Method for improving image quality in a HUD system, polarizing element, and HUD system equipped with such a polarizing element

Abstract

The present invention relates to a method for improving image quality in a head-up display (HUD) system, the head-up display (HUD) system comprising: an image display device (1); a reflective element (3) having a semi-transparent first reflective surface (3a) and at least one semi-transparent second reflective surface (3b) substantially parallel thereto; and a polarization-dependent reflective layer (13) and / or an anti-reflection layer (14) and / or an optically birefringent layer (15), wherein the second reflective surface (3b) is located on the opposite side of the first reflective surface (3a) from the image display device (1); the reflective element (3) generates reflected light beams (12a, 12b) from an incident light beam (11) that originates from various points of an image generated by the image display device (1) and arrives on the reflective surface (3a, 3b), and the reflected light beams (12a, 12b) are reflected from the incident light beam (11) and arrive on the reflective surface (3a, 3b), The present invention relates to a polarization element and a HUD system, the polarization element and the HUD system comprising: a reflecting element for reflecting a portion of the incident light beam (11) reflected by the reflecting element (3) toward the design detection point (23 a), the reflecting element (3) determining an optimal polarization state for each incident light beam (11) that is reflected by the reflecting element (3) toward the design detection point (23 a), such that the ratio of the intensity of the reflected light beam (12 a) first reflected by the first reflecting surface (3 a) to the intensity of the reflected light beam (12 b) first reflected by the second reflecting surface (3 b) is minimized; and a polarizing element for setting the polarization state of the incident light beam (11) reflected by the reflecting element (3) toward the design detection point (23 a) according to the predetermined optimal polarization state.

Description

[Technical field]

[0001] The present invention relates to a method for improving image quality in a Head-up display (HUD) system, comprising an image display device and a reflective element, the reflective element having a partially transmissive first reflective surface and at least one parallel partially transmissive second reflective surface, the second reflective surface being disposed on an opposite side of the first reflective surface from the image display device, the reflective element generating reflected light beams from incident light beams arriving at the reflective surface starting from multiple points of an image generated by the image display device, and reflecting a portion of the reflected light beams towards a design detection point.

[0002] The invention also relates to a polarizing element adapted to implement the method according to the invention. The present invention further relates to a HUD system comprising a polarizing element according to the present invention. [Background technology]

[0003] HUDs were originally developed for military use to allow pilots to keep their eyes on the route and not look in other directions. Conventional head-down displays (HDDs) require pilots to look at the instrument panel from time to time, and can cause fatigue when they suddenly have to focus on something close after looking at something far away. HUDs also solve the problem of having to focus on something close by creating a distant image plane. Although originating from military technology, HUDs are now also used in the automotive industry and are available to civilian users. By projecting an image onto a reflective element, or combiner (a special windscreen or special plate specially equipped for this purpose), drivers can obtain information about speed, navigation and radio settings, error messages and other driving information while driving (information that they would normally have to find by looking at the dashboard). They can also predict dangerous situations (e.g. road obstacles, road bends in poor visibility, pedestrians stepping off the sidewalk, etc.) by looking at them. Currently, HUDs are mainly installed in luxury cars, but their range of application is expanding rapidly.

[0004] The most common HUD solution installed in a vehicle (see FIG. 1) essentially consists of an image display device 1 and an aspheric mirror 2, where the reflective element 3 is the windshield. In this embodiment, the aspheric mirror 2 is for correcting the distortion of the reflective element 3, which is curved due to the windscreen design, and for focusing the diverging light beam of the image display device 1, which is a digital projector that forms a far image plane and produces a projected image 4 that the driver sees at a comfortable distance as if he is "floating" in front of the vehicle. FIGS. 2 and 3 show an enlarged view of an incident light beam 11 coming from the image display device 1. This light beam is reflected from both the first and second reflective surfaces 3a and 3b of the reflective element 3, thereby giving rise to first and second reflected light beams 12a, 12b. The reflected light beam 12a produces a first image 8, while the reflected light beam 12b also produces a second image 9. The simultaneous appearance of the two images 8, 9 results in unpleasant and even confusing results. Figure 2 shows the appearance of the two images 8, 9, and Figure 3 shows the intensity of the reflected light beams 12a, 12b for unpolarized light. Those skilled in the art will know that due to multiple reflections, even more scattered light may appear than shown in the figures. There are several known solutions to eliminate one of the two images 8, 9 formed and to reduce the intensity of one of the two light beams 12a, 12b relative to the other (i.e. to improve the contrast ratio).

[0005] The intensity reflected at each surface is governed by the so-called Fresnel law. At a reflecting surface, the incident light beam 11 and the reflected light beam 12 define a surface, the so-called incident surface 10. As known to those skilled in the art, the ratio of the intensities of the reflected light beam 12 and the incident light beam 11 is the reflection coefficient R=I R / I B (I B is the intensity of the incident beam, I RThe reflected beam intensity (θ) depends on the angle of incidence, θ, the refractive index n of the materials on either side of the surface (n=1 for air, n=1.518 for some glasses), and the relationship of the plane of incidence 10 to the polarization of the light beam. These relationships are shown in Figure 4. Linearly polarized light is S-polarized if it is polarized perpendicular to the plane of incidence 10, and P-polarized if it is polarized parallel to the plane of incidence 10. The so-called Brewster angle θ B is also known. This is the reflection coefficient R S does not become zero, and the reflection coefficient R P is the angle of incidence that characterizes a material interface where R drops to zero. (In particular, when the light is not purely S or P polarized, the polarization coefficient R S and R P (It consists of a reflective surface and a reflective mirror). HUDs that work on the principle of polarization take advantage of this effect and are designed at Brewster's angle. However, Figures 5 and 6 show that illuminating the windscreen only at Brewster's angle with S and P polarization still does not make the system suitable for a head-up display. This is because in the case of S polarization, the projected image will remain a ghost image (Figure 5), or in the case of P polarization in the absence of reflection, the reflected light will not reach the observer's eye (Figure 6), and therefore the projected image will be invisible.

[0006] To solve the problems shown in Figures 5 and 6, known HUD systems adopt the solution of changing the reflective properties at at least one of the surfaces 3a, 3b of the reflective element 3 depending on the polarization. In one known solution, a special thin film system is formed on one or more surfaces 3a, 3b of the reflective element 3. The function of the formed high reflection reflective layer 13 or low reflection anti-reflection layer 14 is to increase or decrease the light reflection at its given surface 3a, 3b for a certain type of polarization (see Figures 7, 8). At and near Brewster's angle, for the reflective element 3 illuminated with light of the given type of polarization, it is possible to achieve a high reflection contrast ratio and thus enjoy the reflected image, but the useful range is limited. In one possible embodiment, the windscreen is coated with, for example, 12 nm Ag and 63 nm SiO2 and illuminated with an S-polarized light beam 11.

[0007] The required thin film systems are usually applied on a foil (e.g. WCF = Windshield Combiner Film from 3M) which is laminated to the appropriate surface to be coated during production. A cheaper solution is a reflective film that can be applied to the windscreen after production, which can also be worked on by the user. An important aspect that cannot be overlooked when designing the reflective layer 13 is the windshield's transmission, which is regulated by countries (minimum 75% in Europe, minimum 70% in the USA). If a windscreen coated with a thin film system reflects too much light and transmits too little, the transmission will fall below the legal value and it will lose its marketability.

[0008] Another known solution (FIGS. 9 and 10) is to produce a polarization-dependent reflection by rotating the plane of polarization of the light beam passing through the first reflecting surface 3a. For example, an optical birefringent layer 15 (lambda / 2 waveplate), preferably in the form of a film, with an optical axis 150 is laminated to the first reflecting surface 3a of the reflecting element 3, which is illuminated with polarized light at an angle close to the Brewster angle. A birefringent layer 15 of suitable thickness (with an optical axis at an angle of 45° to the plane of incidence coplanar with the plane of the drawing) rotates the polarization plane of the illuminating light by 90°, i.e. from S-polarized to P-polarized and vice versa. An incident light beam 11 travelling at an angle close to the Brewster angle will thus be reflected with a lower intensity from one surface 3a or 3b than from the other surface, depending on its polarization. With S-polarized light (FIG. 9), the reflection from the first reflecting surface 3a is greater, while with P-polarized light (FIG. 10), the reflection from the second reflecting surface 3b is greater. As deviations from Brewster's angle become increasingly greater, the effect decays more and more and ghost images appear, limiting the useful angular range.

[0009] Another problem with this solution including the birefringent layer 15 is that the angle between the entrance surface 10 and the optical axis 150 of the birefringent layer 15 is different for each direction (Figures 18a and 18b) when there is a deviation from the design direction. As a result, the exact 90° polarization rotation for which the birefringent layer is designed is not met in each direction. Figure 18a shows three light beams 11a, 11b, and 11c reaching the user's eye from different points in the perceived image. Of the three light beams shown, the first light beam 11a travels in the design direction 21, and for this light beam the angle between the optical axis 150 and the entrance surface 10 corresponds to the angle Φ0 = 45 degrees, which is optimal for this HUD system. This angle does not change along the vertical direction for the light beams 11 incident on the reflective element 3 closer to one of the horizontal edges of the image seen by the user. On the other hand, if an incident light beam 11 is viewed that is offset from the vertical centerline of the image in the horizontal direction, such as light beam 11b, the angle between the incidence surface 10b and the optical axis 150 is smaller, and therefore the angle Φ1 associated with light beam 11b is smaller than the angle Φ0, i.e. Φ1<45 degrees. Similarly, for an angle offset to the left of the vertical centerline of the image, such as light beam 11c, this angle is larger, and the angle Φ2 between the incidence surface 10c and the optical axis 150 is larger than the angle Φ0, i.e. Φ2>45 degrees. The birefringent film rotates the polarization by 90 degrees only if the condition Φ0=45 degrees is met. However, for different angles Φ, the rotation of the polarization is no longer exactly 90 degrees, and the intensity ratio of the light beams 12a, 12b reflected from the two reflecting surfaces 3a, 3b deviates from the ideal ratio, and therefore the ghost image ratio increases. In Figure 18b, the curves on the right side show the same angle values ​​over the projected image area. At the vertical centerline of the image, the angle is equal to 45 degrees between the bottom and top of the image. For a light beam 11 offset to the right from the vertical centerline of the image, the various curves show light beams 11 whose optic axis forms an angle less than 45 degrees with the incidence surface 10. In this figure, the angle that the optic axis 150 of the birefringent layer 15 forms with the incidence surface 10 is 40 degrees along the first curve, 35 degrees along the second curve, 30 degrees along the third curve, and 25 degrees along the fourth curve, moving from the center to the right.For light beams 11 offset to the left of the vertical centerline of the image, the various curves show light beams 11 whose optic axes form angles greater than 45 degrees with respect to the plane of incidence 10. In this figure, the angles that the optic axis 150 of the birefringent layer 15 forms with the plane of incidence 10 are 50 degrees along the first curve, 55 degrees along the second curve, 60 degrees along the third curve, and 65 degrees along the fourth curve, moving left from the center. For each light beam 11, the angular deviation of the optic axis 150 from the ideal angle of 45 degrees leads to a proportional degradation in image quality, i.e., an increase in the ghost image ratio.

[0010] FIG. 21a shows the ghost image ratio at the deviation in vertical and horizontal angles for the system according to FIG. 9 illuminated by S-polarized light. Depending on the individual, a ghost image ratio of more than 3-5% can be unpleasant. In this figure, the boundaries of the areas with the same ghost image ratio are shown with solid lines, and the value of the ghost image ratio is displayed at each boundary line. The figure shows lines belonging to the areas with ghost image ratios of 3%, 5%, 10%, 20%, 30%, 40% or 50%, where inside each line, the value of the displayed ghost image ratio is smaller or decreases toward the inside, and outside each line, this value is larger or increases toward the outside. It can be clearly seen from this figure that a ghost image ratio of 3% is only achieved within a small area, i.e., within the vertical range of approximately +9° to -9° and horizontal range of approximately -7° to +7°. The angles shown in this figure are the angles that enter the observer's (user's) eye and are enclosed by the light beams that define a field of view that can be considered substantially "ghost image free."

[0011] Windshields, which are widely used today, consist of two layers of glass and a layer of polyvinyl butyral (PVB) enclosed between them. (The PVB layer performs a safety function: its purpose is to prevent glass fragments from falling on the passengers in case of an accident.) The birefringent layer 15 is also typically sandwiched between the two glass layers in order to protect it (FIGS. 11 and 12). This does not significantly deteriorate the contrast, but it does better protect the birefringent layer 15 from mechanical influences. Such solutions are known, for example, from patent documents US 10416447 B2 and EP 0424950 A2.

[0012] Another known solution is to place two λ / 2 waveplates (films) between two glass layers of a windscreen (front glass), with an air gap (FIG. 13) (US Pat. No. 7,839,574 B2) or a reflective thin film system (FIG. 14) (US Pat. No. 7,123,418 B2, US Pat. No. 6,952,312 B2) between them. FIG. 13 shows a flat windscreen with two λ / 2 birefringent layers 15, 15' with an air gap between them, encapsulated in a PVB layer 16, which is illuminated with P-polarized light. The angle of the film optical axis with respect to the plane of incidence is 45° for one film and 135° for the other film. FIG. 14 shows a flat windscreen with two λ / 2 birefringent layers 15, 15' with a reflective layer 13 between them, encapsulated in a PVB layer 16, which is illuminated with P-polarized light. The optical axis 150 of one birefringent layer 15 forms an angle of 45° with respect to the plane of incidence, and the optical axis 150 of the other birefringent layer forms an angle of 135°. The advantage of these two solutions is that they work with P-polarized light, i.e. polarized sunglasses do not block the visibility of the image. The first mentioned system based on the principle of P-polarized light has never been widespread, because in rainy weather, due to any kind of external dirt, the light is scattered on the outside, which deteriorates the image. In contrast, in the two solutions just mentioned, rainy weather is not an obstacle, since the strongest reflection occurs in the PVB layer 16. The mechanical protection of the film or films arranged between the two glass layers is provided by the outer glass layer.

[0013] To further improve the contrast ratio, U.S. Patent No. 5,999,314 discloses the application of reflective or anti-reflective (anti-reflective) layers 13, 14 to the exterior surfaces of the glass sheets surrounding the birefringent layer 15. These thin films on the exterior surfaces also provide the ability to change or adjust the Brewster angle.

[0014] The difficulty with the polarized method employed in some of the known solutions presented above is that it only tolerates deviations from the ideal angle over a small angular range, and as the deviation from the ideal angle increases (moving towards the edge of the image), the image quickly degrades and ghost images appear. Proper operation is typically only achieved within a range of ±3-9°, i.e., about the size of the palm of the hand on the windshield surface (the exact values ​​vary depending on the method employed and the definition of "proper operation" as already discussed with respect to Figure 21a).

[0015] From the HUD system with the image display device 1 and the reflective element 3 described above and shown diagrammatically in FIG. 15, it can generally be said that there is only one so-called design direction 21 in which it works without defects, i.e. without ghost images. Conventional HUD systems are designed such that the displayed image is ghost-free when viewed from the design direction, i.e. the design direction 21 is the direction for which the HUD system is optimized. The more vertically and horizontally one deviates from this direction, the more intense the ghost images appear. The useful area 22 shown in FIG. 15 is the spatial angle range in which ghost images are still not annoying. Depending on the individual, a ghost image ratio of more than about 3-5% becomes inconvenient (uncomfortable).

[0016] In the solution shown in Fig. 17a, the light leaving the image display device 1 passes through a simple linear polarizer 19, i.e. this polarizer 19 produces a homogeneous polarization. The polarized light beam reflects from the first mirror 17 and the second mirror 18 towards the reflective element 3, from which it is reflected towards the user's eye. The second mirror 18 is preferably an aspheric mirror producing a converging light beam from the projected diverging light beam. In the known solution, the optical axis of the linear polarizer 19 is the same at all points, so that the polarization axis of the linearly polarized light beam passing through it also points in the same direction. Thus, as the user moves towards the edge of the perceived image, i.e. deviating from the design direction 21, the polarization plane of the incident light beam 11 also becomes increasingly rotated with respect to the plane of incidence 10 (see Figs. 16a and 16b). FIG. 16a shows three incident light beams 11a, 11b, 11c having the same polarization and reaching the user's eye from different points of the image generated by the image display device 1. Of the three light beams shown, light beam 11a travels in an ideal direction, i.e., in the design direction 21, and the polarization direction of this light beam coincides with the ideal polarization direction of the present HUD system. That is, the angle β0=90 degrees that the polarization axis of light beam 11a makes with the incidence plane 10a of light beam 11a. This angle does not change along the vertical direction for many light beams 11 that are incident on the reflective element 3 closer to one of the horizontal edges of the image that the user is looking at. On the other hand, when looking at light beams that deviate from the vertical centerline of the image in the horizontal direction, as in the case of light beam 11b, for light beam 11 that is offset from the vertical centerline of the image to the right in the horizontal direction, this angle decreases, and therefore the angle β1 for light beam 11b is smaller than the angle β0, i.e., β1<90 degrees. Also, for angles to the left of the vertical center line of the image, such as light beam 11c, this angle increases, so that the angle β2 of light beam 11c is smaller than the angle β0, i.e., β2>90 degrees. Since at a given incidence plane 10 the incident light beam 11 can be considered to be purely S-polarized only when β0=90 degrees, no discussion can be made about purely S-polarized light beams at different angles. In Fig. 16b, the curves on the right show the same angle value β in the projected image area.At the vertical centerline of the image, between the bottom and top of the image, this angle is equal to 90 degrees. For a light beam 11 horizontally offset to the right of the vertical centerline of the image, the viewing angles for which the polarization axis makes an angle less than 90 degrees with the plane of incidence 10 are shown along the different curves. The angle that the polarization axis of the light beam 11 makes with the plane of incidence 10 to which that light beam 11 belongs is 85 degrees along the first curve, 80 degrees along the second curve, 75 degrees along the third curve, and 70 degrees along the fourth curve (going to the right from the vertical centerline). Similarly, for a light beam 11 horizontally offset to the left of the vertical centerline of the image, the light beams 11 with an angle greater than 90 degrees with the plane of incidence 10 are shown along the different curves. The angles that the polarization axis of light beam 11 makes with the plane of incidence 10 are (going left from the vertical centerline) 95 degrees along the first curve, 100 degrees along the second curve, 105 degrees along the third curve, and 110 degrees along the fourth curve. For each polarized light beam 11, any deviation in angle of the polarization axis from the ideal 90 degree angle leads to a degradation in image quality, i.e., an increase in the ghost image ratio, in proportion to the magnitude of the deviation. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] U.S. Patent No. 10,416,447 [Patent Document 2] European Patent Application Publication No. 0424950 [Patent Document 3] U.S. Patent No. 7,839,574 [Patent Document 4] U.S. Patent No. 7,123,418 [Patent Document 5] U.S. Patent No. 6,952,312 [Patent Document 6] U.S. Patent No. 5,999,314 Summary of the Invention

[0018] The inventors have found that in prior art HUD systems, the reflective element 3 is illuminated with homogenous light, i.e. light that is polarized in the same way for each light beam 11, and thus the polarization is only appropriate for the light beams reflected in the design direction 21. That is, when moving towards the edge of the image, due to the rotation of the entrance face 10, the polarization of the light beams 11 arriving there is less optimal than in the design direction, leading to the appearance of ghost images. In other words, in current HUD systems, only the center of the image is free of ghost images, limiting the area in which an image of acceptable quality can be displayed.

[0019] The inventors have also recognized that in existing HUD systems, the polarization axis of the incident light beam 11 belonging to the reflected light beam 12 coming from the edge of the viewed image and the plane of incidence 10 are in an angular relationship with each other that does not correspond to the design angle.

[0020] The invention is further based on the discovery that by utilizing a properly designed polarizing element placed in the optical path of the incoming light beam, the polarization state of the light beam 11 can be set according to a predetermined optimal polarization state corresponding to the propagation direction of the light beam 11. In this optimal polarization state, a substantially ghost-free image can be achieved, even at the edges of the image.

[0021] It is an object of the present invention to provide a method and apparatus in which the drawbacks of the above-mentioned conventional solutions are overcome. Another object of the present invention is to improve the image quality of current HUD systems, in particular to reduce the ghost image ratio and to increase the area which can be considered substantially free of ghost images.

[0022] Thus, the goal of the present invention is not to simply shine homogeneously polarized light on the reflective element 3, but to transform the HUD system so that polarized light arrives at each point on the reflective element 3 with the degree of polarization required to minimize the ghost image ratio. For this purpose, light with linear or elliptical (including circular) polarization may be appropriate, depending on how the HUD operates.

[0023] According to the present invention, the above object is achieved by a HUD system including an image display device, a reflective element having a semi-transmissive first reflective surface, at least one semi-transmissive second reflective surface substantially parallel thereto, and a layer selected from the group consisting of a polarization-dependent reflective layer, an anti-reflective layer, and an optically birefringent layer, wherein the second reflective surface is located on the opposite side of the image display device from the first reflective surface, the reflective element is adapted to generate a reflected light beam from an incident light beam originating from each point of an image generated by the image display device and incident on the reflective surface, and to reflect at least a part of the reflected light beam toward a design detection point, and the reflective element is arranged with respect to the image display device such that a part of the incident light beam reflected toward the design detection point reaches the first reflective surface at a Brewster angle. In the optical path between the image display device and the reflective element, the system further includes a polarizing element that sets the polarization state of the incident light beam reflected by the reflective element toward the design direction point so that the polarization state is such that the intensity ratio calculated by dividing the minimum value of the intensity of the reflected light beam first reflected by the first reflective surface and the intensity of the reflected light beam first reflected by the second reflective surface by the maximum value of these two types of intensities is the minimum value.

[0024] An image display device (also called a projection unit) typically produces an image having a width defined in a first direction and a height defined in a second direction substantially perpendicular to the first direction. The light beam emitted therefrom may pass through additional polarizing optical elements before reaching a light-transmitting and reflective reflective element (i.e. a windscreen), from which the reflected or transmitted light enters the user's eye. In a preferred embodiment, the image display device projects substantially linearly polarized light onto the reflective element. For example, the image display device may include a linear polarizer. As with all windscreens, the reflective element reflects light to a significant extent with one polarization characteristic and transmits light to a significant extent with another, different polarization characteristic. The optical element located in the optical path of the projected linearly polarized light beam sets the polarization state of the incident light beam reflected in the direction of the design detection point (i.e. the light beam that defines the projected image), as described above. This is achieved by controlling the vertical and / or horizontal angles (α) enclosed in the design direction pointing towards the design point. V , α H ) can be understood as modifying the polarization properties of these light beams in response to the

[0025] Another object of the invention is a method as claimed in claim 1 for improving the image quality of a HUD system. The invention further relates to a polarizing element as claimed in claim 6, which is adapted to carry out the method according to the invention.

[0026] According to the present invention, the polarization properties of the light beam 11 incident on the reflective element 3 are set differently for each incidence direction by appropriately designed polarizing elements so that ghost images are minimized after reflection from the reflective element 3. With such a solution, the contrast ratio of the ghost images can be significantly reduced for each pixel, i.e. the image area that the user perceives as free of ghost images can be significantly increased.

[0027] Some preferred aspects of the invention are set out in the dependent claims. Further details of the invention will be explained below with reference to the accompanying drawings. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a schematic diagram of a conventional HUD system. [Diagram 2] FIG. 1 is a schematic diagram showing a ghost image of a conventional HUD system. [Diagram 3] FIG. 3 shows the intensity of the reflection according to FIG. 2; [Figure 4] 1 is a curve showing the change in the reflection coefficient of a pure P-polarized beam and a pure S-polarized beam at an air-glass medium boundary versus the incidence angle. [Diagram 5] FIG. 13 shows the results of using pure S-polarized light and a simple windscreen. [Figure 6] FIG. 13 shows the results of using pure P-polarized light and a simple windscreen. [Figure 7] FIG. 1 shows the result of applying a reflective element with a highly reflective reflective layer on a first reflective surface and a low reflective anti-reflective layer on a second reflective surface of the reflective element. [Figure 8] FIG. 13 shows the result of applying a reflective element with a highly reflective reflective layer on the second reflective surface and a low reflective anti-reflective layer on the first reflective surface of the reflective element. [Figure 9] FIG. 13 shows the results of using an optically birefringent layer and S-polarized light on a first reflective surface of a reflective element. [Figure 10] FIG. 13 shows the results of using a birefringent layer and P-polarized light on a first reflective surface of a reflective element. [Figure 11] FIG. 13 shows the results of forming the reflective element as a multi-layer windscreen and using a PVB film, a birefringent interlayer, and S-polarized light. [Figure 12] FIG. 13 shows the results of forming the reflective element as a multi-layer windscreen and using a PVB film, a birefringent interlayer, and P-polarized light. [Figure 13] FIG. 13 shows the results of forming the reflective element as a two-layer windscreen and using two birefringent layers between the two layers with an air layer between them. [Figure 14] FIG. 13 shows the results of forming a reflective element as a two-layer windscreen and using two birefringent layers between the two layers and a reflective layer between them. [Figure 15] FIG. 1 is a schematic diagram showing areas free of ghost images in a conventional HUD system. [Figure 16a] FIG. 1 illustrates how the angle between the plane of incidence and the polarization axis of the incident light beam changes depending on the direction of the reflected light beam in a conventional HUD system. [Figure 16b] FIG. 16b illustrates the inclination of the polarization axis of the incident light beam highlighted in FIG. 16a relative to the plane of incidence. [Figure 17a] FIG. 1 illustrates how the angle between the plane of incidence and the polarization axis of the incident light beam changes depending on the direction of the reflected light beam in a conventional HUD system. [Figure 17b] FIG. 2 illustrates how the angle between the plane of incidence and the polarization axis of the incident light beam remains constant as the direction of the reflected light beam changes in a HUD system according to the present invention. [Figure 18a] FIG. 1 illustrates that in a conventional HUD system, the angle between the plane of incidence and the optical axis of the birefringent layer on the reflective element varies depending on the direction of the reflected light beam. [Figure 18b] FIG. 18b illustrates the inclination of the plane of incidence of the light beam highlighted in FIG. 18a relative to the optical axis of the birefringent layer of the reflective element. [Figure 19a] FIG. 1 illustrates a conventional HUD system with only a linear polarizer placed after the image display device, showing the optic axis of the birefringent layer on the reflective element. [Figure 19b] FIG. 1 illustrates a HUD system according to the present invention comprising a linear polarizer placed after an image display device followed by a polarizing element according to the present invention, showing the optical axes of the birefringent layers placed on the reflective element and on the polarizing element, respectively. [Figure 19c] FIG. 2 is a schematic diagram showing a curved polarizing element according to the present invention. [Figure 20a] FIG. 1 is a schematic diagram of an image display device and light beams formed by polarizers in a conventional HUD system. [Figure 20b]2 is a schematic diagram of a light beam produced by an image display device and a first exemplary polarizing element of a HUD system according to the present invention; FIG. [Figure 20c] 2 is a schematic diagram of a light beam produced by an image display device and a second exemplary polarizing element of a HUD system according to the present invention. [Figure 20d] 2 is a schematic diagram of a light beam generated by an image display device and a third exemplary polarizing element of a HUD system according to the present invention. [Figure 20e] 13 is a schematic diagram of a light beam generated by an image display device and a fourth exemplary polarizing element of a HUD system according to the present invention. [Figure 21a] FIG. 20b shows projected image ghost image ratio values ​​for the HUD system shown in FIG. 20a. [Figure 21b] FIG. 20b shows projected image ghost image ratio values ​​for the HUD system shown in FIG. [Figure 21c] FIG. 20C shows projected image ghost image ratio values ​​for the HUD systems shown in FIGS. 20C, 20D, and 20E. [Figure 22] 1 is a schematic diagram of a preferred embodiment of a HUD system according to the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] 1-17a and 21a illustrate prior art HUD systems and their problems, which have already been described in detail under the heading Background of the Invention of the specification. For the sake of clarity, reference will be made below to the previously described figures only to the extent necessary.

[0030] The method according to the invention comprises, for example as shown in Fig. 17b, an image display device 1 and a reflective element 3, the reflective element 3 having a semi-transparent first reflective surface 3a, at least one semi-transparent second reflective surface 3b substantially parallel thereto, and a polarization-dependent reflective layer 13 and / or an anti-reflection layer 14 and / or an optically birefringent layer 15. The second reflective surface 3b is located on the opposite side of the first reflective surface 3a from the image display device 1, i.e. the light beam generated by the image display device 1 first reaches the first reflective surface 3a and is transmitted therethrough to the second reflective surface 3b. In the context of the invention, the polarization-dependent reflective layer 13 and the anti-reflection layer 14 respectively refer to layers (e.g. thin films, coatings, etc.) whose reflection is maximum or minimum for a certain polarization of light (e.g. S-polarized light). That is, these layers reflect (reflective layer 13) or transmit (anti-reflection layer 14) a certain type of polarized light with high efficiency. The reflective layer 13 and the anti-reflective layer 14 may be disposed on or between the surfaces 3a, 3b as previously described.

[0031] In a preferred embodiment, the reflective element 3 is provided as a windscreen (front glass) of a vehicle, with its inner surface being the first (reflective) surface 3a and its outer surface being the second (reflective) surface 3b. The reflective surfaces 3a, 3b are adapted to generate reflected light beams 12 from incident light beams 11 arriving at the reflective surfaces 3a, 3b originating from various points of the image generated by the image display device 1, and to reflect a portion of the reflected light beams 12a, 12b towards a certain designed detection point 23a located within the designed detection area 23. It should be noted that the image generated by the image display device is the image that is perceptible on the screen or other surface of the image display device. The image seen by the user of the HUD system is referred to as the projected image. The reflective element 3 is preferably positioned such that at least a portion of the incident light beam 11 arrives at the surface 3a, 3b at the Brewster angle. It should be noted that in the context of this specification, the designed detection area 23 means the area that includes the spatial positions from which the user expects to see the image generated by the HUD system, and is therefore a design parameter. Therefore, the design detection area 23 depends on the environment of the HUD system. For example, in the case of a HUD system designed to be installed in a certain type of vehicle, it depends on the seat position of the driver's seat relative to the windscreen. In practice, for example, the eye positions of users (drivers) of various heights (seated heights) may be determined when they are seated in the driver's seat of a certain type of vehicle in the driving position (i.e., when the driver's seat is set to the correct height and distance for comfortable driving). The area that includes the determined eye positions may be considered as the design detection area 23, and for example, the center point of the design detection area 23 may be considered as the design detection point 23a. It is also conceivable to determine only the design detection point 23a. For example, the design detection point 23a may correspond to a point located at the midpoint between the eyes of a user of average sitting height sitting in the driver's seat in the driving position. Another possibility is to determine the design detection point 23a for a certain user. By default, the design direction 21 mentioned above faces the design detection point 23a when the HUD system is used as intended.It should also be noted that the reflective element 3 reflects only some of the light beams emanating from various points of the image generated by the image display device 1 in the direction of the design detection point 23a, while the remaining beams of light are reflected in other directions. However, since only these light beams reach a hypothetical user looking from the design detection point 23a (thereby forming the projected image), the method according to the invention only deals with the incident light beams 11 that create these light beams, and in the further steps of the method, these light beams are referred to as the incident light beams 11 reflected in the direction of the design detection point 23a.

[0032] According to the present invention, an optimal polarization state is determined for the incident light beam 11 reflected by the reflecting surfaces 3a, 3b in the direction of the design detection point 23a. This optimal polarization state is the polarization state for which the intensity ratio of the two reflected light beams 12a, 12b, the reflected light beam first reflected by the first reflecting surface 3a and the reflected light beam first reflected by the second reflecting surface 3b, is minimal when a given incident light beam 11 is reflected. In the case shown in FIG. 12, this intensity ratio is zero for the light beams 12a and 12b reflected by the reflecting surfaces 3a, 3b (because the P-polarized light beam 11 incident at Brewster's angle passes completely through the reflecting surface 3a and therefore there is no reflection). That is, the minimum intensity ratio of the reflected light beams 12a, 12b is also zero. From the point of view of the HUD system, this is an ideal case, since no ghost images are generated. As used herein, the reflected light beam 12b first reflected by the second reflecting surface 3b is understood to mean the light beam that passes through the first surface 3a and is first reflected by the second surface 3b. For simplicity, higher order reflected light beams as shown in FIG. 5, which are reflected between the first and second surfaces 3a, 3b and further reflected by the second surface 3b in the direction of the design detection point 23a, are not taken into account. The optimal polarization state is determined for each incident light beam 11 reflected by the two reflecting surfaces 3a, 3b in the direction of the design detection point 23a. Will be .

[0033] As known to those skilled in the art, the polarization state of the light beam 11 can be represented by the so-called Jones vectors, i.e. the amplitudes of oscillation of the light beam components and their phase angles (if the phase difference of the Jones vectors X and Y is 0° or 180° it is called linear polarization, if it is 90° it is called circular polarization; in all other cases it is called elliptical polarization).

[0034] In one exemplary embodiment, as already shown with respect to Figures 7 and 8, one or both of the first and second reflecting surfaces 3a, 3b are provided with a reflecting layer 13 and / or an anti-reflection layer 14 designed for a particular type of linearly polarized light (S or P). The reflection and transmission of the surfaces 3a, 3b of the reflecting element 3 provided with the reflecting layer 13 are as follows: For each material interface, for each direction, the reflection and transmission coefficients, i.e. the so-called Fresnel coefficients, can be determined. That is, for the reflecting element 3 (e.g. glass), the following coefficients can be determined: for the first surface 3a: r 12 ,t 12 ,r 21 ,t 21 , and for the second surface 3b: r 23 ,t 23 ,r 32 ,t 32 These coefficients are complex numbers that depend on the angle of incidence and the complex refractive index of the material on both sides of the surface, as well as the structure of the thin film applied to the surface. Furthermore, these coefficients also depend on the polarization, and for S-polarized light (r 12S ,t 12S ,r 21S ,t 21S ,r 23S ,t 23S ,r 32S ,t 32S ) and P-polarized light (r 12P ,t 12P ,r 21P ,t 21P ,r 23P ,t 23P ,r 32P ,t 32P ) can be determined separately.

[0035] The Jones vector of the incident beam is given by

[0036]

number

[0037] And when the strength is expressed by the following formula:

[0038]

number

[0039] For the first (E1) and second (E2) reflected light beam, the following Jones vectors are obtained:

[0040]

number

[0041] where

[0042]

number

[0043] is the matrix of propagation in reflecting element 3. The intensities of the reflected light beams 12a and 12b are respectively given by the following equations:

[0044]

number

[0045] where d is the thickness of the reflective element 3, κ is the extinction coefficient of the material (e.g., glass) filling the space between the two reflective surfaces, and λ 0 is the wavelength of the illuminating light beam. The ghost image ratio is equal to the ratio of the minimum intensity to the maximum intensity, given by:

[0046]

number

[0047] Near the Brewster angle, CR occurs when X=0 or Y=0. ghost will be minimized, which is exactly the case for P or S polarization, respectively. Thus, in this embodiment, the determined optimal polarization state for the light beam 11 is P or S polarization, respectively, depending on the type of the reflective layer 13 (or anti-reflection layer 14). Thus, in order to keep the intensity ratio of the reflected light beams 12a, 12b to a minimum, i.e., to minimize the ghost image ratio, the reflective element 3 must be illuminated at each point with a P or S polarized light beam 11. That is, the polarization axis of the linearly polarized light beam 11 must be set relative to the orientation of the incidence face 10 of that given light beam 11. This can be determined individually, taking into account the parameters of the HUD system (e.g., the geometry, arrangement, etc. of the reflective element 3), as will be obvious to those skilled in the art.

[0048] In another possible embodiment, the reflective element 3 is provided with at least one optical birefringent layer 15 with an optical axis 150. This optical birefringent layer 15 is arranged in front of and parallel to the second reflective surface 3b, as already explained with reference to Figures 9, 10 and 18a, 18b. To achieve a minimum ghost image ratio, the embodiment using the birefringent layer 15 requires an elliptically polarized light beam 11, whose polarization, approaching the reflective element 3 obliquely and passing through the birefringent layer 15, undergoes a phase shift such that the beam 11 reaches the second reflective surface 3b with S or P polarization. The ghost image ratio will thus be minimized.

[0049] If the reflective element 3 is coated with a birefringent layer 15 on its first surface 3a, the equation in the above example becomes:

[0050]

number

[0051] Δ is the phase shift of the birefringent layer 15, which depends on the angle of incidence and direction of the light beam 11. φ is the angle between the plane of incidence 10 and the optical axis 150, which also depends on the direction of the light beam. Similarly, the intensity can be calculated in this case as follows:

[0052]

number

[0053] In design direction 21, Δ=π and φ=45°, so we have the following equation:

[0054]

number

[0055] Then, R2 is:

[0056]

number

[0057] It can be seen that the terms for the reflection of S-polarized and P-polarized light are reversed compared to the previous case. The ghost image ratio is:

[0058]

number

[0059] The reflection coefficient of P-polarized light is always smaller than that of S-polarized light (r P <r S ), whereas the opposite is true for transmission (t P >t S ). Furthermore, at Brewster's angle, r P =0 and t P = 1, and near the Brewster angle, r P ≒0 and t P ≒1. Therefore, when X=0 or Y=0, the ghost image ratio CR ghostwill be minimized. Either for P-polarized light (in which case R1 < R2) or simply for S-polarized light (in which case R1 > R2), it is exactly in that state. That is, in the design direction 21, the reflection element 3 must be irradiated with one of these polarizations in order to minimize the ghost image ratio. Outside the design direction 21, at other incident angles 10, the conditions of Δ = π and φ = 45° are not satisfied, and in the expression of CR ghost the terms of reflection of S-polarized light and P-polarized light are not simply interchanged, and their combination is obtained. Then, the optimal polarization state can be determined by minimizing the following expression.

[0060] [Number]

[0061] In the above formula, a and b are complex numbers. The ghost image ratio CR ghost It should be noted that the optimal polarization state that minimizes is not necessarily linearly polarized light. Therefore, the phases of X and Y will not necessarily be the same. When the phases of X and Y are not the same, it is called elliptically polarized light. That is, in the method according to the present invention, the polarization state of the light beam 11, that is, the relative phases X and Y, are formed so as to maintain the minimum of CR ghost before reaching the reflection element 3.

[0062] In the next step of the method according to the present invention, the polarization state of the incident light beam 11 reflected in the direction of the design detection point 23a by the reflecting surfaces 3a and 3b is set (set) according to the optimal polarization state already determined by the polarization element 20 arranged in the optical path of the incident light beam 11. When the optimal polarization state is determined for a single representative light beam 11 instead of a plurality of light beams 11 emitted from a predetermined image area, then the polarization states of all the light beams emitted from that predetermined image area are set to correspond to the optimal polarization state determined for the representative light beam 11.

[0063] In an embodiment with a reflective layer 13, the optimal polarization state of the incident light beam 11 is created by linearly polarizing each of the incident light beams 11 reflected in the direction of the design detection point 23a by the polarizing element 20 so that each of the incident light beams 11 reaching the first reflective surface 3a is S-polarized.

[0064] In the embodiment with the birefringent layer 15, creating an optimal polarization state for the incident light beam 11 is achieved by elliptically polarizing the incident light beam 11 reflected in the direction of the design detection point 23a by the polarizing element 20 so that each of the incident light beams 11 passing through at least one optical birefringent layer 15 and reaching the second reflecting surface 3b is S-polarized or each is P-polarized.

[0065] In a particularly preferred embodiment, an LCD panel 34 with controllable pixels is provided as the polarizing element 20, in which the incident light beam 11 reflected in the direction of the design detection point 23a passes through the pixels of the LCD panel 34. The polarization state of the transmitted incident light beam 11 is set according to a predetermined optimal polarization state by controlling the pixels. As known to those skilled in the art, each pixel of the LCD panel 34 acts as an elementary polarizer, where the optical axis and / or phase shift can be varied pixel by pixel according to the desired polarization characteristics by controlling the voltage applied to each pixel, i.e., by controlling the LCD panel 34.

[0066] The inventors have recognized that the image created by the HUD system may not always be viewed by the user from the same position (e.g., leaning sideways while driving) or the position of the user's eyes may be substantially different from the design detection point 23a for some other reason (e.g., the design detection point 23a is previously determined for an average user, and the actual user's physique is substantially different therefrom). In this case, a HUD system optimized for a fixed design detection point 23a may not work properly because the plane of incidence 10 of the light beams entering the user's eyes will be rotated from their previous position, and the predetermined optimal polarization state of the light beam 11 will not provide the minimum ghost image ratio. The advantage of the LCD panel 34 is that the polarization characteristics can be dynamically changed, so that the polarization state of the light beam 11 passing through a pixel can be set as desired. Thus, in a particularly preferred embodiment, the design detection point 23a is selected according to the current observation position of the user using the HUD system. In other words, the design detection point 23a is adjusted in real time to the user's current viewing position, preferably a point located at the midpoint between the user's eyes, and the optimal polarization state of the light beam 11 reflected towards the currently determined design detection point 23a is determined accordingly. By controlling the LCD panel 34, the polarization characteristics of the light beam 11 passing therethrough are set according to the thus determined optimal polarization state. Determining the user's current viewing position can be achieved, for example, by observing the user with one or more cameras using face recognition and motion tracking algorithms, as will be obvious to those skilled in the art.

[0067] The invention further relates to a polarizing element 20 designed to implement the method according to the invention, which determines the polarization properties of the light beam 11 reaching the reflecting element 3 and defining the projected image according to the vertical and horizontal angles α between said light beam 11 and the optical axis, the design direction 21. V , α H The amount of the change is changed depending on the amount of the change.

[0068] In one exemplary embodiment, where the reflective element 3 preferably comprises a reflective layer 13 and / or an anti-reflection layer 14, the polarizing element 20 is a linear polarizing filter with a plurality of polarization axes, where the directions of the polarization axes at different points of the polarizing filter are different. Preferably, the polarizing element 20 has a substantially flat surface. Such a configuration can be seen, for example, in Fig. 17b and Fig. 20c. Here, the polarizing element 20 arranged in the optical path of the light beam 11, starting from the image display device 1 and passing through a linear polarizer 19, is configured such that its polarization axis varies from point to point, and thus the polarization axis of the transmitted light beam 11 also varies depending on the exit position of the light beam 11. Thus, in this example, the light beam 11 passing through the polarizing element 20 will be a linearly polarized light beam with different polarization axes. These light beams will reach the different points of the surfaces 3a, 3b of the reflective element 3 as pure S-polarized or pure P-polarized beams, depending on the design of the reflective element 3 (see below). The optimal polarization state to be created by the polarizing element 20 depends on the design of the reflective element 3. For example, if the first surface 3a of the reflective element 3 is coated with a reflective layer 13 having a higher reflectivity for S-polarized light than for P-polarized light, and the second surface 3b is coated with an anti-reflective layer 14 having a low reflectivity for both polarizations, the light beam 11 reaching the reflective element 3 must be a substantially pure S-polarized beam. If the second surface 3b of the reflective element 3 is coated with a reflective layer 13 having a higher reflectivity for S-polarized light than for P-polarized light, and the first surface 3a is coated with an anti-reflective layer 14 having a low reflectivity for both polarizations, the light beam 11 reaching the reflective element 3 will also be a pure S-polarized beam. On the other hand, if the first surface 3a of the reflective element 3 is coated with a reflective layer 13 having a higher reflectivity for P-polarized light than for S-polarized light, and the second surface 3b is coated with an anti-reflective layer 14 having a low reflectivity for both polarizations, the light beam 11 reaching the reflective element 3 will be a substantially pure P-polarized beam.Finally, if the second surface 3b of the reflective element 3 is coated with a reflective layer 13 having a higher reflectivity for P polarized light than for S polarized light, and the first surface 3a is coated with an anti-reflection layer 14 with low reflectivity for both polarizations, then the light beam 11 reaching the reflective element 3 must still be a purely P polarized beam.

[0069] In another exemplary embodiment in which the reflective element 3 is provided with at least one birefringent layer 15, the polarizing element 20 is an optical birefringent element with multiple optical axes, the phase shift of which is different at different points of the birefringent element, which adjusts (modulates) the polarization properties of the light beam passing through its surface differently depending on the position of incidence, so as to ensure that the transmitted light beam 11 is an elliptically polarized beam with a predetermined polarization direction. That is to say, the light, preferably linearly polarized, from the image display device 1 is modulated by the polarizing element 20 with a phase shift of different degrees at different positions according to the optimal polarization state to be achieved. The ellipticity of the resulting elliptically polarized light beam can be varied as required between linear and circular polarization, as previously described with respect to the Jones vector formula.

[0070] Depending on the birefringent layer 15 used for the reflective element 3 and the polarization direction (S or P) of the light entering the reflective element 3, different shapes of the polarizing element 20 can be used as a polarization modulator in the HUD system according to the present invention between the image display device 1 or its linear polarizer 19 and the reflective element 3. That is, the polarizing element 20 can be formed as a plane or an arbitrarily curved surface, as shown in Fig. 20b and Fig. 20d. The exact shape of the polarizing element 20 formed as a birefringent layer mainly depends on the distance between the observer and the polarizing element 20 and the shape of the reflective element 3. For example, if the light entering the reflective element 3 is P-polarized light, the polarizing element 20 used as a polarization modulator preferably forms a substantially planar surface. On the other hand, if the light reaching the reflective element 3 is S-polarized light, the polarizing element 20 preferably forms a non-planar curved surface (see Figs. 19b and 20b), and its exact curvature (arc shape) can be determined, for example, by numerical simulation. The linearly polarized beams arriving at the curved surface travel as different elliptically polarized light beams 11 due to the tilted incidence angle, tilted optical axis and tilted polarization plane. In one such embodiment, the radius of curvature R of the circular arc is preferably half the distance A+B between the polarizing element 20 and the designed detection point 23a, and the intersection point of the radius of curvature R is on the side of the polarizing element 20 facing the reflective element 3, as shown in Figure 19c.

[0071] In a preferred embodiment shown in Fig. 19b, the polarizing element 20 is a birefringent element, whose optical axis 150' is substantially at an angle of 90 degrees with the optical axis 150 of the birefringent layer of the reflective element 3. By adjusting these two optical axes 150, 150' perpendicular to each other (rotating one by +45° and the other by -45° from the perpendicular axis of symmetry), the wavelength dependency can be reduced, so that achromatic behavior can be achieved. This is because the two birefringent layers are designed for one and the same constant wavelength. For different wavelengths, the light beam undergoes different phase shifts, which leads to color fringing of the system. When the optical axes 150, 150' are perpendicular to each other, the amount of error introduced by the light beam after passing through the polarizing element 20 configured as a birefringent layer is compensated by the birefringent layer 15 arranged in the reflective element 3.

[0072] In a particularly preferred embodiment shown in Fig. 20e, the polarizing element 20 is an LCD panel 34 with individually controllable pixels for producing both linearly and elliptically polarized beams of any optical axis, whose polarization properties can be dynamically changed by controlling the voltage applied to each pixel. In the embodiment shown in Fig. 20e, the optical element placed in the optical path of the light, preferably pre-linearly polarized light produced by the image display device 1, is a controllable LCD panel 34 designed and controlled so that the phase shift can be varied point-by-point according to the desired polarization properties. No polarizing filters are required in this setup, since the liquid crystal pixels of the LCD panel 34 are used for polarization modulation. By controlling each pixel, the polarization of the transmitted light is changed point-by-point, rather than changing the brightness as in conventional LCD displays. The linearly polarized light coming from the linear polarizer 19 can thus be modulated in each direction by controlling the pixels. Since the optical axis of the birefringent liquid crystal of the LCD panel 34 can be rotated to the desired extent, the LCD panel 34 is essentially a controllable version of the polarizing element 20 of Fig. 20d. The parameters needed to control the pixels of an LCD can be determined by measurement or modeling.

[0073] The invention further relates to a HUD system comprising a polarizing element 20 according to the invention. The HUD system comprises an image display device 1 and a reflective element 3 having a partially translucent first reflective surface 3a and at least one substantially parallel partially translucent second reflective surface 3b, and further a polarization-dependent reflective layer 13 and / or an antireflection layer 14 and / or an optically birefringent layer 15. In a particularly preferred embodiment, the reflective element 3 is a windscreen of a vehicle, where the first reflective surface 3a is the inner surface of the windscreen and the second reflective surface 3b is the outer surface of the windscreen. The image display device 1 is a digital projector or a digital screen known per se. Optionally, as will be obvious to a person skilled in the art, additional optical deflectors are arranged between the image display device 1 and the reflective element 3, such as a first mirror 17 and / or a second mirror 18 as shown in Fig. 17b.

[0074] In a preferred embodiment, the HUD system comprises a reflective element having a first optically birefringent layer 15 with an optical axis 150 and a polarizing element 20 formed as a second birefringent element with an optical axis 150', the first and second optical axes 150, 150' being substantially at a 90 degree angle with respect to each other. In a particularly preferred embodiment shown in FIG. 22, the polarizing element 20 is an LCD panel 34, and the HUD system comprises one or more digital cameras 40 for determining the user's current viewing position and a central IT unit 50 associated therewith, the central IT unit 50 being configured to control the LCD panel 34. The term central IT unit 50 is used herein in a broad sense to encompass any hardware device capable of receiving, processing and preferably storing digital data. In one possible example, the central IT unit 50 is a computer with a storage medium for storing computer programs and data received from the digital camera 40, and a central processing unit (processor) for processing the received data and running the computer programs. A face recognition and motion tracking algorithm known per se may be run in the central IT unit 50, which identifies the current spatial position of the user's face based on image information provided by one or more cameras 40, and thus determines the current detection point 23a. Knowing the position of the detection point 23a, the plane of incidence 10 through which the light beams 12a, 12b reflected from the reflective element 3 pass and reach the user's eye can be determined. That is to say, the optimal polarization state of the light beam 11, which results in the minimum ghost image ratio, can also be determined by the central IT unit 50. The optimal polarization state can be set by appropriately controlling an LCD panel arranged in the optical path of the light beam 11.

[0075] 21a-c show the ghost image ratios caused by the conventional HUD system and the HUD system of the present invention. As already mentioned, for the HUD system of FIG. 19a or FIG. 20a, it can be considered that there is no ghost image (CR ghostThe range of the chromatic aberration (<3%) is very limited (±9° vertically and ±7° horizontally) as can be clearly seen in FIG. 21a. In contrast, the HUD system according to the present invention shown in FIG. 20b is considered to be free of ghost images (CR ghost The ghost image free range (<3%) is substantially increased (±9° vertically and ±25° horizontally) as can be clearly seen in Figure 21b. The ghost image ratio for the HUD systems resulting from the embodiments of Figures 20c, 20d and 20e is shown in Figure 21c. For these HUD system designs, the ghost image free range is further improved (±9° vertically and over ±30° horizontally) as can be clearly seen in Figure 21c.

[0076] As is clear from the above description, the present invention significantly reduces the ghost image ratio in HUD systems that work on the polarization principle, or significantly increases the range where ghost images are considered to be free. The proposed solution is compatible with existing systems and can also be implemented in new HUD devices with significantly larger image sizes. In some cases, the horizon seen by the driver can be completely covered, so that a so-called AR (Augmented Reality) experience can also be created. The novel solution also has the great advantage that it can be combined with P-polarized HUD systems (which also work with polarized sunglasses). Of course, the solution is not only applicable to automobiles (motor vehicles), but also to windscreens, for example, of aircraft, ships, etc.

[0077] Various modifications of the above-described embodiment will be apparent to those skilled in the art without departing from the scope of protection of the invention as defined in the appended claims. [Explanation of symbols]

[0078] 1: Image display device 2: Aspherical mirror 3: reflective element, 3a: first reflective surface, 3b: second reflective surface 4: Projected image 8: First image, 9: Second image 10:Incidence plane 11, 11a, 11b, 11c: incident light beam 12, 12a, 12b: Reflected light beam 13: Reflective layer 14: Anti-reflection layer 15, 15': Birefringent layer 16:PVB layer 17, 18: Mirror 19: Linear polarizer 20: Polarizing element 21:Design direction 22: Useful area 23: Design detection area 23a: Design detection points 34: LCD panel 40: Digital camera 50: Central IT Unit 150, 150': optical axis of birefringent layer

Claims

1. A method for improving an image quality of a HUD system, in particular by reducing ghost images of the HUD system, comprising an image display device (1) and a reflective element (3) having a semi-transparent first reflective surface (3a), at least one semi-transparent second reflective surface (3b) substantially parallel thereto, and a layer selected from the group consisting of a polarization-dependent reflective layer (13), an anti-reflection layer (14) and an optically birefringent layer (15), where the second reflective surface (3b) is located on an opposite side of the image display device (1) from the first reflective surface (3a), the reflective element (3) being configured to generate reflected light beams (12a, 12b) from each incident light beam (11) emanating from different points of an image generated by the image display device (1) and incident on the reflective surface (3a, 3b), and to reflect at least a part of the reflected light beams (12a, 12b) towards a design detection point (23a), the method being characterized by: - for each incident light beam (11) reflected by the reflective element (3) in the direction of the design detection point (23a), determining the optimal polarization state that minimizes the ratio of the intensities of the reflected light beams (12a, 12b) calculated by dividing the minimum of the intensity of the reflected light beam (12a) first reflected by the first reflective surface (3a) and the intensity of the reflected light beam (12b) first reflected by the second reflective surface (3b) by the maximum of these two intensities, when reflecting that given incident light beam (11); and - setting the polarization state of each incident light beam (11) reflected by the reflecting element (3) in the direction of the design detection point (23a) according to said predetermined optimal polarization state by a polarizing element (20) arranged in the optical path of the incident light beam (11).

2. 2. A method according to claim 1, characterized in that the reflection coefficient of at least one of the first and second reflecting surfaces (3a, 3b) is changed by a layer selected from a polarization-dependent reflective layer (13) and an anti-reflective layer (14), and the creation of an optimal polarization state of the incident light beam (11) is performed by linearly polarizing each of the incident light beams (11) reflected in the direction of the design detection point (23a) by a polarizing element (20) in such a way that each of the incident light beams (11) arriving at the reflecting surface (3a, 3b) with the changed reflection coefficient is S-polarized or P-polarized depending on the type of the layer.

3. 2. The method according to claim 1, characterized in that the reflective element (3) is provided with at least one optical birefringent layer (15) having an optical axis (150), which optical birefringent layer (15) is arranged in front of and parallel to the second reflective surface (3b), and the creation of an optimal polarization state of the incident light beam (11) is performed by elliptically polarizing the incident light beam (11) reflected in the direction of the design detection point (23a) by a polarizing element (20) so that each of the incident light beams (11) passing through the at least one birefringent layer (15) and reaching the second reflective surface (3b) is S-polarized or each is P-polarized.

4. 4. The method according to claim 1, further comprising providing, as the polarizing element (20), an LCD panel (34) with controllable pixels, passing an incident light beam (11) through the pixels of the LCD panel (34) and setting the polarization state of the transmitted incident light beam (11) according to a predetermined optimum polarization state by controlling the pixels.

5. 5. The method of claim 4, further comprising providing at least one digital camera, detecting a current observation position of a user of the HUD system using the digital camera, selecting design detection points (23 a) according to the detected current observation position of the user, and determining an optimal polarization state using the selected design detection points (23 a).

6. A polarizing element (20), characterized in that it is adapted to carry out the method according to any one of claims 1 to 5.

7. 7. The polarizing element (20) according to claim 6, characterized in that the polarizing element (20) is a linear polarizing filter with a plurality of polarization axes, the directions of which differ at different points of the polarizing filter.

8. 7. The polarizing element (20) according to claim 6, characterized in that the polarizing element (20) is an optical birefringent element having a plurality of optical axes (150'), the plurality of optical axes (150') and / or the phase shift thereof being different at different points of the birefringent element.

9. A polarizing element (20) according to any one of claims 6 to 8, characterized in that the polarizing element (20) has a plane or curved surface.

10. 7. The polarizing element (20) according to claim 6, characterized in that the polarizing element (20) is an LCD panel (34) having individually controllable pixels.

11. A HUD system comprising an image display device (1) and a reflective element (3) having a semi-transmissive first reflective surface (3a), at least one semi-transmissive second reflective surface (3b) substantially parallel thereto, and one layer selected from the group consisting of a polarization-dependent reflective layer (13), an anti-reflection layer (14) and an optically birefringent layer (15), wherein the second reflective surface (3b) is located on the opposite side of the first reflective surface (3a) from the image display device (1), and the reflective element (3) reflects various different portions of an image generated by the image display device (1). a first reflecting surface (3a, 3b) that reflects at least a portion of the reflected light beam (12a, 12b) toward a design detection point (23a), the reflecting element (3) being disposed relative to an image display device (1) such that a portion of the incident light beam (11) reflected toward the design detection point (23a) reaches the first reflecting surface (3a) at a Brewster angle, the HUD system being characterized in that: The HUD system comprises a polarizing element (20) arranged in an optical path between an image display device (1) and a reflective element (3), for adjusting the polarization state of each incident light beam (11) reflected by the reflective element (3) toward a design detection point (23a) in such a way that, just at the time of reflection of the incident light beam (11) while passing through the polarizing element (20), an intensity ratio calculated by dividing the minimum value of the intensity of the reflected light beam first reflected by a first reflective surface and the intensity of the reflected light beam first reflected by a second reflective surface by the maximum value of these two intensities is minimized.

12. 12. The HUD system of claim 11, comprising a reflective element (3) having an optically birefringent layer (15) with a first optical axis (150), and the polarizing element (20) is a birefringent element with a second optical axis (150'), wherein the first and second optical axes (150, 150') are at an angle of substantially 90 degrees to each other.

13. 13. The HUD system according to claim 11 or 12, characterized in that the polarizing element (20) is an LCD panel (34), the HUD system further comprising one or more digital cameras (40) for determining a current observation position of a user of the HUD system, and a central IT unit (50) connected to the cameras, the central IT unit (50) being configured to control the LCD panel (34).

14. The HUD system according to any one of claims 11 to 13, characterized in that the image display device (1) is a digital projector or a digital display.

15. 15. A HUD system according to any one of claims 11 to 14, characterized in that the reflective element (3) is a windshield of a vehicle, the first reflective surface (3a) is an inner surface of the windshield, and the second reflective surface (3b) is an outer surface of the windshield.

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