Apparatus for generating virtual images with speckle reducing light mixing rods

Abstract

The invention relates to a device for generating a virtual image (VB), having an image generation unit (1) for generating an image, which has a light source (14R, 14G, 14B) emitting coherent light and a speckle reduction element. The speckle reducing element is a speckle reducing light mixing rod (17).

Description

Technical Field

[0001] This invention relates to an apparatus for generating virtual images, which has a speckle-reducing light mixing rod. Background Technology

[0002] A head-up display (HUD), also known as a head-up display, is a display system in which an observer can maintain their gaze direction because the content to be displayed is superimposed onto their field of vision. Due to the complexity and cost of such systems, they were initially used primarily in the aviation industry, but are now also widely installed in the automotive sector.

[0003] Head-up displays (HUDs) typically consist of an image generating unit (PGU), an optical unit, and a mirror unit. The PPU generates an image using at least one display element. The optical unit guides the image to the mirror unit. The mirror unit is a partially reflective, light-transmitting glass. Therefore, the observer sees both the content displayed by the PPU as a virtual image and the real world behind the glass. In the automotive industry, windshields are commonly used as mirror units, and their curved shape must be taken into account during display. Through the interaction of the optical and mirror units, the virtual image is a magnified display of the image generated by the PPU.

[0004] An observer can only view the virtual image from the position of what is called the eyebox. The eyebox is an area whose height and width correspond to the theoretical viewing window. As long as one of the observer's eyes is inside the eyebox, that eye can see all elements of the virtual image. Conversely, if the eye is outside the eyebox, the virtual image is only partially visible or completely invisible to the observer. The larger the eyebox, the fewer restrictions the observer faces when choosing a seating position.

[0005] Optical units typically include multiple mirrors to minimize the required space. Light emitted from the image generation unit is reflected by folded mirrors onto curved mirrors, which then reflect the light toward the windshield. Currently used curved mirrors are implemented as essentially flat plates with a large curvature, depending on the desired optical function. Such curved mirrors are manufactured, for example, by injection molding or pressure molding.

[0006] The image generation unit has a light source, which for color display either has one or more light sources emitting white light, or multiple light sources emitting different colors of light. For example, a laser light source using red, blue, and green light.

[0007] To generate good and uniform images, laser-based image generation units typically require well-mixed (collimated) laser light in the illumination system. To achieve this, a light mixing rod can be used, which can be designed to be hollow or filled. By placing a scatterer with directional scattering capabilities in front of the light mixing rod, the angular range of the output can be controlled. If the light mixing rod is of sufficient length (for a sufficient number of reflections), a very uniform field is obtained at the output surface, which is then imaged or transmitted to an image generator, such as a DMD (Digital Micromirror Device) or LCOS (Liquid Crystal On Silicon). This enables the generation of clear and efficient illumination.

[0008] In the case of coherent light, such as coherent light emitted by a laser source, unwanted speckle patterns are generated during the process. These patterns should be reduced or eliminated to obtain a good image impression.

[0009] Against this backdrop, DE 10 2022 205 445 A1 describes an apparatus for generating virtual images, comprising an image generation unit having multiple laser sources emitting coherent light of different colors and a speckle reduction element, the function of which is not described in detail in this document. US 9,335,604 B2 describes an apparatus for generating virtual images in which an active speckle reduction element is installed. Additionally, an optical waveguide for carrying image information is installed, but this waveguide is not suitable for reducing speckle effects. Summary of the Invention

[0010] The objective of this invention is to provide an improved apparatus for generating virtual images.

[0011] This task is solved by the manner described in the claims and the description of embodiments. Preferred embodiments of the invention are also given herein.

[0012] An apparatus for generating virtual images includes an image generation unit for producing images, the image generation unit having a light source emitting coherent light and a speckle reduction element, characterized in that the speckle reduction element is a speckle-reducing light mixing rod. This has the advantage that the functions of light mixing and speckle reduction are implemented in a single component. None of the aforementioned documents disclose equipping a light mixing rod with speckle reduction functionality. Therefore, according to the invention, the image generation unit is illuminated by light with speckle reduction achieved through the light mixing rod.

[0013] According to one design, a first diffuser is arranged on the input surface of an optical mixing rod, and additional speckle reduction elements are also arranged within the optical mixing rod. This has the advantage that the light entering the optical mixing rod is already partially scattered, thus the entire length of the optical mixing rod is used to reduce coherence, thereby reducing the speckle effect. According to the invention, multiple speckle reduction measures are integrated into the optical mixing rod, and these measures are also based on different effects, such as scattering, partial reflection, or other effects.

[0014] According to one design, an additional speckle-reducing element is an additional diffuser. This has one of the advantages: the diffuser is a tested and industrially available component. When using a hollow light mixing rod, one or more additional diffusers are arranged within its cavity. When using a solid light mixing rod, which consists of two or more solid sub-rods, additional diffusers are arranged between these sub-rods. Advantageously, the input and / or output surfaces of the light mixing rod or sub-rods have roughened surfaces. In this case, the diffuser function is implemented directly on the light mixing rod or sub-rod. In this case, it is not necessary to arrange and install a separate component, i.e., the diffuser.

[0015] According to one design, the light mixing rod has at least one beam splitter. This has one of the advantages: generating light paths of different lengths through one or more beam splitters results in uncorrelated speckle patterns that, when superimposed, reduce or even completely eliminate the speckle effect perceptible to the observer, i.e., below the observer's perceptibility threshold. Combinations of beam splitters and diffusers are also within the scope of the invention.

[0016] According to one design, the light mixing rod is a hollow rod with a beam-dispersing coating on its inner surface. This has one of the advantages: the coating can be manufactured at a lower cost. An inward-reflecting coating, i.e., towards the interior of the light mixing rod, is advantageously provided on the outer surface of the light mixing rod.

[0017] According to one design, the transmittance of the spectrophotometric coating is equal to or greater than its reflectance. It has been shown that, in this case, coherence and consequently the speckle effect are reduced particularly effectively. Favorable reflectance-to-transmittance ratios are 50:50, 40:60, or 30:70.

[0018] According to one design, the walls of a hollow light mixing rod have a non-uniform thickness. This has one of the advantages: the different wall thicknesses result in more varied light paths, thereby effectively reducing coherence and the resulting speckle effect.

[0019] Advantageously, the wall thickness continuously increases and then decreases again around the periphery of the hollow rod. This shape is relatively easy to manufacture, for example, by placing a hollow rod made of glass horizontally during glass cooling. Here, a slight flow of glass occurs due to gravity, resulting in a thickness greater in the lower region than in the upper region.

[0020] Advantageously, the thickness varies in stages around the perimeter. This near-discontinuous shape also leads to a reduction in coherence and speckle effects. For example, it can be manufactured by splicing and fusing together prefabricated glass tube halves of different thicknesses. This results in steep steps along the connecting lines.

[0021] Equally advantageous is the wall thickness, which varies continuously or in steps along the length of the hollow bar. The manufacturing method and advantages are similar to those of a wall thickness varying around the perimeter.

[0022] According to one design, the length of the optical mixing rod and the scattering angle of at least one diffuser are coordinated. This has one advantage: through proper fitting, the speckle effect can be reduced without complex additional measures. It has proven advantageous that the diffuser arranged on the input surface has a scattering angle on the order of approximately 20°, while the diffuser arranged inside the optical mixing rod has a smaller scattering angle in the range of 5° to 10°, and the cross-sectional area of ​​the optical mixing rod is approximately 100 mm². 2 At that time, there was a distance of 30 mm between the two diffusers.

[0023] A head-up display according to the invention has a device according to the invention for generating virtual images.

[0024] Further features of the invention will be apparent from the following description and the appended claims in conjunction with the accompanying drawings. Attached Figure Description

[0025] Figure 1 A head-up display for motor vehicles according to the prior art is schematically shown;

[0026] Figure 2 A portion of the image generation unit is shown schematically;

[0027] Figure 3 Another implementation of the image generation unit is illustrated schematically;

[0028] Figure 4 An image generation unit according to the invention is shown;

[0029] Figure 5 Another image generation unit according to the invention is shown;

[0030] Figure 6Another image generation unit according to the invention is shown;

[0031] Figure 7 A cross-sectional view of the light mixing rod is shown;

[0032] Figure 8 A cross-sectional view of another light mixing rod is shown;

[0033] Figure 9 The longitudinal section of the light mixing rod is shown;

[0034] Figure 10 A longitudinal section of another light-mixing rod is shown;

[0035] Figure 11 A longitudinal section of another light-mixing rod is shown;

[0036] Figure 12 A cross-section of another light-mixing rod is shown; and

[0037] Figure 13 A perspective view of the light mixing rod is shown. Detailed Implementation

[0038] To better understand the principles of this invention, embodiments will be explained in detail below with reference to the accompanying drawings. In the drawings, the same reference numerals are used for the same or functionally identical elements, and they are not necessarily redescribed in every drawing. It should be understood that this invention is not limited to the illustrated embodiments, and the described features can be combined or modified without departing from the scope of protection defined by the claims.

[0039] First, combined Figures 1 to 3 This paper elucidates the basic concept of a head-up display with a coherent light source.

[0040] Figure 1 A schematic diagram of a head-up display (HUD) for a motor vehicle according to the prior art is shown. The HUD has an image generation unit 1, an optical unit 2, and a mirror unit 3. A light beam SB1 is emitted from a display element 11, which is reflected by a folding mirror 21 onto a curved mirror 22, which then reflects the beam towards the mirror unit 3. The mirror unit 3 is shown here as the windshield 31 of a motor vehicle. From there, a light beam SB2 is directed toward the observer's eye 61.

[0041] An observer sees a virtual image VB, located above the hood of the vehicle or even in front of it. Through the interaction of optical unit 2 and mirror unit 3, the virtual image VB is a magnified display of the image displayed by display element 11. Here, speed limit signs, current vehicle speed, and navigation instructions are symbolically shown. All elements of the virtual image are visible to eye 61 as long as eye 61 is within the rectangular eyebox 62. If eye 61 is outside the eyebox 62, the observer will only be partially or completely unable to see the virtual image VB. The larger the eyebox 62, the fewer restrictions the observer faces when choosing a seating position.

[0042] The curvature of the curved mirror 22 serves two purposes: firstly, it prepares the optical path, ensuring a larger image and a larger eye box 62; secondly, the curvature of the curved mirror compensates for the curvature of the windshield 31, so that the virtual image VB corresponds to a magnified reproduction of the image displayed from the display element 11. The curved mirror 22 is rotatably mounted via a support structure 221. This rotation of the curved mirror 22 moves the eye box 62, thereby adjusting its position to match the position of the eye 61. The folding mirror 21 is used to lengthen the path of the light beam SB1 between the display element 11 and the curved mirror 22, while the optical unit 2 remains compact. The optical unit 2 is isolated from the environment by a transparent cover plate 23. This protects the optical elements of the optical unit 2 from dust, for example, within the vehicle's interior space. An optical film 24 or coating is also present on the cover plate 23 to prevent incident sunlight SL from reaching the display element 11 through the mirrors 21 and 22. Otherwise, the display element could be temporarily or permanently damaged due to the generated heat. To prevent this, for example, the infrared portion of the sunlight SL is filtered using the optical film 24. The sunshade 25 is used to block light incident from the front, which would otherwise not be reflected from the cover 23 to the windshield 31, potentially causing glare for the observer. In addition to sunlight SL, other stray light sources 64 may also reach the display element 11.

[0043] Figure 2 A portion of the image generation unit 1 for a head-up display is schematically shown. Laser diodes LD1-LD3 are provided for three complementary colors. The light emitted by these laser diodes is guided via optical waveguides WG1-WG3 to display elements TFT1, TFT2, and TFT3 for their respective color portions. Here, the optical waveguides WG1-WG3 are exemplarily shown on a dichroic prism DP. In this example, all laser diodes LD1-LD3 for the three complementary colors are arranged below the dichroic prism DP. The laser diodes LD1-LD3 are stacked in a space-saving manner. Therefore, the optical waveguides WG1-WG3 have different lengths.

[0044] The preparation of light is based on optical waveguide technology, also known as waveguide technology. This means that coupling, homogenization, and illumination are based on optical waveguides, specifically optical waveguides WG1 to WG3, performed individually for each color, thus potentially eliminating the need for switchable grids. Consequently, many structural elements in the light guide can be omitted.

[0045] Figure 3 Another embodiment of the image generation unit 1 is schematically shown, which has 14R, 14G, and 14B light sources emitting coherent light. In the figure, a controllable mirror unit 73 can be seen in the image generation unit 1, which functions as a display element 11. The mirror unit 73 consists, for example, a two-dimensional arrangement of micromirrors, which are controlled to be positioned in one of two locations. Therefore, the light beam incident on the mirror unit is modulated in a pixel grid to generate a virtual image. This is a DMD. According to another variant, the controllable mirror unit 73 consists of a mirror adjustable about multiple axes, which is controlled such that the incident laser beam is reflected according to the two-dimensional grid, thereby generating a virtual image VB.

[0046] The light beam illuminating the micromirrors of mirror unit 73, or the laser beam illuminating the mirror adjustable around multiple axes, originates from light sources 14R, 14G, and 14B. Here, light sources 14R, 14G, and 14B are shown schematically in boxes. These light sources can be designed as conventional light sources, such as light-emitting diodes (LEDs), or as laser light sources. The solution according to the invention is particularly meaningful in that speckle may occur when light sources 14R, 14G, and 14B emit coherent light, and this speckle can be reduced by the measures according to the invention.

[0047] Figure 4The image generation unit 1 according to the invention is shown. Light sources 14R, 14G, and 14B are designed as laser diodes. The light emitted by the light sources is collimated, shown here through lens 151. The light emitted by the three light sources is combined into a common propagation direction through mirror 161 or through two dichroic mirrors 162 and 163. The light reaches and passes through the light mixing rod 17. Diffusers 171, 172, and 173 are arranged in or on the light mixing rod 17. After leaving the light mixing rod 17, the light passes through lens 155 and is deflected towards mirror unit 73 by mirror 166. Here, the light first passes through another lens 156 and RTIR prism 18. This guides the light to mirror unit 73 at an angle suitable for mirror unit 73, so that when the micromirror position of mirror unit 73 is "ON", the light enters RTIR prism 18 at such an angle that it is reflected as shown, pointing to the right of optical unit 2 in the figure. Light illuminating the micromirror in the "OFF" position is reflected and thus does not re-enter RTIR prism 18. Specifically, light is absorbed in a light trap (not shown). The term RTIR prism stands for "reverse total internal reflection" prism. This is a component commonly used in DMD projectors.

[0048] To generate a good and uniform image, the laser-based image generation unit 1 typically needs to thoroughly mix the (collimated) laser light in the illumination system. To achieve this, a light mixing rod 17 can be used. The light mixing rod can be designed to be hollow or filled / solid. If a diffuser, here diffuser 171, is placed in front of the light mixing rod, it provides targeted scattering, generating additional optical paths LW1. By appropriately selecting the scattering characteristics of diffuser 171, the angular range of the output end of the light mixing rod 17 can be controlled. The scattering characteristics of diffuser 171 include its scattering angle α1, which is denoted here between the two outermost optical paths LW1 it induces. If the light mixing rod 17 has a sufficient length L (for a sufficient number of reflections), a very uniform field is obtained at the output surface of the light mixing rod 17, which is then imaged or transmitted to an image generator (e.g., as a mirror unit 73 formed as a DMD or LCOS). This enables the generation of sharp and efficient illumination.

[0049] In the case of coherent light, such as the laser light used in the embodiments, undesirable speckle patterns are generated during the process, which should be reduced or eliminated. According to the invention, further diffusers 172 and 173 are incorporated within the light mixing rod 17 as scatterers for this purpose. Their scattering angles α2 and α3 are also marked here between their respective outermost optical paths LW2 and LW3. Depending on the so-called scattering pattern, diffusers 172 and 173 ensure the generation of additional optical paths LW2 and LW3. These are uncorrelated and additive, ultimately leading to a reduction in speckle contrast. The more diffusers 171-173 used, the lower the speckle contrast. If the light mixing rod 17 is designed as a hollow rod, the diffusers 172 and 173 are uniformly distributed along its length L. If the light mixing rod 17 is designed as a solid rod, it consists of sub-rods 1701, 1702, and 1703, in the illustrated embodiment of which the lengths L1, L2, and L3 are equal. A diffuser 172 and a diffuser 173 are arranged between sub-bars 1701, 1702 and 1703 respectively.

[0050] The advantage of the inventive solution is that the speckle reduction element is a passive component. In contrast, an active speckle reduction element requires a movable component or actively controlled variable element to switch between at least two states. This requires structural space, may lead to undesirable vibrations, and consumes energy.

[0051] Figure 5 An image generation unit 1 according to the invention is shown. This image generation element differs from the embodiment shown in the previous figure in the design of the light mixing rod 17. A single diffuser 171 is provided here. The light mixing rod 17 is hollow internally and has a glass layer 174 of a first thickness D1 on its upper wall and a glass layer 175 of a second thickness D2 on its lower wall. In the figure, the first thickness D1 is less than the second thickness D2. A beam-splitting coating 176 is provided on the inner surface 170 of the glass layers 174, 175, pointing towards the internal cavity of the light mixing rod 17. The beam-splitting coating has, for example, a ratio of reflectivity R (also called reflectance) to transmittance T (also called transmittance) of R / T = 50 / 50, or R / T = 40 / 60, or R / T = 30 / 70, or other suitable ratio. Light from the diffuser 171 is therefore partially reflected at the inner surfaces of the glass layers 174, 175 and partially passes through the glass layers. This creates an additional light path. Light rays arriving at the outer surfaces of glass layers 174 and 175 from the interior are reflected by a reflective coating 177 disposed there. The reflective coating 177 has a reflectivity R > 90%. Each ray of light arriving at the beam-splitting coating 176 is again partially reflected and partially transmitted there, thus creating additional light paths.

[0052] This generates multiple light paths, each producing an unrelated speckle pattern that is then summed. To generate multiple light paths within the hollow light mixing rod 17, a beam-splitting coating 176 is preferably applied to the first boundary layer. Even without such a coating, additional light paths will be generated. The beam-splitting coating 176 presets a suitable ratio of reflectivity R to transmittance T, adapted to the respective boundary conditions. To enhance the effect, glass layers of different thicknesses D1 and D2 are used.

[0053] Here, the light mixing rod 17 can also be designed to be hollow or filled. In a filled design, the filling material can be either a material with the same refractive index as the outer shell of the light mixing rod 17 or a material with a different refractive index. Glass is preferably used as both the wall and filling material. The advantage of a design with the same refractive index is that the reflection and transmission characteristics, especially the R / T ratio, are entirely defined by the spectrophotometric coating 176. Different speckle reduction behaviors can still be achieved with the same material combination. The advantage of a design with different refractive indices is that if the boundary layer already has the desired reflection and transmission characteristics, the spectrophotometric coating 176 can be completely omitted. It is also advantageous that, although the spectrophotometric coating 176 is used, it is only used for fine-tuning of the reflection and transmission characteristics.

[0054] Figure 6 An embodiment of the invention is shown, which combines features of the two embodiments previously described. This combination achieves maximum speckle contrast reduction because it combines the effects of increasing the number of different optical paths from both implementations. It should be noted that the lengths L of the light mixing rod or the lengths L1, L2, L3 of the sub-rods 1701, 1702, 1703, as well as the scattering angles α1, α2, α3 used, play a crucial role. To obtain sufficiently good results, the two parameters must be coordinated.

[0055] Figure 7 A cross-sectional view of the light mixing rod 17 is shown. Here, the light mixing rod 17 is a hollow rod. Its inner surface 170 is provided with a beam-splitting coating 176. It can be seen that the wall 1704 of the light mixing rod has a continuously varying thickness D, with the minimum thickness D1 taken in the lower part of the figure and the maximum thickness D2 taken in the upper part of the figure.

[0056] Figure 8A cross-sectional view of another light mixing rod 17 is shown. Here, the light mixing rod 17 consists of an upper half 1705 and a lower half 1706. The upper half 1705 has a thickness D2, which is greater than the thickness D1 of the lower half 1706. The two halves 1705 and 1706 have substantially constant thicknesses D2 and D1, respectively. The thickness varies only in the transition region between the two halves 1705 and 1706, transitioning from a small thickness D1 to a large thickness D2 over a short distance. Exemplarily, the region within the inner surface 170 is filled with material, so that the light mixing rod 17 is a solid component. The inner surface 170 also represents a boundary layer 1761. The material on one side of the boundary layer 1761 has a different refractive index than the exterior. Therefore, the boundary layer 1761 acts as a beam splitter because it partially reflects light that strikes it and partially transmits light.

[0057] As can be seen, the wall 1704 of the light mixing rod 17 has uneven thicknesses D1 and D2.

[0058] Figure 9 The longitudinal section of the light mixing rod 17 is shown, and the wall 1704 of the light mixing rod 17 increases in thickness from the minimum thickness D1 to the maximum thickness D2 from left to right along the length L of the light mixing rod 17.

[0059] Figure 10 A longitudinal section of another light mixing rod 17 is shown. This consists of four sub-rods 1700-1703, each with walls of different thicknesses D0-D3. At the transitions between sub-rods 1700-1703, the thicknesses change continuously but steeply, i.e., the thickness changes continuously over a short distance. It can be seen that the thickness first decreases from left to right and then increases again. The light mixing rod shown here can be equipped with a diffuser, a beam-splitting coating, or, in a solid design, a partially reflective boundary layer. Suitable combinations can also be achieved here, as in other examples.

[0060] Figure 11 A longitudinal section of another light mixing rod 17 is shown. This rod consists of three sub-rods 1701-1703, each with walls of different thicknesses D1-D3. A diffuser 171 is arranged on the input surface of the left sub-rod 1701. Diffusers 172 and 173 are arranged at the transition between sub-rods 1701 and 1703.

[0061] Figure 12 A cross-section of another light mixing rod 17 is shown. It can be seen that this is not circular, but rather has an approximately rectangular cross-section. Its wall 1704 has an almost constant thickness D in this cross-section.

[0062] Figure 13 A perspective view of the light mixing rod 17 is shown. The light mixing rod consists of two sub-rods 1701 and 1702, each of which has an input surface that serves as a diffuser 171 and 172.

[0063] Any type of projector typically includes a light preparation unit in its illumination system between the light sources 14R, 14G, 14B and the display element 11 to sharply and uniformly illuminate the image generator, such as based on LCOS, TFT (Thin Film Transistor), DMD, or similar methods. Two common methods for adequately mixing the light sources 14R, 14G, 14B are the use of multi-lens arrays (MLAs) and the use of a light mixing rod 17. Especially in laser light sources, speckle effects (in the image) arise due to coherence, which are undesirable. To suppress speckle effects, a moving diffuser is mounted in front of the light mixing rod, which generates different scattering patterns in the time domain that are uncorrelated with each other, thus reducing contrast due to speckle. Because this is a moving component, this solution is also called an active déscribing method. Active déscribing methods are expensive due to the short lifespan of the moving component and the need for more space, such as to accommodate the diffuser wheel. To generate a good and uniform image, laser-based projectors typically require adequate and good mixing of the (collimated) laser light in the illumination system. To achieve this, a light mixing rod can be used, which can be designed to be hollow or filled. According to the invention, a diffuser with targeted scattering function is integrated into the light mixing rod 17, preferably arranged directly in front of the input face of the light mixing rod. In this case, the angular range of the output end can be controlled. If the light mixing rod 17 has a sufficient length L, allowing for a sufficient number of reflections, a very uniform field is obtained at the output surface, which is then imaged or transmitted to an image generator, i.e., a display element 11, for example, implemented as a DMD or LCOS. According to the invention, sharp and efficient illumination can thus be generated. Preferably, a plurality of diffusers 172, 173 are arranged in the light mixing rod 17.

Claims

1. An apparatus for generating a virtual image (VB), the apparatus having an image generating unit (1) for producing an image, the image generating unit having a light source (14R, 14G, 14B) emitting coherent light and a speckle reducing element, characterized in that, The speckle reduction element is a speckle-reducing light mixing rod (17).

2. The apparatus of claim 1, wherein, A first diffuser (171) is arranged on the input surface of the light mixing rod (17), wherein another speckle reduction element is arranged in the light mixing rod (17).

3. The apparatus of claim 2, wherein, The additional speckle reduction element is an additional diffuser (172, 173).

4. The device of any of the preceding claims, characterized in that, The light mixing rod (17) has at least one beam splitter (176, 1761).

5. The apparatus of claim 4, wherein, The light mixing rod (17) is a hollow rod, and the inner surface (170) of the hollow rod is provided with a light-dispersing coating (176).

6. The apparatus according to claim 5, characterized in that, The transmittance (T) of the spectral coating (176) is equal to or greater than the reflectance (R) of the spectral coating.

7. The apparatus according to any one of claims 4 to 6, characterized in that, The wall (1704) of the light-mixing rod (17) designed as a hollow rod has a non-uniform thickness (D1, D2).

8. The apparatus according to any one of the preceding claims, characterized in that, The lengths (L, L1, L2, L3) of the light mixing rod (17) are matched with the scattering angles (α1, α2, α3) of at least one diffuser (171, 172, 173).

9. A head-up display having means according to any one of the preceding claims.

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