Substrate-guided optical devices

The present invention relates to substrate-sguided 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.

The invention cam be implemented to advantage in a large number -of imaging applications, such as, for example, head-mounted and head-up displays (L3MD s and HUD’s), cellular phones, compact displays, 3-D dispEays, compact beam expanders, as well as non-imagzing applications such as fTat-panel indicators, compact illuminators and scanners.

Background of the Indention

One of the important applications for compact optical elements is- in HMD’s wherein an optical module serves both as an imaging Bens 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 liglrt 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 displary comprises an array of elements (p-ixels) 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 see-thrrough applications, ^respectively. Typically, a conventional, tree-space optical module is used for these purposes. As the desired field-of-viesw (FOV) of the system increa_ses, such a conventZonal optical module becomes larger, heavier and bulkier,' and- therefore, even for moderate performance devices, izmpractical. This is a major drawback for all kinds of displays but especially in head—mounted applications, whereim the system must necessarily be as light and as compact as possible.

The strive for compactness has led to several different com_plex optical solutions, all of whicta, on the one hand, are still ncot sufficiently comjwact for most

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WO 2005/024491 practical applications, and, on the other hand, suffer major drawbacks in Menns of manufacturability. Furthermore, the eye-motion-box (EMB) ofthe optical viewing angles resulting from these designs is usually very small - typically less thazn 8 mm. Hence, the performance of the optical system is very sensitive, even ~to small movements ofthe optical system relative to the eye of the viewer, and does mot allow sufficient pupil motion for conveniently reading text from such displays.

Disclosure of the Invention

The present invention facilitates the structure and fabrication of very compact light-guide optical elements (LOE) for, amongst other applications, head—mounted displays. The invention allows relatively wide FOV’s 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 offer-ed 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 tiaving specialized configurations.

The invention also enables the construction of improved HUD’s. HLJD’s have become popular and they miow play an important role,, not only in most modesm combat aircrafts, but also in civi lian aircrafts, in which HUD systems have become a key component for low-visibility landing operation. Furthermore, there havee recently been numerous proposals and designs for HUD’s in automotive applications where they can potentially assist the driver in driving and navigation duties. Nevertheless, state-of-the-art HUD’s suffer several significant drawbacks. All HUES’s of the current designs require a display source that must be offset a significant distance from the combiner to ensure thiat the source illuminates the entire combiner surface. As a result, the combiner-projector HUD system is necessarily bulky and Marge, and requires considerable installation space, making it inconvenient for installation and, at times, even unsafe to use. The large optical aperture of conventional HUDs also pose a significant optical clesign challenge, rendering the HUD’s with either a compromising performance, or leading to high cost wherever high-performance is

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WO 2005/024491 required. The cErromatic dispersion of high-quality holographic HUD’s is of particular concern.

An important application of the present invention relates to its imple~rnentation in a compact HUD, which alleviates the aforementioned drawbacks. In the HUD design of the current invention, the combiner is illuminated with a compact display source that can b-e attached to the substrate. Hence, the overall system is very compact and can readily be installed in a variety of configurations for a wi<L.e range of applications. In addition, the chromatic dispersion of the display is negligible and, as such, can operate with wide spectral sources, including a conventional white-light source. In additiom the present invention expanxIs the image so that the actdve area of the combiner can be much larger than the area tliat is actually illuminated toy the light source.

A further application of the present invention is to provide a compact display with a wide FOV for mobile, hand-held application such as cellular pMiones. In today's wireless Enternet-access market, sufficient bandwidth is availab le for full video transmission. The limiting factor remains the quality of the display within the end-user’s devices. The mobility requiremer*t restricts the physical size of the displays, and the result is a direct-display wifi* a poor image viewing quiality. The present invention enables, a physically very connpact display with a very Inrge virtual image. This is a key feature in mobile comnxnnications, and especially for mobile Internet access, solving one of the main limitations for its practical implementation. Thereby, the present invention enables the viewing of the digital content of a full format Internet pa_ge within a small, hand-held device, such as a cellular phone.

A broad otoject of the present invention, Wherefore, is to alleviate the drawbacks of state-of-the-art compact optical display devices and to provide otiher optical components and systems having improved performance, according ~to specific requirements.

The invenation therefore provides an optical device, comprisiiMg a flighttransmitting substrate having at least two major surfaces parallel to eacta other and

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PCT/IL2004/000813 edges; optical meanss for coupling light waves located in a fi^ld-of-view into said substrate by internal reflection, and at least one partially reflecting surface located in said substrate whieh is non-parallel to said major surfaces of the substrate, characterized in tha~t at least one of said major surfaces is coated with a dichroic coating.

Brief Description olf the Drawings

The invention is described in connection with certain prreferred embodiments, with reference to tBne following illustrative figures so that i_t 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 ancd conceptual aspects of the invention. In thi~s 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 ta_ken with the drawings are to serve as direction to those skilled in the art as to how tine several forms of the invention may be ennbodied in practice.

In the drawirings:

Fig. 1 is a side view of a prior art folding optical device;

Fig. 2 is a side viesw of an embodiment of a LOE, in accordance with the present invention;

Figs. 3A and 3B illustrate the desired reflectance and transmittance characteristics of selectively reflecting surfaces used in the present invention for two ranges of incident angles;

Fig. 4 illustrates the reflectance curves as a function of wavelength for an exemplary dichroic coating for P-polarization;

Fig. 5 illustrates a reflectance curve as a function of wavelemgth for an exemplary dichroic coating for S-polarization;

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Fig. 6 illustrates the reflectance curves as a function of incident angle for an exemplary dichroic coating;

Figs. 7 is a diagram illustrating detailed sectional views of an exemplary arra^y of selectively reflective surfaces;

Fig. 8 illustrate s the reflectance curves as a function of incident angle for an another dichroic coating;

Fig. 9 illustrates an exemplary embodiment of the* present invention embedded in a standard- eye-glasses frame, and

Fig. 10 illustrates an exemplary HUD system in accordance with the present invention;

Detailed Description of Preferred Embodiments

Fig. 1 illustrates a conventional folding optics arrangement, wherein the substrate 2 is illuminated by a display source 4 . The display is collimated by a collimating lens 6. The light from the display source 4 is coupled into substrate 2 by a first reflecting surface 8, in such a way that the main ray 10 is parallel to the sub· str ate plane A second reflecting surface 12 couples the light out of the substrate an*d into the eye of a -viewer 14. Despite the compactness of this configuration, it snuffers significant drawbacks; in particular only a very limited FOV can be affectecS. As shown in Fig. 1, the maximum allowed off-axis arvgle inside the substrate is:

₊ (1) «max ⁼ ~-1 ’ wherein T is the substrate thickness;

d_eye is the desired exit-pupil diameter, and

I is thes distance between reflecting surfaces 8 and 12.

With angles higher than the rays are reflected from the substrate surface before arriving at the reflecting surface 12. Hence, the reflecting surface 12 —will be illuminated a~t an undesired direction and ghost images appear.

Therefore, the maximum achievable FOV “with this configuration is:

, (2) wherein v is -the refractive index of the substrate.

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Typically th_e refractive index values lie in the range of 1.5-1.6.

Comrmonly, the diameter of the eye pupil is 2-6 mm. To a_ccommodate movement «or misalignment of the display, a larger exit-pupil diameter is necessary. Taking the minimum desirable value at approxiimately 8 to 10 mm, the distance between the optical axis of the eye and the side of the head, I, is, typicallyz, between 40 and 80 mm. Consequently, even for a small FCV of 8°, the desirred substrate thickness would be of the order of 12 mm.

Methods have been proposed to overcome the above problem. TThese include, utilizing a_ magnifying telescope inside the subistrate and non-paraallel coupling directions. Even with these solutions, however, and even if only one reflecting surface is considered, the system thickness remam_s limited by a similar value. The FOV is limited by the diameter of the projection of the reflective surfTace 12 on the substrate jlane. Mathematically, the maximui» achievable FOV, due to this limitation, is expressed as;

Ttana^-cf^ (3)

FOV „-----ii--2S ^x max n ’ . ^eye wherein is the angle between the reflectin_g surface and the monnal to the substrate [plane, and

R_ey^ is the distance between the eye of the -viewer and the substrate (typically, about 30-^40 mm).

Practically tana^. cannot be much larger than 1; hence, for the same parameter's described above for a FOV of 8°, the : required substrate thxickness here is on the order of 7 mm, which is an improvement on the previous limit. Nevertheless, as the desired FOV is increased, the substrate thickness increases rapidly. For instance, for desired FOVs of 15° and 30° the substrate limiting thickness is 18 mm or 25 mm, respectively.

Teo alleviate the above limitations, the present invention utiliszes an array of selectiveLy reflecting surfaces, fabricated within ai LOE. Fig. 2 illusti—ates a sectional view of air. LOE according to the present inventicon. The first reflectirag surface 16 is

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PCT/IL22004/000813 illuminated by a collimated input plane wave 18, emanating from a display light source (not shown) located behind the device, wherein the plane wave ZL8 is one of a set of light -waves located within a given FOV to be coupled into th-e LOE. The reflecting suuface 16 reflects the incident liglrt from the source such tiMat the light is trapped inside a planar substrate 20 by to<al internal reflection. After several reflections off the surfaces of the substrate, the trapped wave reaches an array of selectively reflecting surfaces 22, which couple the light wave 23 out o if the substrate into the EMCB 24 of a viewer. For avoiding ^host images, the output light wave 23 should be plane wave, otherwise, different rays representing a singlie point at the display soucce will arrive at the EMB 24 of the viewer at different incid-.ent angles and ghost images that interfere with the primary image will be seen by Are viewer. In order to present this phenomenon, the output light wave 23, and hence Bhe input wave 18, should fee plane waves. That is, the angular deviation between two» different rays located on “the same light wave should be less than a_res, wherein a_r&s is the angular resolution 'of the optical device. Usually, for most visual systems, c^. is ~ 1-2 milliradians, but different devices can yield di fferent angular resolutions.

Assigning that the central wave of the source is coupled out of tfee substrate 20 in a direction normal to the substrate surface 26, and the off-axis angle of the coupled wave inside the substrate 20 is a/™ then the angle between the refl-ecting surfaces and the sub»strate plane is:

a A W “^r2 2 '

As can be seen in Fig. 2, the trapped rays arrive at the reflecting surfaces from two distinc-t directions 28, 30. In this particular embodiment, the trapyped rays arrive at the reflecting surface from one of these directions 28 after an ewen number of reflections from the substrate surfaces 26, whierein the incident angle [5_ref between the trapped ray* and the normal to the reflecting surface is:

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S (5) βref ^— ^tn ^— &sur2

The trapped rays arrive at the reflecting surface from the second direction 30 aftee- an odd number of reflections from th_e substrate surfaces 2^, where the off-axis angle is a ’_in = 180^o-a^ and the incident angle between the trappe d ray and the normal to tine reflecting surface is:

1^ = ^-0..2-^ ^.,-02,.2=^^ . («)

In order to prevent undesired reflec tions and ghost image s, the reflectance for one of these two directions should be negligible. The desired discrimination between the two incident directions can be achieve d if one angle is signi_ficantly smaller then the ether one. It is possible to provide a coating with very lov=v reflectance at high inci dent angles and a high reflectance for low incident angles. This property can be exploited to prevent undesired reflectioEis and ghost images by eliminating the refleectance in one of the two directions . For example choosing 25° from Equations (5) and (6) it can be calculated tkiat:

β'* = 105° ; = 50° ; a’* = *30° ; = 25° . (7)

If now a reflecting surface is determined for which P’_r^/is not reflected but &_ref is, the desired condition is achieved . Fig. 3A and 3B Illustrate the desired refleectance behavior of selectively reflecting surfaces. While the ray 32 (Fig. 3A), having an off-axis angle of $_ref~ 25°, is partially reflected and is coupled out of the substrate 34, the ray 36 (Fig. 3B), which arrives at an off-axis angle of 3^-75° to the reflecting surface (which is equivalent to β’η>/~ 105°), is transmitted through the reflecting surface 34 without any notable reflection.

Figs. 4 and 5 show the reflectance curves of a dichroic- coating designed to achfieve the above reflectance characteristics, for four different incident angles: 20°, 25°-, 30° and 75°, with P-polarized and S-polarized light respectively. While the reflectance of the high-angle ray is negligible over the entire relevant spectrum, the

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PCT<TL2004/000813 rays at off-axis angles of 20°, 25° and 30°, obtain almost constant refl-ectance of 26%, 29% and 32% respectively, for P-polarized light, and 32%, 228% and 25% respectively, for S-polarized. light, over the same spectrum. Evidently, reflectance decreases with the obliquity of the incident rays for P-polarized light and increases for S-polarized light.

Fig. 6 illustrates the reflectance curves of the same dichroic coating, as a function of the incident angle for both polarizations at wavelength λ=550 nm. There are two significant regions in this graph: between 65° and 80° where “the reflectance is very low, and between 15° and 40° where the reflectance changes monotonically with decreasing incident angles (increasing for P-polarized light ancl decreasing for S-polarized light). Hence, as long as one can ensure that the entire angular spectrum □f where very low reflections are desired, will be located insi<Me the first region, while the entire angular spectrum of where higher reflections are required, will be located inside the second region, for a given FOV, one can ensure the reflection of only one substrate mode into the eye of the viewer and a ghost-free innage.

There are some differences between the behaviors of the two polarizations. The main differences are that the region of high angles, where the reflectance is very low, is much narrower for the S-polarization and that it is much more difficult to achieve a constant reflectance for a given angle over the entire spec-tral bandwidth for the S-polarized light than for the P-polarized light. It is therefore preferable to design the LOE only for the P-polarized light. This would be satisfactory for a system using a polarized display source, such as an LCD, or for a system where the output brightness is not crucial an.d the S-polarized light can be filtered out. However, for an unpolarized display source, like a CRT or an OLED, or for a system where the brightness is critical, S-polarized light cannot be neglected and it anust be taken into account during the design procedure. Another difference is ttaat the monotonic behavior of the S-polarized light at the angular spectrum of where higher reflections are required, is opposite to that of the P-polarized light, that is, the reflectance for the S-polarized light increases with the obliquity of the incident rays.

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Thi s contradictory behavior of the two polarizations at the angular spectrum of β«/ coirfd be utilized during the optic=al design of the system to achieve the desired reflectance of the overall light accor ding to the specific requirements of each system.

Assuming that the coupled wave illuminates the entire area of the reflecting surface, after reflection from the surface 16, it illuminates an area of 2Si = 23”tan(a) on the substrate surface. On the ot her hand, the projection of a reflection surface 22 oil the substrate plane, is S2 — T ianfosuri)· To avoid either overlapping or gaps beiween the reflecting surfaces, the projection of each surface is adjacerf to its neighbor. Hence, the number N of reflecting surfaces 22 tbeiough which each coupled _raZy passes during one cycle (i.e., between two reflections from the same surface of the sui-bstrate) is:

_^2T-cot(g_wl) .

S₂

In this example, where = 25° and = 25°, tfrie solution is N = 2; that is, each ray passes through two different surfaces during one cycle.

The embodiment describee! above with regard to. Tig. 7 is an example of a method for coupling the input waves into the substrate. Input waves could, “however, a Iso be coupled into the substrate t>y other optical means, iwicluding, but not limited to, folding prisms, fiber optic bundles, diffraction gratings, an_d other solutions.

Also, in the example illustrated in Fig. 2, the input waves and the imatge waves aure located on the same side office substrate. Other configurations are envisioned in v=vhich the input and the image waves could be locate*! on opposite sidles of the substrate. It is also possible, in certain applications, to couaple the input waves into the substrate through one of the substrate’s peripheral sides.

Fig. 7 is a detailed section_al view of an array of selectively reflective surfaces which couple light, trapped insid«e the substrate, out and into the eye of a viewer. As ean be seen, in each cycle the coupled ray passes through, reflecting surface s 43, at an ^ngle of α’_ίΛ = 130°, whereby the angle between the ray and the nonanal to the reflecting surfaces is ~75°. Th© reflections from these surfaces are negligible. In

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PCT/IL2004/000813 addition, the ray passes twice through the reflecting surface 44, in each cycle, at an angle of a,,, = 50°, whea-e the incident angle is 25°. Part of the emergy of the ray is coupled out of the substrate. Assuming that one array of two seLectively reflecting surfaces 22 is used to couple the light onto the eye of a viewer, the maximal FOV is:

&OV (9) max n

Hence, for the s ame parameters of the examples above, the limiting substrate thickness for an FOV of 8° is in the order of 2.8 mm; for FOVs of 15° and 30°, the limiting substrate thicl-cness is 3.7 mm and 5.6 mm, respectively'. These are more favorable values than tZhe limiting thickness of the state-of-the-art solutions discussed above. Moreover, more than two selectively reflecting surfaces can be used. For example, for three selectively reflecting surfaces 22, the limiting substrate thickness for FOVs of 15° and 30° is approximately 2.4 mm and 3.9 mm, respectively. Similarly additional reflecting surfaces may be introduces to, amongst other· advantages, reduce foe limiting optical thickness further.

For configuration where a relatively small FOV is requir&d, a single partially reflecting surface can be sufficient. For example, for a system with foe following parameters: Reye ⁼ 25 mm ; = 72° and T= 5 nun, a moderate FOV of 17 can be achieved even with s single reflecting surface 22. Part of the rays will cross the surface 22 several tim_es before being coupled out into the desired direction. Since the minimal propagation angle inside the substrate to achieve the tot-al-intemal reflection condition for BK7 material or similar is a_in(_min) = 42°, the propagation direction of the central angle of the FOV is a._in(cen) = 48°. Consequently, the projected image is not normal to the surface but is rather inclined to 12° off-axis. Nevertheless, for many applications this is ac ceptable.

Unfortunately». this solution is not always feasible. For ma_ny other applications there is a constraint that the projected image should be norwnal to the substrate surface. Another problem, which is associated with the total internal reflection condition, is the maximal FOV of the image that can be trapped! inside the substrate.

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Unfortunately, dt is very difficult to acteieve very low reflectance for off-axis angles exceeding 82°. Assuming that the reqinired FOV angle inside th«e substrate is o.fok the maximal incoident angle between the central wave and the nomnal to the reflecting surface is

Assumimg an external FOV of 30°, which corresponds to ~20° inside the substrate, yields β'„^ = 72°. Inserting thus value into Eq. (6) yields «/„=48°, and hence the minimal required angle of the trappe d wave is a-1 ·» = ^ai„ ~= 38° . (11) in 2r

Clearly, this angle cannot be trapped inside BK7 or other similar materials. It is true that therre are flint optical materials with higher refractive indices, which can exceed 1.8, however, the transparency of these materials is usuaLly not high enough for substrate-mcade optical elements. Ar·other possible solution is Co coat the substrate surfaces not wi<h regular anti-reflection coatings but with angular—sensitive reflecting coatings that trsp the entire FOV inside the substrate even for lowver angles than the critical angle. ZIt must be noted that ev en for a non see-through applications, where one of the sufcstrate surfaces can be opaque and hence can be coated with a conventional rezflectmg surface, the other surface, the one which is next to the eyes of the viewer, should be transparent, at least for the angles of the required external FOV. Therefore, the wequired reflecting coating should have very high reflectance for the region of angles lower than the critical angle and veiy high reflectance for the entire FOV of the ima_ge.

Fig. 8 sImows the reflectance curves of a dichroic coating designed to achieve the above refle ctance characteristics, aa a function of the incident angle, for both polarizations at the wavelength λ=55Ο nm, where the angle is measured in air. >

Evidently, there are two significant regions in this graph: bet“ween 30° and 90° (equivalent to 2lC° - 42° inside the substrate) where the reflectanc e is very high; and

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PCT/EL2004/000813 between 0° and 22° (equivalent to 0° - 15° inside ths substrate) where the reflectance Ls very low. Hence, as long as one can ensure that tfce entire angular spectrum of α,η, where very high reflections are desired, will be loc ated inside the first region, while the entire angular spectrum of exterior FOV, whewe essentially zero reflections are required, will be located inside the second region, for a given FOV, one can ensure that the entire FOV will be trapped inside the substr ate by internal reflections and that the viewer can see the whole image. It is importanzt to note that since the fabrication process of the LOE usually involves cementing optical elements and since the required angular-sensitive reflecting coating is applied to the substrate surface only after the LOE body is complete, it is not possible to utilize the conventional hot-coating procedures that may damage the cenaented areas. Fortunately, novel thin-film technologies, as ion-assisted coating procedures, can also be used for cold processing. Eliminating the need to heat parts allows cemented parts, such as LOEs, to be safely coated.

In general, LOE offer several important adzvantages over alternative compact optics for display applications, which include:

1) The input display source can be located very close to the substrate, so that the overall optical system is very compact and lightweight, offering an unparalleled form-factor.

2) In contrast to other compact display configurations, the present invention offers flexibility as to location of the input display source relative to the eyepiece. This flexibility, combined with the ability to locate the source close to the expanding substrate, alleviates the need to use an off-axis oj*tical configuration that is common to other display systems. In addition, since the imput aperture of the LOE is much smaller than the active area of the output aperture, the numerical aperture of the collimating lens 6 is much smaller than required for a comparable conventional imaging system. Consequently a significantly mo«re convenient optical system can be implemented and the many difficulties associated with off-axis optics and high

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PCT/IL2004/000813 numerical-aperture lenses, such as field or chromatic aberrations can be compensated for relatively easily and efficiently.

3) The reflectance coefficients of the selectively reflective surfaces in the presemnt invention are essentiality identical over the entire relevant spectrum. Hence, both monochromatic and polychromatic, light sources may be used as display sources. Ttie LOE has a negligible wavelength-dependence ensuring high-quality color displays with high resolutions.

4) Since each point from the input display is transforrmed into a plane wave that is reflected into the eye of the viewer from a large part of the reflecting array, tHae tolerances on the exact location of the eye can be signifLcantly relaxed. As such, tBae viewer can see the entire FOV, and the EMB can be significantly larger than in oth_er compact display configurations.

5) Since a large part of the intensity from the display source is coupled into tfee substrate, and since a large portion of this coupled energy is “recycled” and coupled out into the eye of the= viewer, a display of comparatively high brightness can fee achieved even with disjwlay sources with low power consumption.

Fig. 9 illustrates an embodiment of the present indention in which the LOE 20 is embedded in an eye-glasses frame 58. The display souirce 4, the collimating lens 6, and the folding lens 60 are assembled inside the arm portions 62 of the eye-glass«es frame, just next to the edge of the LOE 20. For a case im which the display source is an electronic element siach as a small CRT, LCD, or OLED, the driving electronics ¢54 for the display source might be assembled inside the back portion of the arm 62. A power supply and data interface 66 is connectable to arm 62 by a lead 68 or oth er communication means including radio or optical transmission. Alternatively, a battery and miniature data link electronics can be integrated in the eye-glasses frame .

The embodiment described above can serve in both see-through and non-see-through systenxs. In the latter case opaque layejrs are located in front of ttie LOE. It is not necessary to occlude the entire LOE, topically only the active area, where the display is visible needs to be blocked. As suc h, the device can ensure th_at

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1_5 ithe peripheral vision of the user is maintaiiMed, replicating the viewing experiena.ce of a -computer or a television screen, in which such peripheral vision serves an important cognitive function. Alternatively, a variable filter can be placed in front of the system in such a way that the viewer can control tJtie level of brightness of the light ennerging from the external scene. This variable filter could be either a mechanically controlled device such as a folding filter, or two rota_ting polarizers, an electronically controlled device, or even an automatic device, ^whereby the transmittance of the Citer is determined by the brightness of the external background. One method to achEeve the required variable transmittance filter is t® use electrochromic materials in order to provide electrical control of optical transmittance, wherein materials with electrically controllable optical properties are incorpoirated into laminated structures.

There are some alternatives as to the precise way in which an LOE can be utilized in this embodiment. The simplest option is to use a single element for one eye. Another option is to use an element and a display source for each eye, tout with the same image. Alternatively it is possible to project two different parts of Aie same image, with some overlap between the tw-o eyes, enabling a wider FOV. Yet another possibility is to project two different scenes, one to each eye, in order to create a stereoscopic image. With this altematiwe, attractive implementations are possible, including 3-dimensional movies, advanced virtual reality, training systems an<3. others.

The embodiment of Fig. 9 is just an example illustrating the simple implementation of the present invention. Since the substrate-guided optical dement, constituting the core of the system, is wery compact and lightweight, it could be installed in a vast variety of arrangements. Hence, many other embodiments are also possible including a visor, a folding. display, a monocle, and many more. This embodiment is designated for applications where the display should be neazr-to-eye: head-mounted, head-worn or head-carrie®.

The embodiment described above is a mono-ocular optical system, thsat is, the image is projected onto a single eye. There are, however, applications, such as head-up displays (HUD), wherein it is clesired to project an image onto both eyes.

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Until recently, HUD systems have been used mainly in advanced combat and civilian aircrafit. There have been numerous proposals and designs, of late, £o install a HUD in fromt of a car driver in order to assist in driving navigation or to project a thermal image into his eyes during-low-vi sibility conditions. Current aerospace HUD systems are vezry expensive, the price of a single unit being in the order of hundreds of thousaznds of dollars. In addition, the existing systems are very large, heavy, and bulky, and are too cumbersome for installation in a small aircraft let alone a car. LOE-tuased HUD potentially provide the possibilities for a very compact, self-contained HUD, that can be «readily installed in confined spaces. It also simplifies the construction and manufacturing of the optical systems related Λο the HUD and therefore is a potentially suitable for both improving on aerospace HUD’s, as well as introducing a compact, inexpensive, consumer version for the automotive industry.

Fig. 10 illustrates a metbiod of materializing an HUD system based on the preseiMt invention. The light from a display source 4 is collimate d by a lens 6 to infinity and coupled by the first reflecting surface 16 into substrate 20. After reflection at a second reflecting array (not shown), the optical waves impinge on a third reflecting surfaces 22, which couples the light out into the eyes 24 of the viewer. The overall system can be very compact and lightweight, of the size of a large postcard having a thickness of a few millimeters. The display source, having a volurrae of a few cubic centimeters, can be attached to one of tine comers of the substrate, where an electric wire can transmit the power and data to the system. It is expected that the installation of the presented HUD system vwill not be more complicated than the installation of a simple commercial audio system. Moreover, since there is no need for an external display source for image projection, the necessity to install components in unsafe places is avoided.

The embodiments illustrated in Fig. 10 can be implemented for other applications, in addition to HUD systems for vehicles. One possible utilization of these embodiments is as a flat display for a computer or television. The main unique characteristic of such a display is that the image is not located at the; screen plane, but

WO 2005/024491

PCT7II .2004/000813 is focused at infinity or to a similarly convenient distance. One of the main drawbacks of existing computer displays is that the user has to focus his eyes at a very close distance of between 40 and 60 cm, while the natural focus of ®, healthy eye is to infinity. Many people suffer from headaches after working for a long duration of time at a computer. Many others who work frequently with computers tend to develop myopia. In addition, some people, who suffer from t>oth myopia and hyperopia, need specia_l spectacles for work with a computer. A flat display, based on the present invention, could be an appropriate solution for people wl~io suffer from the above-described probLems and do not wish to work with a head—mounted display. Furthermore, the present invention allows for a significant reduction in the physical size of the screen, /^s the image formed by the LOE is larger ttian the device, it would be possible to izmplement large screens on smaller frames. T~‘his is particularly important for mobile a_pplications such as lap and palm-top computer's.

Yet another pojssible implementation of this embodiment is as a screen for a personal digital assistance (PDA). . The size of the existing con_ventional screens which are presently u sed, is under 10 cm. Since the minimal distance where these displays can be read is on the order of 40 cm, the obtainable FOV is under 15°; hence, the information contemt, especially as far as text is concerned, on_ these displays is limited. A significant improvement in the projected FOV can t>e made with the embodiment illustrate*! in Fig. 10. The image is focused at infini<y, and the screen can be located much c loser to the eyes of the viewer. In addition, since each eye sees a different part of tire total filed-of-view (TFOV), with an overlap at its center, another increase in the TFOV may be achieved. Therefore, a display with an FOV of 40° or larger is feasibl e.

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 o~ther specific forms without departing from the spirit or essential attributes thereof. Tfoe present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated

VWO 2005/024491

PCT/IL2004/000813 by the appended claims rather than by the foregoing description, and all changes whajch come within the meaning and range of equivalency of⁷ the claims are therefore intended to be embraced therein.