Night vision viewing systems

A night vision viewing system, suitable for mounting on an observer's headgear (11), wherein light rays from a forward scene pass to the observer's eye (E) via a path comprising a forward looking objective lens (23), an inclined reflector (35) which directs the rays passing through the objective onto an outwardly sideways looking input face of an image intensifier (31), and a fibre optic stub (33) which directs rays from the output face of the intensifier onto a second reflector (27) which directs the rays downwardly into an eyepiece (21) which is in the observer's forward line of sight. The eyepiece incorporates an internal surface (37) having both light reflective and light transmissive properties which directs rays entering the eyepiece from the intensifier to the observer's eye. The fibre optic stub serves to effect rotation of the intensified image to present an intensified image of correct orientation to the observer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to night vision viewing systems. 
More particularly, the invention relates to night vision viewing systems of 
a kind suitable for support on headgear to provide a wearer of the 
headgear with an intensified view of a scene ahead of the wearer 
superimposed on and in register with the wearer's direct view of said 
scene. 
2. Description of Related Art 
A known such system comprises an eyepiece arranged to intercept a forward 
line of sight of an observer using the system and an optical sub-system 
including a forward looking objective lens fixed with respect to said 
eyepiece and having an optical axis offset with respect to said forward 
line of sight, and an image intensifier means disposed optically between 
said objective lens and said eyepiece so that light rays of an intensified 
real image of a scene forward of the observer viewed by said objective 
lens are directed into said eyepiece via an input surface thereof, thereby 
to provide the observer with a view of said intensified image which is 
superimposed on and in register with the observer's direct view of said 
forward scene through said eyepiece. 
Such a night vision viewing system is hereinafter referred to as a night 
vision viewing system of the kind specified. 
Known night vision viewing systems in which the system is supported on 
headgear have certain inherent shortcomings when used in environments such 
as the cockpit of a high performance aircraft. Inevitably the centre of 
gravity of the night vision system is at a position forward of the neck 
pivot position of the headgear wearer. The weight of the night vision 
system is, generally speaking, of the order of 2 lbs; and, heretofore the 
moment arm about the neck pivot position has been around five inches. In 
static conditions therefore the user must develop a balancing torque of 
about 10 pounds inches. Under high gravitational force manoeuvre 
conditions any unbalanced mass of quite small magnitude may constitute a 
danger to the wearer. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a night vision viewing 
system of the kind specified wherein these shortcomings are alleviated. 
According to the present invention, in a night vision viewing system of the 
kind specified said image intensifier means is arranged so as to be 
sideways looking with respect to said forward line of sight. 
Preferably said light rays are directed downwardly from said sub-system 
onto said input surface, said input surface being an upper surface of the 
eyepiece. 
In one particular system in accordance with the invention said optical 
sub-system further includes a first light reflecting means supported above 
said input surface of the eyepiece which serves to direct light rays 
received from an output face of said image intensifier means onto said 
input surface; and a second light reflecting means which serves to direct 
light rays received from said forward scene via said objective lens onto 
an input face of said image intensifier means. In such a system image 
rotation means is preferably provided between said output face and said 
first light reflecting means. Said image rotation means suitably comprises 
a coherent fibre optic stub optically connected at one end to said output 
face. 
In one particular embodiment of said particular system in accordance with 
the invention said first light reflecting means has light transmitting 
properties as well as light reflecting properties, and the system includes 
means for displaying a real image at a position from which light rays can 
pass via said reflecting means into said eyepiece via said input surface 
for viewing by the observer. 
In a night vision viewing system according to the invention said image 
intensifying means is preferably arranged to be outwardly looking with 
respect to the observer's forward line of sight through said eyepiece. 
The invention also provides a night vision viewing system comprising two 
night vision viewing systems according to the invention, one for each eye 
of the observer. 
In a night vision viewing system in accordance with the invention the or 
each eyepiece is preferably of the kind comprising a body of light 
refractive material having: a substantially flat upper input surface via 
which said rays enter the eyepiece; fore and aft substantially flat 
surfaces spanned by said input surface; and an internal surface between 
said fore and aft surfaces which is concavely curved towards said aft 
surface and which possesses both light reflecting and light transmitting 
properties; said input, fore, aft and internal surfaces being angularly 
mutually disposed so that said rays from said intensified real image, 
after entering said body, are internally reflected forwardly towards said 
internal surface, are reflected at said internal surface rearwardly 
towards said aft surface, and finally are transmitted through said aft 
surface to an observer's design eye position. 
Such an eyepiece is hereafter referred to as an eyepiece of the kind 
specified. 
Night vision viewing systems incorporating eyepieces of the kind specified 
are described in GB-A-No. 2108702 and GB-A-No. 2144558. 
In GB-A-No. 2108702 the eyepiece is in two parts; the internal surface is 
in the form of a coating of appropriate light transmissive and light 
reflective properties at a curved interface formed by complementary 
convexly and concavely curved surfaces of the two parts. In such an 
eyepiece, rays entering the refractive body at the upper surface are 
totally internally reflected at the aft surface. 
In GB-A-No. 2144558 the eyepiece is in three parts, there being two 
internal surfaces one of which is the said surface concavely curved 
towards the aft surface and the other of which is a substantially flat 
surface between the concavely curved internal surface and the aft surface. 
With this eyepiece light rays entering the body at the upper surface are 
totally internally reflected at the said other internal surface towards 
the concavely curved surface and are there reflected rearwardly for 
transmission through said other internal surface and then said aft surface 
to the observer's eye. 
The eyepieces of the night vision viewing systems of GB-A-No. 2108702 and 
GB-A-No. 2144558 are functionally similar, but the eyepiece of GB-A-No. 
2144558 may be made substantially more compact than that of GB-A-No. 
2108702 the latter eyepiece having, in the context of headgear systems at 
least, a substantial angle of rake to its fore and aft surfaces, as a 
result of its two-part construction. In the eyepiece of GB-A-No. 2144558 a 
much smaller angle of rake of the fore and aft surfaces may be achieved 
and the eyepiece may be brought closer to the eye position. Accordingly, a 
larger field of view may be so obtained, retaining the overall dimensions 
in the eyepieces; or (perhaps more important) a smaller eyepiece may be 
employed to achieve a field of view equivalent to that obtainable with a 
substantially larger eyepiece of the two-part construction of GB-A-No. 
2108702. 
Thus an eyepiece of the kind described in GB-A-No. 2144558 is in general 
preferred for use in a system according to the present invention. 
It should be understood that whilst the several said surfaces of an 
eyepiece of the kind specified are stated to be substantially flat it may 
be convenient or advantageous to give these surfaces or any of them a 
degree of curvature in order to ameliorate optical defects necessarily 
present in the system. Astigmatism, in particular, must be present; the 
off-axis nature of system in which the eyepiece is employed makes this 
unavoidable. Accordingly references, in this specification, to flat 
surfaces, should be construed with the foregoing in mind, it being a 
matter of preference and not of the essence that the said surfaces should 
be other than flat since corrective optical elements could be located at 
other places in the optical system. 
The combined effect of the several elements employed in a night vision 
system according to the invention is such that the system is of minimal 
weight and of minimal moment arm about the neck pivot position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, the system is mounted on a helmet 11 and is a 
binocular night vision system comprising two night vision systems 13a, 
13b, one for each eye of the wearer of the helmet 11. 
The night vision viewing systems 13a, 13b are secured to a cowl portion 15 
of the helmet 11 which is pivotally connected at a transverse axis to the 
helmet 11 and from which a vizor 17 is dependant. The angular position of 
the cowl 15, and hence the vizor 17 and the systems 13a, 13b is adjustable 
on release of a catch 19. 
The night vision systems 13a, 13b, each comprise an eyepiece 21, a forward 
looking objective lens 23, and an optical sub-system 25 which comprises a 
first inclined reflector 27, an image intensifier 31, a coherent fibre 
optic stub 33 having one of its ends connected to the output face of the 
image intensifier 31 with an optical cement, and a second inclined 
reflector 35. 
The eyepiece 21 consists of light reflective material, preferably a 
transparent plastics material, and includes an internal surface 37 having 
both light transmissive and light reflective properties. The surface 37 is 
at the interface between two parts A and B of eyepiece 21, one part A 
having an optically flat upper surface 39 which constitutes a first input 
surface of the eyepiece 21, an optically flat aft surface 41 and a fore 
surface concavely curved towards the aft surface 41, which fore surface is 
bonded to a complementary aft surface of the part B to form the surface 
37, said other part B having an optically flat fore surface 43 which 
constitutes a second input face of the eyepiece 21. The eyepiece 21 is 
completed by a third wedge shaped part C having a fore major surface 45 in 
parallel spaced relation to the surface 41 and an aft major face 47 
parallel to the surface 43. 
The required optical properties at the surface 37 are suitably conferred by 
a semi-silvered mirror coating or a holographic coating. 
The inclined reflector 27 is supported above the upper surface 39 of the 
eyepiece 21. The image intensifier 31 is laterally positioned with respect 
to the reflector 27 and is sideways outwardly looking, i.e. rightwards for 
the right eye system 13a and leftwards for the left eye system 13b. The 
second inclinded reflector 35 is at the intersection of the optical axis 
of the objective 23 and the longitudinal axis of the image intensifier 31. 
In operation of the binocular night vision system rays from a forward 
distant scene, represented by the upper case letter F in the drawings, are 
received at the objective lens 23 of each of the component night vision 
systems 13a, 13b, and are brought to a focus as an inverted image of the 
scene at an image place coincident with the input face of the image 
intensifier 31 by reflection from the said second reflector 35. 
An intensified representation of the said inverted image is developed at 
the output face of the intensifier 31 and transmitted through the coherent 
fibre optic stub 33 to the reflector 27. 
The reflector 27 is at the intersection of the longitudinal axis of the 
fibre optic stub 33 and a direction normal to the upper surface 39 of the 
eyepiece 21. 
The position of the free end face 49 of the fibre optic stub 33 is 
coincident with the focal plane of concavely curved surface 37 within the 
eyepiece 21. Rays from the end face 45, after reflection at inclined 
reflector 27, enter the eyepiece 21 via the surface 37 and are totally 
internally reflected at the surface 41 of the eyepiece towards the curved 
surface 37. The rays incident on the surface 37 from the surface 41 are 
reflected backwards to be transmitted through the surface 41 and then the 
rear part C of the eyepiece 21 to a design eye position E. 
Since the ray originate, so far as the eyepiece 21 is concerned, at the 
principal focal plane of the eyepiece 21 on intensified virtual image of 
the forward scene as viewed through the objective lens 23 is seen at the 
design eye position. 
The fibre optic stub 33 is of a twisted construction, the angle of twist 
being such that the combined optical effect of the stub 33, the reflector 
27 and the eyepiece 21 on rays from the intensified but inverted 
representation at the intensifier output face of the forward distant scene 
F is such that a virtual erect image of the intensified representation is 
seen at the design eye position E. Moreover, because of the collimation 
effected in the system, essentially by the curved surface 37, the erect 
virtual intensified image, by virture of the parallel relationship of the 
eyepiece overall fore and aft surfaces 43 and 47, is in register with the 
view from the eye position E of the forward distant scene directly through 
the eyepiece 21 provided by light rays from the scene passing through the 
eyepiece 21 via surfaces 43, 37, 41, 43, 45 and 47, in turn. 
In practice the image rotation required in the stub 33 is of the order of 
90.degree., the precise angle being determined by the detailed 
configuration. 
The system further includes a display arrangement of presenting further 
optical data to one eye of the helmet wearer, superimposed on the direct 
and intensified views of the distant scene, the casing of the arrangement 
being referenced 51 in FIGS. 1, 2 and 3. 
Referring to FIG. 5, the arrangement comprises a cathode ray tube (CRT) 53 
and a relay lens arrangement 55 comprising two spaced lens groups 57a, and 
57b and a reflector 59 between the said two spaced lens groups 57a, 57b, 
for bending the optical path between the lens groups 57a, 57b. 
The display arrangement forms a real image of luminous optical data 
presented at the screen of the CRT 53 at image plane 61 above the 
reflector 27 of the night vision viewing system 13b. The reflector 27 has 
light transmissive properties, as well as light reflective properties, so 
that light rays from the image plane 61 pass through the reflector 27 to 
enter the eyepiece 21 of the night vision viewing system 13b via its 
surface 39, thereby presenting a collimated virtual image of the CRT 
screen display at the design eye position E for the helmet wearer's left 
eye. It will be understood in this connection that the optical path length 
from the image plane 61 to the eyepiece internal curved surface 37 is the 
same as the optical path length from the free end face 49 of the fibre 
optic stub 33 to the surface 37 to preserve the collimated character of 
the display. 
The display arrangement may be a biocular one, as depicted in FIG. 6, 
instead of a monocular one. In the arrangement of FIG. 6 rays from a 
single CRT 63 transmitted by a relay lens arrangement 65 and intermediate 
reflector 67 are received by a prism arrangement 69 composed of four prism 
elements 71 having semi-silvered coatings at their interfaces. Two full 
reflectors 73 are disposed laterally with respect to the prism arrangement 
69, one on either side thereof, to reflect rays projected outwards from 
the prism arrangement downwards to respective image planes 75a and 75b 
respectively located above the reflectors 27 of the two night vision 
systems 13a and 13b, both reflectors 27 having light transmissive as well 
as light reflective properties so that a collimated virtual image of the 
CRT screen display is presented to each eye of the helmet wearer. 
It is known to counter-balance, at least to a first order, the first and 
second moments of helmet mounted night vision viewing systems by counter 
weights disposed at the rear of the helmet shell. It is known to utilise, 
as far as practicable, system elements such as power supplies, cables and 
so on for this purpose. 
The embodiment of the present invention shown in the drawings follows this 
practice. To this end helmet 17 has at it rear a compartment 77 housing 
the image intensifier power supplies, batteries and so on (not shown), and 
CRT drive and control electronic circuitry (not shown). Cabling 79 extends 
from the compartment 77 to the binocular night vision system and cabling 
81 links the electronics in the compartment 77 to electronic circuitry, 
e.g. a signal processor (not shown), not mounted on the helmet 11, for 
generating the signals required to display optical data on the screen of 
the CRT 53 or 63 of the display arrangement. 
The configuration of a binocular night vision system according to the 
invention, particular in respect of the position of the or each image 
intensifier and when using an eyepiece or eyepieces of the kind specified, 
offers the possibility of achieving an optimal centre of gravity position 
in relation to the neck pivot position of a user of the system. Moreover, 
the weight of the system is low by comparison with known systems offering 
a similar angular field of view. 
It will be understood that whilst the system described by way of example is 
a binocular night vision viewing system other systems in accordance with 
the invention may comprise a biocular night vision system, or a monocular 
night vision system leaving the other eye of the user by uninterrupted 
view of the forward distant scene, e.g. through the vizor 17. Similarly a 
display arrangement for optical data need not be provided in a night 
vision viewing system in accordance with the invention.