GB2279153A

GB2279153A - Night vision system

Info

Publication number: GB2279153A
Application number: GB8705777A
Authority: GB
Inventors: Stafford Malcolm Ellis
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1986-03-22
Filing date: 1987-03-11
Publication date: 1994-12-21
Anticipated expiration: 2007-03-11
Also published as: GB8705777D0; GB2279153B; GB8607162D0

Abstract

A night vision system adapted for mounting on a user's headgear (11) comprising a collimating eyepiece (19) into which light rays from an image intensifier (31) are directed downwardly to provide a wearer of the headgear with an intensified virtual image at infinity of a scene forward of the wearer superimposed on his view of the scene through the eyepiece. Light rays from the image intensifier pass along a path comprising a reflective element (33) which reflects the rays upwardly onto a reflective imaging means (23) which reflects the rays back dawn towards the rear of the reflective element and forms a real image at an image plane (51) coincident with the principal focal plane of the eyepiece. <IMAGE>

Description

Night Vision systems This invention relates to night vision systems.

More particularly the invention relates to night vision systems of the kind adapted to be mounted on a user's headgear and comprising an electro-optical system including: a collimating eyepiece which provides a wearer of the headgear with a view through the eyepiece of a scene forward of the wearer; an image intensifier having an output face on which an image of said scene is produced in operation; and a light path along which pass light rays from said output face so as to enter said eyepiece in a generally downwards direction; the light rays being then first reflected in the eyepiece towards a combiner in the eyepiece possessing both light transmissive and light reflective properties, and then being reflected by the combiner towards the wearer so as to provide the wearer with a virtual image at infinity of said scene superimposed on said view of said scene through the eyepiece.

UK Patent Specifications Nos. GB 2108702B and GB 2144558B both disclose night vision systems of the kind specified.

It is an object of the present invention to provide a night vision system of the kind specified which can provide a wider angular field of view to the user than has hitherto been possible.

According to the present invention in a night vision system of the kind specified said light path comprises: a reflective element that serves to intercept light from said outputface and which is inclined with respect thereto so as to reflect rays from said output face in a generally upwards direction; and a reflective imaging means, at a position above the said inclined reflective element, that serves to reflect rays incident thereon from said reflective element in a generally downwards direction to the rear of said inclined reflective element so as to form a real image of said scene, as presented at said output face of the image intensifier, at an image plane at least nominally coincident with the principal focal plane of said eyepiece.

Said reflective imaging means suitably comprises a concave reflector and an associated refractive lens arrangement between said reflector and said eyepiece, said reflector at least partly serving to form said real image.

Alternatively, said reflective imaging means may comprise a plane reflector and an associated refractive lens arrangement between said reflector and said eyepiece, said lens arrangement serving to form said real image.

Said eyepiece suitably comprises fore and aft reflective elements positioned substantially along the line of sight of the wearer through the eyepiece, said forward element constituting said combiner. In one such an arrangement said fore and aft elements are constituted by optical films carried on respective transparent carrier members so inclined as to define a downwardly narrowing space therebetween. In an alternative arrangement the eyepiece comprises a body of light refractive material having fore and aft parallel external surfaces, said fore element comprises an optical film inside said body, and said aft element comprises a surface of said body at which total internal reflection occurs.

One night vision system in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows schematically a sectional elevation through a portion of the system; and Figure 2 is a representation of the field of view obtainable with the system of Figure 1.

Referring to Figure 1, the system is mounted on a headgear 11 and comprises electro-optical system 13 fixed with respect to the headgear. The headgear 11 comprises a helmet shell 15 and a vizor 17, which is pivotally mounted on the helmet shell.

The electro-optical system 13 comprises an eyepiece 19 and an electro-optical sub-system 21. The sub-system 21 is contained within a housing 23 having a nose portion 25 and a body portion 27.

The housing 23 is supported (by means not shown) on the vizor 17 with the nose portion 25 of the housing projecting through an aperture in the vizor 17.

The sub-system 21 has, supported within the nose portion 25, an objective lens 29 and an image intensifier 31, in the embodiment illustrated, a so called GEN3 image intensifier. In use, an intensified image of a portion of the scene ahead of a wearer of the headgear appears on the output face 35 of the intensifier 31.

A planar semi-reflective element 33 is positioned rearwards of the image intensifier 31, inclined in a fore and aft direction with respect to the output face 35 of the intensifier 31.

A reflective imaging means 37 is positioned above the element 33 and comprises a concave reflector 39 and an aberration correction lens arrangement represented at 41.

Beneath the element 33 there is a display device 49, e.g. a LED display device, having a display face 49' positioned at the same distance optically from the concave reflector 39 as the output face 35 of the image intensifier 31. Light rays from the output face 35 are reflected by the element 33, and light rays from the display face 49' pass through the element 33 to be incident on the reflective imaging means 37. These incident rays are brought to a focus by the concave reflector 39 (the associated refracting lens arrangement 41 effecting only aberration correction) to form a representation in the form of a real image of the output face 35 and/or the display face 49 at an image plane 51 located below and to the rear of the element 33.

The eyepiece 19 is a collimating eyepiece whose position is such that its principal focal plane is substantially coincident with the image plane 51.

The eyepiece 19 comprises two tuned reflection holographic optical films 43' and 45' constituted by coatings on the facing surfaces of fore and aft glass carrier members 43 and 45 respectively which define a downwardly narrowing space 47. The carrier member 43 is in the form of a shell, concave towards an eye P of a wearer of the headgear 11, and the carrier member 45, which is positioned between the carrier member 43 and the eye P, is a planar member. The holographic films 43', 45' exhibit reflection characteristics to the narrow band radiation from the image intensifier 31, and the display device 49 at the angles which the rays from the image formed at the image plane 51 are Incident on the holographic films.Accordingly the eyepiece operates in an off axis mode so that rays from an image formed at the image plane 51, after successive reflections first at the film 45' and then at the film 43' are transmitted to the eye P and appear to originate at a (virtual) image at a distance, in the line of sight of the user through the films 45', 43' of the eyepiece 19, within the angular resolving power of the user's eye, that is to say at a so-called 'infinity'. At the same time light from the distant scene passes through the films 43' and 45' to the eye P providing the user with a direct view of the distant scene on which his view of the image formed at the image plane 51 is superimposed.

Figure 2 illustrates typical angular fields of view obtained with the system.

Whilst in the above described embodiment the concave reflector 39 of the reflective imaging means 37 forms the required real image at image plane 51, the lens arrangement 41 performing only an aberration correction function, in other arrangements in accordance with the invention the reflective imaging means may utilise both a concave reflector and a refractive lens arrangement to form the required real image. In such an arrangement the lens arrangement may also serve, as before, in aberration correction.Moreover, other optical elements such as optical wedges and lenses, cylindrical, tilted or off centred, as known in the art, may alternatively or additionally be employed in aberration correction, and such other optical elements may be positioned at any suitable position in the light path between the output face 35 of the image intensifier 13 and the eyepiece 19, not only adjacent the reflector 39, as shown in Figure 1. There Is ample space in the light path to accommodate such corrective elements and, accordingly, the system may be made substantially free of optical defects which tend to promote user resistance to adoption of the system.

In still further arrangements the reflective imaging means 37 may comprise a plane, i.e. a flat, reflector and a refractive lens arrangement to form the required real image at image plane 51, and this lens arrangement may, in addition, perform aberration correction, and other optical elements may alternatively or additionally be employed in aberration correction.

Either or both of the films 43', 45' of the eyepiece 19 may comprise a semi-silvered coating on the relevant glass substrate 43 or 45, instead of a holographic film, although preferably at least the rear film 45' is a holographic film.

Whilst necessarily being required to perform a double reflection and collimating function, the eyepiece of a system according to the invention may nevertheless take a different form to that shown in Figure 1. In suitable alternative forms, described and depicted in the above-mentioned UK Patent specifications Nos.

GB 2108702B and GB 2144558B, the eyepiece comprises a body of light refractive material having parallel fore and aft planar external surfaces. A holographic reflective film inside the body performs a function corresponding to the film 43, and the function of the film 45', i.e. the first reflection of rays from the Image plane 51, is effected by total internal reflection within the body of refractive material, e.g. at a glass-air interface. The glass-air interface may be at the aft planar external face, or alternatively may be between air-spaced component parts of the body. The first reflection may alternatively be effected by a second holographic film instead of by total internal reflection.

It will be appreciated that whilst the night vision system described above by way of example comprises a viewing system for one eye only of the user, other systems in accordance with the invention may incorporate two electro-optical systems, one for each eye of the user, i.e. to form a goggle, as described in principle in UK Patent Specifications Nos.GB 2108702B and GB 21445588. In such arrangements only one of the two electro-optical systems will normally need to be provided with a display device such as 49. It will further be appreciated in this connection that in the event that a stand-by display is not a requirement the display device 49 may, of course, be omitted, in which case, a full reflector may be substituted for the semi-reflective element 33.

Claims

1. A night vision system adapted to be mounted on a user's headgear and comprising an electro-optical system including: a collimating eyepiece which provides a wearer of the headgear with a view through the eyepiece of a scene forward of the wearer; an image intensifier having an output face on which an image of said scene Is produced in operation; and a light path along which pass light rays from said output face so as to enter said eyepiece in a generally downwards direction; the light rays being then first reflected in the eyepiece towards a combiner in the eyepiece possessing both light transmissive and light reflective properties, and then being reflected by the combiner towards the wearer so as to provide the wearer with a virtual image at infinity of said scene superimposed on said view of said scene through the eyepiece; and wherein said light path comprises: a reflective element that serves to intercept light from said output face and which is inclined with respect thereto so as to reflect rays from said output face in a generally upwards direction; and a reflective imaging means, at a position above the said inclined reflective element, that serves to reflect rays incident thereon from said reflective element in a generally downwards direction to the rear of said inclined reflective element so as to form a real image of said scene, as presented at said output face of the image intensifier, at an image plane at least nominally coincident with the principal focal plane of said eyepiece.

2. A system according to Claim 1 wherein said reflective imaging means comprises a concave reflector and an associated refractive lens arrangement between said reflector and said eyepiece, said reflector at least partly serving to form said real image.

3. A system according to Claim 1 wherein said reflective imaging means comprises a plane reflector and an associated refractive lens arrangement between said reflector and said eyepiece, said lens arrangement serving to form said real image.

4. A system according to any one of the preceding claims including optical means in said light path serving to correct aberrations introduced by the remainder of said electro-optical system.

5. A system according to Claim 4 when dependent on Claim 2 or Claim 3 wherein said optical means is constituted at least partly by said lens arrangement.

6. A system according to any preceding claim wherein said reflective element is a semi-reflective element and the system further includes a display device below said reflective element, light rays from which device pass through said reflective element and along said light path to be formed into a real image at said image plane.

7. A system according to any one of the preceding claims wherein said eyepiece comprises fore and aft reflective elements positioned substantially along the line of sight of the wearer through the eyepiece, said forward element constituting said combiner.

8. A system according to Claim 7 wherein said fore element is concave towards the wearer.

9. A system according to Claim 8 wherein said aft element is optically flat.

10. A system according to any of Claims 7 to 9 wherein said fore and aft elements are constituted by optical films carried on respective transparent carrier members so inclined as to define a downwardly narrowing space therebetween.

11. A system according to Claim 10 wherein at least said aft element is a holographic film.

12. A system according to any one of Claims 7 to 9 wherein said eyepiece comprises a body of light refractive material having fore and aft parallel external surfaces and said fore element comprises an optical film inside said body.

13. A system according to Claim 12 wherein said aft element comprises a surface of said body at which total internal reflection occurs.

14. A system according to Claim 13 wherein said aft element comprises a glass-air interface.

15. A system according to any one of Claims 12 to 14 wherein said fore element is constituted by a holographic film.

16. A night vision system substantially as hereinbefore described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A night vision system adapted to be mounted on a user's headgear and comprising an electro-optical system including: a collimating eyepiece which provides a wearer of the headgear with a view through the eyepiece of a scene forward of the wearer; an image intensifier having an output face on which an image of said scene is produced in operation; and a light path along which pass light rays from said output face so as to enter said eyepiece in a generally downwards direction; the light rays being then first reflected in the eyepiece towards a combiner in the eyepiece possessing both light transmissive and light reflective properties, and then being reflected by the combiner towards the wearer so as to provide the wearer with a virtual image at infinity of said scene superimposed on said view of said scene through the eyepiece; and wherein said light path comprises: a reflective element that serves to intercept light from said output face and which is inclined with respect thereto so as to reflect rays from said output face in a generally upwards direction; and a reflective imaging means, at a position above the said inclined reflective element, that serves to reflect rays incident thereon from said reflective element in a generally downwards direction to the rear of said inclined reflective element so as to form a real image of said scene, as presented at said output face of the image intensifier, at an image plane substantially coincident with the principal focal plane of said eyepiece.

9. A system according to Claim 8 wherein said aft element is optically flat.