TECHNICAL FIELD

The present disclosure relates to a polarised light display system and an anti-reflection coating. Particularly, but not exclusively, the disclosure relates to a display apparatus for projecting target images and reducing ghost images on a head-up display, such as a windscreen, for use in a vehicle.

BACKGROUND

Heads-up displays (HUDs) are known displays where images are projected onto a transparent surface, such as a windscreen or visor. Such displays are well known in a number of different environments including in vehicles.

Typically, a portion of the light from an image generation means is reflected by the interior surface of a transparent display screen, providing a target image seen by the user. A common setup of this type is depicted in Figure 1. At the same time, a portion of the projected light will simply pass through the display. Considering the refractive index of most transparent display screens, only a small portion of the light from the image generation means is reflected to constitute the target image. Therefore, the efficiency of the display is limited.

It is known to increase the relative portion of reflected light by controlling the polarity of the light to provide for a higher reflectance at the interior surface of the display screen, however such displays are incompatible with many types of conventional polarising eye wear (i.e. sunglasses).

Further, it is well understood that a sub-portion of the light that passes through the interior surface of the display will be reflected back by the exterior surface, creating a second image commonly known as a ghost image. This ghost image blurs the target image, reducing the overall effectiveness of this type of display.

An object of the present invention is to mitigate some of the deficiencies of the prior art mentioned above. SUMMARY OF THE INVENTION

Aspects and embodiments of the invention provide an apparatus as claimed in the appended claims.

An advantage according to an aspect of the invention is that there is provided an imaging system for generating an image on a display surface, the imaging system comprising a picture generation unit configured to emit first polarised light, the first polarised light having a first polarisation, wherein the first polarisation is such that the first polarised light has an electric field along the plane of incidence with the display surface, a first polariser positioned so as to intersect the first polarised light, the first polariser being configured to convert the first polarised light into second polarised light having a second polarisation, wherein the second polarisation is circular polarisation, a second polariser positioned so as to intersect the second polarised light, the second polariser being configured to convert the second polarised light into third polarised light having a third polarisation, wherein the third polarisation is such that the third polarised light has an electric field normal to the plane of incidence with the display surface, and wherein the second polariser is further configured to convert third polarised light reflected from the display surface into second polarised light.

By adopting the above arrangement, the light is polarised so as to maximise reflectance at the display surface, whilst outputting circularly polarised light to the user. Should the user be wearing polarising eyewear (such as sunglasses), a component of the circularly polarised light will still be transmitted such that the projected image remains visible to the user, regardless of the eyewear and its relative orientation to the incident light. Furthermore, by adopting the above arrangement, no constraint is placed on the angle of incidence on the second polariser/display surface, so that any desired angle may be used - minimising the angular extent (and thus the overall size) of the apparatus.

Optionally, the first polariser is located at the output of the picture generation unit. The first polariser can thus be housed together with the picture generation unit, remote from the display surface, thereby minimising the required real-estate taken up in the vicinity of the display surface.

Optionally, the second polariser is located on the display surface. This ensures light reflected from the display passes back through the second polariser without the need for any additional beam manipulation and the associated optical components. Optionally, the first and second polarisers comprise quarter wave plates.

Optionally, the display surface is the interior surface of a windscreen forming a heads up display.

Optionally, the display surface is the interior surface of a visor of head mounted display.

Optionally, the picture generation unit comprises a projector and a diffuser for realising the projected image.

Optionally, the picture generation unit comprises a laser and a 2D scanning mirror.

Optionally, the picture generation unit comprises a holographic unit to produce computer generated holograms and a diffuser for realising the holograms.

Optionally, the picture generation unit comprises a light field unit to produce 3-dimentional light field images. The projection of 3-dimentional images through the imaging system enables such images to be displayed on the screen of the head-up display with the appropriate varying depth so as to produce a convincing representation of a real object.

Optionally, the picture generation unit comprises OLED devices. Such layers are capable of being activated by the application of current, which can be localised and modulated as desired. They can further provide a flexible, multi-colour display

Optionally, the picture generation unit comprises digital light processing digital micromirror devices.

An advantage according to a second aspect of the invention is that there is provided an anti reflection coating for a surface, the coating comprising a stack of four layers, the first and third layers comprising titanium dioxide and the second and fourth layers comprising magnesium fluoride. The anti-reflection coating is be applied to one interface of a transparent display screen so as to prevent a second reflection event and the resulting ghost image.

Optionally, the first layer is in contact with the surface. Optionally, the thickness of the first, second, third and fourth layers are 5.2 nm, 72.0 nm, 7.3 nm and 136.9 nm accordingly. This configuration is effective at preventing reflection of un polarised light.

Optionally, the thickness of the first, second, third and fourth layers are 11.1 nm, 42.4 nm, 17.0 nm and 109.5 nm accordingly. This configuration is effective at preventing reflection of s-polarised light.

Optionally, the surface is the external surface of a windscreen.

Optionally, the surface is the external surface of a visor of a head mounted display.

As such, light can be reflected by the internal surface of the windscreen/visor to provide an image visible to the user, whilst a second reflection from the external surface of the windscreen/visor is prevented.

An advantage according to a third aspect of the invention is that there is provided an image display system comprising the described imaging system, a display surface and the described anti-reflection coating. Such a system combines optimised reflective efficiency whilst reducing the deteriorative effects of the ghost image, providing a high intensity and clearly defined target image to the user.

Other aspects of the invention will be apparent from the appended claim set.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic of the conventional systems found in the prior art;

Figure 2 is a schematic of the imaging system according to an aspect of the invention.

Figure 3 is schematic of an imaging system showing the origin of the ghost image;

Figure 4 is a schematic of the anti-reflection coating according to an aspect of the invention; Figure 5 is a graph showing the refractive index of Schott BK7 glass across the visible wavelength range.

Figure 6 is a graph showing the reflectance for the first reflection (r1) for s and p polarised incident light.

Figure 7 is a graph showing the reflectance (r2) at the exterior surface of the windscreen for s and p polarised incident light.

Figure 8 is a graph showing the extinction ratio across the visible wavelength range for standard setup.

Figure 9 is a graph showing the refractive index of MgF ₂ and Ti0 ₂ across the visible wavelength range.

Figure 10 is a graph showing the reflectance (r2) at the exterior surface of the windscreen for s-polarised incident light.

Figure 11 is a graph showing the reflectance (r2) at the exterior surface of the windscreen for p-polarised incident light.

Figure 12 is a graph showing the extinction ratio for s-polarised incident light under different anti-reflection coating designs.

Figure 13 is a graph showing the extinction ratio for p-polarised incident light under different anti-reflection coating designs.

Figure 14 is a graph showing the extinction ratio for un-polarised incident light under different anti-reflection coating designs.

Figure 15 is a graph showing the reflectance under Design 1 for an incident angle of (a) 40°, (b) 45° and (c) 50°.

Figure 16 is a graph showing the extinction ratio under Design 1 for an incident angle of (a) 40°, (b) 45° and (c) 50°. Figure 17 is a graph showing the reflectance under Design 2 for an incident angle of (a) 40°, (b) 45° and (c) 50°.

Figure 18 is a graph showing the extinction ratio under Design 2 for an incident angle of (a) 40°, (b) 45° and (c) 50°.

Figure 19 is a table giving the thickness of each layer in the proposed anti-reflection coating designs.

DETAILED DESCRIPTION

Imaging System

In an aspect of the invention, there is provided an imaging system forming part of a HUD, for use in a motor vehicle, utilising the windscreen as the display surface. Whilst the following description is described with reference to a motor vehicle, the disclosure and concepts described herein are applicable to other forms of HUD.

Particularly, but not exclusively, the disclosure relates to an apparatus for projecting light onto a display screen, such as a windscreen of a motor vehicle. Example applications can be, but are not limited to, cars, buses, lorries, excavators, exoskeleton suit for heavy-duty tasks, motorcycles, trains, theme park rides; submarines, ships, boats, yachts, jet-skies for see vehicles; planes, gliders for air crafts, spaceships, shuttles for space crafts. Furthermore, the technology can be installed/integrated in a mobile platform such as a driver’s/operator’s head/eye protection apparatus such as a helmet or goggles, made from glass or any other suitable material. Therefore, any activity, which involves wearing protective helmets/goggles, can benefit from this technology. These can be worn by, but are not limited to, motorcyclist/cyclist, skiers, astronauts, exoskeleton operators, military personnel, miners, scuba divers, construction workers. Moreover, it can be used in a standalone environment for game consoles, arcade machines and with a combination of an external 2D/3D display it can be used as a simulation platform. Also, it can be used in institutions and museums for educational and entertainment purposes.

Figure 1 is a schematic of the imaging system according to an aspect of the invention

Figure 1 shows an imaging system 100 made up of a picture generation unit 200 having projection axis 210. Also shown is a first polariser provided by a first quarter wave plate 300 and a second polariser provided by a second quarter wave plate 400, the second quarter wave plate 400 being mounted onto the internal surface of a windscreen 500. The internal surface of the windscreen 500 provides a first interface 510 from air-to-glass.

The first quarter wave plate 300 is located on and arranged perpendicular to the projection axis 210. The second quarter wave plate 400 and windscreen 500 are located on the projection axis, orientated at an angle qi to the projection axis 210. In the depicted embodiment, qi is 45°, however qi may have any value.

The path of the light from the picture generation unit 200, through the first and second quarter wave plates 300, 400 and onto the windscreen 500 is referred to as the optical path 220. The skilled person would understand that any number of intervening reflectors/lens or other optical components may be placed along the optical path 220 between the picture generation unit 200, and the windscreen 500, to manipulate the optical path as necessary (for example, to minimize the overall size of the imaging system 100). As such, the exact relative orientations and positioning of the picture generation unit 200, waveplates 300, 400 and windscreen 500 are not limited to the illustrated embodiments, but rather any arrangement that allows for light to be directed through the waveplates in the manner described below, onto the windscreen and ultimately reflected towards the user 600.

The picture generation unit 200 comprises an image source which generates the image to be displayed on the windscreen 500. The image source in an embodiment is a light engine, or OLED display or any suitable source which generates the image to be displayed and ultimately projected onto the windscreen 500. By way of example, such images may comprise information regarding the car conditions and further information relating to navigation.

Accordingly, in an embodiment the picture generation unit 200 is a holographic unit which produces computer generated holograms for projecting images onto the windscreen 500. In an alternative embodiment, the picture generation unit 200 is a light field unit configured to produce 3-dimensional light field images for projection onto the windscreen 500.

In an embodiment, the picture generation unit 200 further includes optics for manipulating the light through the first and second quarter wave plates 300, 400 and onto the relevant region of the windscreen 500.

In an embodiment, these optics include a Fresnel lens and one or more mirrors and/or beam splitter, though any suitable focussing optics and light redirecting means may be employed. In use, the projector 200 emits p-polarised light, wherein p-polarised light has an electric field oriented parallel to the plane of incidence with the first interface 510 of the windscreen 500. This p-polarised light passes through the first quarter wave plate 300 and is converted into circularly polarised light in a known manner. The operational principles of waveplates and specifically quarter wave plates are well known and are not reproduced here.

The circularly polarised light passes through the second quarter wave plate 400 and is converted into s-polarised light, wherein s-polarised light has an electric field oriented perpendicular to the plane of incidence.

The s-polarised light is incident on first interface 510 of the windscreen 500, s-polarised light having a relatively high reflectance as compared to p-polarised and circularly polarised light.

The reflected s-polarised light travels towards a user 600, back through the second quarter wave plate 400 thereby being converted back into circularly polarised light.

Accordingly, the light directed towards the user is circularly polarised, so that should the user be wearing any polarising eyewear (such as sunglasses), the polarising eyewear will not entirely block the image regardless of their orientation relative to the incident light, with a component of the light being transmitted to the user.

Accordingly, there is provided an imaging system in accordance with an aspect of the invention.

Anti-Reflection Coating

In an aspect of the invention, there is provided an anti-reflection coating 1000 for use in the HUD systems described above.

Figure 4 shows the layer structure of the anti-reflection coating 1000 and its application to a windscreen 500.

The anti-reflection coating 1000 is formed by a stack of four layers (1100, 1200, 1300, 1400) coated on the external surface of the windscreen 500. The external surface of the windscreen 500 provides a second interface 520 from glass-to-air. The first layer 1100 of the anti-reflection coating 1000 is in contact with the second interface 520 of the windscreen 500, with the fourth layer 1400 being the outermost layer. The first and third layers 1100, 1300 are made of titanium dioxide and the second and fourth layers 1200, 1400 are made of magnesium fluoride. The refractive index information of these two materials is shown in Fig. 9.

In an embodiment, the first, second, third and fourth layers 1100-1400 are 5.2 nm, 72.0 nm, 7.3 nm and 136.9 nm thick respectively, providing an optimised reduction in reflectance of unpolarised light. This is referred to as design 1.

In an alternative embodiment, the first, second, third and fourth layers 1100-1400 are 11.1 nm, 42.4 nm, 17.0 nm and 109.5 nm respectively, providing an optimised reduction in reflectance of s-polarised light. This is referred to as design 2.

Examples of the layer thicknesses of design 1 and 2 are provided in the table of Fig. 19.

In an alternative embodiment, the anti-reflection coating is applied to the interior surface 510 of the windscreen 500. In this case, the light being reflected from the second glass-to-air interface 520 at the exterior surface of the windscreen 500 becomes the target image, while the light being reflected from the interior surface 510 of the windscreen 500 becomes the ghost image.

In use, light incident on the first interface 510 of the windscreen 500 is partially reflected (as described above) and partially transmitted through the glass of the windscreen 500. The transmitted light is subsequently incident on the second interface 520 and reflections from this interface produce a ghost image as discussed in detail below.

The anti-reflection coating reduces the reflectance at the second interface 520 and therefore reduces the presence of the ghost image visible to the user 600.

In an embodiment, the anti-reflection coating is used in conjunction with the imaging system described above. Accordingly, there is provided an imaging system with a reflective efficiency that reduces the deteriorative effects of the ghost image, thereby providing a high intensity and clearly defined target image to the user 600. EXAMPLES

The reflectance at the second interface 520 of the windscreen 500 is simulated for each of the two designs and results for s-polarised and p-polarised incident light are shown in Fig. 10 and Fig. 11 , respectively. It can be seen that design 1 is able to reduce the reflectance at this interface for both s-polarised and p-polarised light, while design 2 has a particular better performance for the s-polarised light.

The effect on s-polarised, p-polarised and unpolarised incident light under different anti reflection coating designs was simulated and the results are shown in Fig. 12 - Fig. 14, respectively. It can be seen that design 1 is able to achieve better results for p-polarised and unpolarised incident light while design 2 performs better for s-polarised light.

The simulations described above were performed when qi is fixed at 45°. Simulations under different values of qi were also performed to evaluate the sensitivity of the designs to different incident angles. These simulation are shown in Figs. 15-18.

Figures 15 and 16 show the extinction ratio under Design 1 for three different incident angles, 40°, 45° and 50° denoted as (a), (b) and (c) respectively.

Figures 17 and 18 show the extinction ratio under Design 2 for three different incident angles, 40°, 45° and 50° denoted as (a), (b) and (c) respectively.

In Figures 15, 17 and 18, the lines corresponding to different polarisations of light are labelled with respect to each graph (c) only. Graphs (a) and (b) have similar overall shapes such that it is clear which lines relate to which polarization (e.g. the lowermost line shown in graph (c) of Fig. 18 corresponds to the same polarisation of light for the lowermost line in graphs (a) and (b).

As can be seen from the figures a reasonably good insensitivity to the incident angle is observed for both designs.

Origin of the Ghost Image

Figure 3 shows a schematic drawing of a typical head-up display (HUD) setup in the vehicle environment. The light from the picture generation unit 200 is partially reflected by the interior surface 510 of the windscreen 500, which constitutes the target image seen by the driver, while the rest will be refracted into the windscreen glass. The light being refracted into the windscreen glass will experience a second reflection/refraction at the glass-to-air interface 520 at the exterior surface of the windscreen 500. As shown, the light being reflected back into the windscreen 500 in this case will experience another (the third) reflection/refraction at the interior glass-to-air interface 510 and the light being refracted towards the user 600 will constitute a ghost image. The ghost image will blur the target image and affect the user experience in this application.

The intensity of the target image (It) can be expressed as

4 = ·Ί 4.v

where is the reflectance for the first reflection described in the above and I _IN is the intensity of the input light.

The intensity of the ghost image (Ig) can be expressed as

4 = (1 - _¾ ) _¾ {!- r ₃ )4 _v

where r ₂ and G3 are the reflectance for the second and third reflection described in the above, respectively.

In order to compare the relative intensity between the ghost and target images, we introduce the extinction ratio (ER), which is defined as:

It should be noted that the values of , r ₂ and G3 depends on multiple factors, including the refractive index of the windscreen glass as well as the polarisation states (s or p) and the incident angle of the light.

In this work, the value of qi is assumed to be 45° and the windscreen is assumed to be made of Schott BK7 glass, whose refractive index across the visible wavelength range is plotted in Fig. 8. Given this, the qi is calculated to be -27.5°. For other materials of the windscreen the values may change.

The reflectance for the first and the second reflection is simulated for s-polarised and p- polarised incident light, and the results are plotted in Fig. 6 and Fig. 7 respectively. It can be seen that s-polarised light has higher reflectance in both cases. Accordingly, the light from the projector should be adjusted into s-polarisation for higher efficiency of light utilisation. On the other hand, the ghost image is also stronger for s-polarised incident light as the value of r ₂ is also higher in this case. To compare the relative power between the target and ghost image, the ER is also simulated for s-polarised, p-polarised and unpolarised incident light and the results are shown in Fig. 8. It can be seen that s-polarised incident light also gives a better extinction ratio. However, the intensity of the ghost image is calculated to be less than 1 dB lower than the target image, which means they have a similar level of light intensity. Therefore, the strong presence of the ghost image in this case will seriously affect the quality of the image that the driver perceives. Accordingly, the anti-reflection coating is employed to reduce to the presence of the ghost image, whilst maintaining the high reflectance at the first interface 510 and the resulting strong target image.