Optical wedge redirection apparatus and optical devices using same

An exit pupil expander (904), operable as a numerical aperture expander and suitable for use with high angle of incidence scanned laser projection systems, includes a microlens array (910) and a varied thickness optical element (900). The varied thickness optical element can be configured to transform a principal beam (953) of a received scan cone (952) to be substantially orthogonal with an output of the exit pupil expander (904) or major surface of the microlens array (910). Further, the varied thickness optical element (900) can be configured to cause the received scan cone (952) to exit the varied thickness optical element (900) substantially symmetrically about the principal beam (953). The varied thickness optical element (900) can also be configured to introduce a controlled amount of spread to the received scan cone (952). The varied thickness optical element (900) is useful in correcting distortion, such as keystone distortion introduced by high angle of incidence feed.

BACKGROUND

1. Technical Field

This invention relates generally to optical devices, and more particularly to optical redirection devices.

2. Background Art

Scanned laser projection devices facilitate the production of brilliant images created with vibrant colors. Scanned systems, such as those manufactured by Microvision, Inc., are capable of creating bright, sharp images with a large depth of focus. Additionally, these scanned laser projection systems can be designed with compact form factors at a reasonable cost. These systems consume small amounts of power yet deliver vivid, complex images.

Scanned laser projection devices are frequently used in sophisticated projection systems such as head-up displays and near-to-eye displays. In such applications, lasers present information to a user, either by presenting the information on a projection surface or by delivering the information directly to the user's eye.

One challenge associated with these systems is size reduction. It can be desirable to make the systems smaller, so that the projection systems can be used in compact applications, such as with eyeglasses or goggles. However, as the optical components become smaller, issues can arise. Distortion of images can be introduced. Similarly, optical artifacts can become a problem.

It would be advantageous to have a compact projection system that does not introduce distortion into projected images.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

It will be appreciated that embodiments of the invention described herein may be comprised of one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the systems and applications set forth below. The non-processor circuits may include, but are not limited to, microprocessors, scanning mirrors, image spatial modulation devices, memory devices, clock circuits, power circuits, and so forth. As such, the functions and operative states shown herein may be interpreted as steps of a method. Alternatively, some or all functions employed by the one or more processors to control the various elements herein, including the spatial light modulator, beam translator, and light translation element, could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits, in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. It is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such programs and circuits with minimal experimentation.

FIG. 1illustrates a block diagram generally setting forth the elements of a head-up or near-to-eye projection system100. While embodiments of the invention described herein are suitable for use in any number of different applications, for ease of discussion a near-to-eye projection system will be described to illustrate the operation of the various elements. While the elements may change in size or form, their operation will generally be the same in head-up display systems. Those of ordinary skill in the art having the benefit of this disclosure will readily recognize the scanning engines, beam redirecting devices, and microlens arrays described below may be used in any number of other applications as well. Accordingly, the scope of the claims is not intended to be limited by the illustrative application used for description purposes.

The system100includes a laser projection source101, a scanner102, a control circuit103, an exit pupil expander104, and relay optics105. The system100uses these elements to present information to a user106. In a near-to-eye application, the information will be delivered directly to the user's eye. In a head-up display, a transparent projection surface may be employed upon which information can be presented.

While the laser projection source101can be a simple monocolor laser, it can alternatively comprise multiple lasers or an integrated multicolor laser device. In one embodiment, the laser projection source101includes a red laser, a blue laser, and a green laser. These lasers can be of various types. For example, for compact designs, semiconductor-based lasers can be used, including edge emitting lasers or vertical cavity surface emitting lasers. In other applications, larger, more powerful lasers can be used, alone or in combination. Where multiple lasers are used as the laser projection source101, one or more optical alignment devices (not shown inFIG. 1) may be used to orient the plurality of light beams into a single combined light beam. The alignment devices can further blend the output of each laser to form a coherent, multicolored beam of light. In one embodiment, dichroic mirrors can be used to orient the light beams into the combined light beam. Dichroic mirrors are partially reflective mirrors that include dichroic filters that selectively pass light in a narrow bandwidth while reflecting others.

The control circuit103, which may be a microprocessor or other programmable device, executes embedded instructions to control the scanner102, and optionally the laser projection source101as well. For example, in one embodiment the control circuit103is programmed to control the scanning of the light108received from the laser projection source101to form a desired image on the exit pupil expander104for delivery to the user106.

The laser projection source101delivers light107to the scanner102at an angle of incidence108determined by the physical geometry of the scanner102relative to the light projection source101(and any intermediate optical elements). In one embodiment, the scanner102is configured as a two-axis raster laser scanner capable of scanning the light across in a raster sweep pattern and delivers scanned light in the form of a scan cone109to the exit pupil expander104.

The exit pupil expander104serves as a “numerical aperture” expander that provides the user106with an expanded eye box110within which information may be seen. The expanded eye box110allows the user106to have a comfortable range of head or eye positions over which they are able to receive light from the system100. The eye focuses the light received from the system100and the user106sees a “virtual” image.

In one embodiment, the exit pupil expander104is disposed at an intermediate image plane of the system100. In one embodiment, the exit pupil expander104comprises an ordered array of microstructures or a randomized light diffuser. For example, as will be described in more detail below, in one embodiment the exit pupil expander104can be configured as a micro lens array. The exit pupil expander104can be manufactured from a molded liquid polymer, or may be formed via other methods. The exit pupil expander104may comprise single or complementary glass or plastic beads, or microspheres or nanospheres, or similarly shaped objects capable of functioning as an optical diffuser or lens. The exit pupil expander104may have optical properties resulting from a selected pitch, radius, or spacing of its constituent parts to expand incident light.

The relay optics105then transfer light received from the exit pupil expander104to the user106. The relay optics105, which can comprise one or more devices, are optical transfer devices that direct light from a relay input to a relay output. For example, the relay optics105can include a light-guiding substrate that defines the optical transfer properties of the overall relay. Regardless of application, in general the relay optics105uses a combination of elements with optical power, which may be lenses of curved reflectors, and a transfer medium, which may be free-space or may be a high-index medium surrounded by a low index medium, to transfer the image from the exit pupil expander104intermediate image plane to the viewing space where the observer is able to see the virtual or real image with their eyes.

In many cases, one element of the relay optics105is a combiner, which both reflects light from the display towards the users eyes and transmits light from the world around so that the users sees his normal view of the world overlaid with information from the display. In the case of a head-up display, the combiner element may be the car windshield or some other partial reflector. In the case of near-to-eye systems, the combiner may be a series of partially reflective surfaces, an example of which is described inFIG. 4below.

FIG. 2illustrates a more detailed view of the scanning engine200, which includes the laser projection source101and the scanner102. The illustrative scanning engine200ofFIG. 2is a Microelectromechanical System (MEMS) scanning engine. Examples of MEMS scanning light sources, such as those suitable for use with embodiments of the present invention, are set forth in commonly assigned US Pub. Pat. Appln. No. 2007/0159673, entitled, “Substrate-guided Display with Improved Image Quality,” which is incorporated by reference herein.

InFIG. 4, the MEMS scanning engine200employs three light sources201,202,203. A beam combiner204, which may employ the dichroic mirrors described above, combines the output of light sources201,202,203to produce a combined modulated beam. A variable collimation or variable focusing optical element205produces a variably shaped beam that is scanned by the MEMS scanning mirror206as a scanned light cone207. Examples of MEMS scanning mirrors, such as those suitable for use with embodiments of the present invention, are set forth in commonly assigned, copending U.S. patent application Ser. No. 11/775,511, filed Jul. 10, 2007, entitled “Substrate-Guided Relays for Use with Scanned Beam Light Sources,” which is incorporated herein by reference, and in US Pub. Pat. Appln. No. 2007/0159673, referenced above. The scanned light beam807can then be directed to the buried numerical aperture expander (105).

FIG. 3illustrates the scanning engine200ofFIG. 2delivering light to a relay300and corresponding optics in a near-to-eye application. Specifically, the MEMS scanning engine200launches the scan cone207into the relay optics, which include an exit pupil expander304, a lens305, and an input coupler302. The scan cone207first arrives at the exit pupil expander304. The exit pupil expander304delivers an expanded scan cone301through a lens305to an input coupler302of the optical relay300, which in this illustrative embodiment is a substrate-guided relay. Light then propagates through the optical relay300in accordance with the optical properties of the substrate to an output coupler303. In this embodiment, the light then is redirected from one or more partially reflective layers304to the user's eye306.

FIG. 4illustrates the system ofFIG. 3in operation. An eyewear device400using an optical relay300presents information directly into than eye401of a user402. The optical relay300receives light from an exit pupil expander304. A MEMS scanning engine200delivers light to the exit pupil expander304. The eyewear device400includes lens assemblies403that are coupled to a frame404. In this illustrative embodiment, the optical relay300is integrated into the lens assembly403. In one embodiment, the eyewear device400and optical relay300are configured such that the user402can see images beyond the lens assemblies403at the same time the MEMS scanning engine200is delivering information.

Referring briefly back toFIG. 1, in such projection systems, so long as the angle of incidence107defined between the laser projection source101and the scanner102is small, the system works well and delivers nearly distortion free images to the user106. Experimental testing has shown that, in some cases, the overall size of the scanning engine can be reduced if the angle of incidence107is increased. However, testing has shown that when the angle of incidence increases beyond a design threshold, which can be about fourteen or fifteen degrees, noticeable distortion is introduced due to asymmetry in the scan cone109. The asymmetry results from the large angle of incidence107.

For example,FIG. 5illustrates a scanning engine500where the angle of incidence508is below the design threshold. The laser projection source501delivers light554to the scanner502. The light554comprises a feed beam that is delivered to the reflective surface of the scanner. The scanning action of the reflective surface redirects the light554in a sweep pattern to present an image550on the exit pupil expander505. The sweeping action of the reflective surface creates the scan cone552. A substantially center beam of the scan cone552is referred to as the “principal beam”553. The principal beam553generally defines a pointing direction of the scan cone552, and represents the direction of a feed beam reflected from the scanner502when the scan mirror is at its central rest position. The principal beam553also indicates the direction that the scan cone552propagates.

The light554is delivered to the scanner502at an angle of incidence508that is less than the design threshold. In this illustrative embodiment, the design threshold is about fourteen or fifteen degrees. Accordingly, the angle of incidence508may be nine or ten or eleven degrees. Since the angle of incidence508is below the design threshold, the scan cone552is substantially symmetrical about the principal beam553. Accordingly, the image550appears normal and is substantially free from distortion.

FIG. 6illustrates the scan cone552ofFIG. 5in more detail. As shown inFIG. 6, the scan cone552is substantially proportional in area about the principal beam553. The scan cone552is said to be substantially proportional in area because a first side681of the scan cone552is substantially the same shape and/or length as a second side682of the scan cone552. Similarly, a top side683of the scan cone552is substantially the same shape and/or length as the bottom side684of the scan cone552.

The scan cone552is also substantially symmetrical about the principal beam553. The principal beam553is substantially disposed in the center of the scan cone552, with a first half685of the scan cone552disposed to the left of the principal beam553appearing to be similar in area with a second half686of the scan cone552disposed to the right of the principal beam553. (Note that the halves are shown to the left and right, but could also be shown as being defined above and below the principal beam553.)

Substantially symmetrical scan cones facilitate clear presentation of information without significant distortion. Turning briefly back toFIG. 5, the substantially symmetrical scan cone552, makes the information550projected on the exit pupil expander505clear and legible.

By contrast,FIG. 7illustrates a scanning engine700where the angle of incidence708is above the design threshold. In this illustrative embodiment, the angle of incidence708is about twenty-seven or twenty-eight degrees, far more than the threshold of fourteen to fifteen degrees mentioned above. This large angle of incidence708introduces asymmetry in the scan cone752, which results in the image750on the exit pupil expander705having a combination of pincushion, coma, and keystone distortion. This combined distortion manifests as the image750appearing to have a first side781that is “pinched” relative to the other side782due to the scan cone752being “tilted” about the principal beam753. Since the keystone distortion dominates, the result is an apparent image shape770resembling a keystone of an arch

While the principal beam753still defines the general pointing direction, the scan cone752disposed about the principal beam753is neither substantially symmetrical nor substantially proportional because planar surfaces are effectively being projected on a “tilted” plane due to high angle of incidence scanner feed. This creates a “tilted scan object plane” relative to the principal beam753, which appears as keystone distortion.

FIG. 8illustrates the scan cone752ofFIG. 7in more detail. The scan cone752is said to be substantially non-proportional because a first side881of the scan cone752appears substantially different in shape and/or length relative to a second side882of the scan cone752due to the tilted scan object plane relative to the principal beam753. Similarly, a top side883of the scan cone752substantially appears different in alignment and direction relative to the bottom side884of the scan cone752.

The scan cone752also appears as being substantially asymmetrical. The principal beam753is shown in the center of the scan cone752, with a first half885of the scan cone752disposed to the left of the principal beam753, and a second half886of the scan cone752shown to the right of the principal beam753. In this illustrative embodiment, the first half885and the second half886appear to have substantially different areas as viewed on the exit pupil expander705.

Substantially asymmetrical scan cones hinder clear presentation of information without significant distortion. Turning back toFIG. 7, the high angle of incidence708causes a substantially asymmetrical scan cone752, makes the image750projected on the exit pupil expander705appear distorted. While keystone error can sometimes be corrected in the projection system, it is not always desirable due to tight tolerance requirements and other distortion issues that can arise.

One may also note that the scanning engine700ofFIG. 7is oriented horizontally, while the scanning engine (500) ofFIG. 5was oriented vertically. The scanning engine700is shown as a horizontally fed system because experimental testing has shown that configuring MEMS scanners horizontally relative to the laser projection source allows the scanning engine to be manufactured in a more compact form factor. More compact form factors lend themselves better in many applications, including the near-to-eye application shown inFIG. 4above. However, horizontal alignment frequently requires an angle of incidence that is greater than the design threshold. Consequently, horizontally aligned systems frequently suffer from keystone distortion. It should be noted, however, that vertically aligned systems can also suffer from keystone distortion if the angle of incidence is beyond the design threshold.

Embodiments of the present invention provide a solution to the keystone distortion introduced in high angle of incidence systems, regardless of alignment. Embodiments of the invention are particularly useful in applications where an exit pupil expander and relay optics are employed. Such applications include the virtual image head-up displays and near-to-eye displays described above because both employ exit pupil expanders to create a large exit pupil or “eye boxes” at the user's eye.

While one can somewhat correct keystone distortion by tilting the scanning engine relative to the exit pupil expander (or other projection surface), this option is less than desirable. Tilting causes the principal beam to no longer be in the center of a displayed image. Additionally, the direction of propagation of the scan cone is no longer orthogonal to the exit pupil expander. This greatly complicates the relay optics. Said differently, the design of relay optics can be greatly simplified when the direction of propagation is normal to the exit pupil expander. Embodiments described below accomplish this task: redirection of a scan cone from a high angle of incidence scanning engine such that the scan cone is symmetrical about the principal beam and travels in a direction substantially normal to the exit pupil expander.

Turning now toFIG. 9, illustrated therein is an alternate exit pupil expander904suitable for use with high angle of incidence laser projection sources in accordance with one or more embodiments of the invention. “High angle of incidence” refers to systems where the angle of incidence between scanner902and light projection source901is greater than the design threshold.

The exit pupil expander904ofFIG. 9corrects for keystone distortion in high angle of incidence scanned laser systems without the problems associated with correction techniques applied at the projector level. For example, systems employing the exit pupil expander904can be manufactured less expensively than systems correcting keystone distortion at the projector level. Additionally, the tolerances associated with the manufacture of the exit pupil expander904are not as tight as those associated with projector level correction systems.

Anytime keystone error is corrected, astigmatism distortion can be introduced. In prior art systems, where keystone is corrected in the projection system, the astigmatism distortion causes the beam spot resolution to grow beyond desirable limits. The exit pupil expander904ofFIG. 9prevents this problem because any astigmatism that is generated does not have an opportunity to cause beam growth at the exit pupil expander904. Some growth may occur beyond the exit pupil expander904, but this is generally inconsequential in a head-up or near-to-eye application. Said differently, while the exit pupil expander904ofFIG. 9may introduce some minor astigmatism distortion, it does so at the intermediate image plane of the system rather than at the point of projection. Accordingly, any distortion is occurring at the plane of focus, thereby significantly reducing its impact. Accordingly, focused spots on the exit pupil expander904appear tighter than when using a conventional projection surface and a projection level correction technique.

The illustrative exit pupil expander904ofFIG. 9includes a numerical aperture expander905suitable for use with relay optics in a head-up or near-to-eye application. The numerical aperture expander905includes a first layer912and a second layer924, with a microlens array910disposed therebetween. The microlens array910ofFIG. 9is a complementary microlens array, as it includes microlens pairs that work in tandem. For example, microlens920and microlens921work together, with light exiting microlens920and entering microlens921while passing through the exit pupil expander904.

A varied thickness optical element900is disposed adjacent to microlens array910. In the illustrative embodiment ofFIG. 9, the varied thickness optical element900is configured as a wedge and is attached to the second layer924of the exit pupil expander904. The varied thickness optical element900is configured to transform a principal beam953of a scan cone952received from the scanner902to be substantially orthogonal with the output of the exit pupil expander904. The term “substantially” is used to refer to an angle that is generally orthogonal, but may not be exactly orthogonal due to manufacturing and design tolerances associated with components and the overall system.

Recall from above that the scanner902in a high angle of incidence system creates a scan cone952that is asymmetrically oriented about the principal beam1053. Additionally, in the illustrative embodiment ofFIG. 9, the scanner902is oriented in a non-orthogonal relationship with the microlens array910. Accordingly, the varied thickness optical element900is configured to do two things: First, it steers the received scan cone952such that the principal beam953enters the microlens array910at an angle that is substantially orthogonal with the first layer912and the second layer924. Second, it steers the remaining beams such that the received scan cone952exits the varied thickness optical element substantially symmetrically about the principal beam953. This is shown illustratively inFIG. 9with a first half985of the output cone928having substantially the same area as a second half986of the output cone928.

In one embodiment, the varied thickness optical element900is configured to perform a third task. In certain head-up display, near-to-eye display, and other optical systems, downstream components such as relay optics look to receive beams having a predetermined amount of spread. In one embodiment, the relay optics are configured to perform more optimally when the received light has a predetermined spread associated therewith. For example, in some systems, relay optics are configured to perform better when the output cone928is an output expansion cone having a predetermined spread of between ten and fifteen degrees. Accordingly, in one embodiment the varied thickness optical element900is further configured to cause beams922,923of the received scan cone952to exit the varied thickness optical element1000with a predetermined spread relative to the principal beam953.

In the illustrative embodiment ofFIG. 9, the microlens elements are arranged in accordance with the predetermined spread. For example, microlens element920and microlens element921are arranged with a pitch that corresponds to the predetermined spread. (Note that the lens elements through which the principal beam953passes have no pitch associated therewith. However, in one embodiment all other lens elements are arranged with pitch.)

The exit pupil expander904ofFIG. 9is well suited for use in head-up and near-to-eye displays. As noted above, relay optics are simplified when the received scan cone is substantially symmetrical about the principal beam. Further, as described in the preceding paragraph, head-up optics often perform better when the received scan cone includes a predetermined amount of spread. By varying the thickness of the varied thickness optical element across the width of the element, i.e., by varying the thickness of the wedge in this embodiment, a designer can optimize the varied thickness optical element for a particular system geometry and a particular amount of keystone distortion introduced by the high angle of incidence scanning engine. Thus, in a scanned laser projection system employing the exit pupil expander904ofFIG. 9, the varied thickness optical element900not only effectively eliminates keystone distortion, but also prepares the output expansion cone928to optimize the performance of subsequent optical components in a system.

The exit pupil expander904ofFIG. 9can be manufactured from a variety of materials. Additionally, the numerical aperture expander905can be manufactured from a variety of materials. Illustrative materials include glass and plastic. In one embodiment, the numerical aperture expander905and varied thickness optical element900are manufactured from the same material. In another embodiment, they are manufactured from different materials. The varied thickness optical element900can be attached to the numerical aperture expander1005, or alternatively may be integrated into one of the first layer912or second layer924. For example, where both the varied thickness optical element900and numerical aperture expander905are both manufactured from glass, they can be attached to each other using conventional glass bonding techniques or by using an optical adhesive.

In one embodiment, to simplify manufacture and reduce cost, the varied thickness optical element900is integrated with one of the first layer912or second layer924of the numerical aperture expander905. For instance, a portion of the microlens array910, the second layer924, and the varied thickness optical element can be manufactured as an integrated plastic assembly by way of an injection molding process. This assembly can then be aligned with the remaining portion of the microlens array910and first layer912to complete the assembly. Another advantage of using plastic and injection molding for the components is that it is easier to achieve the necessary tolerances used to define the function of the varied thickness optical element900.

By placing the varied thickness optical element900adjacent to the microlens array910, the varied thickness optical element900is disposed essentially at the intermediate image plane of the overall system shown inFIG. 9. Thus, when the scan cone952is presenting pixilated information along the projection surface, the effect applied by the varied thickness optical element900is applied “spot by spot.” This is another advantage of the exit pupil expander904ofFIG. 9over projector level keystone distortion correction where correction is applied while each beam is going in a different direction and traversing a large portion of the correction device. The result of using the exit pupil expander904ofFIG. 9is shown inFIG. 10, where information1008delivered from a high angle of incidence scanning engine1000no longer suffers from keystone distortion. Instead, the information1008is clear and legible for delivery to subsequent relay optics.

In one embodiment the projection surface (904) ofFIG. 9is manufactured from two halves that must be aligned during assembly. A first half may include half of the microlens array (910) and the first layer (912), while the second half may include the remainder of the microlens array (910), the second layer (924), and the varied thickness optical element (900). To make alignment easier, in one embodiment these halves can be configured with alignment indicators.FIGS. 11 and 12illustrate two examples of alignment indicators suitable for use with one or more embodiments of the invention.

InFIG. 11, the projection surface1104employs mechanical engagement devices1110,1111as alignment indicators. The projection surface1104ofFIG. 11comprises two halves. A first half includes a first substrate1112. The first substrate1112has a first set1114of microlenses either disposed thereon or integrated therewith. The second half includes a second substrate1124and the varied thickness optical element1100, which is integrated with the second substrate1124in this illustrative embodiment. A second set1123of microlenses is either disposed on or integrated with the second substrate1124.

To make alignment of the two halves easier during manufacture, the first half includes a first mechanical engagement device1110, while the second half includes a second mechanical engagement device1111. In this illustrative embodiment, the first mechanical engagement device1110is a male engagement device mounted on a flange1116extending from the first substrate1112. The second mechanical engagement device1111is a female engagement device mounted on another flange1115extending from the second substrate1124. The male and female engagement devices can be nestled to make alignment of the first half and second half easier.

Turning toFIG. 12, illustrated therein is an alternate alignment indicator. InFIG. 12, each half1201,1202of the projection surface includes an indicator1203,1204that is configured to be read by a machine vision alignment device1205. In the illustrative embodiment ofFIG. 12, the indicators1203,1204are configured as small plus or cross marks that are either etched or tooled into each half1201,1202. During manufacture, the machine vision alignment device1205takes visual pictures of the halves1201,1202as they are moved relative to each other. When the indicators1203,1204coincide, such that the picture seen by the machine vision alignment device1205appears as a single indicator, the two halves1201,1202are aligned. Accordingly, they can then be bonded together as described above with reference toFIGS. 2 and 3.

To this point, embodiments of the varied thickness optical element have been exclusively shown as wedges. However, it will be clear to those of ordinary skill in the art having the benefit of this disclosure that embodiments of the varied thickness optical element are not so limited. Turning now toFIG. 13, illustrated therein is just one of the many possible variants that can be constructed without departing from the spirit and scope of the invention.

As shown inFIG. 13, the varied thickness optical element1304is a varied thickness device, in that its thickness varies across its width. However, the varied thickness optical element1304ofFIG. 13is not configured as a wedge. Instead, the varied thickness optical element1304has a major face1301that is non-linear. In this illustrative embodiment, the major face is shown as a convex surface. However, it will be clear to those of ordinary skill in the art having the benefit of this disclosure that other shapes and contours can be applied to achieve different results in the output cone1328. For example, one application may call for a telecentric output cone1328. By varying the contour of the major face1301of the varied thickness optical element, this effect—or other effects—can be easily achieved.

It should also be noted that the “expander” portion of embodiments of the invention need not be complementary. For example, the microlens array (910) ofFIG. 9included two halves. As shown inFIG. 14, the “expander” can be single sided as well. For example, the exit pupil expander surface1401can be either an optical diffuser or single sided microlens array, either of which is single sided.