Patent Publication Number: US-11025871-B1

Title: Driving scanning projector display with pulses shorter than pixel duration

Description:
REFERENCE TO RELATED APPLICATION 
     The present application claims priority from U.S. Provisional application No. 62/829,491, filed on Apr. 4, 2019 and incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to wearable headsets, and in particular to optical components and modules for a wearable display or a near-eye display. 
     BACKGROUND 
     Head-mounted displays (HMDs), near-eye displays (NEDs), and other wearable display systems can be used to present virtual scenery to a user, or to augment real scenery with dynamic information, data, or virtual objects. The virtual reality (VR) or augmented reality (AR) scenery 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 scenery may be dynamically adjusted depending on the user&#39;s head orientation and gaze direction, to provide a better experience of immersion into a simulated or augmented environment. 
     Scanning projector displays provide an image in angular domain, which can be directly observed by a user. The absence of a screen or a display panel in a scanning projector display may allow a significant weight and size reduction of the display system. A scanning projector display may benefit from using a compact, bright, fast, and energy-efficient light source, as well as a corresponding electronic driver configured to operate such a light source. 
     SUMMARY 
     In accordance with the present disclosure, there is provided a driver for providing a succession of powering electric pulses to a light source for providing a light beam comprising a succession of light pulses corresponding to the succession of powering electric pulses. The light beam is coupled to a scanning projector display for displaying an image comprising a plurality of pixels. A duration of a powering electric pulse of the succession of powering electric pulses is less than a pixel time interval during which a scanner of the scanning projector display is directing the light beam to form a corresponding pixel of the image. 
     The driver may be configured to provide the powering electric pulses having at least an amplitude or duration such that a pulse energy of light pulses provided by the light source when driven by the powering electric pulses is equal to a nominal energy of the light beam when the light source is driven at a constant driving current during the entire pixel time interval. 
     In some embodiments, the driver is configured to control energy of the light pulses for displaying the image by controlling an amplitude of the powering electric pulses such that a pulsewidth of the powering electric pulses is no greater than one third of the pixel time interval. In some embodiments, the driver is configured to provide the powering electric pulses in a middle of the corresponding pixel time intervals during which a scanner of the scanning projector display is directing the light beam to the corresponding pixels of the image. 
     In accordance with the present disclosure, there is provided a scanning projector display comprising a light source for providing a light beam comprising a succession of light pulses, a driver operably coupled to the light source for powering the light source for providing the light beam, a scanner optically coupled to the light source for scanning the light beam for displaying an image comprising a plurality of pixels, and a controller operably coupled to the scanner and the driver and configured for operating the driver for providing powering electric pulses to the light source. Durations (i.e. pulsewidth) of the powering electric pulses are less than pixel time intervals during which a scanner of the scanning projector display is directing the light beam to form corresponding pixels of the image. For example, the pulsewidth of the powering electric pulses may be less than 10 nanoseconds, or even less than 5 nanoseconds. The driver may be further configured to provide the powering electric pulses in a middle of the corresponding pixel time intervals during which a scanner of the scanning projector display is directing the light beam to the corresponding pixels of the image. 
     In some embodiments, the light source comprises at least one of a single-mode light source or a multimode light source. The light source may include at least one of a side-emitting laser diode, a vertical-cavity surface-emitting laser diode, a superluminescent light-emitting diode, or another type of a light-emitting diode. Pulse energy of the powering electric pulses may remain lower than a threshold energy equal to the pixel time interval multiplied by a threshold power. In embodiments where the light source comprises a superluminescent LED (SLED), the threshold power may be equal to a lasing threshold electric power of the SLED. In some embodiments, the light source further includes a body supporting the light source, the driver the scanner, and the controller. The body may have a form factor of a pair of glasses. 
     In accordance with the present disclosure, there is further provided a method for displaying an image. The method includes using a scanner to angularly scan a light beam for displaying the image, the light beam including a succession of light pulses corresponding to pixels of the image. A pointing angle of the scanner is determined, and an energy of a light pulse to be emitted is determined in accordance with the determined pointing angle of the scanner. A light-emitting diode (LED) is energized by providing a powering electric pulse to the LED. The powering electric pulse has an energy corresponding to the determined energy of the light pulse to be emitted. A duration of the powering electric pulse is less than a pixel time interval during which the scanner is directing the light beam to form a corresponding pixel of the image. 
     In some embodiments, determining the energy of the light pulse to be emitted comprises determining an amplitude and/or duration of the light pulse to be emitted, and/or determining an amplitude and/or of the powering electric pulse. The powering electric pulse may be provided in a middle of the pixel time interval during which the scanner is directing the light beam to form the corresponding pixel of the image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will now be described in conjunction with the drawings, in which: 
         FIG. 1  is a schematic view of a projection display illustrating its principle of operation; 
         FIG. 2  is a schematic view of the projection display of  FIG. 1  scanning across a pixel of an image; 
         FIG. 3A  is a graph of a scanner&#39;s pointing angle vs. time; 
         FIG. 3B  is a graph of the LED output power vs. time during scanning across the pixel of  FIG. 2 ; 
         FIG. 4  is the LED&#39;s transfer curve illustrating operation of the LED at different driving currents corresponding to different plug efficiency levels; 
         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 using a superluminescent LED (SLED) array as a light source; 
         FIG. 6B  is a schematic top view of the scanning projector display of  FIG. 6A  illustrating a relationship between fields of view provided by individual emitters of the SLED array; 
         FIG. 7  is a flow chart of a method for displaying an image; 
         FIG. 8  is a schematic top view of a near-eye display including a scanning projector display disclosed herein; 
         FIG. 9A  is an isometric view of a head-mounted display (HMD) of the present disclosure; and 
         FIG. 9B  is a block diagram of a virtual reality system including the HMD of  FIG. 11A . 
     
    
    
     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. In  FIGS. 1, 6A, and 6B , similar reference numerals denote similar elements. 
     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. 
     Power consumption is one of key parameters of a near-eye display system. A light source of the scanning projector used to raster the virtual image is a major contributor to power consumption. Projectors that use semiconductor light sources may have poor wall plug efficiency due to a combination of factors. One such factor is a high threshold current for semiconductor devices such as lasers, VCSELs or SLEDs. Below the threshold, no light is produced; and above the threshold, the relationship between output light level and current follows a non-linear relationship, until at some point, the relationship becomes quasi-linear. To optimize overall efficiency, it is desirable to operate in a top portion of the quasi-linear range of the curve, i.e. substantially above the threshold current. However, if the required output light level is too low, the semiconductor light source may be forced to operate in a low-efficiency portion of the electro-optical transfer characteristic, which lowers the wall plug efficiency of the display system. 
     One way to mitigate this deficiency is to operate the device at much higher light output levels, but for a shorter period of time than a pixel time interval, i.e. a time interval when the projector is directing the light beam to a particular pixel of the image being displayed. This enables the display device to be operated further away from the threshold current and, accordingly, at a higher wall plug efficiency. Operation high above the threshold current may be particularly advantageous when superluminescent LEDs (SLEDs) are used as a light source. 
     In SLEDs, the initial relationship between current and output power is highly non-linear. To achieve higher wall plug efficiency, the SLED may be operated in a short-pulse mode, where a peak electric current is at a high-steepness, quasi-linear portion of the SLED electro-optical transfer curve, thereby increasing electrical-to-optical conversion efficiency and thus increasing overall wall plug efficiency of the SLED light source. Care must be taken such that lasing of the SLED does not develop. The lasing or output power instability may develop due to undesired backreflections in the optical path, as well due to driving SLED with too high a current above its lasing threshold. 
     Referring to  FIG. 1 , a scanning projector display  100  includes a solid-state light source  102  for providing a light beam  104 . The solid-state light source  102  may include a single-mode light source or a multimode light source, e.g. a light-emitting diode (LED) or a superluminescent light-emitting diode (SLED), a side-emitting laser diode, a vertical cavity surface-emitting laser diode (VCSEL), etc. An electronic driver  106  is operably coupled to the light source  102  for powering the light source  102 . A scanner  108 , e.g. a tiltable microelectromechanical system (MEMS) reflector, an acousto-optic modulator, a diffractive scanner, etc., is optically coupled to the light source  102  for scanning the light beam  104  generated by the light source  102 . The scanning may be performed in one or two dimensions, e.g. about an X-axis and/or Y-axis perpendicular to the X-axis, where X- and Y-axes are in plane of the MEMS mirror at its normal i.e. unpowered position. Pre-tilt of MEMS mirror may also be used. A pupil replicator  110  provides a light field  115  including multiple laterally displaced parallel copies of the scanned light beam  104 , which repeat the beam angle, i.e. a direction of propagation of the light beam  104  at every moment of time as the light beam  104  is scanned about one or two axes, as the case may be. 
     A controller  112  is operably coupled to the scanner  108  and the electronic driver  106 . The controller  112  is configured for operating the electronic driver  106  for powering the light source  102  in coordination with driving the scanner  108  and reading its position. For example, the controller  112  may cause the scanner  108  to scan the light beam  104  through a succession of beam angles or directions “A” through “G”, while causing the electronic driver  106  to change the brightness of the light source  102  in accordance with an image to be displayed, thus forming an image in angular domain for direct observation by a viewer&#39;s eye  114 . A feedback circuit may be provided to indicate the current MEMS mirror position to the controller  112 . 
     The pupil replicator  110  provides multiple laterally displaced parallel copies of the scanned light beam  104  in directions “A” through “G”, as illustrated. The viewer&#39;s eye  114  receives the light field  115 , and forms an image at the eye&#39;s retina  116  from the corresponding replicated light beams, as shown. A linear position of the beam copies on the eye&#39;s retina  116  is denoted with letters “a” through “g”, and corresponds to the beam angles or directions “A” through “G” of the scanned light beam  104 . In this manner, the eye  114  forms a linear image on the eye&#39;s retina  116  from the image in the angular domain formed by the light field  115 . In some embodiments, the driver  106  is configured to control energy of the light pulses for displaying the image by controlling at least one of amplitude or pulse width of the powering pulses used to energize the light source. 
     Turning to  FIG. 2 , the electronic driver  106  may be configured for providing powering pulses  202  to the light source  102 , such that a pulsewidth of the powering pulses  202  is less than a time interval during which the scanner  108  of the projector display  100  is directing the light beam  104  through an angular range Δα corresponding to a current pixel  204  of the image being displayed. Pixels  206 , including the currently displayed pixel  204 , are shown in  FIG. 2  in an arc configuration, to illustrate that the image being generated is in angular domain where each beam angle corresponds to a pixel of the image. The energy of the powering pulses  202  may be selected in accordance with the current pixel  204  (or pixels) being displayed at any given moment of time. It is noted that the term “pixel” as used herein refers to an element of an image being displayed, which may or may not be related to a “pixel” as an element of a detector array or an element of a visual display panel comprising an array of light-emitting pixels. 
       FIGS. 3A and 3B  illustrate the timing of the powering of the light source  102 . A scanning angle  300  of the scanner  108  ( FIGS. 1 and 2 ) runs through the angular range Δα of the currently displayed pixel  204  ( FIG. 2 ) corresponding to a time interval Δt ( FIGS. 3A and 3B ). In some embodiments, the time duration of a powering pulse  302  is shorter than Δt, and may be much shorter, e.g. no grater than one third of Δt, one tenth of Δt, one hundredth of Δt, and even less than one thousandth of Δt. 
     In some embodiments, a pulse energy of a light pulse provided by the light source  102  when driven by the powering pulse is approximately, e.g. to within 10-20%, equal to a nominal energy of the light beam when the light source is driven at a constant driving current during the pixel time interval. Herein, the term “nominal” energy refers to an energy of the light beam that corresponds to the brightness of a pixel of the currently displayed image. For example, referring specifically to  FIG. 3B , a nominal energy of the light beam  104  accumulated during the time interval Δt is Δt*P 1 , where P 1  is a nominal optical power level, at which the light source  102 , e.g. an SLED, would be normally operated if the light source  102  were to be powered at the power level P 1  throughput the entire time interval Δt. The light energy emitted during the time interval Δt can be represented by a rectangle  301 . Since the time duration of the powering pulse  302  is shorter than Δt, the optical power level P 2  of the powering pulse  302  can be made proportionally higher, such that the light energy is about the same (e.g. within 10-20%) as Δt*P 1 . In other words, approximately the same number of photons is emitted in the powering pulse  302  but during much shorter time interval than Δt. 
     The latter condition may be expressed as 
     
       
         
           
             
               
                 
                   
                     
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     The power levels P 1  and P 2  are plotted in an electro-optical transfer curve  400  of  FIG. 4 . When the light source  102  is driven at a constant or slowly varying optical power level throughout the entire pixel time interval, the power efficiency, represented by a slope of a dashed straight line  401 , may remain rather low. When, however, the light source  102  is driven in a pulsed mode, i.e. by the powering pulses  302 , the corresponding power efficiency, represented by a slope of a solid straight line  402 , may be much higher. Thus, the overall wall plug efficiency of the scanning projector display  100  may be improved while providing a substantially same capability of rendering an image at a pre-defined brightness level. 
     In some embodiments, the pulse energy of the powering electric pulses is lower than a threshold energy equal to the pixel time interval multiplied by some threshold power. For SLED power sources, the threshold power may be represented by an electric power which, when supplied to the SLED, may cause undesired lasing or output power instability due to parasitic feedback. The threshold electric power may be computed as square of a lasing threshold electric current divided by an effective electric resistance of the SLED p-n junction at that current. The threshold energy may also need to remain below a threshold related to optical safety, i.e. below a safe light energy to be directed to the viewer&#39;s eye  114 , as required by a laser safety standard, for example. In some embodiments, the pulsewidth may be less than 10 nanoseconds or shorter, e.g. less than 5 nanoseconds and even less then 2 nanoseconds. The driver  106  and/or the controller  112  may be configured to provide the powering pulse in a middle of the pixel time interval during which the scanner  108  of the projector display  100  is directed to a corresponding pixel of the image, for a better centering of the displayed pixels of the image. 
     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 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 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 , a first actuator  551  may be disposed under an edge of the reflector  510  to tilt the reflector  510  about X-axis. A 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. 
     Turning to  FIG. 6A , a scanning projector display  600  includes a multi-emitter light source assembly  602  including e.g. a SLED array for providing a diverging optical beam  604 D. An optional collimator  650  is optically coupled to the multi-emitter 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  112  can be operably coupled to the electronic driver  106 , which is coupled to the multi-emitter light source assembly  602 . The controller  112  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, 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  106  is configured for providing powering electric pulses to energize the multi-emitter light source assembly  602 . The controller  112  sends commands to the electronic driver  106  to energize the multi-emitter 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, as explained above with reference to  FIG. 1 . A pulsewidth of the powering electric pulses is less than a time interval during which the 2D MEMS scanner  500  is directing the light beam  604  to form a corresponding pixel of the image. Similarly to what has been explained above with reference to  FIGS. 2 to 4 , this enables one to improve the wall plug efficiency of the multi-emitter light source assembly  602  and the scanning projector display  600  as a whole. 
     In some embodiments, the scanner of the projector display may include a 1D tiltable mirror. For this embodiment, a linear array of light sources may be used 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 multi-emitter light source assembly  602  may include single-mode or multimode light sources, e.g. a side-emitting laser diode, a vertical-cavity surface-emitting laser diode, a superluminescent light-emitting diode, or a light-emitting diode. 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 projector  600  is further illustrated in  FIG. 6B . In this example, the multi-emitter light source assembly  602  includes 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  112  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 projector  600  in a common scan area  630 , as compared to a case of a single emitter. 
     Referring to  FIG. 7 , a method  700  for displaying an image includes using a scanner, such as the scanner  108  of  FIG. 1  or the 2D MEMS scanner  500  of  FIG. 5 , to angularly scan ( FIG. 7 ;  702 ) a light beam e.g. the light beam  104  ( FIG. 1 ) or the collimated light beam  604  ( FIG. 6A ), for displaying an image by rastering the light beam in sync with varying the power level of the light beam. The light beam is pulsed, i.e. it includes a succession of light pulses of variable energy, depending upon brightness of the image pixels energized by the corresponding light pulses. By way of example, the light source  102  emits light pulses when powered by the powering pulses  202  generated by the electronic driver  106  ( FIGS. 1 and 2 ). The controller  112  coordinates the timing of the powering pulses  202  with the angular scanning by the scanner  108 . 
     A pointing angle of the scanner may be determined ( FIG. 7 ;  704 ) e.g. from the driving signal, or from a mirror position sensor reporting a current tilt angle of the reflector  510  ( FIG. 5 ). The pointing angle may be pre-determined by the driving signals, or, for resonant or bi-resonant scanning of the MEMS mirror, obtained from the driving signals and sync signals provided by the mirror position sensors. Furthermore, in some cases, a scanning trajectory of the MEMS mirror may be predicted to enable the proper timing of the light source driving signals to be configured in advance. 
     A parameter of the light pulse to be emitted is determined based on the current pixel(s) of the image to be displayed ( 706 ), that is, the pixel(s) pointed at by the scanner. By way of non-limiting examples, the pulse energy, the pulse amplitude, the pulse duration, or the color of the emitted beam may be determined (for color light sources). The required color may be provided by energizing light sources emitting light at different wavelengths. These multi-color light sources such as an LED, an SLED, etc., may be energized simultaneously, sequentially, or with a programmed delay. The light source  102  is energized by providing a powering electric pulse (e.g. the powering pulse  202  in  FIG. 2 ) to the light source  102 . The determined light pulse amplitude and/or duration may be used to determine an amplitude and/or duration of the corresponding powering electric pulse. In other words, the powering electric pulse has an amplitude and a duration corresponding to the amplitude and duration of the light pulse to be emitted. 
     The parameters of the powering electric pulses to be applied to the light source  102  may be determined e.g. from the electro-optic response curve  400  of  FIG. 4 . As explained above, the duration of the powering pulses is less than the pixel time interval Δt during which the scanner  108  of the projector display  100  is directing the light beam to a pixel (e.g. the currently displayed pixel  204  in  FIG. 2 ) of the image. The pulse may then be emitted ( 708 ). The pulse energy of the emitted pulse corresponds to the brightness of the currently displayed pixel. The process may then proceed to shifting the scanner to a next pixel to be displayed, determining the parameters of the light pulses to be emitted, determining the corresponding electric powering pulse parameters (e.g. amplitude, duration, etc.), and emitting the next pulse. The process repeats until all pixels of the image have been displayed or “painted” by the scanning projector display. 
     Referring now to  FIG. 8 , a near-eye display  800  includes a frame  801  having a form factor of a pair of glasses. The frame  801  supports, for each eye: a light source subassembly  802 , an electronic driver  804  operably coupled to the light source subassembly  802  for powering the light source subassembly  802  for providing at least one light beam, a collimator  806  optically coupled to light source subassembly  802  for collimating the light beam, a scanner  808 , e.g. a tiltable reflector, optically coupled to the collimator  806 , and a pupil replicator  810  optically coupled to the scanner  808 . The light source subassembly  802  may include a substrate supporting an array of single-mode or multimode 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. A controller  805  is operably coupled to the scanners  808  and the electronic drivers  804 . The controller  805  is configured for driving the scanner  808  and reading its position, and operating the electronic drivers  804  for providing powering electric pulses to the light source subassemblies  802  in accordance with the position of the scanner  808 . As explained above, the pulsewidth of the powering electric pulses is less than a time interval during which the scanners  808  of the near-eye display  800  are directing the light beams to form a corresponding pixel of the image. 
     The collimators  806  may include a concave mirror, a bulk lens, a Fresnel lens, a holographic lens, etc., and may be integrated with the light source subassembly  802 . The scanners  808  may include the 2D MEMS scanner  500  of  FIG. 5 , for example, or a pair of 1D tiltable reflectors optically coupled via a pupil relay. The function of the pupil replicators  810  is to provide multiple laterally offset copies of the light beams redirected or scanned by the scanner  808  at eyeboxes  812 , as has been explained above with reference to  FIG. 1 . 
     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  may include electronic drivers and scanning projector displays described herein, e.g. the scanning projector display  100  of  FIGS. 1 and 2 , or the scanning projector display  600  of  FIGS. 6A and 6B . 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 3D 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  may include electronic drivers, light sources, and projector displays disclosed herein. The AR/VR system  950  includes the HMD  900  of  FIG. 9A , an external console  990  storing various AR/VR applications, setup and calibration procedures, 3D 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  915  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  may include, for example, the scanning projector display  100  of  FIGS. 1 and 2 , the scanning projector display  600  of  FIGS. 6A and 6B . The display system  980  may include 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  99 , 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.