Transparent reflective glass for ghosting image suppression for use with reflective display systems

Abstract

The invention relates to a reflective glass for a reflective display system for reflecting a projection image into the eye region of a user, the flat reflective glass being made of a transparent material in which an angle-selective optical element is embedded, the optical element extends in the reflecting glass over the entire surface area of the reflecting glass, and the angle-selective optical element deflects at least a part of the projection light which carries the projection image and is coupled into the reflecting glass via the inner surface at an angle, so that the projection image is reflected on the outer surface of the reflecting glass. The portion of the coupled projection light does not reach into the eye region of the user.

Description

Technical Field

[0001] The present invention relates to a reflective display system that reflects a projected image into a user's eye region via a reflective glass. The present invention further relates to measures for suppressing ghost images caused by double reflections on two boundary surfaces of the reflective glass. Background Art

[0002] Reflective display systems, such as head-up display systems or panoramic head-up display systems, generally use a front windshield to reflect a projected image into the eyes of a user for motor vehicle applications. In a conventional head-up display system, the projection of the projected image is achieved by reflecting a virtual image into the user's eyes through reflection on a reflective glass, and in a panoramic head-up display system, the projection of the projected image is achieved by reflecting a simple image of a self-luminous display device arranged on the upper side of the instrument panel on the reflective glass.

[0003] When reflecting the projection light carrying the projected image, two images are generated due to the inclined incident angle on the front windshield, and these two images can be perceived in the user's eye region. The main image is generated by reflection on the internal air-glass boundary surface, and the ghost image is generated by reflection of the part of the projection light coupled into the reflective glass on the external glass-air boundary surface. The main image and the ghost image generally move vertically relative to each other and cause an unclear image impression.

[0004] To suppress the ghost image, a wedge film is provided in the reflective glass in a conventional reflective display system. The wedge film turns the outer surface of the reflective glass by a small angular value relative to the inner surface and thus superimposes the main image and the ghost image on each other.

[0005] However, this solution is not applicable to a so-called panoramic HUD (PHUD or glass root HUD), that is, a HUD display system with a projection distance or presentation distance in the range of 1 m or less, because the required wedge angle of the wedge film must be significantly larger than that in a conventional reflective display system with a projection distance greater than 2 m (a HUD system with a projection device in the instrument panel).

[0006] Other solutions for suppressing ghost images in the reflective glass can be achieved, for example, by using a polarized, light-reflecting coating of the front windshield. These solutions are sufficient for conventional reflective display systems, but can only be used conditionally in a PHUD because here, the incident angle of the projection light on the reflective glass and the angular distance between the main image and the ghost image are large. Therefore, good image display of a PHUD display system is generally only possible on the lower edge in the black printed area of the front windshield because only the lower edge has been able to achieve reliable suppression of the ghost image so far.

[0007] However, it is desirable to extend the image size of the PHUD display system beyond the black film area into the transparent area of the front windshield, because the current black printed area can achieve a maximum image height of approximately 6 cm, which is too small for different information displays. Summary of the Invention

[0008] The object of the present invention is to provide an improved transparent reflective glass for a reflective display system, which can achieve improved ghost image suppression.

[0009] The above object is solved by a transparent reflective glass for suppressing ghost images according to claim 1 and by a reflective display system according to the dependent claims.

[0010] Other design alternatives are given in the dependent claims.

[0011] According to a first aspect, there is provided a reflective glass for a reflective display system, the reflective glass being configured to reflect a projected image into the eye region of a user, wherein the planar reflective glass is constructed of a transparent material, and an angle-selective optical element is embedded in the transparent material, the optical element extending over the entire surface area of the reflective glass in the reflective glass, and the angle-selective optical element deflects at least a portion of the projected light carrying the projected image and coupled into the reflective glass through the inner surface at an angle such that after reflection on the outer surface of the reflective glass, the portion of the coupled-in projected light does not reach the eye region of the user.

[0012] In order to avoid ghost images in a reflective display system, it is generally necessary to suppress the reflection on the outer side of the reflective glass, or it is necessary to ensure that the projected image reflected there does not reach the eye region of the user.

[0013] To solve the above task, it is provided that the light entering the reflective glass is deflected completely or mostly during its transmission such that the light does not reach the eye region of the user. This is achieved by an optical element in the reflective glass, which has angle-selective transmission characteristics.

[0014] It can be provided that the angle-selective optical element is configured as an angle-selective diffuser, which diffusely transmits the transmitted light with an incident angle exceeding the critical angle and transmits the light with an incident angle below the critical angle without change.

[0015] The functional element of such an angle-selective optical element can be a nematic liquid crystal between two planar carrier films. Thus, the optical element can have a thickness of 50 μm to 300 μm. This sandwich structure of the optical element can be introduced between an inner glass plate and an outer glass plate in order to thereby form a reflective glass. If the reflective glass is part of the front windshield of a motor vehicle, the functional element can be introduced only into a partial region of the front windshield. Alternatively, the functional element can also be applied or laminated directly onto the inner side of a single glass plate. Thus, the angle-selective optical element can be an angle-selective diffuser, which is arranged as a layer within the reflective glass or on the inner side of the reflective glass.

[0016] The angle-selective diffuser only scatters light that is incident at a flat angle of incidence, for example greater than an angle of incidence between 60° and 80°, in particular greater than 70° (in air) and does not affect the light passing through in the remaining angular range. Thus, the light that enters the reflective glass via the inner side of the reflective glass (which would lead to the formation of a ghost image) is scattered, such that only a small fraction of the coupled-in light reaches the eye region of the user. In addition, this causes non-specific imaging due to the diffuse scattering, but only weak, washed-out, non-disturbing light spots.

[0017] Thus, the reflective glass is arranged in a reflective display system with respect to a display device providing an image such that the projection light coupled into the reflective glass is diffusely scattered in the direction of the outer side of the reflective glass when passing through the angle-selective diffuser, such that a part of the light is retained in the reflective glass by means of total internal reflection and only a small fraction reaches the eye region of the user.

[0018] According to an alternative embodiment, the angle-selective optical element can be configured as a holographic element that deflects the incident light at an exit angle greater than the critical angle of total internal reflection on the outer side of the reflective glass.

[0019] As an alternative, a holographic element can also be arranged in the reflective glass as the angle-selective optical element. The holographic element is configured here to deflect the light that is coupled into the reflective glass and reaches the holographic element during its transmission such that the light is retained in the reflective glass by means of total internal reflection and is finally absorbed in the material of the reflective glass. Such holographic deflection elements are known from different manufacturers, for example Holoptics®, Zeiss®, Digilens®, etc.

[0020] Compared with a holographic element, an angularly selective diffuser has the following advantages: The projection light can have a wide wavelength range, and the diffuser can be conventionally implemented as a volume scatterer or holographic. In contrast, using a holographic element has the following advantages: The light that is generally coupled into the reflective glass and causes a ghost image can no longer leave the reflective glass because the light is trapped therein by total reflection. This prevents, as in the case of an angularly selective diffuser, a part of the diffusely scattered light from reaching the user's eye area and thus does not cause an impact on the user's perception of the displayed image. The presentation of the ghost image can be effectively suppressed or attenuated by an angularly selective optical element in the reflective glass, thereby improving the optical image impression of the reflective display system.

[0021] According to another aspect, there is provided a reflective display system having the above-described reflective glass and a planar display device or a projection display unit for providing a projection image, wherein the reflective glass is arranged with respect to the projection light of the projection image such that the projection image reaches the user's eye area reflectively on the inner surface of the projection glass. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Embodiments are described in more detail below with reference to the drawings. In the drawings:

[0023] Figure 1a and Figure 1b A schematic view of a reflective display system in a motor vehicle is shown in a longitudinal cross-section and a top view from the viewpoint of a vehicle occupant;

[0024] Figure 2 A partial cross-sectional view of a front windshield is shown, which is configured as a reflective glass having an angularly selective diffuser; and

[0025] Figure 3 A partial cross-sectional view of a front windshield is shown, which is configured as a reflective glass having a holographic element. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] Figure 1a and Figure 1b A schematic longitudinal cross-sectional view and a view from the perspective of the driver of a motor vehicle having a reflective display system 1 are shown, respectively. The reflective display system 1 has a display device 2, which is arranged on the upper side of the instrument panel 4. The display device 2 includes an upwardly directed display surface 3 for presenting a display image. The display device 2 is arranged on the upper side of the instrument panel 4 below a section of the front windshield 5 that serves as a reflective glass 8. Thus, the lower part of the front windshield 5 can be configured as a reflective area and is the reflective glass 8. In other embodiments, the reflective glass 8 can also be constructed separately from the front windshield 5.

[0027] The reflective glass 8 can extend directly over a large part of the width of the front windshield 5 in the lower region of the front windshield 5. The reflective glass 8 is at least partially transparent and can have a black printed area in the lower region if necessary. The black printed area generally has a height between 3 and 10 cm from the lower edge of the front windshield 5.

[0028] The display device 2 is controlled by the control unit 9 so as to output a desired projection image as a display image, which should be perceived by the vehicle occupant after being reflected by the reflection area of the reflective glass 8.

[0029] The display device 2 is preferably configured as a light-emitting display device, in particular as an LCD display unit with a high-brightness backlight or as a micro-LED display unit or as a projector with a projection surface (screen) located on the instrument panel 4 below the front windshield 5, in order to provide a bright projection image, so that the resulting projection image can be perceived on the corresponding area of the front windshield 5 even in the case of high ambient brightness.

[0030] The display device 2 can be installed in the recess 6 of the instrument panel, so that the display surface 3 of the display device 2 cannot be directly seen by the vehicle occupant and glare of the vehicle occupant due to direct light from the display surface 3 into the eye area B is avoided. In addition, the display surface 3 of the display device 2 is oriented such that the output projection image is reflected on the reflective glass 8 and can be perceived by the vehicle occupant in the eye area B.

[0031] In an alternative embodiment, the display device 2 can also be provided as a projection device (corresponding to a conventional head-up display), which projects the projection image onto the reflective glass 8, where the projection image is focused into the user's eye area B.

[0032] Figure 2 A cross-sectional view of the front windshield 5 of the vehicle is shown, a part of which is configured as a transparent reflective glass 8, and the optical path of the projection light is shown. The reflective glass 8 can thus be part of the front windshield or can also be arranged as a separate element between the front windshield 5 and the display device 2. Additionally, the optical paths of the main image HB and the secondary image NB (produced without the optical element 10 described below) are shown.

[0033] The reflective glass 8 has the following structure in which the optical element 10 is arranged inside the reflective glass 8. The optical element 10 is located in the region where the reflective glass 8 should be transparent to the incident ambient light. In the reflective display system 1 with the reflective glass 8 that forms part of the head-up display (HUD) and is configured as the front windshield 5, the optical element 10 can thus be joined to the black printed area 11 of the front windshield 5. The black printed area can be a light-absorbing element inside the front windshield 5 that extends upward from the lower edge of the front windshield 5. The black printed area 11 can have a height, for example, between 5 and 10 cm starting from the lower edge, and the upper edge of the black printed area 11 is selected such that the line of sight of the motor vehicle driver to the surrounding area in front is not affected.

[0034] The optical element 10 can be configured as an angle-selective diffuser that diffusely scatters the transmitted projection light P incident at an incident angle range greater than the critical angle. The critical angle (in air) can have an angle, for example, between 60° and 80°, preferably 70°. Thus, the ambient light incident at an incident angle smaller than the critical angle can be transmitted without scattering, so that the surrounding environment can be perceived without being affected. In contrast, the projection light P, that is, the light shown by the display device and reaching or coupled into the interior of the front windshield 5 via the inner side of the front windshield 5, is diffusely scattered when it passes through the angle-selective diffuser, as shown by the hatched triangle D in Figure 2 as shown.

[0035] For this purpose, the display device 2 can be arranged with respect to the reflective glass 8 such that all the following angular ranges - in which the projection light P is coupled into the reflective glass 8 via the inner side and refracted in the direction of the angle-selective diffuser 10 - impinge on the diffuser at an angle range greater than the given critical angle. Thus, any projection light P output from the display device 2 and coupled into the reflective glass 8 is diffusely scattered toward the outside of the reflective glass 8, so that by reflection on the outside of the reflective glass (glass-air interface), only a small portion of the diffusely scattered light is reflected back in the direction of the user's eye region. The other part of the diffusely scattered light is reflected in other directions or is trapped by total reflection within the reflective glass 8 or the front windshield 5.

[0036] In Figure 3 similarly shows a cross-sectional view of the front windshield 5 of a vehicle, in which a part is configured as a transparent reflective glass 8, and the optical paths of the projection light P and the main image HB and the sub-image NB are shown. Similar to Figure 2 Figure 2 ​Depending on the implementation, the optical element 10 is configured here as an angle-selective holographic element. The holographic element is constructed such that the transmitted projection light incident on the reflective glass 8 from the inside exits the holographic element at a high exit angle and is directed at an inclined angle towards the outside of the reflective glass 8 (glass-air interface), so that the transmitted projection light is guided inside the reflective glass 8 or the windshield 5 by total reflection and is thus kept trapped.

[0037] Since the holographic element is wavelength-selective, it is preferably provided here that the light source of the display device is configured to be spectrally narrowband in order to achieve the effect of light deflection at a high exit angle and to avoid or reduce artifacts in the holographic element.

[0038] List of reference numerals

[0039] 1 Reflective display system

[0040] 2 Display device

[0041] 3 Display surface

[0042] 4 Instrument panel

[0043] 5 Windshield

[0044] 6 Recess

[0045] 8 Reflective glass

[0046] 9 Control unit

[0047] 10 Optical element

[0048] 11 Black printed area

[0049] P Projection light

Claims

1. Reflective glass (8) for a reflective display system (1), said reflective glass being for reflecting a projected image into an eye region (B) of a user, wherein, The planar reflective glass (8) is constructed of a transparent material in which an angle-selective optical element (10) is embedded or applied, and the optical element extends over the entire surface area of the reflective glass (8) within or on the reflective glass (8). The angle-selective optical element (10) deflects at least a portion of the projection light (P) carrying the projected image and coupled into the reflective glass (8) through the inner surface at an angle such that after reflection on the outer surface of the reflective glass (8), the portion of the coupled-in projection light (P) does not reach the eye region of the user.

2. The reflective glass (8) according to claim 1, wherein, The angle-selective optical element (10) is constructed as an angle-selective diffuser that diffusely transports transmitted light with an incident angle exceeding the critical angle and transmits light with an incident angle below the critical angle without scattering.

3. The reflective glass (8) according to claim 2, wherein, The angle-selective optical element (10) has a nematic liquid crystal between two - in particular parallel - planar carrier elements, in particular carrier films.

4. The reflective glass (8) according to claim 1, wherein, The angle-selective optical element (10) is constructed as a holographic element that deflects the incident light at an exit angle greater than the critical angle of total reflection on the outer side of the reflective glass (8).

5. The reflective glass (8) according to any one of claims 2 to 4, wherein, The angle-selective optical element (10) is introduced into the interior of the reflective glass (8) or applied or laminated onto the inner surface of the glass and forms the inner surface.

6. The reflective glass (8) according to any one of claims 1 to 5, wherein, The reflective glass (8) is part of the front windshield (5) of a motor vehicle.

7. Reflective display system (1), said reflective display system having a reflective glass (8) according to any one of claims 1 to 6 and a planar display device or projection display unit for providing a projection image, wherein, The reflective glass (8) is arranged with respect to the projection light (P) of the projected image such that the projected image reaches the eye region (B) of the user by reflection on the inner surface of the projection glass.

8. The reflective display system (1) according to claim 7, wherein, The display device (2) is constructed as a light-emitting display device, in particular as an LCD display unit with a backlight or as a micro-LED display unit or as a projector having a diffusely reflecting display surface (3) located on the instrument panel (4) below the front windshield (5).

9. Use of a reflective glass (8) according to any one of claims 1 to 5 for a reflective display system (1), said reflective display system having a planar display device (2) or a projection display unit for providing a projected image, wherein, The reflective glass (8) is arranged with respect to the projection light (P) of the projected image such that the projected image reaches the eye region (B) of the user by reflection on the inner surface of the projection glass (8).

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