Patent Publication Number: US-10775884-B2

Title: Gaze calibration via motion detection for eye mounted displays

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. Provisional Application No. 62/507,862, filed on May 18, 2017, the contents of which are hereby incorporated by reference in their entirety. This application is related to U.S. application Ser. No. 15/725,059, filed Oct. 4, 2017, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     This disclosure relates generally to eye-mounted displays and, more particularly, to the calibration of eye-mounted displays. 
     2. Description of Related Art 
     An eye-mounted display has been proposed by Deering. See, e.g. U.S. Pat. No. 8,786,675, “Systems using eye mounted displays.” One type of eye-mounted display includes a tiny projector (or “femtoprojector”) mounted inside a contact lens. The projector projects images onto the retina of a person wearing the lens. A typical femtoprojector is no larger than about a millimeter in any dimension. 
     Eye-mounted displays can be used for augmented reality applications. In augmented reality applications, the images projected by the eye-mounted display change the way a user sees the external environment. For instance, the eye-mounted display can superimpose a virtual object sitting on top of a real-world object. Accordingly, calibrating the eye-mounted display may help improve augmented reality applications that require precision in displaying virtual objects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the disclosure have other advantages and features which will be more readily apparent from the following detailed description and the appended claims, when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an illustration of user wearing an augmented reality system. 
         FIG. 2  is an illustration of a necklace of an augmented reality system. 
         FIG. 3A  is an illustration of an eye wearing a contact lens display of an augmented reality system. 
         FIG. 3B  is a plan view of a contact lens display. 
         FIG. 3C  is a cross-section of a contact lens display on an eye. 
         FIG. 4  is an illustration of a calibration system for the augment reality system. 
         FIG. 5A  is an illustration defining a user&#39;s visual field. 
         FIG. 5B  illustrates an aligned contact lens display. 
         FIG. 5C  illustrates a misaligned contact lens display. 
         FIG. 6  is a flow chart for a method to calibrate a contact lens display relative to a user&#39;s visual axis. 
         FIGS. 7A-7D  illustrate an example of a calibration process using an augmented reality system that does not hold a virtual object stationary in the user&#39;s visual field. 
         FIGS. 8A-8C  illustrate an example of a calibration process using an augmented reality system that holds a virtual object stationary in the user&#39;s visual field. 
     
    
    
     The figures depict various embodiments for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Introduction 
     An augmented reality system can include an electronic contact lens with small projectors, such as femtoprojectors (see, e.g. U.S. Ser. No. 15/570,707 filed on Oct. 30, 2017 for examples of femtoprojector optical designs), to display virtual images to a user. The projectors project images onto the user&#39;s retina that are combined with external light passing through the contact lens from the user&#39;s environment. This enables a user to view virtual images within the real world. 
     If the projectors of the contact lens are misaligned with a user&#39;s gaze or visual axis, a virtual image can be displayed at an unintended location within the user&#39;s environment. For example, if a virtual image of a vase is intended to be displayed on a real-world table, misalignment with the user&#39;s gaze can cause the vase to float slightly over or to the side of the table. 
     The projectors of the contact lens can be calibrated with the user&#39;s gaze to prevent the misalignment of virtual images. A projector can project a calibration image from an initial location, such as an initial pixel within a pixel array of the projector estimated to align with the center of the user&#39;s fovea. The user&#39;s eye is monitored, and if a motion of the eye is detected in response to the calibration image, the projectors of the contact lens can be calibrated based on the detected motion. For simplicity, calibrating the projectors of the content lens is referred to simply as calibrating the contact lens below. 
     The calibration image can be held stationary relative to the user&#39;s external environment. In other words, if the calibration image is displayed off-center from a user&#39;s gaze and the user&#39;s gaze is shifted in order to look at the calibration image, the location from which the calibration image is projected can be shifted in an opposite direction in order to hold the calibration image stationary. Once the user&#39;s gaze at the calibration image is held steady, the shifted location from which the calibration image is projected can be determined to correspond to a center of the user&#39;s gaze. For instance, a particular pixel from a pixel array of the projector corresponding to the shifted location can be determined to correspond to the center of the user&#39;s gaze, and the contact lens can be calibrated based on the determined pixel. It should be noted that the calibration of a contact lens as described herein is separate from but can be combined with as the calibration procedures described in U.S. application Ser. No. 15/725,059. 
     The contact lens can also be calibrated by identifying a magnitude and direction of a user&#39;s eye movement in response to the calibration image. For instance, a pixel corresponding to the center of the user&#39;s gaze can be determined based on the magnitude and direction of the user&#39;s eye movement and based on an initial pixel from which the calibration image is projected. 
     Calibration images can be a single pixel, and the user can be prompted to center the pixel within the user&#39;s gaze. For instance, the user can be prompted via text displayed by the contact lens or audio signal to center the pixel. Alternatively, when the calibration image is displayed off-center from the user&#39;s gaze (e.g., the calibration image is projected away from the center of the user&#39;s fovea), the user may instinctively try to center the calibration image without being prompted. 
     The calibration image can be more than one pixel, but can include an obvious center or focal point, such as an image of a bullseye. The calibration image can also be a set of calibration images projected from different locations. For instance, different calibration images can be projected from multiple different pixels of the contact lens projectors, and the user can be prompted to look at the calibration image that most closely aligns with the center of the user&#39;s gaze. Likewise, the calibration image can be displayed by sequentially displaying a series of pixels to identify a pixel or location within a pixel array that requires the least amount of eye movement from the user, and thus the pixel or location that most closely aligns with the center of the user&#39;s gaze. 
     The contact lens can be calibrated during a dedicated calibration period, for instance at startup of the contact lens or periodically thereafter. The contact lens can also be calibrated in response to a user request, in response to determining that the contact lens is no longer calibrated, or in response to detecting eye movement patterns indicating that calibration is needed. 
     Once calibrated, subsequent images can be projected by the contact lens based on the calibration. For instance, a subsequent image can be projected at or relative to the pixel of a projector determined to correspond to the center of the user&#39;s gaze. Likewise, a subsequent image can be projected from a location based on an offset determined during calibration. 
     Electronic Contact Lens Architecture 
     Turning now to the figures,  FIG. 1  shows a person wearing an augmented reality system  100  including a necklace  110  and an electronic contact lens  120  with an image source, such as one or more femtoprojectors. The contact lens  120  projects images onto the retina of the wearer to create an augmented reality. The necklace  110  and contact lens  120  are wirelessly coupled such that they can provide information to one another. For example, the necklace can provide images and information to the contact lens  120  to project onto the retina and the contact lens  120  provide motion information to the necklace  110 . 
     The AR system of  FIG. 1  does not disturb the wearer because the contact lenses  120  and the necklace  110  are similar to normal contacts lenses and necklaces used in everyday life. For example, the contact lenses  120  may also provide refractive eyesight correction and the necklace  110  may be designed to look like ordinary jewelry. When the AR system  100  is displaying images, reacting to speech or receiving messages, it may provide functions like those of a smartphone. 
     In  FIG. 1 , necklace  110  includes hardware elements  130  distributed about a band of the necklace  110  which allows for a broader range of necklace designs suitable to a variety of aesthetic tastes. In other configurations, the necklace includes hardware elements  130  localized to a pendant of the necklace. Generally, the pendant may be an ornamental object hanging from the necklace  110  that is configured to enclose and conceal the hardware elements  130  of the AR system  100 . 
     While the AR system  100  is illustrated with a necklace  110 , in other embodiments the functions of the necklace  110  described herein can be integrated into another type of wearable device. As an example, the functionality of the necklace  110  can be embedded in a necktie, a scarf, a belt, the brim of a hat, the collar of a shirt, the hood of a jacket, the sleeve of a sweater, the front of a t-shirt, etc. Alternatively, the necklace  110  can be coupled to an external electronic device (not pictured) such as a smart phone and the coupled electronic device may facilitate functionality of the AR system  100 . 
       FIG. 2  shows a necklace  110  of an unobtrusive augmented reality system  100 . In the illustrated embodiment, the necklace  110  includes an electronic module  220  and a coil  210  of conductive material. The coil can be connected or disconnected with a plug  240  and a socket  250 , or may be a single band and not include the plug and socket. Module  220  contains the hardware elements  130  for controlling the augmented reality system  100 . 
     The necklace  110  includes various hardware elements  130  that enable functionality of the AR system  100 . The hardware elements  130  can include one or more of: a power source such as a battery; a modulator to drive a radio-frequency current in the necklace coil that can be used to inductively couple to and communicate with the contact lens  120 ; a data modem to encode data on the radio-frequency signal; sensors such as microphones, cameras, inertial sensors, and GPS receivers; a cellular radio; a Wi-Fi radio; a Bluetooth radio; a graphics processing unit; and a microprocessor and memory. In various configurations, the hardware elements  130  can be included in module  220  and/or may be distributed about the necklace. Alternatively, when the AR system  100  is connected to an external electronic device, any of the sensors, processors and other components mentioned above may be located in the electronic device. The necklace  110  can implement a calibration procedure to calibrate the contact lens  120 , for instance by instructing the contact lens to display a calibration image, to receive data indicative of eye movement, and to identify an alignment of a contact lens projector relative to the user&#39;s gaze or visual axis. Gaze is the direction in which one&#39;s visual axis is pointing. More robustly, a gaze is centered on the user&#39;s visual axis, which refers to the direction defined by the vector originating at an approximate center of the user&#39;s fovea and passing through an approximate center of the user&#39;s pupil. Note that due to the physical shape of an eye, a gaze may not be exactly centered on a user&#39;s visual axis. Calibration of the contact lens is described below in greater detail. 
       FIG. 3A  shows a contact lens  120  mounted on a person&#39;s eye  300 . The contact lens  120  includes a femtoprojector  310  mounted in a contact lens  320 , and a conductive coil  330  near the edge of the contact lens  320  for receiving power and/or data wirelessly from the necklace  110 . The femtoprojector  310  may include electronics for harvesting power from the conductive coil  330  in the contact lens  320 , processing data transmitted to the contact lens  120 , and driving an image source inside the femtoprojector  310 . In one example, the image source is an array of light emitters (e.g., an LED) that projects images onto a wearer&#39;s retina. The femtoprojector  310  can also include optical elements to modify the light that is projected. 
       FIG. 3B  shows the contact lens  120  of  FIG. 3A  in more detail.  FIG. 3B  shows a plan view of a contact lens  120  with multiple femtoprojectors  310 A- 310 C in a contact lens  320 . The conductive coil  330  in the contact lens  320  may have between 1 and about 200 or more turns arranged in a spiral and which extend along a peripheral region of the contact lens  120 . In other configurations, the conductive coil  330  can be arranged in a cylindrical coil or any other looped shape. The conductive coil  330  is connected to the femtoprojectors  310  and other electronics via embedded connectors  322 . The femtoprojectors  310  are located in a central region of the contact lens  120  which is surrounded by the conductive coil  330 . The contact lens  320  is between about 6 mm and about 25 mm in diameter and preferably between about 8 mm and about 16 mm in diameter. 
       FIG. 3B  shows three femtoprojectors  310 A-C in the contact lens  320 , but many femtoprojectors (e.g., up to 50), or only one, may be mounted in such a contact lens  320 . If there is only one femtoprojector  310  in a contact lens  320 , it need not be in the center of the lens. 
     The entire display, made up of all the femtoprojectors  310 , may be a variable resolution display that generates the resolution that each region of the eye can actually see, vastly reducing the total number of individual display pixels required compared to displays of equal resolution and field of view that are not eye-mounted. For example, a 400,000 pixel eye-mounted display using variable resolution can provide the same visual experience as a fixed external display containing tens of millions of discrete pixels. 
     In  FIG. 3B , the contact lens  320  is roughly divided by the dashed circle into an optical zone  322  and a non-optical zone  324 . Components in the optical zone  322  may be in the optical path of the eye  362 , depending on how far open the iris is. Components in the non-optical zone  324  fall outside the aperture of the eye  340 . Accordingly, active optical elements are generally positioned in the optical zone  322  and other elements are positioned in the non-optical zone  324 . For example, femtoprojectors  310  are within the optical zone  322  while the conductive coil  330  is in the non-optical zone  324 . Additionally, the contact lens  320  may also contain other components positioned in the non-optical zone  322  for data processing, data transmission, and power recovery and/or positioning. As an example, an integrated circuit (IC)  318  is positioned in the non-optical zone  324  and is connected to the femtoprojectors  310 . 
     The contact lens  120  can also include other components such as antennae or optical/infrared photodetectors, data storage and buffering, controls, and a calibration system (including, for instance, a motion detection system and a display shift system). The contact lens  120  can include positioning components such as accelerometers used for motion detection, eye tracking, and head tracking. The contact lens  120  can also include data processing components, such as microprocessors, microcontrollers, and other data processing elements. 
       FIG. 3C  shows a cross-sectional view of a contact lens  120  containing a femtoprojector  310  in a contact lens  320 .  FIG. 3C  shows an embodiment using a scleral contact lens that makes contact with the sclera  350  of the eye  300 , though other embodiments can include other suitable contact lens types. The contact lens  320  is separated from the cornea  352  of the user&#39;s eye  340  by a tear layer  354 . The lens  356  of the eye focuses light entering the eye onto the retina  358 . The iris  360  limits the aperture of the eye, that is, the amount of light that enters the lens  356 . 
     The femtoprojector of  FIG. 3C  includes an image source  312  and optical elements  314 . The image source  312  projects an image onto the retina  358  through the optical elements  314 . The optical path  362  from the femtoprojector  310  to the image on the retina  358  is indicated by two arrows. The image  364  is projected onto the retina by the femtoprojector. The lens  356  of the eye  300  also focuses an image of the environment (i.e., an environment image, not illustrated in  FIG. 3C ) onto the retina  358 . Therefore, the image the user sees when using the AR system  100  is a combination of the projected image  364  and the environment image. 
     Electronic Contact Lens Calibration 
       FIG. 4  is a functional diagram of a calibration system  400  for an electronic contact lens. The calibration system  400  includes a femtoprojector  310  (which includes an image source  312  and optical elements  314 ), a motion detection system  410 , and a display shift system  420 . The display shift system  420  receives image data  422  for display by the contact lens  120  and motion data  424  from the motion detection system  420  representative of motion of a user&#39;s eye  300 . The display shift system  420  then selects a location from within the image source  312  (such as a location of pixels  426  in an image source  312 ) from which to project the image data towards the user&#39;s retina. Although the calibration system  400  only includes one femtoprojector, in practice, the calibration system  400  can calibrate any number of femtoprojectors, for instance three monochrome femtoprojectors each corresponding to a different color, such as red, green, and blue. 
     The motion detection system  410  determines the position, velocity, and/or acceleration of an eyeball on which a contact lens is worn. Motion detection may be based on inertial sensors, optical sensors, electrical sensors, magnetic sensors and/or other sensors located in the contact lens and/or in peripheral equipment such as glasses, goggles, headwear, neckwear, clothing and/or external devices. The motion detection component can include an accelerometer, a gyroscope, an inertial measurement unit, a camera, or a magnetic measurement unit. Motion detection may involve forming a best estimate of eyeball position and velocity with the aid of a recursive Bayesian filter such as a Kalman filter. In some embodiments, the motion detection system  410  classifies detected eye motion as either saccade motion or smooth pursuit motion, for instance based on an acceleration or other characteristics of the eye motion. 
     The display shift system  420  selects a location within a pixel array of the image source  312  (“projection location”) to display the image data. For instance, the display shift system  420  may select an initial location to display the image data. The display shift system  420 , in response to detecting motion of the eye, may shift the selected projection location from the initial location in a direction opposite the detected motion. By shifting the projection location, the projected image data is kept stationary relative to the external environment. Likewise, the display shift system  420  may select a projection location to display the image data based on calibration information for the contact lens  120 . For example, during calibration of the contact lens  120 , it may be determined that a center of the user&#39;s gaze corresponds to a particular pixel, or “aligned pixel.” In other words, during calibration, when the projection location is the aligned pixel, the projected image data is aligned with the user&#39;s gaze. Accordingly, the display shift system  420  may select a projection location to display image data relative to the aligned pixel. 
     The retina  358  also includes a fovea  370  with a foveal center  372 . Generally, a person&#39;s visual acuity is highest at the fovea. Therefore, a projected image  364  on the foveal  370  region of the eye is clearer than when a projected image  364  is not on the fovea  370 . Accordingly, it can be beneficial to calibrate the AR system  100  relative to the foveal center  372  (from which the visual axis extends) such that when a virtual object is intended to be displayed at a center of the user&#39;s gaze, the virtual object is projected onto the foveal center  372 . 
       FIG. 5A  shows one way to define a frame of reference for a user&#39;s perception of the external environment, referred to as a “visual field”. A user&#39;s visual field includes everything they see in augmented reality, including both projected images and environment images. As a user&#39;s eye moves to look from object to object, the user&#39;s visual field correspondingly shifts. In other words, the user&#39;s visual field is a perceived image of their augmented reality. 
     In this example, the user&#39;s visual field is defined by an x-y-z coordinate system. The z-axis, or visual axis, projects outwards from the foveal center  372  towards the center of the user&#39;s visual field. Generally, when the user is looking at a point in the external environment, the visual axis  502  (or center of the user&#39;s gaze) is what the user perceives as the center of his visual field. The visual axis intersects an x-y plane at the center of the fovea. 
     The x-axis  504  and y-axis  506  of  FIG. 5A  are orthogonal axes for the user&#39;s visual field. The x-axis  504  corresponds to a “horizontal” orientation and the y-axis  506  corresponds to a “vertical” orientation. That is, when a user&#39;s head is not tilted, horizontal lines in the external environment appear in the users visual field as parallel to the x-axis and vertical lines in the external environment appear in the user&#39;s visual field as parallel to the y-axis. These axes are drawn as straight dashed lines in  FIG. 5A . In the remaining figures, the x- and y-axes are shown as dashed lines, and the origin of the x-y plane (i.e., where the visual axis  502  intersects the x-y plane) is marked by a solid dot. 
       FIG. 5B  shows an example image projected onto a retina. The projected image  364  of  FIG. 5B  is shown as a large bounded square including many smaller squares. The projected image is overlaid on the x-y axes of the user&#39;s visual field, and the dot at the center of the dashed crosshairs represents the visual axis  502 . Although not illustrated, the retina  358  can also receive an image of the environment and, accordingly, the user&#39;s visual field includes the projected image  364  overlaid onto the image of the environment. 
     The projected image  364  can be from a single femtoprojector or from multiple femtoprojectors, each of which includes an image source  312  with multiple pixels  426 . The individual pixels  426  are illustrated as small squares within the projected image  364 . The pixel at the center of the projected image  364 , or centermost pixel  428 , is highlighted. Because the visual axis  502  is aligned with the centermost pixel  428  in  FIG. 5B , the vertical and horizontal cross-hairs intersect the center pixel. Note that the centermost pixel  428  is the pixel at the center of the projected image  364 , and is not necessarily the pixel at the center of the pixel array of the contact lens. 
     In the example of  FIG. 5B , the contact lens  120  is aligned relative to the user&#39;s visual axis  502 . The contact lens  120  is aligned when the centermost pixel  428  of the projected image  364  is projected to a point on the retina  358  corresponding to the visual axis  502  (i.e., the foveal center  372 ). 
       FIG. 5C  illustrates a misaligned contact lens  120 . The contact lens  120  is misaligned when the centermost pixel  428  of the projected image  364  is projected to a point on the retina  358  that does not correspond to the visual axis  502 . Here, the visual axis  502  is offset from the centermost pixel by several pixels relative to the aligned pixel  430 . More explicitly, the aligned pixel  430  is two pixels  426  below and three pixels  426  to the left of the centermost pixel. 
     A calibration process can be implemented to align the image source of a contact lens  120  relative to a user&#39;s visual axis  502 . The calibration process can determine an alignment for the contact lens  120  and can compensate for misalignment when subsequently projecting image data. For example, if a contact lens  120  is physically misaligned relative to a user&#39;s visual axis  502 , image data  422  may not be correctly projected relative to the user&#39;s visual axis  502 . However, the contact lens  120  can be calibrated based on the determined alignment. For instance, the display shift system  420  can shift the location from which the image data  422  is projected based on the determined alignment such that the image data  422  is correctly projected relative to the user&#39;s visual axis  502 . 
       FIG. 6  is a flow diagram of an example method for calibrating the alignment of a contact lens  120  image source relative to a visual axis  502 . The method can include additional or fewer steps, and the steps may be executed in any order. 
     To begin, the calibration process  600  is initiated  610 . There can be any number of methods to initiate  610  the calibration process. For example, the calibration system  400  may determine that the contact lens  120  is misaligned based on detected motion of the eye and may initiate  610  the calibration process  600  in response. Likewise, a user of the AR system may initiate  610  the calibration process  600  with an input (such as with a gesture, voice command, or interaction with an electronic device), the calibration process  600  may automatically initialize  610  when the user powers on the AR system  100 , or the calibration process  600  may initialize  610  periodically or after a defined period of time (e.g., after 4 hours of use). 
     After the calibration process  600  is initialized  610 , the AR system projects  620  a calibration image onto the retina  358 . The calibration image can include a centermost pixel  428 , and the user can be prompted to look at the centermost pixel. In one example, the calibration image is a cross-hairs and the center of the cross-hairs is the centermost pixel  428 . In some embodiments, the calibration image is a single pixel. In other examples, the calibration image can be any other image having an easily determinable image center. In a situation where the contact lens  120  is aligned, the center of the calibration image (i.e., the centermost pixel) is aligned to the user&#39;s visual axis  502 , but when the contact lens  120  is misaligned, the center of the calibration image is not aligned with the user&#39;s visual axis  502 . 
     Next, the user moves their eye to turn their visual field towards the calibration image. The motion detection system  410  detects  630  the motion of the eye as it moves. In various circumstances, the user may adjust their visual field consciously or subconsciously. For example, the AR system  100  may prompt the user to direct his visual axis  502  towards a projected  620  calibration image and in response, the user consciously looks towards the calibration image. In another example, the AR system  100  projects  620  the calibration image and the user directs his visual axis towards the calibration image because the calibration image subconsciously draws the user&#39;s attention. Whatever the case, the motion detection system  410  detects the eye&#39;s motion as the user changes the direction of his visual axis towards the calibration image. 
     Next, the calibration system  400  determines a motion characteristic based on the detected eye motion. The motion characteristic can be any scalar quantity or vector quantity quantifying the detected motion. For example, the motion characteristic may be an acceleration magnitude detected by an accelerometer in the contact lens  120  or may be a velocity and direction detected by a magnetic positioning system in the contact lens  120 . Additionally, the motion characteristic can be a differential motion. That is, the motion characteristic can be a change between two subsequently detected motions. For example, the motion characteristic can be the change in acceleration from an acceleration sensed at a first time and an acceleration sensed at a second time. 
     The motion characteristic may also be a type of motion based on the detected motion. For example, the calibration system  400  can determine that the motion characteristic is smooth motion or saccade motion based on the sensed motion. Smooth motion, such as floater chase motion, is a slow motion of the eye as a user tracks an object through the visual field. Saccade motions are small jittery motions of the eye that subconsciously occur when the eye focuses on an object in the visual field. Generally, smooth motion can indicate that the user is still aligning the user&#39;s visual axis  502  with the calibration image while saccade motion indicates that the calibration image is already aligned with the user&#39;s visual axis. 
     Next, the display shift system  420  adjusts  640  the position of the calibration image by shifting which pixel(s)  426  project the calibration image onto the retina  358  based on the detected motion. That is, the display shift system  420  changes the projection location of the centermost pixel  428  of the calibration image. Thus, the calibration image is projected onto a different portion of the user&#39;s retina and, as a result, the location of the calibration image shifts within the user&#39;s visual field. For example, if the detected motion of the user&#39;s eye is in a first direction, the calibration image can be shifted in a second direction opposite the first direction. In another example, if the user&#39;s gaze shifts three pixels to the right, the calibration image can be projected three pixels to the left of the pixel from which the calibration image was initially projected. 
     In general, adjusting  640  the position of the calibration image based on a detected eye motion reduces the misalignment of the contact lens  120  image source. That is, after adjusting  640  the position of the calibration image, the center of the adjusted calibration image is more closely aligned with the visual axis  502 . In some instances, adjusting  640  the position of the calibration image may unintentionally increase the misalignment. However, the calibration system can iteratively detect  630  motion and adjust  640  the position of the calibration image such that, over time, the contact lens  120  image source becomes aligned with the user&#39;s visual axis  502 . For example, in the event that adjusting  640  the position of the calibration image increases misalignment (e.g., the amount of detected eye motion in response to the calibration image increases), the adjusted position of the calibration image can be determined to be further from alignment than an initial position of the calibration image. In such instances, the position of the calibration image can again be adjusted to be closer to the initial position than the adjusted position. 
     At some point during the calibration process  600 , the calibration system  400  may determine  650  that the contact lens  120  image source is aligned. The contact lens  120  image source is aligned when the center of the projected image is aligned with the visual axis. The calibration system determines  650  that the contact lens  120  image source is aligned based on the motion of the eye detected in response to the calibration image. In one example, the calibration system  400  compares a characteristic of the motion (such as a magnitude of the detected motion) to a corresponding motion characteristic threshold representative of an aligned contact lens image source. In this example, if the detected motion characteristic is within the motion characteristic threshold, the contact lens  120  is determined to be aligned and the location of subsequently displayed images is not adjusted further. The calibration system  400  can determine that the contact lens  120  image source is aligned in response to receiving a corresponding verbal or non-verbal indication from the user, detecting a hand or eye gesture of the user, or receiving an input on a device coupled to the contact lens (such as a necklace or a mobile phone). 
     In another example, the calibration system  400  determines that the contact lens  120  image source is aligned if the motion characteristic is indicative of eye motion corresponding to an aligned system (e.g., saccade motion). In yet another example, the calibration system  400  determines that the contact lens  120  image source is misaligned if the motion characteristic is indicative of eye motion corresponding to a misaligned system (e.g., smooth pursuit motion). In such an example, the calibration system  400  determines that in the absence of smooth pursuit motion, the contact lens  120  image source is aligned. 
     It should be noted that the calibration system  400  can determine that the contact lens  120  image source is misaligned in response to eye motion being detected within a threshold amount of time (e.g., ˜10 ms) of displaying a calibration image. Eye motion detected after the threshold amount of time has passed may be disregarded, as it may be in response to stimulus other than the calibration image. In some embodiments, the calibration system  400  can determine that the contact lens  120  image source is misaligned in response to detecting net eye motion over an interval of time (such as 200 ms to 500 ms), and can determine that the contact lens  120  image source is aligned in response to detecting no net eye motion over the interval of time. In some embodiments, the calibration system  400  aligns the contact lens  120  image source in response to motion of the user&#39;s eye that falls within a frequency band of 0 Hz to 50 Hz. 
     Once the contact lens  120  is aligned, the calibration process  600  determines  660  a calibration parameter associated with the alignment. In one example, the calibration parameter is the location of an aligned pixel (e.g., the pixel that corresponds to the user&#39;s visual axis). In another example, the calibration parameter is an amount of pixel offset between the projection location of the aligned pixel and the projection location of an initial centermost pixel (e.g., the pixel thought to correspond to the user&#39;s visual axis prior to calibrating the contact lens). 
     After calibration, the calibration parameter is used by the display shift system  420  to project subsequent images onto the retina  358  such that the contact lens  120  is aligned. For example, for an image to be projected in alignment with the user&#39;s visual axis, the centermost pixel of the image can be projected at a projector location determined during calibration to correspond to the user&#39;s visual axis (e.g., the aligned pixel). In this example, the calibration parameter is the pixel location of the aligned pixel. Similarly, if the calibration parameter is a pixel offset from an initial projection location determined to correspond to the user&#39;s visual axis, the display shift system  420  can shift a subsequent image by the pixel offset. In either case, the projected image is aligned with the visual axis  502  despite the contact lens being initially misaligned. 
     Once calibration is completed, the calibration parameter can be stored as part of a user&#39;s profile for the eye-mounted display. In addition, calibration results over time can be recorded and analyzed to identify calibration patterns for a user, for a population of users, or for particular models of contact lens. 
       FIGS. 7A-7D  show an example of a calibration process  600  using an AR system  100  that does not hold a virtual object stationary in the user&#39;s visual field. In other words, the virtual object is consistently projected onto a stationary location of the user&#39;s retina. 
     In the example of  FIGS. 7A-7D , a user of the AR system is in Washington, D.C., looking at the White House. In  FIG. 7A , the user notices that the contact lens  120  is misaligned. Before initiating  610  the calibration process  600 , the user focuses the user&#39;s visual axis  502  (indicated by a dot) on the center peak of the White House roof such that the White House is in the user&#39;s visual field  720 . 
     When initiating  710  the calibration process  700 , the AR system  100  instructs a contact lens projector to display a calibration image  710  onto the user&#39;s retina  358 . The AR system then instructs the user to focus on the calibration image  710  (for instance, via a speaker included within the necklace  110 ). Because the contact lens  120  is misaligned, the calibration image  710  is not aligned with the visual axis  502  of the user. In this example, the calibration image  710  is a small cross-hairs whose center is projected from an initial pixel of the image source  312 . For reference, a representative boundary  740  of the image source  312  is also illustrated (as a large box), although the user may not see the boundary. 
     In  FIG. 7B , the user&#39;s eye rotates upwards and to the right to look at the calibration image  710 . The rotation of the user&#39;s eye causes the visual axis of the eye to move (indicated by arrow  730 A). The image source  312  similarly moves (arrow  732 A). Because the AR system  100  does not keep the calibration image  710  stationary in the user&#39;s visual field  720 , the calibration image  710  also moves upwards and to the right. 
     The motion detection system  410  detects the eye&#39;s motion and determines a motion characteristic representative of the detected motion. In this example, the motion characteristic is the magnitude of acceleration of the eye when the user attempts to look at the calibration image  710 . The display shift system  420  can adjust the projection location of the calibration image  710  based on the magnitude of acceleration. Alternatively, the motion characteristic can indicate that the motion of the user&#39;s eye is “smooth motion” in response to the magnitude of the acceleration of the eye corresponding to acceleration representative of smooth motion. 
     In some embodiments, the motion of the user&#39;s eye can be determined to be smooth motion in response to an acceleration of the eye being below an acceleration threshold, in response to a velocity of the eye being below a velocity threshold, in response to the motion of the user&#39;s eye being performed over a time interval greater than a threshold time interval, in response to the motion of the eye being a linear motion, in response to the motion of the eye being relatively constant (e.g., where the ratio of vector standard deviation over a time interval and the mean value of eye velocity over the time interval is less than 0.1), in response to the motion of the eye being floater chase motion, or in response to the motion of the eye including fewer than a threshold number of jump motions. In response to determining that the eye motion is smooth motion, the display shift system  420  can adjust the projection location of the calibration image based on the detected motion. In contrast, in response to determining that the eye motion is not smooth motion, the calibration system  400  can determine that the contact lens is already calibrated, or can ignore the detected motion entirely. 
       FIG. 7C  illustrates the location of the adjusted calibration image  710  in the user&#39;s visual field  720 . The contact lens  120  is still misaligned, albeit to a smaller degree. The calibration image  710  is now located to the left and above the users visual axis  502 . The user&#39;s eye moves upwards and to the left in an attempt to look at the calibration image  710 . The second motion of the visual axis is indicated by arrow  730 B, and the second motion of the image source  312  is indicated by arrow  732 B. 
     The motion detection system  410  detects the second eye motion and determines a second motion characteristic representative of the detected second motion. In this instance, because the magnitude of acceleration of the user&#39;s eye is smaller than the iteration illustrated in  FIGS. 7A and 7B , the display shift system  420  adjusts  640  the projection location of the calibration image  710  to a smaller degree. 
       FIG. 7D  illustrates the location of the second adjusted calibration image  710  in the user&#39;s visual field. In this example, the contact lens  120  is aligned because the user&#39;s visual axis  502  is aligned with the center of the calibration image  710 . Because the calibration image  710  is centered on the user&#39;s visual axis  502 , the user&#39;s eye motion effectively stops. Thus, motion characteristics representative of subsequently detected motions may drop below a threshold corresponding to misalignment, or may indicate saccade motion, and the calibration system  400  determines  650  that the contact lens  120  is aligned. The calibration system  400  can determine  660  a calibration parameter, such as the pixel offset between the projection locations of the center of the calibration image  710  in  FIG. 7A  and  FIG. 7D . 
       FIG. 8A-8C  show an example of a calibration process  600  using an AR system  100  that holds a virtual object stationary relative to an external environment within the user&#39;s visual field. 
     As shown in  FIG. 8A , the user&#39;s visual field  820  again includes the White House, and the user&#39;s visual axis  502  is centered on the peak at the center of the White House roof. A representative boundary  840  of the image source  312  is illustrated. The calibration system  400  determines that the contact lens  120  is misaligned based on eye motions sensed by the motion detection system  410 . The contact lens  120  displays the message “Calibration necessary, initiating calibration” in the user&#39;s visual field  820 . After displaying the message the calibration system initiates  610  the calibration process  600  by projecting a calibration image onto the user retina. Because the contact lens  120  is misaligned, the calibration image  810  is displayed on a stone outcrop rather than the visual axis  502 . 
     In  FIG. 8B , the user&#39;s eye rotates downwards and to the right to look at the calibration image  810 . The rotation of the user&#39;s eye causes the visual axis of the eye to move (indicated by arrow  830 A). The image source  312  moves in a corresponding manner (indicated by arrow  832 A). Because the AR system  100  keeps the calibration image  810  stationary relative to the external environment within the user&#39;s visual field  820 , shifting the user&#39;s gaze does not affect the location of the calibration image  810 . In some cases, this is because the display shift system  420  adjusts the projection location of the calibration image  810  based on the sensed  630  motion of the eye. Generally, the display shift system  420  will shift the projection location in the opposite direction of the sensed motion. That is, if the eye moves down and to the right, the projection location of the calibration image shifts up and to the left. In  FIG. 8B , the visual axis of the user is still not completely aligned with the calibration image. 
     In  FIG. 8C , the user&#39;s eye rotates further downwards and to the right to look at the calibration image  810 . The calibration image  810  is held stationary relative to the user&#39;s external environment, and thus remains on the stone outcropping of the White House. However, the user&#39;s eye has now fully rotated towards the calibration image  810  such that the visual axis  502  of the user is aligned with the calibration image. The second motion of the visual axis is indicated by arrow  830 B, and the second motion of the image source  312  is indicated by arrow  832 B. 
     After a period of time during which no subsequent eye motion is detected, the calibration system  400  can determine that the user&#39;s visual axis is aligned with the calibration image  810 , and can determine a calibration parameter based on the sum of the eye motions  830 A and  830 B. For instance, the calibration system  400  can determine a pixel offset representative of the sum of the eye motions, and can display subsequent images at the pixel offset relative to the initial image source location from which the calibration image  810  was projected (e.g., the image source location corresponding to the calibration image  810  in  FIG. 8A ). Alternatively, the calibration system  400  can determine that the image source location from which the calibration image  810  is projected (e.g., the image source location corresponding to the calibration image  810  in  FIG. 8C ) corresponds to user&#39;s visual axis, and display subsequent images relative to the determined image source location. 
     Although the detailed description contains many specifics, these should not be construed as limiting the scope of the invention but merely as illustrating different examples. It should be appreciated that the scope of the disclosure includes other embodiments not discussed in detail above. For example, the technologies described are applicable to eye-mounted displays other than the specific types described above. Examples include displays in contact lenses other than scleral contact lenses, and also intraocular displays. Various other modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus disclosed herein without departing from the spirit and scope as defined in the appended claims. Therefore, the scope of the invention should be determined by the appended claims and their legal equivalents.