GB2612320A

GB2612320A - Head-up display unit adapted to high working temperatures and high backlight intensity

Info

Publication number: GB2612320A
Application number: GB2115425.7A
Authority: GB
Inventors: Pasca Andrei
Original assignee: Continental Automotive Technologies GmbH
Current assignee: Continental Automotive Technologies GmbH
Priority date: 2021-10-27
Filing date: 2021-10-27
Publication date: 2023-05-03
Also published as: GB202115425D0

Abstract

A head-up display unit 1 has a picture generating unit 10 creating a virtual image VB displayed to a viewer. An optical unit 14 projects the image towards an eyebox 4. The picture generating unit 10 includes a first liquid crystal layer 110, a front polarizer 113, a back polarizer 114, a reflective polarizer 116, a second liquid crystal layer 117 and a backlight pre-polarizer 131. The reflective polarizer is upstream of the back polarizer and downstream of the second liquid crystal layer. The picture generating unit may also include a single black mask layer downstream of the first liquid crystal layer. A control circuit may drive the first and second liquid crystal layers with matching drive signals. The reflective polariser may have a reflectivity of 50% and be a metal grid polariser. There may also be a third liquid crystal layer and a frontside reflective polariser which is positioned between the front polariser of the first liquid crystal layer and the third liquid crystal layer. A vehicle may have such a head-up display unit for generating an image for a user of the vehicle. The head-up display unit is adapted to high working temperatures and a high backlight intensity.

Description

Head-up display unit adapted to high working temperatures and high backlight intensity The present invention is related to a head-up display unit being adapted to high working temperatures, especially temperatures above 90°C, and to high backlight intensity. The invention is further related to a vehicle comprising such a head-up display.

A head-up display, also referred to as a HUD, is a display system in which the viewer can maintain his viewing direction, because the contents to be displayed are displayed in his field of vision. While such systems, due to their complexity and costs, were originally mainly used in the aviation sector, they are now also being used in large scale in the automotive sector, as they allow easy reading of critical information without the need to change the eye gaze from the road to an in-cabin instrument cluster.

Head-up displays generally consist of a picture generating unit (PGU), an optical unit and a mirror unit. The picture generating unit generates the image to be displayed. The optical unit directs the image to the mirror unit. The mirror unit is a partially reflecting, translucent pane, which reflects the light in such way that it reaches the so-called eyebox. The eyebox is an area whose height and width correspond to a theoretical viewing window. As long as an eye of the viewer is inside the eyebox, all elements of the virtual image are visible to the eye. If, on the other hand, the eye is outside the eyebox, the virtual image is only partially or not at all visible to the viewer. The larger the eyebox is, the less restricted the viewer is in his choice of sitting position.

The viewer sees the content displayed by the picture generating unit as a virtual image and at the same time the real world behind the translucent pane. In the automotive sector, the windscreen often serves as a mirror unit, the curved shape of which must be taken into account in the representation. However, the mirror unit may also be a dedicated screen, called combiner. Through the interaction of the optical unit and the mirror unit, the virtual image is an enlarged representation of the image generated by the picture generating unit.

The picture generating unit and the optical unit of a head-up display are typically arranged in a common housing assembly, which may also include electronic components necessary for operation of the head-up display. The housing assembly may be sealed with a cover glass and protects the various components from damage as well as dust or other environmental influences. Furthermore, the housing assembly helps to simplify mounting of the head-up display in a vehicle, as all necessary components can be installed in a single production step.

An important requirement for head-up display applications is the necessary brightness of the virtual image, which needs to be in order of at least 5 000 cd/m2, but preferably in the range 10 000 to 15 000 cd/m2. Current head-up display solutions typically comprise a picture generating unit that uses a liquid crystal display (LCD) for creating the image to be displayed. In view of all the components that are placed in the optical path of the system, this high brightness requirement translates to a brightness requirement on the top surface of the display panel that is about five to ten times larger, i.e. 50 000 to 100 000 cd/m2. Given the typical transmittance of an LCD panel of around 5 to 6%, this translates to a backlight requirement in the order of 1 000 000 to 2 000 000 cd/m2. The high brightness requirements for the display lead to a series of problems when an LCD is used in a head-up display, which are mainly due to the low transmittance.

A first issue is the high thermal impact due to the heat generated by the backlight unit and the luminous power dissipated by the absorption in the display panel of around 95% of the incoming light. Critical aspects in this regard are the clearing temperature of the liquid crystal and the temperature performance of the polarizers attached to the liquid crystal panel. If the temperature of the polarizers exceeds a certain limit, the polarizer may lose its light polarizing properties and the display becomes bright where it should be dark or vice versa. If the temperature of the liquid crystal exceeds the clearing temperature, the liquid crystal may lose its light polarizing properties and the display becomes transparent.

A second issue is the high power consumption required for achieving the high brightness of the backlight, which usually is in the order of 10 W or more for problematic applications, independent of the actual display content.

Head-up displays require very bright backlight in order to generate a virtual image that is visible even under bright outside light conditions. In HUD most display areas are black, as usually only symbols are shown. This means that a large part of the backlight is dissipated in the display causing heat. Liquid-crystal materials do not operate properly above a certain temperature, the so-called clearing temperature. It is desired to keep the liquid-crystal materials temperature below the clearing temperature.

US 2013/279016 Al relates to preventing LCD of a HUD from overheating. A reflective polarizer is arranged downstream the output polarizer of the liquid-crystal panel, thus reflecting light that otherwise would be blocked by the output polarizer. Thus, heating of the output polarizer is reduced. This means that the reflective polarizer is used to prevent a part of the sunlight with a polarization perpendicular with the front polarizer to hit the display.

EP 1 126 292 A2 relates to an optical polarizer consisting of a combination of reflective polarizer and dicroic polarizer. Such polarizer is for example used in an LCD. This patent describes the construction of the combination between a reflective polarizer and a normal polarizer and names several possible applications. Typically, the reflective polarizers are used to improve the brightness of a standard LCD by recycling a part of the reflected light -that means that the light that is reflected back from the display will re-enter the backlight structure where its polarization plane will be re-aligned by the structure to match the polarization plane required by the back polarizer of the display. This is also the application described in EP 1 126 292 A2.

EP 3 451 044 Al relates to a head-up display using two LCD panels.

It is an object of the present invention to provide an improved solution for a head-up display with high brightness.

This object is achieved by a head-up display unit according to claim 1 and a vehicle according to claim 7. The dependent claims include advantageous further developments and improvements of the present principles as described below.

A head-up display unit according to the invention comprises a picture generating unit for creating an image to be displayed as a virtual image to a viewer and an optical unit for projecting the image to be displayed towards an eyebox. The picture generating unit comprises a first liquid crystal layer with a front polarizer and a back polarizer, a reflective polarizer a second liquid crystal layer, and a backlight pre-polarizer. The reflective polarizer is arranged upstream the back polarizer and downstream the second liquid crystal layer. This has the advantage that for those pixels for which the second liquid crystal layer is in a light blocking state, the light that reaches the polarizer is reflected back, and does not reach the back polarizer of the first display. There is thus less heat dissipation in the back polarizer of the first liquid crystal layer. This avoids additional heating of the first liquid crystal layer and thus makes possible a high brightness head-up display that works properly even under critical heat environment. A further advantage is improving the contrast ratio, allowing for a trade-off between main display contrast and transmittance (which further reduces the amount of power that is dissipated into the stack for same virtual image parameters).

According to an aspect of the invention the picture generating unit further comprises a single black mask layer that is arranged downstream the first liquid crystal layer. This advantageously provides for visible separation of pixels by the single black mask. In this arrangement there is no need to provide a further mask layer as potential pixel crosstalk of the second liquid crystal layer is also prevented by the single mask layer arranged downstream adjacent to the first liquid crystal layer.

A head-up display unit according to the invention advantageously further comprises a controller that drives the first liquid crystal layer and the second liquid crystal layer with matching drive signals. This has the advantage that the second liquid crystal layer provides thermal protection for those pixels of the first liquid crystal layer that are anyway in an OFF state, but provides maximum light flux for those pixels of the first liquid crystal layer that are in an ON state. The controller may be arranged as fixed circuitry, as a dedicated IC, as software running on a processor or in any other form known to the skilled person. The controller preferably generates separate control signals for first and second liquid crystal layer. In another embodiment the controller supplies the same control signal to both, first and second liquid crystal layer. The controller may also be arranged as circuitry that pre-processes a single control signal to generate two different but matching control signals to the first and the second liquid crystal layer, respectively. Pre-processing may for example be attenuating the control signal for one of the liquid crystal layers compared to the control signal for the other one. Such attenuation is for example dependent on the colour that a respective pixel is aligned with.

Advantageously, the reflecting polarizer has a comparatively low reflectivity of 50%, or a reflectivity in the range of 45% to 60%. This provides for good thermal protection in the case of a head-up display where typically only about 10% of the pixels are in an ON state simultaneously. This characteristic of a head-up display makes possible application of a relatively inexpensive reflective polarizer.

Preferably, the reflective polariser is a metal wire grid polariser. This has the advantage that it combines the properties of reflective polariser and heat dissipation. It reflects polarized light and thus prevents this light to heat up the first liquid crystal layer. It also dissipates heat that for other reasons occurs in the first liquid crystal layer so that hot spots in the first liquid crystal layer are avoided by more evenly distribute heat thus keeping the liquid crystal material below the critical temperature.

A head-up display according to a further improvement comprises a third liquid crystal layer and a frontside reflective polarizer, wherein the frontside reflective polarizer is arranged between the front polarizer of the first liquid crystal layer and the third liquid crystal layer. This has the advantage to reduce heating of the front polarizer and thus also of the first liquid crystal layer as impinging sunlight will be reflected.

According to one aspect of the invention, a vehicle comprises a head-up display unit according to the invention for generating an image for a user of the vehicle. The vehicle may, for example, be a car or an aircraft. Of course, the inventive solution can also be used in other environments or for other applications, e.g. in trucks, busses, in railway and public transport, cranes and construction machinery, etc. Further features of the present invention will become apparent from the following description and the appended claims in conjunction with the figures.

Figures Fig. 1 shows a sketch of a state-of-the-art head-up display for a motor vehicle; Fig. 2 shows a sketch of a head-up display according to the invention; Fig. 3 shows a simplified sketch similar to Fig.2; Fig. 4 shows a picture generating unit in a simplified diagrammatic view; Fig. 5 illustrates where incident light on a liquid crystal display gets dissipated; Fig. 6 shows a known liquid crystal display; Fig. 7 shows another configuration of a liquid crystal display; Fig. 8 shows a display configuration according to the invention; Fig. 9 shows a known display application; Fig. 10 shows a configuration according to the present invention; Fig. 11 shows an embodiment according to the invention; Fig. 12 shows another embodiment according to the invention; Fig. 13 shows a diagram of a part of a head-up display; Fig. 14 shows a further embodiment according to the invention; Fig. 15 shows another advantageous implementation.

Detailed description

For a better understanding of the principles of the present invention, embodiments of the invention will be explained in more detail below with reference to the figures. Like reference numerals are used in the figures for the same or equivalent elements and are not necessarily described again for each figure. It is to be understood that the invention is not limited to the illustrated embodiments and that the features described may also be combined or modified without departing from the scope of the invention as defined in the appended claims.

Fig. 1 shows a sketch of a state-of-the-art head-up display unit 1 for a motor vehicle. The head-up display unit 1 has a picture generating unit 10, an optical unit 14 and a mirror unit 2. A beam SB1 emanates from a liquid crystal display 11 of the picture generating unit 10 and is reflected by a folding mirror 21 onto a curved mirror 22, which reflects it in the direction of the mirror unit 2. The mirror unit 2 is shown here as a windscreen 20 of the motor vehicle. From there, the beam SB2 is directed towards the eye 3 of an observer.

The observer sees a virtual image VB, which is located outside the vehicle above the bonnet or even in front of the vehicle. Due to the interaction of the optical unit 14 and the mirror unit 2, the virtual image VB is an enlarged representation of the image displayed by the display 11. Here a symbolic speed limit, the current vehicle speed and navigation instructions are displayed. As long as the eye 3 is located inside the eyebox 4 indicated by a rectangle, all elements of the virtual image VB are visible to the eye 3. If the eye 3 is located outside the eyebox 4, the virtual image VB is only partially or not at all visible to the viewer. The larger the eyebox 4 is, the less restricted the viewer is in his choice of the seating position.

The curvature of the curved mirror 22 is adapted to the curvature of the windscreen 20 and ensures that the image distortion is stable over the entire eyebox 4. The curved mirror 22 is rotatably supported by a bearing 221. By rotating the curved mirror 22, it is possible to shift the eyebox 4 and thus to adjust the position of the eyebox 4 to the position of the eye 3. The folding mirror 21 ensures that the distance travelled by the beam SB1 between the display 11 and the curved mirror 22 is long, while, at the same time, the optical unit 14 remains compact. The optical unit 14 and the picture generating unit 10 are accommodated in a housing assembly 15 and may be separated from the environment by a transparent cover 23. The optical elements of the optical unit 14 are thus protected, for example, against dust inside the vehicle. An optical foil or polarizer 24 can be located on the cover 23. The display 11 is typically polarized and the mirror unit 2 acts like an analyser. The purpose of the polarizer, or glare trap, 24 is to influence the amount of sunlight entering the head-up display. A light trap 25 serves to securely shield light reflected from the road or to block light from different sources placed at car height level so that it does not reach the observer. In addition to sunlight SL coming from the sun 5, also light from another source of interference might reach the display 11.

Fig. 2 shows a sketch of a head-up display according to the invention. The head-up display is similar to the head-up display of Fig. 1, but the liquid crystal display 11 is diagrammatically depicted as being provided with a front polarizer 113, a first liquid crystal layer 110, a back polarizer 114, a reflective polarizer 116, a second liquid crystal layer 117 and a backlight pre-polarizer 131 arranged in this order when viewed in upstream direction. The light passes from a backlight unit 13 (not shown here) downstream through backlight pre-polarizer 131, second liquid crystal layer 117, reflective polarizer 116, back polarizer 114, liquid crystal layer 110, and front polarizer 113.

Fig.3 shows a simplified sketch similar to Fig.2. Here, only a single mirror, folding mirror 21, is shown. A Head-up Display, also referred to as HUD, is used to place information in the field of view of the user in such a way as to appear integrated in the surrounding environment. The intention is to provide the information in a way that does not require the user to significantly change the eye gaze direction and/or focus distance. HUDs are of particular importance in avionics and the automotive fields where they allow the vehicle's operator to glance important aspects of the trip without taking the eyes away from the path ahead. For this purpose, state of the art HUDs are required to deliver this information clearly discernible, independent of the ambient light level. This translates in high brightness requirements in excess of 10,000 cd/m2 or even 15,000 cd/m2 for the virtual image produced by the HUD. One of the preferred architectures for HUDs, as depicted in the figure, comprises an image generating unit 10, an intermediate mirror, the folding mirror 21, and a transparent HUD screen, the mirror unit 2. As a note, this diagram should be understood as one of the possible embodiments and should not be regarded as restrictive to the present invention.

The image generating unit 10 is responsible for producing the symbols that shall be visible to the end user. The optical system from the depicted embodiment, consisting of the folding mirror 21 and the HUD screen, mirror unit 2, is designed in such a way as to produce a virtual image VB from the images generated by the generator unit 10 that are magnified and seen at a certain distance in front of the mirror unit 2. As a note, the mirror unit 2 is formed with the aid of an optically clear medium in order to not impede the visibility of the end user's environment. This mirror unit 2 may be a dedicated component, typically called combiner, or it may be formed as a region on the vehicle's windshield.

Fig. 4 shows the picture generating unit 10 in a simplified diagrammatic view. The light source illuminates via a light control structure 16 a liquid crystal display 11 on which an image is displayed that corresponds to the virtual image VB to be shown to the user. Given the topology of the optical system used to create the virtual image VB based on the images produced by the picture generating unit 10, the brightness of the symbols displayed by the image generating unit should be around five times larger than the required virtual image brightness. This translates in brightness levels in excess of 50,000 cd/m2 to 75,000 cd/m2 at the surface of the picture generating unit 10. Typically, the image picture generating unit 10, as seen in the figure, consists of a backlight source, backlight unit 13, some light control structures 16, not shown here in detail, like diffusers, brightness enhancement films, micro-lens arrays, but without being limited to these enumerated components, and a transmissive liquid crystal display 11, also referred to as LCD. The backlight pre-polarizer 131 may also be arranged as part of the light control structure 16.

Given the description of one of the preferred embodiments for the state of the art HUD it is clear that the transmissive LCD has to operate with very large brightness levels. Compared to 150 cd/m2 to 500 cd/m2 employed for most other display applications, ranging from flat panel TVs to computer monitors, mobile phones or fully programable instrument clusters, it is evident that in HUD applications, the display panel must operate with brightness levels 50 to 200 times larger than typical applications.

Generating large amounts of light, even with highly efficient LED light sources, requires large powers for the backlight unit. A typical electrical power value required by the backlight unit is situated around 10W-15W. Out of this, about a third is converted into luminous power, with the rest of 6-10W being thermally dissipated.

Given the close proximity between the liquid crystal display 11 unit and the backlight unit 13, a large fraction of the thermally dissipated power will heat up the display panel, including the liquid crystal layer 110. Moreover, since the images typically generated by HUDs are using only a limited panel area for bright symbols (around 10% of the display area) and the transmittance of the panel in the bright symbols areas is typically 10% or below, also the 3 to 5W of the luminous power produced by the backlight unit 13 are dissipated mostly inside the LCD panel. This dissipated power further increases the temperature of the display panel. Another major contributing factor for the thermal problem is sunlight SL radiation coming from the exterior of the system. Given the architecture of the optical path of a typical HUD application, the display panel would be directly visible from various positions, most notably from directly above the windscreen 20. This means that at certain times of the day and for some specific vehicle positions, the front side of the display may be directly illuminated by sunlight SL with brightness levels that are typically much larger than the light produced by the backlight unit 13. Again, this luminous power given by the sun would be mostly dissipated inside the display panel which normally has a black appearance.

Fig.5 illustrates in a simplified manner the positions where the incident light on both sides of a known liquid crystal display 11 gets dissipated. As it can be seen, the incident sunlight SL enters the liquid crystal display 11 panel through the front polarizer 113. The light that has the polarization plane aligned with that of the front polarizer 113 is able to enter the rest of the display structure. It should be noted that the optical path of the HUD may contain some other elements 24, not shown here, that already select only the light that is aligned with the front polarizer 113 in order to limit the amount of sunlight SL that can reach the display surface. An important fraction (around 70%) of the light that enters the panel from the front side will be dissipated inside the colour filter 12 layer. The rest of the light that is able to cross the colour filter 12 layer would enter the liquid crystal layer 110. However, given that most probably the incident sunlight SL lands on a black display area, the liquid crystal material is typically in the relaxed position and would not rotate the orientation of the polarization plane. With this, the incident sunlight SL will pass through it without substantial alteration and will reach the back polarizer 114 layer.

Considering the orientation of the polarization planes of the front polarizer 113 and the back polarizer 114, this means that the remaining fraction of the sunlight SL (around 25 -30%) will be absorbed by the back polarizer 114. Looking at the backside of the display 11, the backlight BL that has the polarization plane aligned with the back polarizer 114 is able to enter the display 11 panel. As was the case for the frontside illumination, the optical path of the system may contain elements that already select the proper polarization plane of the backlight BL in order to limit the amount of light that would be absorbed by the display 11 panel. From the backlight BL that enters the display 11 panel, most of it will pass through the liquid crystal layer 110 (in the order of 90%) and would reach the colour filter 12 layer. Here, again, a large fraction will be absorbed and only around 25 -30% of the incident backlight BL will be able to cross the colour filter 12. Since most of the display surface must be black, the polarization plane of the backlight BL that is able to cross the colour filter 12 layer must be perpendicular to the polarization plane imposed by the front polarizer 113. This means that the remaining backlight BL fraction will be absorbed by the front polarizer 113. From this description, it is evident that the incoming light will be dissipated (i.e. transformed into heat) mostly by the colour filter 12 layer (for both, front and back light), the back polarizer 114 layer (for front light) and the front polarizer 113 layer (for backlight). Given the high brightness requirements and since almost all of the incident light is absorbed in the display 11 panel, the LCD in HUD applications can reach very high temperatures, close to the limiting values of the system's components. Additionally, the display 11 panel receives also an important fraction of the heat that is dissipated by the backlight unit 13. A particularly difficult condition for the HUD system is at the start-up of the system after the vehicle was stopped or parked for a long period under direct sunlight during summer. In this situation, the temperature reached inside the vehicle can be in the order of 60 to 80 degrees centigrade. If sunlight SL is also shining directly on the display surface, the temperature reached by the panel is even higher.

After this parked condition, if the user wants to use the HUD system, given the high ambient light levels, the backlight unit 13 of the system must provide very high intensity levels that, associated with the images typically shown on the display 11 panel and the conversion efficiency of the unit, will further lead to an increase in the panel temperature. Of course, in parallel with this, the end user will set the vehicle's controls in order to reduce the inside temperature to acceptable levels. However, given the thermal inertia of the system's components, there will be a race between the rising temperature induced by the HUD operation and the decreasing ambient temperature. The lower the thermal inertia of the display 11 panel, the quicker the panel will reach dangerous temperatures before the vehicle's ambient temperature decrease to lower values. From the LCD components, the most affected are the front polarizer 113 and back polarizer 114 films (with maximum temperatures in the order of 105 degrees) and the liquid crystal layer 110 (with the clearing temperature typically above 110°C for HUD applications). It is clear that in the previously described scenario, the display 11 panel's temperature approaches dangerously these levels and the system may need to decrease severely the brightness of the virtual image VB or even to shut down in order to prevent any damage to the display.

Even if the application employs a system configuration with a reflective polarizer as described with regard to Fig.6 below, there is still a large amount of backlight that gets dissipated inside the panel. The main advantage of that application is that it moves the place where the light is dissipated as heat from the front polarizer (which is typically of organic chemical composition and has a lower temperature tolerance before permanent damage) towards the colour filter layer (typically of inorganic chemical composition, so with a much higher tolerated temperature). This delays the moment when the panel used in HUD applications would reach dangerous temperature levels, allowing for the overall vehicle temperature to reach normal levels, but this is not a permanent solution. There could still exist use cases where the HUD is required to provide high brightness images at permanently high operating temperature levels which will still result in the inoperability or even in the damage of the system. As the described known solution is not targeted at improving the thermal behaviour of the display panel used in HUD applications, even if it prevents the backlight to reach the front display panel in regions that must remain black, the light is still dissipated in a polarizer film. Furthermore, because the total transmittance of the display stack-up is lower than that of a single panel, the amount of backlight required is a few times larger than for the normal application. Even more, the same polarizer film dissipates also the incoming solar radiation resulting in even higher thermal stress for the polarizing layer. With these, the known configuration is even more prone to damage than the original system.

The present invention seeks to improve the thermal behaviour of the display panels used in HUD applications such that the risk of damage due to thermal stress is no longer of concern, allowing for the even brighter HUD applications of the future.

Fig.6 shows a known liquid crystal display 11. In the figure, relative sizes are not to scale. Assuming that all optical elements are ideal, the white backlight BL first passes through a light polarizing layer, the back polarizer 114, that selects only the light that has its polarization plane aligned with that required by the display. At this point, about 50% (a factor of 0.5) of the incoming unpolarized light is absorbed as its polarization plane is not aligned with the display polarizer.

After this, the remaining backlight passes a supporting structure 17 and enters the active pixel area, the liquid crystal layer 110. Pixel driving elements 111 are driven by a driver (not shown here) according to the data to be displayed. The pixel driving elements 111 cause the liquid crystal substance of the liquid crystal layer 110 to rotate polarisation depending on an applied signal. The polarization plane of the light passing the liquid crystal layer 110 will be modulated by the data to be displayed with the aid of the liquid crystal layer 110. After the modulation, the light passes through the colour filter 12 layer where it gets filtered so that only the required spectral components, i.e. red through colour filter element 120R, green through colour filter element 120G or blue through colour filter element 120B, may exit the display panel. At this point, about 70% (i.e. a factor of 0.7) of the light that is incident on the colour filter elements 120R, 120G, 120B gets absorbed, so only around 30% (that is a factor of 0.3) of the incoming modulated light may pass through. Additionally, the colour filter 12 layer also delimits the individual colour subpixels with the aid of a black opaque material of which pixel aperture walls 121 of a black matrix that separate the colour specific filter elements 120R, 120G, 120B.

This sets in the end the pixel aperture ratio, that is, the ratio between the active pixel area and the total pixel area (typically around 0.6 to 0.8, or, in percent 60% to 80%) and further limits the amount of light that is capable of exiting the display.

As the pixel aperture walls 121, of which the black matrix is made of, are arranged in the same layer in which the colour filter 12 is arranged, in this example the colour filter 12 layer may also be referred to as a black mask layer 19.

Finally, the modulated and coloured light passes through another support structure 17, is either reflected by the frontside reflective polarizer 116' or passes through the frontside reflective polarizer 116' and enters another polarizer layer, the front polarizer 113. Front polarizer 113 transforms the light polarization variations induced by the liquid crystal layer 110 into brightness variations, resulting in the desired colour image. The display transmittance in the ideal case, where no other optical material induces losses may be estimated by multiplying the above factors resulting in around 50% x 30% x (60% to 80%) which is 9% to 12%. As a note, all subsequent descriptions will use a less detailed description of the display panel, invoking only layers that have an active role in the present invention. Additionally, the relative sizes of the involved elements are not necessarily drawn to scale.

A frontside reflective polarizer 116' layer is aligned such that it reflects the light that has the polarization plane perpendicular to that required by the front polarizing layer 113. Considering two adjacent pixels for ease of illustration with one pixel in an ON configuration, pixel driving element in ON-state 111-0N, (i.e. set to a clear state) and the other one in an OFF configuration, pixel driving element in OFF-state 111-OFF, (i.e. in an opaque state) as depicted in the figure.

The frontside reflective polarizer 116' layer is aligned such that unmodulated backlight BLU that is not modulated by the liquid crystal layer 110 (which otherwise would have been absorbed by the front polarizer 113) gets reflected towards the backlight unit 13 as reflected unmodulated backlight BLU-R. This configuration reduces substantially the amount of light that gets dissipated in the front polarizer 113 layer. Furthermore, since several percent of the backlight that is not used to produce the HUD image is reflected back towards the backlight unit 13 where it can be recycled, the configuration also reduces to some extent the total amount of light that gets dissipated inside the display structure, further reducing the thermal stress for the display.

The ON pixel 111-0N, once the modulated backlight BLM passes through the colour filter 12, will reach the frontside reflective polarizer 116' layer. Given that the frontside reflective polarizer 116' is aligned to reflect the light that has the polarization plane perpendicular to that of the front polarizer 113 and that the backlight was modulated by the pixel, it results that the modulated backlight BLM will pass through this layer with only a minor influence and, in the end, will exit the display as passed modulated backlight BLM-P. For the OFF pixel 111-OFF however, once the light reaches the frontside reflective polarizer 116', since it is just unmodulated backlight BLU that has the polarization plane aligned with that required by the reflective part of the additional film it will be reflected as reflected unmodulated backlight BLU-R towards the backside of the display. This effectively means the above calculated 15% of the polarized backlight will no longer be absorbed by the front polarizer 113, decreasing in this way the instant thermal load on the layer.

Fig.7 shows another configuration that has the potential to reduce the amount of light that is dissipated inside the display panel in HUD applications e.g. as known from EP 3 451 044 Al. It uses two liquid crystal panels 110,110', respectively, with front polarizers 113,113' and back polarizers 114,114', respectively, to generate the source for the virtual image. The front polarizer 113 of the upper liquid crystal panel 110 and the back polarizer 114' of the lower liquid crystal panel 110' have the same polarisation orientation. The front polarizer 113' of the lower liquid crystal panel 110' and the back polarizer 114 of the upper liquid crystal panel 110 have the same polarisation orientation, which is orthogonal to the orientation of front polarizer 113 and back polarizer 114'. The purpose of the configuration in this example is to increase the contrast ratio for HUD application but it has the side effect that the backlight incident to the structure in regions that must remain black is prevented to enter the main display panel. Instead, it gets dissipated at the back polarizer 114 layer of the front display panel.

Fig. 8 shows a display configuration according to the invention. It is suggested to limit the light absorption in the front display panel consisting of front polarizer 113, first liquid crystal layer 110 and back polarizer 114, by interposing between the display panel and the backlight unit (which is not shown here) an additional liquid crystal panel having a second liquid crystal layer 117 and on its frontside a reflective polarizer layer 116. Control circuitry 34 is diagrammatically shown, providing a first drive signal DS1 to the first liquid crystal layer 110 and a second drive signal DS2 to the second liquid crystal layer 117. Both drive signals DS1, DS2 match each other.

For example, both have the same status ON or status OFF for display layer pixels that correspond to the same pixel of the virtual image VB. Drive signals DS1 and DS2 might be identical, one of these might be attenuated, or otherwise be different but still matching.

In state of the art high brightness HUD applications, in order to decrease the amount of backlight that is incident to the backside of the display without altering the resulting virtual image brightness, the backlight unit contains a pre-polarizing film aligned with the same orientation as the display back polarizer. For this reason, all descriptions from the present invention assume that the backlight incident on the HUD display panel is already polarized. Fig.8 illustrates the backlight pre-polarization with the additional pre-polarizer 131 placed at a certain distance from the thermal protection LCD, the second liquid crystal layer 117. However, for the purpose of this present invention, the exact position of this backlight pre-polarization layer 131 is not critical, the same system behaviour resulting if the pre-polarization layer 131 is placed downstream in the backlight unit or if it is directly affixed to the backside of the thermal protection panel, here: the lower side of the second liquid crystal layer 117. The diagrams shown here employ both configurations, without departing from the scope of the present idea. At the interface between the first liquid crystal layer 110 and the second liquid crystal layer 117, the combination between the reflective polarizer 116 layer and the absorption back polarizer 114 behaves as follows: The reflective polarizer layer 116reflects the light with a specific orientation of the polarization plane and lets the light with the orientation of the polarization plane perpendicular to the first one to pass through. According to the invention, the reflective polarizer 116 layer is aligned such that it reflects the light that has the polarization plane perpendicular to that required by the back polarizer 114 layer of the first liquid crystal layer 110. This requirement can be better understood by comparing the known display application with the proposed invention.

Fig. 9 shows a known display application. In this known application, considering two adjacent pixels for ease of illustration with one pixel in an ON configuration (i.e. pixel driving element 111-ON set to a clear state) and the other one in an OFF configuration (i.e. pixel driving element 111-OFF set in an opaque state) as depicted in the figure, it can be seen that the front polarizer 113 layer basically absorbs the unmodulated backlight BLU-A passing through the OFF pixel (right side). For the ON pixel (left side), the backlight BL that enters the display, polarized according to the back polarizer 114, gets its polarization plane rotated by the liquid crystal layer 110 in such a way as to align it with that required by the front polarizer 113. Once the modulated backlight passes through the colour filter 12 layer, since it is properly aligned with the front polarizer 113 it will be able to exit the pixel from the front side of the display. For the OFF pixel, since the polarization plane of the backlight BL is not rotated by the liquid crystal layer 110, the unmodulated light BLU that passes the colour filter 12 will be absorbed by the front polarizer 113.

Assuming a properly pre-polarized backlight and some typical values for the light transmission or reflection coefficients of the different display components (around 90% transmission for a polarizer for the light properly aligned, around 85% transmission for the liquid crystal and all other clear layers (like glass supporting structure 17), 30% for the colour filter 12, 70% given by the pixel aperture ratio, around 16% (i.e. 0.9 x 0.85 x 0.3 x 0.7) of the incoming polarized backlight will be directly absorbed by the front polarizer 113 and around 60% (given by the difference between the colour filter 12 incident light fraction 0.9 x 0.85 and the front polarizer 113 incident light fraction 0.9 x 0.85 x 0.3 x 0.7) would be absorbed by the colour filter 12 layer (delimited by the pixel's aperture black mask). It can be seen that around 60% of the incoming pre-polarized backlight is absorbed at the colour filter 12 layer, leading to an increase of the temperature directly inside the display structure. Since for the ON configuration the light is able to exit the pixel through the front polarizer 113, the amount of pre-polarized backlight that is used to form the virtual image of the HUD can be simply calculated from the fraction computed above, considering also the attenuation induced by the front polarizer 113 as 0.9 x 0.85 x 0.3 x 0.7 x 0.9, resulting in around 14.5% of the backlight exiting the display through its front surface.

This situation changes for the configuration according to the present invention, as depicted in Fig.10. As a note, the figure depicts a stack of two displays, the front display (or main display, shown in the upper part of the figure) having the same role as for the state of the art HUD panel while the backside display is the thermal protection display. In the depiction below, without imposing restrictions to the current invention, the thermal protection display is arranged such that the polarization plane of the incoming light is left unchanged for a transmissive display state and rotated by 90 degrees for the non-transmissive display state. In addition, the thermal protection display is depicted with the backlight pre-polarizer 131 layer affixed to its backside. It should be clear to anyone skilled in the art that this depiction is done only for illustration purposes as real systems can have different positions for the backlight pre-polarizer 131 or different operation mode for the second liquid crystal layer 117 (e.g. normally white or normally black).

Additionally, an important aspect for the thermal protection LCD is the absence of the black mask that creates the pixel apertures and of the colour filter layer. The elimination of these layers is suggested in order to reduce the amount of light that is dissipated inside the structure and comes without side effects on the quality of the generated image. For a normal display, the black mask layer is essential because it eliminates the crosstalk between the adjacent pixels, leading to a clear, highly detailed image.

For the thermal protection LCD, with second liquid crystal layer 117, there is no need to create such a sharp pixel definition as the purpose of the panel is to make only a coarse definition of the backlight that will be modulated at a later stage by the main panel. Here, the thermal protection LCD is configured in such a way as to match the ON or OFF status of the pixels on the main display panel. For positions corresponding to the ON pixel, the thermal protection LCD is configured to allow the unmodulated backlight BLU passing through it to be aligned with the back polarizer 114 layer of the main display. In this way, the assembly is able to produce the bright objects in the virtual image.

For the areas corresponding to OFF pixels (right side of the figure), the thermal protection LCD is configured in such a way as the light passing through it is rotated by 90 degrees relative to the back polarizer 114 layer of the main display and, aligned with the reflective polarizer layer 116. In this configuration, a very large fraction of the incoming backlight BL will no longer exit the thermal protection LCD towards the main display panel, but the modulated backlight BLM will be reflected back as reflected modulated backlight BLM-R, where it can be recycled in the backlight unit.

Assuming similar values for the thermal protection LCD as for the main panel, that is, around 85% transmission for the second liquid crystal layer 117 and all other clear layers, around 90% transmittance of the reflective polarizer 116 for the light that is able to traverse it and 50 to 100% reflectance for the light that is aligned with the reflective polarizer 116 plane, it is possible to calculate the impact of the total stack compared to the state of the art solution.

The reflectivity of 50% of the reflective polarizer layer corresponds to an equivalent contrast ratio of the thermal protection LCD of around 2 to 1. Additionally, the larger the reflectivity of the layer, the larger the equivalent contrast ratio of the thermal protection LCD, given by 1 / (1 -reflectivity). In the ideal case of reflectivity 100%, the contrast ratio would be infinite.

With these, for the ON pixel it is possible to calculate the fraction of the pre-polarized backlight that passes through the display stack as: (0.85 x 0.9) x (0.9 x 0.85 x 0.3 x 0.7 x 0.9). In the previous relation, the first parenthesis is given by the thermal protection LCD with the second liquid crystal layer 117 and the second parenthesis is given by the main LCD with the first liquid crystal layer 110. This results in a fraction of 11.1% transmittance of the stack, compared to 14.5% transmittance for the state of the art displays used in HUD applications, measured in pre-polarized backlight Of the backlight is not pre-polarized, the transmittance fractions would be half of those, that is 5.5% for the proposed invention, compared to 7.3% for the state of the art HUD displays).

It can be seen that for the ON pixel, the transmittance of the stack reduces to around 76.5% of the transmittance of the state of the art displays, requiring an increase of the backlight power in order to produce the same virtual image brightness in the order of 1.31 times that of the state of the art applications. At first sight, this may seem to indicate a larger thermal stress than for typical applications, however, by studying also the behaviour of the stack for the OFF state pixels, it should be obvious there is a net gain for the present proposed invention.

For the OFF pixels, the fraction of pre-polarized backlight that reaches the backside of the main panel can be calculated as 0.85 x (1 -0.5) to (1 -1) x 0.9, considering also the attenuation inside the reflective polarizer 116 of the light that is able to pass through it. For the less efficient reflective polarizer 116 layer, with a reflectivity of only 50%, the amount of light that reaches the backside of the main panel results as 38.3% of the pre-polarized backlight. Assuming in a typical HUD image that only around 10% of the shown pixels are in a clear state, with the rest of them being in a black state, the amount of backlight that is absorbed in the state of the art display application results as the sum of the fraction of the light that is absorbed in the clear display areas (i.e. 10% of the surface) and the fraction of light that hits a black area (90% of the surface, with the light fully absorbed in the panel), that is, 0.1 x (1 - 0.145) + 0.9, or 98.6%.

For the presently described example of the invention having only 50% reflectivity for the reflective polarizer 116, the amount of backlight BL that is absorbed in the main panel in the same virtual image conditions (i.e. same image, same brightness) results as (0.1 x (1 -0.111) + 0.9 x 0.383) x 1.31. It is important to note also the backlight scaling factor of 1.31 needed to achieve the same virtual image brightness. This translates in a fraction of only 56.8% of the backlight of state of the art displays being absorbed in the main panel.

With these, it is clear that the present invention is able to almost halve the amount of backlight that is absorbed in the main panel with the first liquid crystal layer 110 compared to state of the art solutions, even for reflective polarizers 116 that are not very efficient. If the reflective polarizer 116 has a reflectivity of around 80% (i.e. an equivalent contrast ratio of 5 to 1), the amount of backlight that reaches the backside of the main panel in the black areas results as 15.3% of the initial backlight BL. In this situation, assuming the same virtual image conditions, the main panel absorbs only 29.7% of the backlight, resulting in around three times reduction compared to the state of the art applications. Considering the total light that is absorbed inside the entire display stack for the present invention, it is necessary to calculate the light that is reflected back from the thermal protection LCD as 0.85 x 0.5 to 1 x 0.85, knowing that the light will pass two times through the clear layers having the total transmittance of 85%. For a 50% reflectivity of the reflective polarizer, around 36.1% of the backlight is reflected back from the black main display areas. Considering the fraction of 38.3 that is able to exit the thermal protection LCD from its front surface, the amount of backlight that gets absorbed in thermal protection panel is given by the difference between the incident light, the transmitted light and the reflected light in the black main panel areas, that is 1 -0.383 -0.361, or 25.6% of the pre-polarized backlight. Considering also the backlight scaling ratios, the equivalent amount of backlight that is absorbed in the back panel for the same image brightness in the black areas of the main panel is 25.6% x 1.31 = 33.5%. For the bright image areas, the light that is dissipated inside the thermal protection panel can be calculated in a similar fashion as the difference between the incident light and the amount of light that is able to exit from the front surface of the thermal protection panel, that is 1 -0.765, or 23.5%. Considering the backlight scaling factor, the equivalent amount of pre-polarized backlight absorbed in the thermal protection panel corresponding to clear display areas results as 30.8%.

Combining these numbers with values for the light that is absorbed by the main panel for the same 50% reflectivity of the reflective polarizer and same distribution of bright to black pixels, results in 0.1 x 30.8% + 0.9 x 33.5% + 56.8%, or a total of 90% of the equivalent backlight being absorbed by the entire display stack of the proposed invention. Compared with the figure of 98.6% typical of state of the art displays for HUD applications, it is clear that even the usage of a relatively inefficient reflective polarizer 116 results in a decrease in the thermal load of the display stack for the same virtual image parameters.

By using a more efficient reflective polarizer 116 with a reflectivity of 80% of the light that has its polarization plane aligned with the reflective layer, the light that is reflected back from the thermal protection display is given by 0.85 x 0.8 x 0.85, or 57.8%. With this and considering also the light that is able to exit the front side of this display, the amount of light that is dissipated inside this panel becomes 1 -0.153 - 0.578, or 26.9%. Considering the backlight scaling factor, the equivalent proportion of pre-polarized backlight that is absorbed becomes 35.3%. Knowing that for the clear state the transmittance of the stack remains basically the same, the total light that is absorbed in the present invention display stack becomes 0.1 x 30.8% + 0.9 x 35.3% + 29.7%, or around 64.6% of the equivalent backlight.

It should be noted that further improvements are possible because, as detailed in EP 3 451 044 Al, the equivalent contrast ratio of the display stack increases. For instance, if the reflectivity of the reflective layer is 80%, the equivalent contrast ratio of the thermal protection LCD is around 5:1. That is, the equivalent contrast ratio of the entire display stack would be five times larger than for the initial main panel. This allows a trade-off between the transmittance of the main panel and its contrast ratio, without affecting the final optical performance of the HUD application. For instance, the transmittance of the main panel may increase by around 20% while its contrast ratio decreases by around two to three times. In these circumstances, using a reflective polarizer 116 with 80% reflectivity still provides a better contrast ratio for the proposed invention compared to state of the art displays, requiring only a minor increase in the backlight power (i.e. only 10% increase for same virtual image brightness, compared to the 30% calculated previously where the trade-off was not considered). It is clear that the proposed invention is able to reduce the amount of backlight that gets dissipated inside the display stack, even if the needed amount of backlight for the same virtual image brightness is increased.

In fact, the proposed invention, considering also the transmittance improvements resulting from the trade-off between main display transmittance and contrast ratio, can be used to produce virtual images that can be two or more times brighter than the state of the art HUD applications (i.e. 20,000 to 30,000 cd/m) for the same or even lower thermal load in the system. This feature is particularly useful for parallax barrier 3D HUD applications where the required backlight brightness is several times that of typical 2D HUD applications. This is because the parallax barrier substantially reduces the amount of light that is used to form the virtual 3D object compared to normal applications. By adding the thermal protection LCD to the display stack used in 3D HUD applications, the resulting image brightness can be substantially higher than what the standard solutions can provide, creating a substantial competitive advantage. When compared to the state-of-the-art displays, by having an additional thermal protection display with a reflective polarizer affixed to its front surface an important fraction of the incoming light is no longer dissipated in display structure. This significantly reduces the thermal load at which the display is subjected. Considering also careful trade-offs between the contrast ratio and the display transmittance of the main panel, the present invention is able to provide much brighter virtual images at basically the same thermal load as current state of the art applications.

Fig. 11 shows an embodiment according to the invention. The simple implementation according to this embodiment has the thermal protection display with second liquid crystal layer 117 and reflective polarizer 116 directly bonded to the main panel with front polarizer 113, colour filter 12, first liquid crystal layer 110 and back polarizer 114, and no backlight pre-polarization layer 131. This configuration assumes that the backlight unit (not shown here) provides already a pre-polarized backlight.

The present invention may work also in combination with other methods employed for modifying the thermal inertia of the front polarizer layer 113 like that shown in Fig.12. Here, an additional front side reflective polarizer 116' is placed at the front side of the main display panel in order to deflect back from the main front polarizer 113 the light that was still able to enter the main display panel in the black regions, see also the explanation given above with respect to Fig.6. As a note, in this configuration, the front reflective polarizer 116' must be configured in such a way as to reflect back the backlight that is not modulated by the main panel. Depending on the actual implementation of the thermal protection LCD, this means the front polarizer 113 can be rotated by 90 degrees or in the same orientation compared to the same layer used for the thermal protection LCD.

Fig. 13 shows a diagram of a part of a head-up display. A backlight unit 13 generates light that passes through the liquid crystal display 11 and a light collimator 32. In case the solar irradiation is important and contributes with a large fraction to the thermal load of the display panel, the same thermal protection LCD with a third liquid crystal layer may be bonded on the top side of the main panel. In state of the art HUD applications, in order to prevent the reflection of the sunlight SL entering the HUD optical path along the main optical axis 31 and reaching the display 11 surface back towards, potentially, the end user of the system, the display 11 assembly is typically placed at an angle relative to the main optical axis 31, as seen in the figure showing that the normal direction ND is inclined to the HUD main optical axis 31.

The light collimator 32 prevents the light generated by the HUD having a high inclination relative to the optical axis to exit the system and thus, prevents unwanted reflections inside the vehicles cabin. In the same time, the collimator 32 also limits the amount of external light, e.g. sunlight SL, that is capable to reach the display 11 surface, as again, light having a high inclination relative to the optical axis 31 will be blocked by the collimator's walls. This effectively limits the positions from which the sunlight SL can pose a problem to the display assembly. In addition to this, typically there will also be a polarizer film placed in the optical path upstream to the display unit. This polarizer is aligned such that the polarized light produced by the HUD is able to pass through it with very little attenuation. However, the polarizer will substantially reduce (by around 50%) the amount of external light that can enter the HUD system by eliminating the light with the polarization plane perpendicular to the polarizer plane.

As any light that is reflected back in a direction parallel to the main optical axis 31 is, from the optical system's perspective, indistinguishable from the useful light SB1 generated by the display 11 unit, the end user may be affected by it as this reflected sunlight SL may obscure the information that must be displayed by the HUD system. For this reason, state of the art HUD systems have the display 11 tilted relative to the optical axis 31 so any sunlight SL that gets reflected from the display 11 surface is deviated towards a light trap 33 from the collimator's walls. In this way, the front surface of the display 11 unit can have a higher reflectivity than other state of the art displays used in non-HUD applications without significant impact on the images produced for the end-user. In fact, the higher the reflectivity, the lower the amount of sunlight SL-P that can pass into the display 11 and reach the colour filter 12 layer, easing the thermal stress at which the panel is subjected. However, with state of the art approaches where the higher reflectivity is achieved with the aid of some coatings on the front side of the display, also the brightness of the images generated by the HUD unit is affected as the reflective layer is not able to discriminate between the useful light and the stray light.

Fig. 14 shows a further improved embodiment according to the invention. In this embodiment, the situation changes for the configuration proposed in the present invention as the thermal protection display, having the third liquid crystal layer 118 and a frontside reflective polarizer 116", is able to controllably reflect the stray light coming from the exterior of the HUD system without affecting the areas that produce the images for the virtual image. With the proposed configuration, the pre-polarized solar light SL that still is able to enter the front side of the display 11 stack will be reflected towards the light trap 33 and will not affect the visibility of the virtual image.

As seen for the backlight case, this implementation is capable to reduce by an important fraction the amount of-sunlight SL that is dissipated inside the colour filter 12 layer, substantially reducing the thermal load at which the display 11 is subjected.

Another advantageous implementation is shown in Fig.15. It uses two thermal protection displays 711, 811. A first thermal protection display 711 having the second liquid crystal layer 117 and the reflective polarizer 116, and being placed on the backside of the main panel. Another thermal protection display 811 having the third liquid crystal layer 118 and the frontside reflective polarizer 116", and being placed on the front side of the main panel. The purpose of the backside thermal protection display 711 is to limit the amount of backlight BL that is able to enter the main display panel having the first liquid crystal layer 110. Pre-polarized backlight is reflected by black regions of the backside thermal protection display 711 as reflected unmodulated backlight BLU-R. Polarized sunlight SL is reflected by black regions of the front thermal protection display 811 as reflected sunlight SL-R. The front side protection display 811 thus limits the amount of sunlight SL that reaches the colour filter 12 layer of the main panel.

Claims

Patent claims 1. A head-up display unit (1), the head-up display unit (1) comprising: - a picture generating unit (10) for creating an image to be displayed as a virtual image (VB) to a viewer; and - an optical unit (14) for projecting the image to be displayed towards an eyebox (4); the picture generating unit (10) comprising: - a first liquid crystal layer (110); -a front polarizer (113); - a back polarizer (114); - a reflective polarizer (116); - a second liquid crystal layer (117); and -a backlight pre-polarizer (131); wherein the reflective polarizer (116) is arranged upstream the back polarizer (114) and downstream the second liquid crystal layer (117).
2. The head-up display unit (1) according to claim 1, the picture generating unit (10) further comprising a single black mask layer (19) arranged downstream the first liquid crystal layer (110).
The head-up display unit (1) according to claim 1 or 2, further comprising control circuitry (34) that drives first liquid crystal layer (110) and second liquid crystal layer (117) with matching drive signals (DS1,DS2).
4. The head-up display unit (1) according to anyone of the preceding claims, the reflecting polarizer (116) having a reflectivity of 50%.
5. The head-up display unit (1) according to claim 1, wherein the reflective polariser (116) is a metal grid polariser.
6. The head-up display unit (1) according to anyone of the preceding claims, further comprising a third liquid crystal layer (118) and a frontside reflective polarizer (116"), wherein the frontside reflective polarizer is arranged between the front polarizer (113) of the first liquid crystal layer (110) and the third liquid crystal layer (118).
7. A vehicle with a head-up display unit (1) according to any of claims 1 to 5 for generating an image for a user of the vehicle.