GB2149140A

GB2149140A - Head-up display systems

Info

Publication number: GB2149140A
Application number: GB8426957A
Authority: GB
Inventors: Colin Kenward Marshall
Original assignee: GEC Avionics Ltd
Current assignee: Allard Way Holdings Ltd
Priority date: 1983-10-31
Filing date: 1984-10-25
Publication date: 1985-06-05
Also published as: GB8426957D0; FR2554246A1; GB2149140B; FR2554246B3

Abstract

A head-up display system comprising a combiner (5), a projector unit (1, 2, 3) effective to projective light within a selected wavelength band onto the combiner (5), and an element (9) interposed in the light path between the combiner (5) and an observer's viewing position (O). The element (9) is transmissive only to light within the selected wavelength band. This system may be incorporated into goggles or a visor on headgear. <IMAGE>

Description

SPECIFICATION Display systems This invention relates to display systems more particularly to head-up display systems.

Such display systems comprise a combiner through which an observer can view a scene forward of the observer and a projector unit effective to project light representing a display onto the combiner to provide the observer by reflection at the combiner with a view of the display superimposed on his view through the combiner of said scene.

Such display systems find particular application in aircraft.

In such an application, it is sometimes found that, although normally satisfactory, under certain conditions the brightness of the image of the display presented to the observer is inadequate.

For example, the need to be able to deploy ground attack aircraft not only under good visibility, i.e. daytime, conditions of poor visibility, such as at night, has led to the development of a 'round the clock' airborne ground attack system which comprises a forward looking infra-red sensing device and a head-up display whose projector unit is operable in either a raster scan mode or in a cursive mode in order to enable a pilot to control a pod launched weapon.

At night, with the projector unit of the head-up display operating in the raster scan mode, infra-red radiation from the terrain over which the aircraft is flying is received by the sensor. A scan converter section of the infrared sensing device serves to produce from the infra-red image of the terrain a corresponding visual image presented at the screen of the projector unit. In distinction from the presentation of cursive data, the terrain representation, being produced on the combiner by raster scanning of the projector unit screen, is of relatively low brightness. At night this presents no problem as against a dark background the brightness of the scene presented is well suited to the human eye. In daytime condition the infrared sensing device is useful for obtaining data from visually covert terrain characteristics, for example from camouflaged tanks.In daytime conditions, however, especially bright daylight, the image of the display on the combiner is barely distinguishable. This problem is exacerbated with the use of a diffractive optic combiners as these combiners have high transmissivity to light from the outside scene compared to conventional combiners. Such combiners are often used to provide a wide angle head-up display in azimuth, examples of such combiners being disclosed in US Patent No. 3,940,204 and UK Patent No. 2,088,079.

It is an object of the present invention to provide a head-up display system wherein this problem is alleviated.

According to a first aspect of the invention a head-up display system comprises: a combiner through which an observer can view a scene forward of the observer; a projector unit effective to project light representing a display and within a selected wavelength band onto said combiner to provide the observer by reflection at the combiner with a view of the display superimposed on his view through the combiner of said scene and an element adapted to be interposed in the light path between said combiner and the observer and which is transmissive substantially only to light within said selected wavelength band.

Preferably said combiner comprises a tuned reflective optical coating which is substantially non-transmissive to light from said scene within said selected wavelength band.

Said element suitably comprises a body of light transmissive material, having on the surface remote from said viewing position an optical coating substantially transmissive to light only within said selected wavelength band. In such an element preferably the surface of said body adjacent to said observer carries a non-reflective optical coating.

Said element is suitably incorporated in an eyepiece of goggles to be worn by the observer.

Alternatively said element is incorporated in a visor on headgear to be worn by the observer.

Preferably said display system includes a removable neutral density filter adapted to be interposed in the light path between said observer and said combiner.

According to a second aspect of the invention, a method of using a head-up display system of the kind comprising a combiner through which an observer can view a scene forward of the observer; a projector unit effective to project light representing a display and within a selected wavelength band onto said combiner to provide the observer by reflection at the combiner with a view of the display superimposed on his view through the combiner of said scene, comprises interposing in the light path between the combiner and the observer an element which is transmissive substantially only to light within said selected wavelength band, thereby to enhance the brightness of the observers view of the display relative to the brightness of the observer's view of the scene.

Said method suitably further comprises the step of interposing in said light path a neutral density filter.

One head-up display system in accordance with the invention is described hereafter with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of the display system; and Figure 2 is a diagram illustrating the charac teristics of a filter employed in the system of Figure 1.

Referring to Figure 1, the system comprises a projector unit comprising a cathode ray tube 1, and a lens system 2, the cathode ray tube having a screen 3 coated with a narrow wavelength band 'green' light emitting phosphor. The cathode ray tube 1 is arranged to display an image produced by scanning an infra-red image produced by a forward looking infra-red sensing device (not shown) from infra-red radiation emitted by a terrestrial object, for example a tank which may not be optically visible due to camouflage. The system further comprises a diffractive optic combiner 5 positioned adjacent to the windscreen 6 of an aircraft such that the pilot, positioned at view point 0, may view the distant screen through the windscreen through the combiner 5 and also see the image on the screen 3 which has projected onto the combiner superimposed on the view of the distant scene.The combiner 5 comprises a tuned reflective optical element, and thus has the property of efficiently reflecting light at particular combinations of wavelength and angle of incidence. The combiner includes a holographic coating responsive to the same wavelength band as the light emitted by the phosphor of the screen 3,and thus reflects green light within this wavelength band and is so angled with respect to the optical axis of the system that green light projected onto it from the cathode ray tube 1 is reflected towards the observer, whilst green light incident on the combiner from a distant scene is reflected away from the observer, light in other wavelength bands from the distant acene being transmitted by the combiners.

As so far described, the system is of known form for a head-up display system employing a diffractive optic combiner. The system shown in Figure 1 however differs from known systems in that in the light path between the viewpoint 0 and the combiner 5 there is interposed a removeable thin body of light transmissive material 7. The body 7 carries on its surface remote from the viewpoint 0 an optical coating 9 having the spectral response depicted in Figure 2. As can be seen in Figure 2, the coating 9 constitutes an interference filter, which selectively transmits green light within a narrow wavelength band, this wavelength band being included in the wavelength band of the phosphor of the screen 3. On the surface of the body 7 adjacent to the observer 0 there is provided an anti-reflection coating 11.

In use of the system during daylight conditions since the combiner 5 is non-transmissive to green light from the distant scene, and the coating 9 is non-transmissive to the light from the distant scene which has been transmitted by the combiner 5, light from the screen 3 of the cathode ray tube will be presented to the pilot against a background approximating that experienced in night time conditions. The green daylight component which is incident on the coating 9 from directions not intercepted by the combiner will however be transmitted to the viewing position 0. Such light will be clearly distinguishable from the light from the screen 3, and will appear as a frame within which an image corresponding to the image on the screen 3 is presented to the observer.The anti-reflection coating 11 will prevent veiling glare arising from the ambient light within the aircraft being reflected from the side of the element facing the viewing positions.

The efficiency of the above described system clearly depends on the optical density of the coating 9. In some circumstances, e.g. in bright desert conditions, it may be useful to provide at least one neutral density filter in the light path between the viewpoint 0 and the combiner 5 to further attenuate the light from the outside screen seen by the observer using the head-up display system. Such a filter is indicated as 1 3 in Figure 1, although it will be appreciated that the filter may be positioned anywhere along the light path between the viewpoint 0 and combiner 5.

Generally, however, it will be convenient to fit such a filter by some form of retaining means (not shown) immediately adjacent to the coating 9.

The body 7, complete with its optical coatings 9, 11, together with the retaining means for the neutral density filter 1 3 is suitably one of two such arrangements incorporated respectively in the two eyepieces of goggles (not shown) to be worn by the pilot. Alternatively the body 7 could be a suitably coated visor on a helmet to be worn by the pilot (also not shown), the headgear being provided with retaining means for the neutral density filter 13. Such arrangements could however cause problems with the visibility of the Head Down displays within the aircraft. These generally take the form of cathode ray tube displays as well as the older conventional analogue type instruments having dial displays with moving pointers. A cathode ray tube display having a phosphor screen of the same type as the screen 3 of the head-up display clearly presents no problems.

Instruments which are illuminated by ambient lighting within the aircraft will however become invisible to the pilot as viewed through the goggles or visor as their brightness will be less than the outside world which has itself been obliterated. One solution would be to use localised mercury vapour lamps emitting light at the same wavelength as the phosphor screen 3 and therefore visible through the interference filter coating 9.

Alternatively where a head down instrument is not discernable data provided by such instruments might be presented to the pilot using the head-up display.

It will be appreciated that whilst the headup display system described herebefore by way of example has particular application in daytime ground attack roles, it also has application as a training aid for trainee pilots unused to night time flying. By progressively increasing the optical density of any neutral density filters used in the system, a trainee pilot may become used to a progressively limited view of the outside world.

It will also be appreciated that whilst the head-up display system described by way of example uses an image derived from data collected by a forward looking infra-red sensing device, the invention is equally applicable to head-up displays in which the image projected onto the combiner screen is derived from a different source.

It will also be appreciated that whilst the invention is particularly applicable to head-up displays incorporating a combiner of the diffractive optic type, the invention is also applicable to any combiner which is effective to reflect light within a selected wavelength band from a distant scene away from the observer, and to reflect light within the selected wavelength band projected onto the combiner from a projector unit towards the observer.

Claims

1. A head-up display system comprising a combiner through which an observer can view a scene forward of the observer; a projector unit effective to project light representing a display and within a selected wavelength band onto said combiner to provide the observer by reflection at the combiner with a view of the display superimposed on his view through the combiner of said scene, and an element adapted to be interposed in the light path between said combiner and the observer and which is transmissive substantially only to light within said selected wavelength band.

2. A display system according to Claim 1 wherein said combiner comprises a tuned reflective optical coating which is substantially non-transmissive to light from said scene within said selected wavelength band.

3. A display system according to Claim 2 in which said element comprises a body of light transmissive material, having on the surface remote from said observer an optical coating substantially transmissive to light only within said selected wavelength band.

4. A display system according to Claim 3 in which the surface of said body adjacent to said observer carries a non-reflective optical coating.

5. A display system according to any one of the preceding claims in which said element is incorporated in an eyepiece of goggles or spectacles to be worn by the observer.

6. A display system according to any one of Claims 1 to 3 in which said element is incorporated in a visor on headgear to be worn by the observer.

7. A display system according to any one of the preceding claims which includes a removeable neutral density filter interposed in the light path between said observer and said combiner.

8. A head-up display system substantially as hereinbefore described with reference to the accompanying drawings.

9. A method of using a head-up display system of the kind comprising a combiner through which an observer can view a scene forward of the observer; a projector unit effective to project light representing a display and within a selected wavelength band onto said combiner to provide the observer by reflection at the combiner with a view of the display superimposed on his view through the combiner of said scene, the method comprising interposing in the light path between the combiner and the observer an element which is transmissive substantially only to light within said selected wavelength band, thereby to enhance the brightness of the observer's view of the display relative to the brightness of the observer's view of the scene.

10. A method according to Claim 9 including the step of interposing in said light path a neutral density filter.

11. A method of using a head-up display system substantially as hereinbefore described with reference to the accompanying drawings.