CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisional application Serial No. 63/334,519 filed April 25, 2022, the disclosure of which is hereby incorporated in its entirety by reference herein.

TECHNICAL FIELD

[0002] Embodiments relate to a method and system for assessing the axial length of a subject’s eye, such as for monitoring the progression of myopia.

BACKGROUND

[0003] Myopia, or short sightedness, is extremely prevalent throughout the world, affecting approximately 30% of the population. Elongation of the eye is the main cause of myopia and represents the most important risk factor for myopia-related eye complications, such that determining the progression of elongation provides an indicator for the progression of myopia. While the progression of myopia has previously been monitored by the change in refractive error, axial length has been established as an important, critical measurement in monitoring the progression and control of myopia.

[0004] In order to measure axial length, prior methods have assumed a direct conversion between the radius of rotation of an eye and its axial length. For example, a geometrical model of measuring the radius of rotation of an eye has been described previously (Perkins et al., Brit. Y. Ophthal. (1976) 60, 266; Dick et al., Clinical & Experimental Optometry 73.2: March/April 1990, p. 43-50). These geometric models assume that the eye has a perfect spherical shape and that the elongation of the eye is symmetrical along a given axis. However, such prior art models are over-simplistic and therefore inaccurate, as the assumption of a direct conversion between radius of rotation of the eye and its axial length is not observed in practice. [0005] Current methods for measuring axial length include biometric instruments, ultrasound, optical coherence tomography (OCT) imaging and interferometric methods. Such instruments and methods suffer from one or more disadvantages, such as requiring contact with the eye, being too expensive for many practitioners, and requiring a high level of operational skill. Furthermore, these prior methods rely on a fixed location of the subject’s head in space which limits their usability, especially in the case of children who are a main target group for monitoring progressive myopia.

SUMMARY

[0006] In one or more embodiments, a method for assessing changes in an axial length of an eye of a subject positioned at a distance from a display includes conducting a baseline measurement including: (a) displaying at least one target on the display; (b) determining a distance of a corneal apex of the subject from the display (d), a distance between the corneal apex and a center of a pupil of the subject (ea), and a distance between a point on an x axis of the display which is perpendicular to the corneal apex to the at least one target on the display (ca); (c) calculating an eye globe radius (Re) and an eye globe diameter (De); and (d) calculating and registering an initial axial length (ALe) of the baseline measurement. The method further includes conducting at least one subsequent measurement at a later point in time from the baseline measurement, the at least one subsequent measurement including: (e) carrying out steps (a) to (d) of the baseline measurement to obtain new values of the eye globe radius (Re) and the eye globe diameter (De); (f) calculating a new true axial length (ALe’) based on the new values of the eye globe radius (Re) and the eye globe diameter (De); and (g) calculating a difference between the initial axial length (ALe) and the new true axial length (ALe’ ) to obtain a change in axial length.

[0007] In one or more embodiments, the method may include repeating steps (a) to (c) at least n times, wherein n is equal or greater than 1 ; obtaining n values of the eye globe radius (Re) and the eye globe diameter (De), and calculating average values of the eye globe radius (Re) and the eye globe diameter (De) from n values of the eye globe radius (Re) and the eye globe diameter (De), respectively; and calculating the initial axial length (ALe) and the new true axial length (ALe’) based on the average values of the eye globe radius (Re) and the eye globe diameter (De).

[0008] In one or more embodiments, the method may further include correlating at least one of the eye globe diameter (De) and the average value of the eye globe diameter (De) with values of the eye globe diameter (De) from a lookup table and determining the new true axial length (ALe’). The lookup table may include one or more of: 1) a theoretical model of a purely spherical eye; 2) an actual measurement database of deviations versus axial length change; 3) genetic factors of the subject; 4) gender of the subject; 5) absolute axial length value of the subject; 6) absolute refraction of an eye of the subject; 7) age of the subject; 8) corneal curvature of the eye of the subject; 9) real position of the eye of the subject in x, y, z space and head tilts; and 10) ethnicity of the subject.

[0009] In one or more embodiments, the theoretical model may include a derivation of a radius of the eye from a given line of sight distance from the eye to the display. The at least one target may be displayed along an axis including one or more of an x axis of the display, a y axis of the display, or an oblique axis. The change of axial length may provide an indication for a progression of myopia.

[0010] In one or more embodiments, the method may include confirming with an eye tracker that a head of the subject is within an allowed headbox volume. The method may further include calibrating the eye tracker prior to initiating the baseline measurement.

[0011] In one or more embodiments, a method for assessing axial length of an eye of a subject includes: (a) displaying at least one target on a screen; (b) determine a distance (d) of a corneal apex of the subject from the at least one target; (c) determining a distance (ea) between the corneal apex and a center of a pupil of the subject; (d) determining a distance (ca) between the at least one target and a point on the screen which is perpendicular to the corneal apex; (e) calculating an eye globe radius (Re) and an eye globe diameter (De); and (f) determining axial length (ALe) based on the eye globe diameter (De). [0012] In one or more embodiments, a method for assessing axial length of an eye of a subject includes: (a) displaying at least one target on a screen; (b) determining a distance (d) of a corneal apex of the subject from the at least one target; (c) determining a distance (ea) between the corneal apex and a center of a pupil of the subject; (d) determining a distance (ca) between the at least one target and a point on the screen which is perpendicular to the corneal apex; (e) calculating an eye globe radius (Re) and an eye globe diameter (De); (f) carrying out steps (a) to (e) n times, wherein n is an integer greater than one; (g) obtaining n values of the eye globe radius (Re) and the eye globe diameter (De); (h) calculating average values of the eye globe radius (Re) and the eye globe diameter (De) based on n values of the eye globe radius (Re) and the eye globe diameter (De); and (i) determining an axial length (ALe) based on the average values of the eye globe radius (Re) and the eye globe diameter (De).

[0013] In one or more embodiments, the method may include correlating at least one of the eye globe diameter (De) and the average value of the eye globe diameter (De) with values of the eye globe diameter (De) from a lookup table and determining the axial length (ALe). The lookup table may include one or more of: 1) a theoretical model of a purely spherical eye; 2) an actual measurement database of deviations versus axial length change; 3) genetic factors of the subject; 4) gender of the subject; 5) absolute axial length value of the eye of the subject; 6) absolute refraction of the eye of the subject; 7) age of the subject; 8) corneal curvature of the eye of the subject; 9) real position of the eye of the subject in x, y, z space and head tilts; and 10) ethnicity of the subject.

[0014] In one or more embodiments, a method for assessing axial length of an eye of a subject includes providing a display screen configured to be positioned at a known distance from the subject, an eye tracker in proximity to the display screen, and a controller in communication with the display screen and the eye tracker; at an initial time point, performing a baseline measurement including displaying at least one target on the display screen using the controller and detecting a first deviation of a gaze position of the eye from the at least one target using the eye tracker and the controller; performing a plurality of baseline measurements to complete a baseline measurement session at the initial time point; averaging the first deviations from the baseline measurement session using the controller to determine a baseline deviation; at a later time point, performing a repeat measurement including displaying at least one target on the display screen using the controller and detecting a second deviation of the gaze position of the eye from the baseline deviation using the eye tracker and the controller; performing a plurality of repeat measurements to complete a repeat measurement session at the later time point; averaging the second deviations from the repeat measurement session using the controller to determine a measured deviation; and correlating the measured deviation with a change in axial length of the eye from the initial time point to the later time point using the controller.

[0015] In one or more embodiments, correlating the measured deviation with the change in axial length may be performed using a lookup table. The lookup table may include one or more of: 1) a theoretical model of a purely spherical eye; 2) an actual measurement database of deviations versus axial length change; 3) genetic factors of the subject; 4) gender of the subject; 5) absolute axial length value of the eye of the subject; 6) absolute refraction of the eye of the subject; 7) age of the subject; 8) corneal curvature of the eye of the subject; 9) real position of the eye of the subject in x, y, z space and head tilts; and 10) ethnicity of the subject.

[0016] In one or more embodiments, the theoretical model may include a derivation of a radius of the eye from a given line of sight distance from the eye to the display screen. The method may further include confirming with the eye tracker that a head of the subject is within an allowed headbox volume. The method may also further include calibrating the eye tracker prior to initiating the baseline measurement.

[0017] In one or more embodiments, the at least one target may be displayed along an axis including one or more of an x axis of the display screen, a y axis of the display screen, or an oblique axis. The eye may include a right eye or a left eye, and the first deviations and the second deviations may be averaged separately for the right eye and the left eye to generate averaged right eye deviations and averaged left eye deviations. The method may include determining if the averaged right eye deviations and the averaged left eye deviations differ beyond a threshold and, if so, providing an alert. Displaying at least one target may include dynamically moving at least one target from an initial position to a final position to facilitate a gaze of the subject being directed at the at least one target. [0018] In one or more embodiments, a method for assessing axial length of an eye of a subject includes providing a display screen configured to be positioned at a known distance from the subject, an eye tracker in proximity to the display screen, and a controller in communication with the display screen and the eye tracker; displaying at least one target on the display screen at an initial time point using the controller and detecting a first deviation of a gaze position of the eye from the at least one target using the eye tracker and the controller; averaging a plurality of first deviations at the initial time point using the controller to determine a baseline deviation; displaying at least one target on the display screen at a later time point using the controller and detecting a second deviation of the gaze position of the eye from the baseline deviation using the eye tracker and the controller; averaging a plurality of second deviations at the later time point using the controller to determine a measured deviation; and correlating the measured deviation with a change in axial length of the eye from the initial time point to the later time point using the controller for monitoring a progression of myopia in the eye of the subject.

[0019] In one or more embodiments, correlating the measured deviation with the change in axial length may be performed using a lookup table. The lookup table may include one or more of: 1) a theoretical model of a purely spherical eye; 2) an actual measurement database of deviations versus axial length change; 3) genetic factors of the subject; 4) gender of the subject; 5) absolute axial length value of the eye of the subject; 6) absolute refraction of the eye of the subject; 7) age of the subject; 8) corneal curvature of the eye of the subject; 9) real position of the eye of the subject in x, y, z space and head tilts; and 10) ethnicity of the subject.

[0020] In one or more embodiments, the theoretical model may include a derivation of a radius of the eye from a given line of sight distance from the eye to the display screen. At least one target may be displayed along an axis including one or more of an x axis of the display screen, a y axis of the display screen, or an oblique axis.

[0021] In one or more embodiments, a system for assessing axial length of an eye of a subject includes a display screen configured to be positioned at a known distance from the subject; an eye tracker disposed in proximity to the display screen for detecting a gaze position of the eye with respect to the display screen; and a controller in communication with the display screen and the eye tracker, the controller configured to generate at least one target on the display screen at an initial time point, to record a first deviation of the gaze position from at least one target, to average a plurality of first deviations at the initial time point to determine a baseline deviation, to generate at least one target on the display screen at a later time point, to record a second deviation of the gaze position of the eye from the baseline deviation, to average the second deviations at the later time point to determine a measured deviation, and to correlate the measured deviation with a change in axial length of the eye from the initial time point to the later time point for monitoring a progression of myopia in the eye of the subject.

[0022] In one or more embodiments, the controller may be configured to correlate the measured deviation with the change in axial length using a lookup table. The lookup table may include one or more of: 1) a theoretical model of a purely spherical eye; 2) an actual measurement database of deviations versus axial length change; 3) genetic factors of the subject;

4) gender of the subject; 5) absolute axial length value of the eye of the subject; 6) absolute refraction of the eye of the subject; 7) age of the subject; 8) corneal curvature of the eye of the subject; 9) real position of the eye of the subject in x, y, z space and head tilts; and 10) ethnicity of the subject.

[0023] In one or more embodiments, the theoretical model may include deriving a radius of the eye from a given line of sight distance from the eye to the display screen. The system may further include occlusion glasses configured to be worn by the subject for automatically blocking vision of one eye toward the display screen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIGURE 1 is a schematic illustration of a system for assessing axial length of a subject’s eye according to one or more embodiments;

[0025] FIGURE 2 is a flow chart depicting steps of a baseline measurement session according to one or more embodiments;

[0026] FIGURE 3 is a flow chart depicting the steps for determining change in the axial length of a subject according to one or more embodiments; [0027] FIGURE 4 is a diagram illustrating a theoretical model for determining axial length according to one or more embodiments;

[0028] FIGURE 5 is a flow chart depicting steps of a baseline measurement session according to one or more embodiments; and

[0029] FIGURE 6 is a flow chart depicting steps of a repeat measurement session according to one or more embodiments.

DETAILED DESCRIPTION

[0030] As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

[0031] Embodiments disclosed herein provide a method and system for assessing the change in the axial length of a subject’s eye over time. The method and system account for the shortcomings of the prior art by evaluating axial length changes in multiple dimensions, accounting for asymmetrical eye elongation which may not be detected when measured in only one dimension as in prior methods. Furthermore, the disclosed method and system compensate for the deviations of eye shape from a perfect spherical shape. The method and system also advantageously allows for a convenient, free space measurement with no restraint of the subject’s head required.

[0032] With reference first to FIG. 1, a system 10 for assessing the axial length of a subject’s eye E is schematically illustrated according to one or more embodiments. The system 10 includes an eye tracker 12 positioned near or integrated with a display screen 14, where a subject S is located at a distance from the eye tracker 12 and display screen 14 as shown. The display screen 14 may be stationary (e.g., a computer monitor) or movable/portable (e.g., a tablet or a mobile phone). The subject’s head may be disposed in an unrestrained manner within a defined three-dimensional (x, y, z) headbox volume 16 in free space, within which an eyebox reference frame 18 is defined for each eye E (see also, FIG. 4). According to the disclosed embodiments, the eye tracker 12 is employed to determine the gaze position of the subject’s eye E with respect to the display screen 14. The eye tracker 12 also determines the location of the subject’s eye E in space relative to the display screen 14. This latter location includes the distance of the eye E from at least one target 20 shown on the display screen 14.

[0033] As illustrated in FIG. 1 , at least one target 20 may comprise two spaced targets (A and B), but may alternatively comprise any array of dots, images, or other visual stimuli in order to elicit movement or gaze of the eye E. The display and position of the targets 20 are generated by a controller 22 (e.g., personal computer or digital processor) in communication with the display screen 14, where the controller 22 is also in communication with the eye tracker 12 to receive data therefrom regarding the gaze direction/position and location of the eye E in space, including the distance of the eye E from the targets 20 on the display screen 14. The communication between the controller 22 and the display screen 14 and eye tracker 12 may be wired or wireless (e.g., Bluetooth®). The controller 22 includes digital memory 24, wherein a stored algorithm and/or a lookup table along with image processing capability allow the controller 22 to determine the radius of rotation R of the eye E and provide an assessment of axial length change with time, or eye elongation, as described further below.

[0034] The eye tracker 12 follows the position and movements of the eye E in a noncontact manner. As is known in the art, the eye tracker 12 may function, for example, by illuminating the eye E (i.e., pupil) with a near-infrared or infrared light source to generate a reflection on the cornea, wherein the reflection is optically monitored or recorded (i.e., by a sensor or an infrared camera) to determine eye rotation. The subject S is presented with stimuli, such as targets 20, to elicit eye movements which can then be detected and analyzed using image processing. Any suitable eye tracker may be used with system 10, for example, an eye tracker by Tobii AB (Danderyd, Sweden). [0035] The use of an eye tracker 12 in the disclosed system and method enables the assessment of eye radius of rotation R, gaze position, and calibration with correlation to multiple targets 20 along at least one axis, but not limited to a single axis. Hence, the measurement can be made in one, two or three dimensions (x, y, z) which imparts enhanced accuracy to the method and system disclosed herein as opposed to the prior art. Measurement in three dimensions also advantageously removes the necessity of affixing head position in order to avoid movement, such as by a chin rest or other means.

[0036] In one or more embodiments, the gaze position may be measured multiple times for each target 20 and for each eye E. As shown in FIG. 1 , shutter or occlusion glasses 26 may be configured to be worn by the subject S and used to automatically block the vision of one eye toward the display screen 14 while testing the other eye, facilitating measurement of each eye independently and automatically.

[0037] The method and system according to the embodiments disclosed herein may be used for comparing between measurements performed at different times (e.g., every 6 months) in order to determine whether there is a relative change in the value of the axial length. The method and system may also be used to provide an estimate of initial axial length, with some confidence interval, which may be used as a screening tool for certain myopia cases.

[0038] The first measurement of the subject’s eye E may be used as a baseline and all future measurements may be compared to the baseline measurement in order to calculate the relative eye elongation. The first or any subsequent measurements using the system 10 may be done following calibration of the eye tracker 12, or instead may be performed without eye tracker 12 calibration. Calibrated results may be used to assess axial elongation on subsequent visits by monitoring for any changes from the original calibrated state. In order to facilitate calculation, the first baseline measurement could be used to calibrate the eye tracker 12 to the targets 20 on the display screen 14, so that all future measurements may be shown as deviation from the targets 20. [0039] In accordance with one or more embodiments and with reference to the flow chart depicted in FIG. 2 and the diagram of FIG. 4, the baseline measurement may comprise the following steps:

(a) Position the subject at a distance from the display within the allowed x, y, z headbox and remove glasses (if subject is wearing glasses);

(b) Display at least one target (e.g. A) on the screen, such as across a main meridian (e.g. x-axis) (FIG. 2, block 201);

(c) Determine the distance d, ea and ca (FIG. 2, block 202). (d) is the distance of the corneal apex from the x-axis (which is displayed on the screen); (ea) is the distance between the corneal apex and the center of the pupil; (ca) is the distance between the point on the x-axis which is perpendicular to the corneal apex to the target on the x-axis (e.g. (A));

(d) Calculate (Re) (FIG. 2, block 203);

(e) (Optional) Repeat steps (b) and (c) at least n times (n is equal or greater than 1) to obtain average values of (Re) (FIG. 2, block 204). The repeated measurements and calculations in steps (c) and (d) can be done on different positions of the targets on the x-axis, in which case the values of (ea) and (ca) will be with respect to the position of the target;

(f) Based on the values of (Re) calculate an “average (Re)” and an average (De) (FIG. 2, block 205);

(g) The De and/or average De calculated in step (f) is the baseline (De) for all future measurements (FIG. 2, block 206);

(h) Correlate average (De) with (De) values from lookup tables and determine true Axial Length (ALe) (FIG. 2, block 207); and

(i) Register the true (ALe) of the initial session (FIG. 2, block 208). [0040] According to one or more embodiments, before initiating the baseline and/or subsequent measurements, a check is performed, such as by the eye tracker 12 in communication with the controller 22, to ensure that the subject’s head is within an allowed x, y, z headbox volume 16. If not, the subject’s head may be repositioned before proceeding. Furthermore, an optional calibration of the eye tracker 12 may be performed.

[0041] Referring now to FIG. 3, a flow chart 300 for determining change in the axial length of a subject is shown. Determining a change of axial length according to one or more embodiments may comprise the following steps:

1. Carry out steps (a) to (h) of the baseline session (FIG. 3, block 301);

2. Calculate new true Axial Length (ALe’) (FIG. 3, block 302); and

3. Calculate the difference between (ALe) and (ALe’), i.e. AALe. AALe is an indicator of the progression of myopia.

[0042] FIG. 4 is a diagram illustrating a theoretical model for estimating axial length, and therefore eye elongation, for monitoring myopia progression. In the diagram shown, A and B are the coordinates of displayed targets 20 along the x axis of the display screen 14 as generated by the controller 22. Variables a and b are the line of sight distances from the center of the eye globe E to the gaze positions A and B, respectively. Variable d is the distance (z axis) from the display screen 14 to the corneal apex of the eye globe E as received by the controller 22 from the eye tracker 12.

[0043] With continuing reference to FIG. 4, variable c is the calculated distance between the two target positions A and B as determined by the controller 22. Variables ca and cb are the distances between gaze coordinates at 2 gaze points A and B with reference to vertical distance d as calculated by the eye tracker 12 and the controller 22. Variables ea and eb are the distances from the line of sight vectors a and b, respectively, to an intersection point (corneal apex point) of the distance d vector with an x axis of the eyebox reference frame 18. Variables a and P are the angles between the line of sight vectors a and b, respectively, and the x axis of the display screen 14 as calculated by the controller 22. Variable Re is the radius of the eye globe E calculated by the controller 22.

[0044] As such, for a given distance a, and with the assumption that the eye globe E is a perfect sphere, Re can be calculated as follows: ca d + Re ea Re

Re * ca = d * ea + Re * ea

Re * (ca — ea) = d * ea

> _r > d*ea

Radius of rotation Re = - ca-ea

Diameter of rotation De = Re * 2

[0045] As described above with reference to FIG. 3 and also with reference to FIG. 6, these measurements and calculations may be repeated for subsequent time periods to determine Re’ and the differences between Re and Re’, or change in axial length, over time to monitor the progression of myopia. For greater accuracy and averaging, the above calculations can also be performed for a given distance b and b’. Furthermore, while this example describes the calculation of Re and Re’ only on the x axis, it is understood that the corresponding process and calculations can be made to calculate Re and Re’ on the y axis or any other axis, as described above. Of note is that the location of the eye tracker 12 is irrelevant, as it is compensated for in its internal calculations.

[0046] The above describes the calculation of (Re) based on the assumption that the target is on the x axis. The calculations similarly apply for cases in which the target is on the y axis or any combinations of x and y axes.

[0047] The diagram and calculations with reference to FIG. 4 refer to displaying a target at position A for measuring ca and ea. For greater accuracy and averaging, the calculation can be performed also by presenting a target at different locations, e.g. at a second location B by measuring cb and eb.

[0048] With reference now to FIG. 5, a flow chart of a baseline measurement session 30 at an initial time point according to one or more embodiments is shown. To begin the baseline measurement session, the subject S is positioned at a known distance from the display screen 14 (block 32). This step may include removing the subject’s glasses, if any. A check is performed, such as by the eye tracker 12 in communication with the controller 22, to ensure that the subject’s head is within an allowed x, y, z headbox volume 16 (block 34). If not, the subject’s head may be repositioned before proceeding (block 36). As an optional next step, calibration of the eye tracker 12 may be performed (block 38). During such a calibration, the subject S may be shown targets 20 on the display screen 14, where the subject’s gaze position errors across the display screen 14 (in relation to a theoretical eye tracker 3D model) are detected and received by the eye tracker 12 and controller 22, and an interpolation is performed across the display screen 14. If an estimate of the absolute radius of rotation is required, the pre-calibrated deviation in relation to the targets may be measured.

[0049] With continuing reference to FIG. 5, the controller 22 initiates a baseline measurement by displaying at least one target 20 across an axis of the display screen 14, and first deviations of the gaze position from the targets 20 are detected and recorded by the eye tracker 12 and controller 22 (block 40). After calibration of the eye tracker 12 (block 38), any repeated deviations are expected to be smaller. In one or more embodiments, the target 20 could move dynamically from an initial position (for instance, the center of the display screen 14) to a final position to facilitate or verify that the subject’s attention and gaze are on the target 20 during the baseline measurement.

[0050] The axis could be an x axis (i.e., right and left) of the display screen 14, a y axis (up and down) of the display screen 14, an oblique axis, or another axis. In one or more embodiments, the right eye and the left eye are tested separately during each baseline measurement, such as with the aid of occlusion glasses 26 as described above. Once the baseline measurement is complete, a counter value is increased (block 42) and then the controller 22 compares the counter value to a desired total number of baseline measurements (block 44). If the counter value has not reached the desired total number of baseline measurements, another baseline measurement may be initiated (block 40). A subsequent baseline measurement can display targets 20 along the same axis as a previous baseline measurement or along a different axis from a previous baseline measurement.

[0051] When the counter value is equal to the desired total number of baseline measurements, the first deviations from all baseline measurements are averaged in any known method such as average, median, etc. (block 46). In one or more embodiments, the first deviations for the right eye and the deviations for the left eye are averaged separately. Next, the averaged right eye deviations and the averaged left eye deviations are compared by the controller 22 to determine if they differ beyond a threshold (block 48). If so, an alert may be provided, such as by displaying a warning to the subject S on the display screen 14 (block 50). This concludes the baseline measurement session, wherein the averaged first deviations result in a baseline deviation that serves as a starting reference for the monitoring of myopia in the subject’s eye E.

[0052] Referring now to FIG. 6, a flow chart of a repeat measurement session 60 at a later time point according to one or more embodiments is shown. To begin the repeat measurement session, the subject S is positioned at a known distance from the display screen 14 (block 62). This step may include removing the subject’s glasses, if any. A check is performed, such as by the eye tracker 12 in communication with the controller 22, to ensure that the subject’s head is within an allowed x, y, z headbox volume 16 (block 64). If not, the subject’s head may be repositioned before proceeding (block 66).

[0053] With continuing reference to FIG. 6, the controller 22 initiates a repeat measurement by displaying at least one target 20 across an axis of the display screen 14, and second deviations of the gaze position from the baseline deviation are detected and recorded by the eye tracker 12 and controller 22 (block 68). In one or more embodiments, the target 20 could move dynamically from an initial position (for instance, the center of the display screen 14) to a final position to facilitate or verify that the subject’s attention and gaze are on the target 20 during the repeat measurement.

[0054] The axis could be an x axis (i.e., right and left) of the display screen 14, a y axis (up and down) of the display screen 14, an oblique axis, or another axis. In one or more embodiments, the right eye and the left eye are tested separately during each repeat measurement, such as with the aid of occlusion glasses 26 as described above. In one nonlimiting example, when the second deviation of the gaze position from the baseline deviation is recorded, comparisons may be made between like axes and like eyes.

[0055] Once the repeat measurement is complete, a counter value is increased (block 70) and then the controller 22 compares the counter value to a desired total number of repeat measurements (block 72). If the counter value has not reached the desired total number of repeat measurements, another repeat measurement may be initiated (block 68). A subsequent repeat measurement can display targets 20 along the same axis as a previous repeat measurement or along a different axis from a previous repeat measurement.

[0056] When the counter value is equal to the desired total number of repeat measurements, the second deviations from all repeat measurements are averaged (block 74). In one or more embodiments, the second deviations for the right eye and the deviations for the left eye are averaged separately. Next, the averaged right eye deviations and the averaged left eye deviations are compared by the controller 22 to determine if they differ beyond a threshold (block 76). If so, an alert may be provided, such as by displaying a warning to the subject S on the display screen 14 (block 78). The averaged second deviations result in a measured deviation that can be correlated to a relative axial length change for the subject’s eye E (block 80), as described further below. This concludes the repeat measurement session, wherein the relative axial length change provides an assessment of the progression of myopia in the subject’s eye E since a previous measurement session. Additional repeat measurement sessions can be performed after further intervals of time have elapsed to continue to monitor myopia progression.

[0057] The correlation between the measured deviation and the relative axial length change may be accomplished by using an empirically-built lookup table which may include a plurality of parameters or an artificial intelligence (Al) non-analytical correlation. In one or more embodiments, the lookup table may include one or more of the following parameters: 1) a theoretical model of a purely spherical eye; 2) an actual measurement database of deviations versus axial length change; 3) genetic factors of the subject; 4) gender of the subject; 5) absolute axial value of the subject; 6) absolute refraction of the subject; 7) age of the subject; 8) corneal curvature of the subject; 9) real position of the eye(s) in x, y, z space and head tilts during the repeat measurements and session; and 10) ethnicity of the subject. The factors compiled into the lookup table or artificial intelligence correlation may be generated independently through experimental data collection. Of course, the list above is not intended to be limiting or exhaustive, and other factors and/or data may also be included in the lookup table.

[0058] According to one or more embodiments, the display may be a computer screen, tablet screen, mobile device screen e.g. mobile phone, virtual reality (VR) headset or any device configured for displaying images.

[0059] In accordance with one or more embodiments, the calculations disclosed herein are based on the assumptions that the eye globe is a perfect sphere.

[0060] For simplicity, the calculations assume that the eye tracker 12 is located at the same distance from the eye as the screen 14. However, the same methodology may apply to cases in which the eye tracker 12 is not located at the same distance as the screen 14. In such cases, appropriate adjustments may be applied to similar calculation methods.

[0061] The method and system disclosed herein evaluate the axial length of a subject’s eye. Comparing between at least two measurements at different times provides an indication of the changes in axial length (eye elongation) of the subject’s eye in order to monitor the progression of myopia. Initial axial length may also be estimated, providing a “red flag” for certain myopia cases. The eye tracking-based measurement system and method disclosed herein allows for an affordable, child-friendly solution which may be done in an open space without head constraint using a digital environment. [0062] While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.