Computer-implemented method for determining a sequence of gaze directions and a device for tracking gaze direction
The integration of fundus imaging with DPI systems simplifies gaze tracking calibration and enhances accuracy by using retinal imaging to determine gaze directions, addressing the challenges of existing DPI methods.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- INOKO VIDI SP ZOO
- Filing Date
- 2026-01-21
- Publication Date
- 2026-07-30
AI Technical Summary
Existing gaze tracking techniques, particularly those using Dual Purkinje Image (DPI), face challenges in laborious calibration processes that require patient cooperation, are prone to errors due to assumptions about eye movement centers, and are cumbersome for individuals with disabilities or fatigue.
A method and device that integrate a fundus imaging system with a DPI system to simplify calibration by determining gaze directions using both anterior and retinal images, eliminating the need for precise patient alignment through synchronized optical axes and polynomial relationships between image pixels and screen coordinates.
Simplifies calibration, reduces errors, and provides accurate gaze direction tracking for diverse populations, including those with disabilities, by leveraging retinal imaging to determine actual gaze directions without requiring precise patient alignment.
Smart Images
Figure IB2026050558_30072026_PF_FP_ABST
Abstract
Description
Computer-implemented method for determining a sequence of gaze directions and a device for tracking gaze directionField of the invention
[0001] The invention concerns a computer-implemented method for determining a sequence of gaze directions and a device for tracking gaze direction. The invention is particularly applicable in medical diagnostics and in the implementation of humanmachine interfaces, in particular in virtual and augmented reality systems.State of the art
[0002] Gaze tracking techniques are known in the prior art and are used in a wide range of fields, including medical diagnostics and virtual reality solutions.
[0003] Techniques for determining and tracking gaze direction based on the observation of so-called Purkinje images, i.e. , images of a light source formed as a result of reflection from the outer surface and at least one of the inner surfaces of the anterior chamber of the eye, are known. These techniques are referred to as DPI (Dual Purkinje Image). DPI devices adapted for gaze tracking are available. They usually illuminate the anterior chamber from the side, obliquely. Some of these solutions are known from US20030123027A1 , US11003244, and EP3918571. The disadvantages of these systems include a laborious calibration process requiring determination of eye model coefficients individually for each patient.
[0004] Recording the calibration signal is problematic because it requires cooperation of the examined patient, who must understand the instructions, which may be problematic in children, the elderly, and patients with medical conditions, and correctly perform instructions relating to gaze direction (errors, diseases, and paralysis may prevent correct task execution).
[0005] It further assumes that the eye trajectory within the region covered by the gaze during fixation on a calibration target has a center of mass coinciding with the center of the fixation target. This assumption is not necessarily true, since the eye registers not a point on the retina, but an area.
[0006] The process is laborious in that it requires the patient to repeatedly and precisely direct the gaze in specific, different gaze directions. This requires concentration and, in a state of fatigue or disorientation, is prone to error, since regardless of the instructions, the patient may or may not direct the gaze in the indicated direction at a given moment.
[0007] EP4096497 discloses a method for determining a retinal trajectory, in which successive frames representing images of a portion of the retina are recorded using a first imaging device, and at least a first segment of the retinal trajectory is determined using relative displacements of at least a first subset of successive frames and recalculation thereof into coordinates corresponding to the time of frame acquisition. The invention further relates to an imaging device for use with the method according to the invention and to a computer program. The invention also relates to an imaging system adapted for sufficiently rapid acquisition of retinal images. Other systems are also known that enable eye movement tracking using fundus imaging devices configured to acquire retinal images. Available fundus imaging devices include fundus cameras, optical coherence tomography (OCT) tomographs, laser scanners, scanning laser ophthalmoscopes, and others.Problem to be solved
[0008] The purpose of the invention is to solve the problem of laborious calibration in DPI (Dual Purkinje Image) techniques and the problem of effective provision of information useful for observation of eye movement.Summary
[0009] A computer-implemented method for determining a sequence of gaze directions, comprising the step of directing a light source onto an anterior chamber of the eyeball, the step of recording, using a camera, an image of the anterior chamber of the eyeball as well as an image of the light source reflected from an outer surface of the anterior chamber of the eyeball and an image of the light source reflected from an inner surface of the anterior chamber of the eyeball, to obtain a sequence of images, and a step of applying an eyeball model together with calibration coefficients to determine a first gaze direction, is characterized in that it further comprises a step of recording an image of a portion of the retina of the eyeball. This portion of the retina of the eyeball corresponds to a predetermined range of gaze directions represented by an angle. The method further comprises the step of calculating a second gaze direction based on the position of the portion of the fundus in the recorded image, the step of calculating the aforementioned calibration coefficients based on at least one determined second gaze direction, and the step of determining an output sequence of gaze directions based on the sequence of determined first and / or second gaze directions.
[0010] The gaze tracking device according to the invention comprises a processing unit, a light source adapted to direct light onto the anterior chamber of the eye, and a recording system for recording a series of images of the anterior chamber of the eye together with an image of the light source reflected from the outer surface of the anterior chamber of the eyeball and an image of the light source reflected from an inner surface of the anterior chamber of the eyeball. The processing unit is adapted to apply an eyeball model together with calibration coefficients to determine a first gaze direction, characterized in that it further comprises a fundus imaging device connected to the processing unit for recording an image of a portion of the retina corresponding to a predetermined range of gaze directions represented by an angle. The processing unit is adapted to determine a second gaze direction based on the position of the portion of the retina of the eyeball in the image from the fundus imaging device and to determine the aforementioned calibration coefficients based on the second gaze direction.
[0011] Advantageously, the recording system and the fundus imaging device operate along a single optical axis. This solution significantly simplifies calibration and eliminates errors associated with calibration of the relative position and orientation of the systems.
[0012] Preferably, the device further comprises a display system for displaying messages to the patient. Connection of the display system to the processing unit ensures efficient communication with the patient and accelerates the calibration procedure and the examination.
[0013] Advantageously, the display system operates along the same optical axis as the recording system and the fundus imaging device.
[0014] Advantageously, the display system, the recording system, and the fundus imaging device are synchronized by the processing unit.Description of the drawings
[0015] The subject matter of the invention is illustrated in exemplary embodiments in the drawings, wherein Fig. 1 shows a schematic diagram of the device according to the invention, Fig. 2 schematically shows the eyeball, Fig. 3a and Fig. 3b show the operation of gaze direction analysis based on displacement of a portion of the retinal image, Fig. 4 schematically shows the optical path in the device according to an embodiment of the invention, Fig. 5 shows an image of the eye with indicated characteristic points, and Fig. 6a and Fig. 6b show exemplary time courses of thecoordinates of the eyeball trajectory as a function of time counted in frames obtained by sequential recording of the first and second gaze directions.Description of embodiments
[0016] The purpose of the eye tracking system is to determine which point on a screen the subject is looking at - to determine the gaze direction. The point of gaze and the gaze direction are related to each other by the axis of vision - one of the defined axes in the optical system of the eye, along which a beam of light propagates between the point at which the gaze is directed (the point of gaze) and the point on the retina, which approximately corresponds to the macula and, more precisely, to the preferred fixation point, i.e. , an area on the retina that is different for each individual.
[0017] A schematic diagram of the device for tracking gaze direction according to the invention is shown in Fig. 1 , with the observed eye schematically indicated. The method according to the invention is performed using this device. The eyeball 200, together with the parts relevant to the further description, is shown in Fig. 2.
[0018] The device according to an embodiment of the invention comprises a processing unit 190, which is a microcomputer adapted for acquisition, generation, and processing of signals. Connected to the processing unit is a light source 111 adapted to direct light onto the anterior chamber 210 of the eye 200, and a recording system 112 for recording a series of images of the anterior chamber 210 of the eye 200. In the image of the anterior chamber of the eye, images of the light source reflected from its surfaces are visible. The light source reflected from the surfaces of the anterior chamber 210 forms Purkinje images. In this embodiment of the invention, the so-called first Purkinje image is used, which is the reflection of the light source 111 from the outer surface 211, i.e., the anterior surface of the cornea, and the so-called fourth Purkinje image, which is the reflection from an inner surface, which in this embodiment is the posterior surface 242 of the lens 240. Alternatively, reflections from other surfaces may be used, for example from the inner surface 212 of the anterior chamber 210 of the eyeball 200 - the second Purkinje image. After calibration based on the position of these images, the gaze direction corresponding to eye rotation can be determined. The light source 111 and the recording system 112 together form a DPI module for tracking gaze direction based on the position of the Purkinje images.
[0019] The gaze tracking device further comprises a fundus imaging device 120 connected to the processing unit 190 for recording an image of a portion of the retinapredetermined angle a of incident light directions. It is advantageous that, for a gaze direction directed substantially straight ahead, the image includes a portion of the retina 220 with a significant number of contrast features facilitating displacement analysis. This portion of the retina contains the highest density of photoreceptors and is directed toward the objects on which the patient focuses the gaze. This portion is located in the vicinity of the so-called fovea 223, but usually does not coincide exactly with the fovea 223. This is advantageous in that the fovea is readily identifiable in the image and, based on its displacement in the image 320 from the fundus imaging device 120, the gaze direction can be readily determined. This is illustrated in Fig. 3a and Fig.3b schematically illustrating operation of the fundus imaging device 120.
[0020] The fundus imaging device 120 is directed substantially straight toward the eyeball 200, which it observes through a semi-transparent mirror 301 arranged at an angle. The device observes an area on the retina. Depending on the position of the eyeball 200, this area includes a portion of the retina 220 corresponding to the range of angles a. The image 320 from the device has boundaries corresponding to extreme positions within the range a. In the experiment shown in Fig. 3a and Fig. 3b, a screen presenting two points spaced apart by an angle a is displayed in the mirror 301.
[0021] In Fig. 3a, the gaze is directed at point 328. The image 320 shows the first portion 221 of the retina 220. The fovea 223 is visible in a first extreme position in the image 320.
[0022] In Fig. 3b, the gaze is directed at point 328. The image 320 shows the second portion 222 of the retina 220. The fovea 223 is visible in a second extreme position in the image 320.
[0023] The processing unit 190 ensures synchronization of the recording system 112, the fundus imaging device 120, and the display system 130, on which instructions for the patient or points or images prompting the patient to direct the gaze in a specific direction are displayed.
[0024] A schematic diagram of the optical path of the gaze tracking device according to an embodiment of the invention is shown in Fig. 4. In this device, the recording system 112, the fundus imaging device 120, and the display system 130 are integrated so that they share a common optical axis passing through the adjustable lens L3 and the mirror DM1. As a result, eye movements in accordance with instructions displayed to the patient by the display system 130 are observed from the same direction by the recording system 112 and the fundus imaging device 120.
[0025] The processing unit 190 receives data from the recording system 112 and the fundus imaging device 120. The processing unit 190 is adapted to determine the second gaze direction based on the position of portions 221, 222 of the retina 220 in the image 320 from the fundus imaging device 120. In this embodiment, this displacement is determined as proportional to displacement of the retina in the image within the range of angles, as illustrated in Fig. 3a and Fig. 3b.
[0026] The processing unit is adapted to determine the aforementioned calibration coefficients based on the second gaze direction obtained from the fundus imaging device 120 in this manner. It performs a calibration procedure known in the prior art, with the proviso that assumed directions corresponding to instructions displayed on the display system 130 are not substituted, but actual gaze directions, i.e. , the second gaze directions determined by the fundus imaging device 120, are substituted. This eliminates error resulting from lack of precision in determining the patient’s gaze direction toward the pattern.
[0027] The processing unit 190 is adapted to determine the first gaze direction based on analysis of Purkinje images reflected from the surfaces of the anterior chamber of the eye 200 and the obtained calibration coefficients. This is performed using the DPI technique known in the prior art, for example from US20030123027A1 , with several modifications.
[0028] It is important that, in the device according to the embodiment, the recording system 112 is directed substantially straight toward the eye from the center of the measuring system. The light sources 111 generating Purkinje images are positioned laterally
[0029] Recording an image of the eye from the center, i.e., substantially straight toward the eye, is important because:• the image of the eye is symmetrical with respect to the range of motion - image distortions introduced by eye rotation are similar in all directions,• all optical systems of the measuring system and the eye are coaxial, whereby calibration of the systems relative to each other is not required, and consequently the calibration procedure is less complex and image distortions are reduced,• the fundus imaging device 120 shares with the recording system 112 the anterior pupil plane of the eye on which Purkinje images are formed, since light is physically reflected from the cornea and the lens, as shown in Fig. 5..
[0030] The recording system 112 for recording an image of the anterior chamber of the eyeball 200 in this embodiment is a camera. This device is adapted to record an image of the eye together with reflections formed on the surfaces of the anterior chamber 210, including the lens 240. Light from the light sources 111 is reflected from the surfaces of the anterior chamber of the eye in such a manner that it is visible against the dark background of the pupil 230. This reflected light passes through the adjustable lens L3, the semi-transparent mirror DM1, the semi-transparent mirror DM2, and the adjustable lens L2 to the recording system 112 in the form of a camera with a CMOS sensor. A person skilled in the art is capable of routinely proposing various alternative recording devices for detecting displacement of the anterior chamber of the eye known in the prior art.
[0031] The fundus imaging device 120 connected to the processing unit 190 for recording an image of a portion of the retina 220 of the eyeball 200 is a laser scanner scanning within the range of angles a of gaze directions. Fig. 3a and Fig. 3b show scanning of an eyeball rotating in one plane, but by tracking displacements on the two-dimensional image 320 resulting from the scanning, deviation in two planes is determined in an identical manner - proportionally to displacement of portions 221, 222 of the retina visible in the image 320 relative to the range of scanning angles. Radiation from the LS scanner passes through lens L8, the beam splitter BS, the MEMS scanner, lens L6, lens L5, the PGS scanner, lens L4, the semi-transparent mirror DM1, the adjustable lens L3, the anterior chamber 210, the pupil 230, and the lens 240, and impinges on the retina 220 - in its portions 221, 222 - is reflected and returns through the lens 240, the pupil 230, the anterior chamber 210, the lens L3, the semi-transparent mirror DM1, the lens L4, the PGS scanner, the lens L5, the lens L6, the MEMS scanner, the beam splitter BS, and the lens L7 to the APD, in which the image 320 is formed.
[0032] The display system 130 for presenting stimuli to the patient in this embodiment is a screen, but other solutions for optically delivering stimuli, for example by laser projection, may be used. It is advantageous that the image / stimulus is presented on the retina 220 of the eye on the same plane / surface that is imaged by the fundus imaging device 120. The images displayed on the screen 301 pass through lens L1, lens L9, the semi-transparent mirror DM2, the semi-transparent mirror DM1, the adjustable lens L3, the anterior chamber 210, the pupil 230, and the lens 240, and impinge on the retina 220.
[0033] Focus in the recording system 112, the fundus imaging device 120, and the display system 130 is set independently on the respective parts of the eye. The recording system 112 is focused on reflections visible against the background of the pupil, whereas the fundus imaging device 120 and the display system 130 are focused on the retina. It is preferable that focus adjustment be independent, whereby different wavelengths of light may be conveniently used in each of the systems.
[0034] An image of the eye recorded by the camera constituting the recording system 112 is shown in Fig. 5.
[0035] According to the invention, by means of the light sources 111, the recording system 112, and the processing unit 190, the gaze direction is determined using the DPI method based on the relative position of Purkinje images P1 and P4 recorded against the background of the pupil, as shown in Fig. 5, as well as their position relative to its center. Differences in position are scaled in image units - pixels - and are mapped to rotation angles and gaze directions in the calibration procedure. In solutions known in the prior art, the camera observes the eyeball from the side or from below, which enables avoidance of the eyelashes and the nose.
[0036] Calibration consists in recording images while the patient looks at points on the screen having known coordinates. By using the fundus imaging device 120, it is possible to determine the actual coordinates at which the patient is looking or to determine an average if the patient has difficulty maintaining accurate and stable fixation. Subsequently, the coefficients of a functional relationship transforming pixels of the image into screen coordinates - gaze angles - are determined. In this embodiment, a polynomial relationship (K-th order polynomial) of the X and Y coordinates of the point at which the gaze is directed, known in the prior art, is adopted based on raw data obtained from analysis of Purkinje images:x = SSi1auxkyl, k, I 6 [0, K]>= SSi1WV fc. / e [0, / C]
[0037] In this embodiment, the raw data are represented by the vector P = [x, y] = p4 - p1. Where p1 and p4 are the coordinates of the first and fourth Purkinje images, respectively. Alternatively, P = pc - p1 is often used, where pc denotes the center point 231 of the pupil 230.
[0038] The coefficients akiand bkiin the calibration are determined, for example, by a least squares method. Easy acquisition of known gaze directions using the fundusimaging device facilitates formulation of an overdetermined system of equations based on observation of patient activity, without a laborious calibration procedure requiring precise gaze direction. The choice of polynomial order depends on the application. Second-order polynomials usually perform well; however, in some cases it is justified to use higher-order polynomials. In limited ranges of applications, a linear relationship may also be sufficient. The higher the polynomial order, the greater the number of calibration gaze directions determined by tracking of the retinal image that must be used. For a second-order polynomial, calibration according to 9 points / gaze directions is convenient. Use of retinal analysis to determine these gaze directions precisely, instead of requiring precise gaze from the patient, constitutes a significant simplification and at the same time results in improved accuracy. Enforcing precise gaze at 9 or more consecutive points is laborious even for rested and compliant patients.
[0039] It is advantageous in the calibration procedure to select those fragments of the eye trajectory that are of the highest quality and will introduce the smallest errors into the procedure, i.e., those for which the smallest contribution of the artifacts described above is expected. In practice, this concerns data recorded during fixation on a point, i.e., in particular, data acquired in the vicinity of blinks, low-quality data, etc., should not be used. Such calibration based on a rigid table of points would be very laborious. However, by tracking gaze directions using the fundus imaging device 120 throughout the calibration process, significantly more points can be obtained and the best ones selected, or overdetermined systems of equations can be formulated and the error reduced.
[0040] In the DPI technique, it is assumed that the eye moves as a rigid body -movement of each feature within the area is linearly related to rotation. This assumption is not necessarily valid due to artifacts associated with oscillatory motion of the lens caused by inertia and suspension on elastic fibers, as well as lens accommodation, changes in its thickness, and changes in the position of the reflection point P4 and the position of the pupil. After calibration, use of two systems for determining gaze directions allows the effect of lens oscillation to be captured and measured as a difference between gaze directions as a function of time.
[0041] Exemplary time courses of two coordinates of the trajectory recorded within the range of applicability of gaze tracking based on retinal imaging (FET) and gaze tracking based on Purkinje images are shown in Fig. 6. It is clearly visible that thetrajectory tracked on the basis of Purkinje images exhibits oscillations - this results from vibrations of the anterior chamber of the eyeball during rapid eye movements. These vibrations have a significantly smaller influence on retinal imaging. Thus, by recording, in accordance with the invention, sequences of the first gaze direction (DPI) and the second gaze direction (FET), information about the mechanics of the eyeball can be obtained. In practice, when applying the method according to the invention after calibration, the first gaze direction may always be used - the DPI technique operates over the widest range of angles. Both the first and second gaze directions may be used, which provides valuable diagnostic information, or, depending on orientation of the eyeball, the second gaze direction may be selected as more accurate within the range of retinal image tracking and, outside this range, the first gaze direction as less accurate.
[0042] When determining gaze direction based on the retinal image, there is no calibration problem for the patient, since only the device requires calibration. Calibration consists in determining a relationship between a signal controlling image formation (e.g., a signal controlling the scanners) and the angle between beams forming extreme points of the image. This makes it possible to generate an image with a known angular size. This angular size directly corresponds to rotation of the gaze direction.
[0043] Calibration of the fundus imaging device 120 for determining gaze directions may be performed according to the scheme shown in Fig. 3a and Fig. 3b. The laser scanner constituting the fundus imaging device 120 sweeps a light beam - Fig. 3a and Fig. 3b show extreme beams. The retinal image is obtained by sweeping its area with the beam. The extreme beams enter the eye at angles differing by an angle a and form an image having an angular size a. Depending on axial length of the eyeball, the geometric size of this range on the retina will vary - for an average eye it is approximately 300 pm / degree. The eyeball 200 looking at the edge of the scanning range, i.e. , at point 328, sets the axis of vision parallel to one of the extreme beams. In this eye position, the macula 223 is visible at the edge of the image. When the eye rotates by an angle a, the axis of vision becomes parallel to the other extreme beam, the subject sees the other edge of the scanning area, i.e., point 329, and in the image 320 of the retina 220 the macula is located at the opposite edge of the retinal image. For a retinal image having an angular size defined in this manner, eye rotation in angular units is equal to the product of the angular size of the image and the relativedisplacement of a selected feature visible in the image relative to the image size. There is no dependence on a particular patient; hardware calibration concerns only control of the angles of the scanning beams. Points on the screen at which the subject is looking may also be converted into angular units, since the relationship between screen coordinates and angular coordinates of the beams depends solely on optics of the systems and may be calculated or calibrated by hardware.
[0044] During calibration, it is sufficient that the patient look at points 328 and 329 on the screen at a known angular separation, images of the eye are recorded, differences in feature coordinates are calculated, and these are mapped to coordinates of points on the screen.
[0045] The configuration shown in Fig. 4 is exemplary. It is apparent to a person skilled in the art that directions and deflection points of the optical paths may be configured differently using a number of elements known in the prior art, including lenses, objectives, or beam-splitting elements such as dichroic mirrors, beam-splitting films, and others. The system may employ free-space optics or fiber optics. In particular, in the fundus imaging device 120, illumination and detection may be implemented by means of a fiber-optic splitter.Industrial applicability of the invention
[0046] The invention is applicable in diagnosis of eye conditions and in assessment of eye dynamics and organs responsible for eye motility. The invention is also applicable in augmented reality and virtual reality solutions, enabling precise and rapid gaze direction tracking and thereby faster and more precise interfaces.
[0047] The invention further enables tracking, within a certain angular range, not only orientation of the eyeball 200, but also directly the gaze direction. Comparison of results under dynamic conditions provides valuable information about the condition of the apparatus controlling the lens of the eyeball.
[0048] The invention provides a device and a method for quantitative determination of changes in gaze direction also for individuals who cannot be examined using conventional video trackers alone due to calibration problems, including children, elderly persons, patients, persons with dementia, uncooperative subjects, or persons with impaired vision.
[0049] By means of the invention, advantages of the technologies are combined - a wide range of measured angles from Purkinje images with high precision and simple calibration of a system operating on the basis of the retinal image.
[0050] Additionally, the invention provides access to information about relationships between signals recorded from the anterior part and the posterior part of the eye -enabling determination of movement of, for example, the lens relative to the eyeball and, consequently, obtaining information about biomechanical properties of the human eye or ocular biometry.
[0051] By comparing information about gaze direction obtained from the front (anterior chamber) and from the back (retina) of the eye, it is possible to obtain information about ocular biometry (its geometric dimensions and distances between light-reflecting layers relative to one another) and about accommodation of the lens and its mechanical properties, since knowing angles of rotation of the eye determined from the posterior part of the eye, it is possible to separate a component related to movement of the eyeball from movements of the lens relative to the eyeball derived from measurements of the anterior part.
[0052] The processing unit referred to in the present description may be implemented in the form of any device capable of performing data processing, such as a microcontroller, microcomputer, general-purpose processor, a Field Programmable Gate Array (FPGA), or an Application Specific Integrated Circuit (ASIC). Implementation of this processing unit may include both hardware and software.
[0053] The method according to the invention may be implemented as a computer program which, when executed on the described processing unit, controls operation of the recording system, the fundus imaging device, and the display system. The computer program may be stored on a data carrier such as non-volatile memory (e.g., flash memory, SSD) or on an optical medium, or may be transmitted in the form of a digital signal.
[0054] Execution of the computer program on the processing unit enables implementation of all stages of the method according to the invention, including recording of data, processing thereof to obtain an image, and displaying the image on the display system. The software may also enable control of parameters of the recording system and the fundus imaging device, ensuring flexibility and adaptation of system operation to specific user requirements or operating conditions.
[0055] In this manner, the invention may be implemented both in hardware form and in software form, while maintaining the possibility of adaptation to different device configurations and application environments.
Claims
Patent claims1. A computer-implemented method for determining a sequence of gaze directions, comprisinga step of directing a light source onto an anterior chamber (210) of an eyeball (200),a step of recording, by means of a camera, an image of the anterior chamber (210) of the eyeball (200) includingan image of a light source reflected from the outer surface (211) of the anterior chamber (210) of the eyeball (200), and an image of the light source reflected from an inner surface (212, 242) of the anterior chamber (210) of the eyeball (200), so as to obtain a sequence of the images,a step of applying an eyeball model (200) including calibration coefficients to determine a first gaze direction,characterized in that it further comprisesa step of recording an image of a portion (221, 222) of the retina (220) of the eyeball (200), wherein said portion (221, 222) of the retina (220) of the eyeball (200) corresponds to a predetermined range of gaze directions represented by an angle (a),a step of calculating a second gaze direction based on the position of the portion (221, 222) of the retina (220) of the eye in the recorded image, a step of calculating said calibration coefficients based on at least two determined second gaze directions, anda step of determining an output sequence of gaze directions based on the sequence of determined first and / or second gaze directions.
2. A gaze tracking device comprising a processing unit (190) and a light source (111) connected thereto and adapted to direct light onto an anterior chamber (210) of an eye (200), and a recording system (112) for recording a series of images of the anterior chamber (210) of the eye (200) includingan image of the light source reflected from the outer surface (211) of the anterior chamber (210) of the eyeball (200), andan image of the light source reflected from an inner surface (212) of the anterior chamber (210) of the eyeball (200),wherein the processing unit (190) is adapted to apply an eyeball model (200) together with calibration coefficients to determine a first gaze direction, characterized in thatit further comprises a fundus imaging device (120) connected to the processing unit (190) for recording an image of a portion (221, 222) of the retina (220) corresponding to a predetermined solid angle (a) of gaze directions, whereinthe processing unit (190) is adapted to determine a second gaze direction based on the position of the portion (221 , 222) of the retina (220) of the eyeball (200) in the image from the fundus imaging device (120) and to determine said calibration coefficients based on the second gaze direction.
3. The device according to claim 2, wherein the recording system (112) and the fundus imaging device (120) operate along a single optical axis.
4. The device according to claim 2 or 3, wherein it further comprises a display system (130).
5. The device according to claim 4, wherein the display system (130) operates along the same optical axis as the recording system (112) and the fundus imaging device (120).
6. The device according to any one of claims 1 to 5, wherein the display system (130), the recording system (112), and the fundus imaging device (120) are synchronized by the processing unit (190).