COMPACT HEAD-MOUNTED DISPLAY SYSTEM
Field of the Invention
The present invention relates to substrate-guided optical devices, and
particularly to devices which include a plurality of reflecting surfaces carried by a
common light-transmissive substrate, also referred to as a light-guide element.
The invention can be implemented to advantage in a large number of imaging
applications, such as portable DVDs, cellular phones, mobile TV receivers, video
games, portable media players or any other mobile display devices.
Background of the Invention
One application for compact optical elements concerns head-mounted displays
(HMDs), wherein an optical module serves both as an imaging lens and a combiner,
wherein a two-dimensional image source is imaged to infinity and reflected into the
eye of an observer. The display source may originate directly from a spatial light
modulator (SLM), such as a cathode ray tube (CRT), a liquid crystal display (LCD),
an organic light emitting diode array (OLED), a scanning source or similar devices,
or indirectly, by means of a relay lens, an optical fiber bundle, or similar devices.
The display source comprises an array of elements (pixels) imaged to infinity by a
collimating lens and transmitted into the eye of the viewer by means of a reflecting,
or partially reflecting, surface acting as a combiner for non-see-through and seethrough
applications, respectively. Typically, a conventional, free-space optical
module is used for these purposes. As the desired field-of-view (FOV) of the system
increases, however, such a conventional optical module becomes larger, heavier and
bulkier, and therefore, even for a moderate performance device, is impractical. This
is a major drawback for all kinds of displays and especially in head-mounted
applications, wherein the system should necessarily be as light and as compact as
possible.
The strive for compactness has led to several different complex optical
solutions, all of which, on the one hand, are still not sufficiently compact for most
practical applications, and, on the other hand, suffer major drawbacks with respect to
manufacturability. Furthermore, the eye-motion-box (EMB) of the optical viewing
angles resulting from these designs is usually very small - typically less than 8 mm.
Hence, the performance of the optical system is very sensitive, even for small
movements of the optical system relative to the eye of a viewer, and does not allow
sufficient pupil motion for comfortable reading of a text from such displays.
The teachings included in Publication Nos. WO 01/95027, WO 03/081320,
WO2005/024485, WO2005/024491, WO2005/024969, WO2005/124427,
WO2006/013565, WO2006/085309, WO2006/085310, WO2006/087709,
WO2007/054928, WO2007/093983, WO2008/023367, WO2008/129539,
WO2008/149339 and WO2013/175465, all in the name of Applicant, are herein
incorporated by reference.
Disclosure of the Invention
The present invention facilitates the exploitation of very compact light-guide
optical element (LOE) for, amongst other applications, HMDs. The invention allows
relatively wide FOVs together with relatively large EMB values. The resulting
optical system offers a large, high-quality image, which also accommodates large
movements of the eye. The optical system disclosed by the present invention is
particularly advantageous because it is substantially more compact than state-of-theart
implementations and yet it can be readily incorporated even into optical systems
having specialized configurations,
A broad object of the present invention is therefore to alleviate the drawbacks
of prior art compact optical display devices and to provide other optical components
and systems having improved performance, according to specific requirements.
In accordance with the present invention, there is provided an optical system,
comprising a light-transmitting substrate having at least two major surfaces and
edges; an optical prism having at least a first, a second and a third surface, for
coupling light waves having a given field-of-view into the substrate by total internal
reflection; at least one partially reflecting surface located in the substrate, the partially
reflecting surface being orientated non-parallelly with respect to the major surfaces of
said substrate, for coupling light waves out of the substrate; at least one of the edges
of the substrate is slanted at an oblique angle with respect to the major surfaces; the
second surface of the prism is located adjacent to the slanted edge of the substrate,
and a part of the substrate located next to the slanted edge is substantially transparent,
characterized in that the light waves enter the prism through the first surface of the
prism, traverse the prism without any reflection and enter the substrate through the
slanted edge.
Brief Description of the Drawings
The invention is described in connection with certain preferred embodiments,
with reference to the following illustrative figures so that it may be more fully
understood.
With specific reference to the figures in detail, it is stressed that the particulars
shown are by way of example and for purposes of illustrative discussion of the
preferred embodiments of the present invention only, and are presented in the cause
of providing what is believed to be the most useful and readily understood description
of the principles and conceptual aspects of the invention. In this regard, no attempt is
made to show structural details of the invention in more detail than is necessary for a
fundamental understanding of the invention. The description taken with the drawings
are to serve as direction to those skilled in the art as to how the several forms of the
invention may be embodied in practice.
In the drawings:
Fig. 1 illustrates a span of optical rays which are coupled into an LOE, according
to the present invention;
Fig. 2 illustrates a span of optical rays which illuminates the input aperture of
an LOE;
Fig. 3 illustrates a prior art side view of an exemplary coupling-in mechanism
comprising a prism optically attached to one of the major surfaces of the LOE;
Fig. 4 is an another schematic diagram illustrating a side view of a prior art
exemplary coupling-in mechanism comprising a prism optically attached to one of the
major surfaces of the LOE;
Fig. 5 illustrates a span of optical rays illuminating the input aperture of an LOE
wherein one of the edges of the LOE is slanted at an oblique angle with respect to the
major surfaces;
Fig. 6 is a schematic diagram illustrating another system with a span of optical
rays illuminating the input aperture of an LOE, wherein one of the edges of the LOE is
slanted at an oblique angle with respect to the major surfaces;
Fig. 7 is a schematic diagram illustrating an embodiment of an optical system
coupling-in input light waves from a display light source into a substrate, having an
intermediate prism attached to the slanted edge of the LOE, in accordance with the
present invention;
Fig. 8 illustrates another embodiment of an optical system coupling-in input
light waves from a display light source into a substrate, having an intermediate prism
attached to the slanted edge of the LOE, in accordance with the present invention;
Fig. 9 is a schematic diagram illustrating a device for collimating input light
waves from a display light source by utilizing a polarizing beamsplitter, in accordance
with the present invention, and
Fig. 10 is a schematic diagram illustrating a device for collimating input light
waves from liquid crystals on silicon (LCOS) light source, in accordance with the
present invention and
Figs. 11A and 11B are two embodiments showing a top view of eyeglasses
according to the present invention.
Detailed Description of Preferred Embodiments
The present invention relates to substrate -guided optical devices, in particular,
compact HMD optical systems. Usually, a collimated image having a finite FOV is
coupled into a substrate. As illustrated in Fig. 1, the image inside an LOE or,
hereinafter, a substrate 20 contains a span of plane waves having central waves 14
and marginal waves 16 and 18. The angle between a central wave 14 of the image
and the normal to the plane of the major surfaces 26, 32 is a jn . The FOV inside the
substrate 20 is defined as2- Aa . Consequentially, the angles between the marginal
waves 16 and 18 of the image and the normal to the plane of the major surfaces are
a in +Aa and ain - Aa , respectively. After several reflections off the surfaces 26, 32
of the substrate 20, the trapped waves reach an array of selectively reflecting
surfaces 22, which couple the light waves out of the substrate into an eye 24 of a
viewer. For simplicity, only the rays of the central waves 14 are plotted as being
coupled-out from the substrate.
The object of the present invention is to find a light wave coupling-in
mechanism which is different to the coupling-in mechanism of the prior art and
having more compact dimensions. In Fig. 2, there is illustrated a span of rays that
have to be coupled into substrate 20, with a minimal required input aperture 21. In
order to avoid an image with gaps or stripes, the points on the boundary line 25,
between the edge of the input aperture 21 and the lower surface 26 of the
substrate 20, should be illuminated for each one of the input light waves by two
different rays that enter the substrate from two different locations: one ray 30 that
illuminates the boundary line 25 directly, and another ray 31, which is first reflected
by the upper surface 32 of the substrate before illuminating the boundary line 25.
The size of the input aperture 21 is usually determined by two marginal rays: the
rightmost ray 34 of the highest angle of the FOV, and the leftmost ray 36 of the
lowest angle of the FOV.
A possible embodiment for coupling the marginal rays into the substrate 20 is
illustrated in Fig. 3. Here, the input light waves source 38, as well as a collimating
module 40, e.g., a collimating lens, are oriented at the required off-axis angle
compared to the major surfaces 26, 32 of the substrate 20. A relay prism 44 is
located between the collimating module 40 and the substrate 20 and is optically
cemented to the lower surface 26 of the substrate 20, such that the light rays from the
display source 38 impinge on the major surface 26 at angles which are larger than the
critical angle, for total internal reflection inside the substrate. As a result, all the
optical light waves of the image are trapped inside the substrate by total internal
reflection from the major surfaces 26 and 32. Although the optical system illustrated
here is simple, it is still not the most compact coupling-in mechanism. This is an
important point for optical systems which should conform to the external shape of
eyeglasses, as well as to hand-held or other displays.
In order to minimize the dimensions of the collimating module 40, the aperture
Df of the input surface 46 of the coupling-in prism 44 should be as small as possible.
As a result, the dimensions of the coupling-in prism would also be minimized
accordingly, while the coupled rays of the entire FOV will pass through the couplingin
prism 44.
As illustrated in Fig. 4, in order for the rightmost ray 34 of the highest angle of
the FOV to pass through the prism 44, the aperture DL of the output surface 21 of the
prism 44 must fulfil the relation
wherein d is the thickness of the substrate 20.
In addition, in order for the leftmost ray 36 of the lowest angle of the FOV to
pass through the prism 44, the angle asurl between the left surface 48 of the prism 44
and the normal to the major surface 26 of the substrate 20 must fulfil the relation
For minimizing the chromatic aberrations of the optical waves passing through
the prism 44, it is advantageous to orient the input surface 46 of the coupling-in prism
44 to be substantially normal to the central wave 14 of the image. As a result, the
angle aslir2 between the entrance surface 46 of the prism 44 and the normal to the
major surface 26 of the substrate 20 is
Taking the inequality of Eq. 2 to the limit, in order to minimize the dimensions
of the prism 44 yields the following internal angles of the prism: the angle between
the surfaces 46 and 21 is \ the angle between surface 48 and 21 is
Consequentially, the angle between surfaces 46 and 48 is
Utilizing these values yields
Taking the inequality of Eq. 1 to the limit and inserting it in Eq. 4 yields
Although the optical system illustrated in Figs. 3 and 4 seems to be simple, it
is still not the most compact coupling-in mechanism, since it is important for such
optical systems to conform to the external shape of displays such as eyeglasses or
hand-held displays.
Fig. 5 illustrates an alternative embodiment of coupling light waves into the
substrate through one of its edges. Here, the light waves-transmitting substrate 20
has two major parallel surfaces 26 and 32 and edges, wherein at least one edge 50 is
oriented at an oblique angle with respect to the major surfaces and wherein asur3 is the
angle between the edge 50 and the normal to the major surfaces of the substrate.
Usually the incoming collimated light waves are coupled directly from the air, or
alternatively, the collimating module 40 (Fig. 3) can be attached to the substrate 20.
As a result, it is advantageous to couple the central wave 14 normal to the slanted
surface 50 for minimizing chromatic aberrations. Unfortunately, this requirement
cannot be fulfilled by coupling the light directly through surface 50. Usually, even
for coupled images having a moderate FOV, the angle ain (Fig. 3) between the
central wave 14 of the image and the normal to the plane of the major surfaces has to
fulfil the requirement n . As a result, if the central wave 14 is indeed normal
to the slanted surface 50, then the relation must be fulfilled.
Consequentially, the outcome will be the fulfillment of the relations in the system
an d, for a comparatively wide FOV, even
Fig. 6 illustrates the complex situation wherein the maximal angle between the
trapped rays and the major surfaces 26, 32 is larger than the angle between the input
surface 50 and the major surfaces. As illustrated, the points on the boundary line 25,
between the edge of input aperture 50 and the lower surface 26 of substrate 20, are
illuminated only by the leftmost ray 35 of the wave that directly illuminates the
boundary line 25. The other marginal ray 34, which impinges on the edge 51 of the
input surface 50, is first reflected by the upper surface 32 prior to illuminating the
lower surface at a different line 52 which is located at a distance Ax from the
boundary line 25. As illustrated, the gap Ax is not illuminated at all by the trapped
rays of the marginal wave 34. Consequentially, dark stripes will appear and the
coupled-out waves and the image quality will be significantly inferior.
This situation is solved by the embodiment shown in Fig. 7. An intermediate
prism 54 is inserted between the collimating module 40 (Fig. 3) and the slanted
edge 50 of the substrate. One of the prism's surfaces 56 is located adjacent to the
slanted edge 50 of the substrate 20. In most cases, the refractive index of the
intermediate prism should be similar to that of the substrate 20. Nevertheless, there
are cases wherein a different refractive index might be chosen for the prism, in order
to compensate for chromatic aberrations in the system. The incoming light waves are
coupled directly from the air, or alternatively, the collimating module 40, can be
attached to the intermediate prism 54. In many cases, the refractive index of the
collimating module 40 is substantially different than that of the substrate 20, and
accordingly, is different from that of the prism 54. Therefore, for minimizing the
chromatic aberrations, the input surface 58 of the prism 54 should be oriented
substantially normal to the central light wave of the incoming ray. In addition, the
leftmost ray of the lowest angle of the FOV should pass through the prism 54. As a
result, the conditions of Eqs. (2) and (3) should be fulfilled also for the configuration
of Fig. 7. To eliminate the undesired phenomena of dark stripes as described with
reference to Fig. 6, the relation
must be satisfied, namely, the angle between the slanted edge of the substrate and the
normal to the major surfaces of the substrate is larger than the highest angle of the
FOV. Accordingly, the aperture D s of the output surface 56 of the prism 54 must
fulfil the relation
Apparently, since the light waves enter the prism 54 through the entrance
surface 58 of the prism, directly cross the prism without any reflections and enter the
substrate through the slanted edge 50, the expansion of the active area Dp of the
entrance surface 58 in relation to the aperture Ds of the exit surface 56, is minimal. In
addition, as described above, in order for the leftmost ray 36 (Fig. 4) of the lowest
angle of the FOV to pass through the prism 54, the angle a suri between the left
surface 60 of the prism 54 and the normal to the major surface 26 of the substrate
must also fulfil the relation of Eq. (2), namely, the angle between the surface 60 of
the prism 54 and the normal to the major surfaces of the substrate, is smaller than the
lowest angle of the FOV. Therefore, when the relations of Eqs. (2), (6) and (7) are
fulfilled, the coupled-in light waves from the entire FOV will completely cover the
major surfaces of the substrate without any stripes or gaps.
As illustrated in Fig. 8, by taking the inequalities of Eqs. (2), (6) and (7) to the
limit, the internal angles of the prism 54 are: the angle between the surfaces 56
and 58 is 2a in - 90° +Aa and the angle between surface 56 and 60 is
180°-2ain . Consequentially, the angle between surfaces 58 and 60 is 90°-Da.
Utilizing these values yields
wherein DP is the active area of the input surface 58 of the intermediate
prism 54.
Therefore, by comparing Eqs. (5) and (8), the relation between the active areas
Dp and DT of the input surfaces of the prisms 54 and 44 of the prior art system of
Fig. 4, respectively, is:
Apparently, for a narrow FOV, that is, Da « ain , the improvement is
negligible. However, for a relatively wide FOV the active area DP of the prism 54
should be reduced considerably compared to the active area DT of the prism 44. For
example, for and the reduction ratio of Eq. (9) has a significant
value ofD
In the embodiment illustrated in Fig. 3, the collimating module 40 is shown to
be a simple transmission lens, however, much more compact structures utilizing
reflective lenses, polarizing beamsplitters and retardation plates may be employed. In
such a structure, the fact that in most microdisplay light sources, such as LCDs or
LCOS light sources, the light which is linearly polarized, is exploited by optical
component 61, as illustrated in Fig. 9. As shown, the s-polarized input light waves 62
from the display light source 64, are coupled into a light-guide 66, which is usually
composed of a light waves transmitting material, through its lower surface 68.
Following refiection-off of a polarizing beamsplitter 70, the light waves are coupledout
of the substrate through surface 72 of the light-guide 66. The light waves then
pass through a quarter-wavelength retardation plate 74, reflected by a reflecting
optical element 76, e.g., a flat mirror, return to pass again through the retardation
plate 74, and re-enter the light-guide 66 through surface 72. The now p-polarized
light waves pass through the polarizing beamsplitter 70 and are coupled out of the
light-guide through surface 78 of the light-guide 66. The light waves then pass
through a second quarter-wavelength retardation plate 80, collimated by a
component 82, e.g., a lens, at its reflecting surface 84, return to pass again through
the retardation plate 80, and re-enter the light-guide 66 through surface 78. The now
s-polarized light waves reflect off the polarizing beamsplitter 70 and exit the lightguide
through the exit surface 86, attached to the intermediate prism 54. The
reflecting surfaces 76 and 84 can be materialized either by a metallic or a dielectric
coating.
In the embodiment illustrated in Fig. 9, the display source can be an LCD
panel, however, there are optical systems, especially wherein high brightness imaging
characteristics are required, where it is preferred to utilize an LCOS light source
device as a display light source. Similar to LCD panels, LCOS light source panels
contain a two-dimensional array of cells filled with liquid crystals that twist and align
in response to control voltages. With the LCOS light source, however, the cells are
grafted directly onto a reflective silicon chip. As the liquid crystals twist, the
polarization of the light is either changed or unchanged following reflection of the
mirrored surface below. This, together with a polarizing beamsplitter, causes
modulation of the light waves and creates the image. The reflective technology
means that the illumination and imaging light beams share the same space. Both of
these factors necessitate the addition of a special beamsplitting optical element to the
module, in order to enable the simultaneous operations of the illuminating, as well as
the imaging, functions. The addition of such an element would normally complicate
the module and, when using an LCOS light source as the display light source, some
modules using a frontal coupiing-in element or a folding prism, would become even
larger. For example, the embodiment of Fig. 9 could be modified to accommodate an
LCOS light source by inserting another beamsplitter between the display source 64
and the beamsplitter 66. However, this modified version may be problematic for
systems with a comparatively wide FOV, wherein the focal length of the collimating
module is shorter than the optical path of the rays passing through the of double
beamsplitter configuration.
To solve this problem, as seen in Fig. 10, a modified optical component 90 is
provided, wherein only one reflecting surface 84 is located adjacent to surface 78 of
the light-guide 66. Hence, the optical path through this light-guide 66 is much
shorter. As shown, the s-polarized light waves 92, emanating from a light source 94,
enter the prism 96, reflect off the polarizing beamsplitter 98 and illuminate the front
surface of the LCOS light source 100. The polarization of the reflected light waves
from the "light" pixels is rotated to the p-polarization and the light waves are then
passed through the beamsplitter 98, and consequentially, through a polarizer 102
which is located between the prisms 96 and 66 and blocks the s-polarized light which
was reflected from the "dark" pixels of the LCOS light source 100. The light waves
then enter the prism 66 and pass through the second beamsplitter 70, are coupled out
of the prism through surface 78 of the prism 66, pass through a quarter-wavelength
retardation plate 80, collimated by a collimating lens 82 at its reflecting surface 84,
return to pass again through the retardation plate 80, and re-enter the prism 66
through surface 78. The now s-polarized light waves reflect off the polarizing
beamsplitter 70 and exit the prism 66 through the exit surface 86, which is attached to
the intermediate prism 54.
Returning now to Fig. 9, wherein the viewer's eye 24 is located at the same
side of the slanted edge 50, the dimensions of the optical prism 66 are substantially
extended over the lower major surface 26 of substrate 20 and only slightly extended
over the upper surface 32. This slight extension can be completely eliminated with a
proper design, for instance, by slightly increasing the angle asmi of the slanted
edge 50.
For the embodiment which is illustrated in Fig. 10, however, the optical
component 90 is substantially extended over the lower surface 26 of the substrate 20,
as well as over the upper surface 32.
As illustrated in Fig. 11A, this unique configuration may be preferred for
optical systems wherein a collimating module is composed of the optical component
90 of Fig. 10, having prisms 66 and 96. Optical component 90 is installed between
the eyeglasses frame 104 and the substrate 20. In this case, the viewer's eye 24 is
located on the opposite side of the slanted edge 50 of the substrate 20. The light
waves are coupled into the substrate 20 through the slanted edge 50 towards the
major surface 32, from which surface 32, it bounces towards the partially reflecting
surfaces 22 and from there exit the substrate through the major surface 32 towards the
viewer's eye 24. Even though there is a front extension 106 of the optical component
90 to the front part of the eyeglasses, the rear extension 108 of the prism 96 is
minimal, and the entire optical component 90, can easily be integrated inside the
frame 104 of the eyeglasses.
Seen in Fig. 1IB is a modification based on the optical module illustrated in
Fig. 9, wherein the viewer's eye 24 is located on the same side of the slanted edge 50
of the substrate 20. The light waves emanating from the optical component 90 are
coupled into the substrate 20 through the slanted edge 50, enter the substrate 20
towards the major surface 26, from which surface it bounces towards the major
surface 32 and from there it continues towards the partially reflecting surfaces 22, and
exit the substrate though the major surface 32 towards the viewer's eye 24.
It will be evident to those skilled in the art that the invention is not limited to
the details of the foregoing illustrated embodiments and that the present invention
may be embodied in other specific forms without departing from the spirit or
essential attributes thereof. The present embodiments are therefore to be considered
in all respects as illustrative and not restrictive, the scope of the invention being
indicated by the appended claims rather than by the foregoing description, and all
changes which come within the meaning and range of equivalency of the claims are
therefore intended to be embraced therein.

WHAT IS CLAIMED IS:
1. An optical system, comprising:
a light-transmitting substrate having at least two major surfaces and edges;
an optical prism having at least a first, a second and a third surface, for coupling
light waves having a given field-of-view into the substrate by total internal reflection;
at least one partially reflecting surface located in the substrate, the partially
reflecting surface being orientated non-parallelly with respect to the major surfaces of
said substrate, for coupling light waves out of the substrate;
at least one of the edges of the substrate is slanted at an oblique angle with
respect to the major surfaces;
the second surface of the prism is located adjacent to the slanted edge of the
substrate, and
a part of the substrate located next to the slanted edge is substantially
transparent,
characterized in that the light waves enter the prism through the first surface of
the prism, traverse the prism without any reflection and enter the substrate through
the slanted edge.
2. The optical system according to claim 1, wherein the field-of-view is defined
by a lowest and a highest angle of the light waves coupled into the substrate.
3. The optical system according to claim 2, wherein the angle between the
slanted edge of the substrate and the normal to the major surfaces of the substrate is
larger than the highest angle of the field-of-view.
4. The optical system according to claim 2, wherein the angle between the third
surface of the prism and the normal to the major surfaces of the substrate is smaller
than the lowest angle of the field-of-view.
5. The optical system according to claim 1, wherein the first surface of the prism
is substantially normal to the central wave of the light waves.
6. The optical system according to claim 1, wherein the coupled-in light waves
from the field-of-view substantially cover the entire major surfaces of the substrate
without forming any stripes or gaps.
7. The optical system according to claim 1, further comprising a collimating
module.
8. The optical system according to claim 7, wherein the collimating module is
composed of a light waves transmitting material, having at least one light waves
entrance surface, at least one light waves exit surface and a plurality of external
surfaces.
9. The optical system according to claim 8, wherein the first surface of the prism
is positioned adjacent to the exit surface of the collimating module.
10. The optical system according to claim 8, wherein the collimating module
comprising:
at least one light waves reflecting surface carried by the optical device at one
of said external surfaces;
at least one retardation plate carried by the optical device on at least a portion
of an external surface;
at least one light waves collimating component covering at least a portion of at
least one of the retardation plates, and
at least one light waves polarizing beamsplitter, disposed at an angle to at least
one of the light waves entrance or exit surfaces.
11. The optical system according to claim 7, further comprising a display light
source.
12. The optical system according to claim 11, wherein light waves emerging from
the display source are collimated by the collimating module and coupled into the
substrate through the prism.
13. The optical system according to claim 1, wherein the refractive index of the
prism is similar to the refractive index of the substrate.
14. The optical system according to claim 1, wherein the refractive index of the
prism is different than the refractive index of the substrate.
15. The optical system according to claim 1, wherein the refractive index of the
prism is substantially different than the refractive index of the collimating module.
16. The optical system according to claim 1, wherein the light waves are coupled
out from the substrate by at least one partially reflecting surface into an eye of a
viewer.
17. The optical system according to claim 16, wherein the viewer's eye is located
at the same side of the slanted edge of the substrate.
18. The optical system according to claim 16, wherein the viewer's eye is located
at the opposite side to the slanted edge of the substrate.
19. The optical system according to claim 16, wherein the collimating module is
substantially extended beyond the two major surfaces of the substrate.