Image intensifier binocular

Method of designing a compact binocular night vision instrument with an electronic image intensifier (2) which is oriented so that its symmetry axis forms a right angle with the line of sight (19) between the night vision binocular and the scene to be observed. The total forward length thereby becomes particularly short, thus reducing the downward torque which is troublesome when the night vision binocular is carried like a pair of spectacles, attached to the head (generally called night vision goggles). The total forward length may be even further reduced if the symmetry axis of the objective lens (1) is also at right angles with the line of sight (19). These measures makes feasible a simple beam splitter system where the optical axis through one of the eye pieces (13B) intersects the common optical axis beam splitter (14), and to design an instrument enclosed in a single housing having a smooth exterior shape which is easy to grasp.

BACKGROUND OF THE INVENTION 
Electronic nightvision devices, where a scene is projected by a fast 
objective lens on the photo-cathode of an image intensifier and where a 
phosphorous screen of the same intensifier is viewed through a magnifier 
or a microscope, are used in the dark, with the night sky and the stars as 
the only illumination. 
Such an instrument may have one eyepiece or it may be binocular, whereby 
the light from the phosporous screen is divided by a beam splitter to two 
eyepieces so that the observer may watch the phosphorous screen with both 
eyes. The device may be designed for handheld use--usually with several 
times magnification--or it may be fitted to the head like spectacles or 
attached to a helmet. In this latter application the device is called 
night visiion goggles, usually with unit magnification so that the 
observer is viewing the night scene in natural scale. 
Since the observer in many applications must carry the night vision goggles 
for extended time periods, it is particularly important that they be as 
light as possible and that they not protrude unnecessarily in front of the 
observer, because the associated downward torque would cause neck strain. 
However, the components of the goggles objective lens, image intensifier 
tube, microscope unit, batteries etc.--are by necessity comparatively 
heavy and are according to prevailing design principles, so oriented that 
the whole device protrudes about 12-20 cm in front of the eyes of the 
observer. The experience with such an instrument is often that the front 
weight is embarrassing: Furthest away from the observer is a fast and thus 
comparatively heavy objective lens, followed by an image intensifier and 
finally a binocular microscope system with a beam splitter closest to the 
observer. The optical axis through the objective lens and the image 
intensifier are thus parallel with the line of sight against the scene 
being viewed. 
SUMMARY OF THE INVENTION 
According to the invention, the optical axis of the image intensifier and 
possibly also the optical axis of the objective lens are oriented at right 
angles relative to the line of sight. Through this simple measure, the 
length of the device in front of the observer is significantly reduced to 
about 6 cm, i.e., less than half of a conventional design. The center of 
gravity is moved correspondingly closer to the observer, which makes it 
considerably easier to carry the night vision goggles without observer 
fatigue, this being the main purpose of the invention. 
The invention furthermore enables significant simplification of the design 
of the binocular microscope system through which the phosphorous screen is 
being viewed: The orientation of the image intensifier in a horizontal 
plane and at right angles relative to the line of sight makes the entrance 
of rays into the microscope asymmetric in relation to the observer, i.e., 
from the side of the instrument. This fact makes it feasible to arrange a 
more efficient beam splitter system with very few optical components. 
Conventional nightvision goggles have symmetric microscope systems such 
that the rays from the image intensifier enter the microscope viewing 
system at the center. Such an arrangement necessitates the use of a larger 
number of optical components than is the case according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS 
The single FIGURE shows an exemplary embodiment of the invention. The 
instrument has the approximate shape of a horizontal bar having a 
substantially rectangular profile in front of the eyes of the observer. 
The instrument lacks such delicate protruding details as the objective 
lens. The binocular microscope system, including the eyepieces 13A and 
13B, is located in the same horizontal plane as the eyes of the observer. 
The objective lens 1 and the image intensifier 2 are located in a plane 
immediately above. The largest dimension of the instrument is along a 
horizontal direction from side to side, where it however does not collide 
with an eventual helmet (in a vertical direction), nor creates any 
annoying torque (in a forward direction along the line of sight). 
The two eyepieces 13A and 13B, the beam splitter prism 14, the eyepiece 
prisms 15 and 16, the collimator lens 7 and the transport lens 8 are all 
located in the lower plane. 
The collimator lens 7 projects the image intensifier screen 6 at infinite 
image distance while the transport lens 8 thereafter focuses the rays on 
the image planes of the eyepieces 13A and 13B after the ray partition in 
the beam splitter prism 14. The light between the screen 6 and the 
collimator lens 7 is transferred by way of the two mirrors or prisms 
17,18. These enable the image intensifier 2 to be located above the 
microscope system so that the optical axes through the image intensifier 
and the common ray portion of the microscope define a plane at right 
angles to the line of sight 19. 
The mirrors 17 and 18 may, however, be twisted so that the microscope 
instead comes above or on the side of the image intensifier 2. 
The selected beam splitter 14 allows large exit pupils to be used and 
reflects part of the light beam in a direction away from the observer. The 
eyepiece prism 15 will, however, reflect this same light beam two more 
times, causing it to intersect a line defined by the common optical axis 
of the microscope through the beam splitter prism 14. This light beam 
finally passes the eyepiece 13B to one eye of the observer. 
That light beam being transmitted through the beam splitter 14 will reach 
the second eyepiece 13A via a single reflection in the prism or mirror 16. 
In front of the image intensifier tube 2, the objective lens 1 is located, 
which via the mirror or prims 20 projects an image of the scene on the 
photocathode 3. This scene to be watched is located on the line of sight 
19 leaving the instrument. 
In some applications--as when it is desirable to increase the objective 
lens 1 diameter or locate it in a symmetric position between the eyes--it 
can be advantageous to place the mirror or prism 20 between objective lens 
1 and photocathode 3. The optical axis of the objective will in thise case 
coincide with the line of sight 19. The prism or mirror 20 may 
alternatively be positioned within the objective lens 1. It may also be 
advantageous to let the axis through the image intensifier 2 deviate from 
the horizontal plane, however maintaining the right angles relative to the 
line of sight 19. 
The beam splitter 14 may be arranged in several other ways, for example, it 
may be located between lenses 7 and 8. 
The design shown and described is thus different from other night vision 
goggles, being more compact--particularly forward along the line of sight. 
It is consequently easier to fit on the head or in a helmet, while at the 
same time the observer gets less tired during prolonged use. The night 
vision goggles according to the invention are moreover less costly to 
produce because the optical components included are comparatively few and 
all of them are assembled inside a single housing.