Night vision viewing systems

A helmet mounted night vision viewing system comprising a goggle arrangement supported from above the face aperture of the helmet, the goggle comprising two independent viewing arrangements, one for each eye of a wearer of the helmet, each viewing arrangement comprising an optical projector (11, 13, 15) arranged to project an intensified image of a scene ahead of the helmet wearer downwardly to an eyepiece (17) in the form of a body of refractive material (20) in front of the wearer's eye wherein light from the optical projector is reflected at an aft surface (23), towards a curved reflective surface (25) from which the light is reflected back towards the aft surface for transmission therethrough to the wearer's eye. The optical projectors each comprise an objective lens (11), image intensifier (13) and roof prism (17), the intensifier being positioned between the objective and prism so as to allow the eyepiece to be relatively small and close to the wearer's eye in the interest of lightness and compactness, thus rendering the system suitable for use in high performance aircraft.

This invention relates to night vision viewing systems. 
More particularly the invention relates to helmet mounted night vision 
viewing systems of a kind suitable for use in high performance aircraft. 
For such use, a viewing system, in addition to providing suitable optical 
performance, is required to be light and compact due to the high 
accelerations experienced in high performance aircraft. 
One particular viewing system suitable for such use comprises a helmet 
having a face aperture and a night vision goggle arrangement supported on 
the helmet from a position above said face aperture, the goggle 
arrangement comprising two independent viewing arrangements, one for each 
eye of a wearer of the helmet. Each viewing arrangement comprises a frame 
part and, supported by the frame part, an optical projector and an 
eyepiece. The optical projector comprises an objective, a roof prism and 
an image intensifier arranged, when the helmet is being worn, to project 
an intensified image of a portion of a scene ahead of the wearer in a 
generally downwards direction in relation to said face aperture. The 
eyepiece is positioned below said projector, in front of a respective one 
of the eyes of the wearer, and comprises a body of light refractive 
material having an upper surface, and fore and aft further surfaces 
spanned by said upper surface, said fore further surface possessing light 
reflecting properties and being concavely curved towards said aft further 
surface. Said upper surface and fore and aft further surfaces are 
angularly mutually disposed so that rays of said intensified image from 
said optical projector enter said body at said upper surface, are 
substantially totally internally reflected by said aft further surface 
towards said fore further surface, are reflected at said fore further 
surface rearwardly towards said aft further surface, and finally are 
transmitted through said aft further surface to the wearer's eye, thereby 
to provide the wearer with a view of said intensified image. 
Such a night vision viewing system is hereafter referred to as a night 
vision viewing system of the kind specified. 
One night vision viewing system of the kind specified is disclosed in GB 
No. 2,108,702-A. In the system therein disclosed the roof prism in the 
projector, which serves to fold the optical path through the projector, is 
between the objective and the image intensifier of the projector. It is a 
necessary consequence of this relative disposition of the elements of the 
goggle that the goggle provides a large eye relief, i.e. the space between 
the wearer's eyes and the rearmost surfaces of the eyepieces is large. 
This is desirable in that it enables the wearer to see round the eyepieces 
to view cockpit instruments and other parts of the real world scene, but 
this is only achieved at the expense of relatively large eyepieces and 
some loss of compactness. 
It is an object of the present invention to provide an improved form of 
night vision viewing system of the kind specified wherein this problem is 
alleviated. 
According to the present invention, in a night vision viewing system of the 
kind specified, in the optical projector of each viewing arrangement, the 
image intensifier is positioned between the objective and the roof prism. 
Such a configuration of the optical projector elements has a notable and 
unexpected beneficial effect permitting a design which is ergonomically 
very attractive. Thus the configuration enables a smaller, (but still 
sufficient) eye relief to be provided than is possible with the system 
disclosed in GB No. 2,108,702-A. This in turn means that the eyepieces may 
be made smaller and lighter with enhancement, rather than loss, of optical 
performances. 
In a preferred arrangement in accordance with the invention the roof prisms 
and eyepieces are located at least partly within the face aperture of the 
helmet. The overhang of the system beyond the helmet is thus reduced. 
It will be understood that the reduced weight and greater compactness of a 
system according to the present invention compared with the system of GB 
No. 2,108,702-A is very attractive from a wearer's viewpoint in high 
performance aircraft where linear and angular accelerations experienced 
are very high. 
In one particular system according to the invention in each said eyepiece 
said fore further surface posseses both light reflecting and light 
transmitting properties and is positioned between fore and aft parallel 
planar external surfaces of said body of refractive material, thereby 
providing the wearer with a view through the eyepiece of the scene ahead 
of the wearer on which said view of the intensified image is superimposed.

The goggle has two light paths to the pilot's right and left eyes, 
respectively. Referring to FIG. 1, in each path there is an optical 
projector comprising an objective lens 11, an image intensifier 13 and a 
roof prism 15, and an eyepiece 17. 
An inverted laterally reversed image of the distant scene ahead of a wearer 
of the helmet on which the goggle is mounted, represented in FIG. 1 by the 
letter F, is directed onto the image intensifier 13 by the objective lens 
11. The intensified image at the output of the intensifier 13 is inverted, 
laterally reversed and folded through 90.degree. by the roof prism 15. The 
exit face 16 of the roof prism 15 is in approximately parallel 
relationship to an input face 19 of the eyepiece 17. 
The eyepiece 17 includes a central body 20 of light refractive material, 
preferably a transparent plastics material, the upper surface of the body 
20 constituting the input face 19 of the eyepiece 17. The body 20 also has 
an optically flat output face 23 and a collimating light reflective 
surface 25 concavely curved towards the face 23. A substantial part of the 
light from each point of the image of the outside world formed at the exit 
face 16 of the roof prism 15 is, after refraction at the input face 19 of 
the eyepiece 17, incident on the output face 23 at angles greater than the 
critical angle from glass to air. 
Such rays are, of course, internally reflected, the internally reflected 
rays being incident on the collimating reflective surface 25. After 
reflection at the surface 25 substantially all of the reflected light is 
transmitted to the helmet wearer's eyes by way of the output face 23. The 
wearer is thus provided with a view of the intensified image of the 
distant scene produced by the image intensifiers 13. 
The reflective surface 25 is a surface at the interface between the central 
body 20 of light refractive material and a further body 27 of light 
refractive material bonded to the body 20 by an optical cement, the body 
27 having an optically flat forward surface 29. 
The surface 25 has light transmitting as well as light reflecting 
properties. It may be a semi-silvered mirror coating or it may be a 
holographic coating. The latter variety coating has the advantage of 
offering better light transmission of the scene viewed directly through 
the eyepieces 17 as further described below. 
The eyepieces 17 also each include a wedge-shaped further body 30 of light 
refractive material disposed with one of its major faces in parallel 
spaced relation to the face 23 of the central body 20. The other major 
face 31 of the body 30 is parallel to the surface of the body 27 and 
constitutes the output face of the eyepiece 17. 
The helmet wearer is thus provided with a view of the distant scene 
directly through the eyepieces 17 on which his view of the intensified 
image is superimposed. 
Referring now also to FIGS. 2 to 5, in a practical embodiment of the goggle 
described above the optical elements in each light path to the observer 
are supported in relatively fixed relationship by a frame 33. 
Each objective lens 11 is housed within a cylindrical forward portion 35 of 
the frame 33. The associated image intensifier 13 is within a central 
cylindrical housing part 37. At the rear of the housing part 37 there is a 
dog-house 39 containing the roof prism 15, and a case 41 supporting the 
eyepiece 17 is secured to the dog-house 39 below the roof prism 15, 
intensified light being directed downwardly by the roof prism 15 onto the 
eyepiece. Light from the image at the exit face 16 of the prism 15 is 
presented to the observer after double reflected within the eyepiece 17, 
as previously described. 
The two frames 33 are hingedly connected to a bracket 45 releasably 
connected to the front of the pilot's helmet 47, above the face aperture 
49 of the helmet. The goggle is angularly displaceable about the hinge 
axis between an operational position in which the goggle is shown in full 
lines in the drawings, and a stowed position S shown by chain-dashed lines 
in FIG. 3. 
As may be seen, substantial parts of the dog-houses 39 containing the roof 
prisms 15 and of the eyepieces 17 with their cases 41 lie within the face 
aperture 49 of the helmet 47 under the front rim of the helmet. The eye 
clearance, indicated by C in FIG. 3, is accordingly less than that in the 
prior arrangement of GB No. 2,108,702-A previously mentioned, and thus 
permits the eyepieces 17 to be greatly reduced in size and weight without 
affecting the field of view. 
The optical systems for the right and left eyes respectively have divergent 
optical axes.