Optical waveguide for a display device

Abstract

The disclosure relates to an optical waveguide for a display device and to a device for generating a virtual image using such an optical waveguide. The optical waveguide has two or more partial optical waveguides adapted to different wavelengths. At least one of the partial optical waveguides in this case has an optical filter. The optical filter is adapted to the wavelengths used.

Claims

1. An optical waveguide comprising: a first optical waveguide adapted to transmit light of a first wavelength; a second optical waveguide adapted to transmit light of a second wavelength; a third optical waveguide adapted to transmit light of a third wavelength; and an optical filter having three narrowband transmission bands corresponding to the first wavelength, the second wavelength, and the third wavelength.

2. The optical waveguide as claimed in claim 1, wherein the optical filter is a dielectric filter, and wherein the light of the first wavelength is red light, the light of the second wavelength is green light, and the light of the third wavelength is blue light.

3. A device for generating a virtual image, the device comprising: an image-generating unit for producing an image; and an optics unit for projecting the image onto a mirror unit for generating the virtual image; and an optical waveguide for expanding an exit pupil, the optical waveguide comprising: a first optical waveguide adapted to transmit light of a first wavelength; a second optical waveguide adapted to transmit light of a second wavelength; a third optical waveguide adapted to transmit light of a third wavelength; and an optical filter having three narrowband transmission bands corresponding to the first wavelength, the second wavelength, and the third wavelength.

4. The device as claimed in claim 3, wherein the optical filter is a dielectric filter, and wherein the light of the first wavelength is red light, the light of the second wavelength is green light, and the light of the third wavelength is blue light.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 schematically shows a head-up display according to the prior art for a motor vehicle;

(2) FIG. 2 shows an optical waveguide with two-dimensional enlargement;

(3) FIG. 3 schematically shows a head-up display with an optical waveguide;

(4) FIG. 4 schematically shows a head-up display with an optical waveguide in a motor vehicle;

(5) FIG. 5 illustrates the incidence of stray light into an optical waveguide according to the disclosure with an optical filter;

(6) FIG. 6 schematically shows the transmission curve of the optical filter from FIG. 5;

(7) FIG. 7 illustrates the incidence of stray light into an optical waveguide according to the disclosure with three optical filters; and

(8) FIG. 8 schematically shows the transmission curves of the optical filters from FIG. 7.

(9) Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

(10) The concept of a head-up display with an optical waveguide will be explained with reference to FIGS. 1 to 4.

(11) FIG. 1 shows a schematic diagram of a head-up display according to the prior art for a motor vehicle. The head-up display has an image generator 1, an optics unit 2, and a mirror unit 3. A beam bundle SB1 emanates from a display element 11 and is reflected by a folding mirror 21 onto a curved mirror 22 that reflects it in the direction of the mirror unit 3. The mirror unit 3 is illustrated here as a windshield 31 of a motor vehicle. From there, the beam bundle SB2 travels in the direction of an eye 61 of a viewer.

(12) The viewer sees a virtual image VB that is located outside the motor vehicle above the engine hood or even in front of the motor vehicle. Due to the interaction of the optics unit 2 and the mirror unit 3, the virtual image VB is an enlarged representation of the image displayed by the display element 11. A speed limit, the current vehicle speed, and navigation instructions are symbolically represented here. As long as the eye 61 is located within the eyebox 62 indicated by a rectangle, all elements of the virtual image are visible to that eye 61. If the eye 61 is outside the eyebox 62, the virtual image VB is only partially visible to the viewer, or not at all. The larger the eyebox 62 is, the less restricted the viewer is when choosing their seating position.

(13) The curvature of the curved mirror 22 is adapted to the curvature of the windshield 31 and ensures that the image distortion is stable over the entire eyebox 62. The curved mirror 22 is rotatably mounted by a bearing 221. The rotation of the curved mirror 22 that is made possible thereby makes it possible to displace the eyebox 62 and thus to adapt the position of the eyebox 62 to the position of the eye 61. The folding mirror 21 serves to ensure that the path traveled by the beam bundle SB1 between the display element 11 and the curved mirror 22 is long and, at the same time, that the optics unit 2 is nevertheless compact. The optics unit 2 is delimited with respect to the environment by a transparent cover 23. The optical elements of the optics unit 2 are thus protected for example against dust located in the interior of the vehicle. An optical film or a polarizer 24 is furthermore located on the cover 23. The display element 11 is typically polarized, and the mirror unit 3 acts like an analyzer. The purpose of the polarizer 24 is therefore to influence the polarization in order to achieve uniform visibility of the useful light. An anti-glare protection 25 serves to reliably absorb the light reflected via the interface of the cover 23 so that the observer is not dazzled. In addition to the sunlight SL, the light from another stray light source 64 can also reach the display element 11. In combination with a polarization filter, the polarizer 24 can additionally be used to block out incident sunlight SL.

(14) FIG. 2 shows a schematic spatial illustration of an optical waveguide 5 with two-dimensional enlargement. In the lower left region, an input coupling hologram 53 can be seen, by means of which light L1 coming from an image-generating unit (not shown) is coupled into the optical waveguide 5. The light propagates therein in the drawing to the top right, according to the arrow L2. In this region of the optical waveguide 5, a folding hologram 51 that acts similarly to many partially transmissive mirrors arranged one behind the other and produces a light bundle that is expanded in the Y-direction and propagates in the X-direction is located. This is indicated by three arrows L3. In the part of the optical waveguide 5 that extends to the right in the figure, an output coupling hologram 52 is located, which likewise acts similarly to many partially transmissive mirrors arranged one behind the other and, indicated by arrows L4, couples light upward in the Z-direction out of the optical waveguide 5. In this case, an expansion takes place in the X-direction, so that the original incident light bundle L1 leaves the optical waveguide 5 as a light bundle L4 that is enlarged in two dimensions.

(15) FIG. 3 shows a three-dimensional illustration of a head-up display with three optical waveguides 5R, 5G, 5B, which are arranged one above the other and each stand for an elementary color red, green, and blue. Together they form the optical waveguide 5. The holograms 51, 52, 53 present in the optical waveguide 5 are each wavelength-dependent, meaning that one optical waveguide 5R, 5G, 5B is used in each case for one of the elementary colors. An image generator 1 and an optics unit 2 are shown above the optical waveguide 5. The optics unit 2 has a mirror 20, by means of which the light produced by the image generator 1 and shaped by the optics unit 2 is deflected in the direction of the respective input coupling hologram 53. The image generator 1 has three light sources 14R, 14G, 14B for the three elementary colors. It can be seen that the entire unit shown has a small overall structural height compared to its light-emitting surface.

(16) FIG. 4 shows a head-up display in a motor vehicle similar to FIG. 1, except here in a three-dimensional illustration and with an optical waveguide 5. It shows the schematically indicated image generator 1, which produces a parallel beam bundle SB1 that is coupled into the optical waveguide 5 by the mirror plane 523. The optics unit is not shown for the sake of simplicity. A plurality of mirror planes 522 each reflect a portion of the light incident on them in the direction of the windshield 31, the mirror unit 3. The light is reflected thereby in the direction of the eye 61. The viewer sees a virtual image VB above the engine hood or at an even farther distance in front of the motor vehicle. With this technology, too, the entire optical unit is incorporated in a housing that is separated with respect to the environment by a transparent cover. As with the head-up display from FIG. 1, a retarder can be arranged on this cover.

(17) FIG. 5 illustrates the incidence of stray light into an optical waveguide 5 according to the disclosure with an optical filter 70. FIG. 6 schematically shows the transmission curve of the optical filter 70. The optical waveguide 5 is a constituent part of a device for generating a virtual image VB for a viewer 61. In this example, the device is a head-up display of a motor vehicle. The optical waveguide 5 is used here for the two-dimensional expansion of an exit pupil. The windshield 31 of the motor vehicle acts as the mirror unit 3. The stray light source 64 is the sun in this case.

(18) In some examples, as shown, the optical waveguide 5 has three partial optical waveguides 5R, 5G, 5B optimized for different wavelengths R, G, B. The wavelengths preferably correspond to the colors red, green, and blue. The uppermost partial optical waveguide 5R in FIG. 5 has an optical filter 70, for example a dielectric filter. As can be seen from the schematic transmission curve in FIG. 6, in which the transmission coefficient T is plotted over the wavelength , the optical filter 70 has three narrowband transmission bands that correspond to the three wavelengths R, G, B used. The partial optical waveguides 5R, 5G, 5B are illuminated by light sources 14R, 14G, 14B, which each emit the associated colors. When using laser light, only very narrowband spectra are available for the supported wavelengths R, G, B of the light sources 14R, 14G, 14B.

(19) In head-up displays based on holographic optical waveguides 5, in addition to the useful light, the specular and holographic reflections of the sun are also reflected as the stray light source 64 at the glass plates of the respective partial optical waveguides 5R, 5G, 5B via the mirror unit 3 to the eye 61. As a rule, this leads to at least an impairment of the image perception, and often even to direct, safety-relevant dazzling of the user of the system. The use of the optical filter 70 ensures that only the narrowband regions of the stray light can penetrate as far as the partial optical waveguides 5R, 5G, 5B. This significantly reduces the proportion of the potentially reflected sunlight.

(20) FIG. 7 illustrates the incidence of stray light into an optical waveguide 5 according to the disclosure with three optical filters 70R, 70G, 70B. FIG. 8 schematically shows the transmission curves of the optical filters 70R, 70G, 70B. Partial image (a) shows the transmission curve of the first optical filter 70R viewed from the stray light source 64, partial image (b) shows the transmission curve of the second optical filter 70G, and partial image (c) shows the transmission curve of the third optical filter 70G. As in FIG. 6, the transmission coefficient T is plotted over the wavelength in the transmission curves.

(21) In this variant, one optical filter 70R, 70G, 70B is provided for each partial optical waveguide 5R, 5G, 5B. The first partial optical waveguide 5R viewed from the stray light source 64 has on the upper side a first optical filter 70R that transmits all three colors, while the second optical filter 70G of the subsequent partial optical waveguide 5G is transmissive only to two colors. The third optical filter 70B of the third partial optical waveguide 5B transmits only one color. Depending on the possibility of the production process, these optical filters 70R, 70G, 70B can also be applied directly onto the interfaces of the partial optical waveguides 5R, 5G, 5B in order to avoid further reflections and to minimize the amount of energy incident on the sum of all interfaces.

(22) A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other implementations are within the scope of the following claims.

LIST OF REFERENCE SIGNS

(23) 1 Image generator/image-generating unit 11 Display element 14, 14R, 14G, 14B Light source 2 Optics unit 20 Mirror 21 Folding mirror 22 Curved mirror 221 Bearing 23 Transparent cover 24 Optical film/polarizer 25 Anti-glare protection 3 Mirror unit 31 Windshield 5, 5R, 5G, 5B Optical waveguide 51 Folding hologram 52 Output coupling hologram 522 Mirror plane 523 Mirror plane 53 Input coupling hologram 54 Substrate 55 Cover layer 56 Hologram layer 61 Eye/viewer 62 Eyebox 64 Stray light source 70, 70R, 70G, 70B Optical filter L1 . . . L4 Light , R, G, B Wavelength S1 Receiving an image to be displayed S2 Determining regions without image contents to be represented S3 Switching the electrode array according to the specific regions SB1, SB2 Beam bundles SL Sunlight T Transmission coefficient VB Virtual image