Axially asymmetric image source for head-up displays

Abstract

To provide an improved head-up display system for providing a suitably sized output pupil.SOLUTION: An image projection apparatus for a head-up display comprises: a digital mirror device comprising a plurality of pivotable mirrors; an illumination source configured to illuminate the pivotable mirrors with incident light distributed spatially across the mirrors and angularly across an input cone; and an optical waveguide aligned such that light from the illumination source is coupled into the waveguide after reflection from the pivotable mirrors. The input cone is asymmetric and has a larger cone angle in an axis than the maximum tilt angle of the pivotable mirrors in the axis.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present disclosure relates to head-up displays, and in particular to image sources for such displays. [Background technology]

[0002] A head-up display utilizes a transparent component through which the user looks to superimpose an image onto the user's actual view of their surroundings. The user's eyes receive light from their surroundings in the normal way, and the user's eyes also receive light from the head-up display system. Thus, the image from the head-up display is superimposed onto the actual surroundings.

[0003] The transparent components of a head-up display may either be mounted in a fixed location on the equipment being used by the user (e.g., the cockpit of an aircraft), or may be mounted on the user's head (e.g., as glasses or on a helmet) so that the components move with the user's head and therefore remain in a fixed location relative to the user's eyes.

[0004] A fundamental requirement of a head-up display is to couple light from an image source to the location required for viewing by the user, known as the output pupil. This can be achieved using a waveguide system such as that shown diagrammatically in cross-section in Figure 1.

[0005] Image projection systems 11, 12 project images into a waveguide 10 which couples the image to a user's eye 1, which is placed at the system's exit pupil. The waveguide body 10 is optically transparent, so that the user views the real world through the waveguide and the image from the projection systems 11, 12 is superimposed onto their field of view.

[0006] The projection optics 12 are positioned to distribute the projected image across the input pupil of the waveguide. The near field at that pupil represents the spatial distribution of light across the pupil, while the far field represents the image information (each pixel of the image is represented by a discrete ray angle). The image displayed to the user (i.e., at the exit pupil) is desirably focused far away from the eye (infinity), as this is the appropriate focusing distance of the real-world image onto which the projected image will be superimposed.

[0007] The input grating 13 couples the light beam 14 into the waveguide body 10. The input grating 13 may be a diffraction grating that diffracts the light beam 14 to an angle that confines it within the waveguide 10 by total internal reflection. The output grating 15 diffracts the light back to its original angle, so that it exits the waveguide and propagates to an exit pupil positioned to coincide with the location of the eye 1. The output grating 15 may also be a diffraction grating. The waveguide 10 and the input and output gratings 13 and 15 are configured to preserve the light beam direction between the entrance and exit pupils, and to preserve the color and angle image information. That is, the far field of the light at the exit pupil coincides with the far field at the entrance pupil. Other coupling structures for the input and output gratings 13 and 15 may be utilized, as known in the art.

[0008] For clarity of illustration, a single ray 14 is shown, but it will be understood that this is representative of multiple rays over a range of angles.

[0009] The input light is also spatially distributed, so the light ray 14 also represents a light ray distributed across the exit pupils produced by the image projection systems 11,12.

[0010] To allow flexibility in aligning the user's eyes with the device, the exit pupil produced by the waveguide 10 should be large enough so that the entire field of view can be seen from a range of locations known as the "eyebox". For example, an exit pupil larger than 10 mm in diameter may be desirable. The waveguide system may be designed to expand the entrance pupil so that the exit pupil is larger than the entrance pupil, thus reducing the entrance pupil requirements of the image projection system 11, 12. This expansion is commonly known as pupil replication. The expansion should be performed while preserving the far field and maintaining constant brightness across the near field.

[0011] This expansion may be achieved utilizing both diffractive structures 15 and 16 in waveguide 10. Pupil expansion may be provided in either one dimension or two perpendicular dimensions of the pupil using techniques known in the art.

[0012] Pupil expansion in two dimensions presents more design challenges than one-dimensional expansion: achieving uniform illumination across the near field of the exit pupil with two-dimensional expansion can be difficult, especially over a wide chromaticity range, and requires precise design and fabrication of the waveguide optics.

[0013] In contrast, one-dimensional expansion is simpler, but limits the available exit pupil size in one dimension to that provided by the projection source. Figure 2a shows a schematic diagram of an entrance pupil 200 that is expanded in a waveguide 201 in only one dimension. The exit pupil 202 is expanded in the x direction, while in the y direction the pupil size is maintained, resulting in an asymmetric eyebox 203. As a result, when combining a one-dimensional expansion waveguide with a projection device, the exit pupil provided by the projector should typically be larger in the orthogonal axis to the waveguide expansion, as shown in Figure 2b for the entrance pupil 204, the waveguide 205 for 2D expansion, and the exit pupil 206, providing a symmetric eyebox 207.

[0014] Reflective microdisplay devices can be used in conjunction with other complex optical elements that can be used to create a reasonably large exit pupil, but such systems can be physically large and can cause degradation of the achievable image quality.

[0015] FIG. 3 shows a schematic diagram of a single DMD mirror 300 illuminated by an input cone 301. The mirror is shown in a flat state where it produces an output cone 302. The mirror 300 tilts about an axis 303 perpendicular to the plane of the drawing. In the on state, the mirror 300 tilts counterclockwise to produce an output cone 304. In the off state, the mirror tilts clockwise to produce an output cone 305. The style of DMD shown in FIG. 3 is often known as a "diamond pixel" architecture. In other types of DMDs (digital mirror devices), each mirror is movable about a point such that the mirror surface moves along multiple axes between states; such DMD devices are commonly known as "tilt and roll" devices.

[0016] Only the on-state output cone 304 should be collected by the image projection optics, so the output cone 304 should not overlap with the flat-state output cone 302 and the off-state output cone 305. A typical mirror of a DMD device such as that of FIG. 3 may be tilted by + / - 12°, which means that the maximum illumination cone angle is 24° (+ / - 12°) to avoid overlap between adjacent output cones. The illumination of the DMD spans the entire area of ​​the micromirror device, which defines the field of view achievable by the projection system. The illumination cone angle defines the pupil size of the projection system. That is, the projection optics converts the spatial distribution of light into an angular distribution (far field) and vice versa after the DMD, so that the eye located at the exit pupil sees the image generated by the DMD.

[0017] 4 shows a paraxial design of a DMD-based image projector to create an exit pupil 400. The DMD is represented at 401. The top half of the figure shows the X-axis of the DMD 401 (where the device is larger) and the bottom half of the figure shows the Y-axis of the DMD 401 (where the mirror array is smaller).

[0018] DMD 401 is uniformly spatially illuminated with light over an angular illumination cone by a light source (not shown). Ray bundles 402 and 403 represent reflections from the central region of 401, and ray bundles 404-407 represent reflections from the edges of 401. The cone angle of bundles 402-407 corresponds to the cone angle of the illumination source at the DMD plane 401.

[0019] A paraxial lens system 408 focuses the light from the DMD to form an exit pupil 400 .

[0020] In an example configuration, the DMD array 401 may be 10 mm by 6.25 mm. The optical system may be designed to illuminate the entire DMD array 301, thereby providing a 40° by 25° field of view through the projection lens. Using a maximum illumination cone angle of 24°, this provides a 6 mm diameter exit pupil 300.

[0021] Using expansion in one dimension in a waveguide system would limit the example system exit pupil (eyebox) to 6 mm in the non-expanded dimension, which may be insufficient, thus requiring two-dimensional expansion.

[0022] Therefore, there is a need for an improved head-up display system to provide an exit pupil of a suitable size.

[0023] The embodiments described below are not limited to implementations that solve any or all of the shortcomings of known systems. Summary of the Invention

[0024] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0025] An image projection apparatus for a head-up display is provided, the apparatus comprising: a digital mirror device having a plurality of rotatable mirrors; an illumination source configured to illuminate the rotatable mirrors with incident light spatially distributed across the mirrors and angularly distributed across an input cone; and an optical waveguide aligned such that light from the illumination source is coupled into the waveguide after reflection from the rotatable mirror, wherein the input cone is asymmetric and has a cone angle about an axis that is greater than the maximum tilt angle of the rotatable mirrors at that axis.

[0026] The rotatable mirror may rotate on a single axis.

[0027] A rotatable mirror has four sides in plan and can rotate about an axis between two vertices of the periphery of the mirror.

[0028] The digital mirror device may be a Diamond Pixel device.

[0029] The input cone angle may be 24° along an axis perpendicular to the mirror's pivot axis and may be greater than 24° along an axis parallel to the mirror's pivot axis.

[0030] The image projection apparatus for the head-up display may further comprise a lens apparatus for projecting an output of the digital mirror device onto an exit pupil, where the exit pupil is spatially asymmetric.

[0031] The image projector may project its output into the waveguide for coupling into the exit pupil of the display.

[0032] The waveguide may expand the entrance pupil through pupil duplication to provide a larger exit pupil to the user.

[0033] The disclosure provided herein can provide a waveguide that is smaller and lighter than other waveguide designs, yet does not create stray light paths.

[0034] Embodiments of the invention will now be described, by way of example, with reference to the following drawings, in which: [Brief description of the drawings]

[0035] [Figure 1] FIG. 1 shows a cross-sectional view of the waveguide structure. [Figure 2a] FIG. 2a shows a schematic diagram of one-dimensional expansion. [Figure 2b] FIG. 2b shows a schematic diagram of two-dimensional expansion. [Diagram 3] FIG. 3 shows a schematic of the reflections from the DMD mirrors. [Figure 4] FIG. 4 shows a paraxial projection system. [Diagram 5] FIG. 5 shows a schematic diagram of a DMD array. [Figure 6] FIG. 6 shows a single mirror from the DMD array. [Figure 7] FIG. 7 shows a single mirror from a DMD array with asymmetric illumination. [Figure 8] FIG. 8 shows the exit pupil from the DMD mirror with asymmetric illumination. [Figure 9] FIG. 9 shows a paraxial projection system with asymmetric illumination. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Further details, aspects and embodiments of the present invention will now be described, by way of example only, with reference to the drawings in which elements are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For ease of understanding, like reference numerals have been included in the respective drawings.

[0037] FIG. 5 shows a plan view of a DMD array 500, where the tilt axis of each mirror is about a corner-to-corner line of symmetry, and the mirrors tilt about a single axis. This type of DMD is one example of the type of device that may be used in this disclosure, but other types of DMD arrays may also be used. For example, DMDs in which the mirrors rotate on a single axis but with different alignment of the axes may be used, or DMDs in which the mirrors rotate about more than one axis may be used. FIG. 6 shows a close-up of a single mirror 600 and its pivot axis 601 along the Y axis. The mirror 600 is illuminated by an input cone 602, and cone 603 represents the output cone.

[0038] FIG. 7 shows a schematic diagram of a single mirror 700 of the array illuminated to create an asymmetric exit pupil 701. The input illumination cone 702 is arranged to be asymmetric. In the Z axis (out of the plane of the drawing), the input cone is constrained to the 24° mentioned above so that there is no overlap between the on / flat / off output cones. The illumination cone is asymmetric such that the cone angle parallel to the Y axis is larger than the cone angle in the Z axis. Due to the orientation of this configuration, it is not necessary to angularly constrain the size of the exit pupil (except by relying on conventional optical design limitations). In an example configuration, the illumination cone may be 24°×40°. In general, the cone angle in an axis may be larger than the maximum mirror tilt angle in that axis. In a particular example, the mirror tilts about only one axis. FIG. 8 shows the exit pupil for each of the mirror states, showing the pupil expanded in the Y direction while maintaining a lack of interaction between states in the X direction. In one example, the mirror rotates about the Y axis of FIG. 7 with no movement about the other axes.

[0039] Figure 9 shows a paraxial system utilizing the asymmetric illumination described above. The DMD device 900 has the same dimensions as described in connection with Figure 2, and the same field of view of 40° x 25° is provided at the exit pupil 901. On the X-axis, the illumination angle is the same 24° as described in connection with Figure 3, but on the Y-axis, the exemplary ray bundles 902, 903, 904 have a larger cone angle, e.g., 40°. The paraxial projection system 905 provides a paraxial exit pupil 901 of 6 mm x 10.25 mm.

[0040] The asymmetric exit pupil 901 of FIG. 9 may be projected into a waveguide of a HUD for projection to a user. Expansion in only one dimension may be provided by the waveguide to provide an exit pupil for a 20 mm (expanded from 6 mm) by 10.25 mm system. Thus, expansion in only one dimension may provide an acceptable exit pupil size. Thus, a DMD image source system (with its associated advantage in brightness) may be utilized in combination with one dimensional expansion, thus obtaining the benefits of these technologies.

[0041] The above description has been given using an example DMD device having specific parameters. These parameters are given for convenience of explanation only, and the principles described herein apply to devices having different parameters.

[0042] In the above description, a side diamond device was used as an example. However, other types of devices with rotating mirrors can be used according to the same principles. Similarly, a square mirror was used as an example, but other shapes can also be used.

[0043] Although the present invention has been described in relation to some embodiments, it is not intended that the present invention be limited to the specific form described herein. Rather, the scope of the present invention is limited only by the appended claims. In addition, although features may appear to be described in relation to certain embodiments, those skilled in the art will recognize that various features of the described embodiments may be combined in accordance with the present invention. In the claims, the term "comprising" does not exclude the presence of other elements or steps.

[0044] Furthermore, the order of features in the claims does not imply any particular order in which the features must be performed, and in particular the order of individual steps in method claims does not imply that those steps must be performed in that order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude plurals; thus, references to "a", "an", "first", "second", etc. do not exclude plurals. In the claims, the terms "comprising" or "including" do not exclude the presence of other elements.

Claims

1. An image projection device for a head-up display, A digital mirror device comprising a plurality of rotatable mirrors; an illumination source configured to illuminate the plurality of rotatable mirrors with incident light spatially distributed across the plurality of mirrors and angularly distributed across an input cone; a light guide aligned such that light from the illumination source is coupled into the light guide after reflection from a plurality of the rotatable mirrors; Equipped with 1. An image projection apparatus for a head-up display, wherein the input cone is asymmetric and has a cone angle about an axis that is greater than a maximum tilt angle of the rotatable mirror about that axis.

2. The image projection device for a head-up display according to claim 1 , wherein the rotatable mirror rotates on a single axis.

3. 3. The image projection device for a head-up display according to claim 2, wherein the rotatable mirror has four sides in a plan view and rotates around an axis between two vertices of an outer periphery of the mirror.

4. The image projection device for a head-up display according to claim 3 , wherein the digital mirror device is a Diamond Pixel Device.

5. 5. An image projection device for a head-up display as claimed in any one of claims 1 to 4, wherein the angle of the input cone is 24° along an axis perpendicular to the rotation axis of the mirror and is greater than 24° along an axis parallel to the rotation axis of the mirror.

6. 6. An image projection apparatus for a head-up display as claimed in any one of claims 1 to 5, further comprising a lens arrangement for projecting an output of the digital mirror device onto an exit pupil, the exit pupil being spatially asymmetric.

7. A head-up display comprising an image projection device according to any one of claims 1 to 6, wherein the optical waveguide expands in one axis an exit pupil generated by illumination of the digital mirror device.

8. The head-up display with image projection device of claim 1 , wherein the light guide expands the entrance pupil in one dimension via pupil replication to provide a larger exit pupil to a user.

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