GB2240853A

GB2240853A - Head-up display

Info

Publication number: GB2240853A
Application number: GB9102201A
Authority: GB
Inventors: Brian Henry Poole
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1990-02-08
Filing date: 1991-02-01
Publication date: 1991-08-14
Also published as: FR2657969A1; DE4102678A1; GB9102201D0

Abstract

The fold-up element 6 of a head-up display is a holographic, diffractive or Fresnel device which reflects the beam of radiation from the screen 3 of CRT 1 onto a combiner 10 at an angle r that is less than the angle of incidence i. In this way, the instantaneous field of view I of the display can exceed the size of the exit lens 41. The combiner 10' of Fig 2 is a device that reflects the beam of radiation at an angle that is less than the angle of incidence on the combiner so as further to increase the instantaneous field of view I'. <IMAGE>

Description

HEAD-UP DISPLAY APPARATUS This invention relates to head-up display apparatus.

Head-up displays are used in aircraft to present a display representation which is visible at infinity by the pilot in his line-of-sight through the windshield. In this way, the pilot can see the display of instrumentation or other information without having to look down or change focus of his eyes.

These displays usually have a cathode ray tube CRT, on the screen of which is provided the display, and which is located below the pilot's line-of-sight. A lens system and mirror is used to direct a collimated image of the CRT screen onto a combiner which is located in the pilot's line-of-sight. The combiner may be a semi-reflecting mirror or a holographic element which both act to direct the collimated image of the CRT screen into the pilot's line-of-sight whilst enabling the pilot to see through the combiner, so that his view of the scene in front of the aircraft is not obscured.

The size of the instantaneous field of view (IFOV) of the head-up display, that is, the field of view visible to the pilot at any time, without moving his head, is determined by the diameter of the exit lens and the location of the image of the lens. In order to make the IFOV as large as possible, it is desirable for the exit lens to be as large as possible. This, however, leads to an increase in the size of the display housing which may not be acceptable in aircraft where space is limited.

One way of increasing the IFOV without increasing the lens diameter is to form two images of the exit lens, one above the other, by means of two parallel combiners located above one other. Both combiners reflect an image of the exit lens into the pilot's line-of-sight, the two images showing different parts of the screen of the CRT so that the IFOV is increased. In some aircraft, the use of two combiners is not possible because there is insufficient clearance below the windshield. The use of two combiners does also have attendant disadvantages, in that there is a transition at the edge of the two combiners which interrupts the appearance of the display at the edges.Where the upper combiner is viewed through the lower combiner this leads to attenuation both of the display representation and of the external scene, the light from which has to pass through two combiners before reaching the pilot.

It is an object of the present invention to provide head-up display apparatus which can provide an acceptable field of view and be of a low profile, compact construction.

According to the present invention there is provided head-up display apparatus including a display region in which a display representation is provided, a lens systems located in front of the display region and arranged to provide a collimated beam of radiation from the display region along the axis of the apparatus, reflector means located on the axis so that the beam of radiation being incident on the reflector means, the reflector means being arranged to reflect the beam onto combiner means which directs an image of the display region into the line-of-sight of the user, and the reflector means being arranged to reflect the beam at an angle less than the angle of incidence.

The reflector may be a holographic, diffractive or a Fresnel element. The display region is preferably the screen of a cathode ray tube. The lens system may include an exit lens and the instantaneous field of view of the apparatus be greater than the size of the exit lens.

The combiner may be a reflective element and may be arranged such that the angle of incidence of the beam on the combiner means is greater than the angle of reflection of the beam off the combiner means. The combiner may be a holographic element and may include two parallel combiner elements, one above the other.

Aircraft head-up display apparatus in accordance with the present invention, will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows the apparatus in side elevation; and Figure 2 is a side elevation view of a modified form of the apparatus in Figure 1.

The head-up display apparatus includes a monochrome cathode- ray tube CRT 1 which receives electrical signals from a driver unit 2 and provides a display representation accordingly on the screen 3. The display representation provided may be flight information, weapon-aiming information, instrumentation about operation of the aircraft or the like. The screen 3 of the CRT is arranged close to the vertical.

Located directly in front of the screen 3 is a converging Petzval lens systems 4 which includes an inlet lens 40 and an exit lens 41. The lens system 4 provides a collimated beam of radiation along an optical axis 5 normal to the screen 3.

The beam of radiation falls on a fold reflector 6 which is in the form of a holographic element or a diffraction grating which is inclined at an angle of 30 degrees to the optical axis 5 so that the angle of incidence i is 60 degrees. It will be appreciated that other angles could be used. The reflector 6 is fabricated in such a way that the angle of reflection r is less than the angle of incidence. More particularly, the angle of reflection r is arranged to be 30 degrees, so that the beam undergoes a reflection through 90 degrees. The reflector 6, therefore, acts like a plane mirror inclined at 45 degrees to incident radiation. Techniques for fabricating holograhic elements and diffraction gratings with such properties are well known.

The reflector 6 could take forms other than a holographic or diffraction element. For example, it could be a Fresnel stepped mirror having multiple parallel facets arranged at an angle to the plane of the mirror so that the angle of reflection, relative to the normal to the plane of the mirror, is less than the angle of incidence.

The beam of radiation is directed upwardly onto a combiner element 10 which may be of conventional construction. In the present example, the combiner element 10 is a flat, rectangular plate of glass coated on the surface 11 facing the pilot with a layer that is reflective to the radiation emitted by the CRT screen 3, but'which is transparent to other wavelengths. The majority of radiation from outside the aircraft, therefore, passes through the combiner 10 along the pilot's line-of-sight 20 with little attenuation, so that the pilot can view the external scene through the combiner. The combiner 10 is inclined at about 45 degrees to the pilots line-of-sight 20.

The combiner 10 forms a virtual image I of the exit lens 41, on the side of the combiner remote from the pilot. The image I is the same size as the exit aperture 7 of the reflector 6 and is greater than the size of the exit lens 41, because of the shallower angle of the reflector compared with conventional fold-up mirrors used in head-up displays. It will be appreciated that, using conventional fold mirrors, the exit aperture is the same size as the exit lens, limiting the size of the image to the size of the exit lens.

The CRT 1, lens system 4 and the reflector 6 are mounted within a housing 9 on which is supported the combiner 10. The depth of the housing 9 can be small, compared with conventional head-up displays having a single combiner and a similar IFOV, because the field-of-view is greater than that of the exit lens 41.

It will be appreciated that it would be possible to use two parallel combiner elements in the present invention, one above the other, further to increase the IFOV or to reduce the depth of the apparatus.

Instead of using a conventional reflecting combiner, a reflective or diffractive holographic combiner could be used further to increase the IFOV, such as the reflective combiner 10' shown in Figure 2. The holographic combiner 10' is similar to the fold reflector 6 in that the angle of incidence i is greater than the angle of reflection r. In this way, the virtual image I' formed can be made larger than the exit aperture 7.

It will be appreciated that the apparatus is not confined to use in aircraft but could be used in other applications where a similar display representation is required.

Claims

1. Head-up display apparatus including a display region in which a display representation is provided, a lens systems located in front of the display region and arranged to provide a collimated beam of radiation from the display region along the axis of the apparatus, reflector means located on the axis so that the beam of radiation is incident on the reflector means, wherein the reflector means is arranged to reflect the beam onto combiner means which directs an image of the display region into the line-of-sight of the user, and wherein the reflector means is arranged to reflect the beam at an angle less than the angle of incidence.

2. - Head-up display apparatus according to Claim 1, wherein the reflector means is a holographic element.

3. Head-up display apparatus according to Claim 1, wherein the reflector means is a diffractive element.

4. Head-up display apparatus according to Claim 1, wherein the reflector means is a Fresnel element.

5. Head-up display apparatus according to any one of the preceding claims, wherein the display region is provided on the screen of a cathode ray tube.

6. Head-up display apparatus according to any one of the preceding claims, wherein the lens system includes an exit lens, and wherein the instantaneous field of view of the apparatus is greater than the size of the exit lens.

7. Head-up display apparatus according to any one of the preceding claims, wherein the combiner means is a reflective element.

8. ~ Head-up display apparatus according to any one of Claims 1 to 6, wherein the combiner means is arranged such that the angle of incidence of the beam on the combiner means is greater than the angle of reflection of the beam off;the-combiner means.

9. Head-up display apparatus according to Claim 8, wherein the combiner means is a holographic element.

10. Head-up display apparatus according to any one of the preceding claims, wherein the combiner means includes two parallel combiner elements, one above the other.

11. Head-up display apparatus substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

12. Head-up display apparatus substantially as hereinbefore described with reference to Figure 1 as modified by Figure 2 of the accompanying drawings.

13. Any novel feature or combination of features as hereinbefore described.