Helmet supported optical systems with four reflections

A helmet supported optical system which provides an eye of a wearer (11 or 49) of a helmet (7 or 35) with a direct view of the scene forward of the wearer (11 or 49) on which is superimposed collimated optical imagery. The system comprises: an eyepiece (1 or 47a, 47b); means (5 or 37a, 37b, 39a, 39b, 41a, 41b, 43a, 43b) operable to develop a bright data representation at a plane (31or 44a, 44b) substantially congruent with the principal focal plane of an element (21 or 46a, 46b) of the eyepiece (1 or 47a, 47b); and, disposed between the means (5 or 37a, 37b, 39a, 39b, 41a, 41b, 43a, 43b) operable and the eyepiece (1or 47a, 47b), a body (3 or 45a, 45b) of light refractive material. The system has a configuration which facilitates improvement in respect of all up weight and/or moments of the system about the helmet wearer's (11 or 49) neck pivot position vis-a-vis known helmet supported optical systems, as above which do not include the body (3 or 45a, 45b) of light refractive material disposed between the means operable (5 or 37a, 37b, 39a, 39b, 41a, 41b, 43a, 43b) and the eyepiece (1 or 47a, 47b).

Referring to FIGS. 1 to 4, the first system comprises an eyepiece 1, a 
glass body 3 and a cathode ray tube 5, supported by means (not shown) on a 
helmet 7. The eyepiece 1 is supported in front of a position 9 for an eye 
of a wearer 11 of the helmet 7, defined with respect to axes of the helmet 
7. 
The eyepiece 1 comprises a three part 13a,13b,13c composite glass body 
having a light input face 15, a boundary surface 17 opposite the input 
face 15, and essentially flat and parallel fore 18 and aft 19 faces. At 
confronting faces 20a,20b of the parts 13a,13b is an aerially extensive 
region constituted by an optical coating 21 on one of the faces 20a,20b 
possessing both light transmissive and light reflective properties. The 
faces 20a,20b and hence optical coating 21 are concavely, specifically 
substantially spherically, curved towards the aft face 19. The coating 21 
can be a neutral density coating or preferably a dichroic filter coating 
or even a diffractive coating. The parts 13a,13b are united by optical 
cement. The confronting faces 22a,22b, of the parts 13b,13c, are separated 
by a very small air space 23. 
The glass body 3 has a first face 25 by way of which can enter the body 3, 
a second face 27 having a mirror coating and a third face 29. The second 
27 and third 29 faces are inclined at acute angles .theta..sub.1 and 
.theta..sub.2 respectively to the first face 25 and at an obtuse angle 
.theta..sub.3 to one another. 
The cathode ray tube 5 has a flat output face 31. 
The third face 29 of the glass body 3 is positioned parallel to and in 
close proximity with the light input face 15 of the eyepiece 1 and the 
first face 25 of the glass body 3 is positioned parallel to and in close 
proximity with the output face 31 of the cathode ray tube 5. 
The system is such that the output face 31 of the cathode ray tube 5 is 
congruent with the principal focal plane of the coating 21. 
The operation of the system is as follows. 
The wearer 11 directly views a distant forward scene through the eye piece 
1. 
The cathode ray tube 5 develops a bright data representation at its output 
face 31 and light therefrom is incident on and passes through the first 
face 25 of the glass body 3 so as to be incident on and reflected by the 
mirror coating of the second face 27 so as to be incident again on the 
first face 25. The angles of reflection of the light from the second face 
27 are such that the angles of incidence of the light on the first face 25 
are greater than the air/glass critical angle, so that the light is 
totally internally reflected at the first face 25 so as to be incident on 
the third face 29 of the glass body 3. 
The light incident on the third face 29 passes through the third face 29 
and through the light input face 15 of the eyepiece 1 so as to be incident 
on the confronting face 22a of the eyepiece 1. At the confronting face 22a 
the light is totally internally reflected so as to be incident on the 
optical coating 21 of the eyepiece 1 at which the light is partially 
reflected so as to traverse the air space 23 and enter the eye of the 
wearer 11 situated at position 9. 
Thus, an image of the bright data representation developed by the cathod 
ray tube 5 is superimposed on the wearer's view of the distant forward 
scene through the eyepiece 1. 
Since the output face 31 of the cathode ray tube 5, at which face 31 the 
bright data representation is developed, is congruent with the principal 
focal plane of the coating 21 of the eyepiece 1, the light reflected by 
the coating 21 which enters the eye of the wearer 11 will be collimated 
and thus the image of the bright data representation superimposed on the 
wearer's 11 view of the distant forward scene through the eyepiece 1 will 
be a virtual image at infinity and therefore parallax free. 
The bright data representation could, for example, comprise synthetic data 
appropriate for head-up display systems. 
The bright data representation need not, of course, be developed at the 
output face 31 of the cathode ray tube 5 but could, particularly where 
improved collimation is required, for example, be developed at a plane 
defined by a relay lens interposed between the output face 31 of the 
cathode ray tube 5 and the glass body 3. 
Referring to FIGS. 5 and 6, the second system comprises in respect of each 
of two eye positions 33a,33b defined with respect to axes of a helmet 35 
supporting the system, a forward looking objective lens 37a,37b, an amici 
prism 39a,39b, a condenser lens 41a,41b, an image intensifier 43a,43b 
having an output face 44a,44b, a glass block 45a,45b as the glass block 3 
of the first system, and an eyepiece 47a,47b as the eyepiece 1 of the 
first system. The objective lens 37a,37b and the eyepiece 43a,43b both 
look forward in directions parallel to one another and the output face 
44a,44b of the image intensifier 43a,43b is congruent with the principal 
focal plane of an optical coating 46a,46b of the eyepiece 43a,43b 
corresponding to the optical coating 21 of the eyepiece 1 of the first 
system. 
In operation, the lens 37a,37b, prism 39a,39b, lens 41a,41b and intensifier 
34a,43b develop a bright data representation of the distant forward scene 
as seen by the lens 37a,37b at the output face 44a,44b of the intensifier 
43a,43b. 
The second system thus comprises a night vision system wherein a virtual 
image at infinity of the bright data representation of the distant forward 
scene is superimposed on and in register with the view of the wearer 49 of 
the helmet 35 of the distant forward scene through the eyepieces 43a,43b. 
The systems described by way of example exhibit significant improvement 
compared with comparable prior art systems as regards their all up weights 
and moments about the helmet wearer's neck pivot position. This 
improvement arises in part from the presence of the glass body 3 in the 
first described system and the glass bodies 45a, 45b in the second system. 
The presence of such a body in a system being found to assist 
significantly in arriving at a satisfactory design configuration in an 
ergonomic manner. 
It will be understood that requirement for a low all up weight and low 
moment about neck pivot position arises particularly in the context of a 
combat aircraft where the wearer of a helmet system according to the 
invention is likely to be subject to high accelerations.