Electrically controllable device having variable optical qualities or system which is holographic,thermotropic or which has suspended particles

ELECTRO-CONTROLLED DEVICE WITH VARIABLE OPTICAL PROPERTIES OR HOLOGRAPHIC, THERMOTROPIC OR SUSPENSION PARTICLE SYSTEM

The present invention relates to electrically controllable systems with variable optical properties, and more specifically to systems of the glazing type, the light diffusion of which can be modified under the effect of an appropriate electrical supply, such as liquid crystal systems and optical valves.

Incidentally, the invention is similarly applicable to systems which exhibit significant light scattering, but only in a narrow range of viewing angles of incidence, such as holographic systems. It also applies to thermotropic systems, as well as to systems with suspended particles (known by the English abbreviation SPD for suspended particles display).

Liquid crystal systems use a functional film based on a polymer medium in which liquid crystal droplets are dispersed, in particular nematic droplets with positive dielectric anisotropy. The liquid crystals, when the film is turned on, orient themselves along a privileged axis, which allows vision. When switched off, in the absence of alignment of the crystals, the film becomes diffusing and prevents vision. Examples are described in patents EP-88,126, EP-268,877, EP-238,164, EP-357,234, EP-409,442 and EP-964,288. This type of film is generally placed between two substrates, for example two sheets of polymer provided with electrodes, a structure which can then be laminated between two glasses to form a glazing. This type of glazing is marketed by the company SAINT-GOBAIN GLASS under the name “Priva-Lite”. In fact, all devices using so-called NCAP (Nematic Curvilinearly Aligned Phase) or PDLC (Polymer Dispersed Liquid Crystal) crystals can be used.

Optical valve systems generally use a functional film in the form of an optionally crosslinked polymer matrix, in which are dispersed microdroplets containing particles capable of being placed in a preferred direction under the action of an electric or magnetic field. Depending in particular on the potential applied and / or the nature and the concentration of orientable particles, the film will exhibit a variable light transmission, generally associated with a variable light diffusion (under voltage, the particles intercept much less light than when they are off. ). An example thereof is described in patent WO93 / 09460, with a film comprising a crosslinkable polyorganosiloxane matrix and absorbent particles of the polyiodide type.

Many applications have been envisaged for these types of system, for example to equip internal partitions or constitute exterior glazing of buildings, (in particular in offices), or in land (train, car), air ( plane) or maritime.

As mentioned in patent EP-823 653, an original application consists in using them as a rear-projection screen operating in transmission, where the projector is on one side of the screen and the viewer on the other. . The possible uses are numerous: to equip shop windows, for advertising panels, for station or airport display panels, for example. The use of liquid crystal systems makes projection possible without having to obscure the room where the projection takes place. However, with a standard system, the image contrast is not very good. This is the reason why patent EP-823 653 proposes a solution for improving this contrast, which consists in juxtaposing the variable diffusion system with a variable light transmission system of the electrochromic type. When the electrochromic system is switched on, it darkens and gives the image projected on the screen better contrast, and for the viewer, better visual comfort in an environment with natural or artificial light; it makes it possible in particular to absorb the stray light reflected in a diffuse manner on the screen coming from the lighting means placed on the viewer's side.

This solution is attractive because the screen obtained is extremely flexible in its optical properties. However, it also has drawbacks; the manufacture of the screen requiring the juxtaposition of two functional systems is not very simple, the resulting screen itself may have to present a significant thickness, and it is necessary to set up a double system of connections and power supply.

The aim of the invention is therefore to improve the performance of a system with variable light diffusion such as one of those described above, in particular in view of an application as a projection screen, and this by means of means that are simple to manufacture, simple to use and simple to maintain, and in fact less expensive.

The subject of the invention is first of all an element with variable optical properties comprising:

(a) - an electrically controlled variable light diffusion system, of the optical valve or liquid crystal system type, or a suspended particle system or a non-electrically controlled system (a ') such as a holographic or thermotropic system or, which is associated to (b) - at least one absorbent element at least in the visible range.

In the context of the invention, the absorbent element (b) is understood to mean an element which has the desired optical property in the visible light permanently, without requiring an electrical or other control.

It should be noted that thermotropic systems can also be classified among the electrically controlled systems when they are associated with one or more heating elements (for example a conductive layer heating by Joule effect), in order to make them switch at will, and not according to , in particular, the climatic conditions to which they are exposed.

The invention has therefore chosen to add functionality to a standard system (a) or (a '), namely a light absorption that can be chosen at the design stage at the desired level, at a higher level that it has. usually, but without resorting to a system requiring electric piloting. In this way, the manufacture of the assembly is simpler, by having recourse to absorbing elements which can be produced more quickly and more easily than systems of the electrochromic type. An excellent result is obtained in terms of optical performance. Thus, for a given absorption A conferred by the absorbent element, in the case of an application of the device according to the invention to a rear-projection screen operating in transmission, the reflection of the ambient light on the screen is reduced. by a factor A². As the light from the projection light source is also attenuated by a factor of A, the screen contrast gain is A²/ A, i.e. equal to A.

According to one embodiment of the invention, the system (a) is a liquid crystal system as described above, comprising a film of liquid crystal droplets dispersed in a medium and contained by two protective sheets each provided with an electrode. . These sheets can be chosen based on rigid substrates, preferably essentially transparent (glass, sufficiently thick polycarbonate PC), semi-rigid or flexible (thin polymer, such as thin PC). It is possible to have the two sheets of different natures (a glass and a polymer substrate for example).

The preferred variant consists in the two protective sheets being based on a flexible polymer, in particular transparent of the PET (polyethylene terephthalate) type.

The absorbent elements (b) can be chosen of different types. One or more can be used, of the same nature or of different natures, they will therefore “add” light absorption to the system (a) or (a '), over its entire surface or most of its surface.

The absorbent element can first of all consist of a rigid or semi-rigid substrate (glass, polymer) which is tinted in the mass, with suitable dyes. Preferably, T_L from 10 to 60%, for a thickness of the order of 1 to 8 mm, in particular 1.5 to 6 mm. One can very particularly use the range of glasses marketed under the name PARSOL or VENUS by the company SAINT-GOBAIN GLASS, or choose glass compositions as described in the patents ... Generally, it is preferred that the glass or the plastic substrate has a transmission color that is as neutral as possible, especially in grays. In an application as a rear-projection screen, it is these tones that distort the original colors of the projected images the least.

The absorbent element can also consist of a sheet of flexible polymer tinted in the mass, in particular of thermoplastic, that is to say the type of sheet which is used as an interlayer in laminated glazing. It may be polyvinyl butyral PVB, polyurethane PU, etylene vinyllacetate EVA. These tinted sheets have, for example, a light transmission T_L from 25 to 60%, in particular from 30 to 45% for a thickness generally between 0.2 and 0.8 mm (generally around 0.38 mm). For the same reason as above, we will preferably choose a color in neutral transmission, in the grays for example. It is also possible to use a sheet based on a polymer with absorbent properties which is applied to a clear substrate, for example a clear glass.

The absorbent element can also be in the form of a thin absorbent layer arranged on one side of a rigid (glass, polymer), semi-rigid or flexible (polymer) substrate forming part of the assembly. element with variable optical properties. The term “thin layer” is understood to mean a layer of interference thickness (less than 1 μm, in particular between 1 and 100 nm or 2 and 50 nm). It can also be a superposition of thin layers, at least one of which is absorbent in the visible range. They may for example be layers based on nickel oxide and / or iron and / or chromium, which can be deposited by a thermal decomposition technique (pyrolysis in the gas phase, in the form of powder or in liquid phase), by a vacuum technique (cathodic sputtering in particular assisted by a magnetic field) or by sol-gel. When it is a cathodic sputtering process, it can be reactive in the presence of oxidizing elements such as O2 from metal targets or substoichiometric oxide targets, or unreactive from oxide targets . The absorbent layer can also be essentially metallic, for example based on one of the following metals: nickel Ni, chromium Cr, Ni-Cr alloy, steel, Inconel alloy, Au or Au alloy, Ag silver or Ag alloy, copper. It is then deposited preferably by non-reactive cathodic sputtering. It can also be based on metal nitride (s), of the TiN or ZrN type, which can be deposited by CVD or by reactive cathode sputtering in the presence of nitriding elements from metal targets. The absorbent layer can also be based on an electrochromic material, such as optionally hydrated tungsten oxide, optionally hydrated nickel oxide, iridium oxide or optionally hydrated tantalum.

In the case where the system (a) is a liquid crystal system, another type of absorbent member (b) is to use a dye within the functional film. They may very particularly be dichroic dyes which are dissolved within the liquid crystal droplets and / or in the medium in which they are dispersed.

These dichroic dyes are, for example, chosen from the family of diazoquinone derivatives or from that of anthraquinone derivatives. The percentage or weight of dye relative to the liquid crystals, when the latter is dissolved therein, is preferably between 0.1 and 5%, in particular between 0.5 and 2%. In the variant where the dye is in the medium, its percentage by weight by addition to the medium can be between 20 and 30% for example.

In the case where an electrically controllable system (a) is used, the absorbent element may be an absorbent electrically conductive layer forming part of one of the electrodes which supply the functional film with electricity. It may be a layer based on a conductive metal oxide (which can be deposited by pyrolysis or by cathodic sputtering as in the case of the absorbent layer mentioned above). The oxide (or the mixture of oxides) is preferably doped. The fact that it is absorbent comes, in a first variant, from the fact that it is in the reduced state and / or substoichiometric in oxygen (while remaining an electrical conductor). It may be for example indium oxide (doped with tin) and reduced, tin oxide (doped for example with fluorine) and reduced, zinc oxide (doped for example Al) and reduced. It can also be conductive oxides which are intrinsically absorbent, without being in the reduced state: this is the case of tin oxide doped with antimony, which at 30 nm, can have an absorption luminous A from 10 to about 62% depending on the dopant percentage (2.5 to 10% dopant).

In a second variant, the fact that the conductive layer forming part of the electrode is absorbent stems from the combination of its chemical nature and the choice of its thickness. Thus, for a metal oxide-based (doped) layer, the latter can become sufficiently absorbent for the invention if it is sufficiently thick. It is also possible to choose a layer based on a metal of the Ni, Cr, NiCr type, or on metal nitride (TiN, ZrN, etc.), the thickness of which is adjusted in an appropriate manner (for example, a layer of TiN of approximately 25 nm has a light absorption of about 50%).

The absorbent element can also be a film based on plastic material, which is absorbent and which can, for example, be calendered to a clear substrate, in particular clear glass.

According to the invention, the element with optical properties, by the presence of the absorbing element (s) (b) sees its light absorption A_L increase by at least 5% and / or see its light reflection Ri. decrease by at least 5%, in particular by at least 8%.

The element with optical properties as a whole has a light transmission T_L in particular between 10 and 50%, preferably between 20 and 40%: this range of T_L is adequate to obtain the desired absorption effect while maintaining a sufficient level of light transmission, this last point being particularly important when the electrically controlled variable diffusion system is in the "ON" state, that is to say - say in the powered and transparent state.

The preferred embodiment according to the invention consists in the element with variable optical properties also comprising:

(c) - at least one anti-reflective element in the visible (called anti-reflective in the remainder of the text).

The antireflection element (c) can be constituted, according to a first variant, of an alternation of thin layers of interference thickness with high and low refractive indices, according to a sequence (high index layer / low index layer)_not, with n> 1. A high index / low index sequence, quite particularly the first from the substrate on which the antireflection is located, can be replaced by a layer of a material of intermediate index. Each “layer” can be a single layer or a superposition of several layers respectively at high or low index. The layers can be deposited by pyrolysis or by sputtering, like the absorbent layers mentioned above.

Low index layers can be SiO₂, Âl₂O₃ or mix. High index layers can be SnO₂, S13N4, TiO₂, Nb₂O_5d AIN, ZnO. The intermediate index layers can be a mixture of high and low index oxides or of SiON. Examples of suitable anti-reflections are described in patents EP-728 712, EP-911 302, WO97 / 43224, WO00 / 72053, FR99 / 14423. An example is marketed under the name Vision Lite by the company Saint-Gobain Glass II may in particular be a stack of the S13N4 or SnO type.₂/ SiO₂/ Si3N₄or Nb₂O₅/ SiO₂ (the SiO₂ which may include a little metal of the aluminum type or a little boron, in particular if it is obtained by cathodic sputtering). According to an interesting embodiment, the anti-reflection coating can comprise an absorbent layer, for example made of nitride of the TiN or ZrN type: there are thus two functions at the same time, with a single stack of layers which can be deposited. one after the other by the same method of deposit:

anti-reflective effect and absorption.

According to a second variant, the antireflection element is an antireflection film, which can be applied by calendering to the surface of a substrate of the glass or plastic type. It may for example be a polyethylene terephthalate film

PET with anti-reflective coating or cellulose triacetate film.

In both variants, the antireflection coating is effective when it is deposited on the exterior face of at least one of the “exterior” substrates of the element with variable optical properties (the term exterior substrate is understood to mean the one which delimits the element, of which one side is accessible and facing outwards). The addition of an anti-reflective element (at least) is very positive in the context of an application to a rear-projection screen for transmission. Indeed, it will decrease the light reflection, preferably from the outer face of the element as a whole which is intended to be turned towards the viewer. It will therefore make it possible to reduce the light reflection coming from parasitic lighting on the viewer side. Its effect is also beneficial, but much less marked, if it is placed on the outside face which will be on the side of the projector.

According to a third variant, the same type of anti-reflection effect is obtained by superficially modifying the outer surface of at least one of these outer substrates: this modification can consist of a surface texturing, a surface etching leaving calibrated growths. , as described in patent FR 00/08842 filed on July 6, 2000: at increasing depth, there is less and less material and more and more air, which creates on the surface of the material (glass in particular) an intermediate index layer between the material in question and the air , hence an anti-reflective effect.

At first glance, it seems surprising to combine in the same element an element absorbing in the visible (which will therefore help to lower the level of light transmission) and an anti-reflective element (which will on the contrary promote an increase in transmission in the visible. to the detriment of reflection). In fact, the inventors noticed that there was a synergy between these two components, which made it possible to achieve excellent levels of contrast, when the global element was used as a rear projection screen.

The configuration of the “overall” element of the invention can be very varied: one can have recourse to a glazing structure in the narrow sense of the term, that is to say using at least one glass substrate. It can have, in particular, one, two or three glass substrates. It is also possible to replace all or part of these glasses with other rigid substrates of the polycarbonate type.

It is even possible to envisage the case where the element as a whole does not contain rigid substrates, and for example only comprises the functional film, of the liquid crystal type, clamped between two sheets of flexible polymer provided with electrodes (of the liquid crystal type. PET / ITO or PET / SnO₂ doped), without counting of course the connection elements and any peripheral seals.

We then have a flexible screen, which can be rolled up or stretched by a frame or other appropriate tension means, at will, or place it in the immediate vicinity of a traditional glazing.

It is possible to have laminated glazing structures, in particular of the type:

Φ - Optional anti-reflective coating / glass 1 / sheet (s) of thermoplastic polymer / variable light distribution system / sheet (s) of thermoplastic polymer / glass 2 / optional anti-reflective coating, or ® - Optional anti-reflective coating / glass 1 / sheet thermoplastic polymer (s) / variable light diffusion system / thermoplastic polymer sheet (s) / glass 2 / thermoplastic polymer sheet (s) / glass 3 / optional anti-reflective coating.

A single lens configuration can be of the type:

® - Optional anti-reflective coating / glass 1 / thermoplastic polymer sheet (s) / variable light diffusion system / optional protective polymer sheet (s).

In these configurations, it is the glass 1 and / or 2 and / or 3 which can be tinted in the mass and / or be provided with an absorbent layer, or one of the sheets of thermoplastic polymer be tinted in the mass .

As we have seen, a favorable configuration consists in having the glass closest to the viewer which is this absorbent glass (by coloring in the mass or by adding a layer).

A preferred variant of this configuration consists in this absorbing glass which is provided with an anti-reflective coating.

The subject of the invention is the element in its entirety described above (which may include all the connectors, power supplies, seals, peripheral frame known per se and suitable for this type of product), this element forming part of a display screen. retro-projection operating in transmission.

The invention will hereinafter be described in more detail with the aid of the accompanying figures which represent:

Figure 1: a laminated glazing according to the invention with two glasses,

Figure 2: a laminated glazing according to the invention with three glasses.

These figures are extremely schematic and are not to scale for ease of reading.

In all the examples which follow, the liquid crystal system 1 is configured as: PET sheet / electrode / functional film / electrode / PET sheet, the whole being currently used in Priva-Lite glazing from SAINTGOBAIN GLASS.

The functional film comprising the liquid emulsion of nematic liquid crystals is about 10 to 30 µm (preferably 20 to 25 µm) thick. The PET sheets have a thickness of about 175 µm. The two electrodes are made of ITO (indium oxide doped with tin) with a resistivity of about 100 ohms per square.

Some details are given below on liquid crystals and their usable medium. As regards the liquid crystals, these can be of the “NCAP” type, in particular those used in the “Priva-Lite” or “PDLC” glazing which were mentioned above. As a general rule, their birefringence is between 0.1 and 0.2, it is variable in particular depending on the medium used, of the order of 0.1 if the polymer of the medium is of polyurethane (PU) type and of 'order of 0.2 if it is of the polyvinyl alcohol (PVA) type.

The medium is in fact preferably based on a polymer of the PU (latex) and / or PVA family, generally prepared in aqueous phase in a proportion of polymers of 15 to 50% by weight relative to water. .

The elements that are active with respect to light diffusion are advantageously in the form of droplets with an average diameter of between

0.5 and 3 μm, in particular between 1 and 2.5 μm, dispersed in the medium. The size of the droplets depends on a number of parameters, including in particular the ease of emulsifying the active elements in the medium considered. Preferably, these droplets represent between 120 and 220% by weight of the medium, in particular between 150 and 200% by weight, apart from the generally aqueous solvent of said medium.

Particularly preferably, liquid crystals are chosen in the form of droplets with a diameter of approximately 2.5 μm when the medium is based on polyurethane latex (birefringence of approximately 0.1) and with a diameter d. 'about 1 µm when the medium is more based on polyvinylalcohol (birefringence of about 0.2).

The power supply uses voltages between 0 and 110 V.

EXAMPLE 1 - Comparative example

This is a laminated glazing with two glasses according to FIG. 1, of configuration:

Clear glass 3 / EVA 4 / preset liquid crystal system 1 / EVA 5 / clear glass 2

The clear glasses are standard silico-soda-lime glasses marketed by SAINT-GOBAIN GLASS under the name “Planilux” and 2 mm thick. The EVA (ethylene vinyl acetate) sheet is 380 micrometers thick.

The lamination of the liquid crystal system is obtained in a known manner by pressurizing and / or heating to around 100 ° C.

EXAMPLE 2

This is a laminated glazing of the same configuration as in Example 1, but a glass colored in the mass, marketed under the name VENUS VG10 by SAINT-GOBAIN GLASS and of thickness 2, was substituted for the clear glass 2, 1 mm. This VENUS glass has the following characteristics: T_L between 25 and 30%. In the case of the glass used here, the Tl is 26.6% (according to Illuminant D65), its color is gray.

This glazing is illustrated in figure 1, with the liquid crystal system

1, the tinted glass in the mass 2, the clear glass 3, the two sheets of EVA 4, 5.

The projector has also been shown schematically to show the most judicious way of arranging the screen with respect to said projector (the tinted glass in the mass on the side opposite to it).

EXAMPLE 3

Example 3 shows a configuration with three glasses according to figure 2:

compared to the configuration of FIG. 1, it adds a clear glass 8 of 4 mm thick laminated to the glass 2 and provided on the outside face with an anti-reflection coating 7. This anti-reflection coating is composed of the stack of next layers:

SnO₂/ SiO₂/ Nb₂O₅/ SiO₂: Al

It is deposited in a known manner on the glass by cathodic sputtering assisted by a magnetic field.

FIG. 2 thus illustrates another embodiment, where the laminated glazing has three glasses: one adds to the configuration according to FIG. 1, on the side of the tinted glass in the mass 2, a thermoplastic interlayer polyurethane sheet 6 of 0.76 mm thick, allowing the third glass 8 to be leafed, which is a clear glass identical to the clear glass 3. In the case where an anti-reflective coating 7 is used, it is therefore on the outer face of this third lens 8 that it is placed.

EXAMPLE 4

Example 4 uses the configuration of example 3, and adds a second anti-reflective coating 7 '(see figure 2) on the exterior face of clear glass 3. The coating 7' is identical to the coating 7 of example 3. In this example, glasses 3 and 8 have a thickness of 4 mm.

EXAMPLE 5

Example 5 uses the configuration of Example 2 with two glasses according to FIG. 2, but substitutes for VENUS 2 glass colored in the mass another glass tinted in the mass 2 5 mm thick and marketed by SAINT-GOBAIN GLASS under the name “Parsol Gris”. Its characteristics are: T_L 48.5%, gray color.

EXAMPLE 6

Example 6 is a configuration with three glasses according to FIG. 2, taking again in glass 2 the tinted glass of example 5, and adding a clear glass 8 of 4 mm provided with an anti-reflective coating 7 as in example 3 .

EXAMPLE 7

Example 7 uses the configuration of Example 6, and adds on the outer face of the clear glass 3 an anti-reflective coating 7 '(identical to that of Example 4). In this example, clear glasses 3 and 8 are each 4mm thick

EXAMPLE 8

This example is a laminated glazing of the same configuration as in Example 2, to which is added by calendering on the outer face of the tinted glass 2 an anti-reflective film 7 (plastic film) marketed by Murei Danki under the reference ReaLook 2201

EXAMPLE 9

This example is a laminated glazing of the same configuration as in Example 5, to which is added by calendering on the outer face of the tinted glass 2 an anti-reflective film 7 identical to that used in Example 8.

Table 1 below summarizes for these nine examples:

- The type of glass and / or the number of anti-reflective coatings used,

- Their light transmission measured according to the illuminant D₆₅ in the “OFF” state: 15 Tloff, that is to say de-energized, and in the “ON” state: T_Lon, that is, when the functional film is switched on maximum voltage of 110 V

- Their light reflection measured according to the illuminant D₆5, tinted glass side (viewer side if we refer to the figures) in the OFF state: Rloff and in the "ON" state: Rlon

TABLE 1

<td></td><td>Tloff</td><td>Tlon</td><td>Rloff</td><td>Rlon</td>

<td>EX. 1:</td><td rowspan="2"> 71,15</td><td rowspan="2"> 72,69</td><td rowspan="2"> 16,62</td><td rowspan="2"> 17,59</td>

<td>2 clear glasses</td>

<td>EX. 2:</td><td rowspan="2"> 20,5</td><td rowspan="2"> 21,0</td><td rowspan="2"> 5,85</td><td rowspan="2"> 5,95</td>

<td>1 clear glass +1 VENUS glass</td>

<td>EX. 3: 2 glasses</td><td rowspan="2"> 21,20</td><td rowspan="2"> 21,55</td><td rowspan="2"> 1,36</td><td rowspan="2"> 1,53</td>

<td>clear + 1 VENUS glass and 1 anti-reflection</td>

<td>EX. 4 2 glasses</td><td rowspan="2"> 22,25</td><td rowspan="2"> 22,72</td><td rowspan="2"> 1,32</td><td rowspan="2"> 1,41</td>

<td>clear + 1 VENUS glass and 2 anti-reflection</td>

<td>EX. 5 1 1 glass</td><td rowspan="2"> 37,12</td><td rowspan="2"> 38</td><td rowspan="2"> 7,82</td><td rowspan="2"> 8,23</td>

<td>clear + 1 Parsol Gris glass</td>

<td>EX. 6: 2 glasses</td><td rowspan="2"> 37,95</td><td rowspan="2"> 38,69</td><td rowspan="2"> 3,43</td><td rowspan="2"> 3,88</td>

<td>clear + 1 Parsol Gris glass and 1 anti-glare</td>

<td>EX. 7: 2 glasses</td><td rowspan="2"> 39,95</td><td rowspan="2"> 41,09</td><td rowspan="2"> 3,16</td><td rowspan="2"> 3,48</td>

<td>clear + 1 Parsol Gris glass and 2 anti-glare</td>

<td>EX. 8: 1 glass clear + 1 VENUS glass and 1 anti glare on plastic</td><td> 21,15</td><td> 21,51</td><td> 2,42</td><td> 2,48</td>

<td>EX 9: 1 glass clear + 1 Parsol Gris glass and 1 anti-glare on plastic</td><td> 38,25</td><td> 38,85</td><td> 4,86</td><td> 5,25</td>

From these results, it can be seen that the use of a tinted glass instead of a clear glass makes it possible to lower the level of T_L both in the “ON” state and in the “OFF” state very significantly (almost 50% for VENUS glass, more than 30% for Parsol Gray glass). At the same time, it makes it possible to substantially lower the R_Lin the “ON” state and in the “OFF” state (about 8 to 10%, whether it is VENUS glass or Parsol glass), which is spectacular. It was not obvious that the increase in light absorption provided by the tinted glass in the mass is done at the same time to the detriment of the light transmission and of the light reflection.

The combined use of a tinted glass and an anti-reflective coating, the anti-reflective coating preferably being on the tinted glass, allows the level of R to be lowered even further._L, about 4 to 6%, to reach really very low values ​​(of the order of 1 to 3%). Finally, the use of a second anti-reflective coating (on clear glass) makes it possible to further lower the level of Rl of the glazing.

It should be noted that in the configuration with three glasses, it is possible to reverse the position of glasses 2 and 8, that is to say to arrange the colored glass in the mass of outer glass, the intermediate glass being the clear glass. It is of course better to have structures with two glasses rather than three glasses, in particular in terms of weight, size and cost. However, it is more common to deposit stacks of anti-reflective thin films on a clear glass than on an absorbent glass, and it may therefore be easier industrially to adopt the three-glass configuration.