Patent Publication Number: US-2021173190-A1

Title: Folded-beam, low-obliquity beam scanner

Description:
REFERENCE TO RELATED APPLICATION 
     The present application claims priority from U.S. provisional patent application No. 62/944,816 entitled “Folded-Beam, Low-Obliquity Beam Scanner”, filed on Dec. 6, 2019, and incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to optical scanning devices, and in particular to compact optical scanning devices usable in scanning projectors and scanning projector displays. 
     BACKGROUND 
     Head mounted displays (HMD), helmet mounted displays, near-eye displays (NED), and the like are being used increasingly for displaying virtual reality (VR) content, augmented reality (AR) content, mixed reality (MR) content, etc. Such displays are finding applications in diverse fields including entertainment, education, training and biomedical science, to name just a few examples. The displayed VR/AR/MR content can be three-dimensional (3D) to enhance the experience and to match virtual objects to real objects observed by the user. Eye position and gaze direction, and/or orientation of the user may be tracked in real time, and the displayed imagery may be dynamically adjusted depending on the user&#39;s head orientation and gaze direction, to provide an experience of immersion into a simulated or augmented environment. 
     Compact display devices are desired for head-mounted displays. Because a display of HMD or NED is usually worn on the head of a user, a large, bulky, unbalanced, and/or heavy display device would be cumbersome and may be uncomfortable for the user to wear. 
     Projector-based displays provide images in angular domain, which can be observed by a user&#39;s eye directly, without an intermediate screen or a display panel. An imaging waveguide may be used to extend image light carrying the image in angular domain over an eyebox of the display. The lack of a screen or a display panel in a scanning projector display enables size and weight reduction of the display. Projector-based displays may use a scanning projector that obtains image in angular domain by scanning an image light beam of a controllable brightness and/or color. It is desirable to make scanning projectors and optical scanners more compact. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will now be described in conjunction with the drawings, in which: 
         FIG. 1  is a side cross-sectional view of a beam scanner including a reflective polarizer; 
         FIG. 2  is a side cross-sectional view of a beam scanner including a reflective polarizer and a beam-folding mirror; 
         FIG. 3A  is a side cross-sectional view of a beam scanner including a reflective polarizer and a pair of beam-folding mirrors for routing an optical beam around an image-replicating waveguide; 
         FIG. 3B  is a side cross-sectional view of a non-zero obliquity beam scanner including a beam-folding mirror; 
         FIGS. 4A and 4B  are field of view (FOV) plots of the beam scanners of  FIGS. 3A and 3B , respectively; 
         FIG. 5  is a plan view of a 2D scanning microelectromechanical system (MEMS) mirror; 
         FIG. 6A  is a schematic view of a microelectromechanical system (MEMS) scanning projector display; 
         FIG. 6B  is a schematic top view of a scanning projector display with an arrayed light source, illustrating a relationship between fields of view provided by individual emitters of the arrayed light source; 
         FIG. 7  is a three-dimensional view of a packaged beam scanner of  FIG. 3A ; 
         FIG. 8  is a schematic top view of a near-eye display using beam scanners of this disclosure; 
         FIG. 9A  is an isometric view of a head-mounted display of this disclosure; and 
         FIG. 9B  is a block diagram of a virtual reality system including the headset of  FIG. 9A . 
     
    
    
     DETAILED DESCRIPTION 
     While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. All statements herein reciting principles, aspects, and embodiments of this disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
     As used herein, the terms “first”, “second”, and so forth are not intended to imply sequential ordering, but rather are intended to distinguish one element from another, unless explicitly stated. Similarly, sequential ordering of method steps does not imply a sequential order of their execution, unless explicitly stated. In  FIGS. 1, 2 , and  FIG. 3A , similar reference numerals denote similar elements. 
     A tiltable reflector may be used to scan a light beam emitted by a light source to form an image in angular domain for direct observation by a user of a display. As the light beam is scanned, the brightness and/or color of the scanned light beam may be varied in coordination with the scanning. The brightness and/or color of the light beam are set to correspond to brightness and/or color of image pixels being currently highlighted or “painted” by the light beam. The entire image is formed when the light beam is scanned in two dimensions, e.g. over X- and Y-viewing angles, over the entire field of view (FOV) of the user. When the frame rate is high enough, the eye integrates the scanned light beam, enabling the user to see the displayed imagery substantially without flicker. 
     One problem associated with beam scanners is a reduction of field of view (FOV) caused by an oblique angle of incidence of the light beam onto a tiltable reflector of the beam scanner. The oblique incidence angle may be required by the optical geometry used, e.g. to physically separate an impinging light beam from the scanned reflected light beam. The FOV reduction is caused by distortion of the solid angle representing the range of scanning at oblique angles of incidence of light beam at the tiltable reflector. 
     In accordance with the present disclosure, the output (scanned) light beam may be spatially separated from the input optical beam by polarization. This obviates the need in geometrical separation of the beams by oblique angles of incidence, resulting in a compact configuration providing a nearly straight angle of incidence at the tiltable reflector when the latter is in a center (non-tilted) angular position. Low obliquity of the impinging light beam enables the scanning range to be utilized more efficiently. 
     In accordance with this disclosure, there is provided a beam scanner comprising a substrate, a tiltable reflector hingedly supported by the substrate, and a reflective polarizer supported over the tiltable reflector and configured to receive and redirect a light beam towards the tiltable reflector for scanning the light beam. A quarter-wave waveplate (QWP) may be provided in an optical path between the reflective polarizer and the tiltable reflector. The QWP may be configured to change a polarization state of the light beam to an orthogonal polarization state upon double pass propagation of the light beam through the QWP. The QWP may be supported by the reflective polarizer. 
     In some embodiments, the beam scanner may further include a folding mirror configured to receive and redirect the light beam towards the reflective polarizer. The folding mirror may be supported by the substrate. An enclosure enclosing the tiltable reflector may be provided. The enclosure may support the reflective polarizer. In embodiments where the folding mirror is supported by the substrate and optically coupled to the reflective polarizer, the enclosure may include an optical window for receiving and transmitting the light beam through the optical window for impinging onto the folding mirror. 
     In accordance with this disclosure, there is provided a projector comprising a light source and a beam scanner. The light source may be configured for providing a light beam having a characteristic comprising at least one of brightness or color. The beam scanner may include a substrate, a tiltable reflector hingedly supported by the substrate, and a reflective polarizer supported over the tiltable reflector and configured to receive and redirect the light beam towards the tiltable reflector for scanning the light beam. 
     In some embodiments, the projector may further include a QWP in an optical path between the reflective polarizer and the tiltable reflector. The QWP may be configured to change a polarization state of the light beam to an orthogonal polarization state upon double pass propagation of the light beam through the QWP. A first folding mirror may be configured to receive and redirect the light beam towards the reflective polarizer. The first folding mirror may be supported by the substrate. A second folding mirror may be configured to receive and redirect the light beam towards the first folding mirror. An enclosure enclosing the tiltable reflector may be provided. The enclosure may support the reflective polarizer, and may include an optical window for receiving and transmitting the light beam therethrough for impinging onto the first folding mirror. A controller may be operably coupled to the light source and the beam scanner for varying the characteristic of the light beam in coordination with scanning the light beam by the beam scanner. 
     In accordance with this disclosure, there is further provided a display comprising a light source for providing the light beam, a beam scanner described above, and a pupil-replicating waveguide configured to receive the light beam reflected from the tiltable reflector, and to spread the light beam over an eyebox of the display. The pupil-replicating waveguide may be disposed substantially parallel to the substrate of the beam scanner. A first folding mirror may be configured to receive and redirect the light beam towards the reflective polarizer. A second folding mirror may be configured to receive and redirect the light beam towards the first folding mirror and around the pupil-replicating waveguide. 
     Several embodiments of a beam scanner of this disclosure will now be considered. Referring to  FIG. 1 , a beam scanner  100  includes a substrate  102 , e.g. a semiconductor substrate, and a tiltable reflector  104 , e.g. a tiltable mirror and/or grating, hingedly supported by the substrate  102 . A reflective polarizer  106  is supported over the tiltable reflector  104  and is optically coupled to the tiltable reflector  104 . The reflective polarizer  106  may be configured to reflect a light beam  110  of a particular pre-defined polarization state towards the tiltable reflector  104 , and to transmit light of an orthogonal polarization state. The light beam  110  impinging onto the tiltable reflector  104  may then be scanned about one or two axes. A quarter-wave waveplate (QWP)  108  may be disposed in an optical path between the reflective polarizer  106  and the tiltable reflector  104 . The QWP  108  may be configured to change a polarization state of the light beam  110  to the orthogonal polarization state upon double pass propagation of the light beam  110  through the QWP  108 . For example, the QWP  108  may be oriented with its optic axis at  45  degrees w.r.t. a polarization direction of a linearly polarized light beam  110 , such that at double pass propagation, the QWP  108  acts as a half-wave waveplate rotating the linear polarization of the light beam  110  to an orthogonally oriented linear polarization. The QWP  108  may stand separately, or may be supported by the reflective polarizer  106 , as shown—the latter configuration tends to be more compact. 
     A projector  120  includes the beam scanner  100  and a light source  112  optically coupled to the beam scanner  100 . In operation, the light source  112  emits the light beam  110  having a controllable characteristic such as brightness, color, etc. The light beam  110  may be circularly polarized. The light beam  110  emitted by a light source  112  impinges onto the QWP  108 . The QWP  108  is disposed and oriented to convert a circular polarization state into a linear polarization state that is reflected by the reflective polarizer  106 . The reflected linearly polarized light beam  110  propagates again through the QWP  108  and becomes circularly polarized. The circularly polarized light beam  110  is reflected by the tiltable reflector  104  back towards the QWP  108 . This reflection reverses the handedness of the circular polarization. Upon a third propagation of the light beam  110  through the QWP  108 , the light beam  110  becomes linearly polarized at an orientation of the polarization vector that ensures transmission of the light beam  110  through the reflective polarizer  106 . The transmitted light beam  110  propagates towards a pupil-replicating waveguide  114 . The replicating waveguide  114  may include a single waveguide element or a stack of waveguide elements. The pupil-replicating waveguide  114  spreads the light beam  110  over an eyebox  118  of the near-eye display  140 , thus carrying an image in angular domain to the eyebox  118 . Substantially parallel placement of the substrate  102  and the pupil-replicating waveguide  114  may allow a further size reduction of the near-eye display  140 . 
     A controller  116  may be operably coupled to the light source  112  and the beam scanner  100  and configured to vary the brightness and/or color of the light beam  110  in coordination with scanning the light beam  110  with the beam scanner  100 . Together, the pupil-replicating waveguide  114 , the light source  112 , and the beam scanner  100  form a near-eye display  140 . 
     Turning to  FIG. 2 , a projector  220  includes a beam scanner  200 . The projector  220  and the beam scanner  200  are similar to the projector  120  and the beam scanner  100 , respectively, of  FIG. 1 . The beam scanner  200  of  FIG. 2  further includes a first folding mirror  206  in an optical path of the light beam  110  between the light source  112  and the tiltable reflector  104 . The first folding mirror  206  may be supported by the substrate  102 , may extend from the substrate  102 , or may be mounted separately. First folding mirror  206  may be a mirrored prism or a thin mirrored substrate, for example. One advantage of utilizing the first folding mirror  206  is further size reduction of the beam scanner  200  due to the reflective polarizer  106  being disposed at a shallower angle than in the beam scanner  100  of  FIG. 1 . Another advantage of the beam scanner  200  is that the light source  112  may be disposed on the opposite side of the pupil-replicating waveguide  114 , providing size savings for a near-eye display  240 . Substantially parallel placement of the substrate  102  and the pupil-replicating waveguide  114  may allow a further size reduction of the near-eye display  240 . Herein and throughout the rest of this disclosure, the term “substantially parallel” is taken to mean parallel to within 10-15 degrees, for certainty. 
     Referring to  FIG. 3A , a beam scanner  300 A is similar to the beam scanner  200  of  FIG. 2 . A projector  320 A includes the light source  112  optically coupled to the beam scanner  300 A by a second folding mirror  306  in an optical path of the light beam  110  between the light source  112  and the tiltable reflector  104 . One advantage of utilizing two folding mirrors, i.e. the first folding mirror  206  and the second folding mirror  306 , is a possibility of a further size reduction of the beam scanner  300 A. Two folding mirrors  206  and  306  enable redirection of the light beam  110  around the pupil-replicating waveguide  114 . 
     The beam scanner  300 A may further include an optical window  344 . Together with the reflective polarizer  106 , the optical window  344  may provide an enclosure of the tiltable reflector  104  and the first folding mirror  206 , enabling the tiltable reflector  104  to be hermetically packaged, if so required. In the projector  320 A, the light beam  110  impinges onto the tiltable reflector  104  at zero angle of incidence, or at a small non-zero angle, when the tiltable reflector  104  is not tilted. 
     A near-eye display  340 A includes the pupil-replicating waveguide  114  and the projector  320 A optically coupled to the pupil-replicating waveguide  114 . In operation, the tiltable reflector  104  is scanned about one or two axes of tilt. In  FIG. 3A , the tiltable reflector  104  scans the light beam  110  about an axis perpendicular to the plane of  FIG. 3A . A fan of three scanned beams,  110 A,  110 B, and  110 C, is shown to illustrate the scanning. 
       FIG. 3B  depicts a beam scanner  300 B that does not include internal folding mirrors or reflective polarizers. The beam scanner  300 B is considered herein for comparison with the beam scanner  300 A of  FIG. 300A , to show the effects of oblique vs. normal angle of incidence of the light beam  110  onto the tiltable reflector  104  when the latter is at a nominal orientation, i.e. parallel to the substrate  102 . The beam scanner  300 B of  FIG. 3B  includes the tiltable reflector  104  hingedly supported over the substrate  102  and protected by a window  346 . An external folding mirror  348  redirects the light beam  110  emitted by the light source  112  to impinge on to the tiltable reflector  104  through the window  346  at an oblique angle of incidence. 
     A projector  320 B includes the light source  112  optically coupled to the beam scanner  300 B by the external folding mirror  348  disposed in an optical path of the light beam  110  between the light source  112  and the tiltable reflector  104 . A near-eye display  340 B includes the tiltable reflector  104  and a pupil-replicating waveguide  350 . The tiltable reflector  104  reflects the light beam  110  to impinge onto the pupil-replicating waveguide  350  at different angles of incidence. In the projector  320 B, the light beam  110  impinges onto the tiltable reflector  104  at an oblique angle of incidence, i.e. away from the normal angle of incidence. 
       FIGS. 4A and 4B  illustrate the effects of normal angle of incidence ( FIG. 4A ) vs. oblique angle of incidence ( FIG. 4B ) onto the tiltable reflector  104 .  FIG. 4A  shows a zero-obliquity scanning angular area  400 A and an associated inscribed rectangular FOV  402 A (shaded rectangle) in units of tangents of corresponding ray angles θx and θy, TanX and TanY respectively. The zero-obliquity FOV  402 A solid angle is covering most of the angular area  400 A. By comparison,  FIG. 4B  shows an oblique incidence scanning angular area  400 B and an associated inscribed rectangular FOV  402 B (shaded rectangle). The FOV  402 B solid angle occupies a smaller percentage of the angular area  400 B, and has a different aspect ratio. For example, to ensure an FOV of the near-eye display  340 A of  FIG. 3A , the tiltable reflector  104  needs to be tilted within a certain tilt ranges about X-axis and about Y-axis. To ensure the same FOV of the near-eye display  340 B of  FIG. 3B  at a non-zero obliquity, the tiltable reflector  104  needs to have a larger tilt ranges about X- and Y-axes. Thus, the zero- or low-obliquity coupling of the light beam  110  to the tiltable reflector  104  improves the utilization of the scanning range of the tiltable reflector  104 , enabling smaller scanning ranges of the tiltable reflector  104  and/or wider fields of view at the same scanning ranges of the tiltable reflector  104 . It is to be noted that the configurations and advantages disclosed herein may be applicable to various types of projector-based displays, not necessarily near-eye displays. 
     In some embodiments, the beam scanners of the projectors described herein may include microelectromechanical system (MEMS) scanners. Referring to  FIG. 5 , a two-dimensional (2D) microelectromechanical system (MEMS) scanner  500  includes a reflector  510 , e.g. a mirror or a diffraction grating, supported by a pair of first torsional hinges  501  allowing tilting the reflector  510  about X axis. The first torsional hinges  501  extend from the reflector  510  to a gimbal ring  520 , which is supported by a pair of second torsional hinges  502  extending from the gimbal ring  520  to a fixed base  522 , for tilting of the gimbal ring  520  and the reflector  510  about Y axis. Actuators  551 ,  552  may be disposed underneath the reflector  510  and/or the gimbal ring  520  for providing a force for actuating the tilt of the reflector  510  about X and Y axes. The actuators  551 ,  552  may be electrostatic, electro-magnetic, piezo-electric, etc. For electrostatic mirror actuation, the comb drive may be located on the torsional hinge members. For example, in the embodiment shown in  FIG. 5 , the first actuator  551  may be disposed under an edge of the reflector  510  to tilt the reflector  510  about X-axis. The second actuator  552  may be disposed under the gimbal ring  520  for tilting the gimbal ring  520  and the reflector  510  about Y-axis. It is noted that reflector  510  may be offset from a center of a corresponding substrate, if needed. 
     A feedback circuit  554  may be provided for determining the X- and Y-angles of tilt of the reflector  510 . The feedback circuit  554  may measure electric capacitance between the first electrostatic actuator  551  and the reflector  510  to determine the X-tilt, and electric capacitance between the second electrostatic actuator  552  and the gimbal ring  520  to determine the Y-tilt. Separate electrodes may also be provided specifically for the feedback circuit  554 . In some embodiments, the feedback circuit  554  may provide a sync or triggering pulses when the reflector  510  is tilted at a certain X- and/or Y-angle, including zero angle. The reflector  510  of  FIG. 5  corresponds to the tiltable reflector  104  of  FIGS. 1, 2, 3A, and 3B . 
     Turning to  FIG. 6A , a scanning projector display  600  includes a light source assembly  602  for providing a diverging optical beam  604 D. An optional collimator  650  is optically coupled to the light source assembly  602 , to collimate the diverging optical beam  604 D and provide a collimated optical beam  604 . A scanner, such as the 2D MEMS scanner  500  of  FIG. 5 , is optically coupled to the collimator  650 . The controller  612  can be operably coupled to an electronic driver  606 , which is coupled to the light source assembly  602 . The controller  612  is also coupled to the 2D MEMS scanner  500  for tilting the reflector  510  of the 2D MEMS scanner  500 . 
     The collimator  650 , e.g. a lens, a mirror, etc., is optically coupled to the pulsed light source  602  for collimating the diverging optical beam  604 D to obtain the collimated optical beam  604 . Any optical component having optical power, i.e. focusing or collimating power, such as a concave mirror, a diffractive lens, a folded-beam freeform optical element, etc., may be used in the collimator  650 . The reflector  510  of the 2D MEMS scanner  500  is optically coupled to the collimator  650  for receiving and angularly scanning the collimated optical beam  604 . 
     The electronic driver  606  is configured for providing powering electric pulses to energize the light source assembly  602 . The controller  612  sends commands to the electronic driver  606  to energize the light source assembly  602  in coordination with tilting the 2D MEMS scanner  500 , for “painting” or rastering an image in angular domain. When viewed by a human eye, the image in angular domain is projected by the eye&#39;s cornea and lens to become a spatial-domain image on the eye&#39;s retina. 
     In some embodiments, the scanner of the projector display may include a 1D tiltable mirror, and the light source assembly  602  may include a linear array of light sources to provide a plurality of image pixels in a direction perpendicular to the direction of scanning. The linear array of light sources may also be used in a 2D scanner, as well. In some embodiments, the 2D MEMS scanner  500  may be replaced with a pair of 1D tiltable mirrors, one for scanning about X axis, and the other for scanning about Y axis. The two 1D tiltable mirrors may be optically coupled, e.g. via a pupil relay. Other types of scanners may be used, for example diffractive or acousto-optic scanners. 
     The light source assembly  602  may include one or a plurality of single-mode or multimode light sources, e.g. side-emitting laser diodes, vertical-cavity surface-emitting laser diodes, superluminescent light-emitting diodes (SLEDs), or light-emitting diodes. The pulse energy of the light pulse may be selected to be lower than a threshold energy equal to the pixel time interval multiplied by a threshold optical power of the light source. For SLED light sources, the threshold optical power of the SLED may be a lasing threshold optical power of the SLED. 
     The operation of the scanning projector display  600  is further illustrated in  FIG. 6B . In this example, the light source assembly  602  is a multi-emitter assembly including three emitters providing three beams (only chief rays shown)  621  (dotted lines),  622  (solid lines), and  623  (dashed lines). The collimator  650  collimates the beams  621 ,  622 , and  623 . By selecting suitable geometry e.g. distances and focal length of the collimator  650 , the latter may also cause the beams  621 ,  622 , and  623  to impinge onto a center of the reflector  510  at slightly different angles of incidence, for scanning all three beams  621 ,  622 , and  623  together. Since the angles of incidence of the beams  621 ,  622 , and  623  onto the tiltable reflector  510  are different, respective scanning areas  631  (dotted lines),  632  (solid lines), and  633  (dashed lines) of the beams  621 ,  622 , and  623 , respectively, are mutually offset as shown. The controller  612  may take these spatial offsets into account by providing corresponding delays to the driving signals of the three emitters of the multi-emitter light source assembly  602 . Spatial offsets in combination with the delays in energizing individual emitters may be provided such as to effectively triple the spatial resolution of the scanning projector display  600  in a common scan area  630 , as compared to a case of a single emitter. 
     Referring now to  FIG. 7 , a packaged MEMS scanner  700  has the optical configuration of the beam scanner  300 A of  FIG. 3A , and operates similarly to the beam scanner  300 A. The packaged MEMS scanner  700  ( FIG. 7 ) includes a hermetic package or enclosure  702  formed by the reflective polarizer  106 , the optical window  344 , and side walls  704 . The enclosure  702  encapsulates the tiltable reflector  104  and the first folding mirror  206 . The enclosure  702  may be vacuum sealed or filled with an inert gas such as argon, to provide a stable environment for the tiltable reflector  104 . 
     Turning to  FIG. 8 , a near-eye display (NED)  800  includes a frame  801  having a form factor of a pair of glasses. The frame  801  may support, for each eye: a projector  802  for providing display light carrying an image in angular domain, an electronic driver  804  operably coupled to the projector  802  for powering the projector  802 , and a pupil replicator  832 , e.g. a pupil-replicating waveguide assembly, optically coupled to the projector  802 . 
     Each projector  802  may include a beam scanner described herein, for example and without limitation the beam scanner  80  of  FIG. 1 , the beam scanner  200  of  FIG. 2 , the beam scanner  300 A of  FIG. 3A , the beam scanner  300 B of  FIG. 3B , the beam scanner  700  of  FIG. 7 , etc. The tiltable reflector of such a beam scanner may include a MEMS tiltable reflector, for example. In some embodiments, each projector  802  includes the projector  120  of  FIG. 1 , the projector  220  of  FIG. 2 , the projector  320 A of  FIG. 3A , the projector  320 B of  FIG. 3B , etc. Light sources for these projectors may include a substrate supporting an array of single-emitter or multi-emitter semiconductor light sources, e.g. side-emitting laser diodes, vertical-cavity surface-emitting laser diodes, SLEDs, or light-emitting diodes, for providing a plurality of light beams. Collimators of the light sources may include concave mirrors, bulk lenses, Fresnel lenses, holographic lenses, freeform prisms, etc. The pupil replicators  832  may include waveguides equipped with a plurality of surface relief and/or volume holographic gratings. The function of the pupil replicators  832  is to provide multiple laterally offset copies of the display light beams provided by the projectors  802  at respective eyeboxes  812 . 
     A controller  805  is operably coupled to the light sources and tiltable reflectors of the projectors  802 . The controller  805  may be configured to determine the X- and Y-tilt angles of the tiltable reflectors of the projectors  802 . The controller  805  determines which pixel or pixels of the image to be displayed correspond to the determined X- and Y-tilt angles. Then, the controller  805  determines the brightness and/or color of these pixels, and operates the electronic drivers  804  accordingly for providing powering electric pulses to the light sources of the projectors  802  to produce light pulses at power level(s) corresponding to the determined pixel brightness and color. 
     In some embodiments, the controller  805  may be configured to operate, for each eye, the tiltable reflector to cause the light beam reflected from the tiltable reflector to have a beam angle corresponding to a pixel of an image to be displayed. The controller  805  may be further configured to operate the light source in coordination with operating the tiltable reflector, such that the light beam has brightness and/or color corresponding to first pixel being displayed. In multi-light source/multi-emitter embodiments, the controller  805  may be configured to operate the corresponding light sources/emitters in coordination, to provide a larger FOV, an improved scanning resolution, increased brightness of the display, etc., as described herein. For example, in embodiment where the projectors for both of user&#39;s eyes each include two light sources, the controller may be configured to operate each of the two tiltable reflector to cause the second light beam reflected from the tiltable reflector to have a beam angle corresponding to a second pixel of an image to be displayed, and operate the second light source in coordination with operating the tiltable reflector, such that the second light beam has brightness corresponding to the second pixel. 
     Embodiments of the present disclosure may include, or be implemented in conjunction with, an artificial reality system. An artificial reality system adjusts sensory information about outside world obtained through the senses such as visual information, audio, touch (somatosensation) information, acceleration, balance, etc., in some manner before presentation to a user. By way of non-limiting examples, artificial reality may include virtual reality (VR), augmented reality (AR), mixed reality (MR), hybrid reality, or some combination and/or derivatives thereof. Artificial reality content may include entirely generated content or generated content combined with captured (e.g., real-world) content. The artificial reality content may include video, audio, somatic or haptic feedback, or some combination thereof. Any of this content may be presented in a single channel or in multiple channels, such as in a stereo video that produces a three-dimensional effect to the viewer. Furthermore, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof, that are used to, for example, create content in artificial reality and/or are otherwise used in (e.g., perform activities in) artificial reality. The artificial reality system that provides the artificial reality content may be implemented on various platforms, including a wearable display such as an HMD connected to a host computer system, a standalone HMD, a near-eye display having a form factor of eyeglasses, a mobile device or computing system, or any other hardware platform capable of providing artificial reality content to one or more viewers. 
     Referring to  FIG. 9A , an HMD  900  is an example of an AR/VR wearable display system which encloses the user&#39;s face, for a greater degree of immersion into the AR/VR environment. The HMD  900  is an embodiment of the near-eye display  140  of  FIG. 1 , the near-eye display  240  of  FIG. 2 , the near-eye display  340 A of  FIG. 3A , the near-eye display  340 B of  FIG. 3B , or the scanning projector display  600  of  FIGS. 6A and 6B , for example. The function of the HMD  900  is to augment views of a physical, real-world environment with computer-generated imagery, and/or to generate the entirely virtual  3 D imagery. The HMD  900  may include a front body  902  and a band  904 . The front body  902  is configured for placement in front of eyes of a user in a reliable and comfortable manner, and the band  904  may be stretched to secure the front body  902  on the user&#39;s head. A display system  980  may be disposed in the front body  902  for presenting AR/VR imagery to the user. Sides  906  of the front body  902  may be opaque or transparent. 
     In some embodiments, the front body  902  includes locators  908  and an inertial measurement unit (IMU)  910  for tracking acceleration of the HMD  900 , and position sensors  912  for tracking position of the HMD  900 . The IMU  910  is an electronic device that generates data indicating a position of the HMD  900  based on measurement signals received from one or more of position sensors  912 , which generate one or more measurement signals in response to motion of the HMD  900 . Examples of position sensors  912  include: one or more accelerometers, one or more gyroscopes, one or more magnetometers, another suitable type of sensor that detects motion, a type of sensor used for error correction of the IMU  910 , or some combination thereof. The position sensors  912  may be located external to the IMU  910 , internal to the IMU  910 , or some combination thereof. 
     The locators  908  are traced by an external imaging device of a virtual reality system, such that the virtual reality system can track the location and orientation of the entire HMD  900 . Information generated by the IMU  910  and the position sensors  912  may be compared with the position and orientation obtained by tracking the locators  908 , for improved tracking accuracy of position and orientation of the HMD  900 . Accurate position and orientation is important for presenting appropriate virtual scenery to the user as the latter moves and turns in 3D space. 
     The HMD  900  may further include a depth camera assembly (DCA)  911 , which captures data describing depth information of a local area surrounding some or all of the HMD  900 . To that end, the DCA  911  may include a laser radar (LIDAR), or a similar device. The depth information may be compared with the information from the IMU  910 , for better accuracy of determination of position and orientation of the HMD  900  in 3D space. 
     The HMD  900  may further include an eye tracking system  914  for determining orientation and position of user&#39;s eyes in real time. The obtained position and orientation of the eyes also allows the HMD  900  to determine the gaze direction of the user and to adjust the image generated by the display system  980  accordingly. In one embodiment, the vergence, that is, the convergence angle of the user&#39;s eyes gaze, is determined. The determined gaze direction and vergence angle may also be used for real-time compensation of visual artifacts dependent on the angle of view and eye position. Furthermore, the determined vergence and gaze angles may be used for interaction with the user, highlighting objects, bringing objects to the foreground, creating additional objects or pointers, etc. An audio system may also be provided including e.g. a set of small speakers built into the front body  902 . 
     Referring to  FIG. 9B , an AR/VR system  950  includes the HMD  900  of  FIG. 9A , an external console  990  storing various AR/VR applications, setup and calibration procedures,  3 D videos, etc., and an input/output (I/O) interface  915  for operating the console  990  and/or interacting with the AR/VR environment. The HMD  900  may be “tethered” to the console  990  with a physical cable, or connected to the console  990  via a wireless communication link such as Bluetooth®, Wi-Fi, etc. There may be multiple HMDs  900 , each having an associated I/O interface  915 , with each HMD  900  and I/O interface(s)  915  communicating with the console  990 . In alternative configurations, different and/or additional components may be included in the AR/VR system  950 . Additionally, functionality described in conjunction with one or more of the components shown in  FIGS. 9A and 9B  may be distributed among the components in a different manner than described in conjunction with  FIGS. 9A and 9B  in some embodiments. For example, some or all of the functionality of the console  990  may be provided by the HMD  900 , and vice versa. The HMD  900  may be provided with a processing module capable of achieving such functionality. 
     As described above with reference to  FIG. 9A , the HMD  900  may include the eye tracking system  914  ( FIG. 9B ) for tracking eye position and orientation, determining gaze angle and convergence angle, etc., the IMU  910  for determining position and orientation of the HMD  900  in 3D space, the DCA  911  for capturing the outside environment, the position sensor  912  for independently determining the position of the HMD  900 , and the display system  980  for displaying AR/VR content to the user. The display system  980  includes ( FIG. 9B ) an electronic display  925 , for example and without limitation, a liquid crystal display (LCD), an organic light emitting display (OLED), an inorganic light emitting display (ILED), an active-matrix organic light-emitting diode (AMOLED) display, a transparent organic light emitting diode (TOLED) display, a projector, or a combination thereof. The display system  980  further includes an optics block  930 , whose function is to convey the images generated by the electronic display  925  to the user&#39;s eye. The optics block may include various lenses, e.g. a refractive lens, a Fresnel lens, a diffractive lens, an active or passive Pancharatnam-Berry phase (PBP) lens, a liquid lens, a liquid crystal lens, etc., a pupil-replicating waveguide, grating structures, coatings, etc. The display system  980  may further include a varifocal module  935 , which may be a part of the optics block  930 . The function of the varifocal module  935  is to adjust the focus of the optics block  930  e.g. to compensate for vergence-accommodation conflict, to correct for vision defects of a particular user, to offset aberrations of the optics block  930 , etc. 
     The I/O interface  915  is a device that allows a user to send action requests and receive responses from the console  990 . An action request is a request to perform a particular action. For example, an action request may be an instruction to start or end capture of image or video data or an instruction to perform a particular action within an application. The I/O interface  915  may include one or more input devices, such as a keyboard, a mouse, a game controller, or any other suitable device for receiving action requests and communicating the action requests to the console  990 . An action request received by the I/O interface  915  is communicated to the console  990 , which performs an action corresponding to the action request. In some embodiments, the I/O interface  915  includes an IMU that captures calibration data indicating an estimated position of the I/O interface  915  relative to an initial position of the I/O interface  915 . In some embodiments, the I/O interface  915  may provide haptic feedback to the user in accordance with instructions received from the console  990 . For example, haptic feedback can be provided when an action request is received, or the console  990  communicates instructions to the I/O interface  915  causing the I/O interface  915  to generate haptic feedback when the console  990  performs an action. 
     The console  990  may provide content to the HMD  900  for processing in accordance with information received from one or more of: the IMU  910 , the DCA  911 , the eye tracking system  914 , and the I/O interface  915 . In the example shown in  FIG. 9B , the console  990  includes an application store  955 , a tracking module  960 , and a processing module  965 . Some embodiments of the console  990  may have different modules or components than those described in conjunction with  FIG. 9B . Similarly, the functions further described below may be distributed among components of the console  990  in a different manner than described in conjunction with  FIGS. 9A and 9B . 
     The application store  955  may store one or more applications for execution by the console  990 . An application is a group of instructions that, when executed by a processor, generates content for presentation to the user. Content generated by an application may be in response to inputs received from the user via movement of the HMD  900  or the I/O interface  915 . Examples of applications include: gaming applications, presentation and conferencing applications, video playback applications, or other suitable applications. 
     The tracking module  960  may calibrate the AR/VR system  950  using one or more calibration parameters and may adjust one or more calibration parameters to reduce error in determination of the position of the HMD  900  or the I/O interface  915 . Calibration performed by the tracking module  960  also accounts for information received from the IMU  910  in the HMD  900  and/or an IMU included in the I/O interface  915 , if any. Additionally, if tracking of the HMD  900  is lost, the tracking module  960  may re-calibrate some or all of the AR/VR system  950 . 
     The tracking module  960  may track movements of the HMD  900  or of the I/O interface  915 , the IMU  910 , or some combination thereof. For example, the tracking module  960  may determine a position of a reference point of the HMD  900  in a mapping of a local area based on information from the HMD  900 . The tracking module  960  may also determine positions of the reference point of the HMD  900  or a reference point of the I/O interface  915  using data indicating a position of the HMD  900  from the IMU  910  or using data indicating a position of the I/O interface  915  from an IMU included in the I/O interface  915 , respectively. Furthermore, in some embodiments, the tracking module  960  may use portions of data indicating a position or the HMD  900  from the IMU  910  as well as representations of the local area from the DCA  911  to predict a future location of the HMD  900 . The tracking module  960  provides the estimated or predicted future position of the HMD  900  or the I/O interface  915  to the processing module  965 . 
     The processing module  965  may generate a 3D mapping of the area surrounding some or all of the HMD  900  (“local area”) based on information received from the HMD  900 . In some embodiments, the processing module  965  determines depth information for the 3D mapping of the local area based on information received from the DCA  911  that is relevant for techniques used in computing depth. In various embodiments, the processing module  965  may use the depth information to update a model of the local area and generate content based in part on the updated model. 
     The processing module  965  executes applications within the AR/VR system  950  and receives position information, acceleration information, velocity information, predicted future positions, or some combination thereof, of the HMD  900  from the tracking module  960 . Based on the received information, the processing module  965  determines content to provide to the HMD  900  for presentation to the user. For example, if the received information indicates that the user has looked to the left, the processing module  965  generates content for the HMD  900  that mirrors the user&#39;s movement in a virtual environment or in an environment augmenting the local area with additional content. Additionally, the processing module  965  performs an action within an application executing on the console  990  in response to an action request received from the I/O interface  915  and provides feedback to the user that the action was performed. The provided feedback may be visual or audible feedback via the HMD  900  or haptic feedback via the I/O interface  915 . 
     In some embodiments, based on the eye tracking information (e.g., orientation of the user&#39;s eyes) received from the eye tracking system  914 , the processing module  965  determines resolution of the content provided to the HMD  900  for presentation to the user on the electronic display  925 . The processing module  965  may provide the content to the HMD  900  having a maximum pixel resolution on the electronic display  925  in a foveal region of the user&#39;s gaze. The processing module  965  may provide a lower pixel resolution in other regions of the electronic display  925 , thus lessening power consumption of the AR/VR system  950  and saving computing resources of the console  990  without compromising a visual experience of the user. In some embodiments, the processing module  965  can further use the eye tracking information to adjust where objects are displayed on the electronic display  925  to prevent vergence-accommodation conflict and/or to offset optical distortions and aberrations. 
     The hardware used to implement the various illustrative logics, logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but, in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Alternatively, some steps or methods may be performed by circuitry that is specific to a given function. 
     The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments and modifications, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.