CN113966483A

CN113966483A - Projection device for head-up display system

Info

Publication number: CN113966483A
Application number: CN202180001988.1A
Authority: CN
Inventors: J·哈根; K·菲舍尔
Original assignee: Saint Gobain Glass France SAS
Current assignee: Saint Gobain Glass France SAS
Priority date: 2020-05-15
Filing date: 2021-05-04
Publication date: 2022-01-21
Also published as: WO2021228621A1; DE202021004074U1

Abstract

The invention relates to a projection device for a head-up display (HUD), comprising at least a windscreen panel (10) having a HUD region, wherein the windscreen panel (10) has an outer glass panel (1) and an inner glass panel (2) joined to each other by a thermoplastic interlayer (3), a projector (4) directed towards the HUD region, wherein the radiation of the projector (4) is predominantly P-polarized, the windscreen panel comprising a reflective coating (20) for reflecting radiation emitted by the projector (4), wherein a first retardation layer (6.1) is present for converting the polarization of radiation transmitted through the retardation layer (6.1), wherein a first retardation layer (6.1) is arranged within the HUD region and the reflective coating (2) is arranged between the first retardation layer (6.1) and the outer glass pane (1) or the inner glass pane (2).

Description

Projection device for head-up display system

The present invention relates to a projection apparatus for a head-up display.

Modern vehicles are increasingly equipped with so-called head-up display (HUD, "head-up display") technology. A head-up display is a display system that projects additional information in the form of an image in its field of view to the driver of the vehicle. The head-up display is composed of a projector (imaging unit) and a plurality of optical modules for deflecting or reflecting (reflecting) an image onto a projection surface or a reflection surface. Composite glass panes, in particular windshields of vehicles, are often used as projection surfaces. Although the image is projected onto the windshield panel, the image floats at a distance above the vehicle hood as perceived by the driver's human eye.

In this way, additional information can be projected into the driver's field of view, for example the current driving speed, navigation or warning cues, which the driver can perceive without having to change his direction of sight. Therefore, the head-up display can contribute significantly to improving traffic safety.

Generally, the image produced by the projector is composed of polarized, in particular S-polarized, light radiation. The S-polarized light impinges on the composite glass sheet at a specific angle of incidence and is at least partially both refracted into the composite glass sheet and reflected as S-polarized light into the driver' S field of view. However, the reflected image does not fade or show up in unwanted reflections (so-called ghosting).

The angle of incidence of the S-polarized radiation is typically about 65%, which is approximately equal to the brewster angle of the air/glass transition (57.2 ° for soda lime glass). The problem here is that the projector image is reflected at the two outer transitions from air to glass and from glass to air. In addition to the desired main image, therefore, a slightly displaced secondary image, the so-called ghost image ("ghost") appears. This problem is mitigated by arranging the surfaces of the windscreen panels at an angle to each other. This is achieved by using a wedge shaped interlayer in the lamination of a windscreen panel designed as a composite glass panel. Thereby, an overlap of the main image and the ghost image can be achieved. Composite glasses with wedge-shaped membranes for HUDs are known, for example, from WO 2009/071135 a1, EP 1800855B 1 or EP 1880243 a 2.

Wedge-shaped films are expensive and therefore the manufacturing of such composite glass sheets for HUDs is rather expensive. There is therefore a need for a HUD system that is feasible with windshields that do not include wedge shaped membranes. Thus, for example, a HUD projector can be operated with P-polarized radiation that is substantially not reflected at the surface of the glass sheet due to radiation near the brewster angle. As a reflective surface for P-polarized radiation, the windshield plate has instead a reflective coating. The reflective coating is arranged to reflect both S-polarized radiation and P-polarized radiation, wherein it reflects significantly more S-polarized radiation than P-polarized radiation.

DE 102014220189 a1 discloses a HUD projection device with P-polarized radiation and a metal layer as a reflective structure. WO 2019/046157 a1 and US 2017/242247 a1 also disclose HUD systems with P-polarized radiation. Here, a reflective coating with at least two metal layers is used.

US 6,744,478B 1 discloses a HUD system in which a liquid crystal display generates a light beam directed at the windscreen panel. The windshield has an optical rotating layer on a first surface of a transparent plate. The rotation layer comprises a liquid crystal polymer. The reflective layer is disposed on an inner side of an inner glass sheet of the windshield sheet.

US 2009/195875 a1 discloses a HUD system with a windscreen panel, wherein a birefringent layer is arranged in or on the windscreen panel.

It is an object of the present invention to provide a HUD projection device with a reflective coating which has a good reflectivity for P-polarized radiation in the visible spectral range and improves the projection of images.

According to the invention, the object of the invention is achieved by a projection device according to claim 1. Preferred embodiments follow from the dependent claims.

The projection device for a head-up display of the present invention includes a windshield having a HUD region and a projector. The windshield has an outer glass pane and an inner glass pane which are joined to one another by a thermoplastic interlayer. The projector directed towards the HUD area of the windscreen panel emits predominantly P-polarized radiation. Furthermore, the windscreen panel has a reflective coating for reflecting radiation emitted by the projector. Furthermore, the windscreen panel comprises a first retarder for converting the polarization of radiation transmitted through the retarder, wherein the first retarder is arranged within the HUD region and the reflective coating is arranged between the first retarder and the outer or inner glass panel.

It was surprisingly found that this arrangement of reflective coating and retardation layer leads to a significantly better reflection. A color-neutral display of the HUD projection and an overall impression of color neutrality of the glass plate are thereby achieved.

A first retardation layer is preferably provided for converting the P-polarized beam of transmitted optical radiation into an S-polarized beam. By rotating from P-polarization to S-polarization, the full efficiency of the reflective coating is exploited. In other words, the retardation layer retards light polarized parallel to a particular axis of the component by half a wavelength relative to light polarized perpendicular thereto. Particularly good results are achieved thereby.

In a preferred embodiment, the first retardation layer has at least one optically anisotropic material or optically birefringent material, in particular quartz or mica. Other materials which are suitable in principle are, for example, calcite (CaCO3), lithium niobate (LiNbO3), ruby (Al2O3), rutile (TiO2) and zircon (ZrSiO 4). The first retardation layer is preferably arranged between the inner glass plate and the outer glass plate.

In a further preferred embodiment, the windshield has a second retarder for converting the polarization of the light transmitted by the retarder in the HUD region.

The reflective coating is preferably arranged between the first retardation layer and the outer or inner glass plate, wherein the reflective coating reflects significantly more S-polarized radiation than P-polarized radiation.

In particular, the reflective coating is arranged between the first retardation layer and the second retardation layer. This significantly reduces image distortions and thus enables an optically optimal display of the image for the observer.

The reflective coating is a stack of thin layers. The thin-film stack is composed of a layer sequence of thin individual layers. The thin-layer stack comprises at least one silver-based conductive layer. The silver-based conductive layer gives the reflective coating substantial reflective properties and furthermore infrared reflection and electrical conductivity. The silver-based conductive layer may also be referred to as a silver layer for simplicity.

The conductive layer is designed to be silver-based. The conductive layer preferably contains at least 90% by weight of silver, particularly preferably at least 99% by weight of silver, very particularly preferably at least 99.9% by weight of silver. The silver layer may comprise a dopant such as palladium, gold, copper or aluminum. The geometric layer thickness of the silver layer is preferably at most 15 nm, particularly preferably at most 14 nm, very particularly preferably at most 13 nm. This makes it possible to achieve a favorable reflectivity in the infrared range without excessively reducing the transmission in the visible range. The geometric layer thickness of the silver layer is preferably at least 5 nm, particularly preferably at least 8 nm. Particularly preferably, the silver layer has a geometric layer thickness of 10 nm to 14 nm or 11 nm to 13 nm.

In an advantageous embodiment, the reflective coating does not comprise a dielectric layer having a refractive index of less than 1.9. Thus, all dielectric layers of the reflective coating have a refractive index of at least 1.9. A particular advantage of the present invention is that the required reflective properties can only be achieved by a relatively high refractive index dielectric layer. Since, for low-refractive-index layers with a refractive index of less than 1.9, silicon oxide layers with a low deposition rate in magnetic field-assisted cathode deposition can be considered in particular, the reflective coatings of the invention can be produced so rapidly and cost-effectively.

The reflective coating comprises, above and below the silver layer, independently of one another, a dielectric layer or a sequence of dielectric layers, respectively, having a refractive index of at least 1.9. The dielectric layer can be designed, for example, on the basis of silicon nitride, zinc oxide, tin-zinc oxide, silicon-metal mixed nitrides, such as zirconium silicon nitride, zirconium oxide, niobium oxide, hafnium oxide, tantalum oxide, tungsten oxide or silicon carbide. The oxides and nitrides may be deposited stoichiometrically, substoichiometrically or superstoichiometrically. They may have dopants, for example aluminum, zirconium, titanium or boron.

The optical thickness of the upper dielectric layer or layer sequence is preferably from 100 nm to 200 nm, particularly preferably from 130 nm to 170 nm. The optical thickness of the lower dielectric layer or layer sequence is preferably from 50 nm to 100 nm, particularly preferably from 60 nm to 90 nm. Good results are thus obtained.

In principle, it is sufficient if the HUD region of the windshield is provided with a reflective coating. However, other areas may also be provided with a reflective coating. The windscreen panel can be provided with a reflective coating over substantially the entire surface, which may be preferred due to manufacturing.

In one embodiment of the invention, at least 80% of the surface of the glass sheet is provided with a reflective coating of the invention. In particular, the reflective coating is applied over the entire surface of the pane surface, except in the surrounding edge regions and optionally also in the local regions, which regions, as communication, sensor or camera windows, should ensure the transmission of electromagnetic radiation through the windscreen pane and are therefore not equipped with a reflective coating. The surrounding uncoated edge region has, for example, a width of up to 20 cm. This edge region prevents direct contact of the reflective coating with the surrounding atmosphere, thereby protecting the reflective coating in the interior of the windshield panel from corrosion and damage.

Other possibilities and indications of the manufacture and parameters of the reflective coating, as applicable in the present invention, are also known from EP 19212006.1, which has not been previously published, the disclosure of which is incorporated in its entirety.

The projector is disposed on the inner space side of the windshield panel and irradiates the windshield panel through the inner space side surface of the inner glass panel. The projector is directed at the HUD region and illuminates the region to produce a HUD projection. According to the invention, the radiation of the projector is mainly P-polarized, i.e. has a proportion of P-polarized radiation of more than 50%. The higher the proportion of P-polarized radiation in the total radiation of the projector, the stronger the intensity of the desired projected image and the weaker the intensity of the unwanted reflections on the surface of the windscreen panel. The proportion of P-polarized radiation of the projector is preferably at least 70%, particularly preferably at least 80%, in particular at least 90%. In a particularly advantageous embodiment, the radiation of the projector is substantially pure P-polarized, i.e. the proportion of P-polarized radiation is 100% or deviates therefrom only insignificantly. The description of the polarization direction is based here on the plane of incidence of the radiation on the windshield. P-polarized radiation denotes radiation whose electric field oscillates in the plane of incidence. S-polarized radiation means radiation whose electric field oscillates perpendicular to the plane of incidence. The plane of incidence is spanned by the vector of incidence in the geometric center of the illuminated area and the surface normal of the windshield.

The radiation of the projector preferably strikes the windshield at an angle of incidence of 45 ° to 75 °, in particular 60 ° to 70 °. In an advantageous embodiment, the angle of incidence differs from the brewster angle by at most 10 °. At this time, the P-polarized radiation is only insignificantly reflected on the surface of the inner glass sheet on the inner space side of the windshield sheet so that no ghost image is generated. The angle of incidence is the angle in the geometric center of the HUD region between the vector of incidence of the projector radiation and the surface normal on the interior space side (i.e., the surface normal on the exterior surface on the interior space side of the windshield panel). In the case of soda lime glass, which is commonly used for window glass panels, the brewster angle of the air/glass transition is 57.2 °. Ideally, the angle of incidence should be as close as possible to the Brewster's angle. However, for example, an angle of incidence of 65 ° can also be used, which is customary for HUD projection devices, can be realized in a vehicle without problems and deviates from the brewster angle only to a small extent, so that the reflection of P-polarized radiation increases only insignificantly.

Since the reflection of the projector radiation takes place substantially on the reflective coating and not on the outer glass-plate surface, there is no need to arrange the outer glass-plate surfaces at an angle to each other in order to avoid ghost images. Therefore, the outer surfaces of the windshield panels are preferably arranged substantially parallel to each other. The thermoplastic intermediate layer is preferably not designed as a wedge for this purpose, but rather has a substantially constant thickness, in particular also in the vertical direction between the upper edge and the lower edge of the windshield, as do the inner and outer glass panes. In contrast, the wedge-shaped intermediate layer has a variable thickness, in particular an increased thickness, in the vertical direction between the lower edge and the upper edge of the windshield panel. The intermediate layer is typically formed from at least one thermoplastic film. The production of the windscreen panel is designed to be more advantageous, since standard membranes are significantly more cost-effective than wedge-shaped membranes.

The outer and inner glass panes are preferably made of glass, in particular soda-lime glass, as is common for window panes. However, these glass plates can in principle also be made of other types of glass (e.g. borosilicate glass, quartz glass, aluminosilicate glass) or transparent plastics (e.g. polymethyl methacrylate or polycarbonate). The thickness of the outer and inner glass sheets can vary widely. Preference is given to using glass plates having a thickness of from 0.8 mm to 5 mm, preferably from 1.4 mm to 2.5 mm, for example glass plates having a standard thickness of 1.6 mm or 2.1 mm.

The outer glass sheet, inner glass sheet and thermoplastic interlayer may be clear and colorless, but may also be colored or tinted. In a preferred embodiment, the total transmission through the windshield (along with the reflective coating) is greater than 70%. The term total transmission is based on the method for measuring the light transmission of a motor vehicle glazing as defined in ECE-R43, appendix 3, clause 9.1. The outer and inner glass sheets may be unstressed, partially prestressed or prestressed independently of each other. If at least one of the glass sheets should be prestressed, this can be thermally or chemically prestressed.

In an advantageous embodiment, the outer glass plate is colored or tinted. The outer reflectivity of the windscreen panel can thereby be reduced, whereby the impression of the glass panel is more pleasant for the outside observer. However, in order to ensure a predetermined light transmission (total transmission) of 70% of the windshield plate, the outer glass plate should preferably have a light transmission of at least 80%, particularly preferably at least 85%. The inner glass sheet and the intermediate layer are preferably clear, i.e. not coloured or tinted. For example, green or blue colored glass may be used as the outer glass sheet.

The windshield panel is preferably curved in one or more directions in space, as is common for automotive glass panels, with a typical radius of curvature of about 10 cm to about 40 m. But the windscreen panel can also be flat, for example when it is arranged as a glass panel for a bus, train or tractor.

The thermoplastic interlayer comprises at least one thermoplastic polymer, preferably Ethylene Vinyl Acetate (EVA), polyvinyl butyral (PVB) or Polyurethane (PU) or mixtures or copolymers or derivatives thereof, particularly preferably PVB. The intermediate layer is typically formed from a thermoplastic film. The thickness of the intermediate layer is preferably from 0.2 mm to 2 mm, particularly preferably from 0.3 mm to 1 mm.

The windscreen panel can be manufactured by methods known per se. The outer and inner glass sheets are laminated to each other by an interlayer, for example, by autoclave, vacuum bag, vacuum ring, calendering, vacuum laminator, or combinations thereof. The joining of the outer glass pane and the inner glass pane is usually carried out here under the influence of heat, vacuum and/or pressure.

The reflective coating is preferably applied to the surface of the glass plate by Physical Vapor Deposition (PVD), particularly preferably by sputtering ("sputtering"), very particularly preferably by magnetic field-assisted sputtering ("magnetron sputtering"). The coating is preferably applied prior to lamination. Instead of applying a reflective coating to the surface of the glass plate, it is in principle also possible to provide the reflective coating on a carrier film, which is arranged in an intermediate layer.

The invention is explained in detail below with the aid of figures and examples. The figures are schematic and not to scale. The drawings are not intended to limit the invention in any way.

Wherein:

figure 1 shows a top view of a composite glass plate of a projection device of this type,

figure 2 shows a cross-section through a projection device of this type,

figure 3A shows a schematic view of an S-polarized light beam passing through a windscreen panel of a projection device,

figure 3B shows a schematic view of a P-polarized light beam passing through the windscreen panel of the projection device,

fig. 4 shows a section through a windscreen panel of a projection device according to the invention.

In all cases, the description with numerical values should not be understood as an exact value, but also include a tolerance of +/-1% to +/-10%.

Fig. 1 and 2 each show a detail of a projection device of this type for a HUD. The projection device comprises a windscreen panel 10, in particular of a passenger car. Furthermore, the projection device has a projector 4, which is directed to the region of the windscreen panel 10. This region is commonly referred to as HUD region B. In this region, an image generated by the projector 4 can be projected, which is perceived by an observer 5 (for example a vehicle driver) as a virtual image on the side of the windscreen panel 10 facing away from him, when the eyes of the observer 5 lie within the so-called eye movement range E.

The windscreen panel 10 is formed by an outer glass pane 1 and an inner glass pane 2, which are joined to each other by means of a thermoplastic interlayer 3. The lower edge U of which is arranged downwards towards the engine of the passenger car and the upper edge O of which is arranged upwards towards the top. The outer glass pane 1 faces the outside environment in the mounted position and the inner glass pane 2 faces the vehicle interior space.

Fig. 3A shows a schematic view of a beam of S-polarized light when the beam is partially reflected on the reflective coating 20 of the windscreen panel 10. The outer pane 1 of the windscreen panel 10 has an outer side surface I, which in the mounted position faces the external environment, and an inner side surface II, which in the mounted position faces the interior space. Likewise, the inner glass pane 2 has an outer side surface III, which in the mounted position faces the outside environment, and an inner space side surface IV, which in the mounted position faces the inner space. The outer glass plate 1 and the inner glass plate 2 are made of soda lime glass, for example. The outer glass plate has, for example, a thickness of 2.1 mm, and the inner glass plate 2 has a thickness of 1.6 mm or 2.1 mm. The intermediate layer 3 is formed, for example, from a PVB film having a thickness of 0.76 mm. Apart from possible surface roughness, which is common to the profession, PVB films have a substantially constant thickness, which are not designed as so-called wedge films.

The outer side surface III of the inner glass pane 2 is provided with a reflective coating 20, which is provided as a reflective surface for the projector radiation (and possibly additionally as an infrared-reflective coating).

When such a windscreen panel 10 is illuminated by S-polarized light radiation and this light radiation impinges on the windscreen panel 10 at an angle of incidence α of about 65 ° (which is close to the so-called brewster angle), this radiation is mainly reflected on the surfaces IV, III and I.

Fig. 3B shows a schematic view of a beam of P-polarized light when the beam is partially reflected on the reflective coating 20 of the windscreen panel 10.

When such a windscreen panel 10 is illuminated by P-polarized light radiation and this light radiation impinges on the windscreen panel 10 at an angle of incidence α of about 65 °, this radiation is only insignificantly reflected on the surfaces I and IV. The main reflection occurs on the reflective coating 20. It acts as a reflecting surface for the radiation of the projector 4 used to generate the HUD projections.

Figure 4 illustrates one embodiment of a windshield panel 10 designed according to this invention. In contrast to fig. 2, the windscreen panel 10 according to the invention has a reflective coating 20 and a first retarder 6.1 for converting the polarization of the radiation transmitted through the retarder.

The reflective coating 20 is arranged between the first 6.1 and the second 6.2 retardation layer. Two retardation layers 6.1 and 6.2 with a reflective coating 20 are applied on the PVB film 3.1.

The intermediate layer 3 is thus a stack here, which has a reflective coating 20, two retardation layers 6.1 and 6.2 and a PVB film 3.1. The reflective coating 20 is arranged centrally between the retardation layers 6.1 and 6.2. The first retarder 6.1 is located above the reflective coating 20. A second retardation layer 6.2 and a PVB film 3.1 are located below the reflective coating 20. The interlayer 3 joins the inner glass pane 2 and the outer glass pane 1.

The reflective coating 20 is arranged as a reflective surface for the P-polarized radiation emitted by the projector 4.

The first 6.1 and the second 6.2 retardation layer each comprise quartz. Each having a thickness of about 28 μm. The first 6.1 and second 6.2 retardation layers each cover the entire area of the HUD region. The two retardation layers 6.1 and 6.2 are designed to be transparent.

The projector 4 emits P-polarized radiation according to the invention. Since the projector 4 illuminates the windscreen panel 10 at an angle of incidence of 65 °, the radiation is only insignificantly reflected on the surface IV of the windscreen panel 10. However, the reflective coating 20 is optimized for reflection of P-polarized radiation. The P-polarized radiation passes through the first retarder 6.1 before it impinges on the reflective coating 20. The polarization of the radiation is converted in this case. The P-polarization of the radiation is rotated to S-polarization. A part of the S-polarized radiation is reflected on the reflective coating 20 and another part is transmitted.

The reflected S-polarized radiation passes through the first retardation layer 6.1 again and changes its polarization again, so that the reflected radiation emerges from the windscreen panel 10 as P-polarized radiation.

The transmitted S-polarized radiation also passes through the second retardation layer 6.2. The transmitted S-polarized radiation again changes its polarization such that the transmitted radiation emerges as P-polarized radiation on the surface I of the outer glass plate 1.

The HUD projection enhancement is visible by pure reflection of S-polarized radiation. This result is surprising and surprising to the person skilled in the art.

The reflective coating 20 is a stack of thin layers. The reflective coating 20 comprises a silver-based conductive layer. A metal barrier layer is disposed directly over the conductive layer. On which an upper dielectric layer sequence is arranged, which consists, from the bottom up, of an upper matching layer, an upper refractive index increasing layer and an upper anti-reflection layer. Below the conductive layer a lower dielectric layer sequence is arranged, which from top to bottom consists of a lower matching layer, a lower refractive index increasing layer and a lower anti-reflection layer.

The materials and layer thicknesses are known from the examples below.

The layer sequence of the reflective coating 20 according to examples 1 to 5 is shown in table 1 together with the material and the geometric layer thickness of the individual layers. The dielectric layers may be doped independently of one another, for example with boron or aluminum.

TABLE 1

。

In contrast to fig. 2, the intermediate layer 3 has a PVB film 3.1. Alternatively or additionally, the intermediate layer 3 may have a further film made of a thermoplastic polymer, preferably EVA, PU or mixtures or copolymers or derivatives thereof. The PVB film has a substantially constant thickness of about 0.38 mm.

The retardation layers 6.1 and 6.2 comprise, for example, quartz. They have a thickness of about 28 μm (micrometers). The retardation layers 6.1 and 6.2 cover the entire area of the HUD region. Alternatively, the retardation layers 6.1 and 6.2 may comprise rutile. Here, the thickness is about 870 nm (nanometers). The retardation layers 6.1 and 6.2 are designed to be transparent.

List of reference numerals:

1 outer glass plate

2 inner glass plate

3 thermoplastic interlayer

3.1 PVB films

4 projector

5 observer/vehicle driver

6.1 first retardation layer

6.2 second retardation layer

10 windscreen panel

20 reflective coating

Upper edge of the O-windscreen panel 10

Lower edge of the U windscreen panel 10

B HUD region of the windscreen Panel 10

E eye movement range

Outer side surface of the outer glass pane 1 facing away from the intermediate layer 3

II side surface of the outer glass pane 1 facing the interior space of the intermediate layer 3

III outer surface of the inner glass pane 2 facing the intermediate layer 3

IV the inner space side surface of the glass pane 2 facing away from the intermediate layer 3.

Claims

1. Projection device for a head-up display, comprising at least

A windscreen panel (10) with a HUD region, wherein the windscreen panel (10) has an outer glass panel (1) and an inner glass panel (2) joined to each other by a thermoplastic interlayer (3),

a projector (4) directed towards the HUD region, wherein the radiation of the projector (4) is predominantly P-polarized,

wherein the windscreen panel comprises a reflective coating (20) for reflecting radiation emitted by the projector (4), and

having a first retarder (6.1) for converting the polarization of radiation transmitted through the retarder (6.1), wherein the first retarder (6.1) is arranged within the HUD region, and

a reflective coating (20) is arranged between the first retardation layer (6.1) and the outer glass plate (1) or the inner glass plate (2).

2. A projection device as claimed in claim 1, wherein the windscreen panel (10) has a second retarder layer (6.2) for converting the polarization of light transmitted through the retarder layer (6.2) in the HUD region.

3. A projection device according to claim 2, wherein the reflective coating (20) is arranged between the first retardation layer (6.1) and the second retardation layer (6.2).

4. A projection device according to any one of claims 1 to 3, wherein the first retardation layer (6.1) is arranged for converting a P-polarized beam of transmitted optical radiation into an S-polarized beam.

5. A projection device according to any one of claims 1 to 4, wherein the first retardation layer (6.1) comprises quartz.

6. A projection device according to any one of claims 1 to 5, wherein the first retardation layer (6.1) comprises mica.

7. A projection device according to any of claims 1 to 6, wherein the first retardation layer (6.1) is arranged between the inner glass plate (2) and the outer glass plate (1).

8. The projection apparatus according to any of claims 1 to 7, wherein the reflective coating (20) has an electrically conductive layer.

9. The projection device of claim 8, wherein the conductive layer comprises silver.

10. A projection device according to any one of claims 1 to 9, wherein the radiation of the projector (4) is substantially pure P-polarized.