Compact Head-Mounted Display System Having Uniform Image
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 lighttransmissive
substrate, also referred to as a light-guide optical element (LOE).
The invention can be implemented to advantage in a large number of imaging
applications, such as, for example, head-mounted and head-up displays, cellular phones,
compact displays, 3-D displays, compact beam expanders as well as non-imaging
applications such as flat-panel indicators, compact illuminators and scanners.
Background of the Invention
One of the important applications for compact optical elements is in head-mounted
displays, wherein an optical module serves both as an imaging lens and a combiner, in
which a two-dimensional display is imaged to infinity and reflected into the eye of an
observer. The display can be obtained directly from either a spatial light modulator (SLM)
such as a cathode ray tube (CRT), a liquid crystal display (LCD), an organic light emitting
diode array (OLED), or a scanning source and similar devices, or indirectly, by means of a
relay lens or an optical fiber bundle. The display 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-seethrough
and see-through applications, respectively. Typically, a conventional, free-space
optical module is used for these purposes. Unfortunately, as the desired field-of-view
(FOV) of the system increases, such a conventional optical module becomes larger,
heavier, bulkier, and therefore, even for a moderate performance device, is impractical.
This is a major drawback for all kinds of displays, but especially in head-mounted
applications, wherein the system must 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 one hand, are still not sufficiently compact for most practical applications,
and, on the other hand, suffer major drawbacks in terms of 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 to small movements of the optical system relative to the eye of the
viewer, and do not allow sufficient pupil motion for conveniently reading text from such
displays.
The teachings included in Publication Nos. WO01/95027, WO03/081320,
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, WO2013/175465, IL 232197, IL 235642, IL 236490 and IL 236491, all
in the name of Applicant, are herein incorporated by references.
Disclosure of the Invention
The present invention facilitates the design and fabrication of very compact LOEs
for, amongst other applications, head-mounted displays. The invention allows relatively
wide FOVs together with relatively large eye-motion-box values. The resulting optical
system offers a large, high-quality image, which also accommodates large movements of
the eye. The optical system offered by the present invention is particularly advantageous
because it is substantially more compact than state-of-the-art implementations, and yet it
can be readily incorporated even into optical systems having specialized configurations.
A further application of the present invention is to provide a compact display with a
wide FOV for mobile, hand-held applications such as cellular phones. In today's wireless
internet-access market, sufficient bandwidth is available for full video transmission. The
limiting factor remains the quality of the display within the device of the end-user. The
mobility requirement restricts the physical size of the displays, and the result is a directdisplay
with poor image viewing quality. The present invention enables a physically very
compact display with a very large virtual image. This is a key feature in mobile
communications, and especially for mobile internet access, solving one of the main
limitations for its practical implementation. The present invention thereby enables the
viewing of the digital content of a full format internet page within a small, hand-held
device, such as a cellular phone.
The broad object of the present invention is therefore to alleviate the drawbacks of
state-of-the-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 therefore provided an optical
device, comprising a light-transmitting substrate having an input aperture, an output
aperture, at least two major surfaces and edges, an optical element for coupling light waves
into the substrate by total internal reflection, at least one partially reflecting surface located
between the two major surfaces of the light-transmitting substrate for partially reflecting
light waves out of the substrate, a first transparent plate, having at least two major surfaces,
one of the major surfaces of the transparent plate being optically attached to a major
surface of the light-transmitting substrate defining an interface plane, and a beam-splitting
coating applied at the interface plane between the substrate and the transparent plate,
wherein light waves coupled inside the light-transmitting substrate are partially reflected
from the interface plane and partially pass therethrough.
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 is a side view of an exemplary prior art light-guide optical element;
Figs. 2A and 2B are diagrams illustrating detailed sectional views of an exemplary
array of selectively reflective surfaces;
Fig. 3 is a schematic sectional-view of a reflective surface with two different
impinging rays, according to the present invention;
Fig. 4 illustrates a sectional view of an exemplary array of selectively reflective
surfaces wherein a transparent plate is attached to the substrate edge;
Fig. 5 is a schematic sectional-view of a reflective surface according to the present
invention, illustrating the actual active aperture of the surface;
Fig. 6 illustrates the active aperture size of the reflecting surfaces as a function of the
field angle, for an exemplary LOE;
Fig. 7 illustrates detailed sectional views of the reflectance from an exemplary array of
selectively reflective surfaces, for three different viewing angles;
Fig. 8 illustrates the required distance between two adjacent reflecting surfaces as a
function of the field angle, for an exemplary LOE;
Fig. 9 is another schematic sectional-view of a reflective surface with two different
impinging rays, according to the present invention;
Fig. 10 illustrates a sectional view of an exemplary array of selectively reflective
surfaces having a wedged transparent plate is attached to the substrate edge;
Fig. 11 is another schematic sectional-view of a reflective surface with two different
impinging rays, according to the present invention, wherein the two rays are reflected from
two partially reflecting surfaces;
Fig. 12 is yet another schematic sectional- view of a reflective surface with two
different impinging rays, according to the present invention, wherein the two rays are
coupled into the LOE remotely located and coupled-out of the LOE adjacent to each other;
Figs. 13A and 13B are schematic sectional- views of a beam splitting surface
embedded inside a light-guide optical element;
Fig. 14 is a graph illustrating reflectance curves of a beam splitting surface as a
function of incident angles, for an exemplary angular sensitive coating for s-polarized light
waves;
Fig. 15 is a further graph illustrating reflectance curves of a beam splitting surface as
a function of incident angles, for an exemplary angular sensitive coating for s-polarized light
waves;
Fig. 16 is a schematic sectional-view of two different beam splitting surfaces
embedded inside a light-guide optical element;
Fig. 17 is another schematic sectional-view of a beam splitting surface embedded
inside a light-guide optical element wherein partially reflecting surfaces are fabricated inside
the transparent attached plate, and
Figs. 18A and 18B are yet further schematic sectional-views of embodiments of a
beam-splitting surface embedded inside a light-guide optical element wherein the coupling
in, as well as the coupling-out elements are diffractive optical elements.
Detailed Description of Embodiments
Fig. 1 illustrates a sectional view of a light-guide optical element (LOE), according
to the present invention. The first reflecting surface 16 is illuminated by a collimated
display 18 emanating from a light source (not shown) located behind the device. The
reflecting surface 16 reflects the incident light from the source such that the light is trapped
inside a planar substrate 20 by total internal reflection. After several reflections off the
surfaces 26, 27 of the substrate, the trapped light waves reach an array of partially
reflecting surfaces 22, which couple the light out of the substrate into the eye 24, having a
pupil 25, of a viewer. Herein, the input surface of the LOE will be defined as the surface
through which the input light waves enter the LOE and the output surface of the LOE will
be defined as the surface through which the trapped light waves exit the LOE. In addition,
the input aperture of the LOE will be referred to as the part of the input surface through
which the input light waves actually pass while entering the LOE, and the output aperture
of the LOE will be referred to as a part of the output surface through which the output light
waves actually pass while exiting the LOE. In the case of the LOE illustrated in Fig. 1,
both of the input and the output surfaces coincide with the lower surface 26, however,
other configurations are envisioned in which the input and the image light waves could be
located on opposite sides of the substrate, or on one of the edges of the LOE. Assuming
that the central light wave of the source is coupled out of the substrate 20 in a direction
normal to the substrate surface 26, the partially reflecting surfaces 22 are flat, and the offaxis
angle of the coupled light wave inside the substrate 20 is aί , then the angle su 2
between the reflecting surfaces and the normal to the substrate plane is:
"in
sur2
As can be seen in Fig. 1, the trapped rays arrive at the reflecting surfaces from two
distinct directions 28, 30. In this particular embodiment, the trapped rays arrive at the
partially reflecting surface 22 from one of these directions 28 after an even number of
reflections from the substrate surfaces 26 and 27, wherein the incident angle Pref between
the trapped ray and the normal to the reflecting surface is:
The trapped rays arrive at the reflecting surface from the second direction 30 after
an odd number of reflections from the substrate surfaces 26 and 27, where the off-axis
angle is a ' in = 180°-ain and the incident angle between the trapped ray and the normal to
the reflecting surface is:
P ref = in ~ sur2 = ~ in ~ =
wherein the minus sign denotes that the trapped ray impinges on the other side of the
partially reflecting surface 22.
As illustrated in Fig. 1, for each reflecting surface, each ray first arrives at the
surface from the direction 30, wherein some of the rays again impinge on the surface from
direction 28. In order to prevent undesired reflections and ghost images, it is important that
the reflectance be negligible for the rays that impinge on the surface having the second
direction 28.
An important issue that must be considered is the actual active area of each
reflecting surface. A potential non-uniformity in the resulting image might occur due to the
different reflection sequences of different rays that reach each selectively reflecting
surface: some rays arrive without previous interaction with a selectively reflecting surface;
other rays arrive after one or more partial reflections. This effect is illustrated in Fig. 2A.
Assuming that, for example, o „ = 50°, the ray 80 intersects the first partially reflecting
surface 22 at point 82. The incident angle of the ray is 25° and a portion of the ray's energy
is coupled out of the substrate. The ray then intersects the same selectively partially
reflecting surface at point 84 at an incident angle of 75° without noticeable reflection, and
then intersects again at point 86 at an incident angle of 25°, where another portion of the
energy of the ray is coupled out of the substrate. In contrast, the ray 88 shown in Fig. 2B,
experiences only one reflection 90 from the same surface. Further multiple reflections
occur at other partially reflecting surfaces.
Fig. 3 illustrates this non-uniformity phenomenon with a detailed sectional view of
the partially reflective surface 22, which couples light trapped inside the substrate out and
into the eye 24 of a viewer. As can be seen, the ray 80 is reflected off the upper surface 27,
next to the line 100, which is the intersection of the reflecting surface 22 with the upper
surface 27. Since this ray does not impinge on the reflecting surface 22, its brightness
remains the same and its first incidence at surface 22 is at the point 102, after double
reflection from both external surfaces. At this point, the light wave is partially reflected
and the ray 104 is coupled out of the substrate 20. For other rays, such as ray 88, which is
located just below ray 80, the first incidence at surface 22 is before it meets the upper
surface 27, at point 106 wherein the light wave is partially reflected and the ray 108 is
coupled out of the substrate. Hence, when it again impinges on surface 22, at point 110
following double reflection from the external surfaces 26, 27, the brightness of the
coupled-out ray is lower than the adjacent ray 104. As a result, all the rays with the same
coupled-in angle as 80 that arrive at surface 22 left of the point 102 have lower brightness.
Consequently, the reflectance from surface 22 is actually "darker" left of the point 102 for
this particular couple-in angle.
It is difficult to fully compensate for such differences in multiple-intersection
effects nevertheless, in practice, the human eye tolerates significant variations in
brightness, which remain unnoticed. For near-to-eye displays, the eye integrates the light
which emerges from a single viewing angle and focuses it onto one point on the retina, and
since the response curve of the eye is logarithmic, small variations, if any, in the brightness
of the display will not be noticeable. Therefore, even for moderate levels of illumination
uniformity within the display, the human eye experiences a high-quality image. The
required moderate uniformity can readily be achieved with the element illustrated in Fig. 1.
For systems having large FOVs, and where a large EMB is required, a comparatively large
number of partially reflecting surfaces is required, to achieve the desired output aperture.
As a result, the non-uniformity due to the multiple intersections with the large number of
partially reflecting surfaces becomes more dominant, especially for displays located at a
distance from the eye, such as head-up displays and the non-uniformity cannot be
accepted. For these cases, a more systematic method to overcome the non-uniformity is
required.
Since the "darker" portions of the partially reflecting surfaces 22 contribute less to
the coupling of the trapped light waves out of the substrate, their impact on the optical
performance of the LOE can be only be negative, namely, there will be darker portions in
the output aperture of the system and dark stripes will exist in the image. The transparency
of each one of the reflecting surfaces is, however, uniform with respect to the light waves
from the external scene. Therefore, if overlapping is set between the reflective surfaces to
compensate for the darker portions in the output aperture, then rays from the output scene
that cross these overlapped areas will suffer from double attenuations, and darker stripes
will be created in the external scene. This phenomenon significantly reduces the
performance not only of displays which are located at a distance from the eye, such as
head-up displays, but also that of near-eye displays, and hence, it cannot be utilized.
Fig. 4 illustrates an embodiment for overcoming this problem. Only the "bright"
portions of the partially reflecting surfaces 22a, 22b and 22c are embedded inside the
substrate, namely, the reflecting surfaces 22a, 22b and 22c no longer intersect with the
lower major surface 26, but terminate short of this surface. Since the ends of the reflecting
surfaces are adjacent to one another over the length of the LOE, there will be no gaps in
the projected image, and since there is no overlap between the surfaces there will be no
gaps in the external view. There are several ways to construct this LOE, one of which is to
attach a transparent plate 120 having a thickness T, preferably by optical cementing, to the
active area of the substrate. In order to utilize only the active areas of the reflective
surfaces 22 in the correct manner, it is important to calculate the actual active area of each
partially reflective surface and the required thickness T of the plate 120.
As illustrated in Fig. 5, the bright aperture D„ of the reflective surface 22n in the
plane of the external surface 26, as a function of the coupled-in angle o , is:
ot( ) + (¾ )
Since the trapped angle o can be varied as a function of the FOV, it is important
to know with which angle to associate each reflecting surface 22n, in order to calculate its
active aperture.
Fig. 6 illustrates the active aperture as a function of the field angle for the following
system parameters; substrate thickness d=2 mm, substrate refractive index v=1.51, and
partially reflecting surface angle a su =64°. In consideration of the viewing angles, it is
noted that different portions of the resulting image originate from different portions of the
partially reflecting surfaces.
Fig. 7, which is a sectional view of a compact LOE display system based on the
proposed configuration, illustrates this effect. Here, a single plane light wave 112,
representing a particular viewing angle 114, illuminates only part of the overall array of
partially reflecting surfaces 22a, 22b and 22c. Thus, for each point on the partially
reflecting surface, a nominal viewing angle is defined, and the required active area of the
reflecting surface is calculated according to this angle. The exact, detailed design of the
active area of the various partially reflective surfaces is performed as follows: for each
particular surface, a ray is plotted (taking refraction, due to Snell's Law, into
consideration) from the left edge of the surface to the center of the designated eye pupil 25.
The calculated direction is set as the nominal incident direction and the particular active
area is calculated according to that direction.
As seen in Fig. 5, the exact values of the reflecting surfaces active areas can be
used to determine the various distances T between the left edge 102 of the bright part of
each reflecting surface 22 and the lower surface 26. A larger active area dictates a smaller
inter-surface distance. This distance represents the thickness of the plate 120 (Fig. 7) that
should be attached to the lower surface of the LOE. As illustrated in Fig. 5, the distance T
as a function of the coupled-in angle o , is:
T = - Dn -cot(« ) 5
Fig. 8 illustrates the required thickness Jof the plate 120 as a function of the field
angle, for the same parameters as set above in reference to Fig. 6. It is worthwhile setting
the thickness T as the maximal calculated value to assure that the phenomenon of dark
stripes will be avoided in the image. Setting a too thick plate 120 will cause an opposite
effect, namely, the appearance of bright stripes in the image.
As illustrated in Fig. 9, two light rays, 122 and 124, are coupled inside the substrate
20. The two rays are partially reflected from surface 22a at points 126 and 128,
respectively. Only ray 122, however, impinges on the second surface 22b at point 130 and
is partially reflected there, while ray 124 skips over surface 22b without any reflectance.
As a result, the brightness of ray 124, which impinges on surface 22c at point 134, is
higher than that of ray 122 at point 132. Therefore, the brightness of the coupled-out ray
138 from point 134 is higher than that of ray 136 which is coupled-out from point 132, and
a bright stripe will appear in the image. Consequently, an exact value of the thickness T
should be chosen to avoid dark as well as bright stripes in the image.
As illustrated in Fig. 10, a possible embodiment for achieving the required
structure, wherein the thickness T of the plate 120 depends on the viewing angle, is to
construct a wedged substrate 20', wherein the two major surfaces are not parallel. A
complementary transparent wedged plate 120' is attached to the substrate, preferably by
optical cementing, in such a way that the combined structure forms a complete rectangular
parallelepiped, i.e., the two outer major surfaces of the final LOE are parallel to each other.
There are, however, some drawbacks to this method. First of all, the fabrication process of
the wedged LOE is more complicated and cumbersome than the parallel one. In addition,
this solution is efficient for systems having small EMB, wherein there is a good matching
between the viewing angle and the lateral position on the substrate plane. For systems
having a large EMB, however, namely, wherein the eye can move significantly along the
lateral axis, there will be no good adjustment between the viewing angle and the actual
thickness of the plate 120'. Hence, dark or bright stripes may be seen in the image.
This occurrence of dark or bright stripes due to the structure of the partially
reflective surfaces in the LOE is not limited to the surface which creates this phenomenon.
As illustrated with reference to Fig. 3, the brightness of the coupled ray 88, which is
reflected twice by surface 22a, is lower at point 110 than that of ray 80, which is reflected
only once from surface 22a at point 102. As a result, the brightness of the reflected wave
112 is lower than that of the adjacent ray 104. As illustrated in Fig. 11, however, not only
the brightness of the reflected wave from surface 22a is different, but also the brightness of
the transmitted rays 140 and 142 is different. As a result, the brightness of the reflected
rays 144 and 146 from surface 22b, at points 148 and 150, respectively, will be different in
the same way and a dark stripe will be created also at this region of the image, as well.
Naturally, this dissimilarity between the rays will continue to propagate in the LOE to the
next partially reflective surfaces. As a result, since each partially reflective surface creates
its own dark or bright stripes, according to the exact incident angle, for an LOE having a
large number of partially reflecting surfaces, a large amount of dark and bright stripes will
be accumulated at the far edge of the output aperture of the LOE, and consequently, the
image quality will be severely deteriorated.
Another source for unevenness of the image can be the non-uniformity of the image
waves which are coupled into the LOE. Usually, when two edges of a light source have
slightly different intensities this will hardly be noticed by the viewer, if at all. This
situation is completely different for an image which is coupled inside a substrate and
gradually coupled-out, like in the LOE. As illustrated in Fig. 12, two rays 152 and 154 are
located at the edges of the plane wave 156, which originates from the same point in the
display source (not shown). Assuming that the brightness of ray 152 is lower than that of
ray 154 as a result of a non-perfect imaging system, this non equality will hardly be seen
by direct viewing of the plane wave 156 because of the remoteness between the rays.
However, after being coupled into the LOE 20, this condition is changed. While the ray
154 illuminates the reflecting surface 16 just right to the interface line 156 between the
reflecting surface 16 and the lower major surface 26, the right ray 152 is reflected from
surface 16, totally reflected from the upper surface 27, and then impinges on the lower
surface 26 just left to the interface line 158. As a result, the two rays 152 and 154
propagate inside the LOE 20 adjacent to each other. The two exit rays 160 and 162, which
originated from rays 152 and 154, respectively, and reflected from surface 22a, have
accordingly different brightness. Unlike the input light wave 156, however, the two
different rays are adjacent to each other, and this dissimilarity will easily be seen as a dark
stripe in the image. These two rays 164, 165 will continue to propagate together, adjacent
to each other, inside the LOE and will create a dark stripe at each place that they will be
coupled out together. Naturally, the best way to avoid this unevenness is to assure that all
the coupled light waves into the LOE have a uniform brightness over the entire input
aperture for the entire FOV. This demand might be very difficult to fulfil for systems
having large FOV as well as wide input apertures.
As illustrated in Figs. 13A and 13B, this unevenness problem may be solved by
attaching a transparent plate to one of the major surfaces of the LOE, as described above
with reference to Fig. 4. In this embodiment, however, a beam splitting coating 166 is
applied to the interface plane 167 between the LOE 20 and the transparent plate 120. As
illustrated in Fig. 13A, two light rays, 168 and 170, are coupled inside the substrate 20.
Only ray 168 impinges on the first partially reflective surface 22a at point 172 and is
partially reflected there, while ray 170 skips over surface 22a, without any reflectance. As
a result, assuming that the two rays have the same brightness while coupled into the LOE,
ray 170 which is reflected upward from the lower major surface 26 has a higher brightness
then ray 168 which is reflected downward from the upper surface 27. These two rays
intersect each other at point 174, which is located at the interface plane 167. Due to the
beam splitting coating which is applied thereto, each one of the two intersecting rays is
partially reflected and partially passes through the coating. Consequently, the two rays
interchange energies between themselves and the emerging rays 176 and 178 from the
intersection point 174 have a similar brightness, which is substantially the average
brightness of the two incident rays 168 and 170. In addition, the rays exchange energies
with two other rays (not shown) at intersection points 180 and 182. As a result of this
energy exchange, the two reflected rays 184 and 186 from surface 22b will have
substantially similar brightness and the bright stripe effect will be significantly improved.
Similarly, as illustrated in Fig. 13B, two light rays, 188 and 190, are coupled inside
the substrate 20. Only ray 188, however, impinges on the first partially reflective surface
22a at point 192 and partially reflected there before being reflected by the upper surface
27. As a result, assuming that the two rays have the same brightness while coupled into the
LOE, ray 190 which is reflected downward from the upper major surface 27, has a higher
brightness then ray 188. These two rays, however, intersect each other at point 194 which
is located at the interface plane 167 and exchange energies there. In addition, these two
rays intersect with other rays at the points 196 and 198 which are located on the beam
splitting surface 167. As a result, the rays 200 and 202 which are reflected from surface
22a and consequentially the rays 204 and 206 which are reflected from surface 22b, will
have substantially the same brightness, and therefore, the dark stripes effect will be
significantly decreased. This improved uniformity of brightness effect is applicable also for
dark and bright stripes, which are caused by a non-uniform illumination at the input
aperture of the LOE. As a result, the brightness distribution of the optical waves, which is
trapped inside the LOE, is substantially more uniform over the output aperture of the LOE
than over the input aperture.
As illustrated in Fig. 13A the light rays 184, 186, which are reflected from surface
22a, intersect with the beam splitting surface 167, before being coupled out from the LOE.
As a result, a simple reflecting coating cannot be easily applied to surface 167 since this
surface should also be transparent to the light-waves that exit the substrate 20 as well as
transparent to the light wave from the external scene for see-through applications, namely,
the light-waves should pass through plane 167 at small incident angles, and be partially
reflected at higher incident angles. Usually, the passing incident angles are between 0° and
15° and the partially reflecting incident angles are between 40° and 65°. In addition, since
the light rays cross the interface surface 167 many times while propagating inside the LOE,
the absorption of the coating should be negligible. As a result, a simple metallic coating
cannot be used and a dielectric thin-film coating, having a high transparency has to be
utilized.
Fig. 14 illustrates for s-polarization the reflectance curves as functions of the
incident angles for three representative wavelengths in the photopic region: 470 nm, 550
nm and 630 nm. As illustrated, it is possible to achieve the required behavior of partial
reflectance (between 45% and 55%) at large incident angles between 40° and 65° and low
reflectance (below 5%) at small incident angles, for s-polarized light-waves. For ppolarized
light-waves, it is impossible to achieve substantial reflectance at incident angles
between 40° and 65°, due to the proximity to the Brewster angle. Since the polarization
which is usually utilized for an LOE-based imaging system, is the s-polarization, the
required beam splitter can be fairly easily applied. However, since the beam splitting
coating should be substantially transparent for light waves from the external scene which
impinge on the interface surface at low incident angles and which are substantially non
polarized, the coating should have low reflectance (below 5%) at small incident angles also
for p-polarized light waves.
A difficulty still existing is that the LOE 20 is assembled from several different
components. Since the fabrication process usually involves cementing optical elements,
and since the required angular-sensitive reflecting coating is applied to the light-guide
surface only after the body of the LOE 20 is complete, it is not possible to utilize the
conventional hot-coating procedures that may damage the cemented areas. Novel thin-film
technologies, as well as ion-assisted coating procedures, can also be used for cold
processing. Eliminating the need to heat parts, allows cemented parts to be safely coated.
An alternative is that the required coating can simply be applied to transparent plate 120,
which is adjacent to the LOE 20, utilizing conventional hot-coating procedures and then
cementing it at the proper place. Clearly, his alternative approach can be utilized only if the
transparent plate 120 is not too thin and hence might be deformed during the coating
process.
There are some issues that should be taken into consideration while designing a
beam splitting mechanism as illustrate above:
a. Since the rays which are trapped inside the LOE are not only totally reflected
from the major surfaces 26 and 27, but also from the internal partially reflecting
interface plane 167, it is important that all three of these surfaces will be
parallel to each other to ensure that coupled rays will retain their original
coupling-in direction inside the LOE.
b. As illustrated in Figs. 13A and 13B, the transparent plate 120 is thinner than the
original LOE 20. Unlike the considerations which were brought regarding to the
uncoated plate in Figs. 7-10, wherein the thickness of plate 120 is important for
uniformity optimization, however, here the thickness of the coated plate might
be chosen according to other considerations. On one hand, it is easier to
fabricate, coat and cement a thicker plate. On the other hand with a thinner plate
the effective volume of the LOE 20, which is practically coupled the light
waves out of the substrate, is higher for a given substrate thickness. In addition,
the exact ratio between the thicknesses of the plate 120 and the LOE 20 might
influence the energy interchange process inside the substrate.
c. Usually, for beam splitters which are designated for full color images the
reflectance curve should be as uniform as possible for the entire photopic
region, in order to abort chromatic effects. Since, however, in the
configurations which are illustrated in the present invention the various rays
intersect with each other many times before being coupled out from the LOE
20, this requirement is no longer essential. Naturally, the beam-splitting coating
should take into account the entire wavelengths spectrum of the coupled image,
but the chromatic flatness of the partially reflecting curve may be tolerated
according to various parameters of the system.
The reflectance-transmittance ratio of the beam-splitting coating should not
necessarily be 50 -50 . Other ratios may be utilized in order to achieve the
required energies exchange between the darker and the brighter rays. Moreover,
as illustrated in Fig. 15, a simpler beam-splitter coating can be utilized, wherein
the reflectance is gradually increased from 35% at an incident angle of 40° to
60% at an incident angle of 65°.
The number of the beam-splitting surfaces which are added to the LOE is not
limited to one. As illustrated in Fig. 16, another transparent plate 208 may be
cemented to the upper surface of the LOE, wherein a similar beam-splitting
coating is applied to the interface plane 210 between the LOE 20 and the upper
plate 208, to form an optical device with two beam splitting surfaces. Here, the
two unequal rays 212 and 214 intersect with each other at point 215 on the
coated interface plane 210 along with other intersections with other rays at
points 216 and 217. This is in addition to the intersections on the lower beamsplitting
interface planel67. As a result, it is expected that the uniformity of the
reflected rays 218 and 220 will be even better than that of the embodiments of
Figs. 13A and 13B. Naturally, the fabrication method of the LOE having two
beam-splitting interface planes is more difficult than that of having only a
single plane. Therefore, it should be considered only for systems wherein the
non-uniformity problem is severe. As before, it is important that all of the four
reflecting surfaces and planes 26, 27, 167 and 210, should be parallel to each
other.
The transparent plate 120 should not be necessarily fabricated from the same
optical material as the LOE 20. Furthermore, the LOE might be fabricated of a
silicate based material while, for the sake of eye safety, the transparent layer
may be fabricated of a polymer based material. Naturally, care should be taken
to ensure optical qualities of the external surfaces and to avoid deformation of
the transparent plate.
So far it was assumed that the transparent plate is totally blank. However, as
illustrated in Fig. 17, partially reflecting surfaces 222a and 222b, may be
fabricated inside the plate 120, in order to increase the useable volume of the
LOE. These surfaces should be strictly parallel to the existing surfaces 22a and
22b and oriented at exactly the same orientation.
All the various parameters of the above embodiments, such as, the thickness and
the optical material of the plate 120, the exact nature of the beam-splitting coating, the
number of the beam-splitting surfaces and location of the partially reflecting surface inside
the LOE, could have many different possible values. The exact values of these factors are
determined according to the various parameters of the optical system as well as the specific
requirements for optical quality and fabrication costs.
So far, it was assumed that the light waves are coupled out from the substrate by
partially reflecting surfaces, which are oriented at an oblique angle in relation to the major
surfaces, and usually coated with a dielectric coating. As illustrated in Fig. 18A, however,
there are systems wherein the light waves are coupled into and out from the substrate
utilizing diffractive elements 230 and 232, respectively. The same uniformity issues that
were discussed above should also be relevant to this configuration. As illustrated, the two
rays 234 and 236 from the same point in the display source are coupled into the substrate
238 remotely located from each other at the two edges of the coupling-in element 230. The
rays are coupled-out by the coupling-out element 232 located adjacent to each other.
Therefore, any dissimilarity between the rays will be easily seen in the coupled-out wave.
In addition, in order to validate a uniformed coupled-out image the diffractive efficiency of
the coupling-out element 232 is increased gradually. As a result, different rays from the
same point source might pass through different locations in the element 232 before being
coupled-out the element and hence will have different brightness in the image. Another
source for the unevenness can be caused by the fact that the ray 234 is partially diffracted
out of the substrate at the right edge 240 of the grating 232 while ray 236 impinges on the
lower surface just left of the grating, and hence, is not diffracted there. As a result, for all
the coupling-out locations in the grating 232 for the two adjacent rays 234 and 236, ray
236 will have a higher brightness and this difference will easily be seen.
Fig. 18B illustrates a similar approach to solve these issues. As illustrated, a
transparent plate 242 is cemented to the upper surface 244 of the substrate 238, wherein the
interface surface 246 is coated with a beam-splitting coating similar to the above-described
coatings.

CLAIMS
1. An optical device, comprising:
a light-transmitting substrate having an input aperture, an output aperture, at least
two major surfaces and edges;
an optical element for coupling light waves into the substrate by total internal
reflection;
at least one partially reflecting surface located between the two major surfaces of the
light-transmitting substrate for partially reflecting light waves out of the substrate;
a first transparent plate, having at least two major surfaces, one of the major surfaces
of the transparent plate being optically attached to a major surface of the light-transmitting
substrate defining an interface plane, and
a beam-splitting coating applied at the interface plane between the substrate and the
transparent plate,
wherein light waves coupled inside the light-transmitting substrate are partially
reflected from the interface plane and partially pass therethrough.
2. The optical device according to claim 1, wherein light waves coupled out the
substrate by the partially reflecting surface, substantially pass through the interface plane
without any significant reflectance.
3. The optical device according to claim 1, wherein the major surfaces of the lighttransmitting
substrate are parallel to the major surfaces of the first transparent plate.
4. The optical device according to claim 1, wherein the beam-splitting coating has
substantial reflectance at large incident angles and low reflectance at small incident angles.
5. The optical device according to claim 4, wherein the beam-splitting coating has low
reflectance at incident angles between 0° and 15° and substantial reflectance at incident
angles higher than 40°.
6. The optical device according to claim 4, wherein the beam-splitting coating has
reflectance higher than 35% at incident angles higher than 40° and reflectance lower than
10% at incident angles lower than 15°.
7. The optical device according to claim 1, wherein the beam-splitting coating is applied
to a major surface of the light-transmitting substrate.
8. The optical device according to claim 7, wherein the beam-splitting coating is applied
utilizing cold-coating process.
9. The optical device according to claim 1, wherein the beam-splitting coating is applied
to one of the surfaces of the first transparent plate.
10. The optical device according to claim 4, wherein the reflectance of the beam-splitting
coating is substantially constant at incident angles higher than 40° and lower than 60°.
11. The optical device according to claim 4, wherein the reflectance of the beam-splitting
coating is not constant at incident angles higher than 40°.
12. The optical device according to claim 11, wherein the reflectance of the beamsplitting
coating increases as a function of the incident angle at incident angles higher than
40°.
13. The optical device according to claim 4, wherein the reflectance of the beam-splitting
coating at large incident angles is substantially uniform for the entire photopic region.
14. The optical device according to claim 1, wherein the transparent plate is thinner than
the light-transmitting substrate.
15. The optical device according to claim 1, further comprising a second transparent
plate which is optically attached to the other major surface of the light-transmitting
substrate, defining a second interface plane.
16. The optical device according to claim 15, wherein a beam-splitting coating is applied
at the second interface plane.
17. The optical device according to claim 16, wherein the beam-splitting coating has
substantial reflectance at large incident angles and low reflectance at small incident angles.
18. The optical device according to claim 1, wherein the light-transmitting substrate and
the transparent plate are fabricated of the same optical material.
19. The optical device according to claim 1, wherein the light-transmitting substrate and
the transparent plate are fabricated of two different optical materials.
20. The optical device according to claim 1, wherein the transparent plate is fabricated of
a polymer based material.
21. The optical device according to claim 1, wherein the optical element for coupling
light waves into said substrate is a diffractive element.
22. The optical device according to claim 1, wherein the least one partially reflecting
surface located between the two major surfaces of the light transmitting-substrate, is a
diffractive element.
23. The optical device according to claim 1, wherein the least one partially reflecting
surface located between the two major surfaces of the light transmitting-substrate, is
oriented at an oblique angle in relation to the major surfaces of the substrate.
24. The optical device according to claim 23, wherein the least one partially reflecting
surface is coated with a dielectric coating.
25. The optical device according to claim 1, comprising a plurality of partially reflecting
surfaces located between the two major surfaces of the light transmitting-substrate,
wherein the partially reflecting surfaces are parallel to each other.
26. The optical device according to claim 1, wherein the brightness distribution of optical
waves coupled inside the substrate is substantially more uniform over the output aperture
of the substrate than over the input aperture.