, the polymeric transparent matrix is a pre-polymer or a polymer dissolved in a solvent.

[47] In a variant of the invention, the polymeric transparent matrix is a material having adhesive properties, preferably an optical adhesive like e.g. a thiol-ene optical adhesive.

[48] Within the scope of the present description and appended claims, with the term “adhesive properties of the material/layer” or“adhesive material/layer” a material/layer capable of generating an adhesive strength to a substrate of at least 0.05 N/cm is meant.

[49] Advantageously, the specific polymer matrix identified by the applicants allows achieving both, adhesive properties and a high degree of transparency and, consequently, a diffusive layer with very high optical transmission properties.

[50] According to an alternative variant of the invention, the polymeric transparent matrix does not have adhesive properties and the coating step comprises the deposition of at least one internal adhesive layer of a transparent adhesive material between the at least one first flexible substrate film and the layer of scattering system.

[51] Preferably, the transparent adhesive material of the internal adhesive layer is an optical polymeric adhesive like e.g. a thiol-ene optical adhesive.

[52] In a variant of the invention, at least one film between the first and the second flexible substrate films is a film chosen between a transparent film, a reflective film, an at least partially absorbing film and a peelable film.

[53] According to a variant of the invention, at least one film between the first and the second flexible substrate film is a polymeric film, like e.g. a PET (polyethylene

terephthalate) film or a PEN (polyethylene naphthalate) film. [54] Within the scope of the present description and appended claims with the expression “transparent polymeric film” a polymeric film is meant that is characterized by a light transmittance in the visible wavelength range of at least 50%, preferably of at least 70% and more preferably, of at least 90%. Herein, as defined in the Standard Terminology of

Appearance, ASTM international, E 284 - 09a, in general, the total transmittance is the ratio of the flux transmitted at all forward angles to the incidence flux.

[55] Within the scope of the present description and appended claims with the expression “reflective polymeric film” a polymeric film is meant that is characterized by a light total reflectance in the visible wavelength range of at least 50%, preferably of at least 70% and more preferably, of at least 90%. Herein, as defined in the Standard Terminology of

Appearance, ASTM international, E 284 - 09a, in general, the total reflectance is the ratio to the incident flux of the radiant or luminous flux reflected at all angles within the hemisphere bounded by the plane of measurement.

[56] Within the scope of the present description and appended claims with the expression “absorbing polymeric film” a polymeric film is meant that is characterized by a light absorption in the visible wavelength range of at least 50%, preferably of at least 70% and more preferably, of at least 90%. Herein, as defined in the Standard Terminology of

Appearance, ASTM international, E 284 - 09a, in general, the absorptance is ratio of the absorbed radiant or luminous flux to the incident flux.

[57] In an advantageous manner, the coating step comprises the deposition of two internal adhesive layers of a transparent adhesive material between the first and the second flexible substrate films and the layer of scattering system, respectively.

[58] Preferably, both layers of the two internal adhesive layers are coated onto the same first or second flexible substrate film, wherein the layer of scattering system is sandwiched between the two internal adhesive layers during deposition.

[59] Advantageously, the slot die coating allows the simultaneous deposition of a plurality of layers thereby achieving a multi-layer deposition at once and accordingly, a production time optimization. [60] According to an alternative variant of the invention, a first internal adhesive layer of the two internal adhesive layers is coated onto the first flexible substrate film and a second internal adhesive layer of the two internal adhesive layers is coated onto the second flexible substrate film.

[61] In a variant of the invention, the coating step comprises dropping the scattering system between the first and the second flexible substrate films so that at least one between the first and the second flexible substrate films drags a layer of scattering system between the two substrate films thereby simultaneously performing the coating and the lamination of both substrate films.

[62] This particular coating conveniently assures the creation of a highly uniform diffusive layer in terms of thickness of the same, during the lamination step itself. This allows production time optimization and the implementation of the production process by means of a compact system.

[63] In a variant of the invention, the scattering system additionally comprises microparticle spacer elements.

[64] In a further variant of the invention, the microparticle spacers are spherical microparticles, rod-shaped microparticles, disc-shaped microparticles or a combination thereof.

[65] In a further variant of the invention, the microparticle spacers have a size comprised within 5 to 500 micrometers

[66] Advantageously, in case of lamination of a second substrate film, the spacer elements allow to define a precise inter-layer thickness. In fact, if pressure is applied to the multi-layer film structure during lamination, the spacer elements prevent that irregularities of the layer thickness are generated, e.g. due to a not perfectly homogeneous pressure.

[67] In a variant of the invention, the solidification step comprises the polymerization of the diffusive layer.

[68] In a further variant of the invention, the polymerization of the diffusive layer takes place by UV curing and/or temperature curing and/or electron beam curing. [69] In another variant of the invention, the UV curing is performed by means of at least one emitting device which emits in the UV-visible range (100 nm - 600 nm), by preferably controlling the temperature during photopolymerization.

[70] In a further variant of the invention, the UV curing is performed by means of at least one emitting device which emits with UV light intensities in the range from 1 to

600 mW/cm2.

[71] In an alternative variant of the invention, the temperature curing is performed by means of electrical, microwave, IR heat sources or a combination thereof.

[72] In a variant of the invention, the temperature curing is performed at a temperature ranging from room temperature to 80 °C.

[73] In an alternative variant of the invention, the scattering system comprises an already polymerized polymer dissolved into a solvent and the solidification step comprises the drying of the solvent.

[74] In a variant of the invention, the chromatically diffusing multi-layer film structure comprises at least one additional layer coupled to the at least one first and/or the at least one second flexible substrate film and/or to the chromatic diffusive layer, the at least one additional layer being chosen between: an internal adhesive layer, an external adhesive layer, a peelable layer, a nanoparticle-based Rayleigh-like diffusing coating layer, a microparticle- based diffusing coating layer, a coating layer combining nanoparticle-based Rayleigh-like diffusing and microparticle-based diffusing, an antireflective coating layer and/or a mirror coating layer.

[75] In a further variant of the invention, the chromatically diffusing multi-layer film structure comprises an external transparent adhesive layer applied to the outer surface of one of the first and/or second flexible substrate films.

[76] In a variant of the invention, the chromatically diffusing panel structure comprises a second panel element and the chromatically diffusing multi-layer film structure is sandwiched between the first and the second panel element.

[77] In this case, the multi-layer film structure is advantageously protected against atmospheric agents like UV light, dust, humidity and so on, which could change its chromatic and optical properties. Furthermore, the resulting stratified panel structure may be strong enough in order to fulfill architectural requirements such as fire resistance, shock resistance, scratch resistance and the like.

[78] In a variant of the invention, the first panel element of the chromatically diffusing panel structure is provided with at least one of: a nanoparticle -based Rayleigh-like diffusing coating, a microparticle-based diffusing coating, a coating combining nanoparticle-based Rayleigh-like diffusing and microparticle-based diffusing, an antireflective coating, and a mirror coating.

[79] In a further variant of the invention, the reflective coating and/or the antireflecting coating is applied to an outer face of the first panel element.

Brief Description of the Drawings

[80] With reference to the attached drawings, further features and advantages of the present invention will be shown by means of the following detailed description of some of its preferred embodiments.

[81] According to the above description, the several features of each embodiment can be unrestrictedly and independently combined with each other in order to achieve the advantages specifically deriving from a certain combination of the same. In the drawings:

Fig. 1A is a schematic illustration of a first plant for producing a chromatically diffusing film structure according to the invention;

Fig. 1B is a schematic illustration of a second plant for producing a chromatically diffusing film structure according to the invention;

Fig. 1C is a schematic illustration of a third plant for producing a chromatically diffusing film structure according to the invention;

Fig. 2 is a flow chart of the process for producing a chromatically diffusing film structure according to the invention;

Figs. 3A to 3E show schematic cross-sections of some embodiments of a

chromatically diffusing film structure according to the invention; Figs. 4A to 4D respectively show a schematic cross-section of a first, a second and a third embodiment of a diffusing panel comprising the chromatically diffusing film structure according to the invention.

Figs. 5A and 5B are schematic drawings of a sun-sky imitating lighting system using a chromatically diffusing panel in transmission and in reflection, respectively.

Detailed Description

[82] The following is a detailed description of exemplary embodiments of the present disclosure. In the figures and in the following description, identical reference numerals or symbols are used to indicate constructive elements with the same function. Moreover, for the sake of clarity of illustration, it is possible that some references are not repeated in all of the figures.

[83] While the invention can be subject to modifications, or be implemented in alternative ways, in the drawings some embodiments are shown which will be discussed in detail in the following. However, it should be understood that there is no intention to limit the invention to the specific embodiments described, but on the contrary, the invention is meant to cover all the modifications or alternative and equivalent implementations which fall within the scope of protection of the invention as defined in the claims.

[84] Expressions like“example given”,“etc.”,“or” indicate non-exclusive alternatives without limitation, unless expressly differently indicated. Expressions like“comprising” and “including” have the meaning of“comprising or including, but not limited to” unless expressly differently indicated.

[85] With regard to figure 1A, a first basic configuration of a plant for implementing the process for producing a polymeric multi-layer film structure for chromatic diffusion according to the present invention is generally indicated with 100.

[86] Plant 100 comprises a coating stage 110 in which a coating apparatus 106 performs the coating of a first transparent flexible substrate film 101 with a layer of a scattering system 103 comprising a curable matrix into which a plurality of scattering elements is dispersed, wherein the scattering elements are configured to preferentially scatter short- wavelength components of impinging light with respect to long-wavelength components of impinging light thereby implementing a Rayleigh-like scattering.

[87] As used herein, the expression“scattering elements” defines a dispersion of nanoparticles suitable to provide light scattering. Moreover, as used herein, the term “nanoparticles” defines inorganic or organic particles having an average dimension in the range of 5 to 350 nm.

[88] In the embodiment of figure 1A, the coating apparatus 106 is an apparatus for slot die coating configured to achieve a coating of the first substrate film 101 with a layer 103 having a thickness in the range of 5-500 pm and a high uniformity degree. Apparatuses

implementing other coating techniques, such as roller coating, spray coating, knife coating, or metering rod coating, and so on may be used. In general, any suitable roll-to-roll coating technique may be used.

[89] The first 101 flexible substrate film is preferably a polymeric film, like e.g. a PET (polyethylene terephthalate) film or a PEN (polyethylene naphthalate) film.

[90] The first 101 flexible substrate film is fed to the plant 100 by means of a first 104 winding roll and a plurality of idle rolls which may be heated.

[91] Downstream of the coating stage 110, the plant 100 comprises a solidification stage 120 which may be implemented as a curing stage, preferably UV curing, temperature curing, electron beam curing or a combination thereof.

[92] In the explanatory embodiment herein described, the curing stage 120 is implemented as a combination of UV and temperature curing. The curing stage 120 of the example comprises one or more lamps 121 which emit in the UV-visible range (100 nm - 600 nm) and a heating chamber 122 for controlling the temperature during photopolymerization. The heating chamber 122 may be provided with electrical, microwaves or IR heat sources.

[93] Downstream of the curing stage 120 the so-obtained chromatically diffusing multi layer film structure 10 can be fed to further processing stages, like e.g. a cutting stage or a stage in which the multi-layer film structure 10 is laminated to a selected sheet, or to a rewinding stage 130 as shown in the exemplary embodiment of figure 1A. [94] With regard to figure 1B, a second basic configuration of a plant for implementing the process for producing a polymeric layer structure for chromatic diffusion according to the present invention is generally indicated with 100’.

[95] The plant 100’ of figure 1B differs from the plant 100 of the first basic configuration in that the coating stage 110 is followed by a lamination stage 115 in which the coated first substrate film 101,103 is brought into a side by side configuration with a second 102 flexible substrate film.

[96] The second 102 flexible substrate film is fed to the plant 100 by means of a second 105 winding roll and a plurality of idle rolls which perform the lamination of the first coated substrate film 101,103 with the second substrate film 102. Also the idle rolls feeding the second 102 flexible substrate film may be heated.

[97] In detail, the first coated substrate film 101,103 and the second substrate film 102 are brought into a side by side configuration through a couple of lamination rolls 108,109. The lamination rolls 108,109 may be heated rolls and/or may exert a small pressure on the resulting film structure in order to assure a uniform coupling between the first coated substrate film 101,103 and the second substrate film 102. Accordingly, a multi-layer film structure comprising a diffusive layer of scattering system 103 sandwiched between two substrate films 101,102, namely a chromatic diffusive layer 103, is created.

[98] With regard to figure 1C, a third basic configuration of a plant for implementing the process for producing a polymeric layer structure for chromatic diffusion according to the present invention is generally indicated with 100”.

[99] The plant 100” of figure 1C differs from the plants 100,100’ of the first and of the second basic configurations in that the coating and lamination stages 110,115 are implemented as single stage comprising an apparatus 107 for performing a gravitational dropping of the scattering system 103. The dropping apparatus 107 is positioned so as to drop the scattering system 103 between the two lamination rolls 109,108 through which the first 101 and the second 102 flexible substrate films are fed in order to be brought into a side-by-side configuration. [100] The gravitational dropping of the scattering system 103 between the two lamination rolls 108,109 determines an accumulation of scattering system 103 at the entry between the couple of lamination rolls 108,109.

[101] A thin layer of accumulated scattering system 103 is dragged between the two films 101,102 by the forward feeding of the two films 101,102 themselves so as to create a layer of scattering system 103, i.e. a chromatic diffusive layer 103. Accordingly, a simultaneous coating with the scattering system 103 and lamination of the two films 101,102 takes place, and the multi-layer film structure 10 to be fed to the subsequent curing stage 120 is created.

[102] The process 200 for producing a chromatically diffusing multi-layer film structure 10 for generating a sun-sky-imitating effect according to the present invention is schematically shown in figure 2 and comprises the following steps.

[103] During a first step 210, the scattering system 103 is prepared by dispersing a plurality of scattering elements into a pre-polymer or a polymer matrix dissolved in a suitable solvent. Herein, in the context of polymerization, the term“pre-polymer” refers to any kind of polymer precursor able to form a polymer as the host material of the scattering elements. It can be, for example, a monomer, an oligomer, a short chain polymer or a mixture of these three components. Suitable pre-polymers for the present invention are precursors able to form a polymer having excellent optical transparency. It can be selected from thermoplastic, thermosetting and photocurable resins. Suitable pre -polymers may belong (but are not limited) to the following categories: esters, aldehydes, phenols, anhydrides, epoxides, acrylates, vinyls, alkenes, alkynes, styrenes, halides, amides, amines, anilines, phenylenes, aromatic hydrocarbons, and siloxanes. In addition, fluorinated polymer precursor may be used forming fluoropolymer (homopolymers or copolymer) having in many cases non-stick properties. A wide variety of commercial useful pre -polymers are available, such as thiol- ene optical adhesives, e.g. of the NOA series from Norland optics Inc. and UV curable adhesives and sealants e.g. from Croda International Plc. or Henkel AG & Co. KGaA.

[104] The scattering elements comprise organic or inorganic nanoparticles or a

combination of thereof. The material for the nanoparticles may be made with one or more materials that are transparent and/or essentially do not absorb light in the visible range. In the case of nanoparticles having an organic nature, the inventors are referring to polymers (optionally crosslinked) while in the case of nanoparticles having an in organic nature, reference is preferably made to metal oxides (e.g. Ti02, Si02, ZnO, Zr02, Fe203, A1203, Sb2Sn05, BΪ203, Ce02 or a combination thereof) with a single-phase structure or a core/shell structure. The external surface of the nanoparticle is preferably functionalized with a specific organic coating (surfactant molecules) or with dispersing agents in order to guarantee an optimal compatibility and dispersion in the polymer matrix. A poor

compatibility of nanoparticles with the pre-polymer matrix may lead to the formation of large aggregate/agglomerates and inhomogeneous distribution that will strongly affect the scattering properties of the diffusive layer 103 and consequently of the resulting

embodiment.

[105] The nanoparticles may be monodisperse or polydisperse, they may be spherically shaped or shaped otherwise. In any case the effective diameter d of the nanoparticles falls within the range [5 nm-350 nm], such as [10 nm-250 nm], or even [40 nm-l80 nm], or [60 nm-l50 nm], where the effective diameter d is the diameter of the equivalent spherical particle, namely the effective diameter spherical particle having similar scattering properties as the aforementioned nanoparticles.

[106] The dispersion process may use low/high shear mixing equipment such as magnetic stirrer and/or static mixers and/or other mixing equipment like e.g. sonicators.

[107] The pre-polymer or polymer material used for preparing the matrix is a material that is transparent per se and does not absorb light, i.e. the matrix without scattering elements is transparent and does not absorb light, i.e. its absorption in the visible wavelength range can be considered negligible.

[108] The refractive indexes of the two materials (matrix and scattering elements) are different, and this mismatch on the refractive index combined with the diameter and the areal density (number per square meter) of the scattering elements are responsible of the Rayleigh- like scattering phenomenon, i.e. are the parameter that define the cross section of the scattering phenomenon in the chromatic diffusive layer of composite material 103. The amount of the impinging light scattered from the chromatic panel increases by increasing one of these parameters.

[109] To scatter, the nanoparticles have a real refractive index n_p sufficiently different from that of the matrix _¾, (also referred to as host material) in order to allow light scattering to take place. For example, the ratio m between the particle and host medium refractive indexes

Yl

(with m º— ) may be in the range 0.5 < m £ 2.5 such as in the range 0.7 < m < 2.1 or

nh

0.7 < m < 1.9 .

[110] Moreover, the chromatic effect is based on the number of nanoparticles per unit area seen by the impinging light propagating in the given direction as well as the volume-filling-

4 d

fraction /. The volume filling fraction f is given by / =— 7T(— ) p with p [meter ] being the number of particles per unit volume. By increasing/, the distribution of nanoparticles in the diffusing layer may lose its randomness, and the particle positions may become correlated. As a consequence, the light scattered by the particle distribution experiences a modulation which depends not only on the single-particle characteristics but also on the so-called structure factor. In general, the effect of high filling fractions is that of severely depleting the scattering efficiency. Moreover, especially for smaller particle sizes, high filling fractions impact also the dependence of scattering efficiency on wavelength, and on angle as well.

One may avoid those“close packing” effects, by working with filling fractions / < 0.4 , such as / < 0.1 , or even / < 0.01 .

[111] The chromatic effect is further based on a number N of nanoparticles per unit area of the chromatic diffusive layer 103 in dependence of an effective particle diameter D = d . Thereby, d [meter] is the average particle size defined as the average particle diameter in the case of spherical particles, and as the average diameter of volume-to-area equivalent spherical particles in the case of non-spherical particles, as defined in [T.C. GRENFELL, AND S.G. WARREN, "Representation of a non-spherical ice particle by a collection of independent spheres for scattering and absorption of radiation". Journal of Geophysical Research 104, D24, 31,697-31,709. (1999)]. The effective particle diameter is given in meters or, where specified in nm.

In some embodiments used in transmission mode:

In other embodiments used in reflection mode:

3.47 x 10 -29 m² + 2

N ³ — N min = [meters ²], ( D given in [ meters ] ) and N £

D m 2 - 1

-27

1.03 x 10 m² + 2 7.71x 10 -28 m² + 2

N max = [meters ²], e.g. N £ N = [meters ²] .

D^t m - 1 D^b m² - 1

[112] The step 210 of dispersing the scattering elements into the polymer matrix may comprise the addition of spacer elements 13 such as spherical elements of a size of some micrometers (5 pm to 500 pm), to the mixture of scattering elements and the polymer matrix, thereby defining an effective layer thickness by the (large) pm-particles.

[113] In the alternative, the spacer elements 13 may be pm-particles in the form of rods, discs or any other suitable geometry.

[114] In a subsequent step 220, at least one flexible substrate film 101,102 is coated with at least a thin uniform layer of the scattering system 103, i.e. the dispersion containing the scattering elements.

[115] In case the scattering system 103 is made starting from a non-adhesive matrix, the coating step comprises the coating of an additional internal layer of transparent adhesive material (not shown).

[116] Accordingly, a multi-layer film structure 101,103 is obtained comprising at least a first flexible substrate layer 101 and a flexible chromatic diffusive layer 103 responsible for the Rayleigh-like scattering of the impinging light, and optionally an internal adhesive layer. [117] Then, the multi-layer film structure 101,103 is subject to solidification (step 240). By way of example, in the plants of figures 1A to 1C, the multi-layer film structure 101,103 is subject to curing by controlling both, the temperature and the UV radiation to which the structure 101,103 is subjected. The solidification step 240 performed through curing initiates and accelerates the polymerization of the pre-polymer matrix. UV light intensities are, for example, in the range from 1-600 mW/cm2 and the temperature is set in the range of room temperature to 80 °C.

[118] Accordingly, a chromatically diffusing multi-layer film structure 10,10’ comprising at least a substrate layer 11,16 made of a flexible polymeric film and a chromatic diffusive layer 12 made of the solidified scattering system 103 is obtained.

[119] According to an alternative embodiment, the scattering system 103 comprises an already polymerized polymer dissolved into a suitable solvent and the solidification step 240 comprises the drying of the solvent.

[120] Optionally, before solidification, the coated substrate film 101,103 is bonded together with a second substrate film 102 (lamination step 230), which is possibly as well coated with a layer of scattering system 103 (e.g. if the scattering system 103 is applied by means of the gravitational dropping apparatus 107). In this case, after solidification 240, a multi-layer film structure 10” is obtained having two external flexible substrate layers 11 and at least one flexible chromatic diffusive inter-layer 12 responsible for the Rayleigh-like scattering of the impinging light.

[121] In case the couple of lamination rolls 108,109 are configured to exert a pressure on the multi-layer film structure 101,103,102 during lamination, this step 230 contributes to the definition of the diffusive layer 12 thickness.

[122] Otherwise, the diffusive layer 12 thickness is set during the coating step 220, in particular if the coating takes place by slot die coating or curtain coating or further similar techniques capable of precisely depositing a uniform layer of composite material 103 onto the first substrate film 101. The diffusive layer 12 thickness is set for example in the range from 5 pm to 500 pm which also defines the final areal density of the scattering elements (number per square meter). [123] A plurality of versions of the so-obtained chromatically diffusing multi-layer film structure 10, 10’, 10”, 10’” are shown in figures 3 A to 3E.

[124] According to a first embodiment shown in figure 3 A, the chromatically diffusing multi-layer film structure 10 comprises a substrate layer 11 made of a flexible and transparent polymeric film, and a chromatic diffusive and adhesive flexible layer 12. The film structure 10 of figure 3A can be wound to form a reel in order to be directly bonded onto e.g. a panel element 21 (as shown in figure 4 A) after unwinding of the reel. In this case, the transparent substrate layer 11 provides protection of the diffusive layer 12 e.g. from atmospheric agents, without preventing light transmission or reflection.

[125] According to a second embodiment shown in figure 3B, the chromatically diffusing multi-layer film structure 10” comprises a peelable layer 16 made of a polymeric film, and a chromatic diffusive and adhesive layer 12. In this case, the film structure 10 of figure 3B can be provided in sheets and can be bonded onto e.g. a panel element 21 after removal of the peelable layer 16. Preferably, a second panel element 21 is laminated there above in order to sandwich and protect the diffusive layer 12 e.g. from atmospheric agents as shown in figure 4B.

[126] According to a third embodiment shown in figure 3C, the chromatically diffusing multi-layer film structure 10”’ comprises two external substrate layers 11 made of a flexible and transparent polymeric film, and a chromatic diffusive and adhesive layer 12 sandwiched there between. In this case, the film structure 10 of figure 3C can be bonded onto e.g. a panel element 21 or sandwiched between two panel elements 21 after application of a transparent adhesive layer 24 as shown in figure 4C.

[127] According to a fourth embodiment (not shown), the chromatically diffusing multi layer film structure comprises two external layers 11,16 and a chromatic diffusive and adhesive layer 12 sandwiched there between, wherein a first external layer 11 is made of a flexible and transparent polymeric film and a second external layer 16 is a peelable layer.

[128] A fifth embodiment shown in figure 3D, differs from the embodiment of figure 3C for the presence of spherical spacer elements 13 in the diffusive layer 12, which strongly simplifies the creation of a diffusive layer 12 with a highly uniform thickness. [129] According to a sixth embodiment shown in figure 3E, the chromatically diffusing multi-layer film structure 10”’ comprises two external substrate layers 11 made of a flexible and transparent polymeric film, and a chromatic diffusive non-adhesive layer 12 sandwiched there between by interposition of two internal transparent adhesive layers 14.

[130] Moreover, the chromatically diffusing multi-layer film structure 10’” of figure 3E is provided with an optional external transparent adhesive layer 15 for simplification of a subsequent application onto e.g. a panel element 21 as shown in figure 4D. Even if not shown, in addition to the external adhesive layer 15, a peelable layer 16 applied thereon may be also provided.

[131] The transparent adhesive polymeric internal 14 and/or external 15 layers are preferably chosen between PVB, EVA, thiol-ene optical adhesives or similar.

[132] EXAMPLE:

[133] Phase 1 - Preparation of the nanoparticles dispersion and pre-polymer scattering system

[134] Nanoparticles of Zinc Oxide available on the market with nominal diameterdOO nm were mixed in Ethanol (as solvent) in a ratio of 1:3 in wt%. The Zinc Oxide powder was functionalized by adding Stearic Acid (as surfactant molecules) to the Zinc Oxide - Ethanol mixture in a concentration of about 5%wt. The mixture with the addition of Stearic Acid was vigorously stirred for 3 days.

[135] The obtained colloidal dispersion was then added to NOA65 (Norland Optical Adhesives) of the producer Norland Products Inc. as monomer/pre-polymer matrix in a variable concentration from 1 to 10% wt depending on the desired optical effect.

[136] Phase 2 - Production of the chromatically diffusing multi-layer film structure of figure 3C

[137] The obtained scattering system 103 was loaded in a coating apparatus 106 configured to coat with high uniformity a layer of the pre-polymer/nanoparticles mixture with a thickness of 60 pm on a PET substrate film 101 advancing at a speed of V=0,5m/min.

[138] The coated substrate film 101,103 was laminated with a second PET substrate film 102 advancing at the same speed. [139] The sandwiched pre-polymer/nanoparticles mixture was then photo- polymerized by means of an UV lamp 121 emitting at 365 nm with an intensity of about 5mW/cm2 at a controlled temperature of 30 °C thereby obtaining the chromatically diffusing multi-layer film structure 10” of figure 3C.

[140] According to an embodiment, the multi-layer film structure 10 of figure 3 A is used to cover a selected panel element 21 according to the final application (optional step 250) thereby obtaining a chromatically diffusing panel structure 20 as shown in figure 4A.

[141] The panel element 21 is configured to provide for the required transmission, i.e. a transparent panel such as a glass sheet or PMMA sheet (or sheet made of another polymer).

[142] Fig. 4B shows a chromatically diffusing stratified panel structure 20’ in which the adhesive chromatically diffusing multi-layer film structure 10’ of figure 3B is stratified between two glass panels 21. In the panel structure of Fig. 4B a glass panel 21 is additionally provided with a mirror coating 22 on its external surface (optional).

[143] The mirror coating 22 can be applied on the outer surface of the stratified panels 21 in order to obtain high quality surface chromatic stratified mirror. A typical way to produce mirror is to deposit metals such as aluminum or silver on a glass surface. The reflection efficiency depends on the deposited material and the quality of the reflected image depends on the flatness/roughness of the glass.

[144] Fig. 4C shows a chromatically diffusing stratified panel structure 20” in which the adhesive chromatically diffusing multi-layer film structure 10” of figure 3C is stratified between two glass panels 21.

[145] A conventional lamination process of a pair of, for example, 3 mm thick glass panels 21 and the chromatically diffusing multi-layer film structure 10” of figure 3C with the interposition of a commercial EVA film 24 between the glass panels 21 and the multi-layer structure 10” starts with bringing the layers in close contact. That assembly is then introduced, for example, in a plastic bag and a low vacuum is applied to the system in order to remove any air in the bag. The vacuum-packed bag can then be introduced in an oven and the temperature be raised to 85 °C (with a raising rate of, for example, 3.5°C/min). The assembly is maintained at that temperature for about 10 min. Subsequently in a second step, the temperature is further raised to about l25°C (raising rate 3.5°C/min) and maintained at that temperature for about 30 min. The assembly is than cooled to room temperature in, e.g. about 20 min and the stratified glass panel structure 20 is removed from the plastic bag.

[146] Fig. 4D shows a chromatically diffusing stratified panel 20”’ in which the chromatically diffusing multi-layer film structure 10”’ of figure 3E is applied onto a glass panel 21 and wherein the glass panel 21 is provided on its external surface with an antireflective coating 23. The antireflective coating 23 is a surface treatment that allows increasing the regular transmittance of a material. This treatment has to be optimized on a defined wavelength range (visible range for the present application) and strongly depends on the optical properties of the materials facing the antireflective coating. An antireflective coating optimized for the interface glass-air in the visible spectrum, makes the panels shown in Fig. 4D more efficient in terms of transmittance and decrease the intensity of the reflected scene.

[147] In general, different types of glass panels 21 may be used such as normal float glass, tempered glass, surface etched glass and similar. Moreover, the outer surface of the glass panels 21 can be provided with a micro structure thereby achieving the effect of blurring the perceived scene behind the micro structure surface. This property may be desired because unwanted structures beyond the panel structure 20 and images reflected on the panel structure 20 will be perceived blurry. Accordingly, a possible sharp variation of luminance is smoothed by the micro structure-based surface, thereby enhancing the depth perception.

[148] The chromatically diffusing multi-layer film structure 10 according to the invention can be used in sun- sky-imitation lighting systems as the ones hereafter described by way of example.

[149] In connection with Fig. 5 A, a lighting system 40 in transmission mode with respect to a panel structure 20 comprising the chromatically diffusing multi-layer film structure

10, 10’, 10”, 10’” of the invention is disclosed, and in connection with Fig. 5B, a lighting system 60 in reflection mode with respect to a chromatically diffusing multi-layer film structure 10, 10’, 10”, 10”’ is disclosed. [150] Referring to Fig. 5A, a lighting system 40 is illustrated schematically in a cut view of a room 30. In detail, the lighting system 40 comprises a first light source 41 comprised in a box 46 and configured to emit light in an emission solid angle to form a light beam 42 propagating along a main light beam direction 43. Moreover, first light source 41 emits light in the visible region of the light spectrum, for example, with wavelengths between 400 nm and 700 nm.

[151] Bottom unit 44 comprises a diffused light generator (i.e. chromatically diffusing panel structure 20, 20’, 20”, 20’”) based on a chromatically diffusing multi-layer film structure 10, 10’, 10”, 10”’ according to the invention and operates as a Rayleigh-like diffuser 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 at the wavelength 450 nm (blue) at least 1.2 times, for example at least 1.4 times, such as at least 1.6 times more efficiently than light in the wavelength range around 650 nm (red), wherein a diffusion efficiency is given by the ratio between the diffused light radiant power with respect the impinging light radiant power.

[152] The chromatically diffusing multi-layer film structure 10, 10’, 10”, 10”’ will separate an incident light beam 42 of light source 41 in four components, particularly in:

a transmitted (directed non-diffuse) component (light beam 42A), formed by light rays that pass through and do not experience significant deviations, e.g. is formed by light rays experiencing a deviation smaller than 0.1 °; a luminous flux of the transmitted component is a significant fraction of the overall luminous flux incident on diffused light generator 20, 20’, 20”, 20”’;

a forward diffuse component, formed by scattered light propagating into a lightwell 45/room 30 (with the exception of that light beam direction and of directions differing from that light beam direction by an angle smaller than 0.1 °); a luminous flux of the forward diffuse component corresponds to a blue skylight fraction generated from the overall luminous flux incident on the multi-layer film structure 10, 10’, 10”, 10”’;

a backward diffuse component, formed by scattered light propagating into box 46; a luminous flux of the backward diffuse component is, in general, in the range of but preferably less than the blue skylight fraction; and

a reflected component, formed by reflected light and propagating along a direction at a mirror angle into box 46, a luminous flux of the reflected component depends, for example, on the incident angle of the light beam onto the chromatically diffusing multi-layer film structure 10, 10’, 10”, 10’”.

[153] In an alternative embodiment (not shown) requiring a very large bottom unit, the bottom unit may be formed just by a chromatically diffusing film structure 10, 10’, 10”, 10”’ according to the invention. Accordingly, the bottom unit would be flexible and very light and could subtend a large area without requiring dedicated constructional support structures.

[154] Referring to Fig. 5B, aspects of an optical setup as well as the perceptive aspects of illuminations systems as generally described herein are described for a reflective lighting system 60.

[155] Lighting system 60 comprises again light source 61, configured to emit light in an emission solid angle to form a light beam 62 (in Fig. 5B delimited by dashed lines 62’) propagating along a main light beam direction 63 (also referred to as main beam axis).

[156] Lighting system 60 further includes a reflector unit 64 that couples the light originating from light source 61 to a region 67 to be lit up. In general, reflector unit 64 comprises a reflective structure 68 providing a reflective surface 68A and a chromatically diffusing multi-layer film structure 10, 10’, 10”, 10”’ laminated to the reflective surface 68A.

[157] Reflective surface 68 A is generally any type of optical acting interface that reflects light having passed through the multi-layer film structure 10. Due to reflective surface 68A, light of light beam 62 having passed the being incident on reflective surface 68A is redirected to pass again through the chromatically diffusing multi-layer film structure 10, thereafter forming an illuminating light beam 62A (in Fig. 5B delimited by dashed lines 62A’).

[158] In order to quantify the sun-sky imitation phenomenon achieved by means of the chromatically diffusing multi-layer film structure 10 of the invention the regular

transmittance property T(l) of the material at a certain wavelength can be considered. [159] To obtain a sun- sky-imitation lighting system, some particular range of regular transmittance are required. Note that both the first material (the matrix) and the second material (nanoparticles) are almost non-absorbing in the visible range, so the portion of the light that is not regularly transmitted is totally scattered in the Rayleigh-like scattering mode. Regarding the transmission configurations as the one described in Fig. 5A the regular transmittance for the blue T[450 nm]may be in general within the range [0.05-0.9]. In particular in some embodiments aiming at a pure clear sky the range would be [0.3-0.9], such as [0.35-0.85] or even [0.4-0.8]; in the embodiments aiming at a Nordic sky the range would be [0.05-0.3], such as [0.1-0.3] or even [0.15-0.3].

[160] Considering that in the reflection configuration (Fig. 5B) the nano-loaded scattering inter-layer is crossed twice by an impinging light (due to the presence of the mirror). The regular transmittance for the blue T[450 nm] of a chromatic stratified panel before the mirroring of the outer surface may be in general within the range [0.2-0.95]. In particular in some embodiments aiming at a pure clear sky the range would be [0.55-0.95], such as [0.6- 0.92] or even [0.62-0.9]; in the embodiments aiming at a Nordic sky the range would be [0.2-0.55], such as [0.3-0.55] or even [0.4-0.55]. The transmittance of a pure clear sky is higher than the one of a Nordic sky. For example, considering the same light source impinging on two chromatic stratified panels, one in the pure clear sky configuration and one in Nordic configuration, the chromatic properties in the sun-sky effect will be different. The sky in the Nordic configuration will be whitish compared to the one in the pure clear sky.

The sun in the Nordic configuration will be more yellow than the one in the pure clear sky.

[161] Although the preferred embodiments of this invention have been described herein, improvements and modifications may be incorporated without departing from the scope of the following claims.

Claims

1. A process (200) for producing a chromatically diffusing multi-layer film structure (’,10”, 10’”) for generating a sun- sky-imitating effect in lighting systems (40,60) comprising the steps of:

- coating (220) at least one first flexible substrate film layer (101,102) with at least a scattering system (103) thereby creating a multi-layer film structure (101,102,103) comprising a first flexible substrate layer (101,102) and at least one coating layer of scattering system (103) having substantially uniform thickness, wherein the scattering system (103) comprises a dispersion of scattering elements configured to preferentially scatter short-wavelength components of impinging light with respect to long-wavelength components of impinging light thereby implementing a light scattering in Rayleigh-like regime, wherein the scattering elements are dispersed in a polymeric transparent matrix and wherein a number N of scattering elements per unit area of the scattering system coating layer (103) in dependence of an effective particle diameter D = d _¾ ( D given in [ meters ] ), is

7.l3 x l0 ²⁹ m² + 2

within the range defined by N ³ N_mm [meters ²], and

D ⁶ m²— 1

1.56x10 ²⁷ m² + 2

N £ N max meters ²] ;

D⁶ m 2 - 7 [

1

or within the range defined by

2

3.47 x 10 -29 m² + 2

N ³ N = [meters ²], and

D^t m 2 - 1

7.7lx l0 m² + 2

N < W =

D^E m 2 - i [meters ²]

1

- laminating (230) the at least one first flexible substrate film (101,102) coated with the layer of scattering system (103) with a second flexible substrate film (102,101) so as to sandwich the layer of scattering system (103) between the pair of flexible substrate films (101,102); and - subjecting (240) the multi-layer film structure (101,102,103) to

solidification so as to obtain a chromatically diffusing film structure (10”, 10’”) at least comprising a first and a second flexible substrate layers (11,16) and a flexible chromatic diffusive layer (12) coupled to and sandwiched between the first and the second flexible substrate layers (11,16) and obtained through solidification of the coating layer of scattering system (103).

2. The process (200) of claim 1, wherein at least one first film between the first and the second flexible substrate films (102,101) is transparent, preferably a polymeric transparent film.

3. The process (200) of claim 1 or 2, wherein the scattering elements of the scattering system (103) comprise organic and/or inorganic nanoparticles which are transparent and/or do not substantially absorb light in the visible range and wherein the scattering elements of the scattering system (103) have an average size smaller than 350 nm, preferably having a peak at particle sizes below 350 nm in the particle size distribution.

4. The process (200) of any one of claims 1 to 3, wherein the scattering elements of the scattering system have refractive index n_p and the matrix of the scattering system has refractive index _¾, wherein the ratio m between the particle and host medium

Yl

refractive indexes (with m º— ) is in the range of 0.5 < m < 2.5 , preferably in the range of nh

0.7 £ m £ 2.1 , more preferably in the range of 0.7 £ m £ 1.9 .

5. The process (200) of any one of the preceding claims, wherein the scattering system (103) additionally comprises spacer elements (13) having a size comprised within 5 to 500 micrometers, preferably spherical and/or rod-shaped and/or disc-shaped elements.

6. The process (200) of any one of the preceding claims, wherein the polymeric transparent matrix is an adhesive material, preferably an optical adhesive, like e.g. a thiol-ene optical adhesive, in form of a pre-polymer or of a polymer dissolved in a solvent.

7. The process (200) of any one of the preceding claims, wherein the coating step (220) comprises dropping the scattering system (103) between the first and the second flexible substrate films (101,102) so that at least one of the pair of flexible substrate films (101,102) drags a layer of scattering system (103) between the two films (101,102) thereby simultaneously performing the coating and the lamination of the films (101,102).

8. The process (200) of any one of claims 1 to 6, wherein the coating step (220) comprises at least one between roll-to-roll coating processes such as slot die coating, spray coating, knife coating, roller coating, gravure coating or metering rod (Meyer bar) coating.

9. The process (200) of any one of the preceding claims, wherein the coating step (220) comprises the deposition of at least one internal adhesive layer (14) of a transparent adhesive material between the at least one coating layer of scattering system (103) and at least one of the first and the second flexible substrate films (101,102); and

wherein, optionally, the transparent adhesive material of the at least one internal adhesive layer (14) is an optical polymeric adhesive, like e.g. a thiol-ene optical adhesive.

10. The process (200) of any of the preceding claims, wherein the solidification step (240) comprises

- polymerizing the transparent matrix when at least part of the polymeric transparent matrix comprises a pre-polymer, by e.g. UV curing and/or temperature curing and/or electron beam curing; and/or - drying the polymeric transparent matrix by evaporation of a solvent, when at least part of the polymeric transparent matrix is a polymer dissolved into the solvent.

11. The process (200) of any of the preceding claims, wherein the scattering elements of the scattering system (103) comprise inorganic nanoparticles with its external surface having been functionalized with an organic coating, wherein optionally the organic coating used for functionalizing the external surface of the inorganic nanoparticles comprises surfactant molecules.

12. The process (200) of any of the preceding claims, wherein the flexible chromatic diffusive layer (12) has a thickness comprised within 5 pm to 500 pm.

13. The process (200) of any of the preceding claims, wherein at least one second film between the first and second flexible substrate films (101,102) is a film chosen between:

- a transparent film,

- a reflective film,

- an at least partially absorbing film, and

- a peelable film;

- a polymeric film, like e.g. a polyethylene terephthalate film or a polyethylene naphthalate film.

14. A chromatically diffusing multi-layer film structure ( 10” , 10”’ ) for generating a sun- sky-imitating effect in lighting systems comprising:

at least one first and at least one second flexible substrate layers (11,16); and at least one flexible chromatic diffusive layer (12) coupled to and sandwiched between the at least one first and at least one second flexible substrate layers (11,16), the chromatic diffusive layer (12) comprising a solidified scattering system comprising a dispersion of scattering elements configured to preferentially scatter short- wavelength components of impinging light with respect to long-wavelength components of impinging light thereby implementing a light scattering in Rayleigh-like regime, wherein the scattering elements are dispersed in a polymeric transparent matrix and wherein a number N of scattering elements per unit area of the scattering system coating layer (103) in dependence of an effective particle diameter D = d _¾ ( D given in [ meters ] ), is within the range defined by d

or within the range defined by

15. The structure (10”, 10”’) of claim 14, further comprising at least one additional layer (14,15,16,22,23) coupled to the at least one first and/or the at least one second flexible substrate film (11,16) and/or to the chromatic diffusive layer (12), between:

- an internal adhesive layer (14);

- an external adhesive layer (15);

- a peelable layer (16);

- a nanoparticle-based Rayleigh-like diffusing coating layer;

- a microparticle-based diffusing coating layer;

- a coating layer combining nanoparticle-based Rayleigh-like diffusing and microparticle-based diffusing;

- an antireflective coating layer;

- a mirror coating layer.

16. A chromatically diffusing panel structure (20, 20’, 20”, 20”’) comprising a first panel element (21) bonded to a chromatically diffusing film structure (10”, 10’”) according to any one of claims 14 or 15.

17. The chromatically diffusing panel structure (20, 20’, 20”, 20”’) of claim 16, further comprising a second panel element (21), the chromatically diffusing multi layer film structure (10”, 10”’) being sandwiched between the first and the second panel elements (21).

18. A lighting system (40,60) comprising:

a light source (41,61) configured to generate a visible light beam;

a chromatically diffusing film structure (10”, 10’”) according to claims 14 or

15 or a chromatically diffusing panel structure (20, 20’, 20”, 20’”) according to claims

16 or 17, illuminated by the light source (41,61), wherein

a portion of the light beam forms an illuminating light beam by passing through the chromatically diffusing film structure (10”, 10’”) or through the chromatically diffusing panel structure (20, 20’, 20”, 20’”) essentially unscattered, and a portion of the light of the light beam is Rayleigh-like scattered by the scattering elements within the

chromatically diffusing film structure (10”, 10”’) or the chromatically diffusing panel structure (20, 20’, 20”, 20”’).