Note: Descriptions are shown in the official language in which they were submitted.
Z~043g
Description
A Helmet Mounted Display Having
Dual Interchangeable Optical Eyepieces
Technical Field
This invention relates to helmet mounted
displays, and more particularly to a helmet mounted
display having dual interchangeable optical
eyepieces.
Background Art
The use of helmet mounted displays (HMDs) in
modern high-performance aircraft and rotorcraft is
well known. The increasing complexity of these
crafts has led to an increased burden on the pilot
to visually interpret flight data from a large
number of sources. The HMD helps to alleviate this
burden by providing in the pilot's forward field of
view a dispiay of information essential for the
pilot's performance of such tasks as target --~
acquisition and weapon delivery. The HMD allows him
to spend more time piloting the craft in a head-up
~i mode, i.e., looking out at the exterior scene and
not looking down as often at the instrument panel. -
The information displayed by the HMD typically
consists of symbols relating to pilotage and weapon
targeting. This symbol information is fed by the
onboard flight~computer to a cathode ray tube (CRT)
image source. The CRT image is then projected
through a series of optical components, typically
including partially reflective/ partially
transmissive optical components located in front of
Hlg35-GC
2C~0439
the pilot's eyes. Viewing through the partially ~ ;
transparent components, sometimes referred to as a
"combiner", the pilot is presented with a virtual
image of the CRT image projected in his view of the -
5 external "real world" scene. :::~
Depending on the ambient light conditions in
which the pilot is flying, different requirements
are placed on the optical design of the HMD. For
the relatively high brightness daytime light (as
compared to nighttime light), the combiner must have
high transparency (see-through) since the pilot
views the external scene as well as the projected
symbol information. Consequently, the display ~ -
source must have high brightness (e.g., a CRT --
written in the stroke mode) so as to produce enough
contrast in the projected symbols.
For nighttime conditions, when flying by the
"naked eye" is dangerous or impossible, night vision
aids are required. In the present art, pilots
20 utilize Image Intensifier devices employed in Night -
Vision Goggles (NVGs). An advantage of the HMD is
that the nighttime viewing function can be
accomplished with the HMD optical system. For such
use, the external scene may be sensed by, for
example, image intensified television of forward
looking infrared devices. The output of these
devices i8 electronically processed and fed to an
image pro;ection source such as a raster mode CRT.
The proces~ing may also include the addition of
symbol data to the sensed image of the exterior
scene. The resulting CRT image is pro~ected in the
pilot's forward field of view through the HMD
pro~ection optical components.
-- 2 --
~:~' :'
-
2~)~0439
The raster mode CRT image~ are typically much
lower in luminance than the visual symbol
information produced by the stroke written CRT for
the daytime situation. (Typical stroke written
5 luminances can be 100 times as bright as typical
raster mode luminances.) Thus, the highly
transmissive, only partially reflective combiner
used in a HMD designed for day usage is necessarily
inefficient in transferring light from the CRT to
the eye. (For some typical combiner designs,
brightnes~ transfer from the CRT to the eye can be
less than ten percent). Consequently, the high
see-through viewing optics commonly used in prior
art HMD designs are not optimally suited for night ~ ;
flying conditions.
Another approach to night vision capability
involves the direct incorporation on the helmet of
image intensifier devices directly coupled to the
HMD optics. Such apparatus is disclosed and claimed
in a copending U.S. patent application of the same
assignee entitled: DIRECT INCORPORATION OF NIGHT
VISION IN A HELMET MOUNTED DISPLAY, U.S. Serial No.
(Attorney docket No. H1936GC) filed on even date
herewith by Fournier et al. There, the brightness
levels provided by the image intensifiers are on the
order of 1% or less of the typical CRT raster
brightness, and the brightness transmitted to the
eye from the high see-through viewing optics can be
inadequate for normal vision, much less for piloting
an aircraft.
The HMD prior art attempts to solve the
luminance transfer problem by using a refractive
relay system which uses only a single combiner
. .
20~0439
(e.g., the IHADSS HMD from Honeywell). However,
these HMDs introduce other problems, e.g., the
diameters of the relay lenses tend to be large, and
the eye relief (i.e., the distance from the
observer's eye to the nearest XMD optical component)
tends to be short. These dimensions are undesirable
when attempting to design an HMD to meet the
geometries imposed on the HMD by the human head.
Two completely different optical systems can be
used to meet the requirements of both day and night
viewing but this requires considerable extra
hardware which is both costly and difficult to stow
in the aircraft. It also raises some concerns about
the changeover from one HMD to another when
transitioning from daylight to nighttime. For
example, when the pilot is on a mission that begins
during daylight and runs through dusk into night,
the pilot must replace the entire day HMD with the ;
entire nighttime HMD. This changeover may occur at
a critical time and may be so cumbersome as to
require the pilot to land the aircraft to accomplish
- the changeover.
Disclosure of Invention
An ob;ect of the invention is to provide a HMD
comprising an arrangement of helmet mounted image
pro~ection optical components common for both
daytime and nighttime use and a pair of
interchangeable eyepieces, a first eyepiece
optimized for daytime light conditions, the day
30 eyepiece having high see-through transmission and ~ -`
intended primarily for viewing high luminance stroke -~
written data pro~ected in the external daytime
., -
20~043g
scene, a second eyepiece optimized for nighttimelight conditions, the night eyepiece being opaque
and providing for efficient transmission of the
light from the image source to the eye, the night
eyepiece being intended for use in viewing very low
luminance images such as are provided by image
intensifiers used in, for example, night vision
goggles, whereby it is a simple matter of
interchanging between the eyepieces depending on the
ambient light conditions.
Other objects of the invention include having
the eyepieces and common relay optics provide for
control of astigmatism and chromatic and spherical - ~ ~;
aberrations, and having the overall HMD provide for
reduced weight and bulk.
According to the invention, apparatus for
displaying an image in the forward field of view of
a human eye includes an image source for generating
a visual image, and an arrangement of optical
components disposed along an optical axis and
mounted to a helmet worn by a human, the arrangement
~ having a fir6t portion comprising optical components
:~ that are common to both daytime and nighttime use,
the arrangement also having a pair of
interchangeable eyepiece portions, a first one of
the:pair comprising optical components optimized for
daytime light conditions, a second one of the pair
comprising optical components optimized for
nighttime use, each one of the pair being disposed,
when mounted to the helmet, along the optical axis
following the fir~t portion of optical components to
- 5 -
2~)~0~39
present the image to the human forward field of
view.
These and other objects, features and
advantages of the present invention will become more
apparent in light of the detailed description of a
best mode embodiment thereof, as illustrated in the
accompanying drawing.
., ".
Brief Description of Drawing
Fig. 1 illustrates a perspective view of a
pilot of a modern high-performance aircraft wearing
helmet mounted display apparatus typical of that of
the prior art;
Fig. 2 illustrates a perspective view of HMD
apparatus in accordance with the present invention;
Fig. 3 is a cross sectional view of a portion
of the HMD apparatus;
Fig. 4 illustrates an optical ray trace of a
preferred embodiment of optical components;
Fig. 5 illustrates a second optical ray trace
of a preferred embodiment of optical components;
Fig. 6 illustrates an optical ray trace of an
alternative embodiment of optical components; and
Fig. 7 illustrates a second optical ray trace ;,
of an alternative embodiment of optical components.
':
Best Mode for Carrying Out the Invention
Fig. 1 illustrates a perspective view of an
aviator piloting à modern high-performance aircraft
while wearing a helmet mounted display (HMD) 10
typical of that found in the prior art. In the HMD
30 10, flight information is viewed through partially -
transparent optical eyepieces 11,12 located along
2~0439
the pilot's forward line of sight The flight
information is provided at the image surface of one
or more CRTs (not shown) to a series of optical
components (not shown) that relay the image to the
eyepieces 11,12. The CRTs and projection optics can
all be helmet mounted, or some portion of the -
display components can be located in the cockpit.
The HMD illustrated in Fig. ~ is solely for
daytime light conditions. For low luminance
nighttime conditions, the pilot desires an
intensified image of the exterior scene 80 as to
enable him to pilot the craft to the best of his
ability. In this case, the pilot is required to
remove the entire daytime HMD and replace it with an
entire HMD designed for nighttime light conditions.
This changeover can be awkward and dangerous when
performed during flight. Thus, in accordance with
the present invention, a HMD is provided having
helmet mounted pro~ection optics that allow for both
daytime and nighttime usage with a reduced amount of
necessary HMD component changeover.
Fig. 2 illustrates a perspective view of a
preferred embodiment of a HMD 20 in accordance with
the present invention. The HMD 20 mounts to the
outer surface of a known type aviator's helmet 21,
such as the model HGU55 provided by Gentex Corp. of
California. The helmet provides an opening in the
outer surface in proximity to the facial area. The
HMD comprises two CRT image sources 22,23 together
- 30 with a corresponding pair of identical optical
component arrangements 24,25, one for each eye.
Each CRT generates images of pilotage symbol
information. The optical component arrangement is
_ 7 _
2~0439
described in detail hereinafter with respect to the
cross-sectional illustration of Fig. 3, and the
optical ray diagram of Figs. 4,5.
Each arrangement 24,25 comprises, in part, a ;
"relay optic" portion 24a,25a h~ving optical
components (not visible) enclosed in aluminum and
used in both day and night HMD configurations. Each
arrangement also comprises a daytime "eyepiece"
portion 24b,25b having optical components optimized
for daytime light conditions, and a nighttime
eyepiece (not shown) 24c,25c having optical
components optimized for nighttime light conditions.
The daytime eyepieces 24b,25b mount in a first
interchangeable binocular goggle assembly 28a, and
the nighttime eyepieces 24c,25c mount in a second
interchangeable binocular goggle assembly 28b (not
shown). It is to be understood that the goggle
assemblies 28a,28b are similar in structure; the
~; difference lies in the optical components comprising
the eyepiece portions as described in detail
hereinafter with respect to Figs. 4,5 and Ta~les ~ -
I,II. Fig 2 illustrates the HMD with the goggle
assembly 28a separated from the helmet 21.
The goggle assembly mates with each relay optic
portion 24a,25a by engagement slides 29,30. A known
type, first ball detent 31 holds the goggle assembly
to a mounting block 32 on the front of the HMD. The
ball detent 31 and engagement slides 29,30 allow the
pilot to quickly remove the goggle assembly from the -
helmet. The front mounting block 32 attaches to the
front of the helmet using either a fixed screw mount ~;
or a second ball detent 33. A first rod 34 connects
the two relay optic portions together. The rod 34 -
- 8 -
: ~ ~
:::
2~)~0439 ~ ~
engages a hook 35 on the crown of the helmet. The
second ball detent 33 and rod/hook 34/35 permit the
pilot to quickly disengage the entire HMD 20 from
the helmet 21.
A second rod 36 passes through the front
mounting block 32 and connects to the two relay
optic portions. The first and second rods 34,36
permit the relay optic portions to slide
horizontally, thereby allowing the pilot to align
the two relay optic portions for his particular eye
spacing. This eye spacing is commonly referred to
as the interpupillary distance (IPD). The second
rod 36 has a knob (not shown) at one end to
facilitate the IPD adjustment through a range of
15 58.9 - 73.3 mm, which is suitable for a wide range
of pilot head sizes.
In order to allow IPD adjustment, the optical
components of the eyepieces are mounted in segmented
portions 39a,39b of the goggle assembly 28. The
segmented portions 39a,39b slide relative to one
another when a retaining screw 40 is loosened.
Thus, to adjust the IPD, the pilot loosens the
retaining screw 40 and ad~usts the knob on the
second rod 36 until the IPD is correct for his ~ ;~
25 particular eye spacing. Then the pilot retightens ;
the retaining screw.
Fig. 3 is a cross-sectional view of either one
of the optical component arrangement 24,25. The CRT
22 presents a visual image of flight information on
30 a plano concave fiber optic faceplate 41 that is a `
part of the CRT. The CRT is typically a Model
H-1380, one inch diameter, miniature CRT provided by
Huqhes Aircraft Company, Industrial Products
_ g _ "
2~)~0439
Division, Carlsbad, California. The CRT drive
electronics (not shown) are well known and are
located in the aircraft cockpit. The drive
electronics can operate the CRT in either the stroke
(high brightness) mode and raster mode. The CRT
image information is presented to the drive
electronics by the on-board flight computer. The
drive electronics connect to the CRT by a shielded
electrical cable 42. The CRT 22 attaches to the
relay optic portion 24a by means of a flange 43
secured with adhesive to the CRT and a nut assembly
44 which mates with threads 45 on the relay optic
portion. This attachment point is located at an
entrance aperture of the relay optic portion, as
illustrated by the split line 46.
Proceeding along an optical axis 50, a pair of
glass optical lenses 53,54 are positioned after the
CRT faceplate 41. The first lens 53 is positioned
with a machined seat 55. The ~econd lens 54 rests
against the first lens. A first tubular spacer 56
follows the second lens, followed by a third lens
58, a seccnd tubular spacer 59, and a second pair of
lenses 61,62. The two lenses comprising each of the
first and second lens pairs are normally positioned
next to each other and are made of different types
of glass or plastic material so as to reduce
chromatic abberrations.
A split line 63 designates a physical break in
the relay optic portion; the segment to the left of
the split line 63 is either press-fitted or secured
with adhesive into the segment to the right of the ~ ~ -
split line. A lens retaining nut 64 is positioned
-- 10 --
Z0~0439
inside the housing to hold the lenses 53,54,58,61,62
and spacers 56,59 in place.
Attached to a back surface 65 is a fold mirror
66 which is used to direct (fold) the optical axis
50 downward in the relay optic portion. The mirror
66 is attached using conventional optical component
mounting technique~ known in the art so as to
provide a low stress mount. A sixth lens 69 i8
positioned by a second lens retaining nut 70. The
relay optic portion then physically terminates at a
split line 72. Below the split line 72 i~ the
daytime eyepiece 24b. The split line can also be
considered illustrative of the location of an
intermediate image focal plane 72a of the relay
optic portion, and of an input aperture 72b of the
eyepiece.
Located underneath the split line 72 ic a first
eyepiece lens 74 positioned against a machined seat
75a with a retaining nut 75b. Also within the
eyepiece is a combiner 76 and a beamsplitter 77.
The beamsplitter 77 is held in place with clips 78a,
78b. The optical axis is illustrated as terminating
at a focal point 79 at the observer's eye (i.e., ;;
with an observer wearing the HMD of the present ~-
invention). It is to be understood that the optical
components comprising the nighttime eyepiece 25c are
positioned inside the eyepiece with similar types of ~ -~
machined seats and retainer nuts. ~ - -
~ ' ~1 1 ! ' ' :
Fig. 4 illustrates an optical ray!trace of a
3a preferred embodiment of the optical component
arrangement 24,25. The relay optic portion
components are above the split line 72, while the
daytime eyepiece components are below the split
:.~
.
Z0~0439
line. Also, surfaces and inter-component spacings
of each component are enumerated in Fig. 4.
TABLE I
.: ~
Surface Radius Curve Thick/ Mat'l
5No.fmm) TypeDist ~mm)
8440.000000CV 61.863200 AIR
8535.488000CX 3.000000 F4
8621.770000CV 1.792300 AIR
8733.991000CX 6.000000 SIO2
108885.875000CX 21.905600 AIR
89201.985000CV 5.500000 SIO2 ;
9024.617000CX 5.209200 AIR
91175.000000CX 4.500000 SIO2
9254.941000CX 1.433000 AIR
159330.044000CV 3.000000 F4
94384.855000CX 34.294000 AIR
-- --19.000000 AIR
96221.803000CV 5.000000 SIO2
9774.673000CX 81.356900 AIR
20982000.000000 CX 5.000000 ACRYL
~; 99221.803000CV 33.643100 AIR
100 -- --34.801353 AIR
101I28.165000 CV 35.941353 AIR
100 -- --3.000000 ACRYL
25Io32 ____ 5I.420000 AIR
Table I lists the prescription data for the ~;
optical~ components. Listed in order from left to
riqht are (1) the surface number, (2) the radius of
curvature in mm, (3) the type of curvature (CV ~
COncaVQ CX~- conve~), (4) the distance to the next
surface or thickness in mm, and (5) the type of
material between the surface and the next surface.
The type and radius of curvatures of the optical
~ . . . .
' ~'"
Zl)~0439
components is chosen in part to control astigmatism
and spherical aberrations.
Thus, referring to Fig. 4 and Table I, the
image produced by the CRT is presented on an outer
surface 84 of the plano concave (CV) fiber optic
faceplate 41 having a radius of curvature of 40 mm.
The faceplate is located approximately at an
entrance pupil 46a of the relay optic portion. The
optical rays then travel through air a distance of
10 61.8632 mm to a first surface 85 of the first glass
lens 53. All distances listed in Table I are
measured from the centers of each component. The
first surface 85 of the first lens has a convex
shape and a radius of curvature of 35.488 mm. Also,
the lens is 3.0 mm thick and is made of F4 glass.
The physical characteristics and spatial disposition
of the remaining optical components are determined
from Fig. 4 and Table I in a similar manner.
;~ It is to be noted that surface 95 is that of
20 the fold mirror, whose surface comprises an ~-
aIuminized reflective coating. Also, surface 100 is
listed twice in Table I in accord with the path ~ ~
taken by the light which is first reflected from ~ -
partially reflective surface 100 to surface 101,
then reflected from partially reflective surface 101
back toward the eye, passing through the
beamsplitter defined by surfaces 100 and 102. The
first listing indicates a 34.801353 mm ray travel
distance to sùrface 101, whereas the second listing
indicates a beamsplitter thickness of 3 mm. Also,
surface 103 indicates the exit pupil of the optical `
rays. The exit pupil is approximately 8 mm in
diameter.
- 13 - ~
:; . ..,~,.,
X~û439 : -
The relay optics are designed to produce a
focused CRT image at the point in the optical path
in proximity to the intermediate image focal plane
72a. The focused image has a magnification range of
0.5 - 4 of ~he image at the CRT faceplate 41. The
focused image at the focal plane 72a is at a
distance of 100 - 400 mm (i.e., the focal length of
the relay optics) along the optical path 50 from the
faceplate.
The daytime eyepiece essentially creates a
virtual image, in the observer's forward field of
view, of the focused image at the focal plane. This
virtual image is focused at a distance from the
observer's eye of from one meter to infinity, which
results in the image appearing in focus to the eye
; of the observer. Thus, the observer does not have ~-
to refocus his eyes to view the image generated by
the CRT that i8 superimposed on the observer's view
of~the external terrain. The virtual image of the ~;
occupies a portion of the observer's visual field
having a minimum subtense at the eye of ten (10)
degrees. The focal length of the eyepiece is --
approximately 100 mm, resulting in an overall
optical path length from the faceplate to the eye of
200 - 400 mm.
The CRT faceplate is shown with 19 mm diameter
which i8 the active image area of the miniature CRT. ~
The lens diameters are chosen to contain the rays -. -
with margin to permit retention in the relay optic
portions. The lenses comprising the relay optic
portion are all glass; either F4 or fused silica ~ ~;
; (SI02). The eyepiece components are all acrylic
plastic. Plastic elements were chosen for weight
- 14 -
: . : .,:
2~)P0439
and safety reasons. However, it is to be understood
that the eyepiece lenses can be glass without
detracting from the scope of the present invention.
All components other than the folding mirror 66
have a known antireflective coating. In addition,
the coatings on the surface 101 of the combiner 76 -
and the surface 100 of the beamsplitter 77 are
adjusted for a reflectivity of 20~-60% (40%
preferred) for visible light in the wavelength range
10 of 400 - 700 nanometers. The resulting partial
transmissivity of the beamsplitter and combiner
allow the observer to view external scenes disposed
beyond the daytime eyepiece. The day eyepiece can
be termed to be catadioptric due to the use of
partially transmissive/ partially reflective optical
components. Each optical component in Fig. 4 can be
built from the prescription data of Table I using
known techniques.
Fig. 5 illustrates an optical ray trace of the
optical components comprising the nighttime eyepiece
24c. Table II lists the corresponding prescription
data. Sincè the relay optic portion components are
similar as those of Fig. 4, the entries in Table II
for surfaces 84-96 are similar to those in ~able I.
Note that surface 114 of component 114a is an
aluminized reflective surface which totally reflects
the optical rays and blocks transmission of optical
rays of the external scene disposed beyond the night
eyepiece. Thus, component 114a is essentially
opaque. Also, the image projected into the
observer's forward field of view occupies an angle ~ ,Y;
in the observer's visual field having a minimum
subtense of twenty (20) degrees.
,
:: ' . .
~ . :~.:
- 15 -
2~)~0439
TABLE ~I
SurfaceRadius CurveThick/ Mat'l
No. (mm) Ty~eDist (mm)
8440.000000 CV61.863200 AIR
8535.488000 CX3.000000 F4
8621.770000 CV1.792300 AIR
8733.991000 CX6.000000 SI02
8885.875000 CX21.905600 AIR
89201.985000 CV5.500000 SIO2
9C24.617000 CX5.209200 AIR
91175.000000 CX4.500000 SIO2 ~-
9254.941000 CX1.433000 AIR
9330.044000 CV3.000000 ~4
94384.855000 CX34.294000 AIR
~5 95 -- --19.000000 AIR
96221.803000 CV5.000000 SIO2
9774.673000 CX93.000000 AIR
1121863.457201 CX7.000000 ACRYL
11339.515000 CX21.000000 AIR
114 -- __20.500000 AIR
115 -- --4.000000 ACRYL
11683.478000 CX0.500000 AIR
11739.515000 CX4.500000 ACRYL
118 -- --25.000000 AIR ~ - ;
103 -- ~
Fig. 6 illustrates an optical ray trace of an
alternative embodiment of a relay optic portion 120a ~ ~
and a daytime eyepiece 120b, separated at a split - - -
line 123. Table III list the corresponding
prescription data. Fig. 7 illustrates the
alternative embodiment nighttime eyepiece 120c.
Table`IV list the corresponding prescription data.
The alternative embodiments are comprised entirely ~ ~-
of conventional plastic optical elements. It should ~ -
be noted that the alternative embodiments
illustrated have a longer optical path length than
- 16 -
::: -
: ~.
Z~110439
the preferred embodiments of Figs. 4,5. Thisrequires the CRTs 22,23 and a portion of the relay
optic components to be mounted on the back side of
the helmet 21 for best fit to the outer surface of
the helmet.
TABLE III
Surface Radius Curve Thick/ Mat'l
No. (mm) Ty~e ~ist (mm~
84 40.000000 CV42.360000 AIR
10130149.880000 CX3.000000 POLYC
131 53.660000 CV18.418400 AIR
132110.449000 CX7.000000 ACRYL
133 57.279000 CX21.000000 AIR
134 ~ 43.000000 AIR
15135316.793000 CX4.000000 ACRYL
136316.793000 CX69.000000 AIR
137163.725000 CX7.000000 ACRYL
138 42.692000 CX0.000000 AIR ~
139 42.692000 CV2.000000 POLYC - -
20140135.158000 CX130.000000 AIR
141 -- --76.000000 AIR
142 65.596000 CV4.200000 ACRYL
143 55.139000 CX64.000000 AIR
~i 144 -- --25.300000 AIR
25145135.158000 CV26.500000 AIR
144 -- --3.000000 ACRYL
}46 -- --47.000000 A R ,~,.
.
.
- 17 -
: ~
: , :
~) iL0439
.
TABLE IV
SurfaceRadius CurveThick/ Mat'l
No. (mm) ~ypeDist (mm)
84 40.000000 CV42.360000 AIR
130149.880000 CX3.000000 POLYC
131 53.660000 CV18.418400 AIR
132110.449000 CX7.000000 ACRYL
133 57.279000 CX21.000000 ~ AIR
134 ~ 43.000000 AIR
135316.793000 CX4.000000 ACRYL
136316.793000 CX69.000000 AIR
137163.725000 CX7.000000 ACRYL
138 42.692000 CX0.000000 AIR
139 42.692000 CV2.000000 POLYC
140135.158000 CX130.000000 AIR
141 -- --114.000000 AIR :. -
173 84.312000 CX8.000000 POLYC .:.
174 84.312000 CX23.000000 AIR
175 -- --21.000000 AIR .
176 34.836000 CX5.000000 ACRYL
177 -- --24.000000 AIR ~ :
103 -- __ __
~- ,
The preferred embodiment of the daytime optical
components provide for data display in a 30-35
degree monocular field of view with 36% see-through
luminance transmission, and approximately 6% : .
luminance transfer from the CRT (60~ beamsplitter
transmission, 60% combiner transmission). The
nighttime optical components provide for :
approximately 40 degree monocular field of view with
no combiner see-through. The HMD is designed for
binocular viewing using two eyepieces with 100%
overlap of the left and right visual fields. Also,
the the optic~ are designed to accomodate a CRT ~:~
image source having an active image diameter in the
range of 16 - 25 mm, with the projection optical ~
- 18 - :
- :
2~)~0439
lenses having an effective focal length in the range
of 15 - 55 mm.
It will be appreciated by those skilled in the
art of optical design that it i8 a significant
design task to develop an HMD optical system that
produces an image of quality suitable for pilotage
while incorporating interchangeable eyepieces having
such different optical characteristics as the day
and night eyepieces herein disclosed. Considering
the optics of a particular configuration (i.e., the
common relay optics together with either the day
eyepiece or night eyepiece), the day and night
optics have substantially different effective focal
lengths, and the Petzval surfaces differ
sub~tantially. That is, the day and night optics
ideally would have an image input surface (the CRT
;- faceplate) of substantially different curvature.
Also, simultaneous control of various aberrations i8
a difficult task.
As disclosed, a CRT is used as the image
;~ source; however, other image sources could be used.
A CRT was chosen because of size, weight, cost,
resolution, and brightness considerations. Also, as `~
illustrated, the CRT is mounted on the helmet. ~ ~
25 However, the image source can be mounted off the ~-
helmet and not detract from the scope of the present -~
invention. In this case, the image would then be
presented to the helmet mounted projection optics by
a flexible fiber optic cable.
As illustrated, the daytime and nighttime
eyepieces are comprised of plastic optical elements. `~
Plastic was chosen for weight and safety reasons.
However, these components can be made of a suitable
2~)~0439
light weight glass; the choice of material is not
critical to the practice of the present invention.
Also, the eyepieces are disclosed as being housed
together on sliding segments in an interchangeable
goggle assembly. However, each eyepiece can be
mounted individually to the corresponding relay
optic portion in a suitable interchangeable
structure. By retaining the connection (in the form
of the two mounting rods) between the two housing
portions, the desired binocular unit with IPD
adjustment can be achieved. However, by removing
the connection between the housing portions, a
monocular HMD may be obtained by mounting only one
arrangement 24 to the helmet 21. Ths resulting
monocular HMD arrangement is within the scope of the
present invention.
The material enclosing the relay optic portion
illustrated in Fig. 3 is aluminum. However, a
molded plastic or other suitable material that
provides structural integrity consistent with low
weight may be utilized. The choice of material
itself forms no part of the present invention.
Although the invention has been shown and
described with respect to a best mode embodiment ~ -
thereof, it should be understood by those skilled in
the art that the foregoing and various other
changes, omissions, and additions in the form and
detail thereof may be;~made therein without departing
from the spirit and scope of the invention.
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