Patent Description:
An eye tracker is a device for estimating eye positions and eye movement. Eye tracking systems have been used in research on the visual system, in psychology, psycholinguistics, marketing, and as input devices for human-computer interaction. In the latter application, typically the intersection of a person's point of gaze with a desktop monitor is considered. <CIT> discloses a head mounted display with eyetracking capability. <CIT> discloses a see-through computer display with improved optics. <CIT> discloses an HMD apparatus with an adjustable eye tracking device.

Preferred embodiments of the invention are stipulated in the dependent claims. While several embodiments and/or examples have been disclosed in this description, the subject-matter for which protection is sought is strictly and solely limited to those embodiments and/or examples encompassed by the scope of the appended claims. Embodiments and/or examples mentioned in the description that do not fall under the scope of the claims are useful for understanding the invention. Various embodiments of methods and apparatus for eye tracking in virtual and mixed or augmented reality (VR/AR) applications are described. A VR/AR device such as a headset, helmet, goggles, or glasses (referred to herein as a head-mounted display (HMD)) is described that includes a display (e.g., left and right displays) for displaying frames including left and right images in front of a user's eyes to thus provide 3D virtual views to the user. The HMD includes left and right optical lenses (referred to herein as eyepieces) located between the display and the user's eyes. The eyepieces form a virtual image of the displayed content at a design distance which is typically close to optical infinity of the eyepieces.

The HMD includes an eye tracking system for detecting position and movements of the user's eyes. The eye tracking system includes at least one eye tracking camera (near-IR (NIR) cameras) positioned at each side of the user's face and pointed towards the eye-facing surfaces of the respective eyepieces, an illumination source (an NIR light source) that emits NIR light towards the user's eyes, and hot mirrors located between the eye-facing surfaces of the eyepieces and the user's eyes. In accordance with the invention, positioning the hot mirror surfaces on the eye-facing surfaces of the eyepieces allows the eye tracking cameras to be placed at the sides of the user's face (e.g., at or near the user's cheek bones) without having to image through the eyepieces.

The light sources of the HMD emit NIR light to illuminate the user's eyes. A portion of the NIR light is reflected off the user's eyes to the hot mirrors located on the eye-facing surfaces of the eyepieces of the HMD. The hot mirrors reflect at least a portion of the NIR light, while allowing visible light to pass. The NIR cameras, for example located at or near the user's cheek bones capture images of the user's eyes reflected by the hot mirrors.

Images captured by the eye tracking system may be analyzed to detect position and movements of the user's eyes, or to detect other information about the eyes such as pupil dilation. For example, the point of gaze on the display estimated from the eye tracking images may enable gaze-based interaction with content shown on the near-eye display of the HMD. Other applications may include, but are not limited to, creation of eye image animations used for avatars in a VR/AR environment.

This specification includes references to "one embodiment" or "an embodiment. " The appearances of the phrases "in one embodiment" or "in an embodiment" do not necessarily refer to the same embodiment. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure.

"Comprising. " This term is open-ended. As used in the claims, this term does not foreclose additional structure or steps. Consider a claim that recites: "An apparatus comprising one or more processor units. " Such a claim does not foreclose the apparatus from including additional components (e.g., a network interface unit, graphics circuitry, etc.).

"Configured To. " Various units, circuits, or other components may be described or claimed as "configured to" perform a task or tasks. In such contexts, "configured to" is used to connote structure by indicating that the units/circuits/components include structure (e.g., circuitry) that performs those task or tasks during operation. As such, the unit/circuit/component can be said to be configured to perform the task even when the specified unit/circuit/component is not currently operational (e.g., is not on). The units/circuits/components used with the "configured to" language include hardware - for example, circuits, memory storing program instructions executable to implement the operation, etc. Reciting that a unit/circuit/component is "configured to" perform one or more tasks is expressly intended not to invoke <NUM> U. § <NUM>, paragraph (f), for that unit/circuit/component. Additionally, "configured to" can include generic structure (e.g., generic circuitry) that is manipulated by software or firmware (e.g., an FPGA or a general-purpose processor executing software) to operate in manner that is capable of performing the task(s) at issue. "Configure to" may also include adapting a manufacturing process (e.g., a semiconductor fabrication facility) to fabricate devices (e.g., integrated circuits) that are adapted to implement or perform one or more tasks.

"First," "Second," etc. As used herein, these terms are used as labels for nouns that they precede, and do not imply any type of ordering (e.g., spatial, temporal, logical, etc.). For example, a buffer circuit may be described herein as performing write operations for "first" and "second" values. The terms "first" and "second" do not necessarily imply that the first value must be written before the second value.

"Based On" or "Dependent On. " As used herein, these terms are used to describe one or more factors that affect a determination. These terms do not foreclose additional factors that may affect a determination. That is, a determination may be solely based on those factors or based, at least in part, on those factors. Consider the phrase "determine A based on B. " While in this case, B is a factor that affects the determination of A, such a phrase does not foreclose the determination of A from also being based on C. In other instances, A may be determined based solely on B.

" When used in the claims, the term "or" is used as an inclusive or and not as an exclusive or. For example, the phrase "at least one of x, y, or z" means any one of x, y, and z, as well as any combination thereof.

Various embodiments of methods and apparatus for eye tracking in virtual and mixed or augmented reality (VR/AR) applications are described. A VR/AR device such as a headset, helmet, goggles, or glasses (referred to herein as a head-mounted display (HMD)) is described that includes a display (e.g., left and right displays) for displaying frames including left and right images in front of a user's eyes to thus provide 3D virtual views to the user. The HMD includes left and right optical lenses (referred to herein as eyepieces) located between the display and the user's eyes. The eyepieces form a virtual image of the displayed content at a design distance which is typically close to optical infinity of the eyepieces. The HMD includes an eye tracking system (which may also be referred to as a gaze tracking system) for detecting position and movements of the user's eyes, or for detecting other information about the eyes such as pupil dilation. The point of gaze on the display estimated from the information captured by the eye tracking system may, for example, allow gaze-based interaction with the content shown on the near-eye display. Other applications may include, but are not limited to, creation of eye image animations used for avatars in a VR/AR environment.

Embodiments of an eye tracking system for HMDs according to the invention include at least one eye tracking camera (near-IR (NIR) cameras) positioned at each side of the user's face (e.g., at or near the user's cheek bones) and pointed towards the eye-facing surfaces of the respective eyepieces, an illumination source (an NIR light source) that emits NIR light towards the user's eyes, and hot mirrors located between the eye-facing surfaces of the eyepieces and the user's eyes. A hot mirror may be defined as a mirror that acts as a dichroic filter to reflect light in the near-infrared range while allowing visible light to pass. According to the invention, the hot mirrors are implemented as a dichroic filter coating on the eye-facing surfaces of the eyepieces; the eye-facing surfaces of the eyepieces may be, but are not necessarily, planar. Positioning the hot mirror surfaces on the eye-facing surfaces of the eyepieces and thus between the eyepieces and the user's eyes allows the eye tracking cameras to be placed at the sides of the user's face (e.g., at or near the user's cheek bones) without having to image through the eyepieces.

While embodiments of an eye tracking system for HMDs are generally described herein as including at least one eye tracking camera positioned at each side of the user's face to track the gaze of both of the user's eyes, an eye tracking system for HMDs may also be implemented that includes at least one eye tracking camera positioned at only one side of the user's face to track the gaze of only one of the user's eyes.

<FIG> and <FIG> illustrate eye tracking systems for VR/AR HMDs. A VR/AR HMD <NUM> includes a display <NUM> and two eyepiece lenses <NUM>, mounted in a wearable housing. The user looks through the eyepieces <NUM> onto the display <NUM>. The eyepieces <NUM> form a virtual image of the displayed content at a design distance which is typically close to optical infinity of the eyepieces <NUM>. To fit eye tracking cameras <NUM> into the HMD <NUM> housing, two different camera optical paths have been used. As shown in HMD 100A of <FIG>, in a first camera optical path, the cameras <NUM> capture light through the eyepiece <NUM>. As shown in HMD 100B of <FIG>, in a second camera optical path, the cameras <NUM> have a direct view of the user's eyes.

Referring to HMD 100A of <FIG>, the cameras <NUM> are positioned such that a frontal view of the eyes <NUM> is captured through the eyepieces <NUM>. In order to remove the cameras <NUM> from the user's field of view, hot mirrors <NUM> are positioned between the eyepieces <NUM> and the display <NUM> to fold the camera <NUM> optical paths away from the visible light display <NUM> optical paths. NIR light source(s) <NUM> may be positioned in the HMD 100A (e.g., around the eyepieces <NUM>, or elsewhere in the HMD 100A) to illuminate the user's eyes <NUM> with NIR light.

Referring to HMD 100B of <FIG>, the cameras <NUM> do not look through the eyepieces <NUM>, but instead have direct views onto the user's eyes <NUM>. For this optical path, cameras <NUM> are typically mounted at the side of the user's nose, the side of the cheek-bone <NUM>, or on top or bottom of an eyeglass-frame. Physical constraints of the HMD 100B housing may determine which position is suitable for a given system. Since the eyepieces <NUM> are close to the user's eyes <NUM>, there is not enough space to place a hot mirror to fold the cameras <NUM> away from the user's face as it is done in HMD 100A. As a consequence, the cameras <NUM> do not have a frontal view onto the user's eyes <NUM>. Thus, the incident angle of a camera <NUM>'s on-axis chief ray on the nominal cornea plane which is parallel to the display <NUM> plane may be substantially less than <NUM> degrees.

The camera optical paths shown in <FIG> and <FIG> have advantages and disadvantages. The through-the-eyepiece view of <FIG> allows a more centered view of the eye, but has to deal with distortions in the eye images introduced by the eyepiece. The direct view of <FIG> does not pass through the eyepiece, but may look onto the eye from a tilted position which may cause reduced detection accuracy of eye features at extreme gaze angles due to distortion, insufficient depth-of-field, and occlusions.

<FIG> illustrates a VR/AR HMD <NUM> that implements an eye tracking system that includes hot mirrors <NUM> located between the eye-facing surfaces of the eyepieces <NUM> and the user's eyes <NUM>, and eye tracking cameras <NUM> located at the sides of the user's face (e.g., at or near the user's cheek bones <NUM>), according to some embodiments. VR/AR HMD <NUM> may include a display <NUM> and two eyepieces <NUM>, mounted in a wearable housing. The user looks through the eyepieces <NUM> onto the display <NUM>. The eyepieces <NUM> form a virtual image of the displayed content at a design distance which is typically close to optical infinity of the eyepieces <NUM>. The eye tracking system may, for example, be used to track position and movement of the user <NUM>'s eyes. In some embodiments, the eye tracking system may instead or also be used to track dilation of the user <NUM>'s pupils, or other characteristics of the user <NUM>'s eyes. NIR light source(s) <NUM> (e.g., NIR LEDs) may be positioned in the HMD <NUM> (e.g., around the eyepieces <NUM>, or elsewhere in the HMD <NUM>) to illuminate the user's eyes <NUM> with NIR light. The hot mirrors <NUM> are positioned on the eye-facing surfaces of the eyepieces <NUM>. The optical paths for the eye tracking cameras <NUM> provide a direct view of the eyes <NUM> via reflection off the hot mirrors <NUM>. The eye tracking system of <FIG> thus provide direct-view (i.e., not through the eyepieces <NUM>) eye tracking cameras <NUM> that capture a portion of NIR light emitted by light sources <NUM>, reflected off the user's eyes, and reflected by hot mirrors <NUM> located on the eye-facing surface of the eyepieces <NUM> to the cameras <NUM> to image the user's eyes <NUM>. The eye tracking system of <FIG> provides an eye tracking camera optical path that enables a smaller tilt-angle as in the camera optical path shown in <FIG>, but that avoids looking through the eyepieces as in in the camera optical path shown in <FIG>.

In some embodiments, the HMD eye tracking system of <FIG> may use eyepieces <NUM> with flat or convex eye-facing surfaces and hot mirrors <NUM> positioned on the eye-facing surfaces of the eyepieces <NUM>. By positioning the hot mirrors <NUM> on the eye-facing surfaces of the eyepieces <NUM>, the camera optical path can be folded, resulting in a larger incident angle of the camera axis on the center pupil location (closer to <NUM> degrees) than in direct-view eye tracking camera architectures as shown in <FIG>.

The hot mirrors <NUM> may be implemented as a coating on the eye-facing surfaces of the eyepieces <NUM>. The characteristics of the hot mirror need to take reflectivity over a variety of angles in the near-infrared (NIR) spectrum into account, as well as over the visible spectrum. Example hot mirror specifications are described in TABLE <NUM>, including > <NUM>% reflectivity of NIR at a nominal camera incident angle on the hot mirror (e.g. <NUM> degrees), less than <NUM>% reflectivity of visible light at <NUM> degrees, and less than <NUM>% reflectivity at incident angles > <NUM> degrees.

In some embodiments, the display <NUM> emits light in the visible light range and does not emit light in the NIR range, and thus does not introduce noise in the eye tracking system.

TABLE <NUM> provides parameters for an example embodiment of a hot mirror <NUM>, and is not intended to be limiting. The values given in the cells may be varied within reasonable limits while still achieving similar results. For example, the value for near-infrared light given in the first column, row four (<NUM> +/-<NUM>) may be <NUM> nanometers (nm) +/-<NUM>, or other values may be used. As another example, the Incident angle of <NUM> degrees given in the third column, row one and/or the Incident angle of <NUM> degrees given in the fourth column, row one may be varied by a few degrees.

<FIG> shows a side view of an example HMD <NUM> that implements an eye tracking system as illustrated in <FIG>, according to some embodiments. Note that HMD <NUM> as illustrated in <FIG> is given by way of example, and is not intended to be limiting. In various embodiments, the shape, size, and other features of an HMD <NUM> may differ, and the locations, numbers, types, and other features of the components of an HMD <NUM> may vary. The eye tracking system may, for example, be used to track position and movement of the user <NUM>'s eyes. In some embodiments, the eye tracking system may instead or also be used to track dilation of the user <NUM>'s pupils, or other characteristics of the user <NUM>'s eyes. Information collected by the eye tracking system may be used in various VR or AR system functions. For example, the point of gaze on the display <NUM> may be estimated from images captured by the eye tracking system; the estimated point of gaze may, for example, enable gaze-based interaction with content shown on the near-eye display <NUM>. Other applications of the eye tracking information may include, but are not limited to, creation of eye image animations used for avatars in a VR or AR environment. As another example, in some embodiments, the information collected by the eye tracking system may be used to adjust the rendering of images to be projected, and/or to adjust the projection of the images by the projection system of the HMD <NUM>, based on the direction and angle at which the user <NUM>'s eyes are looking. As another example, in some embodiments, brightness of the projected images may be modulated based on the user <NUM>'s pupil dilation as determined by the eye tracking system.

As shown in <FIG>, HMD <NUM> may be positioned on the user <NUM>'s head such that the display <NUM> and eyepieces <NUM> are disposed in front of the user <NUM>'s eyes <NUM>. One or more NIR light source(s) <NUM> (e.g., NIR LEDs) may be positioned in the HMD <NUM> (e.g., around the eyepieces <NUM>, or elsewhere in the HMD <NUM>) to illuminate the user <NUM>'s eyes <NUM> with NIR light. In some embodiments, the NIR light source(s) <NUM> may emit light at different NIR wavelengths (e.g., <NUM> and <NUM>). Hot mirrors <NUM> are positioned on the eye-facing surfaces of the eyepieces <NUM>. The hot mirrors <NUM> may be optimized according to the NIR wavelength(s) of the eye tracking camera(s) <NUM>, positions of the cameras <NUM>, and the display <NUM> optics specifications. At least one eye tracking camera <NUM> (e.g., an NIR camera, for example a 400x400 pixel count camera, that operates at <NUM> or <NUM>, or at some other NIR wavelength) is located at each side of the user <NUM>'s face, for example below the user's eye and at or near the user <NUM>'s cheek bones as shown in <FIG>. Note that the location and angle of eye tracking camera <NUM> is given by way of example, and is not intended to be limiting. While <FIG> shows a single eye tracking camera <NUM> located on each side of the user <NUM>'s face, in some embodiments there may be two or more NIR cameras <NUM> on each side of the user <NUM>'s face. For example, in some embodiments, a camera <NUM> with a wider field of view (FOV) and a camera <NUM> with a narrower FOV may be used on each side of the user's face. As another example, in some embodiments, a camera <NUM> that operates at one wavelength (e.g. <NUM>) and a camera <NUM> that operates at a different wavelength (e.g. <NUM>) may be used on each side of the user's face. A portion of NIR light emitted by light source(s) <NUM> reflects off the user <NUM>'s eyes, is reflected by hot mirrors <NUM> to the cameras <NUM>, and is captured by the cameras <NUM> to image the user's eyes <NUM>.

Embodiments of the HMD <NUM> with an eye tracking system as illustrated in <FIG> may, for example, be used in augmented or mixed (AR) applications to provide augmented or mixed reality views to the user <NUM>. While not shown, in some embodiments, HMD <NUM> may include one or more sensors, for example located on external surfaces of the HMD <NUM>, that collect information about the user <NUM>'s external environment (video, depth information, lighting information, etc.); the sensors may provide the collected information to a controller (not shown) of the VR/AR system. In some embodiments, the sensors may include one or more visible light cameras (e.g., RGB video cameras) that capture video of the user's environment that may be used to provide the user <NUM> with a virtual view of their real environment. In some embodiments, video streams of the real environment captured by the visible light cameras may be processed by the controller of the HMD <NUM> to render augmented or mixed reality frames that include virtual content overlaid on the view of the real environment, and the rendered frames may be provided to the projection system of the HMD <NUM> for display on display <NUM>.

Embodiments of the HMD <NUM> with an eye tracking system as illustrated in <FIG> may also be used in virtual reality (VR) applications to provide VR views to the user <NUM>. In these embodiments, the controller of the HMD <NUM> may render or obtain virtual reality (VR) frames that include virtual content, and the rendered frames may be provided to the projection system of the HMD <NUM> for display on display <NUM>.

The controller may be implemented in the HMD <NUM>, or alternatively may be implemented at least in part by an external device (e.g., a computing system) that is communicatively coupled to HMD <NUM> via a wired or wireless interface. The controller may include one or more of various types of processors, image signal processors (ISPs), graphics processing units (GPUs), coder/decoders (codecs), and/or other components for processing and rendering video and/or images. The controller may render frames (each frame including a left and right image) that include virtual content based at least in part on the inputs obtained from the sensors, and may provide the frames to a projection system of the HMD <NUM> for display to display <NUM>. <FIG> further illustrates components of a HMD and VR/AR system, according to some embodiments.

<FIG> is a block diagram illustrating components of an example VR/AR system <NUM> that includes an eye tracking system as described herein, according to some embodiments. According to the invention, a VR/AR system <NUM> includes an HMD <NUM> such as a headset, helmet, goggles, or glasses. HMD <NUM> may implement any of various types of virtual reality projector technologies. For example, the HMD <NUM> may include a VR projection system that includes a projector <NUM> that displays frames including left and right images on screens or displays 2022A and 2022B that are viewed by a user through eyepieces 2220A and 2220B. The VR projection system may, for example, be a DLP (digital light processing), LCD (liquid crystal display), or LCoS (liquid crystal on silicon) technology projection system. To create a three-dimensional (3D) effect in a 3D virtual view, objects at different depths or distances in the two images may be shifted left or right as a function of the triangulation of distance, with nearer objects shifted more than more distant objects. Note that other types of projection systems may be used in some embodiments.

In some embodiments, HMD <NUM> may include a controller <NUM> configured to implement functionality of the VR/AR system <NUM> and to generate frames (each frame including a left and right image) that are displayed by the projector <NUM>. In some embodiments, HMD <NUM> may also include a memory <NUM> configured to store software (code <NUM>) of the VR/AR system that is executable by the controller <NUM>, as well as data <NUM> that may be used by the VR/AR system <NUM> when executing on the controller <NUM>. In some embodiments, HMD <NUM> may also include one or more interfaces (e.g., a Bluetooth technology interface, USB interface, etc.) configured to communicate with an external device <NUM> via a wired or wireless connection. In some embodiments, at least a part of the functionality described for the controller <NUM> may be implemented by the external device <NUM>. External device <NUM> may be or may include any type of computing system or computing device, such as a desktop computer, notebook or laptop computer, pad or tablet device, smartphone, hand-held computing device, game controller, game system, and so on.

In various embodiments, controller <NUM> may be a uniprocessor system including one processor, or a multiprocessor system including several processors (e.g., two, four, eight, or another suitable number). Controller <NUM> may include central processing units (CPUs) configured to implement any suitable instruction set architecture, and may be configured to execute instructions defined in that instruction set architecture. For example, in various embodiments controller <NUM> may include general-purpose or embedded processors implementing any of a variety of instruction set architectures (ISAs), such as the x86, PowerPC, SPARC, RISC, or MIPS ISAs, or any other suitable ISA. In multiprocessor systems, each of the processors may commonly, but not necessarily, implement the same ISA. Controller <NUM> may employ any microarchitecture, including scalar, superscalar, pipelined, superpipelined, out of order, in order, speculative, non-speculative, etc., or combinations thereof. Controller <NUM> may include circuitry to implement microcoding techniques. Controller <NUM> may include one or more processing cores each configured to execute instructions. Controller <NUM> may include one or more levels of caches, which may employ any size and any configuration (set associative, direct mapped, etc.). In some embodiments, controller <NUM> may include at least one graphics processing unit (GPU), which may include any suitable graphics processing circuitry. Generally, a GPU may be configured to render objects to be displayed into a frame buffer (e.g., one that includes pixel data for an entire frame). A GPU may include one or more graphics processors that may execute graphics software to perform a part or all of the graphics operation, or hardware acceleration of certain graphics operations. In some embodiments, controller <NUM> may include one or more other components for processing and rendering video and/or images, for example image signal processors (ISPs), coder/decoders (codecs), etc..

Memory <NUM> may include any type of memory, such as dynamic random access memory (DRAM), synchronous DRAM (SDRAM), double data rate (DDR, DDR2, DDR3, etc.) SDRAM (including mobile versions of the SDRAMs such as mDDR3, etc., or low power versions of the SDRAMs such as LPDDR2, etc.), RAMBUS DRAM (RDRAM), static RAM (SRAM), etc. In some embodiments, one or more memory devices may be coupled onto a circuit board to form memory modules such as single inline memory modules (SIMMs), dual inline memory modules (DIMMs), etc. Alternatively, the devices may be mounted with an integrated circuit implementing system in a chip-on-chip configuration, a package-on-package configuration, or a multi-chip module configuration.

In some embodiments, the HMD <NUM> may include one or more sensors <NUM> that collect information about the user's environment (video, depth information, lighting information, etc.). The sensors <NUM> may provide the information to the controller <NUM> of the VR/AR system <NUM>. In some embodiments, sensors <NUM> may include, but are not limited to, visible light cameras (e.g., video cameras).

As shown in <FIG>, HMD <NUM> may be positioned on the user's head such that the displays 2022A and 2022B and eyepieces 2220A and 2220B are disposed in front of the user's eyes 2292A and 2292B. NIR light sources 2230A and 2230B (e.g., NIR LEDs) may be positioned in the HMD <NUM> (e.g., around the eyepieces 2220A and 2220B, or elsewhere in the HMD <NUM>) to illuminate the user's eyes 2292A and 2292B with NIR light. Hot mirrors 2242A and 2242B are positioned on the eye-facing surfaces of the eyepieces 2220A and 2220B. Eye tracking cameras 2240A and 2240B (e.g., NIR cameras, for example 400x400 pixel count cameras) are located at each side of the user's face, for example at or near the user's cheek bones as shown in <FIG>. Note that the location of eye tracking cameras 2240A and 2240B is given by way of example, and is not intended to be limiting. In some embodiments, there may be a single eye tracking camera <NUM> located on each side of the user's face. In some embodiments there may be two or more NIR cameras <NUM> on each side of the user's face. For example, in some embodiments, a wide-angle camera <NUM> and a narrower-angle camera <NUM> may be used on each side of the user's face. A portion of NIR light emitted by light sources 2230A and 2230B reflects off the user's eyes 2292A and 2292B, is reflected by hot mirrors 2242A and 2242B to respective eye tracking cameras 2240A and 2240B, and is captured by the eye tracking cameras 2240A and 2240B to image the user's eyes 2292A and 2292B. Eye tracking information captured by the cameras 2240A and 2240B may be provided to the controller <NUM>. The controller <NUM> may analyze the eye tracking information (e.g., images of the user's eyes 2292A and 2292B) to determine eye position and movement, pupil dilation, or other characteristics of the eyes 2292A and 2292B.

The eye tracking information obtained and analyzed by the controller <NUM> may be used by the controller in performing various VR or AR system functions. For example, the point of gaze on the displays 2022A and 2022B may be estimated from images captured by the eye tracking cameras 2240A and 2240B; the estimated point of gaze may, for example, enable gaze-based interaction with content shown on the displays 2022A and 2022B. Other applications of the eye tracking information may include, but are not limited to, creation of eye image animations used for avatars in a VR or AR environment. As another example, in some embodiments, the information obtained from the eye tracking cameras 2240A and 2240B may be used to adjust the rendering of images to be projected, and/or to adjust the projection of the images by the projector <NUM> of the HMD <NUM>, based on the direction and angle at which the user's eyes are looking. As another example, in some embodiments, brightness of the projected images may be modulated based on the user's pupil dilation as determined by the eye tracking system.

In some embodiments, the HMD <NUM> may be configured to render and display frames to provide an augmented or mixed reality (AR) view for the user at least in part according to sensor <NUM> inputs. The AR view may include renderings of the user's environment, including renderings of real objects in the user's environment, based on video captured by one or more video cameras that capture high-quality, high-resolution video of the user's environment for display. The AR view may also include virtual content (e.g., virtual objects, virtual tags for real objects, avatars of the user, etc.) generated by VR/AR system <NUM> and composited with the projected view of the user's real environment.

Embodiments of the HMD <NUM> as illustrated in <FIG> may also be used in virtual reality (VR) applications to provide VR views to the user. In these embodiments, the controller <NUM> of the HMD <NUM> may render or obtain virtual reality (VR) frames that include virtual content, and the rendered frames may be provided to the projector <NUM> of the HMD <NUM> for display to displays 2022A and 2022B.

<FIG> is a high-level flowchart illustrating a method of operation of an HMD that includes an eye tracking system as illustrated in <FIG>, according to some embodiments. As indicated at <NUM>, light sources of the HMD emit NIR light to illuminate a user's eyes. As indicated at <NUM>, a portion of the NIR light is reflected off the user's eyes to hot mirrors located on eye-facing surfaces of optical lenses (eyepieces) of the HMD. As indicated at <NUM>, the hot mirrors reflect at least a portion of the NIR light, while allowing visible light to pass. As indicated at <NUM>, NIR cameras located at or near the user's cheek bones capture images of the user's eyes reflected by the hot mirrors. The arrow returning from element <NUM> to element <NUM> indicates that the eye tracking process may be a continuous process as long as the user is using the HMD.

Claim 1:
A system, comprising:
a head-mounted display, HMD (<NUM>), configured to display visual content for viewing by a user, wherein the HMD comprises:
at least one display screen (<NUM>) positioned in front of the user's left and right eyes (<NUM>) and configured to display frames of visual content for viewing by the user;
left and right optical lenses (<NUM>) positioned between the at least one display screen and the user's left and right eyes such that the user views the display screen through the left and right optical lenses;
one or more infrared or near-infrared light sources (<NUM>) configured to emit infrared or near-infrared light towards the user's eyes;
left and right hot mirrors (<NUM>) positioned between the left and right optical lenses and the user's left and right eyes on the eye-facing surfaces of the left and right optical lenses such that the hot mirrors reflect infrared or near-infrared light returned from the user's eyes and allow visible light from the at least one display screen to pass through the left and right optical lenses and subsequently through the left and right hot mirrors to the user's eyes; and
left and right infrared or near-infrared cameras (<NUM>) configured to capture a portion of the infrared or near-infrared light reflected by the left and right hot mirrors to generate images of the user's eyes.