Patent Publication Number: US-2021169322-A1

Title: System for measuring binocular alignment with adjustable displays and eye trackers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present Application is a continuation-in-part of, and therefore claims benefit from U.S. patent application Ser. No. 16/579,826, entitled: “Method and System for Measuring Binocular Alignment”, by Jeffrey P. Krall, and Aric Plumley, filed on Sep. 23, 2019; which is a continuation of U.S. patent application Ser. No. 15/696,161, entitled: “Method and System for Measuring Binocular Alignment”, by Jeffrey P. Krall, and Aric Plumley, filed on Sep. 5, 2017, both Applications are hereby incorporated in their entirety by reference. 
    
    
     FIELD OF INVENTION 
     This invention relates generally to methods and systems for measuring vision acuity, and more particularly, to measuring binocular alignment. 
     BACKGROUND 
     With normal vision, an individual is able to focus at objects located at different distances. Ideally, an individual is able to focus on distant objects, referred to as distance-vision, and on near objects, referred to as near-vision. The optical system of the eye uses numerous muscles to change the focus between these distances. These muscles adjust various aspects of the eye when transitioning between distance-vision and near-vision. The muscle adjustments include making subtle changes to the shape of the crystalline lens to adjust the focus of the lens, rotating the eyeballs to rotate their optical axes, and changing the size of the pupils. 
     Presbyopia is a natural deterioration of near vision, caused by loss of flexibility in the eye&#39;s crystalline lenses as one ages. Presbyopia can be partially compensated by wearing “reading” glasses that correct near-vision refraction errors, so that the eye does not have to focus as strongly when gazing at near objects. Presbyopic persons need different optical corrections for near-vision and for distance-vision. However, using two eyeglasses and changing them frequently is distracting. To avoid continually exchanging eyeglasses, bifocals may be used that offer different optical corrections for near-vision and for distance-vision. The transition between these two vision regions can be abrupt or gradual. The latter eyeglasses are called Progressive Addition Lenses (PALs). Abrupt change bifocals have a visible line separating the two vision regions, while PALs have no lines or edges visible between the regions with different dioptric powers. 
     In spite of all this progress, some types of vision-related discomforts still persist. One of these discomforts is related to a shift of habits in the modern, digital lifestyle. A large and increasing fraction of professions require workers to spend a large and increasing fraction of their working time focusing at close-distance digital interfaces, including computer screens and mobile devices. The same is true for the private lives of many, spending hours playing video games, texting and checking updates on cell phones, among others. All these professional and behavioral shifts rapidly increased the time people spend looking at digital screens, devices, displays, and monitors at a much closer distance than before. The increased time of the eye being trained at near-vision images places excessive demands on the muscles involved in near-vision, often straining them beyond the comfort zone. This can lead to fatigue, discomfort, pain, or even digitally induced migraines. Up to now, there is no widely accepted consensus on the precise causation mechanism of these digital-device related visual discomforts, pains and migraines, even though millions of patients experience these pains every day. Therefore, there is a need for glasses, or other optometric solutions than can provide relief for digital eye discomforts. 
       FIGS. 1-4  illustrate the basic problem of binocular misalignment.  FIG. 1A  illustrates that when we look at a near object, like the shown cross, our vision accommodates in two ways. First, we accommodate the optical power of our eyes  1 - 1  and  1 - 2  to image the near object at a distance L onto the retina of each eyes. This is often called the accommodative response A. Second, we rotate our eyes  1 - 1  and  1 - 2  inward by an angle α, so that the visual axes  2 - 1  and  2 - 2  of the eyes are pointing at the same near object. This response is often called the accommodative convergence AC. For obvious geometric reasons, the angle α of the accommodative convergence AC, relative to the straight forward reference axis, is directly related to the distance L of the accommodative response A: α=α(L). For healthy, well-aligned eyes the ratio of the accommodative convergence AC to the accommodative response A, AC/A, is a geometrically well-defined function, depending on the object distance L and the pupil distance PD of the two eyes. 
       FIGS. 1B-C  illustrate that eyes often display various forms of accommodative misalignments. In  FIG. 1B , the two eyes each turn inward, but to a lesser degree that geometry would require. This leads to the accommodative convergence angle α being less than geometrically necessary by a misalignment angle β. In some detail, the visual axes of the eyes  2 - 1  and  2 - 2  should point into the direction denoted as the necessary accommodative alignment to properly see the near object, but, instead, they turn inward to a lesser degree and instead point to the direction denoted as relaxed or natural accommodative alignment. 
       FIG. 1C  illustrates a case, when this lesser turn is asymmetrical. In the shown case, the visual axis  2 - 1  of the first eye  1 - 1  properly points to the direction of the necessary accommodative alignment, while the visual axis  2 - 2  of the second eye  1 - 2  is turned inward only to the direction of the relaxed or natural accommodative alignment, that is misaligned by the accommodative misalignment angle β. 
       FIGS. 2A-D  illustrate some types of accommodative misalignments. The definitions of misalignments used by different schools of optometry and by monographies show some discrepancies, and the techniques to characterize these misalignments are also varied. Therefore, the here-shown definitions are meant to be illustrative only, and analogues and equivalents are also within the scope of the illustrated terms. 
     To place the discussed misalignments into proper context, first the concept of fusing images is introduced. When our two eyes look at the same object, each eye creates its own visual perception. These perceptions are relayed from the eyes to the visual cortex, where the brain fuses the two images and creates a three dimensional (3D) perception of the viewed object. With optometric diagnostic systems, it is possible to test this image fusing. For example, two separate objects of the same shape can be separately projected into the two eyes with deflections, prisms, and mirrors that make the two projections appear to come from a single object. These visual perceptions will be fused by the brain into a perceived single image. Objects projected in this manner are called fusible objects, presenting fusible images. 
     If in an experiment the distance between the two objects is increased, or the deflection angles are increased, or the shapes of the objects are modified, then the projections into the two eyes start to differ. At some distance, or difference, between the objects, the discrepancy between the visual perceptions of the two eyes exceeds a threshold, and the brain stops fusing the two images into a single perception. Objects with such difference in distance, angle, or shape are called non-fusible objects, presenting non-fusible images. 
     With this preparation,  FIGS. 2A-D  illustrate the concept of fixation disparity, as measured by a test device, often called the Mallet box. The Mallet box displays two vertically aligned bars, and an “X O X” horizontal “anchor”. In some implementations, the two bars can be shifted sideways. In others, adjustable mirrors or prisms are placed in front of the patient&#39;s eye to achieve the same horizontal shift. With appropriate selective optics, the anchor and only one of the bars is shown for the first eye  1 - 1  as a centered bar  5 - 1 - c , and the same anchor plus only the other bar is shown for the second eye  1 - 2  as a centered bar  5 - 2 - c . The anchor and the centered bars  5 - 1 - c  and  5 - 2 - c  are clearly fusible. Accordingly, the brains of patients without accommodative misalignment problems will properly fuse these images. 
       FIG. 28  illustrates that patients with accommodative misalignments will not fuse the images properly. What is typically observed is that, while the images of the anchor, seen by both eyes, are properly fused into a single image, the bars are perceived as shifted. The first eye  1 - 1  perceives a shifted bar  5 - 1 - s , while the second eye  1 - 2  perceives a shifted bar  5 - 2 - s . The angle γ between the line to the image center and one of the visual axes  2 - 1  and  2 - 2  is called fixation disparity. 
       FIGS. 2C-D  illustrate ways to measure the angle needed to counteract, or compensate the fixation disparity. In the system of  FIG. 2C , the two bars are counter-shifted. A counter-shifted bar  5 - 1 - x  is shown for the first eye  1 - 1 , and a counter-shifted bar  5 - 2 - x  is shown for the second eye  1 - 2 . The bars are counter-shifted until the patient perceives the two bars as aligned. The angle corresponding to these counter-shifts, γ*, between the visual axes and line to the counter-shifted bars is measured and is typically referred to as an associated phoria. In the system of  FIG. 2D , the bars are not counter-shifted. Instead, adjustable, or exchangeable prisms  7  are inserted in front of the patient&#39;s eyes. These prisms are adjusted or exchanged until the two bars are perceived as aligned by the patient. Then the prism angles, or the refraction angles of the refracted visual axes, are reported as the associated phoria γ*. 
       FIG. 3  illustrates how increasing a partial associated phoria partially compensates fixation disparity. Strictly speaking, the (full) associated phoria, that fully compensates fixation disparity, is given by the intersect of this curve with the partial associated phoria axis. If human vision were a purely optical process, the partial associated phoria would be simply equal to the negative of the partially compensated fixation disparity. Accordingly, the curve would be a straight line through the origin, tilted by −45 degrees, pointing from the upper left corner to the lower right corner. However,  FIG. 3  illustrates that human vision is much more complex, and perception and image processing play crucial roles in it.  FIG. 3  shows four types of relations between the partially compensated fixation disparity and the partial associated phoria. Visibly, none of these lines are straight, none of them go through the origin, and two of them don&#39;t even intercept the horizontal axis. These type  11  and II relations mean that no amount of partial associated phoria can compensate the fixation disparity in full. Therefore, it remains a substantial challenge to determine the associated phoria that fully compensates a patient&#39;s fixation disparity. A convention is mentioned in closing: the fixation disparity is referred to as “exo”, if the eyes do not turn inward to the necessary degree, while it is referred to as “eso” in those rare cases, when the eyes turn inward too much. 
       FIGS. 4A-C  illustrate a related visual misalignment called disassociated phoria. To characterize disassociated phoria, an experiment similar to that in  FIGS. 2A-D  can be carried out, with the difference that instead of showing fusible images  5 - 1  and  5 - 2 , the optometrists show non-fusible images  6 - 1 - s  and  6 - 2 - s  for the first eye  1 - 1  and the second eye  1 - 2 . In  FIG. 4A , these non-fusible images are the cross and the bar. As  FIG. 41  illustrates, once the eyes are unable to fuse the images, often one or both of the visual axes rotate outward. In the shown asymmetric case, the visual axis  2 - 2  of the second eye  1 - 2  rotates outward by an accommodative misalignment angle δ. This angle δ of the outward rotation is measured and called disassociated phoria. In various applications, as below, the disassociated phoria is distributed over the two eyes evenly, thus the disassociated phoria per eye equaling δ/2. In some cases, e.g. as illustrated in  FIG. 1C , the disassociated phoria δ may manifest itself unevenly and has to be distributed between the eyes accordingly. 
       FIG. 4C  shows a particularly clear case, when simply no image is shown for the second eye  1 - 2 , the view of the second eye  1 - 2  is blocked. This is an extreme case of non-fusible images. As for  FIG. 4B , in response to the block, the visual axis  2 - 2  of the second eye  1 - 2  rotates outward by a measurable disassociated phoria angle δ. 
     As a quantitative characterization of accommodation misalignments, including fixation disparity and disassociated phoria, several practitioners use the misalignment-impacted AC/A ratio. The AC/A is a ratio of the accommodative convergence angle reduced by the fixation disparity, α-δ/2, (expressed with its tangent, in terms of “prism diopters” A), divided by the accommodative distance L, expressed in diopters D. A typical definition of AC is AC=100 tan(α−δ/2), in terms of prism diopters. For an average visual performance, an AC/A ratio of 6-6.5 δ/D is necessary, while, remarkably, in large population segments the average of the misalignment-impacted AC/A ratio was measured to be about 3.5 Δ/D. Clearly, various forms of accommodative misalignment affect a large percentage of the population, and any progress towards relief from this is highly valuable. 
     A startling fact of the corresponding field of optometry is that the associated phoria angles and the disassociated phoria angles, determined by experienced practitioners, show remarkably wide variations. Experiments carried out on the same patient by different optometrists, and sometimes even by the same optometrist at different times, report phoria angles, expressed in prism diopters Δ, with a distribution having a standard deviation as much as 3Δ. (A prism diopter of 1Δ corresponds to a 1 cm prism refraction at 1 meter distance). The large variability of these methods precludes the effective determination and compensation of accommodative misalignments. 
     This exceptionally large standard deviation is probably due to several factors. These include the followings. (1) The methods of determination use the patient&#39;s subjective responses as key inputs. (2) Some methods use central images, while others use peripheral images for determining the associated phoria. The relative accuracy and relevance of these methods was not yet critically evaluated. (3) Most practitioners use a single measurement, or a single method, thus not benefiting from possibly important medical information that can be gleaned from carrying out multiple tests. (4) In a previous exploratory project, Applicants also discovered that the prismatic reaction of the eyes is quite different for moving test images. However, understanding the relation of optimal prismatic corrections based on static and moving test images is in its early stages. (5) While there are several ways to define prismatic misalignments, and they produce different prismatic predictions and diagnoses, eventually a single prism needs to be formed in the spectacles. It is far from obvious how to convert and combine the various diagnostically determined prismatic corrections into a single prism prescription. Applicants are not aware of a critical study that would have evaluated how the efficacy and variability of prism prescriptions depended on the possible combinations of the determined prismatic corrections. 
     For all of the above reasons, determining the prismatic power that optimally compensates accommodative misalignments remains a pressing medical need. 
     SUMMARY 
     To address the above described medical needs, some embodiments include a system to determine a binocular alignment, comprising a first optical unit, including a first display, to display images for a first eye, actuatable along a longitudinal direction according to a simulated distance and an optical power of the first eye, and a first eye tracker assembly, to track a gaze direction of the first eye, adjustable in a horizontal lateral direction to accommodate a pupillary distance of the first eye; and a second optical unit, including a second display, to display images for a second eye, actuatable along the longitudinal direction according to a simulated distance and an optical power of the second eye, and a second eye tracker assembly, to track a gaze direction of the second eye, adjustable in the horizontal lateral direction to accommodate a pupillary distance of the second eye; and a computer, coupled to the first optical unit and the second optical unit, to determine the binocular alignment based on the gaze directions of the first eye and the second eye. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-C  illustrates various accommodative misalignments. 
         FIGS. 2A-D  illustrate method to determine types of accommodative misalignments. 
         FIG. 3  illustrates four types of relationships between fixation disparity and partial associate phoria. 
         FIGS. 4A-C  illustrate methods to determine disassociated phoria. 
         FIG. 5  illustrates a system for determining a binocular misalignment. 
         FIGS. 6A-B  illustrate an embodiment of the system for determining a binocular misalignment. 
         FIG. 7  illustrates an JR image by the eye tracker. 
         FIGS. 8A-B  illustrate an embodiment of the system for determining a binocular misalignment. 
         FIG. 9  illustrates an embodiment of the system for determining a binocular misalignment. 
         FIGS. 10A-B  illustrate embodiments of the accommodation optics. 
         FIG. 11  illustrates a method for determining a binocular misalignment. 
         FIG. 12  illustrates exemplary details of the measuring step. 
         FIGS. 13A-D  illustrate steps of carrying out the measuring step. 
         FIG. 14  illustrates exemplary details of the determining step. 
         FIGS. 15A-C  illustrate steps of carrying out the determining step. 
         FIG. 16  illustrates a below-the-equator embodiment of the method for determining a binocular misalignment. 
         FIG. 17  illustrates a system for determining binocular alignment. 
         FIGS. 18A-B  illustrate embodiments of a first optical unit. 
         FIG. 19  illustrates a view of the system for determining binocular alignment. 
         FIG. 20  illustrates a perspective view of the first optical unit. 
         FIG. 21  illustrates a frontal view of the system for determining binocular alignment. 
         FIG. 22  illustrates an embodiment of the system for determining binocular alignment with a graphical user interface and a patient communication interface. 
         FIG. 23  illustrate an embodiment with an auto-refractor. 
         FIG. 24  illustrate an embodiment of a system for determining binocular alignment. 
         FIG. 25  illustrate an embodiment of a first optical unit. 
     
    
    
     DETAILED DESCRIPTION 
     The systems described in the present patent document address the above articulated medical needs at least in the following aspects. (1) The described system and method determine the prismatic corrections only by objective measurements, without subjective input from the patient. This aspect alone greatly reduces the patient-to-patient and practitioner-to-practitioner variations of the results. In fact, studies on large samples of patients using Applicant&#39;s system and method determined prismatic corrections with a standard deviation reduced from the above mentioned  3 A to well below 1Δ. This significant reduction of the results&#39; standard deviation alone established the here-described method to the status of quantitatively predictive diagnostic methods. (2) The system and method use both central and peripheral test images, because of a newly developed understanding of how the peripheral and the central prismatic corrections are connected. Therefore, the described system and method is a promising platform to determine an optimal compromise prismatic prescription that strikes the best compromise for compensating both central and peripheral accommodative misalignments. (3) The described method has two stages, thus it determines the eventual prismatic correction in a second stage by building on the important misalignment information acquired in the first stage. As such, the method integrates knowledge determined by different methods and benefits from the information determined by all of them. (4) One of the stages of the method involves moving test images. Therefore, the eventually determined prismatic corrections capture and integrate the dynamic prismatic response of the eye as well. (5) The reliable repeatability and small variability of the above mentioned large scale study provided a compelling argument that Applicants&#39; method combined the outputs of different methods in an objective and effective manner to produce a single optimized and objective prismatic correction. The here-described five aspects provide advantages individually and in combinations. 
       FIGS. 5-10  illustrate a system  10  for determining a binocular alignment, and  FIGS. 11-16  illustrate a corresponding method  100  for determining the binocular alignment. 
       FIG. 5  illustrates that in some embodiments, the system  10  for determining a binocular alignment can comprise a stereo display  20 , to project visible images for a first eye  1 - 1  and a second eye  1 - 2 ; an accommodation optics  30 , to modify the projected visible images according to an apparent distance; an eye tracker  40 , to track an orientation of the first eye  1 - 1  and the second eye  1 - 2 ; and a computer  50 , coupled to the stereo display  20 , the accommodation optics  30  and the eye tracker  40 , to manage a determination of the binocular alignment. In what follows, the eyes will be labeled as first eye  1 - 1  and second eye  1 - 2 . This labeling can correspond to a left eye and a right eye, or vice versa. 
       FIG. 6A  shows a detailed illustration of some embodiments of the system  10 . In some embodiments, the eye tracker  40  can include infrared light emitting diodes, or IR LEDs,  42 - 1  and  42 - 2 , positioned close to a front of the system  10 , to project infrared eye-tracking beams on the first eye  1 - 1  and the second eye  1 - 2 , as well as infrared light sources  44 - 1  and  44 - 2 , to illuminate the first eye  1 - 1  and the second eye  1 - 2  with an infrared imaging light. The infrared eye-tracking beams and the infrared imaging light get both reflected from the eyes  1 - and  1 - 2 . The eye tracker  40  can further include infrared (IR) telescopes  46 - 1  and  46 - 2 , with infrared (IR) cameras  48 - 1  and  48 - 2 , to detect the infrared eye-tracking beams and the infrared imaging light, reflected from the first eye  1 - 1  and the second eye  1 - 2 . 
     Many of the elements of the system  10  are included in pairs, e.g., the infrared telescopes  46 - 1  and  46 - 2 . For simplicity of presentation, such pair of elements will be referred to only by their lead identifiers where doing so does not lead to misunderstanding, such as “the infrared telescope  46 ”, abbreviating “the infrared telescopes  46 - 1  and  46 - 2 .” 
       FIG. 7  illustrates a resulting IR image  49 , as detected, or sensed, by the IR camera  48 . In this embodiment, there are four IR LEDs  42 - 1 , . . .  42 - 4  for each eye separately. To avoid clutter, the “−1” or “−2” indicating a particular eye, is omitted in the description of  FIG. 7 . The “−1” . . . “−4” notation here refers to the four IR LEDs, all projecting IR eye tracking beams onto the same eye. The four IR LEDs  42 - 1 , . . .  42 - 4  project four IR eye-tracking beams onto the eye, which reflect from the cornea, creating four so called Purkinje spots P 1 - 1 , . . . P 1 - 4  in the IR image  49 . The “P 1 ” notation refers to the reflection from the proximal surface of the cornea. The higher indexed Purkinje spots P 2 , . . . refer to reflections from deeper lying surfaces inside the eye, such as reflections from the proximal and distal surfaces of the capsule. The here-described embodiments utilize the P 1  Purkinje spots, while other embodiments may employ higher indexed Purkinje spots. 
     The reflected IR imaging light of the IR light source  44  is detected by the IR camera  48  as well. The four Purkinje spots P 1 - 1 , . . . P 1 - 4  overlaid on the detected reflected IR imaging light together form the IR image  49 , as shown. 
     In some embodiments, the eye tracker  40  can include an image recognition system  52 , to determine an orientation of the first eye  1 - 1  and the second eye  1 - 2 , using the detected infrared eye tracking beams, forming the Purkinje spots P 1 - 1 , . . . P 1 - 4 , and the detected infrared imaging light, together forming the IR image  49 . The image recognition system  52  can extract, for example, an image of the contour of a pupil  3 , using edge-recognition methods. Then it can determine an orientation of the eye  1  from the center of the pupil  3 . Separately, it can determine the orientation of the eye from the Purkinje spots P 1 - 1 , . . . P 1 - 4 . Finally, it can employ a weighing algorithm to determine a “best result” orientation by combining the two determined orientations, using various well known image recognition and analysis techniques. The image recognition system  52  can be a separate processor, a separate application specific integrated circuit, or it can be implemented as a software deployed in the system-managing computer  50 . 
       FIGS. 6A-B  illustrate that the system  10  can further include infrared-transmissive visible mirrors  24 - 1  and  24 - 2 , one for each eye, to redirect the projected visible images  26 - 1  and  26 - 2 , from the stereo display  20  to the first eye  1 - 1  and the second eye  1 - 2 ; and to transmit the reflected infrared eye tracking beam and the infrared imaging light, together  45 - 1  and  45 - 2 , from the first eye  1 - 1  and the second eye  1 - 2 . In these embodiments, stereo display screens  22 - 1  and  22 - 2  of the stereo display  20  can be positioned peripheral to a main optical pathway of the system  10 , and the infrared telescopes  46 - 1  and  46 - 2  of the eye tracker  40  can be positioned in the main optical pathway of the system  10 . For reference, the accommodation optics lenses  34 —mirror  24 —IUR telescope  46  axis for each eye is typically referred to as the main optical pathway in this embodiment. Also, for clarity&#39;s sake, in figures where the optical paths and beam are shown, some labels have been simplified. 
       FIG. 6B  shows that in this embodiment, the peripheral stereo display screens  22 - 1  and  22 - 2  can project visible images  26 - 1  and  26 - 2  towards the main optical pathway of the system  10 , that are redirected by the infrared-transmissive visible mirrors  24 - 1  and  24 - 2  toward the eyes  1 - and  1 - 2 . At the same time, the reflected IR eye tracking beams and the reflected IR imaging lights, together  45 - 1  and  45 - 2 , reflected from the eyes  1 - 1  and  1 - 2 , are transmitted by the same infrared-transmissive visible mirrors  24 - 1  and  24 - 2  toward the IR telescopes  46 - 1  and  46 - 2  along the main optical pathway of the system  10 . 
       FIG. 8A  illustrates another embodiment, where the location of the stereo display screens  22  and the IR telescopes  46  is exchanged.  FIG. 88  illustrates that this embodiment can include visible-transmissive infrared ( 1 R) mirrors  24 ′- 1  and  24 ′- 2 , to redirect the reflected infrared eye tracking beam and the reflected infrared imaging light, together  45 - 1  and  45 - 2 , reflected from the first eye  1 - 1  and the second eye  1 - 2 , toward the IR telescopes  46 - 1  and  46 - 2 . At the same time, the visible-transmissive infrared mirrors  24 ′- 1  and  24 ′- 2  can transmit the projected visible images  26 - 1  and  26 - 2 , from the stereo display screens  22 - 1  and  22 - 2  of the stereo display  20  to the first eye  1 - 1  and the second eye  1 - 2 . In these embodiments of the system  10 , the stereo display  20  can be positioned in the main optical pathway of the system  10 , and the infrared telescopes  46  of the eye tracker  40  can be positioned peripheral to the main optical pathway of the system  10 . For reference, in this embodiment, the accommodation optics lenses  34 —mirror  24 —stereo display screen  22  axis for each eye is typically referred to as the main optical pathway in this embodiment. 
       FIG. 9  illustrates a variant of the system  10  of  FIGS. 8A-B , in which the stereo display  20  can include a single stereo display screen  22 , and synchronizing glasses  28 . The synchronizing glasses  28  can be shutter glasses or polarized glasses. In this embodiment, the projected visible images  26 - 1  and  26 - 2  of the left and right stereo display screen  22 - 1  and  22 - 2  of  FIGS. 8A-B  are both displayed by the single stereo display screen  22  in a rapidly alternating sequence. The synchronizing glasses  28  can be precisely coordinated with this alternating sequence, allowing the projection of the visible images  26 - 1  and  26 - 2  to the first eye  1 - 1  and the second eye  1 - 2  in a rapidly alternating manner, creating the impression of separate images being projected into these eyes. The synchronizing glasses  28  can be analogous to the 3D glasses used in the projection of 3D movies, and can rely on liquid crystal LCD layers that can rapidly change the circular polarization of the two lenses of the synchronizing glasses  28 . Such systems  10  can achieve smaller footprints for the system  10  that can be advantageous. For optimal operations, a sufficiently wide field of view for the stereo display screen  22  can be helpful. 
     Some embodiments of the system  10  do not need to include the mirrors  24  or  24 ′. In these systems, the eye tracker  40  may include small implementations of the IR cameras  48 , positioned close to the front of the system  10 , slanted at a sufficiently large angle so that the IR cameras  48  do not block the projections by the stereo display screens  22 . The image recognition system  52  of such implementations of the eye tracker  40  can include a geometric transformation unit to determine the direction of the eye visual axes from a substantially slanted IR image  49  and Purkinje spots P 1 , . . . P 4 , possibly some spots even being obscured by the slant. 
     In embodiments of the system  10 , the accommodation optics  30  can include phoropter wheels  32 - 1  and  32 - 2  with a series of accommodation optics lenses  34 - 1  and  34 - 2  of varying optical power. These accommodation optics lenses  34  are useful to simulate the apparent distance for the first eye  1 - 1  and the second eye  1 - 2 . 
     As described below in relation to the method  100 , the system  10  can be employed to project visible images  26  at different apparent distances for a patient. Doing so can involve at least two technical solutions. First, inserting the accommodation optics lenses  34  with their variable optical power into the main optical pathway can create the impression of the projected visible images  26  being farther or closer. Second, projecting the visible images  26 - 1  and  26 - 2  closer or farther from each other can simulate an appropriate vergence of these images, another important factor in making these images appear as being at the apparent distance for the patient. 
     In some embodiments, for the first technical solution, the accommodation optics  30  can include, in place of the phoropter wheel  32 , or in combination with the phoropter wheel  32 , curved mirrors, trial lenses, flip in/flip out lenses, adjustable liquid lenses, deformable mirrors, z-directionally movable mirrors, rotating diffractive optical elements, translating diffractive optical elements, variable focus moire lenses, or focusing lens groups. 
       FIGS. 10A-B  illustrate that for the second technical solution, the accommodation optics  30  can include a pair of rotatable deflectors  36 , rotatable prisms  38 , or adjustable prisms  38  (only one shown), to deflect the projection of the images  26 - 1  and  26 - 2  to the first eye  1 - 1  and the second eye  1 - 2 , to simulate a vergence of the apparent distance for the first eye and the second eye. 
     In some embodiments, the vergence can be simulated not by the above optical elements, but by shifting the projecting of the projected visible images  26 - 1  and  26 - 2  with the stereo display screens  22 - 1  and  22 - 2  towards each other, in other words, projecting them closer to each other. 
     In some systems  10  the accommodation optics  30  and the stereo display  20  can be combined into a single light field display that includes a microlens array, where the projected visible images  26 - 1  and  26 - 2  shown on the stereo display screens  22 - 1  and  22 - 2 , combined with the optical characteristics of the microlens array can be used to vary the apparent distance of the projected visible images  26 - 1  and  26 - 2  as seen by a patient. 
     In some systems  10 , the accommodation optics  30  and the stereo display  20  can be combined into a single light field display that includes a mems scanner, a focus modulator, or a light source. 
     Having described the problem of prismatic or accommodative misalignments and embodiments of the system  10  that were developed to provide progress in the context of the misalignment problems, next, various methods  100  will be described for determining binocular misalignments using embodiments of the system  10 . 
       FIGS. 11-16  illustrate a method  100  of how to use the above described embodiments of the system  10  to determine a binocular alignment of the eyes  1 - 1  and  1 - 2 . 
       FIG. 11  illustrates that some embodiments of the method  100  can include a measuring  120  of a disassociated phoria of the first eye  1 - 1  and the second eye  1 - 2  of a patient at an apparent distance, and a determining  140  of an accommodative convergence of the first eye  1 - 1  and the second eye  1 - 2  at the apparent distance using the measured disassociated phoria. As mentioned earlier, the method  100  is a two-stage method, and thus its results integrate the information and knowledge revealed by the two different stages. 
     As described below in detail, in some embodiments, the measuring  120  can include projecting non-fusible visible images  26 - 1  and  26 - 2  for the first eye  1 - 1  and the second eye  1 - 2  using the stereo display  20  of the system  10 . For the purpose of describing the method  100  more concisely, the visible images  26 - 1  and  26 - 1  of  FIGS. 5-10  will be simply referred to as images  26 - 1  and  26 - 2  in what follows. 
     Examples of projecting non-fusible images in order to determine a disassociated phoria have been described, e.g., in relation to  FIGS. 2C-D . There, the two non-fusible images  6 - 1 - s  and  6 - 2 - s  were of comparable appearance, or dominance. Some embodiments of the method M also involve projecting such non-fusible images of comparable dominance. 
     In other embodiments, the projecting can include projecting a dominant image for the first eye  1 - 1 , and projecting a non-dominant image for the second eye  1 - 2 . As described in relation to  FIGS. 2C-D , the eye  1 - 2  that sees the non-dominant image often starts wandering off after the brain&#39;s efforts to fuse the two non-fusible images fail. In these embodiments, the measuring  120  can include tracking the eyes  1 - 1  and  1 - 2  with the eye tracker  40 , and determining when the wandering eye  1 - 2  eventually achieves a relaxed orientation. Achieving this relaxed state can be inferred, for example, by the eye tracker  40  determining that the movement of the eye  1 - 2  slowed below a threshold, or changed from a directional movement to a random jitter, or came to a halt. Once the eye tracker  40  determined that the eye  1 - 2  reached the relaxed state, the disassociated phoria can be measured by measuring an orientation of at least one of the first eye  1 - 1  and the second eye  1 - 2  by the eye tracker  40 . 
       FIG. 12  describes implementations of these steps in more detail, and  FIGS. 13A-D  illustrate these steps in a particular embodiment. In these embodiments, the measuring  120  can include the followings.
         Projecting  122  a centered image for the first eye with an apparent distance vergence, using a stereo display;   projecting  124  a distributed image for the second eye with an apparent distance vergence, using the stereo display, wherein the centered image and the distributed image are non-fusible;   tracking  126  a rotation of at least one of the first eye and the second eye using an eye tracker;   identifying  128  a relaxed state from a stabilization of the tracked rotation; and   measuring  130  the disassociated phoria by measuring an orientation of at least one of the first   eye and the second eye in the relaxed state using the eye tracker and a computer.       

       FIG. 13A , left panel illustrates that the projecting of a centered image step  122  can include projecting a centered image  201 - 1 , a cross in this case, on the stereo display screen  22 - 1  of the stereo display  20  of the system  10 . The projecting  122  can be done with an apparent distance vergence  206 . A reference axis  202 - 1  is introduced for reference as a central normal that connects a center of the first eye  1 - 1  with a center of the stereo display screen  22 - 1 . With this, the apparent distance vergence  206  can be characterized by an apparent distance vergence angle α=α(L), the angle that a first eye visual axis  204 - 1  makes with the reference axis  202 - 1  when looking at an object that is placed halfway between the two eyes  1 - 1  and  1 - 2  at the apparent distance L. More generally, the apparent distance vergence  206  will be represented by and referred to as the line directed from a center of the first eye  1 - 1  with the angle α(L) relative to the reference axis  202 - 1 , even if the first eye visual axis  204 - 1  is not pointing along this line. 
     The centered image  201 - 1  is centered in the sense that it is moved off the center of the stereo display screen  22 - 1  only by the apparent distance vergence angle α(L) to simulate the apparent distance vergence  206 . For brevity&#39;s sake, sometimes this angle will be only referred to as the vergence angle α. The definition of the first eye visual axis  204 - 1  can incorporate a lens or any other relevant portion of the accommodation optics  30 - 1 , through which the first eye  1 - 1  is observing the centered image  201 - 1 . 
       FIG. 13A , right panel illustrates the projecting of a distributed image step  124  for the second eye  1 - 2 , in this case, a set of irregularly placed balls or spheres of random size and position, without an apparent center. The centered image  201 - 1  is an example of a dominant image, and the distributed image  201 - 2  is an example of a non-dominant image. The centered, dominant image  201 - 1  and the distributed, non-dominant image  201 - 2  are examples of non-fusible images. Alternatively, the stereo display screen  22 - 2  can be simply darkened as another embodiment of the non-fusible distributed image  201 - 2 , instead of the irregularly placed balls, in analogy to the block in  FIG. 4C . 
       FIG. 13B  illustrates that, as described earlier, the second eye  1 - 2  will initially also turn inward by approximately the same apparent distance vergence angle α as the first eye  1 - 1 , but, after the brain fails to fuse the non-fusible central image  201 - 1  and distributed image  201 - 2 , the second eye  1 - 2  wanders away. The eye tracker  40  can execute the tracking step  126  of the second eye  1 - 2  until the optometrist, or an automated program, determines that the wandering second eye  1 - 2  reached a relaxed state from a stabilization of the tracked rotation in the identifying step  128 . This stabilization can be defined in various ways: from the eye coming to a stop, or an amplitude of the eye&#39;s jitter becoming less than a threshold, or a directional rotation of the eye evolving into a directionless wandering. 
     In the measuring step  130 , once the relaxed state has been identified in step  128 , the eye tracker  40  can measure the orientation of the relaxed second eye  1 - 2  by determining the angle S the second eye visual axis  204 - 2  with the apparent vergence  206 . In this measuring step  130 ,  6  the angular deviation of the relaxed second eye  1 - 2  from the apparent distance vergence  206  will be referred to as the disassociated phoria  208 , with its disassociated phoria angle δ. This definition is in close analogy with that of  FIGS. 4B-C . As mentioned before, small differences exist among various practitioner&#39;s definitions of the disassociated phoria. 
     In some related embodiments, the tracking step  126  may involve tracking a rotation of the first eye  1 - 1 , the second eye  1 - 2 , or both. In these embodiments, the disassociated phoria  208  can be defined from measuring  130  a first eye phoria angle δ- 1 , a second eye phoria angle δ- 2 , and determining the disassociated phoria S as some type of a mean of δ- 1  and δ- 2 . 
       FIGS. 13A-B  illustrated that the steps  122 - 130  of the overall measuring step  120  can be performed as a near vision distance, e.g. L being in the range of 40 cm-100 cm. 
       FIGS. 13C-D  illustrate that the same steps  122 - 130  can be also performed as part of a distance vision test, when the apparent distance is L large, and the apparent distance vergence angle is α=0. In related embodiments, L can be in the m-Om range. Expressed in diopters, the method  100  can be performed at near vision distances corresponding to 1-3D, at distance vision distances corresponding to 0-0.5 D. 
     To summarize, the result of the measuring step  120 , the first stage of the method  100 , is the disassociated phoria  208 , with its disassociated phoria angle δ. The second stage of the method  100 , the determining step  140 , carries out additional tests of the prismatic misalignment that build on the just determined disassociated phoria  208 . Therefore, the overall method  100  is a combination of the first and second stages and thus the method  100  integrates two distinct tests of prismatic misalignments, and thus integrates knowledge and data about two different types of the binocular alignment. Doing so promises a qualitatively more complete treatment and a qualitatively better improvement of the visual acuity. 
       FIG. 14  illustrates that the determining step  140  can include a presenting step  142  of a first image for the first eye and a second image for the second eye, with the apparent distance vergence, corrected with the measured disassociated phoria, using the stereo display; wherein the first image and the second image are fusible. 
       FIG. 15A  illustrates that in some implementations of the presenting step  142 , a fusible first image  210 - 1  can be presented on the stereo display screen  22 - 1  for the first eye  1 - 1 , and a fusible second image  210 - 2  can be presented on the stereo display screen  22 - 2  for the second eye  1 - 2 . These fusible images  210 - 1  and  210 - 2  can be peripheral. For example, the peripheral images  210 - 1  and  210 - 2  can be two, essentially identical circular bands, or rings, of balls or planets, as shown. Centers of the fusible images  210 - 1  and  210 - 2  can be shifted towards each other according to the apparent distance vergence  206 , the vergence angle α being corrected by the disassociated phoria δ ( 208 ), as measured in the measuring step  120 . The measured disassociated phoria S can be symmetrically distributed as δ/2-δ/2 between the two eyes, as shown. In these typical cases, the centers of the fusible images  210 - 1  and  210 - 2  can be shifted towards each other according to α-δ/2, the vergence angle α, corrected by the disassociated phoria δ, relative to the reference axes  202 - 1  and  202 - 2 . In response, the first eye visual axis  204 - 1  and the second eye visual axis  204 - 2  typically align with the apparent distance vergence  206 , corrected by the disassociated phoria  208 , as shown by these visual axes  204  pointing towards the centers of the fusible images  210 . 
     In some cases, when the binocular misalignment of the two eyes is asymmetric, the optometrist may have reasons to attribute the measured disassociated phoria δ unevenly between the two eyes. It is also noted that the earlier convention is continued to make the description more comprehensible: the description will refer to a pair of “limitation N−1 and limitation N−2” simply as “limitations N”, where doing so does not lead to confusion. 
     The shift of the fusible images  210  can be impacted by the accommodation optics  30 . The settings of the accommodation optics  30  can depend on L, the accommodative distance, or a spectacle power preferred by the patient, possibly further corrected by a cylinder or aberration. 
     In some embodiments, the fusible first image  210 - 1  and the fusible second image  210 - 2  can be dynamic. In  FIG. 15A , the directed dashed arcs indicate that the rings of planets can be rotating around their center. Experiments have shown that making the peripheral fusible images  210  rotate captures peripheral prismatic effects more reliably and reproducibly. In the presenting step  142  the radius, spatial distribution, coloring, dynamics, and speed of rotation of these fusible images  210  could all be adjusted to provide the alignment information with the optimal weight. 
     In some embodiments, the first image  210 - 1  and the second image  210 - 2  can be static. In some embodiments, the first image  210 - 1  and the second image  210 - 2  can be central. These embodiments may present their own medical advantages. 
       FIG. 14  describes and  FIG. 15B  illustrates that the presenting step  142  can be followed by a projecting step  144 . The projecting step  144  can include a projecting of a first added central image  212 - 1  for the first eye  1 - 1 , and a projecting a second added central image  212 - 2  for the second eye  1 - 2 . These central images  212  can be projected at the center of the fusible images  210 . In the embodiment of fusible images  210  being circulating planets, the added central images  212  can be projected at the center of their circulation, e.g., as a cross, as shown. 
     The projecting  144  of these two added central images  212 - 1  and  212 - 2  can be performed in an alternating manner, using the stereo display  20 . To express the alternating manner of the projecting  144 , only one of the added central images, the cross  212 - 1  is shown with a solid line, and the other added central image,  212 - 2  is shown with a dashed line in FIG.  15 B. The period of the alternating can be selected according to several different criteria, and can be less than 1 second, in a range of 1-100 second, in some cases in a range of 5-10 seconds. 
     Had δ, the angle of the disassociated phoria  208 , measured in step  120 , completely captured the binocular alignments of the eyes  1 , then the eyes  1  would not have needed to adjust to the projecting step  144  of the added central images  212  with the vergence angle α, corrected by the disassociated phoria angle δ/2. This would have manifested itself in that the eye visual axes  204  would have had remained aligned with the vergence angle α, corrected by the disassociated phoria angle δ/2 after the projecting step  144 . 
     However, Applicant&#39;s studies revealed that patients moved and adjusted their eyes  1  in response to the projecting  144  of the added central images  212  with the corrected vergence angle α-δ/2. This led Applicant to the recognition that additional measurements were necessary to determine the remaining, residual prismatic misalignment of the eyes. These additional measurements are described in steps  146 - 154 , as follows.
         Tracking  146  an adjustment of the first eye in response to the projecting of the first added central image, and tracking an adjustment of the second eye in response to the projecting of the second added central image, using an eye tracker.   projecting  148  a shifted first added central image with a first iterative associated phoria, to reduce the adjustment of the first eye, and projecting a shifted second added central image with a second iterative associated phoria, to reduce the adjustment of the second eye, in an alternating manner, using the stereo display and a computer;   tracking  150  an adjustment of the first eye in response to the projecting of the shifted first added central image, and tracking an adjustment of the second eye in response to the projecting of the shifted second added central image using the eye tracker;   determining  152  whether an effective adjustment of the first and second eye is less than an adjustment threshold, and returning to the projecting the shifted first added central image step if the effective adjustment of the first and second eye is greater than the adjustment threshold;   identifying  154  a stabilized associated phoria from the last first iterative associated phoria and the last second iterative associated phoria, if the effective adjustment of the first and second eye is less than the adjustment threshold; and   identifying  156  a sum of the disassociated phoria and the stabilized associated phoria as a correction to the accommodative convergence, corresponding to the apparent distance. These steps are described in some detail next.       

       FIG. 14  describes and  FIG. 15B  illustrates that in order to determine residual prismatic misalignments, the projecting step  144  can be followed by the tracking  146  of an adjustment of the first eye  1 - 1  in response to the projecting of the first added central image  212 - 1 , and tracking an adjustment of the second eye  1 - 2  in response to the projecting of the second added central image  212 - 2 , using an eye tracker  40 .  FIG. 15B  illustrates that the first eye  1 - 1  adjusts to the projecting  144  by rotating the first eye visual axis  204 - 1  with an adjustment angle of the first eye  214 - 1 , denoted by ε- 1 , and the second eye  1 - 2  adjusts by rotating the second eye visual axis  204 - 2  with an adjustment angle of the second eye  214 - 2 , denoted by ε- 2 . From now on, for brevity, the angles will be referenced to the apparent distance vergence corrected by the disassociated phoria, having the angle α-δ/2, instead of the reference axis  202 . The fact that the adjustment angles ε- 1  and ε- 2  were found non-zero, necessitated the subsequent steps of the determining step  140 . 
       FIG. 15C  shows that the determining the accommodative convergence step  140  next includes projecting  148  a shifted first added central image  212 - 1  with a first iterative associated phoria φ(n)−1, to reduce the adjustment of the first eye  1 - 1 , and projecting a shifted second added central image  212 - 2  with a second iterative associated phoria φ(n)−2, to reduce the adjustment of the second eye  1 - 2 . Here the adjustment of the eye can be measured by a change of the adjustment angle ε(n)−1, as elaborated below. 
     For clarity and brevity, in this  FIG. 15C  only the first eye  1 - 1  is illustrated explicitly. The shifted added central images  212  are connected to the eyes  1  and characterized by shifted image axes  216 .  FIG. 15C  shows the first shifted image axis  216 - 1 , connecting the shifted first added central image  212 - 1  to the first eye  1 - 1 . 
     It was described in relation to  FIGS. 2-3  that the fixation disparity γ and the associated phoria γ*, necessary to compensate it, are not simply equal and opposite to each other. In analogy to this recognition, the associated phoria φ(n)−1 is not simply equal and opposite to the adjustment angle of the first eye, ε(n)−1. Therefore, embodiments of the method  100  determine these quantities iteratively, in steps 1, 2, . . . n. The step index is shown in the above definitions as φ(n)−1 and ε(n)−1: the first first iterative associated phoria is denoted with φ(1)−1, the first second iterative associated phoria by φ(1)−2, and so on. Naturally, the “−1” and “−2” indices continue to label the angles of the first eye  1 - 1  and the second eye  1 - 2 , respectively, while the “(1)”, “(2)”, . . . “(n)” indices label the first, second, and n-th steps of the iterative process. 
     As in the projecting step  144 , the projecting  148  of these shifted added central images  212 - 1  and  212 - 2  can be performed in an alternating manner, using the stereo display  20  and the computer  50 . 
       FIG. 15C  further illustrates that the projecting step  148  can be followed by the tracking  150  of an adjustment of the first eye  1 - 1  in response to the projecting of the shifted first added central image  212 - 1 , and tracking an adjustment of the second eye  1 - 2  in response to the projecting of the shifted second added central image  212 - 2 , using the eye tracker  40 . Specifying the presentation to the first eye  1 - 1  only, the tracking step  150  includes the tracking of the adjustment angle ε(n+)−1 of the first eye  1 - 1  in response to the projecting  148  of the shifted first added central image  212 - 1  with the first iterative associated phoria φ(n)−1. 
     This tracking step  150  is analogous to the tracking step  146 . It is distinguished by the iterative step index having grown from (n) to (n+1). In simplified terms, embodiments of the method involve shifting the added central image  212  with the iterative associated phoria φ(n), tracking the responsive adjustment angle ε(n+1) of the eye  1 , determining the adjustment of the eye  1  from the change of the adjustment angle ε(n+1)−ε(n), and then repeating the shifting of the added central image  212  with a new iterative associated phoria ε(n+1), selected in magnitude and sign to reduce the change of the adjustment angle ε(n+1)−ε(n). 
     In some embodiments, the magnitude of φ(n+1)−φ(n) can be chosen to be equal to ε(n+1)−ε(n): |φ(n+1)−φ(n)|=|ε(n+1)−ε(n)|. In some cases, these embodiments may exhibit a slow convergence. Therefore, in some embodiments, |φ(n+1)−φ(n)| can be chosen to be equal to λ|ε(n+1)−ε(n)|: |φ(n+1)−φ(n)|=λ|ε(n+1)−ε(n)|, where λ&lt;1. These embodiments often exhibit good convergence. Other, non-linear, polynomial, non-analytic or analytic relationships can also be employed in various embodiments. 
     After performing these steps  148  and  150  iteratively, the determining step  152  can be performed to determine whether an effective adjustment of the first and second eye is less than an adjustment threshold. Using the above framework, the determining step  152  may evaluate whether the change of the adjustment angle |ε(n+1)−ε(n)|, is less than a threshold. The effective adjustment can be defined in various ways. It can involve the change of the adjustment angle of only one of the eyes: |ε(n+1)−ε(n)| for the eye  1 - 1 ; or the sum of the changes of the adjustment angles for both eyes  1 - 1  and  1 - 2 , or some weighted average, or a non-linear relation. 
     If the change of the adjustment angle |ε(n+1)−ε(n)| is greater than a threshold, then the method can return to the projecting step  148  of the shifted first added central image  212 , as shown in  FIG. 15C . 
     On the other hand, if in step (n), the adjustment of the eye, as characterized by, e.g., the change of the adjustment angle |ε(n)−ε(n−1)|, is found to be less than the threshold, then the iteration can stop and the method can continue with the identifying  154  of a stabilized associated phoria φ from the last first iterative associated phoria φ(n)−1, and the last second iterative associated phoria φ(n)−2. Again, different formulas can be adopted to define the stabilized associated phoria gin this step  154 , for example, φ=(φ(n)−1)+(φ(n)−2). 
     In the preceding embodiments, the disassociated phoria δ and the stabilized associated phoria φ were typically defined for the two eyes together. Thus, the per-eye values are half of the here-defined angles, in symmetrical cases. 
     The identifying step  154  can be followed by the identifying  156  of a sum of the disassociated phoria d and the stabilized associated phoria φ, (δ+φ), as a correction to the accommodative convergence AC, with the accommodative convergence angle α, that corresponds to the apparent distance. With this, the full, or fully corrected, accommodative convergence, determined by the method  100 , can be expressed via the tangent of the corresponding full, or fully corrected, accommodative convergence angle: [α−(δ+φ)/2], in terms of prism diopters Δ. As mentioned earlier, a typical definition of the accommodative convergence is AC=100 tan [α−(δ−φ)/2], in prism diopters Δ. This form shows one of the ways the result of embodiments of the method  100  is a distinct step forward compared to previous methods, where only the disassociated phoria δ was used to correct a, translating into AC=100 tan [α−δ/2]. Another difference compared to previous methods is the particular system  10  and method  100 , by which δ was determined. 
     With the fully corrected AC having been determined by the method  100 , the binocular alignment can be again characterized by the AC/A ratio, the ratio of the accommodative convergence AC to the accommodative response A, to characterize the binocular alignment. This AC/A ratio can be determined for a single distance, or can be formed from AC and A values for multiple distances. For brevity, from here on, the fully corrected accommodative convergence AC will be simply referred to as accommodative convergence AC. 
     In some embodiments, the method  100  can include determining a distance vision accommodative convergence AC(L d ) as an accommodative convergence resulting from performing the method  100  at a distance vision apparent distance L d ; and determining a near vision accommodative convergence AC(L n ) as an accommodative convergence resulting from performing the method at a near vision apparent distance L n . 
     With this preparation, in some embodiments, the binocular alignment of the first eye and the second eye can be characterized by first determining a distance vision accommodative response A(L d ) and a near vision accommodative response A(L n ), in diopters; and then by constructing a ratio of the distance vision accommodative convergence AC(L d ) minus the near vision accommodative convergence AC(L d ), divided by the distance vision accommodative response A(L d ) minus the near vision accommodative response A(L n ), to characterize the binocular alignment of the first eye and the second eye: 
       binocular alignment=[ AC ( L   d )− AC ( L   a )]/[ A ( L   d )− A ( L   n )]  (1)
 
     In some embodiments, the measuring  120  at the apparent distance and the determining  140  at the apparent distance can be performed using the accommodation optics  30 . 
     When the drawbacks of existing methods were described earlier, the subjectivity of the patient&#39;s feedback has been identified as one source of scatter in the data, and reason for limited reproducibility. In this context, it is mentioned that embodiments of the method  100  can be performed without soliciting a substantive response from the patient to determine one of the key quantities or angles. (Of course, non-substantive responses about, e.g., comfort, can very well be part of the method  100 .) This is one of the keys why the method  100  delivers measurements with high reproducibility. 
       FIG. 16  illustrates that in some embodiments, when the method  100  is performed at apparent distances corresponding to near vision, the disassociated phoria and the accommodative convergence corresponding to the near vision can be determined at viewing angles below an equatorial direction  9  by displaying the centered images  201  below the equatorial direction  9 . 
     Applicant&#39;s extensive experimentation demonstrated that when prismatic eye glasses were manufactured based on the accommodative convergence determined by the method  100 , the patients wearing these glasses reported particularly promising reduction of digital-device related visual discomforts, pains and migraines. 
     It is quite likely that this substantial improvement has been achieved, among others, because the method  100  developed and integrated solutions regarding the points (1)-(5) identified earlier as follows.
         (1) The method  100  does not use the patient&#39;s subjective responses as key inputs.   (2) The method  100  uses both peripheral images, e.g. the images  124  and  210 , and central images, e.g. the images  201  and  212 .   (3) The method  100  uses a two-stage method with the measuring step  120  and the determining step  140 , gathering and utilizing information about both central vision and peripheral vision.   (4) The method  100  uses moving test images, e.g. the images  210 .   (5) The method  100  developed a particular definition of the accommodative convergence and the protocol for its determination, e.g. in steps  142 - 156 , and proved with extensive testing that eye glasses prescribed using this definition reduce eye-strain related discomfort particularly efficiently.       

     For all these reasons, the above described system  10  and method  100  offer promising new ways to reduce eye-strain related discomfort, pain and migraines. 
       FIGS. 17-25  illustrate an additional embodiment of a system for determining binocular alignment  310  that shows analogies to the embodiments of the system for determining binocular alignment  10  in  FIGS. 8A-B . Accordingly, analogous parts are labeled with the same labels, with  300  as the base. As an example, the system for determining binocular alignment  310  can be thought of as an embodiment of the system for determining binocular alignment  10 , and thus the various elements and techniques described in relation to system  10  can be applied, adapted and combined with those of the system  310 , and vice versa. For brevity, the system for determining binocular alignment  310  will be sometimes simply referred to as the system  310 . 
       FIG. 17  illustrates a system to determine a binocular alignment  310  that comprises a first optical unit  315 - 1  that includes a first display  322 - 1 , to display images for the first eye  1 - 1 , actuatable along a longitudinal direction according to a simulated distance and an optical power of the first eye  1 - 1 , and a first eye tracker assembly  340 - 1 , to track a gaze direction of the first eye  1 - 1 , adjustable in a horizontal lateral direction to accommodate a first pupillary distance  4 - 1  of the first eye; and a second optical unit  315 - 2 , including a second display  322 - 2 , to display images for the second eye  1 - 2 , actuatable along the longitudinal direction according to a simulated distance and an optical power of the second eye  1 - 2 , and a second eye tracker assembly  340 - 2 , to track a gaze direction of the second eye  1 - 2 , adjustable in the horizontal lateral direction to accommodate a pupillary distance  4 - 2  of the second eye  1 - 2 ; and a computer  350 , coupled to the first optical unit  315 - 1  and the second optical unit  315 - 2 , to determine a binocular alignment based on the gaze directions of the first eye  1 - 1  and the second eye  1 - 2 .  FIG. 17  also shows an xyz coordinate system on the side for an alternative way to characterize directions. Using this xyz coordinate system, the “horizontal lateral direction” is aligned with the x axis, a “vertical lateral direction” is aligned with the y axis, and the “longitudinal direction” is aligned with the z axis. This alignment can be strict, or in some embodiments it can be within a tolerance range, such as plus minus 10 degrees. 
     Different patients have different prescriptions, such as a few diopters of myopia or hyperopia. Some previously described embodiments of the system for determining binocular alignment  10  use phoropter wheels  32 - 1  and  32 - 2  with lenses of differing diopters to simulate these prescriptions, while displaying images for the patients with fixed stereo display screens  22 - 1  and  22 - 2 —see, e.g.,  FIGS. 8A-B . These embodiments measure the binocular alignment at two different nominal distances. These distances are yet again simulated by rotating the phoropter wheels  32 - 1  and  32 - 2  and engaging lenses with diopters representing the simulated distances in addition to the patient&#39;s prescription. 
     Embodiments of the system for determining binocular alignment  310  do not involve phoropter wheels  32 - 1  and  32 - 2 : they perform both of the above functions by having made the displays  322 - 1  and  322 - 2  actuatable along the longitudinal direction according to the simulated distance and the optical power of the eyes  1 - 1  and  1 - 2 . The elimination of the phoropter wheels  32 - 1  and  32 - 2  makes the physical size of the system for determining binocular alignment  310  notably smaller than that of the system for determining binocular alignment  10  which uses phoropter wheels  32 - 1  and  32 - 2 . This is an advantage in an optometrist&#39;s crowded office where physical space is at a premium. Further, using phoropter wheels  32 - 1  and  32 - 2  enables the system for determining binocular alignment  10  to simulate the patient&#39;s prescription only in discrete steps, such as in 1 diopter steps. As a further advantage, the system for determining binocular alignment  310  can actuate the first and second displays  322 - 1  and  322 - 2  essentially continuously along the longitudinal direction, and thus can simulate the patient&#39;s prescriptions continuously with high accuracy, possibly within 0.1 diopter or better. 
     Another challenge of the systems  10  that use the phoropter wheel  32 - 1  design is that when the phoropter wheel  32 - 1  is rotated to engage a new lens to simulate a new distance or new prescription, since the first eye tracker assembly  340 - 1  is seeing the eye  1 - 1  through the lenses of the phoropter wheel  32 - 1 , the magnification changes with the rotation of the phoropter wheel  32 - 1 . This change in the magnification necessitates the recalibration of the image analysis performed by the computer  350 . This recalibration can lead to time lag and potentially coding challenges. In contrast, embodiments of the system for determining binocular alignment  310  that use actuatable first and second displays  322 - 1  and  322 - 2  avoid this need for recalibration, making the operation of the system  310  much easier. 
     In some embodiments, the first display  322 - 1  and the second display  322 - 2  can travel over a longitudinal range in the 50-200 mm range, in some embodiments, in the 75-125 mm range. The closest longitudinal distance of the first and second displays  322 - 1  and  322 - 2  to the first and second eye tracker assemblies  340 - 1  and  340 - 2  can be in the 5-40 mm range, in others, in the 10-30 mm range. As such, in some embodiments, the system for determining binocular alignment  310  can simulate prescription optical powers in a range of −20 D to +20 D, or less, in others in a range of −1 OD to +10 D, or less, in yet other embodiments in an asymmetric range, such as −10 to +20 D, or less. 
     In embodiments, the closer the first and second displays  322 - 1  and  322 - 2  are positioned to the eyes  1 - 1  and  1 - 2 , the larger field of view is perceived by the patient. This field of view can extend at least from −30 degrees to +30 degrees, in others at least from −35 degrees to +35 degrees to even larger values. Accordingly, some embodiments of the system for determining binocular alignment  310  can be also used for visual field tests that have multiple utilities, such as identifying local blind spots, or scotomas, as well as issues with peripheral vision. These symptoms can be indicative of various ailments, such as glaucoma or brain disorders. 
     There are multiple benefits of making at least parts of the first and second optical units  315 - 1  and  315 - 2  adjustable in a lateral direction, and multiple embodiments to achieve this adjustability. As mentioned above, accommodating the different pupillary distances of different patients can be achieved with making the first and second eye tracker assemblies  340 - 1  and  340 - 2  adjustable in the horizontal lateral, “x” direction. Further, in systems where the first and second optical units  315 - 1  and  315 - 2  are fixed, when a patient is prompted to look at a simulated near object, the eyes are looking through the frontal lenses of the system(see e.g. first lens assembly  360 - 1  in  FIG. 18 ) through a nasally offset off-center region. While these frontal lenses provide the designed refractive power, their off-center regions also introduce an unintended prism into the refraction of the light that perturbs the proper determination of the binocular alignment of the patient. Embodiments of the system for determining binocular alignment  310  minimize or even eliminate this problem again by making the first and second eye tracker assemblies  340 - 1  and  340 - 2 , together with their corresponding frontal lenses, horizontally laterally actuatable. In theses systems  310 , when a nearer object is simulated by displaying images for the first and second eyes  1 - and  1 - 2  shifted closer to the center of the system  310 , the first and second eye tracker assemblies  340 - 1  and  340 - 2 , together with their frontal lenses, can be horizontally laterally actuated so that the patient is still looking at the nearer objects through a center of the frontal lens of the system to determine a binocular alignment  310 , thereby avoiding the unintended prismatic effect. 
     The above motivations to introduce horizontal lateral adjustability can be achieved not only by making the first and second eye tracker assemblies  340 - 1  and  340 - 2  adjustable, or actuatable, along the horizontal lateral direction. To begin with, the first and second eye tracker assemblies  340 - 1  and  340 - 2  can be adjustable together with their corresponding frontal lenses, as just mentioned. Further, in some embodiments of the system for determining binocular alignment  310 , the first display  322 - 1  can be also structurally adjustable, or actuatable, together with the first eye tracker assembly  340 - 1 ; and the second display  322 - 2  can be also structurally adjustable, or actuatable, together with the second eye tracker assembly  340 - 2 . When accounting for the adjustability of the frontal lenses as well, in these embodiments, the entire first optical unit  315 - 1  and the second optical unit  315 - 2  can be horizontally adjustable, or actuatable, as shown in  FIG. 17 . 
     Yet another adjustability can be useful as well. Remarkably, there is a notable spread within the population regarding the vertical positions of the left and right eyes: the two eyes are often misaligned vertically by a few millimeters. Such patients can experience problems with aligning their eyes with the first and second optical units  315 - 1  and  315 - 2 . Embodiments of the system to determine a binocular alignment  310  can manage this problem by having the first eye tracker assembly  340 - 1 , with its frontal lenses, to be adjustable in a vertical lateral direction; and the second eye tracker assembly  340 - 2 , with its frontal lenses, to be adjustable in the vertical lateral direction. With the language of the previously defined coordinate system, this translates to the adjustability along the y axis. 
       FIG. 18A  illustrates that in some embodiments of the system for determining binocular alignment  310 , within the first optical unit  315 - 1 , the first eye tracker assembly  340 - 1  can include one or more first infrared light emitting diodes (IR LEDs)  342 - 1 , to project an infrared (IR) eye-tracking beam  342   b - 1  on the first eye  1 - 1 . Further, the first eye tracker assembly  340 - 1  can also include a first infrared (IR) light source  344 - 1 , to illuminate the first eye  1 - 1  with an infrared (IR) imaging light  344   b - 1 . Finally, the first eye tracker assembly  340 - 1  can include a first infrared (IR) camera  348 - 1 , to detect the IR eye-tracking beam  342   b - 1  after reflection from the first eye  1 - 1 , and the IR imaging light  344   b -after reflection from the first eye  1 - 1 , collectively labeled reflected IR beam and IR light  345   b - 1 , through a first infrared (IR) optics  346 - 1 . Naturally, in the system for determining binocular alignment  310 , within the second optical unit  315 - 2 , the second eye tracker assembly  340 - 2  can include one or more second infrared (IR) light emitting diodes  342 - 2 , to project an infrared ( 1 R) eye-tracking beam  342   b - 2  on the second eye  1 - 2 , a second infrared (IR) light source  344 - 2 , to illuminate the second eye  1 - 2  with an infrared imaging light  344   b - 2 , and a second infrared (IR) camera  348 - 2 , to detect the IR eye-tracking beam  342   b - 2  after reflection from the eye  1 - 2 , and the IR imaging light  344   b - 2 , after reflection from the second eye  1 - 2 , collectively labeled reflected IR beam and IR light  345   b - 2 , through a second  1 R optics  346 - 2 . Since the second eye tracker assembly  340 - 2  is analogous to the first eye tracker assembly  340 - 1 , there is no need to show it expressly. For orientation, the xyz coordinate system of  FIG. 17  is also shown, from a perspective rotated relative to that of  FIG. 17 . 
     In embodiments, the number of the first and second IR LEDs  342 - 1  and  342 - 2  can be in the range of 1-10, in some embodiments in the range of 2-4. In embodiments, the first infrared light source  344 - 1  can include a set of individual infrared light emitting diodes, spatially distributed in order to illuminate the first eye  1 - 1  with a dispersed infrared imaging light  344   b - 1 ; and the second infrared light source  344 - 2  can include a set of individual infrared light emitting diodes, spatially distributed in order to illuminate the second eye  1 - 2  with a dispersed infrared imaging light  344   b - 2 . The individual infrared diodes of the first and second infrared light source  344 - 1  and  344 - 2  can be positioned in many different patterns, such as a circle, an arc, a rectangle, and a rectangular array, among others. Their number can be in the range of 1-50, in some embodiments in the range of 5-20. The infrared imaging lights  344   b - 1  and  344   b - 2  can be dispersed, or homogenized in different ways, including by a diffuser, or by a scattering mirror, or by a scattering surface. 
       FIGS. 18A-B  illustrate that the one or more first infrared (IR) light emitting diodes  342 - 1  can be positioned at different positions in the first eye-tracker assembly  340 - 1 . In  FIG. 18A , the first infrared (IR) light emitting diodes  342 - 1  are positioned at a frontal area of the first eye tracker assembly  340 - 1 , close to the first eye  1 - 1 . In these designs, the IR eye tracking beam  342   b - 1  may make a larger angle with the main optical axis of the first optical unit  315 - 1 , possibly complicating the centering of the reflected IR light. In  FIG. 18B , the one or more first infrared (IR) light emitting diodes  342 - 1  are positioned much higher upstream along the optical path, in the proximity of the first infrared (IR) camera  348 - 1 , often close to its central first IR optics  346 - 1 . In these designs, the FR eye tracking beam  342   b - 1  can be well aligned with the main optical axis of the first optical unit  315 - 1 . 
     In some embodiments of the system for determining binocular alignment  310 , the computer  350  can include, or be connected to, an image analysis system  352 , to determine an orientation of the first eye  1 - 1  and the second eye  1 - 2 , using the reflected IR eye tracking beams  342   b - 1  and  342   b - 2 , and using the IR image formed by the infrared imaging lights  344   b - 1  and  344   b - 2 , the reflected beams together labeled  345   b - 1  and  345   b - 2 . The image analysis system  352  can be configured to use the detected reflected infrared eye tracking beams  342   b - 1  and  342   b - 2  to determine Purkinje reflections from the first eye  1 - 1  and the second eye  1 - 2 ; and to use the IR image formed by the infrared imaging lights  344   b - 1  and  344   b - 2  to determine pupillary attributes of the first eye  1 - 1  and the second eye  1 - 2 . The Purkinje reflections can be any one of the so-called P 1 . P 2 , etc. Purkinje reflections, labeled according to which optical surface of the eye they reflect from. One of the often-used Purkinje reflection is P 1 , the reflection from the frontal surface of the cornea. The IR beam  342   b - 1  is often directed by the first IR LEDs  342 - 1  to reflect from the apex of the cornea to yield a central P 1  Purkinje reflection. The determination of the gaze direction can also involve determining one of the pupillary attributes, such as the location of the pupil center, or how much ellipticity the image of the pupil has. When the eye optical axis is aligned with a main optical axis of the first eye tracker assembly  340 - 1 , then the pupil of the eye  1 - 1  will appear as a circle for typical eyes. When the gaze direction of the eye  1 - 1  turns away from this main optical axis by a rotation angle, the same pupil will appear as an ellipse. Analyzing the ellipticity of this ellipse, as e.g. given by the ratio of its minor axis to its major axis, and determining the directions of these axes delivers important information about the gaze direction&#39;s rotation angle. Yet other pupillary attributes can involve imaging the iris and recording the location of a specific feature of the iris. Determining the pupilary attributes can involve edge recognition software to identify the precise edges of the pupils. 
     The operation of these first and second optical units  315 - 1  and  315 - 2  and the image analysis system  352  has been designed by recalling that for many patients, their pupils are not of the same size, they are not completely circular, or completely aligned. For example, for patients whose eyes are not fully aligned, when one of the two eyes is aligned with the optical axis of the corresponding first and second eye tracker assembly  340 - 1  or  340 - 2 , the other eye is not aligned with its corresponding eye tracker optical axis. Finally, the Purkinje reflection may also not come precisely from the apex. 
     In order to determine the gaze directions of the first and second eyes,  1 - 1  and  1 - 2  in spite of all these possible deviations from the ideal situation, the image analysis system  352  is often operated by first instructing the patient to look straight ahead, and then registering and recording the location of the Purkinje reflection P 1  and the pupil center of the patient by the first and second IR cameras  348 - 1  and  348 - 2 . (As at other loci in this document, since the second eye tracker assembly  340 - 2  is analogous to the first eye tracker assembly  340 - 1 , for brevity it is not illustrated in a separate, repetitive figure.) In addition, the ellipticity and other pupillary attributes of the eye can be also recorded. Connecting the location of the Purkinje reflection Pt with the pupil center can be used to define the direction of gaze, or direction of the optical axis of the eye. All these recordings are used to serve as a reference direction for subsequent measurements. This reference-setting step can be then followed by projecting visible images  326 - 1  and  326 - 2  by the first and second displays  322 - 1  and  322 - 2  for the patient, accompanied by re-measuring the Purkinje reflection(s), pupil center and other pupillary attributes like ellipticity in reaction to these images, followed by comparing the Purkinje reflection(s), pupil centers, and the pupillary attributes of the first eye  1 - 1  and the second eye  1 - 2  to the previously determined reference Purkinje reflections, pupil centers, and pupillary attributes of the first eye  1 - 2  and the second eye  1 - 2 . Comparing these measured valued to the reference values is then used to determine the gaze directions and their changes, as described next. 
     In embodiments, the image analysis system  352  can use the location of the centers of the pupil in the xy plane, as determined from the IR image, formed from the reflected IR lights  344   b - 1  and  344   b - 2 , and the locations of the Purkinje reflections P 1  from the apex of the cornea, as determined from the reflected IR beams  342   b - 1  and  342   b - 2 . If the pupil centers overlap, or coincide, with the corneal apexes in the xy plane, then the eye is looking straight forward, as in the reference IR images. When the pupil centers and the corneal apexes arc offset in the xy plane, then from the direction and magnitude of the offsets the image analysis system  352  can determine the rotational angle of the gaze direction of each eye relative to the reference direction. 
     As mentioned earlier, for a fraction of patients, even when they look straight forward, the pupil center and the corneal apex may not coincide even in the reference images. But even in these cases, the image analysis system  352  can take the locations of the pupil center and corneal apex in an image of a rotated eye, then subtract the reference locations of these, and from the so-constructed differences, determine the rotational angle of the gaze direction of the eyes  1 - 1  and  1 - 2  by which the eyes responded to the projected visible images  326 - 1  and  326 - 2 . Other embodiments can determine the gaze directions by other methods, such as other pupillary attributes and/or other Purkinje reflections. Yet other embodiments can use multiple pupillary attributes without Purkinje reflections. Yet others can do the opposite: use multiple Purkinje reflections without pupillary attributes. 
     Since the eyes perform quick saccadic motions many times a second, the gaze directions rapidly vary in time. Therefore, the above-mentioned Purkinje reflections and pupil centers, and possibly other pupillary attributes, are representative of a specific gaze direction if they are measured close to each other in time. And in reverse: if they are measured with a substantial time difference, bigger than 0.1 second, or 1 second, or more, then the gaze direction computed by the image analysis system  352  may be less and less accurate. To increase the accuracy of this computation, in some embodiments the one or more first infrared light emitting diodes  342 - 1  project the infrared eye-tracking beam (IR beam)  342   b - 1  in an alternating manner with the first infrared light source  344 - 1  illuminating with the infrared imaging light  344   b - 1 ; and the one or more second infrared light emitting diodes  342 - 2  project the infrared eye-tracking beam  342   b - 2  in an alternating manner with the second infrared light source  344 - 2  illuminating with the infrared imaging light  344   b - 2 . The frequency of the alternation can be in the 1-1,000 Hz range, in some embodiments in the 10-150 Hz range, in some embodiments in the 60-120 Hz range. With these alternations, the first and second IR cameras  348 - 1  and  348 - 2  can determine the Purkinje reflections and pupil centers, and possibly other pupillary attributes, within 1-1,000 milliseconds of each other, in other embodiments within 6-100 milliseconds, in yet others 8-16 milliseconds. Determining the Purkinje reflections and pupil centers, and possibly other pupillary attributes, so close to each other advantageously increases the accuracy of the computation of the gaze direction by the image analysis system  352 . As mentioned before, in some embodiments of the system for determining binocular alignment  310 , only multiple pupillary attributes are determined, in other embodiments of system  310  only multiple Purkinje reflections. Determining either of these with the above repetition rates also increases the accuracy of the determination of the gaze directions. 
     In some embodiments of the system for determining binocular alignment  310 , the first eye tracker assembly  340 - 1  also includes a first visible-transmissive infrared mirror  324 - 1 , positioned to transmit images from the first display  322 - 1  along the longitudinal direction to the first eye  1 - 1 ; and to redirect the reflected infrared eye-tracking beam  342   b - 1  and the infrared imaging light  344 - 1 , together labeled  345   b - 1 , from the first eye  1 - 1  to the first infrared camera  348 - 1  in a lateral direction; and the second eye tracker assembly  340 - 2  includes a second visible-transmissive infrared mirror  324 - 2 , positioned to transmit images from the second display  322 - 2  along the longitudinal direction for the second eye  1 - 2 : and to redirect the reflected infrared eye-tracking beam and the infrared imaging light, together  345   b - 2 , from the second eye  1 - 2  to the second infrared camera  348 - 2  in the lateral direction. In some embodiments, the first infrared camera  348 - 1  is positioned relative to the first visible-transmissive infrared mirror  324 - 1  in one of a vertical lateral and a horizontal lateral direction; and the second infrared camera  348 - 2  is positioned relative to the second visible-transmissive infrared mirror  324 - 2  in one of the vertical lateral and the horizontal lateral direction. The horizontal lateral direction corresponds to the x axis, and the vertical lateral direction corresponds to the y axis of the xyz coordinate system of  FIGS. 17-18 . 
     There are various eye-tracking display systems available, e.g. in virtual reality goggles, in which the IR eye tracking beam and the projected visible image do not share a common optical path and they do not utilize visible transparent IR mirrors. In these designs, the eye trackers&#39; IR camera is directly pointed at the eye. However, the geometry of the design dictates that these IR cameras are pointed at the eye from a high angle. As such, the eye tracking IR beam often suffers occlusions from longer eyelashes that confounds their image analysis systems and can lead to tracking impasses. Such occlusion problems by the eyelashes are avoided in the present system for determining binocular alignment  310  by making the reflected IR beams and IR imaging lights  345   b - 1  and  345   b - 2  share the main optical path, leaving the eye in a normal/z/longitudinal direction, and then redirected by the first and second visible transparent IR mirrors  324 - 1  and  324 - 2 . 
     As already referenced earlier, when measuring binocular alignment, the first display  322 - 1  is actuatable to a first longitudinal position according to the simulated distance, wherein the first longitudinal position is dynamically corrected according to the optical power of the first eye  1 - 1 ; and the second display  322 - 2  is actuatable to a second longitudinal position according to the simulated distance, wherein the second longitudinal position is dynamically corrected according to the optical power of the second eye  1 - 2 . The first and second displays  322 - 1  and  322 - 2  are actuatable continuously along the longitudinal/z direction, which allows for a more precise correction of the simulated distance according to the optical power, or prescription, of the eyes  1 - 1  and  1 - 2  of the patient. It is notable also that many virtual reality displays achieve economic advantages by using a single display, and display the images for the left and right eyes on corresponding halves of this single display. Such systems, however, do not have the freedom to move the two halves of the display to different z coordinates, even though for most people the prescription in their two eyes are different and thus would call for differing z coordinates. Embodiments of the system for determining binocular alignment  310 , in contrast, are well-suited to handle such different prescriptions as the two displays  322 - 1  and  322 - 2  are independently actuatable. 
     Further, when simulating images at different distances to determine the binocular misalignment at these distances, the horizontal lateral position of the images can be moved accordingly on the first and second displays  322 - 1  and  322 - 2  by the computer  350 . 
       FIGS. 18A-B  also illustrate that the first optical unit  315 - 1  can include a first lens assembly  360 - 1  to receive and guide the infrared eye-tracking beam and the infrared imaging light, both reflected from the first eye and together labeled  345   b - 1 , towards the first infrared camera  348 - 1 , and to reduce at least one of a chromatic aberration, an optical aberration, an optical astigmatism, and a wavefront distortion; and the second optical unit  315 - 2  can include a second lens assembly  360 - 2  to receive and guide the infrared eye-tracking beam and the infrared imaging light, both reflected from the first eye and together labeled  345   b - 2 , towards the first infrared camera  348 - 2 , and to reduce at least one of a chromatic aberration, an optical aberration, an optical astigmatism, and a wavefront distortion. (The elements of the second optical unit  315 - 2  are not shown explicitly for brevity—they are analogous to those of the first optical unit  315 - 1 .) 
     In some embodiments of the system to determine a binocular alignment  310 , the first infrared camera  348 - 1  and the first lens assembly  360 - 1  are adjustable together; and the second infrared camera  348 - 2  and the second lens assembly  360 - 2  are adjustable together. In embodiments where these two elements are not adjustable together, the infrared cameras  348 - 1  and  348 - 2  need to be much larger, so as to be able to retain the high resolution and low distortion of the images even if the first and second lens assemblies  360 - 1  and  360 - 2  have been adjusted to an off-center, misaligned position. And in reverse, in the embodiments where the first and lens assemblies  360 - 1  and  360 - 2  are adjustable together with the first and second infrared cameras  348 - 1  and  348 - 2 , the first and second infrared cameras  348 - 1  and  348 - 2  can be made much smaller since the collinearity with the first and lens assemblies  360 - 1  and  360 - 2  is maintained in spite of the adjustments. The smaller size of the first and second infrared cameras  348 - 1  and  348 - 2  advantageously reduces the size of the entire system to determine a binocular alignment  310 . 
       FIG. 19  illustrates an embodiment of the system for determining binocular alignment  310 . It shows the same elements as  FIG. 17-18 , from the top, y direction, or vertical lateral direction looking down, similarly to  FIG. 17 . In particular, the directions of the longitudinal/z directional actuation, and the horizontal lateral/x direction are well-demonstrated. 
       FIG. 20  illustrates an embodiment of the first optical unit  315 - 1  of the system for determining binocular alignment  310  from a perspective view. Besides the previously described elements, the further element of a first z actuator  347 - 1  is visible, configured to actuate the first display  322 - 1  along the longitudinal/z direction. Further, a first coupling  354 - 1  to the computer  350  is also visible, coupling the first display  322 - 1  to the computer  350  with a set of flexible or deformable communication lines. The first display  322 - 1  can be configured to display images for the first eye  1 - 1  modified according to at least one of an optical power, a cylinder, and a prism of the first eye  1 - 1 ; and the second display  322 - 2  can be configured to display images for the second eye  1 - 2  modified according to at least one of an optical power, a cylinder, and a prism of the second eye  1 - 2 . 
     In some embodiments, the first display  322 - 1  and the second display  322 - 2  may include a liquid crystal display, a light emitting diode (LFD) display, an organic LED display, a quantum dot LED display, a microlens array, a digital mirror device, and a scanning projector micro-electrical-mechanical system. 
       FIG. 21  illustrates a frontal, z directional view of the system for determining binocular alignment  310 . This is what is visible for the patient. The first and second lens assemblies  360 - 1  and  360 - 2  are shown. Beyond that, some embodiments include a nose bridge  370 , located centrally between the first optical unit  315 - 1  and the second optical unit  315 - 2 , configured to receive and immobilize a patient&#39;s nose. 
     Such embodiments provide progress relative to related diagnostic systems. Quite a few related diagnostic systems intend to immobilize the patient&#39;s head and eyes with a variant of a chin rest, where the patient rests her/his chin. However, the chin still acts as an axis of rotation for the patient&#39;s head, and therefore the eyes can still rotate around the rested chin with the chin-eye distance as a radius, causing rotational misalignment with the diagnostic apparatus. This remaining rotational misalignment can be minimized or eliminated by immobilizing the patient&#39;s head and eyes at the nose instead of at the chin. The nose bridge  370 , with its “downward V” shape achieves this function: it immobilizes the patient&#39;s head at the top of the nose, very close to the eyes, instead of at the chin. For this reason, the eyes are much more solidly immobilized relative to the system for determining binocular alignment  310  in such embodiments. 
     Another advantage is demonstrated by  FIGS. 17-21 . Denoting the center of the system for determining binocular alignment  310  as center  311 , for a fraction of patients the pupil center of their first eye  1 - 1  and that of their second eye  1 - 2  are not at an equal distance from the center of symmetry of their heads. These differences can be 1-2 mm, enough to cause notable errors if the measurements are analyzed assuming a symmetric positioning of the eyes  1 - 1  and  1 - 2 . Therefore, in embodiments of the system for determining binocular alignment  310 , it is advantageous to make not only the overall pupillary distance (“PD”) adjustable to accommodate patient-to-patient variations, but also to make the first/left eye mono-pupilary distance  4 - 1 , defined relative to the center  311 , adjustable independently from the second/right eye mono-pupillary distance  4 - 2 , again defined relative to the center  311 . In embodiments this is realized by making the first eye optical unit  315 - 1  adjustable in the horizontal lateral/x direction relative to the nose bridge  370  to accommodate the mono-pupillary distance  4 - 1  of the first eye  1 - 1 , as indicated with a block arrow; and making the second optical unit  315 - 2  adjustable in the horizontal lateral/x direction relative to the nose bridge  370  to accommodate the mono-pupillary distance  4 - 2  of the second eye  1 - 2 . In some cases, it may be possible to achieve this same goal by making only the first and second eye tracker assemblies  340 - 1  and  340 - 2  adjustable in the horizontal lateral direction relative to the nose bridge  370 . 
       FIG. 22  illustrates further features of embodiments of the system for determining binocular alignment  310 . Some embodiments can include a graphical user interface  380 , configured for a medical operator to interact with the computer  350  to manage the determination of the binocular alignment. This graphical user interface  380  can show to the medical operator, such as an optometrist or technician, the infrared images captured by the first and second IR cameras  348 - 1  and  348 - 2 , the movement of the eyes  1 - 1  and  1 - 2 , the available diagnostic steps to choose from, and parameters of the diagnostic procedure to set, among others. 
     Yet-other embodiments of the system for determining binocular alignment  310  can include a patient communication interface  385 , such as a loudspeaker, to instruct a patient to follow steps of the determination of the binocular alignment. These instructions can come from a remote operator, or they can be pre-recorded, and synchronized with the computer  350  projecting specific visible images  326 - 1  and  326 - 2 . Other embodiments of the patient communication interface  385  can include a patient feedback portal, to receive a feedback from the patient. Examples include a push-button, a track wheel, a touchpad, a microphone, and an audio-interactive device. With any of these patient feedback portals, the patient can select a feedback in response to a step of the diagnostic process. In an example, the computer  350  may start adjusting the longitudinal/z direction of the fist display  322 - 1 , and the loudspeaker of the patient communication interface  385  can convey the pre-recorded instruction to the patient: “indicate when the image is clear by pushing the button”. When the patient pushes the button of the patient communication interface  385 , the computer  350  can record the longitudinal/z position of the first display  322 - 1  that is informative regarding the patient&#39;s prescription, or optical power of the eye  1 - 1 . Or, the computer can move projected visible images  326 - 1  and  326 - 2  in a horizontal lateral/x direction on the first and second displays  322 - 1  and  322 - 2 , and ask the patient to indicate through a push-button when the two images  326 - 1  and  326 - 2  are fused, or when the fusion of the two images is broken. The horizontal lateral/x positions of the two images  326 - 1  and  326 - 2  are informative regarding the binocular alignment of the patient&#39;s eyes  1 - 1  and  1 - 2 . 
       FIG. 23  illustrates that in some embodiments, the first eye-tracker assembly  340 - 1  can include a first auto-refractor  400 - 1 , to determine refractive information about the first eye  1 - 1 : and the second eye-tracker assembly  340 - 2  can include a second auto-refractor  400 - 2 , to determine refractive information about the second eye  1 - 2 . As before, the second auto-refractor  400 - 2  can be analogous to the first auto-refractor  400 - 1  and thus does not need to be shown expressly. The refractive information can be simply the refractive power of the investigated eye, needed to perform the method  100 . For example, the prescription of the patient may have changed unbeknownst to her/him since the last examination by the optometrist. Or the optometrist may want to track the degree of accommodation in response to moving the first display  322 - 1  in the longitudinal/z direction. Or the optometrist may want to check a higher order astigmatism or aberration. 
     In embodiments, the first auto-refractor  400 - 1  can include a first wavefront (WF) infrared (IR) light source  402 - 1 , to project a WF IR light  402   b - 1  into the first eye  1 - 1 . This first WF IR light source  402 - 1  can have many different embodiments, including a LED, a LED array, a superluminescent LED called SLED, and an expanded beam laser, among others. The WF IR light  402   b - 1  can be guided through a first collimator  404 - 1 , and a first polarizing beam splitter  406 - 1 , whose transmitting polarization plane is aligned with the polarization plane of the first WF IR light source  402 - 1 . The first WF IR light  402   b - 1  can be coupled into the optical pathway of the first eye tracker assembly  340 - 1  through a first beam splitter  410 - 1 , optionally through an optional first refractor lens  408 - 1 . From here, the WF IR light  402   b - 1  can be guided to the first eye  1 - 1  via the main optical pathway of the first eye tracker assembly  340 - 1  that includes the first visible transparent IR mirror  324 - 1  and the first lens assembly  360 - 1 , as shown in  FIG. 23 . The (typically pencil-beam-like) WF IR light  402   b - 1  then reflects from the retina of the first eye  1 - 1  into a wider spatial angle as a reflected WF IR light  402   r - 1 . As the reflected WF IR light  402   r - 1  propagates through the lens and cornea of the first eye  1 - 1 , its expanding wavefront gets modified by refraction through the lens and the cornea, and thus acquires information about the refractive properties of the lens and cornea of the first eye  1 - 1 . The reflected WF IR light  402   r - 1  propagates back through the main optical pathway of the first eye tracker assembly  340 - 1 , gets split out of it by the first beam splitter  410 - 1 , and is eventually guided by the first polarizing beam splitter  406 - 1  towards a first microlens array  412 - 1 . This first microlens array  412 - 1  is configured to receive and split the reflected WF IR light  402   r - 1  from the first eye  1 - 1  into beamlets. The beamlets are then captured by a first wavefront camera  414 - 1  to be analyzed to determine the refractive information they carry about the first eye  1 - 1 . 
     The above described embodiment of the autorefractor  400 - 1  broadly follows the design of the Shack-Hartmann wavefront analyzers. Other embodiments can use other wavefront analyzing designs, such as Talbot-Moire interferometry, slit lamps technology, Tscherning aberrometry, lensometer technology, and the alike. Lensometer devices can, in fact, capture optical characteristics of the eye beyond the sphere/refractive power. These characteristics include the cylinder power and axis information, among others. 
     Systems for determining binocular alignment  310  that have an autorefractor  400 - 1  offer another useful diagnostic modality. A class of binocular alignment problems is called “accommodation lag”. This refers to the phenomenon when a patient is presented by an object at a presentation distance d 1 , but the patient&#39;s eyes focus at a different distance d 2  that does not equal d 1 . Often d 2  is larger than d 1 : d 2 &gt;d 1 . Systems  310  with an autorefractor  400 - 1  can recognize and diagnose such an accommodation lag. 
     On a higher, conceptual level, a primary goal of the systems for determining binocular alignment  310  is to diagnose and characterize the cooperation and crosslinking of two systems that control human vision: the focusing system that focuses the crystalline lens at the objects at their actual distance by engaging the ciliary muscles; and the vergence system that rotates both eyes to look at the objects at their actual distance by engaging the six extraocular muscles. Embodiments of the systems for determining binocular alignment  310  in  FIGS. 17-23  deliver high quality diagnostic information on these crosslinked systems by several design choices, including: they simulate the objects by the first and second displays  322 - 1  and  322 - 2  that are actuatable in the longitudinal/z direction; they use first and second optical units  315 - 1  and  315 - 2  that are actuatable in the horizontal lateral direction, and, optionally, they include the first and second autorefractors  400 - 1  and  400 - 2 . These design choices make these systems for determining binocular alignment  310  capable of diagnosing and characterizing the cooperation and crosslinking of the focusing system and the vergence system in an integrated, “closed loop” manner. Thus, embodiments of the system to determine a binocular alignment of  310  are configured to determine a vergence response and an accommodative response in an integrated manner by the first display  322 - 1  and the first eye tracker assembly  340 - 1 , and the second display  322 - 2  and the second eye tracker assembly  340 - 2  being configured to determine the vergence response; and the first display  322 - 1  and the first auto-refractor  400 - 1 , and the second display  322 - 2  and the second auto-refractor  400 - 2  being configured to determine the accommodative response. 
     For completeness, finally reference is made to the method of determining binocular alignment  100 , previously described in relation to  FIGS. 11-16 . The computer  350  can be configured to carry out steps of this method  100 . As such, in some embodiments, the computer  350  can be configured to determine a Fixation Disparity of a patient as an amount of angular misalignment between a central target and a peripheral fusion lock of moving targets around an image with a blank center, as part of the determining of the binocular alignment. 
     In some embodiments, the computer  350  can be also configured to determine a Gross Phoria as an average amount of angular misalignment between the first eye  1 - 1  and the second eye  1 - 2  when the first display  322 - 1  and the second display  322 - 2  display dissimilar images with one of the eyes fixated on a target at a time, as part of the determining of the binocular alignment. 
       FIGS. 24-25  illustrate a related embodiment of the system to determine a binocular alignment  310  that comprises a first optical unit  315 - 1 , including a first display  322 - 1 , to display images for a first eye  1 - 1 , actuatable along a lateral actuation direction according to a simulated distance and an optical power of the first eye  1 - 1 , and a first eye tracker assembly  340 - 1 , to track a gaze direction of the first eye  1 - 1 , adjustable in a horizontal lateral direction to accommodate a pupillary distance of the first eye  1 - 1 ; and a second optical unit  315 - 2 , including a second display  322 - 2 , to display images for a second eye  1 - 2 , actuatable along the lateral actuation direction according to a simulated distance and an optical power of the second eye  1 - 2 , and a second eye tracker assembly  340 - 2 , to track a gaze direction of the second eye  1 - 2 , adjustable in the horizontal lateral direction to accommodate a pupillary distance of the second eye  1 - 2 ; and a computer  350 , coupled to the first optical unit  315 - 1  and the second optical unit  315 - 2 , to determine the binocular alignment based on the gaze directions of the first eye  1 - 1  and the second eye  1 - 2 . A notable difference from the embodiments of  FIGS. 17-23  is that the positioning of the fist and second displays  322 - 1  and  322 - 2  is moved from a longitudinal arrangement to a lateral arrangement in the embodiment of  FIGS. 24-25 . This difference changes the form factor and dimension of the overall system  310 , which can be advantageous in a crowded optometrist&#39;s office. The lateral actuation direction can be a horizontal lateral (“x”) direction or a vertical lateral (“y”) direction. In  FIG. 24 , the lateral actuation direction is horizontal, in  FIG. 25 , it is vertical. 
       FIG. 25  illustrates this latter vertical embodiment in more detail, concentrating on the first eye  1 - 1 . The elements of the the system to determine a binocular alignment  310  that are related to the second eye  1 - 2  are analogous and are not shown for clarity. In this system to determine a binocular alignment  310 , the first eye tracker assembly  340 - 1  can include one or more first infrared light emitting diodes  342 - 1 , to project an infrared eye-tracking beam  342   b - 1  on the first eye  1 - 1 ; a first infrared light source  344 - 1 —possibly including several individual LEDs, to illuminate the first eye  1 - 1  with an infrared imaging light  344   b - 1 ; a first infrared camera  348 - 1 , positioned along a longitudinal direction to detect the infrared eye-tracking beam and the infrared imaging light, both reflected from the first eye and collectively labeled  345   b - 1 ; and a first infrared-transmissive visible mirror  324 ′- 1 , to transmit the reflected infrared eye-tracking beam and the infrared imaging light  345   b - 1  from the first eye  1 - 1  to the first infrared camera  348 - 1  along the longitudinal direction; and to redirect images from the lateral actuation direction of the first display  322 - 1  to the longitudinal direction towards the first eye  1 - 1 . The second eye tracker assembly  340 - 2  can include (not shown for clarity) one or more second infrared light emitting diodes  342 - 2 , to project an infrared eye-tracking beam  342   b - 2  on the second eye  1 - 2 : a second infrared light source  344 - 2 , to illuminate the second eye  1 - 2  with an infrared imaging light  344   b - 2 : a second infrared camera  348 - 2 , positioned along the longitudinal direction to detect the infrared eye-tracking beam and the infrared imaging light, both reflected from the second eye, collectively labeled  345   b - 2 ; and a second infrared-transmissive visible mirror  324 ′- 2 , to transmit the reflected infrared eye-tracking beam and the infrared imaging light  345   b - 1  from the second eye  1 - 2  to the second infrared camera  348 - 2  along the longitudinal direction; and to redirect images from the lateral actuation direction of the second display  322 - 2  to the longitudinal direction towards the second eye  1 - 2 . Typically, the beams to and from the eyes  1 - 1  and  1 - 2  are propagating through a first and second lens assemblies  360 - 1  and  360 - 2 . The many variants and modifications of the embodiments of  FIGS. 17-23  can have analogous implementations in the embodiment of  FIGS. 24-25 . For example, the horizontal adjustability can be implemented only for the first and second eye tracker assemblies  340 - 1  and  340 - 2 , or for these assemblies together with the first and second displays  322 - 1  and  322 - 2 , with or without the first and second lens assemblies  360 - 1  and  360 - 2 , just like it was described for the embodiments of  FIGS. 17-23 . 
     While this document contains many specifics, these should not be construed as limitations on the scope of an invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or a variation of a subcombination.