Patent Description:
Automotive head-up displays are used to convey critical vehicle information directly in the field of view of the vehicle's conductor or driver. The information is delivered to the driver at a certain distance so that no or nearly no accommodation of the driver's eye is required when the driver switches from viewing the road in front of him to reading the vehicle's status shown as the virtual image. This approach reduces the reaction time of the driver by several hundreds of milliseconds, thus increasing road safety.

<CIT> relates to a head up display with a lens decentred to the optical axis of the illumination. It shows a decentred Fresnel lens and, as a separate part, a diffusor which is a heat absorbing glass.

<CIT> relates to a liquid-crystal display and a head-up display. It shows a liquid crystal display plane being perpendicular to the optical axis. A polarizing plate is not perpendicular to the optical axis.

<CIT> relates to a display device, an electronic apparatus, and a projection imaging apparatus. It makes use of a wedge prism. However, this wedge prism does not provide a cooling function.

It is an object of the invention to propose a solution for a head-up display with improved sunlight reflection protection.

A head-up display according to the invention is claimed in claim <NUM>.

This head-up display has an image generator with a light source, a tilted display, an aspheric mirror, an optical system and a transmissive screen. An aspheric lens is arranged between the display and the aspheric mirror. This advantageously reduces peak intensity of sunlight falling on the display. Sunlight may enter the head-up display in reverse direction. Although several means to reduce peak intensity are known, there is an ongoing need to further reduce it. The aspheric lens widens the light rays of the sunlight which are nearly parallel to each other, having a maximum deviation of <NUM>°.

Preferably the aspheric lens has a wedge component. This has the advantage that the tilt of the display can be reduced by the wedge angle of the aspheric lens without reducing the ability to reflect sunlight that enters the head-up display along its main optical axis onto a light trap. The reduced tilt of the display increases the contrast visible from such reduced angle. It thus increases contrast of the virtual image generated by the head-up display.

A display glass having a wedged shape is arranged close to or at the display. This has the advantage that the tilt of the display can be reduced by the wedge angle of the display glass without reducing the ability to reflect sunlight that enters the head-up display along its main optical axis onto a light trap. The reduced tilt of the display increases the contrast visible from such reduced angle. It thus increases contrast of the virtual image generated by the head-up display.

The display glass is a cooling glass. Such cooling glass is often applied onto the light exiting surface of the display in order to protect the display from overheating. Combining two functions, here cooling function and wedge shape function in a single element reduces number of parts and assembly costs.

Preferably the display is tilted with regard to a main optical axis of the head-up display by a first angle and the display glass has a wedge angle with regard to said main optical axis, and wherein first angle and wedge angle add to an effective tilt angle. This has the advantage that both effects combine in full, thus increasing contrast very efficiently.

Preferably the first angle is in the range of α = <NUM>° to α = <NUM>° and the wedge angle β is in the rage of <NUM>° to <NUM>°. Commercially available displays exist that emit light along the optical axis if the display is tilted by an angle α in the mentioned range with regard to a plane orthogonal to this main optical axis. A prism with a wedge angle β in the mentioned range leads to optical dispersion that is sufficiently small as to not have a negative influence on the driver's perception of the virtual image. Using a wedged display glass with the mentioned properties has the advantage that the light emitted by the display is further tilted in dependence on the wedge angle. The effective tilt of the display plane with regard to the main optical axis of the light exiting the display glass is thus further enlarged. This effect is desired in order to tilt the image plane of the virtual image with regard to the optical axis. The virtual image plane thus seems to be oriented nearly parallel to the road surface ahead of the vehicle in which the head-up display is installed. Different parts of the virtual image have their optimum sharpness at different distances from the driver. This gives the impression of a three-dimensional head-up display image: Elements displayed in the lower part of the virtual image appear closer to the driver than elements in the upper part or in the middle part. Displayed elements that are meant to augment real objects can thus be arranged close to these objects not only with regard to their position in the virtual image plane, but also in a focal distance that is comparatively close to their respective real object. Objects at the horizon with a big real distance are augmented with elements that are located in the virtual image at a far focal distance. Navigation supporting elements like arrows showing a turning direction to be followed are located in the virtual image at a location very close to the real road position at which the turn should be made, and - at the same time - with a focal distance close to the real distance of this real road position.

Preferably, the display glass is affixed to the tilted display. The display glass is for example glued or bonded in another way directly on the top layer of the display. This advantageously reduces the number of optical surfaces on which undesired reflections may occur.

According to another preferred variant the display glass is the layer of the display that is next to the optically active layer of the display. In case of a liquid crystal display the optically active layer is the liquid crystal which liquid is sandwiched by two transparent planes. According to this preferred variant one of these planes belongs to the wedged display glass. This advantageously reduces the number of optical surfaces, thus reduces the potential for undesired reflections.

Preferably at least one of the display glass and the aspheric lens is made of BK7 glass or is made of plastic. Both are materials that are readily available and have known properties. Thus, their use will be cost efficient.

Further details, advantages and variants of the invention are provided in the following description of exemplary embodiments in the following figures, which show:.

<FIG> shows a vehicle <NUM> using a head-up display <NUM>, in the following also referred to as HUD <NUM>. On the left side an engine hood <NUM> is visible. The HUD <NUM> is arranged below a dashboard <NUM>. Behind the steering wheel <NUM>, the driver's head <NUM> is shown. Between rooftop <NUM> and engine hood <NUM>, a windshield <NUM> is arranged. The windshield <NUM> acts as a transmissive screen <NUM> of the head-up display <NUM>. The HUD <NUM> generates optical beams L1, which are reflected at the transmissive screen <NUM> and fall into the driver's eye <NUM> as long as the eye <NUM> is within an area called eyebox <NUM>. As long as the eye <NUM> is within the eyebox <NUM>, the driver sees a virtual image <NUM> that appears to be outside the vehicle <NUM> in front of the windshield <NUM>.

Automotive head-up displays <NUM> are used to convey critical vehicle information directly in the field of view of the vehicle's conductor or driver. The information is delivered to the driver at a certain distance so that no or nearly no accommodation of the driver's eye <NUM> is required when the driver switches from viewing the road in front of him to reading the vehicle's status shown as the virtual image <NUM>. This approach reduces the reaction time of the driver by several hundreds of milliseconds, thus increasing road safety. At a speed of <NUM>/h a <NUM> delay translates in a covered distance of about <NUM>, which is about <NUM>% of the total stop distance of the vehicle <NUM>. As seen in <FIG>, the head-up display <NUM> creates a virtual image <NUM> ahead of the transmissive screen <NUM>. The transmissive screen <NUM> can be the windshield <NUM> of the vehicle <NUM>, as shown, or a different, dedicated, partially reflective surface separated from the windshield. Such surface is a so-called combiner. The virtual image <NUM> is visible from a limited region of space only, the so-called eyebox <NUM>.

It should be clear to a person skilled in the art that the depictions in the described figures are only simplifications done for ease of understanding. The real-life systems may differ in construction details without departing from the invention described with help of the figures. From this, it is to be understood that the used descriptive words should not be considered only for their basic meaning but also for equivalents.

The same reference signs are used for the same elements shown in the following figures. They are not necessarily described again, except if they differ in function or if such description seems meaningful with regard to the respective embodiment.

<FIG> shows a head-up display <NUM> having an image generator <NUM>, also referred to as image projection system below, that generates an optical beam L0. The optical beam L0 enters a generic optical system <NUM>, here illustrated by an optical mirror <NUM>, from which an optical beam L1 is directed to the transmissive screen <NUM>. In the embodiment shown the optical mirror is an aspheric mirror <NUM>. A part of the light of beam L1 passes through the transmissive screen <NUM>, which is indicated by dotted arrows. Another part is reflected by the transmissive screen <NUM> and thus reaches the driver's eye <NUM> as optical beam L2.

<FIG> shows an image projection system as image generator <NUM> having a light source <NUM>, a diffuser <NUM>, and a display <NUM>, for example a transmissive liquid crystal display (LCD). At the front side of the display <NUM> a front polariser <NUM> is arranged. At the backside of the display <NUM> a back polariser <NUM> is arranged. The optical beam L0 leaves the image generator <NUM>.

HUD systems as depicted in <FIG> and <FIG> usually consist of an image generator <NUM> coupled to an optical system <NUM> that directs the image formed by the image generator <NUM> onto a transparent screen, the transmissive screen <NUM>. In the example shown, the image generator <NUM> is provided with a backlight unit, the light source <NUM>, which for example uses LEDs, laser beam generators or is of any other type of light source. The image generator is further provided with an image forming system that may be a set of sweeping mirrors, micromirror arrays using MEMS technologies or, as shown in <FIG>, an active matrix liquid crystal display constructed on a flat, rigid glass backplane. The essential parts of the image generator <NUM> for an LCD-based system as illustrated in <FIG> are, as backlighting source, the light source <NUM>, together with the diffuser <NUM>. The diffuser <NUM> may be a simple diffuser film or a mixed function film like a combination of a diffuser and a brightness enhancing film, the front and back polarising films <NUM>, <NUM> and an LCD, the image forming display <NUM>.

<FIG> shows an embodiment of an image generator <NUM> of a head-up display <NUM> according to the invention. The image generator <NUM> is provided with a light source <NUM>, a tilted display <NUM>, a display glass <NUM> as cooling glass and a light trap <NUM>. The cooling glass <NUM> has a wedged shape. An aspheric lens <NUM> is arranged between display <NUM> and an aspheric mirror <NUM>. The optical system <NUM>, the aspheric mirror <NUM> and the transmissive screen <NUM> are not shown here.

A liquid crystal display <NUM> is arranged tilted with regard to a main optical axis MOA of the head-up display <NUM>. A cooling glass <NUM> is arranged on the light exiting side of the liquid crystal display <NUM>. The cooling glass <NUM> is shaped as a wedge. Below the liquid-crystal display <NUM>, on its light incoming side, a diffuser <NUM> is arranged. Optical foils <NUM> are arranged between display <NUM> and diffuser <NUM>, but not explained in more detail here. Below the diffuser <NUM>, a lightbox <NUM> is arranged. At the end of the lightbox <NUM> opposite to the diffuser <NUM>, light emitting diodes (LED) are arranged as light source <NUM>. Above the LEDs, lenses <NUM> are arranged. The LEDs as light source <NUM> are placed on a printed circuit board (PCB) <NUM>. The printed circuit board <NUM> acts as electric insulator between light source <NUM> and a heatsink <NUM> as well as a thermal insulator between these. The printed circuit board <NUM> is provided with vias <NUM> filled with metal or another highly thermally conductive medium in order to transport heat generated by the light source <NUM> to the heatsink <NUM>.

Light generated by the light source <NUM> passes through the lens <NUM>, travels through the lightbox <NUM> and passes through diffuser <NUM>, optical foils <NUM> and the liquid display <NUM>. An optical beam L0 leaves the liquid-crystal display <NUM> and passes through the wedge-shaped cooling glass <NUM> in direction to the generic optical system <NUM>, not shown here.

Sunlight SL may enter the head-up display <NUM>, mainly if it passes through the windshield <NUM> parallel to the main optical axis MOA of the head-up display <NUM>. When reaching the cooling glass <NUM> it is reflected. Due to the inclination of the surface of the cooling glass <NUM> with respect to the main optical axis MOA, the reflected sunlight SLR is not parallel to the main optical axis MOA and thus does not reach the driver's eye <NUM> but is absorbed at a light trap <NUM>.

<FIG> shows that the display glass <NUM> is tilted with regard to the main optical axis MOA of the head-up display <NUM> by a first angle α. The cooling glass <NUM> has a wedge angle β with regard to said main optical axis MOA. The first angle α and the wedge angle β add to an effective tilt angle γ. In this figure display <NUM> and display glass <NUM> are arranged with a distance to each other. In another preferred embodiment as shown in the previous figure the wedged display glass <NUM> is arranged directly on the top layer of the display <NUM>. The display glass <NUM> in this case is affixed to the tilted display <NUM>, e.g. by being glued thereon or bonded thereto in an appropriate manner. According to another preferred embodiment the display glass <NUM> is the top layer of the display <NUM>. The display glass <NUM> in this case is the layer of the display <NUM> that is next to the optically active layer of the display <NUM>.

<FIG> shows a schematic side view of the position of a sloped virtual image VBS that is made possible according to the invention as well as an exemplary view that the driver has through the windshield of the vehicle. The head-up display <NUM> has a tilted display <NUM> and a wedged display glass <NUM> (both not shown here) according to the invention. Due to the large effective tilt of the display <NUM>, also the virtual image generated therefrom is a sloped virtual image VBS that is nearly parallel to the road surface. The optical light beam L1 generated by the head-up display <NUM> is reflected by the windshield <NUM> as transmissive screen <NUM> and reaches the driver's eye <NUM>. The driver sees the upper part of the displayed image e.g. at a focal distance of <NUM> (left part of the sloped virtual image VBS in the figure), and the lower part of the displayed image e.g. at a focal distance of <NUM> (right part of the sloped virtual image VBS in the figure).

The upper left part of <FIG> depicts an exemplary view that the driver has through the windscreen of the vehicle. Elements EC, EM, EF of the virtual image are indicated by dashed lines and have partly hashed areas, while real world elements are indicated by continuous lines. A current speed (here: <NUM>/h) is indicated as close element EC at the lower part of the virtual image. It is visible above the hood (bonnet) <NUM> of the vehicle and has a focal distance of about <NUM>. A navigation arrow that indicates a suggested turn to the right is indicated as element EM of medium distance in the middle part of the virtual image VBS. It is visible above the road surface <NUM> at a focal distance of about <NUM>. At a farther distance a pedestrian <NUM> is indicated. Next to the pedestrian <NUM> a warning sign is displayed as far element EF. It augments the reality, here: the pedestrian <NUM>, with important information for the driver, here: a warning, e.g. that this pedestrian <NUM> might cross the road <NUM>. The far element EF of the virtual image has a focal distance of about <NUM>.

<FIG> illustrates schematically the effect of the tilt of a display <NUM>, <NUM>-<NUM>, <NUM>-<NUM> on the sloped virtual image VBS, VBS-<NUM>, VBS-<NUM>. An optical lens <NUM> is arranged on a main optical axis MOA. For simplicity reasons the optical lens <NUM> is a placeholder for all optical elements of the head-up display <NUM> that are arranged between display <NUM> and sloped virtual image VBS. Display <NUM>-<NUM> is tilted about first angle α with regard to a plane orthogonal to the main optical axis MOA. Together with the optics indicated here by the optical lens <NUM> it generates a sloped virtual image VBS-<NUM> that is tilted about an angle α' with regard to a plane perpendicular to the main optical axis MOA. Another display <NUM>-<NUM> is shown in the figure that is tilted by an angle γ. It generates a sloped virtual image VBS-<NUM> that is tilted about an angle γ'. The plane in which the sloped virtual image VBS-<NUM> lies, the plane in which the display <NUM>-<NUM> lies and the plane of the optical lens <NUM> which is orthogonal to the main optical axis MOA intersect in a line, that is a point in the drawing plane of this figure. Similarly, the other display <NUM>-<NUM> and the other sloped virtual VBS-<NUM> are related to each other.

<FIG> schematically shows an arrangement according to the invention. At the end of lightbox <NUM> the display <NUM> is arranged. Next to the display <NUM> a wedged display glass <NUM> is arranged. It has a wedge angle β. In a distance an optical mirror <NUM> is arranged. In the embodiment shown it has an aspheric mirror surface <NUM>. Between the display glass <NUM> and the aspheric optical mirror <NUM> the aspheric lens <NUM> is arranged, In this embodiment it has a plane surface <NUM> at its side facing the display <NUM> and an aspheric surface <NUM> at its side facing the aspheric mirror <NUM>. The aspheric lens is tilted with regard to the main optical axis MOA (not depicted here). Above the optical mirror <NUM> a top cover <NUM> of the head-up display <NUM> is shown. Sunlight SL passes through the transparent top cover <NUM> and is reflected by the aspherical mirror surface <NUM> towards the display glass <NUM>. A part of the sunlight SL is reflected as reflected sunlight SLR on the aspheric surface <NUM> of the aspheric lens <NUM> towards a light trap <NUM>. Another part is refracted when entering the aspheric lens <NUM> at its aspheric surface <NUM> and refracted again at its backside when passing the plane surface <NUM>. The thus remaining sunlight SL travels to the display glass <NUM>. A part of the sunlight SL is reflected as reflected sunlight SLR' on the outer surface of the display glass <NUM> towards a light trap <NUM>. Another part of the sunlight is refracted inside the display glass <NUM>, reflected at its inner surface, refracted when passing its outer surface and directed to the light trap <NUM> as reflected sunlight SLR". As can be seen, a big amount of sunlight SLR, SLR', SLR" is reflected towards the light trap <NUM>. The remaining sunlight that reaches the display <NUM> is already dispersed by the prism shape of the wedged display glass <NUM> and by the effect of the aspheric lens <NUM>. It is thus less focussed on the display <NUM> which prevents small temperature hotspots, but rather leads to temperature blots having a larger area thus causing less heat stress. As the display glass <NUM> is arranged as cooling glass that conducts heat away from the display <NUM>, the arrangement shown here has good heat resilience.

<FIG> shows two embodiments where the display glass <NUM> is the layer of the display <NUM> that is next to the optically active layer <NUM>, <NUM> of the display <NUM>. In the upper part of the figure an LED (Light Emitting Diode) display is shown schematically. LEDs <NUM> are arranged in an LED layer <NUM>, which acts as optically active layer: Only a single LED <NUM> is shown here for simplicity. The LEDs <NUM> are arranged as matrix. The image is generated by turning on or off LEDs <NUM> of the matrix. The display glass <NUM> in this embodiment is provided with lenslets <NUM> at its lower surface. For each LED <NUM> a lenslet <NUM> is available to collect the light emitted from the LED <NUM>. The display glass <NUM> has a wedged shape as already described with regard to previous figures. In the lower part a liquid crystal display is shown. The optically active layer is the liquid crystal layer <NUM> that is sandwiched between a transparent substrate <NUM> and the wedge-shaped display glass <NUM>. Atop of the display glass <NUM> a front polarizer <NUM> is arranged. At the lower surface of the substrate <NUM> a back polarizer <NUM> is arranged.

<FIG> schematically shows an arrangement according to the invention. At the right side a display <NUM> with a wedged display glass <NUM> affixed to its light emitting surface is shown. Two exemplary light rays L0-<NUM>, L0-<NUM> are also depicted. They are emitted from the display <NUM> and pass through (dotted line) the display glass <NUM>. They are refracted when passing the front surface of the display glass <NUM>. Then the light rays L0-<NUM>, L0-<NUM> travel to the aspheric lens <NUM> which they enter through its plane surface <NUM>. They are refracted and again refracted when passing the aspheric surface <NUM>. Then the light rays L0-<NUM>, L0-<NUM> travel to the aspheric mirror <NUM> where they are reflected at its aspheric mirror surface <NUM> to leave the head-up display <NUM> as light rays L1-<NUM>, L1-<NUM> through its top cover <NUM> (not shown here).

<FIG> shows a plano-aspheric lens <NUM> arranged between aspheric optical mirror <NUM> and display <NUM>. Three light bundles LB1, LB2, LB3 that are emitted from three different positions on the display <NUM> are shown. The light bundles LB1, LB2, LB3 are diverged by the aspheric lens <NUM> and changed in their direction, so that at the end the full surface <NUM> of the aspheric mirror <NUM> is covered. For simplicity the further light paths are not shown here.

The invention suggests using a refractive aspheric lens <NUM> in the beam path between a display <NUM> as image source and an aspheric optical mirror <NUM> of a head-up display <NUM>. This head-up display generates a single virtual image <NUM>, VBS which is not oriented in a plane that is nearly orthogonal to the forward direction of the vehicle the head-up display <NUM> is mounted in but is sloped thereto. The upper part of the sloped virtual image VBS has a larger focal distance to the driver of the vehicle than the lower part of the sloped virtual image VBS. This single virtual image VBS is thus comparable to a head-up display that generates two or more image planes at different focal distance to allow augmented reality (AR) functionality where elements displayed by the head-up display that augment real objects can be placed in respective different focal distances as fits best to the real objects' distance to the driver. The head-up display <NUM> according to the invention may be called to provide "quasi-AR functionality", as elements to be displayed can be placed at different focal distances by placing them in different horizontal stripes of the sloped virtual image VBS. Conventional head-up displays that use a TFT (thin film transistor) technology base liquid crystal display can tolerate only a small tilt of the image plane of the virtual image with regard to a plane perpendicular to the optical axis. The optical axis usually nearly coincides with the forward direction of the vehicle the head-up display is mounted at. Such small tilt is not sufficient for the quasi-AR effect desired. If the tilt at such conventional head-up displays is chosen large enough for a quasi-AR effect, then optical performance, contrast and brightness degrade, and undesired heat effects due to sunlight that enters the head-up display increase so that the appearance of the virtual image is no longer acceptable. A solution that allows for a largely tilted virtual image is desired, as such sloped virtual image is a precondition to display augmented reality elements using a single virtual image, in a pseudo-AR head-up display.

The invention relates to such quasi-AR head-up display having an image source, here the display <NUM>, and at least one aspheric optical mirror <NUM>. According to the invention at least an aspheric lens <NUM> is arranged in the beam path between image source and aspheric mirror <NUM>. This leads to a better distribution of the peak intensity of sunlight SL that enters the head-up display <NUM>. This peak intensity on the display <NUM> is reduced and distributed over a larger area. Thus, heating up of the display <NUM> is reduced. This is important in case of a display technology for which overheating is critical, e.g. liquid crystal technology where the liquid crystal material does not work properly above a critical temperature. Also, the amount of sunlight that is reflected by the display <NUM> and may reach the driver's eye <NUM> is reduced. Thus, irritation of the driver is prevented, driving safety is increased. Further, the arrangement according to the invention provides for an enlarged exit pupil, which is a desired effect. The tilt angle of the display <NUM> needed to generate a sloped virtual image VBS as desired for a quasi-AR head-up display is reduced. It is thus made possible to use the display <NUM> in a geometrical orientation in which good contrast and high brightness are reached. Also, the optical performance is improved thereby. It also allows to optimize the space requirement for the head-up display <NUM>. Preferably the aspheric lens <NUM> is a plano-concave lens, or a plano-aspheric lens as shown in some of the figures. Alternative solutions are to use a convexo-concave lens arranged between display <NUM> and aspheric mirror <NUM>, or a convex-aspheric lens. It is also preferred to provide these elements with a wedge form, and/or to combine it with a prism or wedge-shaped display glass <NUM>, which even more increases efficiency.

Claim 1:
Head-up display (<NUM>) having an image generator (<NUM>) with a light source (<NUM>), a tilted display (<NUM>), an aspheric mirror (<NUM>), an optical system (<NUM>) and a transmissive screen (<NUM>), wherein an aspheric lens (<NUM>) is arranged between display (<NUM>) and aspheric mirror (<NUM>), and wherein a wedge-shaped display glass is provided, characterized in that the display glass is a cooling glass (<NUM>) that conducts heat away from the display (<NUM>).