COMPACT HEAD-MOUNTED DISPLAY SYSTEM HAVING UNIFORM IMAGE
There is disclosed an optical device, including 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 reflect ing 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 in side the light-transmitting substrate are partially reflected from the interface plane and partially pass therethrough.
The present invention relates to substrate-guided optical devices, and particularly to devices which include a plurality of reflecting surfaces carried by a common light-transmissive substrate, also referred to as a light-guide 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 INVENTIONOne 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-see-through 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 INVENTIONThe 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 direct-display 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.
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:
As can be seen in
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 α′ in =180°−αin and the incident angle between the trapped ray and the normal to the reflecting surface is:
wherein the minus sign denotes that the trapped ray impinges on the other side of the partially reflecting surface 22.
As illustrated in
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
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
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.
As illustrated in
Since the trapped angle αin 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.
As seen in
T=d−Dn·cot(αsur) (5)
As illustrated in
As illustrated in
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
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
As illustrated in
Similarly, as illustrated in
As illustrated in
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 inFIGS. 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.
- d. 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°. - e. 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 beam-splitting interface plane 167. 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 ofFIGS. 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. - f. 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.
- g. 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
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 light-transmitting 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 beam-splitting 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.
Type: Application
Filed: Feb 10, 2016
Publication Date: Feb 8, 2018
Inventor: Yaakov Amitai (Rehovot)
Application Number: 15/549,603