Method for determining a gaze direction of an eye

Description

[0001] The present invention relates to a method for determining a viewing direction of an eye, a projection device for data glasses, data glasses, a computer program, a machine-readable storage medium and an electronic control device. State of the art

[0002] The purpose of oculography, or eye tracking, is to record eye movement and determine gaze direction. The technology is widely used in consumer research, for example, in observing reading flow to optimize advertising placement.

[0003] In data glasses, such as helmet-mounted or head-mounted displays (HMDs) or head-worn displays (HWDs), the technology is also used to adapt the displayed image content depending on the user's viewing direction. On the one hand, for example, to reduce electrical power, the display of high-resolution image content can be limited to the area of ​​sharpest vision and lower-resolution content can be shown peripherally. On the other hand, especially in augmented reality systems, which are partially or fully transparent systems that combine a natural perception of the environment with the display of virtual content, information, supplements, and hints can be displayed only for viewed objects for reasons of clarity.

[0004] Technologically, the available systems can be divided into the following categories: Camera-based systems: A camera is directed from the spectacle frame onto the eye. Some systems also incorporate one or more infrared light sources, which may be modulated. The direction of gaze is determined from the image data using an eye model, contrast-based detection of the pupil (dark vs. bright pupil), limbus tracking (contrast boundary between cornea and sclera), or localization of the corneal reflex. Systems have also been described that project a pattern (usually a point cloud or line pattern) onto the eye using structured light and determine the direction of gaze based on the characteristic change in shape in the area of ​​the cornea compared to the area of ​​the sclera.

[0005] Scanned laser systems: A laser source integrated into the frame, for example, is guided over the eye by a micro-electro-mechanical (MEMS) mirror. Methods are known that determine the direction of gaze based on the corneal reflex.

[0006] Electrical measurement: The measurable electrical fields created by ion displacement during muscle contraction can be recorded as a so-called electromyogram. When this method is used in the eye area, it is referred to as an electro-oculogram. By processing the derived electrical signals, eye movement can also be determined. Electro-magnetic measurement: If one coil is located in the magnetic field of another coil, the magnetic coupling of the two coils depends on their relative alignment. If one coil is integrated into the eyeglass frame and another coil into a contact lens, the direction of gaze can also be determined.

[0007] DE 10 2016 226 294 A1 describes a method for determining the refractive power of a lens in an eye. In this method, a laser beam is redirected to the lens of the eye via at least one beam deflection unit and at least one coupling element. Based on an evaluation of the backscattered and / or reflected radiation, in particular based on optical feedback interferometry, an optical path length is determined, and the refractive power of the lens is deduced from this. The evaluation of the backscattered and / or reflected radiation is carried out in particular based on optical feedback interferometry. The measurement principle underlying the method is preferably based on the method also known as self-mixing interference (SMI). Disclosure of the invention

[0008] The method is used to determine the direction of gaze of an eye. This is particularly the case with the eye of a user of data glasses.

[0009] Data glasses can be understood as a human-machine interface (HMD). The term data glasses also includes video glasses, a helmet display, or a VR helmet.

[0010] The user's gaze direction is defined by a reference point, which could be, for example, the center of the pupil, and a direction vector along which the eye sees, which could be defined, for example, by a vector through the center of the pupil and a point on the retina, such as the macula. The reference point can change, for example, due to shifts in the glasses relative to the eye.

[0011] In the method, a laser beam emitted by a laser source is scanned via a reflection element and a deflection element over at least two scanning points on the eye, i.e., the surface of the eye. The at least two scanning points are located on the sclera and / or the iris and / or the pupil of the eye.

[0012] The reflection element serves to reflect the light beam onto the deflecting element. A reflection element can be understood, for example, as a mirror, in particular a micromirror or an array of micromirrors, or a hologram. By means of the reflection element, the beam path of the light beam can be adapted to given spatial conditions. For example, the reflection element can be implemented as a micromirror. The micromirror can be designed to be movable, for example, have a mirror surface that can be tilted about at least one axis. Such a reflection element offers the advantage of a particularly compact design. It is further advantageous if the reflection element is designed to change an angle of incidence and, additionally or alternatively, a point of impact of the light beam on the deflecting element. This allows the light beam to sweep across the deflecting element over a wide area, in particular in rows and columns.

[0013] The deflection element can be a holographic element or a freeform mirror.

[0014] A holographic element can be understood, for example, as a holographic optical component (HOE), which can, for example, fulfill the function of a lens, a mirror, or a prism. Depending on the design, the holographic element can be selective for certain wavelengths (light colors) and angles of incidence.

[0015] Available photosensitive materials (e.g., photopolymer films) for creating holographic elements can only be exposed in the visible wavelength range. However, the use of holographic elements in the near-infrared range would be a significant advantage for this process, so that the measurement process is invisible to the user and, for example, does not interfere with the information displayed by data glasses. Using appropriate measures, such as angle correction and / or optical immersion setups, the photosensitive material can be exposed with a suitable function in the visible wavelength range so that the holographic element exhibits the desired optical target function when used in the near-infrared.

[0016] Holographic elements can be manufactured in large quantities, particularly through optical copying processes. If the holographic element fulfills optical functions that can be exposed into the photosensitive material using simple point or collimated light sources, the copying (manufacturing) process is significantly simpler. This allows the holographic element to be manufactured very cost-effectively.

[0017] The holographic element can be transparent, allowing image information on the lens to be overlaid with the surrounding environment.

[0018] In the method, a laser beam emitted by a laser source is scanned via a reflection element and a deflection element over at least two scanning points on the eye.

[0019] Preferably, the laser beam is scanned over a large number of scan points.

[0020] Here, the scan points are distributed on the surface of the eye in such a way that the eye's direction of gaze can be determined. This is preferably the case when the scan points on the surface of the eye cover at least part of the pupil. It is further preferred if the scan points on the surface of the eye cover the entire eye. The terms "covering" or "covering" do not imply that every single point must be scanned. It is sufficient if the envelope of the scanned points is larger than the part of the eye or the entire eye.

[0021] The method uses a self-mixing effect of the scanned laser beam reflected from the eye into the laser source to determine the optical path length from the laser source to the at least two scan points on the surface of the eye and a reflectivity of the eye at the at least two scan points.

[0022] In the self-mixing effect, also called "laser self-mixing," the coherent radiation emitted by a laser is scattered by a surface, and a portion of this radiation returns to the laser cavity, the optical resonator. If twice the distance to the scatterer corresponds to an integer multiple of the wavelength, the backscattered radiation is in phase with the radiation in the laser cavity. It thus adds constructively to the radiation present there, reducing the laser threshold and thus increasing the laser's output power. If the distance from the scatterer and thus the optical path length is changed, positive or negative interference repeatedly occurs within the laser cavity depending on the distance, so that the laser power is modulated in a sinusoidal pattern between a radiation maximum and a radiation minimum.Similar intensity modulations are achieved when the wavelength of the laser is modulated or when the frequency of the backscattered laser light changes, e.g. due to the Doppler effect, which occurs when reflected from moving objects.

[0023] If the optical radiation power is measured by a photodiode (monitoring photodiode), the change in amplitude of the radiation power can be used to determine the change in intensity of the backscattered laser power. By analyzing the number of oscillations, e.g., by counting the zero crossings or the maximum values, the number of oscillations, i.e., the cycles of constructive and destructive interference, can also be determined. With a known laser wavelength and modulation, the distance and any changes in distance between the laser cavity and the scatterer, as well as the scatterer's velocity component parallel to the laser beam, can be determined. In addition to the distance and velocity of the scatterer, the absolute intensity of the backscattered signal can be used to calculate the scatterer's reflectivity.Accordingly, the laser preferably has an integrated photodiode which measures the optical radiation power of the laser.

[0024] A surface profile of the eye is determined by using the self-mixing effect, which causes a modulation of the laser power during scanning of the laser beam across the eye, to determine a change in the optical path length from the laser source to a current scan point on the surface of the eye. This is referred to here as implementation variant 2.

[0025] The laser power modulation is a sinusoidal function of the change in the optical path length from the laser source to the current scan point on the ocular surface. The measured modulation can thus be used to determine the change in the optical path length.

[0026] This means that, based on the known geometric arrangement of the laser source, the reflection element, and the deflection element, as well as the known optical properties of the deflection element, the distance from the laser source to the surface of the eye can be determined for each scan point. Since the geometric arrangement of the laser source, the reflection element, and the deflection element are known, the surface profile of the eye can also be determined, which in turn allows the direction of gaze to be determined.

[0027] A change in the optical path length causes the laser power modulation to pass through several extreme points due to constructive and destructive interference of the self-mixing effect. The relative shape change can be determined by analyzing the number of extreme points passed through.

[0028] Since the wavelength of the laser radiation used is preferably in the near infrared and thus on the order of a micrometer, a change in the surface profile of the eye of, for example, 1 mm causes more than 2000 extreme points to pass through. This shows that the surface profile of the eye can be measured very precisely using this technique. As an alternative to the feature mentioned above, a gradient of a modulation of the laser power can be used to determine a gradient of the surface profile of the eye. Alternatively, a gradient of the change in distance can be linked to a temporal separation of the extremes or zero crossings of the optical laser power. Therefore, the frequency of the temporal extremes can also be used as a measure of the shape and thus of the direction of gaze.

[0029] According to the claimed subject matter, an embodiment according to which different reflectivities of different parts of the eye are determined and the determination of a change in the optical path length are combined.

[0030] This has the advantage that the surface profile of the eye can be determined much more accurately.

[0031] According to a preferred embodiment, the eye's gaze direction is additionally determined based on different reflectivities of different parts of the eye at the at least two scan points. This embodiment is referred to herein as implementation variant 1.

[0032] The different parts of the eye are the sclera, the iris, and the pupil. The different reflectivities of the sclera, iris, and pupil, obtained from the scan of the eye, can be used to advantageously determine the direction of gaze of the eye.

[0033] According to a preferred embodiment, the gaze direction of an eye is additionally determined using the "red-eye effect" by searching for a scanning point on the eye at which the laser beam is reflected by the retina of the eye. This embodiment is referred to herein as implementation variant 1b.

[0034] The well-known "red-eye effect" is caused by a reflection from the retina. If the light source is positioned approximately on the observer's optical axis, the angles of incidence and reflection of the radiation are equal, and a strong reflection is observable. At all other angles, the pupil appears dark, because the angles of incidence and reflection of the radiation do not match, and the radiation power is absorbed by multiple reflections in the eye.

[0035] This feature advantageously allows for an accurate determination of the pupil position, since the reflectivity of the pupil or the retina visible behind it differs greatly from the reflectivity of other parts of the eye.

[0036] According to a further preferred embodiment, the deflecting element is arranged and configured such that the laser beam is deflected by the deflecting element in the same direction for each scanning position of the reflection element. This direction is preferably parallel to at least one viewing direction of the eye and more preferably to a viewing direction straight ahead. The viewing direction straight ahead is defined by the viewing direction being substantially perpendicular to the plane of the spectacle lens.

[0037] This embodiment of the HOE is preferred for implementation variant 1 and implementation principle 2.

[0038] In this case, the incident laser beams, which depend on the scanning angle, are deflected into parallel exiting laser beams. Since the orientation of the deflection element and the reflection element is fixed by the spectacle frame, the exit position of the laser beam from the deflection element in the direction of the eye is also fixed. This advantageously eliminates the need for a further coordinate system transformation depending on the distance of the eye from the deflection element.

[0039] According to yet another preferred embodiment, the deflection element is arranged and configured such that for each scanning position of the reflection element, there is an eye position whose viewing direction is parallel to the propagation direction of the laser beam deflected by the deflection element in this scanning position. In this case, the laser beams coming from the deflection element are preferably convergent and intersect at a point inside the eyeball, preferably at the pivot point of the eye.

[0040] This advantageously ensures that for every viewing angle of the eye, at the point where the gaze encounters the deflection element, a laser beam exists that runs parallel to the visual axis. Furthermore, an increase in resolution is achieved by limiting the angular range.

[0041] According to another preferred embodiment, the deflection element is arranged and configured such that the laser beams deflected by the deflection element for different scanning positions of the reflection element are divergent. Preferably, the beams are divergent and intersect at a point that lies at a point in the viewing direction, preferably in a viewing direction straight ahead.

[0042] This embodiment is particularly preferred for implementation variants 1 and 2. If the area of ​​the deflection element is smaller than the entire eye area, this implementation advantageously allows for an expansion of the measurement range, e.g., to also include skin areas around the eye for calibrating a reference coordinate system. The field of view is defined as the angular range that can be seen from the eye. The eye area is defined as the area in which the eye can be located.

[0043] According to a preferred embodiment, the deflection element is arranged and configured such that the deflection element has at least two different regions, each region of the deflection element deflecting the laser beams incident on it to a respective point on the eye.

[0044] Preferably, the different areas are disjoint.

[0045] The deflecting element preferably has a plurality of regions, wherein the points deflected onto the eye by the plurality of regions are evenly distributed over the eye. According to a further preferred embodiment, such a region is a horizontally running line or scan line of the deflecting element, which is preferably arranged on the spectacle lens. Even more preferably, the deflecting element has a plurality of horizontally running line-shaped regions, wherein each line-shaped region or several line-shaped regions deflects the laser beams incident on it to a point on the eye. There is a one-to-one assignment between the horizontal regions and the scan points on the eye, wherein each region is assigned to only one point and each point is assigned to only one region.

[0046] This embodiment is advantageous if the measurement quality is insufficient in a fast scanning system or, in other words, if the light yield per measuring point is too low.

[0047] The at least two different areas are defined analogously to the eyebox concept of an HOE, which, similar to a parabolic mirror, focuses the rays onto a point on the eye, the so-called eyebox. This advantageously allows the evaluation electronics, especially in implementation variant 1, to average the received signal for a longer period, thereby improving the signal-to-noise ratio.

[0048] In a high-resolution projection system with, for example, 720 lines, there are sufficient measuring points for measuring the viewing angle with a relatively long dwell time at one measuring point.

[0049] The projection device for data glasses comprises a laser source for emitting a light beam. Furthermore, the projection device comprises a deflection element arranged or capable of being arranged on a lens of the data glasses for deflecting the laser beam toward a user's eye and / or for focusing the laser beam. Furthermore, the projection device comprises a reflection element for reflecting the laser beam onto the deflection element and an electronic control unit.

[0050] The projection device is designed, and the electronic control unit is configured, to perform a method for determining a viewing direction of an eye. Thus, the projection device can achieve the same advantages as the method described above.

[0051] The data glasses comprise a lens and a projection device as described above, with the deflection element arranged on the lens. The data glasses, like the projection device, achieve the same advantages as the method described above.

[0052] The invention further comprises a computer program configured to carry out the described steps of the method, in order to be able to carry out the above-described method using the projection device with this computer program. Furthermore, the invention comprises a machine-readable storage medium on which such a computer program is stored, as well as an electronic control unit configured to carry out the steps of the described method using a projection device. Such an electronic control unit can be integrated, for example, as a microcontroller in a projection device or data glasses. Short description of the drawings

[0053] Embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. Figure 1 shows a schematic representation of an arrangement of data glasses according to an embodiment and an eye of a user. Figure 2 shows a schematic distribution of scan points on a scanned eye according to an embodiment of the method. Fig. 3 shows a schematic flow diagram of a method according to an embodiment of the invention. Figures 4 , 5 , 6 , 7 , 8 , and 9 each show a schematic representation of an arrangement of data glasses according to an embodiment and of a user's eye, wherein the physical properties of the deflection element designed as a HOE differ. Figure 10shows a data glasses with a spectacle lens according to the embodiment of the Figures 8 and 9 . Figure 11 shows schematic measurement data of the optical path length as a function of a horizontal scan angle according to an embodiment of the method. Figure 12 and 13 show schematic measurement results of an embodiment of the method. Figure 14 shows a block diagram of an embodiment of a projection device. Embodiments of the invention

[0054] Figure 1shows the basic functionality of the data glasses 400, in particular using implementation variant 1. The data glasses 400 have a spectacle lens 402 and a projection device 100. The projection device 100 has a scanner optics 152 and a deflection element 102, which in this embodiment is designed as a holographic element (HOE). The deflection element 102 is attached to the spectacle lens 402. The scanner optics 152 is arranged in a housing 105 and has a laser source 104, a collimation element 107, a reflection element 112, and an element (not shown) for beam shaping and / or collimation arranged after the reflection element 112.

[0055] A light beam 106 emitted by the scanner optics 152 is sent through an exit window toward the deflection element 102. The light beam 106 deflected by the deflection element 102 then strikes a user's eye 108. The scanner optics 152 is arranged in a housing 105 attached to the eyeglass frame 160 and the temple 150.

[0056] The projection device 100 can perform a method for determining a viewing direction of an eye.

[0057] In a further embodiment, the projection device can simultaneously perform a method for generating a visible image onto the retina and a method for determining a viewing direction of the eye. The optical functions of the HOE in the deflection element can differ for different wavelengths.

[0058] Figure 1shows two exemplary beam paths: a first beam path 131, in which the laser beam 106 strikes the eye 108 in the region of the sclera 114 and which has a first reflectivity, and a second beam path 132, in which the laser beam 106 strikes the eye 108 in the region of the cornea 123 and which has a second reflectivity. The first beam path 131 corresponds to a first mirror position of the reflection element 112, in which the distance from the reflection element 112 to the deflection element 102 is designated by the reference symbol s 11 and the distance from the deflection element 102 to the surface of the eye 108, in this case to the scan point 110 on the sclera 114, is designated by the reference symbol s 12.The second beam path 132 corresponds to a second mirror position of the reflection element 112, in which the distance from the reflection element 112 to the deflection element 102 is designated by the reference symbol s 21 and the distance from the deflection element 102 to the surface of the eye 108, in this case to the scan point 110 on the cornea 123, is designated by the reference symbol s 22. In the illustration of the . Figure 1 In the second mirror position of the reflection element 112, the laser beam 106 hits the eye 108 along the viewing direction 120, so that the laser beam 106 first hits the cornea 123 and then the retina 113.

[0059] When scanning the laser beam 106, it is moved in a zigzag pattern over the eyeball, similar to the scanning of earlier picture tubes, ie with a fast line frequency and a lower column frequency, as Figure 2 simplified. In a real system, for example, 1280 scan points are used, 110 per line and 720 lines.

[0060] Figure 3shows a method 200 for determining the viewing direction 120 of the eye 108. The method 200 starts in step 202 by moving the reflection element 112 into a first scanning position at which the laser beam 106 strikes a first scanning point 110 on the surface of the eye 108. In the subsequent step 204, a reflectivity of the eye 108 at the current scanning point 110 and the optical path length from the laser source 104 to the current scanning point 110 on the surface of the eye are measured. Depending on the embodiment of the method 200, different types of HOEs are suitable for this purpose as deflection elements 102. In the next step 206, a query is made as to whether all measuring points 110 have already been measured or not. If all measuring points 110 have already been measured, the method continues with step 208, in which the viewing direction 120 is determined based on the measured measuring points 110.If not all measuring points 110 have been measured yet, the method continues with step 202, in which the next scan point is set on the reflection element 112. A check mark indicates an affirmative answer to the query, a cross indicates a negative answer to the query.

[0061] The determination of the viewing direction 120 also differs in the respective procedures of implementation variants 1, 1b and 2.

[0062] Figure 4 shows a data glasses 400 and the eye 108, which is in the same position as in Figure 1 However, the HOE of the deflection element 102 is in the embodiment of the Figure 4 designed so that the laser beams 106 are reflected parallel by the deflection element 102. Here, the laser beams 106 are parallel to the temple 150. The direction of the laser beam 106 also corresponds to the viewing direction 120 of the eye 108, which in the Figure 4 is shown.

[0063] This is particularly advantageous for implementation variants 1 and 2. Figure 5 shows as well as Figure 4 a data glasses 400 and the eye 108. In contrast to the embodiment of the Figure 4 is the HOE of the deflection element 102 of the Figure 5 However, it is designed in such a way that the laser beams 106 are reflected convergently by the deflection element 102. This is particularly advantageous for implementation variant 1b and increases the spatial resolution of the measurement.

[0064] Figure 6 shows as well as the Figures 4 and 5 a data glasses 400 and the eye 108. In contrast to the embodiments of the Figures 4 and 5 is the HOE of the deflection element 102 of the Figure 6However, it is designed such that the laser beams 106 are reflected divergently by the deflection element 102. If the area of ​​the HOE is smaller than the entire eye area, this implementation allows the measurement range to be expanded, e.g., to also include skin areas around the eye for calibrating a reference coordinate system.

[0065] This is particularly advantageous for implementation variants 1 and 2.

[0066] Figure 7 shows as well as the Figures 4 , 5 and 6 a data glasses 400 and the eye 108. In contrast to the embodiments of the Figures 4 , 5 and 6 is the HOE of the deflection element 102 of the Figure 7 However, it is designed in such a way that there are several eyeboxes which, similar to a parabolic mirror, focus the rays at a scanning point 110 of the eye.

[0067] This is particularly advantageous when the measurement quality is insufficient in a fast-scanning system. This allows the evaluation electronics to average the received signal for a longer time, especially with implementation principle 1, thus improving the signal-to-noise ratio.

[0068] Another variant of this approach is in Figures 8 , 9 and 10 Here, the eyebox regions or areas 410 on the spectacle lens 402 each extend over an entire line length of a scan line, as Figure 10 . Furthermore, the eyebox regions or areas 410 can extend over multiple scan lines. All laser beams 106 within an eyebox region 410 are each imaged onto a fixed position on the eye 108, the so-called eyebox, as shown in Figure 8 e.g. for eyebox region A and Figure 9 for example, eyebox region G can be seen.

[0069] In a high-resolution projection system with, for example, 720 lines, there are sufficient scan points 110 for measuring the viewing angle with a relatively long dwell time at a measuring point.

[0070] Figure 11 shows an exemplary determination of an optical path length 250 as a function of the horizontal scan angle 260 according to implementation principle 2 using an HOE function according to Figure 4 A contribution 251 of the reference plane of the optical path length 250 describes an influence of the distance between the laser 104 and the deflection element 102. The surface profile 252 of the eye 108, which is also measured as a distance in this case, is superimposed on this. The viewing direction 120 of the eye 108 can be determined from the surface profile 252 of the eye 108.

[0071] Here, evaluation methods can be directly parameters of the curve shape such as inflection points, intersection points, and extreme points, but so-called "template matching" can also be used, in which a model function is adapted to the measured data. A sketch of the results of a measurement according to implementation principle 1b is shown in the Figures 12 and 13 .

[0072] Figure 12 shows the reflected laser intensity plotted over the horizontal scan angle 260 and the vertical scan angle 261. Figure 13 shows a 2D projection of an intensity in the coordinate system of the eye 108. The center position of the pupil 118 and a direction of gaze can then be determined using signal processing methods.

[0073] Since laser 104 and photodiode are housed in the same chip, the red-eye condition is automatically met, so that the laser source and detector are on the same axis. Thus, the received light power increases as soon as the laser beam 106 passes through the pupil 118 to the reflective retina 113. Outside the pupil 118, only a small amount of light power is backscattered because the shape of the eye 108, i.e., the eyeball, reflects most of the power away from the detection axis, i.e., the viewing direction 120.

[0074] If the intensity scans are combined to form a two-dimensional array, for example, the maximum or the center of the reflection maximum can be determined after optional filtering and the viewing direction can be determined from this.

[0075] The filtering can be a spatial smoothing kernel, which smooths the signals so that software looking for a maximum of the intensity distribution does not find an outlier next to the main maximum.

[0076] Figure 14 shows a block diagram of the projection device 100 for the data glasses 400. The projection device 100 has the micromirror 112 with the associated control 170, an infrared laser 104, and a power monitoring photodiode 180 with the corresponding control 181. The photodiode 180 is integrated into the laser 104. Also shown is a microprocessor 190, i.e., an electronic control unit, for executing the program code, i.e., a computer program, for calculating a value 121 for the viewing direction 120, with the associated memory 195 for calibration information. The calculated value 121 for the viewing direction 120 can then be passed on to downstream system units.

Claims

1. Method (200) for ascertaining a direction of view (120) of an eye (108), wherein a laser beam (106) emitted by a laser source (104) is scanned via a reflection element (112) and a deflection element (102) over at least two scan points (110) on the eye (108); a self-mixing effect of the scanned laser beam (106), which is reflected off the eye (108) into the laser source (104), is used to determine, for the at least two scan points (110), the optical path length from the laser source (104) to the at least two scan points (110) on the eye surface, characterized in that a self-mixing effect of the scanned laser beam (106), which is reflected off the eye (108) into the laser source (104), is used to additionally determine the reflectivity of the eye (108) at the at least two scan points (110), and a surface profile (140) of the eye (108) is ascertained by virtue of the self-mixing effect, which causes a modulation of the laser power during a scanning of the laser beam (106) over the eye (108), being used to ascertain a change in an optical path length from the laser source (104) to a current scan point (110) on the eye surface, wherein the at least two scan points (110) are located on the sclera (114) and on the cornea (123).

2. Method (200) according to Claim 1, characterized in that the direction of view of the eye (110) is ascertained on the basis of different reflectivities of different parts of the eye (108) at the at least two scan points (110).

3. Method (200) according to Claim 1 or 2, characterized in that the direction of view of the eye (108) is ascertained by the "red-eye effect" by virtue of searching for a scan point (110) at which an angle of incidence of the laser beam (106) coincides with the direction of view of the eye (108).

4. Method (200) according to any of the preceding claims, characterized in that the deflection element (102) is a holographic optical element (HOE).

5. Method (200) according to any of the preceding claims, characterized in that the deflection element (102) is arranged and configured such that the laser beam (106) is deflected in the same direction by the deflection element (102) for each scan position of the reflection element (112).

6. Method (200) according to any of the preceding claims, characterized in that the deflection element (102) is arranged and configured such that for each scan position of the reflection element (112) there exists an eye position, the direction of view of which is parallel to the direction of propagation of the laser beam (106) deflected in this scan position by the deflection element (102).

7. Method (200) according to any of the preceding claims, characterized in that the deflection element (102) is arranged and configured such that the laser beams (106) that are deflected by the deflection element (102) for different scan positions of the reflection element (112) are divergent.

8. Method (200) according to any of the preceding claims, characterized in that the deflection element (102) is arranged and configured such that the deflection element (102) has at least two different regions (410), wherein each region (410) of the deflection element (102) deflects the laser beams (106) incident thereon to respective points on the eye (108).

9. Projection device (100) for a pair of smartglasses (400), wherein the projection device (100) comprises the following features: a light source (104) for transmitting a laser beam (106); a deflection element (102) arranged or arrangeable on a lens (402) of the smartglasses (400) for deflecting the laser beam (106) in the direction of an eye (108) of the user and / or for focusing the laser beam (106); and a reflection element (112) for reflecting the laser beam (106) onto the deflection element (102), and an electronic controller, characterized in that the projection device (100) is embodied and the electronic controller is configured to perform each step of the method (200) according to any of Claims 1 to 8.

10. Smartglasses (400) with the following features: a lens (402); and a projection device (100) according to the preceding claim, wherein the deflection element (102) is arranged on or in the lens (402).

11. Computer program configured to perform each step of the method (200) according to any of Claims 1 to 8 using a projection device (100) according to Claim 9.

12. Machine-readable storage medium on which a computer program according to the preceding claim is stored.

13. Electronic controller configured as constituent part of the projection device (100) according to Claim 9 to perform each step of the method (200) according to any of Claims 1 to 8.