Headgear with spherical semi-reflecting surface

Headgear incorporating an optical display system comprising, an optical data source mounted within the headgear above the level of the wearer's eyes, an image combiner having a spherical reflecting surface directed inwardly of the helmet and intercepting the wearer's forward line-of-sight, and a prism positioned and dimensioned so that a collimated, aberration free virtual image of the data source is presented to the wearer as a result of reflection of rays transmitted from the data source by way of the prism to the spherical reflector. Such aberration as is introduced by the prism compensates for aberration introduced by the spherical reflector.

This invention relates to headgear for an occupant of a vehicle, for 
example the pilot of an aircraft, incorporating an optical display system 
for superimposing optical data on a distant scene viewed by the wearer. 
According to the present invention, headgear for the occupant of a vehicle, 
incorporates an image combiner part, supported by the headgear and 
comprising a semi-reflecting concave curved surface directed inwardly of 
the headgear, a substantially planar optical data source located at a 
position of the headgear such that, when the headgear is being worn, the 
said source is above the level of the wearer's eyes, and a prism also 
located at a position of the headgear above the level of the wearer's 
eyes, and in which, when the headgear is being worn, the image combiner 
part is at or is movable to a position at which it intercepts the forward 
line-of-sight of an eye of the wearer, and the prism has a shape and 
dimensions such, and the data source, the prism, and the combiner part are 
so positioned relatively to one another that a collimated virtual image of 
the data source is presented to the wearer as a result of transmission 
through the prism and reflection at said semi-reflecting surface, of rays 
from the data source. 
Conveniently the image combiner part is integral with a transparent visor 
supported by the headgear, and may be movable to a position out of said 
forward line-to-sight. The prism may be operative to compensate for 
aberration of said rays from the data source introduced by said 
semi-reflecting surface. 
In a preferred embodiment, the concave semi-reflecting surface is 
spherical, the interface of the prism through which said rays emerge from 
the prism on their path to the wearer's eye then conveniently being so 
disposed with respect to the concave spherical surface that refraction of 
the rays at the interface compensates for spherical aberration introduced 
by reflection at said spherical surface. 
Preferably the headgear includes a refractive interface through which rays 
from the data source are transmitted on their path to the said eye of the 
wearer, this refractive interface being so inclined with respect to the 
plane of the data source as to compensate for distortion of the image of 
the data source due to variations in the optical path length of different 
rays therefrom. 
The said refractive interface may comprise the interface of the prism 
through which said rays enter the prism on their path to said eye of the 
wearer. 
The prism may conveniently be movable to an inoperative position in which 
it lies outside the field of view presented to the said eye of the wearer 
by reflection at said concave surface when the display system is not in 
use. 
In a preferred embodiment of the invention the headgear comprises a 
protective helmet the data source then being mounted in a cavity formed in 
the body of the helmet and protected from the head of the wearer by a 
layer of impact absorbent material.

The protective helmet for the pilot of an aircraft is of known general 
construction. Between the outer plastic skin 11 of the helmet and the 
pilot's head there is a moulded layer 13 of expanded plastic material e.g. 
expanded polyurethane or expanded polystyrene, and a moulded layer 15 of 
rubber. 
A moulded plastic skin 17 is secured to the skin 11 and is so shaped as, 
with the skin 11, to define a cavity 19 occupying the sincipital region of 
the helmet. A visor 21, in one of its extreme pivotal positions, is 
located substantially entirely within the cavity 19; at its other extreme 
pivotal position the visor is, as shown, before the pilot's eyes. 
In the sincipital region of the helmet, the plastic skin 11 and the 
underlying expanded polyurethane or similar layer 13 have a generally 
semi-circular forward boundary 23. A formed steel sheet number 25 secured, 
as by rivets 27, around the skin 11 at its boundary 23 constitutes the 
support member for certain optical elements of the optical display system. 
The visor 21 incorporates a semi-reflective image combiner part 29 of an 
optical display system. As represented the combiner part is a glass 
element let into an aperture formed in the visor during its moulding. The 
part 29 is so located as, when the visor is in its operative position (as 
shown), to intercept the pilot's forward line-of-sight. 
The combiner part 29 has a concave spherical reflector surface 29a which is 
directed inwardly of the helmet; the principal axis 29b of the surface 29a 
is, as shown, inclined at an acute angle with respect to the forward 
line-of-sight of the pilot. Conveniently the visor 21 has, in the vicinity 
of the combiner part, a shape as indicated. 
The optical data source 21 of the optical display system comprises a 
two-dimensional array of light-emissive diodes (LED's), supported on a 
face of a control box 33 which is itself secured to the support member 25 
and houses LED addressing circuitry operable selectively to activate the 
several diodes of the array so as to form any one or more of a range of 
predetermined optical symbols which, it may be wished to present to the 
pilot for his instruction. 
The addressing circuitry within the control box 33 is energizable by 
further electronic elements housed within the rearward part of the cavity 
19 by way of electrical conductors 35 leading through an aperture in the 
support member 25. 
As shown, the data source 31 and the control box 33 to which it is secured 
are within the cavity 19 formed between the member 25 and the protective 
rubber layer 15. 
The virtual image of any optical symbol developed at the planar surface of 
the data source 31 must appear to be superimposed on the distant scene as 
viewed by the wearer of the helmet through the image combiner part 29, 
i.e. the wearer should be presented with a virtual image at infinity of 
the optical data source. For this purpose the optical data source 31 must 
lie in the principal focal plane of the optical display system so that 
light received therefrom as by the wearer's eye is collimated. 
For practical reasons, the virtual image at infinity should be 
substantially free from distortion; it should, for all practical purposes, 
be not different in its proportions, from the display presented at the 
data source 31, and the optical display system must be compatible with the 
location of the source 31 at the position shown. The extremely limited 
space available and the minimisation of the risk of injury to the wearer 
dictates that the range of positions available for the source 31 and other 
parts of the optical system within the helmet are extremely limited. The 
configuration of elements shown in the figures approximates, quite 
closely, to what, it is believed, is the best packaging arrangement for 
the optical display system within such a helmet. 
As shown, the plane of the optical data source is made the principal focal 
plane for the spherical reflector 29a by a prism 37. The manner in which 
the prism is supported with respect to the member 25 is discussed below. 
So far as the optical properties of the prism 37 are concerned, attention 
is particularly directed to the glass-air interface 39, its distance from, 
and its angle of inclination with respect to the principle axis of the 
reflector surface 29a. This relationship is a particularly important one 
in the design of the system. With an optical system in which the virtual 
image of source 31 is viewed `off-axis` through the combiner 29 the 
position of the above interface 39 can be chosen so that spherical 
aberration introduced by the prism 37 at the interface 39 substantially 
compensates for aberration introduced by the spherical reflector surface 
29a. 
The surfaces 41 and 43 of the prism 37 constitute internally reflective 
surfaces, folding the optical path so that the source 31 can be positioned 
as shown and, further, so as to enable the plane of the source 31 to be 
the principal focal plane for the spherical surface 29a. Rays from each 
point of the source 31 are able to enter the prism 37 through the surface 
45. Two such points, 47a, 47b, are indicated; and from each point rays A, 
A' and B, B' are traced to the helmet wearer. 
The plane of the data source 45 is inclined with respect to the interface 
45 so as to compensate for distortion of the data source due to variations 
in the optical path between the wearer's eye and different points on the 
data source introduced by the prism 37. 
It will be noted that the divergent rays e.g. A,A' and B,B' from each point 
are reflected by the spherical reflector surface 29a as parallel rays to 
the wearer. So, for a range of vertical relative positions between the 
observer's eye and the helmet, the virtual image of any display developed 
at the data source 31 appears undisturbed in relation to the distant scene 
as viewed through the combiner part 29. The helmet and the optical display 
system may, properly designed, accommodate, in particular, variations in 
the head dimensions of potential users of the helmet and also of relative 
movement, e.g. under acceleration, between the helmet and the head of its 
wearer. 
The optical display system discussed so far is capable of dealing with 
spherical aberration and coma; it is not intended to deal with astigmatism 
in the system. Astigmatism may, however, be corrected by introducing into 
the optical system a cylindrical element; and this may be done at any one 
of several places. The interface 39 may have a cylindrical curvature in 
its transverse direction; or the reflective surfaces 41 or 43 may be so 
curved. Yet again, there may be a separate cylindrically curved optical 
element in the system. None of these possibilities is expressed in the 
drawing; in any event, the radius of cylindrical curvature would be very 
large, amounting to, perhaps, a radius of about 80 inches. 
These, then, are the optical considerations typically to be borne in mind 
in arriving at a satisfactory optical display system in accordance with 
the invention. As a practical matter it is desirable that the prism 37 
should be displaceable so as, when the sight is not in use, to occupy a 
retracted or parked position. 
To this end the prism 37 is secured to the rotary part 49 of a journal 
bearing 51 the stationary part 53 of which extends through an aperture 55 
in the member 25. Friction washers 57a and 57b are urged by a wavy washer 
59 so as to hold the prism against rotation from the desired position. The 
friction level is nevertheless such that the prism 37 may be rotated from 
the operative position shown in the figures to the parked position 
indicated (FIG. 1) as a chain dotted line. A cap and screw connector 61 
secure the prism support to the support member 25 of the helmet. The cap 
has two abutment portions 63a, 63b, which limit the rotation of the prism 
and, in particular, define the operative and parked positions for the 
prism. 
It will be appreciated that modifications may be made to the optical 
display system without departing from the scope of the invention. Thus, 
although it is preferred for simplicity to employ only a single optical 
element, i.e. the prism 37, for reflectively directing optical data from 
the source onto the image combiner surface 29, and for providing 
compensation for aberration introduced by the surface 29, separate optical 
elements may be used for these purposes. 
It should also be appreciated that the concave semi-reflecting surface of 
the combiner part 29 may be aspherical, e.g. paraboloidal, although it is 
generally preferred to use spherical surfaces since such surfaces are 
simpler to construct, and any optical aberrations introduced thereby can 
be more readily compensated as demonstrated by the simplicity of the 
system described. 
Furthermore, although the data source 31 is shown in the form of an LED 
display, it may take any other suitable form, e.g. a cathode ray tube or 
electroluminescent display. Alternatively the data source may simply 
comprise a screen or the like, to which light signals, generated at a 
location remote therefrom, are transmitted for example, by optical fibres.