The present invention relates to integrated head-mounted display (HMD)
systems, and in particular, to systems that include two combined units: a headmounted
unit and an eyeball tracking unit.
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 other mobile display devices.
Background of the Invention
One important application for compact optical elements, is in HMDs wherein
an optical module serves both as an imaging lens and a combiner, in which a twodimensional
image source is imaged to infinity and reflected into an eye of an
observer. The display source can be obtained directly from, e.g., 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 indirectly, by
means of a relay lens, or an optical fiber bundle. The display source comprises an
array of elements (pixels) imaged to infinity by a collimating lens and transmitted
into the eye of a viewer by means of a reflecting or partially reflecting surface acting
as a combiner for non-see-through and see-through applications, respectively.
Typically, a conventional, free-space optical module is used for these purposes. As
the desired field-of-view (FOV) of a HMD 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 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 in terms of
manufacturabilitv. Furthermore the eve-motion-box (EM of the ontical viewine
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 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 and
WO2008/1 49339, all in the name of Applicant, are herein incorporated by references.
Disclosure of the Invention
The present invention facilitates the exploitation of very compact light-guide
optical elements (LOEs) for, amongst other applications, HMDs. The invention
allows for 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 offered by the present invention is
particularly advantageous because it is substantially more compact than the state-ofthe-
art implementations and yet, it can be readily incorporated even into optical
systems having specialized configurations.
Another optical function which could prove to be useful for HMD designs is
eyeball tracking, or sensing the direction the eyeball is looking at, relative to the
direction of the head. A typical eye tracker will combine a miniature CCD camera
and an infrared LED to illuminate the pupil. By measuring the changes in shape and
position of the pupil, it is possible to perceive the direction in which the viewer's eye
is looking, with very reasonable accuracy once calibrated. Combining measurements
of head position and eye position would solve the problems inherent in existing HMD
technology, since the projected symbols and boresight could be slaved to the
direction in which the viewer is looking, thus retaining existing human tracking
behavior. It will be useful to combine the HMD and the eyeball tracker in the same
optical module.
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 invention there is therefore provided an optical system,
comprising a light-transmitting substrate having at least two major surfaces and
edges, at least one optical means for coupling light waves into the substrate by total
internal reflection, at least two partially reflecting surfaces carried by the substrate
wherein the partially reflecting surfaces are not parallel to the main surfaces of the
substrate, at least one light source projecting light waves located within a first optical
spectrum, and at least one display source projecting light waves located within a
second optical spectrum, characterized in that the light waves from the light source
and light waves from the display source are coupled into the substrate by total
internal reflection.
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, LOE;
Figs. 2A and 2B illustrate desired reflectance and transmittance characteristics
of selectively reflecting surfaces, used in the present invention, for two ranges of
incident angles;
Fig. 3 illustrates a reflectance curve as a function of the incident angle for an
exemplary dielectric coating;
Fig. 4 is a schematic diagram illustrating a detailed sectional view of an
exemplary array of selectively reflective surfaces;
Fig. 5 illustrates a prior art eyeglass HMD device;
Fig. 6 illustrates a side view of an LOE showing light waves scattered from an
eye and coupled back into the LOE;
Fig. 7 illustrates a reflectance curve as a function of the wavelength for a
specific partially reflecting surface;
Fig. 8 illustrates a reflectance curve as a function of the wavelength for another
partially reflecting surface;
Fig. 9 illustrates reflectance curves as a function of the incident angle for two
different partially reflecting surfaces;
Fig. 10 illustrates a side view of an LOE, showing light waves coupled out
through an exit aperture, which are scattered from the eye and coupled back into the
LOE;
Fig. 1 illustrates a side view of an LOE, showing light waves which are
scattered from an eye and coupled back into the LOE, wherein only part of the rays are
coupled out through an exit aperture;
Fig. 12 illustrates reflectance curves as a function of the wavelength for a
reflection filter at two different incident angles;
Fig. 3 illustrates reflectance curves as a function of the wavelength for a
transmission filter at two different incident angles;
Fig. 14 illustrates an optical system combining light waves from a display source
and a light source;
Fig. 15 illustrates a reflectance curve as a function of the wavelength for a
polarizing beam splitter, and
Fig. 16 illustrates a side view of still another embodiment of an LOE having two
adjacent partially reflecting surfaces for coupling out light waves into the viewer's eye.
Detailed Description of Preferred Embodiments
Fig. illustrates a sectional view of a prior art substrate 20 and associated
components (hereinafter also "an LOE"), utilizable in the present invention. An
optical means, e.g., a reflecting surface 16, is illuminated by a collimated display 18,
emanating from a light source (not shown) located behind the LOE. The reflecting
surface 16 reflects incident light from the source, such that the light is trapped inside
a planar substrate 20 of the LOE, by total internal reflection. After several reflections
off the major lower and upper surfaces 26, 27 of the substrate 20, the trapped waves
reach an array of selective reflecting surfaces 22, which couple the light out of the
substrate into an eye 24, having a pupil 25, of a viewer. Herein, the input surface of
the LOE will be regarded as the surface through which the input waves enter the LOE
and the output surface of the LOE will be regarded as the surface through which the
trapped waves exit the LOE. In the case of the LOE illustrated in Fig. 1, both the
input and the output surfaces are on the lower surface 26. Other configurations,
however, are envisioned in which the input and the image waves could be located on
opposite sides of the substrate 20. Assuming that the central wave of the source is
coupled out of the substrate 20 in a direction normal to the substrate surface 26, the
reflecting surfaces 22 are flat, and the off-axis angle of the coupled waves inside the
substrate 20 is in , then the angle a sur2 between the reflecting surfaces and the
normal to the substrate plane is:
a surl = ¾2 (1)
As seen in Figs. 1 and 2A, the trapped rays arrive at the reflecting surfaces
from two distinct directions 28, 30. In this particular embodiment, the trapped rays
arrive at the reflecting surface from one of these directions 28 after an even number
of reflections from the substrate surfaces 26 and 27, wherein the incident angle f
(see Fig. 2A) between the trapped ray and the normal to the reflecting surface is:
2
?re = 90°- - 2) = 90 - .
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 'in = 180o- and the incident angle between the trapped ray and
the normal to the reflecting surface is as indicated in Fig. 2B:
< - , ) = 90°-(180°- ¾ - _ ) ~90· + - . (3)
As further 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.
A solution for this requirement that exploits the angular sensitivity of thin film
coatings was previously proposed in the Publications referred-to above. The desired
discrimination between the two incident directions can be achieved if one angle is
significantly smaller than the other one. It is possible to provide a coating with very
low reflectance at high incident angles, and a high reflectance for low incident angles.
This property can be exploited to prevent undesired reflect!ons and ghost images by
eliminating the reflectance in one of the two directions. For example choosing
ref ~ 25°, then it can be calculated that:
're = 105° ; a in = 50° ; a n = 130° ; a r = 25° . (4)
If a reflecting surface is now determined for which f i not reflected but
r f is, then the desired condition is achieved.
Referring now specifically to Figs. 2A and 2B, these figures illustrate desired
reflectance behavior of selectively reflecting surfaces. While the ray 32 (Fig. 2A),
having an off-axis angle of ref ~ 25°, is partially reflected and is coupled out of the
substrate 34, the ray 36 (Fig. 2B), which arrives at an off-axis angle of e ~ 75° to
the reflecting surface (which is equivalent to ' ~ 105°), is transmitted through the
reflecting surface 34, without any notable reflection. An LOE is usually exploited
not only for a single wave, but for an optical system having a wide FOV. Assuming a
system having a FOV of 30° and an LOE having a refractive index of 1.517, then the
FOV inside the substrate is -20°. As a result, there are two angular regions which are
defined for this specific LOE: a first region of 75° ± 10° where b' is located, and
a second region of 25° ± 10° where re is located.
Fig. 3 illustrates the reflectance curve of a typical partially reflecting surface of
this specific LOE, as a function of the incident angle for S-polarized light with the
wavelength l=550 nm. For a full-color display, similar reflectance curves should be
achieved for all the other wavelengths in the photopic region. There are two
significant regions in this graph: between 65° and 85°, where the reflectance is very
low, and between 10° and 40°, where the reflectance increases monotonically with
increasing incident angles. Hence, as long as, for a given FOV and for a given
spectral region, it can ensured that the entire angular spectrum of b , where very
low reflections are desired, will be located inside the first region, while the entire
angular spectrum of b , where higher reflections are required, will be located inside
the second region, the reflection into the viewer's eye of an embodiment having only
one substrate can be ensured, thus ensuring a ghost-free image.
Fig. 4 is a schematic sectional view of an array of selectively reflecting
surfaces which couple light rays trapped inside the substrate out and into an eye of a
viewer. As can be seen, in each cycle, the coupled rays pass through reflecting
surfaces 38, having a direction of ' , = 130°, whereby the angle between the rays
and the normal to the reflecting surfaces, is ~75°, and the reflections fro these
surfaces are negligible. In addition, in each cycle, the rays 39 pass through the
reflecting surface 22 twice in a direction of in = 50°, where the incident angle is
25° and part of the energy of the ray is coupled out of the substrate.
In general, all the potential configurations of the LOEs considered in the
Publications referred-to above, offer several important advantages over alternative
compact optics for display applications, which include that:
1) the input display source can be located very close to the substrate, so that the
overall optical system is compact and lightweight, offering an unparalleled formfactor;
2) in contrast to other compact display configurations, the LOE technology offers
flexibility as to location of the input display source relative to the eyepiece. This
flexibility, combined with the ability to locate the display source close to the
expanding substrate, alleviates the need to use an off-axis optical configuration that is
common to other display systems. In addition, since the input aperture of the LOE is
much smaller than the active area of the output aperture, the numerical aperture of the
collimating lens is much smaller than required for a comparable conventional
imaging system. Consequently, a significantly more convenient optical system can
be implemented and the many difficulties associated with off-axis optics and high
numerical-aperture lenses, such as field or chromatic aberrations, can be
compensated-for relatively easily and efficiently;
3) the reflectance coefficients of the selectively reflecting surfaces in the present
invention, are essentially identical over the entire relevant spectrum. Hence, both
monochromatic and polychromatic light sources may be used as display sources. The
LOE has a negligible wavelength-dependence, ensuring high-quality color images
with high resolutions;
4) since each point from the input image is transformed into a plane light wave
that is reflected into the eye of a viewer from a large part of the reflecting array, the
tolerances on the exact location of the eye can be significantly relaxed. As such, the
viewer can see the entire FOV, and the EMB can be significantly larger than in other
compact display configurations, and
5) since a large part of the intensity from the display source is coupled into the
substrate, and since a large portion of this coupled energy is "recycled" and coupled
out into an eye of a viewer, a display of comparatively high brightness can be
achieved even with display sources having low-power consumption.
Fig. 5 illustrates a prior art embodiment in which the LOE is embedded in
eyeglass frames 40. The display source 42 and the collimating device 44, which
includes a light waves folding element, are assembled inside arm portions 46 of the
eyeglass frames 40 next to the edge of the LOE. In a case where the display source is
an electronic element, such as a small CRT, LCD or OLED, driving electronics 48 for
the display source, may be assembled with the back portion of the arm 46. A
handheld unit 50 comprising a power supply, a video source and control interface is
connected to arm 46 by a cable 52, which is used for transmitting power, video
signals, audio signals and control commands. Earphones can also be installed in the
eyeglasses to enable the exploitation of audio channels. The handheld unit 50 can be
a portable DVD, a cellular phone, a mobile TV receiver, a video games console, a
portable media player, or any other mobile display device. The unit 50 is referred to
as "handheld , since it is usually operated by the user's hand, but it can be any other
portable device, and it can be affixed to the user's belt or located in a pocket, a pouch,
a purse or hung on the user's neck. In addition to the components which are
embedded in the eyeglass frame, a miniature video camera 54 with, optional optical
zoom capability, can be installed e.g., in the front region of the frame 40. The camera
captures images from the external scene, transfers the video signal to an imageprocessing
unit 56, which can be installed inside the electronics unit 48, and
controlled in real-time by the user. The processed image signal is then transferred to
the display source 42 which projects the image through the LOE into the eye of a
viewer. Other potential elements that can be installed on the frame are a GPS
receiver, an orientation sensor and a coordinate osition sensor, wherein the
processor 56 receiving an input from these sensors is providing a visually sensible
output for displaying on the eyeglass.
Some of the current HMD technology uses head position measurements to
approximate line-of-sight, which may cause significant disparity between what a
viewer is intended to look at, and what the viewer is actually looking at, as a result of
at least ±20° eye movement. Therefore, it is necessary to integrate eyeball tracking
capability into HMDs in some applications. Eyeball tracking is the process of
measuring either the point of gaze or the motion of an eye relative to the head. An
eyeball tracker is a device for measuring eye positions and eye movement. The most
popular method for operating this device is by utilizing an optical method for
measuring eye motion. Light from an external source, typically infrared, is reflected
from the eye and sensed by a video camera, or some other specially designed optical
sensors. The information is then analyzed to extract eye rotation fro changes in
reflections. Video-based eye trackers typically use corneal reflection and the center
of the pupil as features to track over time. As a result, an HMD-eyeball tracker
integrated system would be able to display stereoscopic virtual images as would a
classical HMD, and also be able to track the 'direction of gaze' of a viewer.
In accordance with the present invention, it would be advantageous to
physically combine the two optical units, the HMD and the eyeball tracker.
Moreover, it would be beneficial to utilize the same LOE for projecting the light from
the display source into a viewer's eye, as described above, as well as for illuminating
the eye with light from the eye tracker source, and to collect light which reflects from
the eye into the detector. These two optical units should work properly without
interfering with each other. To achieve this goal, two main characteristics of the
combined optical system are exploited in the present invention: a separate partially
reflecting surface or facet, dedicated for transferring light from a light source to the
inspected eye and backwards, and a light having a wavelength substantially different
from the photopic region utilized for the eye tracking.
Fig. 6 schematically illustrates how one of the surfaces of an LOE can be
utilized to illuminate the viewer's eye 24 for eye-tracking purposes. As illustrated,
light rays from an eyeball tracker 64 having a wavelength of l , which is
substantially different than the photopic region, usually in the near IR region and
preferably at the range of 850-900 nm, are coupled into the LOE by total internal
reflection through the light waves coupling surface 16. In this embodiment, the input
and the image waves are located on opposite sides of the LOE. The light waves are
coupled-out of the LOE by the partially reflecting surface 22a and are directed to
illuminate the viewer's eye 24. After reflecting from the eye 24, rays 60 and 62 are
coupled back into the LOE by the same partially reflecting surface 22a and then
coupled-out of the LOE by the surface 16, back into the eye tracker 64, wherein light
waves are imaged by a detector 66, which analyzes the incoming rays to track the
position of the eye-pupil 25.
In order to avoid ghost images, it is important that only one of the facets of the
surfaces of the LOE (partially reflecting surface 22a in the shown Figure) will reflect
light waves in the range of about lΐ . Otherwise, light waves from other surfaces will
also be reflected from the eye and cause a noise on the detector 66, thus severely
degrading the quality of the imaged eyeball. In addition, reflecting surface 22a
should be transparent to the photopic range in the relevant angular spectra of the
LOE, in the lower region, as well as the upper one.
Figs. 7 and 8 illustrate the reflectance curve o partially reflecting surfaces 22a
and 22b, respectively, at incident angles of 35° and 75° as a function of the
wavelength. As shown in Fig. 7, reflecting surface 22a reflects light waves having a
wavelength of 850 nm with a reflectance of 20% at an incident angle of 35°, while it
is actually transparent for the entire photopic range at both incident angles. At an
incident angle of 35°, reflecting surface 22b partially reflects the light waves in the
photopic range while it is actually transparent at an incident angle of 75° in the
photopic range, as well as at both angles, in the region of 850 nm.
Fig. 9 illustrates the reflectance curve o partially reflecting surfaces 22a and
22b at a wavelength of 550 nm as a function of the incident angle. As illustrated, for
surface 22b there are two significant regions in this graph: between 65° and 85°,
where the reflectance is very low, and between 10° and 40°, where the reflectance
increases monotonically with increasing incident angles, as required for the regular
operation of an LOE. For partially reflecting surface 22a the reflectance is negligible
at both lower and higher relevant angular regions. The actual interpretation of Figs. 7
to 9 is that reflecting surface 22a is solely dedicated for eyeball tracking and does not
at all interfere with the usual operation of the LOE at the photopic region. In
addition, reflecting surface 22b is substantially transparent to the spectral region of
around 850 nm, and hence, does not interfere with the optical operation of eyeball
tracking. All the other partially reflecting surfaces are designed to behave in a similar
manner to that of reflecting surface 22b. That is, the other facets are also transparent
to the spectral region of 850 nm and have optical performance in the photopic region,
as required by the optical design of the LOE acting as a combiner for HMD.
Another problem that should be addressed is the possibility that a ghost image
might also for a single surface. As illustrated in Fig. 10, two different rays from a
single point in the eye 24 are imaged through the LOE. Their optical behavior is,
however, different: while ray 68 is reflected only once from each of the external
surfaces 28 and 30 of the LOE, the ray 70 is reflected twice from each surface. As a
result, these two rays have different optical pathways from partially reflecting
surface 22a to reflecting surface 16, and hence, they cannot be utilized together to
form the image of wavelength ou at the detector 66. Therefore, the rays that are
reflected twice from the external surfaces 28, 30 must be blocked from the
detector 66.
Fig. 11 illustrates how a spatial filter 72 which is located at the reflecting
surface 16, blocks these undesired rays. That is, ray 74 is no longer reflected by
surface 16 via tracker 64 into the detector 66, but rather continues to propagate inside
disturbance for the incoming light waves from the display source which creates the
image that is projected by the LOE into a viewer's eye, it is important that the filter 72
should be transparent to the light waves having a wavelength of lίG , while still being
reflective to the photopic range.
Fig. 12 illustrates the reflectance curve of the filter 72 as a function of the
wavelength at incident angles of 15° and 35°. As illustrated, the filter is highly
reflective for the photopic range, while it is substantially transparent for the spectral
range around 850 nm.
The embodiments described above with regard to the reflecting surface 16 are
examples of a method for coupling the input waves into the substrate. Input waves
could, however, also be coupled into the substrate by other optical means, including,
but not limited to, folding prisms, fiber optic bundles, diffraction gratings, and other
solutions. n some of these methods, which were described in the Publications
referred to above, the input surface of the LOE is not a reflecting surface but rather a
transparent aperture. In these cases, it is required that the filter will be reflective to
light waves having a wavelength of lΐ , while still transparent to the photopic range.
Fig. 13 illustrates the reflectance curve of filter 72 as a function of the
wavelength at incident angles of 15° and 35°. As illustrated, the filter 72 is
substantially transparent for the photopic range, while it is reflective for the spectral
range of about 850 nm.
The combination of a display source with a light source for illuminating an eye
tracker utilizing light waves having a wavelength of l G , is illustrated in Fig. 14. As
shown, the s-polarized input light waves 80 emanating from the light source 78 and
having wavelengths inside the photopic spectrum, are reflected by a reflective
dichroic surface 82, e.g., a beam splitter, associated with a light guide 83, and are
then coupled into a light guide 84 of a combiner 85, usually composed of a light
waves transmitting material, through its lower surface 86. Following reflection of the
light waves off of a polarizing beam splitter 88, the light waves are coupled out of the
light guide 84 through surface 90. The light waves which illuminate bright pixels of
the Liquid Crystal on Silicon (LCOS) 92 then pass through a dynamic quarterwavelength
retardation plate 94, reflected by a reflecting surface of the LCOS 92,
return to pass again through the retardation plate 94, and re-enter the light-guide 84
through surface 90. The now p-polarized light waves pass through the polarizing
beam splitter 88 and are coupled out of the light guide 84 through surface 96. The
light waves then pass through a second quarter-wavelength retardation plate 98,
collimated by a component 100, e.g., a lens, at its reflecting surface 102, return to
pass again through the retardation plate 98, and re-enter the light-guide 84 through
surface 96. The now s-polarized light-waves reflect off the polarizing beam
splitter 88 and exit the light guide through the upper surface 104 of the combiner 85.
In addition, s-polarized input light waves 06 having wavelengths of the light
illuminating source 107, located in the eyeball tracker 108 and have an optical
spectrum different than the photopic spectrum, preferably, in the near R region, pass
through the dichroic surface 82, are coupled into a light guide 84, pass directly
through the polarizing beam splitter 88 and are then coupled out of the light-guide 84
through the upper surface 104. For the relevant angular spectrum, the dichroic
surface 82 has high reflectance for the photopic spectrum and high transmittance for
the spectrum of the light waves 106.
The two spectrally separated s-polarized input light waves 80 and 106 are now
coupled through the reflecting surface 16 of the LOE, by total internal reflection. The
light waves 80 are utilized for forming a virtual image projected by partially
reflecting surfaces 22a-22e into a viewer's eye 24, while the light waves 106 are
utilized to illuminate the eye 24 for eye-tracking. The light waves 106 having the
wavelength of l, , are reflected from the eye 24 coupled again into the LOE by the
partially reflecting surface 22a, coupled out from the LOE through reflecting
surface 16, and as seen, pass again through the polarizing beam splitter 88 and
through the dichroic surface 82, and coupled into the eyeball tracker 108, where they
are focused onto the detector 110.
Fig. 15 illustrates the reflectance pattern of the polarizing beam splitter 88 of
Fig. 14. As shown, the beam splitter 88 has high and low reflectance, for the s- and
p-polarization, respectively, in the photopic range, while having high transmittance
for the s-polarized light having a wavelength of
In all the configurations described so far, the optical reflecting 16 is utilized to
couple light waves from the display source having wavelengths in the photopic range,
as well as light waves from the eyeball tracker 108 having wavelength of l G, into the
LOE, by total internal reflection. There are, however, configurations wherein
different coupling elements are utilized to couple separately the light waves from the
display source and the light waves from the eyeball tracker. These configuration
include, but are not limited to, two different elements wherein the first one is
substantially transparent for the photopic range, while it is reflective for the spectral
range wavelength of l G, and the second element is substantially transparent for the
spectral range wavelength of l , while it is reflective for photopic range.
In all the configurations described so far, the two partially reflecting surfaces,
22a and 22b, are laterally separated. However, there are configurations wherein, for
the sake of compactness or for enlarging the EMB of the optical system, it is required
that the two surfaces will be adjacent to each other.
Fig. 16 illustrates a first partially reflecting surface 22a which is coated on the
surface directed to one side of substrate 20, and a second partially reflecting surface
22b which is coated on the surface directed towards the other side of substrate 20.
The two surfaces are optically attached and laterally separated by a cement layer 112.
Typically, the thickness of the cement layer 112 is in the order of 10 m , hence, the
two surfaces can be considered as being optically disposed in one location. As
illustrated, two different rays 116, 118 from the eyeball tracker 108 and the display
source 92, respectively, (see Fig. 14) are coupled into the substrate 20 by the coupling
reflecting surface 16 and then coupled out of the substrate 20 towards the viewer's
eye 24 by partially reflecting surfaces 22a and 22b, respectively.
In all the hereinbefore described embodiments, the light waves from the
eyeball tracker, as well as from the display source, are coupled into the substrate by
the same coupling-in element. However, there are embodiments wherein, for the
sake of simplicity or because of geometrical constraints, it is required that the eyeball
tracker and the display source will be separated, and hence, the two different light
waves will impinge on the substrate at two different locations.
Fig. 17 illustrates a system wherein the optical waves 116 and 118 from the
eyeball tracker 120 and the display source 122, are separately coupled into the
substrate 20 by two different coupling-in elements, 124 and 126, respectively. While
the coupling-in element 124 is a simple reflecting surface, the coupling-in element
126 is a dichroic beam splitter. It is assumed that the angle between the surfaces 124
and 126 and the major surface 26 of the substrate 20 is about 30°.
Fig. 18 illustrates a reflection pattern of the polarizing beam splitter 126 of
Fig. 17. As shown, the beam splitter 126 has high reflectance for the s-polarization in
the photopic range at an incident angle of 30°, while having high transmittance for
the s-polarized light having a wavelength of l, at the same angle. As a result, light
waves 116, having a wavelength of l , are coupled into the substrate by the
reflecting surface 124 and then pass through the element 126 with negligible
interference.
In some of methods described in the prior art Publications referred to above,
the input surface of the LOE is not a reflecting surface, but rather a transparent
aperture. In these cases, it is required that the second aperture will be reflective to
light waves having a wavelength of l , while still being transparent to the photopic
range.
So far, it was assumed that the main purpose of the eyeball tracker is to
measure eye positions and eye movements. When, however, an eyelid of a viewer's
eye is closed, the pattern of the optical waves which are reflected from the eye, is
significantly changed. The eyeball tracker can easily detect if a viewer's eyelid is
open or closed. Since the LOE based eyeglasses illustrated in Fig. 5 have a seethrough
capability, it is possible to utilize same for automotive applications where
they can potentially assist a drive in driving and in navigation tasks, or can project a
thermal image in the driver's eyes during low-visibility conditions. In addition to
these tasks, the LOE-based eyeglasses, combined with an eyeball tracker, can also
serve as a drowsy driver alert unit, that is, the eyeball tracker device can detect the
driver's blinking patterns and determine how long the driver's eyes stay closed
between blinks. Hence, the system can conclude that the driver is no longer alert and
provide a warning concerning this situation.

CLAIMS
1. An optical system, comprising:
a light-transmitting substrate having at least two major surfaces and edges;
at least one optical means for coupling light waves into the substrate by total
internal reflection;
at least two partially reflecting surfaces carried by the substrate wherein the
partially reflecting surfaces are not parallel to the main surfaces of the substrate;
at least one light source projecting light waves located within a first optical
spectrum, and
at least one display source projecting light waves located within a second optical
spectrum,
characterized in that the light waves from the light source and light waves from
the display source are coupled into the substrate by total internal reflection.
2. The optical system according to claim 1, wherein the first optical spectrum is
different than the second optical spectrum.
3. The optical system according to claim 1, wherein the first optical spectrum is
in the near IR region.
4. The optical system according to claim 1, wherein the second optical spectrum
is in the photopic range.
5. The optical system according to claim 1, wherein the light waves coupled
inside the substrate are light waves of a first angular spectrum and a second angular
spectrum, and wherein angles located within the first angular spectrum are smaller
than angles located within the second angular spectrum.
6. The optical system according to claim 5, wherein a first partially reflecting
surface is partially reflective for light waves located within the first optical spectrum
and within the first angular spectrum, and substantially transparent for light waves
located within the second optical spectrum and within the first and second angular
spectrum.
7. The optical system according to claim 5, wherein a second partially reflecting
surface is partially reflective for light waves located within the second optical
spectrum and within the first angular spectrum, and substantially transparent for light
waves located within the first optical spectrum and within the first and second
angular spectrum and for light waves located within the second optical spectrum and
within said second optical spectrum.
8. The optical system according to claims 6 and 7, wherein the first and second
partially reflecting surfaces couple light waves from the light source and the display
source, respectively, out of the substrate into an eye of a viewer.
9. The optical system according to claim 6, wherein the first partially reflecting
surface couples light waves reflected from the viewer's eye and located within the
first optical spectrum into the substrate, by total internal reflection.
10. The optical system according to claim 9, further comprising an eyeball tracker
wherein the light source is part of the tracker.
11. The optical system according to claim 10, wherein the optical mean couples
light waves reflected from the viewer's eye out of the substrate into the eyeball
tracker.
12. The optical system according to claim 11, further comprising an optical
detector attached to, or embedded in, the eyeball tracker, wherein the light waves
which are coupled into the eyeball tracker are focused into the detector.
13. The optical system according to claim 11, further comprising a first
transmitting light guide, wherein light waves from the light source and from the
display source are coupled into the substrate through the transmitting light guide.
14. The optical system according to claim 10, wherein the transmitting light guide
is a polarizing beam-splitter.
15. The optical system according to claim 13, further comprising a second light
source projecting light waves located in the second optical spectrum.
16. The optical system according to claim 15, further comprising a second
transmitting light guide, wherein light waves from the first light source and from the
second light source are coupled into the first transmitting light guide through the
second transmitting light guide.
17. The optical system according to claim 16, wherein the second transmitting
light guide is a dichroic beam-splitter.
18. The optical system according to claim 15, wherein the display source is an
LCOS.
19. The optical system according to claim 18, wherein the LCOS is illuminated by
light waves from the second light source.
20. The optical system according to claim 1, further comprising an optical filter
located at the means for coupling light waves into the substrate.
21. The optical system according to claim 20, wherein the optical filter is
reflective for the first optical spectrum and is substantially transparent for the second
optical spectrum.
22. The optical system according to claim 20, wherein the optical filter is
reflective for the second optical spectrum and is substantially transparent for the first
optical spectrum.
23. The optical system according to claim 1, wherein the first and second partially
reflecting surfaces are laterally separated from each other.
24. The optical system according to claim 1, wherein the first and second partially
reflecting surfaces are located adjacent to each other.
25. The optical system according to claim 1, wherein at least one surface of the
array of partially reflecting surfaces is partially reflective for light waves located
within the second optical spectrum and within the first angular spectrum, and
substantially transparent for light waves located within the first optical spectrum and
within the first and second angular spectrum and for light waves located within the
second optical spectrum and within the second angular spectrum.
26. The optical system according to claim 1, further comprising a second optical
means for coupling light waves into the substrate by total internal reflection, wherein
light waves from the first light source and light waves from the display source are
coupled into the substrate by total internal reflection by the first and second optical
means, respectively.
27. The optical system according to claim 26, wherein light waves from the first
light source and light waves from the display source impinge on one of the two
optical means at a given incident angle, and wherein one of the two optical means has
high reflectance for light waves in the photopic range at said incident angle and high
transmittance for light waves from the first light source at the incident angle.
28. The optical system according to claim 26, wherein light waves from the first
light source and light waves from the display source impinge on one of the two
optical means at a given incident angle, and wherein one of the two optical means has
high transmittance for the light waves in the photopic range at said incident angle and
high reflectance for light waves from the first light source at the incident angle.
29. The optical system according to claim 10, wherein the eyeball tracker detects
blinking patterns of a viewer.
30. The optical system according to claim 29, wherein the eyeball tracker serves as
a drowsy driver alerting unit.