Head-mounted display device, method for operating the same and medical optical observation system

Abstract

Method for operating a head-mounted display device (100) comprising the steps of: - (S200) acquiring data (D) representative of at least one state variable of the head (13) of a person (O) wearing the head-mounted display device (1), - (S300) evaluating the acquired data (D) to determine the at least one state variable of the head (13), and - (S400) changing a degree of transparency of at least one display device (3) of the head-mounted display device (1) when the at least one state variable of the head (13) corresponds to a predetermined state variable, characterized in that - image data are acquired as data (D) representative of the at least one state variable of the head (13) with at least one camera (108) associated with the head-mounted display device (100), wherein a field of view corresponding to the orientation of the head (13) is recorded with the camera (108), - the at least one state variable of the head (13) includes the viewing direction of the person (O) wearing the head-mounted display device (1) and - to determine the direction of view, the image data are evaluated in order to create a map of the environment in which the orientation and position of the head of the person (O) wearing the head-mounted display device (1) is known.

Description

[0001] The invention relates to a method for operating a head-mounted display device, such as a head-mounted display. Furthermore, the invention relates to a computer program product for implementing such a method, a head-mounted display device, and a medical optical observation system with a head-mounted display device.

[0002] The microsurgery market is currently undergoing a digital transformation. Driven by ergonomic advantages, electronic image enhancement / enhancement, and ever-increasing digital imaging and intraoperative techniques, 2D or 3D monitors are increasingly being used as the primary image source. However, these techniques also have technical disadvantages, such as distance-dependent depth impressions, viewing angle-dependent color reproduction, large space requirements in the operating room, cables that can pose tripping hazards, and hygiene challenges during cleaning. Therefore, the use of HMD systems (head-mounted display systems) is emerging in the medium to long term.

[0003] For the use of such an HMD system in microsurgery, a flexible switch between augmented reality, virtual reality and “free” view is desirable for a surgeon so that he can use the advantages of each mode depending on the situation in order to carry out his work efficiently, effectively and safely.

[0004] Virtual Reality (VR) offers the advantage of complete immersion. The surgeon is not distracted by the environment, and a high-contrast display of information (e.g., image data from a digital surgical microscope or preoperative radiology data) is possible because ambient light has no influence. However, the surgeon simultaneously loses contact with his surroundings: actions in his environment can no longer be visually perceived. It may be necessary to capture and display the environment using additional cameras (so-called VR see-through). This, however, represents a limited view (latency, color representation, resolution, etc.) of the real environment, as the surgeon may have to move the cameras to perceive his surroundings as desired. When wearing VR glasses, the surgeon would also not be able to specifically look at people or devices in the room to read off additional information or interact with them.

[0005] Augmented Reality (AR) systems do not offer immersion, meaning that important information such as image data may not be displayed well, for example due to low contrast, as the ambient lighting has a direct influence. Furthermore, the surgeon can be more easily distracted from his surroundings when he has to concentrate hard on a specific step (e.g. a surgeon inserting a clip to treat an aneurysm). However, AR systems have the advantage that the surgeon can continue to perceive his surroundings as usual (also known as peripheral vision). An AR display in the form of stereo glasses is described, for example, in DE 103 35 369 A1. With this AR display, virtual buttons can be displayed in such a way that the viewer can only see them if they are looking in a certain direction, so that the virtual buttons appear to be fixed in real space.

[0006] During a surgical procedure, there are different phases with different requirements for a visualization system. When using a digital surgical microscope, which is usually used for more complex procedures, a VR system would be beneficial for the surgeon, allowing them to fully concentrate on their task.

[0007] In other phases, such as the beginning of a surgical procedure, an AR system would be beneficial for the surgeon, for example, to see the outline of a craniotomy directly on the patient's head or to visualize the radiological data of the patient's spine on the patient's back so that the correct area of ​​the patient's back can be exposed. During a surgery, it is particularly important for the surgeon to be able to constantly interact with the sterile nurse or the surrounding environment, for example, to receive or hand over instruments or to reposition a device. To do this, the surgeon needs a clear view without obstructions.

[0008] It would be possible to make the display's transparency dependent on the position on the display. For example, a display could be opaque in the upper area and transparent in the lower area, allowing a downward angled view with high transparency, for example, to still be able to see the hands and / or surgical instruments. Looking straight ahead, however, would result in the viewer seeing the opaque upper area of ​​the display, i.e., the area with a transparency level of 0%, which would serve as the pure VR area.

[0009] However, it is more advantageous to be able to switch between unobstructed view, AR and VR depending on the situation. In order to utilize the advantages of both AR and VR systems, AR glasses can be equipped with displays whose degree of transparency can be adjusted between 0% (opaque) and 100% (transparent). Such a device is known, for example, from US 2012 / 0086624 A1. This allows a user to switch between VR, AR and no-AR ("unobstructed view") depending on the situation. Switching between the transparency settings, however, requires interaction by the wearer using a keyboard, mouse, joystick or the like. This solution therefore has the disadvantage that it would interrupt the workflow of the operating person (e.g. a surgeon), since operating the keyboard, mouse, joystick, etc. would require removing or even setting down surgical instruments from the sitting position.An interruption of the workflow can lead to a higher risk for the patient (longer operating time, sterility, etc.).

[0010] US 2015 / 0173846 A1 describes an HMD in which the degree of transparency can be changed according to the orientation and / or position of the HMD. A tracker is provided to detect the gaze direction and / or orientation, which controls the degree of transparency. This tracker can track the head or eyes to determine the user's gaze direction.

[0011] WO 2014 / 197337 A1 describes controlling the degree of transparency of the HMD through movement commands, for which acceleration sensors may be present.

[0012] With reference to the described prior art, the object of the present invention is to provide an advantageous method for operating a head-mounted display device, in particular a head-mounted display, as well as a corresponding computer program product. A further object of the present invention is to provide a head-mounted display device that is easy to operate without interrupting the workflow.

[0013] The first object is achieved by a method according to claim 1 and a computer program product according to claim 6. The second object is achieved by a display device according to claim 7. The dependent claims contain advantageous embodiments of the invention.

[0014] In the method according to the invention for operating a head-mounted display device, the following steps are carried out: - collecting data representative of at least one state variable of the head of a person wearing the head-mounted display device, - evaluating the acquired data to determine at least one state variable of the head, and - Changing a degree of transparency of at least one display device of the head-mounted display device if the determined at least one state variable of the head corresponds to a predetermined state variable.

[0015] The at least one state variable of the head represented by the acquired data includes the viewing direction of the head of the person wearing the head-mounted display device. The viewing direction should not be considered to be the eye position, but rather the orientation of the head, whereby it is assumed that the eyes are looking straight ahead. Within the scope of the method according to the invention, image data is acquired as data representative of the at least one state variable using at least one camera assigned to the head-mounted display device, in particular at least one camera arranged on the head-mounted display device or worn on the head. The at least one state variable of the head includes the viewing direction of the person wearing the head-mounted display device.To determine the viewing direction, the image data is evaluated to create a map of the environment in which the orientation and position of the head of the person wearing the head-mounted display device—and thus their viewing direction—is known. In addition to grayscale or color values ​​for the individual pixels, the image data can also contain distance values ​​assigned to the pixels that indicate the distance from a specific point. Such distance data can be obtained using stereo cameras or ToF (Time of Flight) cameras. Different approaches can be used to create the map of the environment, for example, a so-called SLAM approach, where SLAM stands for "simultaneous localization and mapping."In the SLAM approach, a map of the environment is incrementally created from images of the surroundings taken by the camera attached to the display device. The position and orientation of the head-mounted display—and thus the head of the wearer of the head-mounted display device—is known. The orientation of the head then determines the wearer's gaze direction.

[0016] The head-wearable display device, which can be fixed to the head, for example, by means of a headband or a head strap, can be designed as a head-mounted display. A head-mounted display (also referred to as HMD) is a visual output device worn on the head. It presents images on a display of the display device. The display can, for example, be a display arranged in front of the eyes, the degree of transparency for optical signals or by applying an electrical voltage can be changed, and the image can be viewed via suitable optics arranged between the display and the eye. Such a display can be manufactured, for example, using LCD shutter technology. Alternatively, a conventional display can be used, the image content of which is reflected back to the eye via a reflective surface, the degree of transparency for optical signals can be changed, for example, by applying an electrical voltage.As a further alternative, a conventional display can be used, the image content of which is reflected back to the eye via a conventional beam splitter, wherein the beam splitter is then associated with a device with which the intensity of ambient light incident on the beam splitter can be reduced. The head-mounted display can also have a display with a variable degree of transparency for each eye, a conventional display together with a reflective surface with a variable degree of transparency, or a conventional display together with a conventional beam splitter and a device for reducing ambient light incident on the beam splitter(s). The degree of transparency of the display, the reflective surface, or the device for reducing ambient light incident on the beam splitter can preferably be adjusted independently for the right and left eyes.The degree of transparency of the display, the reflective surface, or the device for reducing ambient light incident on the beam splitter can be adjusted as desired, particularly within a range from 0% (opaque) to 100% (transparent). Holographic displays can also be used as displays in the head-mounted display device.

[0017] As an alternative to the use of physical displays, it is also possible to use virtual displays in the head-mounted display device. In a virtual display, an image is projected directly onto the retina. The head-mounted display device then comprises, in addition to at least one virtual display, a device for reducing ambient light reaching the eyes, wherein the degree of reduction of ambient light reaching the eyes is adjustable. The degree of transparency of the device for reducing ambient light reaching the eyes can preferably be adjusted as desired within a range from 0% (opaque) to 100% (transparent).

[0018] The head-mounted display device can also be a pair of magnifying glasses. In this case, the display arrangement can comprise a beam splitter, via which an image can be superimposed into the beam path of the magnifying glasses and whose transparency can be varied, in particular in the range from 100% transparent to 0% transparent. As with a head-mounted display, it is also possible to use a conventional beam splitter together with a device that can reduce the intensity of ambient light incident on the beam splitter.

[0019] If not only the position of the head-mounted display device but also that of the patient is recorded, the relative orientation between the patient or surgical site on the one hand and the display device on the other can be determined. This allows the position of the patient or surgical site to be determined in the coordinate system of the display device and, for example, a live image from a digital surgical microscope can always be displayed when the surgeon is looking at the surgical site. If the positions of devices such as the surgical microscope are also recorded, the information about these positions can be used, for example, to switch the head-mounted display device to AR mode when looking towards one of the devices and to display a user interface of the device when the surgeon is looking at the device.

[0020] The method according to the invention thus allows gaze-dependent control of the head-mounted display device by comparing the determined viewing direction with a predetermined direction. This allows the head-mounted display device to be operated easily and intuitively without interrupting the workflow.

[0021] According to a further development of the method according to the invention, the data representative of the at least one state variable of the head includes movement data and / or acceleration data of the head-mounted display device, and the at least one state variable of the head includes a head movement of the person wearing the head-mounted display device. For example, the VR mode can be deactivated by shaking the head as a predetermined state variable of the head and activated by nodding as another predetermined state variable of the head, or vice versa. If both the gaze direction and the head movement are determined as state variables of the head, the number of controllable actions can be increased, for example.

[0022] Furthermore, the invention includes a computer program product with program code for carrying out the invention when the program code is loaded into a computer and / or executed in a computer.

[0023] According to the invention, a head-mounted display device for carrying out the method according to the invention is also provided. This device comprises - a recording unit with means for recording data representative of at least one state variable of the head of a person wearing the head-mounted display device, - an evaluation unit for evaluating the acquired data in order to determine at least one state variable of the head, and - a control unit for changing a degree of transparency of at least one display device of the head-mounted display device when the at least one state variable of the head corresponds to a predetermined state variable.

[0024] The at least one state variable of the head includes the viewing direction of the person wearing the head-mounted display device. The viewing direction should not be considered the eye position but rather the orientation of the head, whereby it is assumed that the eyes are looking straight ahead. The head-mounted display device according to the invention is designed to capture image data as data representative of the at least one state variable of the head using a camera arranged on the head-mounted display device or worn on the head, wherein the at least one state variable of the head includes the viewing direction of the person wearing the head-mounted display device.To determine the direction of view, the head-mounted display device is also designed to evaluate the image data in order to create a map of the environment in which the orientation and position of the head of the person wearing the head-mounted display device - and thus their direction of view - is known.

[0025] Additionally, the data representative of at least one state variable of the head can include movement data and / or acceleration data of the head. The at least one state variable of the head then includes a head movement of the person wearing the head-mounted display device. Means for detecting the head movement can be acceleration sensors, for example.

[0026] The evaluation unit and / or the control unit can be integrated into the head-mounted display device or implemented as software on a computer.

[0027] Possible embodiments of the head-mounted display device, such as a head-mounted display or magnifying glasses, have already been described with reference to the method according to the invention. Reference is made to this description.

[0028] The advantages achievable with the head-mounted display device according to the invention result directly from the advantages described with reference to the method according to the invention.

[0029] According to the present invention, a medical-optical observation system comprising a medical-optical observation device and a head-mounted display device according to the invention is also provided. The medical-optical observation device can be, for example, a surgical microscope that uses image sensors to capture object images, which are then transmitted to the head-mounted display device for appropriate display when the at least one state variable of the head corresponds to a specific state variable. For example, the image captured with the medical-optical observation device can be displayed as a VR image in the head-mounted display device if the state variable signals a view toward the surgical site or a specific head movement.The medical-optical observation device can also simply be a camera whose images are transmitted to the head-mounted display device, for example, to be displayed when the wearer of the display device looks in a specific direction or performs a specific head movement. Furthermore, it is possible for the medical-optical observation device to be an endoscope or endomicroscope equipped with at least one image sensor, whose images are transmitted to the head-mounted display device, for example, to be displayed when the wearer of the display device looks in a specific direction or performs a specific head movement.

[0030] Further features, characteristics and advantages of the present invention will become apparent from the following description of an embodiment with reference to the accompanying figures. Fig. 1 shows a head-mounted display together with a surgical microscope. Fig. 2 shows a schematic representation of components of the Fig. 1 shown head-mounted displays. Fig. 3 shows a flowchart of a method for operating the head-mounted display from Fig. 1. Fig. 4 shows that in Fig. 1 shown head-mounted display in operation. Fig. 5 shows a head-mounted display which represents an embodiment of the head-mounted display device according to the invention.

[0031] It will initially be Fig. 1. This shows a head-mounted display device designed as a head-mounted display 1. The head-mounted display 1 functions both as VR glasses and as AR glasses. In the present example, the head-mounted display 1 wirelessly receives images from image sensors 2A, 2B of a medical optical observation device, namely a surgical microscope 2, which are displayed with the head-mounted display 1 as VR images or AR images if certain state variables of the head-mounted display 1 are present. Additionally or alternatively, the head-mounted display 1 can also receive images from other sources, in particular from other medical optical observation devices, and display them as VR images or AR images if certain state variables of the head-mounted display 1 are present.Possible other sources are ordinary cameras, endoscopes, endomicroscopes, image storage with preoperative data or images containing patient data, etc.

[0032] The head-mounted display 1 has a display arrangement 3A, 3B for each eye, each display 5A, 5B and headbands 4. With the headbands 4, the head-mounted display 1 can be placed on the head 13 of the wearer of the head-mounted display 1, for example, a surgeon O (see Fig. 4), attached and worn. The two displays 5A, 5B can be used to display image data in the field of vision of the surgeon O. The displays 5A, 5B are each assigned to one of the surgeon O's eyes and can be controlled differently in order to be able to provide 3D image data spatially, i.e. with a sense of depth. In this example, LCD displays with LCD shutter technology are used as displays 5A, 5B. The LCD shutter technology makes it possible to adjust the degree of transparency of the LCD displays to any setting between 0% (opaque) and 100% (transparent) by applying an electrical control voltage.

[0033] With additional reference to Fig. Two further components of the head-mounted display 1 are described. These are a detection unit 6, an evaluation unit 10, and a control unit 11.

[0034] The acquisition unit 6 is designed to acquire data D representative of a state variable of the head 13 of the operator O (see Fig. 4). Orientation data, e.g., in the form of angle values ​​describing the orientation of the head 13 of the surgeon O (e.g., azimuth angle and polar angle), is recorded as data. For this purpose, three markers 8 arranged on the head-mounted display 1 are used, the positions of which are continuously recorded using a tracking system (not shown). In other variants, only two markers are used. These are generally sufficient to determine the azimuth angle and the polar angle. With three markers 8, a head rotation around the line of sight can also be recorded. From the recorded position of the three markers 8, the orientation of the head-mounted display 1 and thus the orientation of the head 13 can then be determined.

[0035] As an alternative to the described determination of the orientation data using the markers 8, the orientation data can also be recorded using a gyro sensor arranged on the head-mounted display 1.

[0036] In the present example, in addition to the markers 8, an acceleration sensor 9 is also arranged on the head-mounted display 1. With the acceleration sensor 9, head movements such as shaking or nodding the head can be detected as additional state variables of the head 13.

[0037] The orientation data of the head-mounted display 1 are indicative of the viewing direction of a surgeon O who wears the head-mounted display 1 on his head 13 and whose eyes are directed straight ahead. In the present example, the evaluation unit 10 is designed to evaluate the acquired orientation data D in order to determine the viewing direction of the surgeon O, assuming that he is looking straight ahead.

[0038] In addition, in the present example, the evaluation unit 10 is designed to detect head movements based on the acceleration data detected by the acceleration sensor 9 and to compare these with certain predetermined head movements in order to determine whether the surgeon O shakes the head 13 or nods the head 13.

[0039] The control unit 11 is configured to change the degree of transparency of the displays 5A, 5B of the head-mounted display 1 upon receipt of a signal S provided by the evaluation unit 10, indicating the surgeon's viewing direction or, if applicable, a shaking or nodding of the head, if the viewing direction is in a predetermined direction. Changing the degree of transparency of the displays 5A, 5B depending on the determined viewing direction (which is determined from the orientation of the head 13 and the assumption that the eyes are looking straight ahead) or, if applicable, a shaking or nodding of the head, enables intuitive control of the head-mounted display 1 for situational switching between different modes, such as free view, AR, and / or VR. Examples of such modes are: - no transparency of the displays 5A, 5B when viewing centrally to the front with display of the live image of a surgical microscope as a VR display; - full transparency in the lower part of the displays 5A, 5B when looking forward and down to allow viewing of instruments; - full transparency of the displays 5A, 5B when looking to the right, in order to be able to interact with the sterile nurse and / or AR display of preoperative data, e.g. radiological 3D data; - Full transparency of the displays 5A, 5B when looking to the left, in order to be able to interact with visitors and / or AR display of patient data such as pulse, oxygen saturation, etc. - full transparency of the displays 5A, 5B regardless of the viewing direction when shaking the head, - no transparency of the displays 5A, 5B regardless of the viewing direction when nodding the head.

[0040] The examples listed are not exhaustive and are intended only to demonstrate the possibilities of the invention.

[0041] The modes assigned to the gaze directions and / or head movements and / or other state variables of the head 13, if applicable, can be stored for a surgeon in a configuration profile. The configuration profile then specifies which transparency setting should apply to which state variable of the head 13—in this example, for which gaze direction and / or head movement—and which information source should be overlaid / displayed.

[0042] In the present example, the evaluation unit 10 and the control unit 11 are implemented as software on a computer. The data from the tracking system and the data from the acceleration sensor 9 are transmitted wirelessly to the computer. Control signals for controlling the degree of transparency of the displays 5A, 5B are also transmitted wirelessly from the control unit 11 to the displays 5A, 5B. As an alternative to implementation as software on a computer, the evaluation unit 10 and / or the control unit 11 can also be integrated into the head-mounted display 1, e.g., in the form of ASICs (Application Specific Integrated Circuits). If only one of the two units is integrated into the head-mounted display 1 and the other is implemented as software on a computer, communication between the unit integrated into the head-mounted display 1 and the unit implemented as software on a computer is preferably wireless.

[0043] It will now be discussed with additional reference to Fig. 3 explains the operation of the head-mounted display 1.

[0044] In a first step S100 in the present example, a reference direction is defined. For this purpose, the surgeon O can, for example, look in the direction of a predetermined reference point. A screen, an IR beacon, or a marker pointing in the direction of the patient can be used as a reference point. This provides information about the relative position and / or orientation of the coordinate system of the head-mounted display device 1 and a fixed coordinate system, in which the position and orientation of the patient are preferably also defined. If the tracking system determines that the line of sight of the surgeon O is pointing in the direction of the reference point, this direction can be defined as the reference direction, which can be used, for example, to define right and left in the surgeon's coordinate system.

[0045] In step S200, orientation data D of the head-mounted display 1 is then acquired. In the present example, in step S200, in addition to the orientation data D, acceleration data of the acceleration sensor 9 can also be acquired.

[0046] In step S300, the acquired orientation data D are then evaluated with respect to the reference direction in order to determine the viewing direction of the surgeon O. In addition, the acceleration data can be evaluated with respect to a head movement, if necessary.

[0047] Finally, in step S400, the degree of transparency of the displays 5A, 5B is changed if the viewing direction is in a predetermined direction or if a certain head movement is present.

[0048] It will now be discussed with additional reference to Fig. 4 explains an example assignment of viewing directions to different degrees of transparency.

[0049] When the surgeon looks forward (as in Fig. 4) within an angular range I, the displays 5A, 5B are opaque, i.e. they have a transparency level of 0%. If the surgeon O turns his head 13 13 - and thus his line of sight - downwards, the displays 5A, 5B become fully transparent, i.e. they have a transparency level of 100%, e.g. to be able to see instruments. If the surgeon O then turns his head to the right into the angular range II, the displays 5A, 5B remain fully transparent so that the surgeon can interact with a sterile nurse. In addition, pre-operative radiological data can be shown on the displays 5A, 5B. If the surgeon O then turns his gaze to the left into the angular range III, the displays 5A, 5B also remain fully transparent so that interaction with other people can be possible, and if necessary, patient data such as vital signs such as pulse and / or oxygen saturation are shown by overlaying on the displays 5A, 5B.If the surgeon then looks forward and straight ahead again, i.e., not downward, the displays 5A, 5B become opaque again. Furthermore, an image obtained with a digital surgical microscope, for example, is displayed as a VR image. Regardless of the viewing direction, the surgeon O in this example can also bring about full transparency of the displays 5A, 5B by shaking his head. Conversely, by nodding his head 13, he can make the displays 5A, 5B opaque, regardless of the viewing direction.

[0050] An embodiment of the invention is shown in Fig. 5 shows a head-mounted display 100. The head-mounted display 100 according to the embodiment differs from the one shown in the Fig. 1 and Fig. 2 in the way the direction of view is determined. Instead of the markers 8 or a gyro sensor, the Fig.In the embodiment shown in Figure 5, a camera 108 is provided, with which a field of view corresponding to the orientation of the head 13 of the operator O is recorded. A white-light camera is used to pursue a SLAM approach from robotics to determine the direction of view using the image data contained in the recording. SLAM stands for "Simultaneous Localization and Mapping." In the SLAM approach, a map of the environment is incrementally created from recordings of the environment of the head-mounted display 100, in which map the position and orientation of the head-mounted display 100 is known. As alternatives to conventional cameras, depth sensors such as ToF cameras can also be used to create a map of the environment in which the position and orientation of the head-mounted display 100 is known.

[0051] The present invention has been described in detail using an exemplary embodiment for explanatory purposes. However, a person skilled in the art will recognize that deviations from the exemplary embodiment are possible within the scope of the present invention. For example, instead of a head-mounted display, the head-wearable display device can be designed as magnifying glasses, in which content shown on at least one display can be reflected into the magnifying glass beam path by means of a mirror with adjustable transparency. In this case, magnifying glasses are understood to be a magnifying glass attached to a type of glasses frame. Magnifying glasses are used primarily in the fields of medicine and technology. For example, in general, cardiac, visceral, or eye muscle surgery, magnifying glasses are of great advantage to the surgeon for magnifying the surgical area.By changing the transparency of the mirror from transparent to opaque, the content displayed on at least one display can be switched between an AR mode and a VR mode. The degree of transparency can be controlled based on the viewing direction and / or head movements, as described for the example of the head-mounted display. The present invention is therefore not intended to be limited to the described embodiment, but solely by the appended claims. List of reference symbols 1 head-mounted display 2. Surgical microscope 2A, B image sensors 3A,B Playback arrangement 4 headbands 5A,B Screen 6 Recording unit 8 markers 9 Accelerometer 10 Evaluation unit 11 Control unit 13 heads 100 Head-Mounted Display 108 Camera I Angle range II Angular range III Angular range D Data O Surgeon S Signal S100 Set reference direction S200 Capture orientation data S300 Evaluate orientation data S400 Change transparency level

Claims

[1] Method for operating a head-mounted display device (100) comprising the steps of: - (S200) acquiring data (D) representative of at least one state variable of the head (13) of a person (O) wearing the head-mounted display device (1), - (S300) evaluating the acquired data (D) to determine the at least one state variable of the head (13), and - (S400) changing a degree of transparency of at least one display device (3) of the head-mounted display device (1) if the at least one state variable of the head (13) corresponds to a predetermined state variable, characterized by , that - image data are acquired as data (D) representative of the at least one state variable of the head (13) with at least one camera (108) associated with the head-mounted display device (100), wherein a field of view corresponding to the orientation of the head (13) is recorded with the camera (108), - the at least one state variable of the head (13) includes the viewing direction of the person (O) wearing the head-mounted display device (1) and - to determine the direction of view, the image data are evaluated in order to create a map of the environment in which the orientation and position of the head of the person (O) wearing the head-mounted display device (1) is known. [2] Method claim 1, characterized by that the map of the environment is created incrementally by means of a SLAM approach from images of the environment of the camera attached to the display device (100). [3] Method according to claim 1 or claim 2, characterized by that the image data contains not only grayscale or color values ​​for the individual pixels but also distance values ​​assigned to the pixels. [4] Method according to claim 3, wherein the distance data are obtained by means of stereo cameras or by means of ToF cameras. [5] Method according to one of claims 1 to 4, wherein the data (D) representative of at least one state variable of the head (13) includes movement data and / or acceleration data of the head-mounted display device (1) and the at least one state variable of the head (13) includes a head movement of the person (O) wearing the head-mounted display device (1). [6] Computer program product with program code for carrying out the method according to one of claims 1 to 4, when the program code is loaded into a computer and / or executed in a computer. [7] Head-mounted display device (1) with a detection unit (6) with means for detecting data (D) representative of at least one state variable of the head (13) of a person (O) wearing the head-mounted display device (1), with an evaluation unit (10) for evaluating the detected data (D) in order to determine therefrom the at least one state variable of the head (13), and with a control unit (11) for changing a degree of transparency of at least one display device (3) of the head-mounted display device (1) when the at least one state variable of the head (13) corresponds to a predetermined state variable characterized byin that the head-mounted display device (1) is designed to capture image data as data (D) representative of the at least one state variable of the head (13) using at least one camera (108) assigned to the head-mounted display device (1), wherein a field of view corresponding to the orientation of the head (13) is recorded with the camera (108) and wherein the at least one state variable of the head (13) includes the viewing direction of the person (O) wearing the head-mounted display device (1), and to evaluate the image data in order to determine the viewing direction in order to create a map of the environment in which the orientation and position of the head of the person (O) wearing the head-mounted display device (1) is known. [8] Head-mounted display device (1) according to claim 7, characterized bythat the head-mounted display device (1) is designed to incrementally create the map of the environment by means of a SLAM approach from images of the environment of the camera attached to the display device. [9] Head-mounted display device according to claim 7 or claim 8, characterized by that the image data contains not only grayscale or color values ​​for the individual pixels but also distance values ​​assigned to the pixels. [10] Head-mounted display device according to claim 9, wherein stereo cameras or ToF cameras are provided for obtaining the distance data. [11] Head-mounted display device (1) according to one of claims 7 to 10, wherein the data (D) include movement data and / or acceleration data of the head-mounted display device (1) and the at least one state variable of the head (13) includes a head movement of the person (O) wearing the head-mounted display device (1). [12] Medical optical observation system with a medical optical observation device (2) and a head-mounted display device (1) according to one of claims 7 to 11.

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