Method and system for obtaining an intermediate finding of diagnostic relevance for an eye

Abstract

A method for obtaining an intermediate finding of diagnostic relevance for an eye comprising the steps of obtaining a feature of an eye based on data obtained in a first time period by an acquisition

Description

BACKGROUND OF THE INVENTION Eye health check-ups at an optometrist’s lab are carried out typically once every 1 or 2 years by people having already diagnosed eye conditions, e.g. near-sightedness or far-sightedness. In such a long time interval, a new negative eye condition can develop or a present eye condition can worsen. Furthermore, persons not visiting an optometrist or an ophthalmologist on a regular basis may not realise that a negative eye condition is developing. There is therefore a need for a method and a system providing an intermediate finding of diagnostic relevance for an eye, which does not rely on a consultation by an optometrist or by an ophthalmologist (referred to as professionals in the following). However, the lack of availability of diagnostic equipmentof a sufficiently high quality to enable examinations to be performed outside of these consultations can make this challenging, even if a qualified professional were available to analyse the results. Field of the invention The present invention relates to a method and a system for obtaining an intermediate finding of diagnostic relevance for an eye. SUMMARY OF THE INVENTION The invention is defined in the independent claims and the use claim. Optional embodiments of the invention are described in the dependent claims. In the following, reference is being made to one eye only. However, the teaching set forth below can be implemented for each eye separately or both eyes. Disclosed is a method for obtaining an intermediate finding of diagnostic relevance for an eye, comprising the steps of: obtaining, optionally by a feature data unit, a feature of an eye, wherein the feature of the eye is based on data obtained in a first time period by an acquisition unit, wherein the acquisition unit is part of a head-mountable display unit, HMD; obtaining, optionally by a reference data unit, a reference for the feature of the eye, wherein the reference for the feature of the eye is based on data obtained in a second time period by the acquisition unit, wherein the second time period precedes the first time period; and obtaining, optionally by an intermediate finding unit, the intermediate finding of diagnostic relevance for the eye based on the feature of an eye and the reference for the feature of the eye. Since the feature of an eye and the reference for the feature of the same eye are both based on data obtained from the same acquisition unit, the intermediate finding of diagnostic relevance for the eye is obtained from quantities based on data from the same data source allowing to detect time-dependent changes in the feature of the eye without the need to consult a professional due to the comparability of the data. Depending on the intermediate finding of diagnostic relevance for the eye a medical intervention can take place before further symptoms arise. In the above method obtaining the feature of the eye may comprise obtaining, during the first time period, as the data obtained in the first time period a current image of the eye, optionally using an imaging unit of the acquisition unit; detecting a current feature of the eye, optionally using a feature detecting unit, based on the current image of the eye; and obtaining the feature of the eye on the basis of the current feature. This allows to obtain data in the first time period by equipment not restricted to professionals. In particular, the imaging unit can be part of a gaze-tracking arrangement, for example a gaze tracking arrangement of the head-mountable display unit (HMD). The current image of the eye is obtained by the acquisition unit. In the method, obtaining, during the first time period as the data obtained in the first time period a current image of the eye, may comprise illuminating the eye with a predefined light composition, wherein the predefined light composition is generated by an image display unit of the HMD. This allows to image the eye under predefined light conditions in a simple manner, for example using a display element of a HMD. The image display unit optionally emits the predefined light composition by displaying a single, optionally predefined image or a predefined video. Optionally the predefined light composition comprises, or is formed by, light emitted by a light source of non-visible light which can be part of a gaze-tracking arrangement, for example a gaze-tracking arrangement of a HMD. Illuminating the eye with a predefined light composition simplifies comparison of data obtained from different images of the eye since no adjustment due to different light conditions is needed. The image display unit can illuminate one eye or both eyes. In the latter case, the intermediate finding of diagnostic relevance for an eye can be obtained by applying the present teaching to each eye, on the basis of only one image display unit can be used for illuminating of both eyes. However in many cases a pair of displays may be used to provide images to the user, in which case the illumination from each of these displays may be considered independently as appropriate. Optionally, the method comprises further saving to a trend database a current event data set, the current event data set comprising the current feature of the eye, a timestamp of the current feature of the eye, the timestamp indicating, when the current image of the eye, the current feature of the eye is based on, has been acquired, The current event data set may further 2 comprise the current image of the eye and / or of the feature of the eye obtained on the basis of the current feature. As already introduced with respect to the first and second time periods, and as will be introduced with respect to the third time period, data acquired in the respective time periods will be analysed. By saving the current event data set in the trend database this data is readily available for analysis. The trend database can be saved in a memory. In the method, obtaining the feature of the eye can comprise obtaining the feature of the eye on the basis of the current feature of the eye and a historical feature of the eye, wherein the historical feature of the eye is based on data obtained in a third time period by the acquisition unit, wherein the second time period precedes the third time period; and wherein the third time period precedes the first time period. The first time period may be short, in particular the first time period may have the lengths of the time needed to obtain a single image of the eye by the imaging unit. The third time period is longer than the first time period, optionally at least two times as long, optionally a few seconds or minutes long. The feature can be an average of the current feature of the eye and the historical feature of the eye or multiple historical features of the eye. The third time period can be followed directly by the first time period. By using the current feature of the eye and the historical feature of the eye as the basis for the feature of the eye, the influence of fast variations of the currently measured feature of the eye, which may represent variations due the measurement method used and may not represent variations due to changes in the condition of the eye, can be mitigated in the feature of the eye obtained by the method. It is possible that the at least one historical feature of the eye is based on at least one historical image of the eye obtained, optionally using the imaging unit, during the third time period and / or a feature of the eye based on the basis of the historical image of the eye. Therefore, the historical feature of the eye can be obtained easily, for example from data from the trend database, since depending on the timestamp, an element of the current event data set represents e.g. a historical image of the eye and / or a historical feature of the eye. The historical image of the eye is obtained by the acquisition unit, optionally by using the imaging unit. In addition, in the method, obtaining a reference for the feature of the eye may comprise obtaining, optionally by using the imaging unit, during the second time period, a reference image of the eye, and detecting, optionally using the feature detecting unit, a reference feature of the eye based on the reference image of the eye, and obtaining the reference for the feature of the eye based on the reference feature of the eye. The reference for the feature of the eye of the eye can be based on multiple reference features. The reference image of the eye is obtained by the acquisition unit, optionally by using the imaging unit. This represents a simple method step to obtain a reference for the feature of the eye since images, the reference for the feature of the eye is based on, are obtained by the acquisition unit, therefore no conversion due to use of different acquisition units is required. The third time period can start when a user first uses the acquisition unit and can be reset in response to an input of the user, for example when the user has consulted a professional. The length of the third time period can be in the order of seconds or minutes or hours or days. In addition the method step of obtaining the intermediate finding of diagnostic relevance for the eye based on the feature of the eye and the reference for the feature can comprise obtaining a reference threshold; and comparing a difference value indicative of a difference between the feature of the eye and the reference feature of the eye with the reference threshold. This allows to obtain the intermediate finding of diagnostic relevance for the eye (the intermediate finding), by a simple comparison of two numerical values. The intermediate finding may have two values: positive or negative - although further categorisation indicating a degree of positivity or negativity may also be provided in some implementations. The intermediate finding can be positive, e.g. in case the difference value indicative of a difference between the feature data and the reference data (optionally the absolute value of the difference value) is smaller than the reference threshold. The intermediate finding can be negative, e.g. in case the difference value indicative of a difference between the feature data and the reference data (optionally the absolute value of the difference value) is bigger than or equal to the reference threshold. The difference value may be an absolute value of a further value. The method can further comprise in response to obtaining a negative intermediate finding informing the person that the eye should be examined by a professional. Furthermore, obtaining the intermediate finding of diagnostic relevance for the eye based on the feature of the eye and the reference for the feature of the eye may comprise: obtaining a trend threshold; obtaining, based on data from a trend database, a time-dependent trend for the feature of the eye; and comparing the time-dependent trend with the trend threshold. A time-dependent trend for the feature of the eye can be obtained by e.g. linearly fitting the feature of the eye, obtained on the basis of the current feature data saved in the a trend database, as a function of the time indicated by the timestamp and using the slope / gradient of the linear fit function to identify a time-dependent trend. The absolute value of the gradient of the linear fit function can be compared with the trend threshold. When comparing the timedependent trend with the trend threshold and obtaining the intermediate finding, the intermediate finding can be negative when the feature is changing very fast, e.g. in case the absolute value of the gradient of the linear fit function is bigger than the trend threshold, even though the difference between the feature of the eye and the reference for the feature of the eye is smaller than the reference threshold. For clarification, the feature of the eye and the reference for the feature of the eye, and depending on the embodiment optionally the current feature of the eye or / and the historical feature of the eye or / and the reference feature of the eye belong to the same feature category in order to enable a meaningful analysis and / or comparison of the respective features. Intermediate findings can be obtained for different feature categories in parallel. The feature of the eye, and / or the current feature of the eye, and / or the historical feature of the eye and / or the reference for the feature of the eye can each represent a property of a reflection of light from the eye - these properties including a colour, intensity, or shape of reflected light, for example. The feature of the eye and the reference for the feature of the eye, and depending on the embodiment optionally the current feature of the eye or / and the historical feature of the eye or / and the reference feature of the eye, or their respective feature category, can be - a colour or colour component of the respective image of the eye; a colour or colour component of a certain element or section of the respective image of the eye; an average intensity of light reflected from the eye or reflected from a certain element or section of the eye; a value indicative of a deviation of a shape of a reflection from the eye from a reference shape of the reflection from the eye, wherein the reflection is optionally a reflection of a display element. For sake of simplicity, no distinction is made in this application between the respective feature / image and data representing such a feature / image. Furthermore, a system for obtaining an intermediate finding of diagnostic relevance for an eye is disclosed, the system comprising a head-mountable display unit, HMD, comprising an acquisition unit; a feature data unit configured to obtain a feature of an eye, wherein the feature of the eye is based on data obtained in a first time period by the acquisition unit; a reference data unit, configured to obtain a reference for the feature of the eye, wherein the reference for the feature of the eye is based on data obtained in a second time period by the acquisition unit, wherein the second time period precedes the first time period; and an intermediate finding unit, configured to obtain the intermediate finding of diagnostic relevance for the eye based on the feature of the eye and the reference for the feature of the eye. The system is configured to carry out a method as described above. Since the feature of an eye and the reference for the feature of the eye are both based on data obtained from the same acquisition unit, the intermediate finding unit obtains the intermediate finding of diagnostic relevance for the eye on quantities based on data from the same data source allowing to detect time-dependent changes in the feature of the eye without the need to consult a professional due to the comparability of the data. The system can comprise a computing unit, the computing unit comprising the feature data unit and the reference data unit and the intermediate finding unit, wherein the HMD further comprise a gaze-tracking arrangement, wherein the gaze-tracking arrangement comprises the acquisition unit. Thus, such a gaze-tracking arrangement has a dual purpose of gaze-tracking and providing data allowing to obtain an intermediate finding of diagnostic relevance for an eye. The computing unit can comprise a memory for storing the trend database. The gaze-tracking arrangement is part of a head-mountable display unit, HMD, wherein the HMD is optionally a virtual reality HMD or / and an augmented reality HMD. This allows the use of an HMD providing consistent close-range images of the eyes in order to obtain an accurate intermediate finding of diagnostic relevance for an eye. The computing unit may be part of the HMD or may be separate from the HMD. Such consistent close-range images can be provided for example when the acquisition unit comprises an imaging unit configured to obtain an image of the eye, and optionally the imaging unit comprises one or more inwards-facing cameras of the gaze-tracking arrangement directed towards the eye. The image of the eye obtained can be a current image of the eye, a historical image of the eye and or a reference image of the eye, depending on the timestamp of the image of the eye. The timestamp indicates the time and date of when the image of the eye has been acquired by the imaging unit. The lighting conditions for obtaining images of the eye can be controlled in a particular simple manner when the HMD comprises an image display unit, wherein the image display unit is configured to generate a predefined light composition for illuminating the eye and wherein the image display unit is further configured to display an image or video to the eye, e.g. to illuminate the eye with the predefined light composition as described above with reference to the method. This image can be a uniform white (or another colour) image, for example, however it can also be an image obtained during display of content to the user in the HMD, for example during playing of a game or during watching of a video. Such an image obtained during display of content (content image) can be predefined by a predefined time index or a predefined frame index; alternatively, the image may be any image associated with the content, with properties of that image (such as brightness and colour distribution) being considered when determining properties of the measured reflection. Optionally, this content image can be saved in the trend database in a current event data set, wherein the timestamp of this current event data set indicates the time when the current image of the eye, the current feature of the eye is based on, has been acquired while the eye has been illuminated with the predefined light composition by display of the content image to the eye. A video comprises a sequence of content images / frames. Furthermore, the use of such system described above, in particular comprising a HMD, to carry out a method described above is disclosed. The invention will be described in embodiments involving a gaze tracking system. These embodiments however shall not be regarded as limiting for the subject matter of the invention. Gaze tracking systems are used to identify a location of a subject’s gaze within an environment; in many cases, this location may be a position on a display screen that is being viewed by the subject. In a number of existing arrangements, this is performed using one or more inwards-facing cameras directed towards the subject’s eye (or eyes) in order to determine a direction in which the eyes are oriented at any given time. Having identified the orientation of the eye, a gaze direction can be determined and a focal region may be determined as the intersection of the gaze direction of each eye. One application for which gaze tracking is considered of particular use is that of use in head-mountable display units (HMDs). The use in HMDs may be of particular benefit owing to the close proximity of inward-facing cameras to the user’s eyes, allowing the tracking to be performed much more accurately and precisely than in arrangements in which it is not possibly to provide the cameras with such proximity. In addition, in case of closed HMDs, the light conditions, under which the inward-facing cameras operate, can be controlled with limited influence by light generated outside of the enclosed HMD. Gaze detection techniques, e.g. the use of a gaze-tracking arrangement, can be used for obtaining an intermediate finding of diagnostic relevance for an eye according to an embodiment. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates an HMD worn by a user; Figure 2 is a schematic plan view of an HMD; Figure 3 schematically illustrates the formation of a virtual image by an HMD; Figure 4 schematically illustrates another type of display for use in an HMD; Figure 5 schematically illustrates a pair of stereoscopic images; Figure 6a schematically illustrates a plan view of an HMD; Figure 6b schematically illustrates a near-eye tracking arrangement; Figure 7 schematically illustrates a remote tracking arrangement; Figure 8 schematically illustrates a gaze tracking environment; Figure 9 schematically illustrates system for obtaining an intermediate finding of diagnostic relevance for an eye according to an embodiment comprising a gaze tracking system; Figure 10 schematically illustrates a human eye; Figure 11 schematically illustrates a graph of human visual acuity; Figure 12 schematically illustrates obtained features of the eye in a first, a second and a third time period; Figure 13 schematically illustrates an overlap of an initial image and a current image; Figure 14 illustrates a method for obtaining an intermediate finding of diagnostic relevance for an eye according to an embodiment; and Figure 15 illustrates a further system for obtaining an intermediate finding of diagnostic relevance for an eye. DESCRIPTION OF THE EMBODIMENTS Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, embodiments of the present disclosure are described. In Figure 1, a user 10 is wearing an HMD 20 (as an example of a generic head-mountable apparatus - other examples including audio headphones or a head-mountable light source) on the user's head 30. The HMD comprises a frame 40, in this example formed of a rear strap and a top strap, and a display portion 50. As noted above, many gaze tracking arrangements may be considered particularly suitable for use in HMD systems; however, use with such an HMD system should not be considered essential. Note that the HMD of Figure 1 may comprise further features, to be described below in connection with other drawings, but which are not shown in Figure 1 for clarity of this initial explanation. The HMD of Figure 1 may be closed HMD. A closed HMD completely (or at least substantially completely) obscures the user's view of the surrounding environment and blocks completely (or at least substantially completely) light present outside of the enclosed HMD (ambient light) from reaching a user’s eye and / or an inward-facing camera of the HMD. All that the user can see is the pair of images displayed within the HMD, as supplied by an external processing device such as a games console in many embodiments. Of course, in some embodiments images may instead (or additionally) be generated by a processor or obtained from memory located at the HMD itself. The HMD may have associated headphone audio transducers or earpieces 60 which fit into the user's left and right ears 70. The combination of the fact that the user can see only what is displayed by the HMD and, subject to the limitations of the noise blocking or active cancellation properties of the earpieces and associated electronics, can hear only what is provided via the earpieces, mean that this HMD may be considered as a so-called “full immersion” HMD. Note however that in some embodiments the HMD is not a full immersion HMD, and may provide at least some facility for the user to see and / or hear the user’s surroundings. This could be by providing some degree of transparency or partial transparency in the display arrangements, and / or by projecting a view of the outside (captured using a camera, for example a camera mounted on the HMD) via the HMD’s displays, and / or by allowing the transmission of ambient sound past the earpieces and / or by providing a microphone to generate an input sound signal (for transmission to the earpieces) dependent upon the ambient sound. In operation, a video signal is provided for display by the HMD. This could be provided by an external video signal source 80 such as a video games machine or data processing apparatus (such as a personal computer), in which case the signals could be transmitted to the HMD by a wired or a wireless connection. Examples of suitable wireless connections include Bluetooth® connections. Audio signals for the earpieces 60 can be carried by the same connection. Similarly, any control signals passed from the HMD to the video (audio) signal source may be carried by the same connection. Furthermore, a power supply (including one or more batteries and / or being connectable to a mains power outlet) may be linked by a cable to the HMD. Note that the power supply and the video signal source 80 may be separate units or may be embodied as the same physical unit. There may be separate cables for power and video (and indeed for audio) signal supply, or these may be combined for carriage on a single cable (for example, using separate conductors, as in a USB cable, or in a similar way to a “power over Ethernet” arrangement in which data is carried as a balanced signal and power as direct current, over the same collection of physical wires). The video and / or audio signal may be carried by, for example, an optical fibre cable. In other embodiments, at least part of the functionality associated with generating image and / or audio signals for presentation to the user may be carried out by circuitry and / or processing forming part of the HMD itself. A power supply may be provided as part of the HMD itself. Accordingly, the arrangement of Figure 1 provides an example of a head-mountable display system comprising a frame to be mounted onto an observer’s head, the frame defining one or two eye display positions which, in use, are positioned in front of a respective eye of the observer and a display element mounted with respect to each of the eye display positions, the display element providing a virtual image of a video display of a video signal from a video signal source to that eye of the observer. Figure 1 shows just one example of an HMD. Other formats are possible: for example an HMD could use a frame more similar to that associated with conventional eyeglasses, namely a substantially horizontal leg extending back from the display portion to the top rear of the user's ear, possibly curling down behind the ear. In other (not full immersion) examples, the user's view of the external environment may not in fact be entirely obscured; the displayed images could be arranged so as to be superposed (from the user's point of view) over the external environment. An example of such an arrangement will be described below with reference to Figure 4. In the example of Figure 1, a separate respective display is provided for each of the user's eyes. A schematic plan view of how this is achieved is provided as Figure 2, which illustrates the positions 100 of the user's eyes and the relative position 110 of the user's nose. The display portion 50, in schematic form, comprises an exterior shield 120 to mask ambient light from the user's eyes and an internal shield 130 which prevents one eye from seeing the display intended for the other eye. Therefore, the The combination of the user's face, the exterior shield 120 and the interior shield 130 form two compartments 140, one for each eye. In each of the compartments there is provided a display element 150 and one or more optical elements 160. The way in which the display element and the optical element(s) cooperate to provide a display to the user will be described with reference to Figure 3. Referring to Figure 3, the display element 150 generates a displayed image which is (in this example) refracted by the optical elements 160 (shown schematically as a convex lens but which could include compound lenses or other elements) so as to generate a virtual image 170 which appears to the user to be larger than and significantly further away than the real image generated by the display element 150. As an example, the virtual image may have an apparent image size (image diagonal) of more than 1 m and may be disposed at a distance of more than 1 m from the user's eye (or from the frame of the HMD). In general terms, depending on the purpose of the 10 HMD, it is desirable to have the virtual image disposed a significant distance from the user. For example, if the HMD is for viewing movies or the like, it is desirable that the user's eyes are relaxed during such viewing, which requires a distance (to the virtual image) of at least several metres. In Figure 3, solid lines (such as the line 180) are used to denote real optical rays, whereas broken lines (such as the line 190) are used to denote virtual rays. An alternative arrangement is shown in Figure 4. This arrangement may be used where it is desired that the user's view of the external environment is not entirely obscured. However, it is also applicable to HMDs in which the user's external view is wholly obscured. In the arrangement of Figure 4, the display element 150 and optical elements 200 cooperate to provide an image which is projected onto a mirror 210, which deflects the image towards the user's eye position 220. The user perceives a virtual image to be located at a position 230 which is in front of the user and at a suitable distance from the user. In the case of an HMD in which the user's view of the external surroundings is entirely obscured, the mirror 210 can be a substantially 100% reflective mirror. The arrangement of Figure 4 then has the advantage that the display element and optical elements can be located closer to the centre of gravity of the user's head and to the side of the user's eyes, which can produce a less bulky HMD for the user to wear. Alternatively, if the HMD is designed not to completely obscure the user's view of the external environment, the mirror 210 can be made partially reflective so that the user sees the external environment, through the mirror 210, with the virtual image superposed over the real external environment. In this case, the HMD can provide an augmented reality (AR) style experience to the user and is thus can be labelled as an augmented reality HMD. In case the HMD provides additionally or alternatively a virtual reality (VR) style experience, the HMD can be labelled as a virtual reality HMD. In the case where separate respective displays are provided for each of the user's eyes, it is possible to display on each display a respective independent image, wherein the two respective independent images may be stereoscopic images. An example of a pair of stereoscopic images for display to the left and right eyes is shown in Figure 5. The images exhibit a lateral displacement relative to one another, with the displacement of image features depending upon the (real or simulated) lateral separation of the cameras by which the images were captured, the angular convergence of the cameras and the (real or simulated) distance of each image feature from the camera position. In some situations, an HMD may be used simply to view movies and the like. In this case, there is no change required to the apparent viewpoint of the displayed images as the user turns the user's head, for example from side to side. In other uses, however, such as those associated with virtual reality (VR) or augmented reality (AR) systems, the user's viewpoint needs to track movements with respect to a real or virtual space in which the user is located. As mentioned above, in some uses of the HMD, such as those associated with virtual reality (VR) or augmented reality (AR) systems, the user's viewpoint needs to track movements with respect to a real or virtual space in which the user is located. Turning to gaze tracking in such an arrangement, Figures 6a and 6b schematically illustrate two possible arrangements for performing eye tracking on an HMD. The cameras provided within such arrangements may be selected freely so as to be able to perform an effective gaze-tracking method. In some existing arrangements, visible light cameras are used to capture images of a user’s eyes. Alternatively, infra-red (IR) cameras are used so as to reduce interference either in the captured signals or with the user’s vision should a corresponding light source be provided, or to improve performance in low-light conditions. Figure 6a shows an example of a gaze tracking arrangement in which the cameras are arranged within an HMD so as to capture images of the user’s eyes from a short distance. This may be referred to as near-eye tracking, or head-mounted tracking. In this example, an HMD 600 (with a display element 601) is provided with cameras 610 that are each arranged so as to directly capture one or more images of a respective one of the user’s eyes using an optical path that does not include the lens 620. This may be advantageous in that distortion in the captured image due to the optical effect of the lens is able to be avoided. Four cameras 610 are shown here as examples of possible positions in which gaze-tracking cameras may be provided, although it should be considered that any number of cameras may be provided in any suitable location so as to be able to image the corresponding eye effectively. For example, only one camera may be provided per eye or more than two cameras may be provided for each eye. However it is considered that in a number of embodiments it is advantageous that the cameras are instead arranged so as to include the lens 620 in the optical path used to capture images of the eye. Examples of such positions are shown by the cameras 630. While this may result in processing being required to enable suitably accurate tracking to be performed, due to the deformation in the captured image due to the lens, this may be performed relatively simply due to the fixed relative positions of the corresponding cameras and lenses. An advantage of including the lens within the optical path may be that of simplifying the physical constraints upon the design of an HMD, for example. Figure 6b shows an example of a gaze tracking arrangement in which the cameras are instead arranged so as to indirectly capture images of the user’s eyes. Such an arrangement may be particularly suited to use with IR or otherwise non-visible light sources, as will be apparent from the below description. Figure 6b includes a mirror 650 arranged between a display 601 and the viewer’s eye (of course, this can be extended to or duplicated at the user’s other eye as appropriate). For the sake of clarity, any additional optics (such as lenses) are omitted in this Figure - it should be appreciated that they may be present at any suitable position within the depicted arrangement. The mirror 650 in such an arrangement is selected so as to be partially transmissive; that is, the mirror 650 should be selected so as to enable the camera 640 to obtain an image of the user’s eye while the user views the display 601. One method of achieving this is to provide a mirror 650 that is reflective to IR wavelengths but transmissive to visible light - this enables IR light used for tracking to be reflected from the user’s eye towards the camera 640 while the light emitted by the display 601 passes through the mirror uninterrupted. Such an arrangement may be advantageous in that the cameras may be more easily arranged out of view of the user, for instance. Further to this, improvements to the accuracy of the eye tracking may be obtained due to the fact that the camera captures images from a position that is effectively (due to the reflection) along the axis between the user’s eye and the display. Of course, gaze -tracking arrangements need not be implemented in a head-mounted or otherwise near-eye fashion as has been described above. For example, Figure 7 schematically illustrates a system in which a camera is arranged to capture images of the user from a distance; this distance may vary during tracking, and may take any value in dependence upon the parameters of the tracking system. For example, this distance may be thirty centimetres, a metre, five metres, ten metres, or indeed any value so long as the tracking is not performed using an arrangement that is affixed to the user’s head. In Figure 7, an array of cameras 700 is provided that together provide multiple views of the user 710. These cameras are configured to capture information identifying at least the direction in which a user’s 710 eyes are focused, using any suitable method. For example, IR cameras may be utilised to identify reflections from the user’s 710 eyes. An array of cameras 700 may be provided so as to provide multiple views of the user’s 710 eyes at any given time, or may be provided so as to simply ensure that at any given time at least one camera 700 is able to view the user’s 710 eyes. It is apparent that in some use cases it may not be necessary to provide such a high level of coverage and instead only one or two cameras 700 may be used to cover a smaller range of possible viewing directions of the user 710. Of course, the technical difficulties associated with such a long-distance tracking method may be increased; higher resolution cameras may be required, as may stronger light sources for generating IR light, and further information (such as head orientation of the user) may need to be 13 input to determine a focus of the user’s gaze. The specifics of the arrangement may be determined in dependence upon a required level of robustness, accuracy, size, and / or cost, for example, or any other design consideration. Rather than varying only in the location in which cameras are provided, gaze-tracking arrangements may also differ in where the processing of the captured image data to determine tracking data is performed. Figure 8 schematically illustrates an environment in which a gaze-tracking process may be performed. In this example, the user 800 is using an HMD 810 that is associated with the processing unit 830, such as a games console, with the peripheral 820 allowing a user 800 to input commands to control the processing. The HMD 810 may perform eye tracking in line with an arrangement exemplified by Figure 6a or 6b, for example-that is, the HMD 810 may comprise one or more cameras operable to capture images of either or both of the user’s 800 eyes. The processing unit 830 may be operable to generate content for display at the HMD 810; although some (or all) of the content generation may be performed by processing units within the HMD 810. While the connections shown in Figure 8 are shown by lines, this should of course not be taken to mean that the connections should be wired; any suitable connection method, including wireless connections such as wireless networks or Bluetooth®, may be considered suitable. Similarly, while a dedicated processing unit 830 is shown in Figure 8 it is also considered that the processing may in some embodiments be performed in a distributed manner - such as using a combination of two or more of the HMD 810, one or more processing units, remote servers (cloud processing), or games consoles. The processing required to generate tracking information from captured images of the user’s 800 eye or eyes may be performed locally by the HMD 810, or the captured images or results of one or more detections may be transmitted to an external device (such as a the processing unit 830) for processing. In the former case, the HMD 810 may output the results of the processing to an external device for use in an image generation process if such processing is not performed exclusively at the HMD 810. In embodiments in which the HMD 810 is not present, captured images from the camera 840 are output to the processing unit 830 for processing. Figure 9 schematically illustrates a system for performing one or more eye tracking processes, for example in an embodiment such as that discussed above with reference to Figure 8. The system 900 comprises a processing device 910, one or more peripherals 920, an HMD 930, a camera 940, and a display 950. Of course, not all elements need be present within the system 900 in a number of embodiments - for instance, if the HMD 930 is present then it is considered that the camera 940 may be omitted as it is unlikely to be able to capture images of the user’s eyes. The HMD 930 can be an embodiment of HMD 600. As shown in Figure 9, the processing device 910 may comprise one or more of a central processing unit (CPU) 911, a graphics processing unit (GPU) 912, storage (such as memory and / or a hard drive and / or any other suitable data storage medium) 913, and an input / output 914. These units may be provided in the form of a personal computer, a games console, or any other suitable processing device. For example, the CPU 911 may be configured to generate tracking data from one or more input images of the user’s eyes from one or more cameras, or from data that is indicative of a user’s eye direction. This may be data that is obtained from processing images of the user’s eye at a remote device, for example. Of course, should the tracking data be generated elsewhere then such processing would not be necessary at the processing device 910. The GPU 912 may be configured to generate content for display to the user on which the eye tracking is being performed. In some embodiments, the content itself may be modified in dependence upon the tracking data that is obtained - an example of this is the generation of content in accordance with a foveal rendering technique. Of course, such content generation processes may be performed elsewhere - for example, an HMD 930 may have an on-board GPU that is operable to generate content in dependence upon the eye tracking data. The storage 913 may be provided so as to store any suitable information. Examples of such information include program data, content generation data, and eye tracking model data. In some cases, such information may be stored remotely such as on a server, and as such a local storage 913 may not be required - the discussion of the storage 913 should therefore be considered to refer to local (and in some cases removable storage media) or remote storage. The input / output 914 may be configured to perform any suitable communication as appropriate for the processing device 910. Examples of such communication include the transmission of content to the HMD 930 and / or display 950, the reception of gaze-tracking data and / or images from the HMD 930 and / or the camera 940, and communication with one or more remote servers (for example, via the internet). As discussed above, the peripherals 920 may be provided to allow a user to provide inputs to the processing device 910 in order to control processing or otherwise interact with generated content. This may be in the form of button presses or the like, or alternatively via tracked motion to enable gestures to be used as inputs. The HMD 930 may comprise a number of sub-elements, which have been omitted from Figure 9 for the sake of clarity. Of course, the HMD 930 should comprise a display unit operable to display images to a user. In addition to this, the HMD 930 may comprise any number of suitable cameras for eye tracking (as discussed above), in addition to one or more processing units that are operable to generate content for display and / or generate eye tracking data from the captured images. The camera 940 and display 950 may be configured in accordance with the discussion of the corresponding elements above with respect to Figure 8. Turning to the image capture process upon which the eye tracking is based, examples of different cameras are discussed. The first of these is a standard camera, which captures a sequence of images of the eye that may be processed to determine tracking information. The second is that of an event camera, which instead generates outputs in accordance with observed changes in brightness. It is more common to use standard cameras in such tracking arrangements, given that they are widely available and often relatively cheap to produce. ‘Standard cameras’ here refer to cameras which capture images of the environment at predetermined intervals which can be combined to generate video content. For example, a typical camera of this type may capture thirty images (frames) each second, and these images may be output to a processing unit for feature detection or the like to be performed so as to enable tracking of the eye. Such a camera comprises a light-sensitive array that is operable to record light information during an exposure time, with the exposure time being controlled by a shutter speed (the speed of which dictates the frequency of image capture). The shutter may be configured as a rolling shutter (lineby-line reading of the captured information) or a global shutter (reading the captured information of the whole frame simultaneously), for example. However, in some arrangements it may be considered advantageous to instead use an event camera, which may also be referred to as a dynamic vision sensor. Such cameras do not require a shutter as described above, and instead each element of the light-sensitive array (often referred to as a pixel) is configured to output a signal at any time a threshold brightness change is observed. This means that images are not output in the traditional sense - however an image reconstruction algorithm may be applied that is able to generate an image from the signals output by an event camera. While there is an increased computational complexity for generating an image from such data, the output of the event camera can be used for tracking without any image generation. One example of how this is performed is that of using an IR-sensitive event camera; when imaged using IR light, the pupil of the human eye displays a much higher level of brightness than the surrounding features. By selecting an appropriate threshold brightness, the motion of the pupil would be expected to trigger events (and corresponding outputs) at the sensor. Independent of the type of camera that is selected, in many cases it may be advantageous to provide illumination to the eye in order to obtain a suitable image. One example of this is the provision of an IR light source that is configured to emit light in the direction of one or both of the user’s eyes; an IR camera may then be provided that is able to detect reflections from the user’s eye in order to generate an image. IR light may be preferable as it is invisible to the human eye, and as such does not interfere with normal viewing of content by the user, but it is not considered to be essential. In some cases, the illumination may be provided by a light source that is affixed to the imaging device, while in other embodiments it may instead be that the light source is arranged away from the imaging device. As suggested in the discussion above, the human eye does not have a uniform structure; that is, the eye is not a perfect sphere, and different parts of the eye have different characteristics (such as varying reflectance or colour). Figure 10 shows a simplified side view of the structure of a typical eye 1000; this Figure has omitted features such as the muscles which control eye motion for the sake of clarity. The eye 1000 is formed of a near-spherical structure filled with an aqueous solution 1010, with a retina 1020 formed on the rear surface of the eye 1000. The optic nerve 1030 is connected at the rear of the eye 1000. Images are formed on the retina 1020 by light entering the eye 1000, and corresponding signals carrying visual information are transmitted from the retina 1020 to the brain via the optic nerve 1030. Turning to the front surface of the eye 1000, the sclera 1040 (commonly referred to as the white of the eye) surrounds the iris 1050. The iris 1050 controls the size of the pupil 1060, which is an aperture through which light enters the eye 1000. The iris 1050 and pupil 1060 are covered by the cornea 1070, which is a transparent layer which can refract light entering the eye 1000. The eye 1000 also comprises a lens (not shown) that is present behind the iris 1050 that may be controlled to adjust the focus of the light entering the eye 1000. The structure of the eye is such that there is an area of high visual acuity (the fovea), with a sharp drop off either side of this. This is illustrated by the curve 1100 of Figure 11, with the peak in the centre representing the foveal region. The area 1110 is the ‘blind spot’; this is an area in which the eye has no visual acuity as it corresponds to the area where the optic nerve meets the retina. The periphery (that is, the viewing angles furthest from the fovea) is not particularly sensitive colour or detail, and instead is used to detect motion. As has been discussed above, foveal rendering is a rendering technique that takes advantage of the relatively small size (around 2.5 degrees) of the fovea and the sharp fall-off in acuity outside of that. The eye undergoes a large amount of motion during viewing, and this motion may be categorised into one of a number of categories. Saccades, and on a smaller scale micro-saccades, are identified as fast motions in which the eyes rapidly move between different points of focus (often in a jerky fashion). This may be considered as ballistic motion, in that once the movement has been initiated it cannot be altered. Saccades are often not conscious eye motions, and instead are performed reflexively to survey an environment. Saccades may last up to two hundred milliseconds, depending on the distance rotated by the eye, but may be as short as twenty milliseconds. The speed of a saccade is also dependent upon the total rotation angle; typical speeds may be between two hundred and five hundred degrees per second. ‘Smooth pursuit’ refers to a slower movement type than a saccade. Smooth pursuit is generally associated with a conscious tracking of a point of focus by a viewer, and is performed so as to maintain the position of a target within (or at least substantially within) the foveal region of the viewer’s vision. This enables a high-quality view of a target of interest to be maintained in spite of motion. If the target moves too fast, then smooth pursuit may instead require a number of saccades in order to keep up; this is because smooth pursuit has a lower maximum speed, in the region of thirty degrees per second. The vestibular-ocular reflex is a further example of eye motion. The vestibular-ocular reflex is the motion of the eyes that counteracts head motion; that is, the motion of the eyes relative to the head that enables a person to remain focused on a particular point despite moving their head. Another type of motion is that of the vergence accommodation reflex. This is the motion that causes the eyes to rotate to converge at a point, and the corresponding adjustment of the lens within the eye to cause that point to come into focus. Further eye motions that may be observed as a part of a gaze tracking process are those of blinks or winks, in which the eyelid covers the eyes of the user. Such motions may be reflexive or intentional, and can often interfere with eye tracking as they will obscure vision of the eye, and the eye is often not stationary during such a motion. A system for obtaining an intermediate finding of diagnostic relevance for an eye is disclosed as part of the system for performing one or more eye tracking processes shown in Fig. 9. HMD 930 can be an embodiment of HMD 600. The system for obtaining an intermediate finding of diagnostic relevance for an eye can identify, e.g. through a login process, the user of the HMD. The intermediate finding of diagnostic relevance can be any information which is indicative of a change in the properties of a user’s eye or eyes over time; while this may not be sufficient to make a diagnosis due to hardware limitations or the like, this may be enough to suggest that a more detailed examination by a medical professional with specialised equipment is appropriate. The HMD 930 comprises a gaze-tracking arrangement for both eyes, wherein only the gazetracking arrangement on the right hand side will be discussed in detail. The acquisition unit of the gaze-tracking arrangement comprises as an imaging unit at least one of the inward-facing cameras 610, 630. The HMD 600 comprises further as the image display unit the display element 601 for emitting light, which forms, when imaged by the lens 620, a virtual image which can be viewed by the user’s eye. Thus, the display element 601 can illuminate the eye with a predefined light composition, the predefined light composition being the light emitted by the display element 601 when a predetermined image is shown to the right eye of the user. The processing device 910 provides storage containing instructions, which, when executed by the CPU 911 and / or GPU 912 configure the processing device 910 to function as a feature data unit configured to obtain a feature of an eye based on image data obtained in a first time period from the at least one of the inward-facing camera 610, 630. The description refers to features of the eye and the reference for the feature of the eye. Furthermore, and depending on the embodiment, the description refers also the current feature of the eye or / and the historical feature of the eye or / and the reference feature of the eye which represent the same feature category in the respective first, second and third feature example. A feature of the eye may be considered to be any suitable property of the eye as determined by analysis of the reflections - for instance, a measure of the reflection intensity, shape, or colour (or a measure of how the reflection of a displayed image is modified in these respects) may each be considered features of an eye. First feature example The feature category can be a colour or colour component of the respective image of the eye; or a colour or colour component of a certain element or section of the respective image of the eye. To obtain the respective current, historical or reference feature of the eye, the eye is illuminated using the predefined light composition emitted from the display element 601 emitting the same image, for example a white surface, with the same intensity for obtaining each of the images discussed in this example (the current, historical and reference image of the eye). The respective image of the eye or the certain element or section of the respective image of the eye will be described in the following as the region of interest, ROI. Even though changes overtime may be mostly visible in the certain element or section of the respective image of the eye, a change will also be present, when the whole image of the eye is analysed. Figure 12 shows measured features of the eye. The first time period comprises times t with T6<t<T8. The second time period comprises times t with T1<t<T3. The third period comprises times t with T4<t<T6. Thus, the second time period precedes the third time period; the third time period precedes the first time period; and the second time period precedes the first time period. The first, second and third time period may be predefined in length and position. Times t, T1, T2, ... T8 can be used as timestamps. The third time period can be longer than the first time period. While the second time period may have a fixed position in time, the first time period may be defined to comprise the time stamp of the newest current image acquired. The third time period may be directly followed by the first time period. As a consequence, the absolute position of the first and third time period may change. A portion of the light of the predefined light composition is reflected from the eye to one of the inward-facing camera 610, 630. During a first time period the at least one of the inward-facing cameras 610, 630 obtains a current image of the eye. The current image of the eye is an example of data obtained in the first time period by the acquisition unit. The feature data unit analyses the current image of the eye or e.g. the sclera 1040 as the ROI by obtaining e.g. the average intensity of pixels regarded as yellow in the image of the ROI, wherein yellow can be defined as a range in a RGB colour space. Since the eye is illuminated using the predefined light composition emitted from the display element 601, changes in the average intensity of pixels regarded as yellow in the images of the ROI represent a change of the colour of the ROI. The average intensity of pixels regarded as yellow in the image of the ROI is a colour or colour component of the respective image of the eye; or a colour or colour component of a certain element or section of the respective image of the eye depending on the definition used for the ROI. Furthermore, since this quantity is based on a current image of the eye, this quantity is an example of a current feature of the eye. The feature of the eye may be the current feature of the eye. A change in the average intensity of pixels regarded as yellow of the sclera 1040 can indicate a possible liver failure of the user, as this colour change is a symptom of this issue. The intensity of light captured by a pixel (intensity of pixels) can be expressed either in relative intensity units (e.g. the numerical value representing the light intensity in the pixel) or in absolute intensity units, e.g. in Sl-units, of the light captured by the pixel. A current event data set comprising: a timestamp, when the current image of the eye has been acquired; the average intensity of pixels regarded as yellow in the image of the ROI of the current image of the eye (as the current feature of the eye); and the current image of the eye is saved to a trend database in the storage 913, such that the timestamp is a label for the current feature of the eye and for the current image of the eye. The current event data set can comprise the feature of the eye obtained on the basis at least of the current feature of the eye saved in this current event data set (meaning that the current event data set has a timestamp Tx, the feature of the eye saved in this current event data set is not based on any data saved in a current event data set with a timestamp T with Tx<T). This provides a trend database populated with current event data sets labelled with the timestamp. The trend database can be populated in an initialisation step before the start of the method 1400 until at least one current event data set having a time stamp in the first, the second and the third time period can be found in the trend database. The method 1400 is a method for obtaining an intermediate finding of diagnostic relevance for an eye. As discussed below, the current event data sets can be searched for timestamps falling within the second or third time period. In case the timestamp of a current event data set falls within the second time period, due to the time passed, the current image of the current event data set with such a time stamp can be referred to as a / the reference image of the eye and the current feature of the eye of the current event data set with such a time stamp can be referred to as a / the reference feature of the eye. The reference for the feature of the eye can be a reference feature of the eye. In a current event data set with a time stamp in the second time period, the reference feature of the eye is based on the reference image labelled with the same time stamp as the reference feature of the eye. Thus, the reference image labelled with the same time stamp as the reference feature (which in this example is the reference for the feature of the eye) is an example of data obtained in a second time period by the acquisition unit, on which the reference for the feature of the eye is based. The reference for the feature of the eye can be a single reference feature of the eye or an average of a plurality of reference features of the eye. Furthermore, in case the timestamp of a current event data set falls within the third time period, due to the time passed, the current image of the current event data set with such a time stamp can be referred to as a / the historical image of the eye; and the current feature of the eye of the current event data set with such a time stamp can be referred to as a / the historical feature of the eye. Basing the feature of the eye only on the current feature of the eye may lead however to results influenced by image acquisition conditions changing in time. In order to mitigate the influence of such noisy data, the trend database can be used to identify a current event data set with a timestamp in the third time interval. The feature of the eye can be an average of the current feature of the eye and at least one historical feature of the eye. In this case, the feature of the eye is the average intensity of all pixels regarded as yellow in the image of the ROI of the current image of the eye and in the image of the ROI at least of one historical image of the eye. The average intensity of all pixels regarded as yellow in the image of the ROI of the current image of the eye is the current feature of the eye. The average intensity of all pixels regarded as yellow in the image of the ROI in one historical image of the eye is a historical feature of the eye. Therefore, the feature of the eye calculated as above is calculated on the basis of the current feature of the eye and at least one historical feature of the eye. The system for obtaining an intermediate finding of diagnostic relevance for an eye saved, in the trend database, at least one or a plurality of current event data sets during an initial phase of using of the HMD by the user. This initial phase can be the second time period. It is assumed that in the majority of cases no negative condition of the eye is present during this initial phase. The storage 913 contains instructions, which, when executed by the CPU 911 and or GPU 912 configure the processing device 910 to function as a reference data unit calculating the reference for the feature of the eye. In figure 12 the reference for the feature of the eye is indicated by the dashed line as the average of the features of the eye obtained at times T1 and T2. Based on the input from a professional, a physician or from machine learning, a reference threshold is obtained. The reference threshold is a threshold for the change for this feature of the eye. The feature of the eye is compared to the reference of the feature of the eye, for example by obtaining the absolute value of the difference of these two values. The region, where the absolute value of this difference is smaller than the reference threshold is indicated in figure 12 by the dotted region. In case an absolute value of the difference between the feature of the eye (e.g. the average intensity of pixels regarded as yellow in the image of the ROI of the current image) and the reference for the feature of the eye is bigger than or equal to the reference threshold, the intermediate finding is negative. Otherwise the intermediate finding is positive. In figure 12, the above described intermediate finding is always positive. 22 A trend threshold for time-dependent changes of the feature of the eye can be obtained based on the input from a professional, a physician or from machine learning. A time-dependent trend for the feature of the eye can be calculated in the present example at the time Ti (i=2... 8 in the present example) as the absolute value of the slope / gradient of the line fit of the features of the eye obtained between Ti and at T(i-1). In this example, the line fit is a simple connection between two points directly adjacent in time. However, in case more time points are used for the fit, a least squares approximation of the data points to a line can be used to obtain the slope / gradient of the line fit. As seen in Figure 12, the feature of the eye changes quickly between times T4 and T5. Thus, even though the above described intermediate finding based on comparison of the reference threshold and the feature of the eye is positive, an intermediate finding based on comparison of the trend threshold with the trend for the feature of the eye can be negative, in case the trend for the feature of the eye is bigger than or equal to the trend threshold. The system for obtaining an intermediate finding of diagnostic relevance for an eye may inform the user of the HMD, e.g. by a message displayed to the user using the display element 601, that the eye should be examined by a professional or physician in case of a negative intermediate finding. This message may include further information about the cause for the negative intermediate finding and a suggestion for a potential clinical picture leading to the negative intermediate finding, e.g. a liver failure. As shown below, this is only a suggestion for a clinical picture. Before obtaining the next current image of the eye, the first time period and optionally the third time period may be updated by being moved forward by AT in time, such that the next current image of the eye is obtained during the updated first time period. The first time period and optionally the third time period may be updated by the feature data unit. The storage 913 contains instructions, which, when executed by the CPU 911 and or GPU 912 configure the processing device 910 to function as an intermediate finding unit. The intermediate finding unit can compare the absolute value of the difference between the feature of the eye (e.g. the average intensity of pixels regarded as yellow in the image of the ROI of the current image) and the reference for the feature of the eye (e.g. the average intensity of pixels regarded as yellow in the image of the ROI of one reference image (or multiple reference images, see above)) with the reference threshold and obtaining the intermediate finding of diagnostic relevance for the eye as described above. Please note that a user of the HMD may wear a contact lens 1080 as shown in figure 10. The contact lens 1080 may be used to change the colour of the iris 1050 of the user or even the colour of the whole eye visible to the outside as a fashion statement. The use of such a coloured contact lens 1080 may therefore lead to obtaining a negative intermediate finding, even though no medical condition of the eye has changed in the first time and / or third interval in comparison to the second time interval. Therefore, the intermediate finding does not represent any diagnosis since no particular clinical picture can be attributed to the negative intermediate finding. As indicated above, the system for obtaining an intermediate finding of diagnostic relevance for an eye comprises in the present example the gaze-tracking arrangement with an acquisition unit, which in turn comprises as an imaging unit with at least one of the inward-facing cameras 610, 630. The system for obtaining an intermediate finding of diagnostic relevance for an eye comprises in the present example a computing unit. The computing unit comprises the feature data unit, the reference data unit and the intermediate finding unit. The computing unit is embodied in this example by the processing device 910. Second feature example The feature category can be an average intensity of light reflected from the eye or reflected from a certain element or section of the eye, wherein the intensity of light reflected from the eye or reflected from a certain element or section of the eye is measured as the intensity of this light collected by the imaging device. Only the differences with respect to the first feature example will be discussed. In the second example the feature of the eye is the intensity of light reflected from the eye or reflected from a certain element or section of the eye, ROI. In particular, the ROI can be the pupil 1060 or the area corresponding to the lens of the eye. This feature of the eye is measured by obtaining the average intensity of pixels in the image of the ROI. A reduction of the intensity of light reflected from the ROI may indicate formation of a cataract. During the first time period the at least one of the inward-facing cameras 610, 630 obtains a current image of the eye. The feature data unit analyses the ROI by obtaining the average intensity of pixels in the image of the ROI as the feature of the eye (representing the average intensity of light reflected from the ROI). A current event data set comprising a timestamp, when the current image of the eye has been acquired, the average intensity of pixels in the image of the ROI of the current image (as the current feature of the eye), and the current image of the eye are saved to a trend database in the storage 913, such that the timestamp is a label for the current feature of the eye and for the current image of the eye. As discussed above, on the basis of the current event data set in the trend database and the first, second and third time period, the respective feature of the eye, the reference for the feature of the eye, historic and reference images of the eye, historic and reference features of the eye and time-dependent trends for the feature of the eye can be obtained. Based on the input from a professional, a physician or from machine learning, a reference threshold for the change for this feature of the eye and / or a trend threshold can be obtained. Furthermore, an intermediate finding can be obtained by comparing the reference threshold with the feature of the eye and / or by comparing the trend threshold with the time-dependent trend for the feature of the eye. In this example the predefined light composition may be generated by a light source for generating IR light, which is non-visible to the human eye. The light source for generating IR light can be part of a gaze-tracking arrangement, for example a gaze-tracking arrangement of a HMD. Please note that a user of the HMD may wear a contact lens 1080 as shown in figure 10. The contact lens 1080 may be used to change the colour of the iris 1050 of the user or even the colour of the whole eye visible to the outside as a fashion statement. The use of such a coloured contact lens 1080 may therefore lead to obtaining a negative intermediate finding due to absorption of light used to illuminate the eye, even though no medical condition of the eye has changed in the first time and / or third interval in comparison to the second time interval. Therefore, the intermediate finding does not represent any diagnosis since no particular clinical picture can be attributed to the negative intermediate finding. Third feature example Only the differences with respect to the first feature example will be discussed. The feature category of the eye can be a value indicative of a deviation of a shape of a reflection from the eye from a reference shape of the reflection from the eye. The reflection can be the reflection of the display element 601 or of the display element 150 from the eye. An initial image of the eye can be obtained from the at least one of the inward-facing cameras 610, 630, for example before the second time period. The reflection 1320, e.g. of the display element, is identified, preferably by the feature data unit, as the reference shape in the initial image of the eye using e.g. an algorithm for object detection. During the first time period the at least one of the inward-facing cameras 610, 630 obtains a current image of the eye. The value indicative of a deviation of a shape of a reflection from the eye in a current image of the eye from a reference shape of the reflection from the eye can be calculated by a) identifying as the current shape the same reflection 1330, e.g. the reflection of the display element, in the current image of the eye using the algorithm for object detection and b) calculating the surface area of the symmetric difference between the reference shape and the current shape, wherein the initial image and the current image are overlapped in a single image 1310 with all edges and corners overlapping and the overlapped images having the same orientation. The steps a) and b) are preferably carried out by the feature data unit. The reflection 1320 of the display element (being the reference shape) is a rectangle lying in the overlaid image 1310 in the sections 1321 and 1322. The reflection 1330 of the display element (being the current shape) is a rectangle lying in the overlaid image 1310 in the sections 1322 and 1323. The symmetric difference between the reference shape and the current shape is composed of sections 1321 and 1323. The surface area of the symmetric difference between the reference shape and the current shape is the value indicative of a deviation of a shape of a reflection from the eye from a reference shape of the reflection from the eye and thus the current feature of the eye in this example. Thus, the current feature of the eye is the surface area of the symmetric difference between the reference shape and the current shape. A current event data set comprising a timestamp, when the current image of the eye has been acquired, the above defined current feature of the eye, and the current image of the eye is saved to a trend database in the storage 913, such that the timestamp is a label for the current feature of the eye and for the current image of the eye The trend database can be populated in an initialisation step until at least one current event data set having a time stamp in the first, the second and the third time period can be found in the trend database. As discussed above, on the basis of the current event data set in the trend database and the first, second and third time period, the respective feature of the eye, historic and reference images of the eye, historic and reference features of the eye and time-dependent trends for the feature of the eye can be obtained. The reference for the feature of the eye is zero in this example, since this value represents no change between the shape of a reflection from the eye in a current image of the eye and a reference shape of the reflection from the eye. Based on the input from a professional, a physician or from machine learning, a reference threshold for the change for this feature of the eye and / or a trend threshold can be obtained. Furthermore, an intermediate finding can be obtained by comparing the reference threshold with the feature of the eye and / or by comparing the trend threshold with the time-dependent trend for the feature of the eye. Please note that a user of the HMD may wear a contact lens 1080 as shown in figure 10. Without the contact lens 1080 a light ray 1082 from display element 601 or from the display element 150 is reflected on an element of the eye, e.g. the surface of the lens of the eye visible through the pupil 1060 as indicated by the light ray 1084. Light ray 1084 contributes to the reflection of the display element 601 or of the display element 150. When the contact lens 1080 is present, a further reflection of the light ray 1082 on the contact lens 1080 is present as indicated by the light ray 1086. The direction of the light ray 1086 is different from the direction of the light ray 1084. As a result, the shape of the reflection (e.g. the of the display element 601 or of the display element 150) from the eye in a current image of the eye captured by the inward-facing camera 1088 changes depending on the presence or absence of the contact lens 1080. A contact lens may therefore in this example lead to a negative intermediate finding. Therefore, the intermediate finding in this example does not represent any diagnosis, since no particular clinical picture can be attributed to the negative intermediate finding. The inwardfacing camera 1088 can be one of the inward-facing cameras 610, 630. Figure 14 illustrates a method 1400 for obtaining an intermediate finding of diagnostic relevance for an eye according to an embodiment. The discussion of the system for obtaining an intermediate finding of diagnostic relevance for an eye and the discussion of the first, second and the third example of the feature comprise method steps, which can be used to specify the method 1400 further. The first, the second and the third time period can be predefined in an initialisation step before the start of the method 1400. The trend database can be populated in the initialisation step before the start of the method 1400 until at least one current event data set having a time stamp in the first, the second and the third time period can be found in the trend database. In step S1410 a feature of the eye (e.g. the average intensity of pixels regarded as yellow in the image of the ROI in a current image of the eye) is obtained. In step S1420 a reference for the feature of the eye (e.g. the average intensity of pixels regarded as yellow in the image of the ROI for a single current image having a timestamp in the second time period) is obtained. In step S1430, the intermediate finding of diagnostic relevance for the eye is obtained based on the feature of the eye and the reference for the feature of the eye (e.g. using a reference threshold as discussed above). In case the intermediate finding is negative, the person having the eye is informed about the negative intermediate finding. This information can be provided via a HMD as discussed above, however this information can additionally and / or alternatively be provided via email, text message or an acoustic warning. Before the method 1400 returns to step 1410 for obtaining the next feature of the eye, the first time period and optionally the third time period may be updated by being moved forward by AT in time, such that e.g. the next current image of the eye is obtained during the updated first time period. Figure 15 illustrates a further system 1500 for obtaining an intermediate finding of diagnostic relevance for an eye. The system 1500 comprises a head-mountable display unit 1502, with a gaze-tracking arrangement 1504 and an image display unit 1506. The gaze-tracking arrangement 1504 comprises an acquisition unit 1508. The system 1500 comprises further a computing unit 1510 comprising a feature data unit 1512, a reference data unit 1514 and an intermediate finding unit 1516. The computing unit 1510 may be separate from the head-mountable display unit 1502. However, as indicated by the dotted lines, the computing unit 1510 may be comprised by the head-mountable display unit 1502. Data connections between the respective elements or units of the system 1500 are present however are not shown in the figure 15.

Claims

1. A method for obtaining an intermediate finding of diagnostic relevance for an eye, comprising the steps of:obtaining a feature of an eye, wherein the feature of the eye is based on data obtained in a first time period by an acquisition unit, wherein the acquisition unit is part of a head-mountable display unit, HMD;obtaining a reference for the feature of the eye, wherein the reference for the feature of the eye is based on data obtained in a second time period by the acquisition unit, wherein the second time period precedes the first time period; andobtaining the intermediate finding of diagnostic relevance for the eye based on the feature of the eye and the reference for the feature of the eye.

2. The method according to claim 1, wherein obtaining the feature of the eye comprises obtaining, during the first time period, as the data obtained in the first time period a current image of the eye, and detecting a current feature of the eye based on the current image of the eye, and obtaining the feature of the eye on the basis of the current feature.

3. The method according to claim 2, wherein obtaining, during the first time period, as the data obtained in the first time period a current image of the eye comprises illuminating the eye with a predefined light composition, wherein the predefined light composition is generated by an image display unit of the HMD.

4. The method according to claim 2 or 3, wherein the method further comprises saving to a trend database a current event data set, the current event data set comprising the current feature of the eye, a timestamp of the current feature of the eye, the timestamp indicating, when the current image of the eye, the current feature of the eye is based on, has been acquired.

5. The method according to any one of the claims 2 to 4, wherein obtaining the feature of the eye comprises obtaining the feature of the eye on the basis of the current feature of the eye and a historical feature of the eye, wherein the historical feature of the eye is based on data obtained in a third time period by the acquisition unit, wherein the second time period precedes the third time period; and wherein the third time period precedes the first time period6. The method according to claim 5, wherein the at least one historical feature of the eye is based on at least one historical image of the eye obtained during the third time period and / or a feature of the eye based on the basis of the historical image.

7. The method according to any one of the preceding claims, wherein obtaining the reference for the feature of the eye comprises: obtaining during the second time period, a reference image of the eye, detecting a reference feature of the eye based on the reference image of the eye, and obtaining the reference for the feature of the eye based on the reference feature of the eye.

8. The method according to any one of the preceding claims, wherein obtaining the intermediate finding of diagnostic relevance for the eye based on the feature of the eye and the reference for the feature comprises obtaining a reference threshold; and comparing a difference value indicative of a difference between of the feature of the eye and the reference feature of the eye with the reference threshold.

9. The method according to any one of the preceding claims, wherein obtaining the intermediate finding of diagnostic relevance for the eye based on the feature of the eye and the reference for the feature of the eye comprises: obtaining a trend threshold; obtaining, based on data from a trend database, a time-dependent trend for the feature of the eye; and comparing the timedependent trend with the trend threshold.

10. System for obtaining an intermediate finding of diagnostic relevance for an eye, comprising:a head-mountable display unit, HMD, comprising an acquisition unit;a feature data unit configured to obtain a feature of an eye, wherein the feature of the eye is based on data obtained in a first time period by the acquisition unit;a reference data unit, configured to obtain a reference for the feature of the eye, wherein the reference for the feature of the eye is based on data obtained in a second time period by the acquisition unit, wherein the second time period precedes the first time period; andan intermediate finding unit, configured to obtain the intermediate finding of diagnostic relevance for the eye based on the feature of the eye and the reference for the feature of the eye.

11. The system according to claim 10, wherein the system comprises a computing unit, the computing unit comprising the feature data unit, the reference data unit and the intermediate finding unit, wherein the HMD further comprises a gaze-tracking arrangement, wherein the gaze-tracking arrangement comprises the acquisition unit.

12. The system according to claim 11, wherein the acquisition unit comprises an imaging unit configured to obtain an image of the eye.

13. The system according to claim 12, wherein the imaging unit comprises one or more inwards-facing cameras of the gaze-tracking arrangement directed towards the eye.

14. The system according to any one of the claims 10 to 13, wherein the HMD comprises an image display unit, wherein the image display unit is configured to generate a predefined light composition for illuminating the eye and wherein the image display unit is further configured to display an image or video to the eye.5 15. Use of a system according to any one of claims 10 to 13 to carry out a method according toanyone of the claims 1 to 9.

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