WO1995013557A1

WO1995013557A1 - Display systems and methods

Info

Publication number: WO1995013557A1
Application number: PCT/GB1994/002442
Authority: WO
Inventors: Andrew Dames
Original assignee: Scientific Generics Limited
Priority date: 1993-11-08
Filing date: 1994-11-08
Publication date: 1995-05-18
Also published as: GB9322964D0

Abstract

A head-up display system for a vehicle comprises an image source such as an LCD (14) displaying a vehicle operating parameter such as vehicle speed. The source (14) is illuminated by ambient light (18) which, after being modulated by the source (14) so as to carry the image, is projected onto the internal surface of the vehicle windscreen (10) where a component (26) is reflected back towards the driver (24) who sees the image at an apparent position in front of the vehicle superimposed on his normal field of view seen through the windscreen (10).

Description

TITLE: DISPLAY SYSTEMS AND METHODS

This invention relates to display systems for projecting images or information into a second image or field of view, for example where the second image is a view of a real scene as seen through a screen. Such display systems are often described as head-up displays or HUDs. The invention is particularly applicable to HUDs in road vehicles, aircraft and maritime display systems. The invention also concerns a method of displaying an image.

EP A0177134 discloses a head-up display for a vehicle, employing a display panel incorporating a liquid crystal display illuminated by ambient light. Apart from the data presented by the display, the panel is transparent. The panel is positioned on the dash¬ board at the base of the windscreen and the vehicle driver can view the scene ahead of the vehicle by looking through the panel. However, to see the data presented by the display, the driver must glance downwardly from his usual direction of view and, moreover, must refocus his eyes from near infinity to less than 1 metre, the distance of the LCD in front of his eyes. Thus a display panel of this form has considerable disadvantages.

US Patent 5212471 discloses a head-up display including an illuminated image source, polarised light from which (carrying the image) is projected onto the interior surface of a glass windscreen where the light component reflected at the air/glass interface is directed back towards the viewer who sees the image at an apparent position in front of the windscreen and in the field of view seen through the windscreen. The windscreen has a polarity rotator to prevent the formation of a second or ghost image from light reflected at the glass/air interface at the external surface of the windscreen. The disadvantages of this prior arrangement are the need to illuminate the image source, the need for the light projected onto the windscreen to be polarised and the need for a polarity rotator.

According to the invention there is provided a display system comprising a transparent viewing screen, an image source, and light projecting means for projecting the image onto one surface of the screen so that the image is superimposed on the visual field viewed through the screen, wherein the image source is capable of being illuminated by ambient light to provide the image.

The image source preferably comprises a liquid crystal display device and may also include a diffuser between the LCD and the daylight source. The diffuser is preferably transmissive but may alternatively be reflective.

A particular application of a display system according to the invention is in motor road vehicles. In this case, said screen is the windscreen of the vehicle and the source is positioned adjacent an internal surface of the windscreen for illumination by ambient light passing through the windscreen. It will be appreciated that the image may consist of any information or data which it is desired should be projected into the field of view of the vehicle driver. This data may be a vehicle operating parameter such as vehicle speed or may be in the nature of information for assisting route finding, for example a map.

According to another aspect of the invention there is provided a method of displaying an image in a head-up display viewed through a screen, comprising using natural daylight to illuminate an image source and projecting the light from the source onto the screen whence it is reflected towards the viewer who sees the image added to the field of view through the screen.

The invention thus uses the technique of projecting illumination carrying the image onto the windscreen, and in order to provide the required reflection at the air/glass interface the light emerging from the source may be polarised in an appropriate direction so that when the light strikes the windscreen a useable component or portion is reflected back towards the viewer to form the virtual image. However, it may be possible for the light incident on the windscreen not to be polarised but to rely solely on the angle of incidence of the light on the windscreen.

The invention uses a source illuminated by ambient or natural daylight. This has the advantages that no high brightness illumination system is required even when used with an entirely standard windscreen; there is no need to make a dimming system and the use of daylight allows a lightweight compact image generator (normally an LCD and diffuser) permitting novel geometries, eg a light source on the driver's mirror.

Two preferred embodiments of display system according to invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic view of a first embodiment of display system according to the invention,

Figure 2 shows the optical projection system of Figure 1, and

Figure 3 is a diagrammatic view of the second embodiment of display system.

Referrring to Figure 1 , the display system is fitted at the base of the glass windscreen (or windshield) 10 of a car, for example being fitted to the top of the dashboard 12 thereof. The display system comprises an image source in the form of a liquid crystal display (LCD) 14 showing, in digital format, useful data which in this example is vehicle speed, eg in miles per hour or kilometers per hour. The LCD 14 is a transmission LCD and shows data as clear areas against a black background.

The LCD is illuminated by ambient light, ie daylight 18, passing through the transparent windscreen 10, and between the daylight source and the LCD is placed a transmissive diffuser 16. After passing through the diffuser 16 and LCD 14, this illumination (now carrying the image imparted to it by the LCD) is projected onto the internal surface of the windscreen 10 by optical projecting means shown in Figure 1 as comprising simply a plane mirror 20 which reflects the light beam upwardly towards the windscreen to become incident on the latter at a position 22 in the lower part of the field of view of the vehicle driver indicated at 24.

At the position 22 the light is incident on the internal surface of the windscreen at an air/glass interface where a proportion of the light is reflected towards the driver 24. The remainder of the light is transmitted into the glass of the windscreen and subsequently encounters the glass/air interface formed by the external surface of the windscreen. At this second interface a portion is reflected back towards the driver's eyes, the remainder being directed upwardly and away from the windscreen. The light components 26 reflected at the internal and external windscreen surfaces form an image 28 viewed by the vehicle driver. In this embodiment, the image appears 10 to 20cm outside (ie in front of) the windscreen 10, with a vertically separated double image. This can be tolerated if a small amount of information eg speed, in large numerals is to be displayed. To avoid a double image, the image needs to be typically three metres in front of the screen, where the two images tend to be superimposed.

At the position 22 a proportion of the light must be reflected towards the vehicle driver for the image to be seen. This can be achieved by arranging for the light incident on the windscreen to be polarised in a plane such that the vibrations are in and out of the paper of Figure 1 and arranging for the light to strike the windscreen at an angle close to the Brewster angle. Under these conditions, about 15% of the light is reflected back towards the vehicle driver at the internal surface of the windscreen, a further proportion being reflected back towards the vehicle driver at the external surface of the windscreen. The problem of the second reflection forming a double or ghost image is solved by ensuring that the image is at a distance of about three or four metres in front of the windscreen.

Normal LCD displays of the form encountered in pocket calculators normally produce light polarised at an angle of 45° to the direction of polarisation required in the display system of Figure 1. The direction of polarisation in such known LCDs is set both by the input and output polarisers and by the brushing on the inside of the glass of the LCD. To achieve the direction of polarisation required by the inventive display system, the input and output polarisers and the alignment layers can be rotated to give a white on black display with the required perpendicular polarisation. For even lower losses the output polariser can be omitted, relying solely on the angle of the windscreen as the polarisation analyser.

A lamp 30 illuminates the LCD to illuminate the LCD image at night.

A practical example of the light projecting means for projecting the image onto the internal surface of the windscreen is shown in Figure 2 where components corresponding to those shown in Figure 1 are denoted by the same reference numerals.

Ambient light 18 is shown passing through the windscreen 10 after which it is diffused by the diffuser and reaches the LCD 14 where the dynamic image is imparted to it. The light beam then strikes successive mirrors 32, 34, 36, 38 and 40. The mirros 34, 38 are provided by a common reflective plane mirror. The light passes through an imaging lens 42 which increasing the apparent distance of the virtual image of the LCD to about 3m in front of the windscreen. From the lens 42, the light reflects off the internal surface of the windscreen. Optical baffles 44 are provided to ensure that the light takes the required path through the series of mirrors 32 to 40.

The LCD (with diffuser) and projecting means are incorporated into a compact module which is neatly positioned at the base of the windscreen. The module does not intrude into the driver's field of view but projects an image which is seen by the vehicle driver, superimposed on the view seen through the windscreen, at a position which appears to be slightly ahead of the vehicle and in the lower part of the field of view, so that the driver is constantly aware of the image without it intruding and without the need for the driver to refocus or make adjustment from viewing the scene in the field of view through the windscreen.

In the second embodiment of display system shown in Figure 3, the diffuser and LCD 14 are located at the top of the glass windscreen 10, the LCD being illuminated by daylight 18 passing through the transparent windscreen. After passing through the LCD 14, the light (now modulated to carry the image) strikes a mirror 46 located adjacent the LCD at the top of the windscreen. The mirror 46 directs the light downwardly along a path 48 across the vertical extent of the windscreen to a concave mirror 50 forming an image reflector positioned near the lower edge of the windscreen 10. The mirror 50 directs the light to a position 22 on the internal surface of the windscreen, where components 26 reflected at the internal and external windscreen surfaces form an image 28 which appears to the vehicle driver 24 to be between about two and four metres in front of the windscreen, superimposed on the lower part of the field of view of the windscreen. As before, night illumination is provided by a lamp 30 which illuminates the LCD 14 after dark, for example being included in the electrical circuit of the vehicle instrumentation lighting.

In both embodiments described, daylight is modulated to carry the image. Instead of using daylight in this direct way, the daylight may be used to stimulate optical emission in an indirect way, eg by the use of fluorescence which can enhance colour and brightness.

The described embodiments use transmissive diffusers to diffuse the daylight before the latter reaches the LCD, but reflective diffusers may alternatively be used.

For best operation of the system, the light collector (ie the diffuser) should be as close to the horizontal as possible.

A natural light head-up display system in accordance with the invention is compact and cheap, is passive in the sense that no lamp is required, uses a standard windscreen and tracks ambient ligh levels.

Claims

1. A display system comprising a transparent viewing screen, an image source, and light projecting means for projecting the image onto one surface of the screen so that the image is superimposed on the visual field viewed through the screen, wherein the image source is capable of being illuminated by ambient light to provide the image.

2. A display system according to claim 1, wherein the image source comprises a liquid crystal display.

3. A display system according to claim 2, wherein the liquid crystal display is transmissive and modulates the light incident on the source.

4. A display system according to claim 2 or 3 and including a diffuser for diffusing the ambient light before the latter reaches the liquid crystal display.

5. A display system according to claim 1 or 2, wherein the light projected onto the screen is derived indirectly from the ambient light.

6. A display system according to any of the preceding claims, wherein the projecting means comprise an imaging lens to increase the apparent distance of the image.

7. A display system according to any of the preceding claims, wherein the light incident on the screen is polarised in a plane ensuring reflection of a proportion of the light back towards the viewer.

8. A motor road vehicle having a display system according to any of the preceding claims, said screen being the windscreen of the vehicle and the source being positioned adjacent an internal surface of the windscreen for illumination by ambient light passing through the windscreen.

9. A motor road vehicle according to claim 8, wherein the image is a dynamic image of a vehicle operating parameter such as vehicle speed.

10. A method of displaying an image in a head-up display viewed through a screen, comprising using natural daylight to illuminate an image source and projecting the light from the source onto the screen whence it is reflected towards the viewer who sees the image added to the field of view seen through the screen.