WO2025093135A1

WO2025093135A1 - Ophthalmic microscope device with a digital viewing device

Info

Publication number: WO2025093135A1
Application number: PCT/EP2023/080739
Authority: WO
Inventors: Tobias Trenschel; Imam USMANI; Claudio DELLAGIACOMA; Stefan Oelckers
Original assignee: Haag Streit GmbH
Current assignee: Haag Streit GmbH
Priority date: 2023-11-03
Filing date: 2023-11-03
Publication date: 2025-05-08
Anticipated expiration: 2026-05-03

Abstract

The ophthalmic microscope device, in particular a surgical ophthal- mic microscope, comprises a stand (2), a stereoscopic microscope unit (16) movably mounted to the stand (2), and a stereographic viewing device (18) movably mounted to the stand (2). The microscope unit (16) comprises variable magnification optics (36) and a stereoscopic camera device (40). The viewing device (18) comprises a ste- reographic display device (43) electrically connected to the camera device (40) as well as left and right eyepiece lens systems (46L, 46R). The eyepiece lens systems (46L, 46R) afocally project display areas (44L, 44R) of the display device (43) into left and right exit regions (48L, 48R). Each eyepiece lens system (46L, 46R) is de- signed to support an exit region (48L, 48R) much larger than those found in conven- tional ophthalmic microscopes and to provide, within all of the exit region (48L, 48R), a large field-of-view and high resolution. This allows the user to move their head freely while using the microscope device.

Description

Ophthalmic microscope device with a digital viewing device

Technical Field

The invention relates to an ophthalmic microscope device, in particular to a surgical ophthalmic microscopes, having a stand, a stereoscopic microscope unit, and a viewing device. The viewing device is movably mounted to the stand and is adapted to display a stereoscopic image to the user.

Background Art

Ophthalmic microscopes are used to perform investigations and/or surgical procedures on the human eye. They comprise magnification optics for magnifying the target and a viewing device, such as an eyepiece, for stereoscopically viewing the target.

Each channel of the eyepiece, i.e., the left as well as the right channel, has an exit pupil where an afocal image of the target is provided and where the user's pupil has to be placed for viewing.

In ophthalmology, the amount of light used for illuminating the target is limited because the patient's eye must not be exposed to excessive amounts of light. Hence, the exit pupil of each channel of the eyepiece is typically small, e.g., with a radius of 2 mm, in order to concentrate all the available light and send it into the observer's eye.

This is particularly true for surgical ophthalmic microscopes, i.e., for microscopes used for viewing the patient's eye during eye surgery, where the surgeon needs to have a very clear view of the target they operate on.

US 7784946 describes a surgical ophthalmic microscope having a viewing device with a stereo display. The images from the stereo display can be viewed through a conventional eyepiece. Hence, even though electronic displaying is used, the surgeon experiences the same "look-and-feel" as when using a conventional (analog) surgical ophthalmic microscope.

Disclosure of the Invention The problem to be solved by the present invention is to provide an ophthalmic microscope, in particular a surgical ophthalmic microscope, with superior user experience.

This problem is solved by the ophthalmic microscope device of claim 1.

Accordingly, the ophthalmic microscope device comprises at least the following elements:

- A stand: This stand is provided to support at least some of the components of the microscope device. It may, for example, be placed on the floor of an operating theater, but it may, e.g., also be mounted to a wall or ceiling of the room.

- A stereoscopic microscope unit comprising variable magnification optics and a stereoscopic camera device: This part of the microscope device obtains an image of the patient's eye, magnifies it, and projects it onto the camera device. The camera device is stereoscopic, i.e., it records a stereoscopic image, e.g., by using two cameras or distinct areas of a single camera.

- A viewing device: This is the part of the microscope device through which the user observes the stereoscopic image of the patient's eye. It is mov- ably mounted to the stand, i.e., it is, in operation, supported by the stand, either directly or indirectly, but the relative position of the viewing device in respect to the stand can be adjusted by the user to adapt it to their needs and for more comfortable viewing. The viewing device comprises at least the following parts:

- A left display area connected to a left channel of the stereoscopic camera device: The left display area is adapted to display the image of the left channel of the camera device. It has a resolution of at least 1000 x 1000 pixels.

- A left eyepiece lens system attributed to the left display area and to a left exit region. The left eyepiece lens system is adapted and structured to project the left display area into the left exit region.

- A right display area connected to a right channel of the stereoscopic camera device: The right display area is adapted to display the image of the right channel of the camera device. Again, it has a resolution of at least 1000 x 1000 pixels.

- A right eyepiece lens system attributed to the right display area and to a right exit region. The right eyepiece lens system is adapted and structured to project the right display area into the right exit region.

Each eyepiece lens system (i.e., the left as well as the right eyepiece lens system) is designed such that "good" viewing conditions are generated in the exit region. The exit region is defined as the volume in space over which these "good" viewing conditions are met. These "good" viewing conditions are quantitatively expressed as follows:

- The full field-of-view angle a of the associated display area in the exit region is at least 35°. In other words, the display area extends over a comparatively large field-of-view. This allows the user to view it at good resolution provided that the modulation transfer function (see below) of the lens system is sufficient.

- The modulation transfer function between the associated display area and the associated exit region is at least 0.5 for all angular frequencies below 11.1 cycles/deg at any point in the associated exit region as defined below. In other words. This condition assures that the modulation transfer function of the lens system provides, in the exit region, a resolution sufficient to easily distinguish fine details displayed by the display area.

According to the claim, the exit region extends at least over a cuboid that spans

- at least 4 mm along a Z-direction corresponding to a viewing axis of the viewing device,

- at least 4 mm along a horizontal X-direction,

- at least 4 mm along a Y-direction extending perpendicularly to the X- and Z-directions.

Further, the cuboid is at a distance of at least 17 mm from a userfacing end of the eyepiece lens system (i.e., from the end of the eyepiece lens system that is, in operation, closest to the user and farthest away from the associated display area).

In other words, the exit region extends at least over a cuboid having the specified, comparatively large extension and distance from the user-facing end of the eyepiece. At least in this spatial volume, the high-quality viewing conditions are maintained.

This concept is based on the understanding that such a design gives the user freedom for placing their eyes. It radically deviates from the conventional design of ophthalmic microscopes where the exit pupil is small and provides good resolution for the well-centered user's eye only. It not only extends the exit pupil over a larger volume of space, but it also guarantees a good resolution and field-of-view over all of that volume. This allows the user to place their head comparatively freely in front of the viewing device, without the narrow lateral and axial restrictions dictated by conventional eyepieces.

The concept is further based on the understanding that display technology is required for generating the image to be viewed by the user. In conventional analog systems, the light reflected from the patient's eye would not be sufficient to illuminate such a large, high-quality exit region.

The claimed microscope is found to increase operating convenience because it becomes easy for the user to switch between a position where they observe the microscope image and a position where they view the environment. This is particularly advantageous for surgical ophthalmic microscope devices because, during an operating procedure, the surgeon needs to be able to quickly change between viewing the microscopic image and viewing other parts of the operating theater.

By placing the cuboid at a distance of at least 17 mm from the userfacing end of the eyepiece lens system, even users wearing glasses can take advantage of the additional freedom that the system provides.

It must be noted that the good viewing conditions may extend beyond the claimed cuboid, e.g., in axial direction because, for an afocal system, good viewing quality may be easy to maintain over large axial distances.

Advantageously, the modulation transfer function MTF used for assessing the above criteria is averaged over all spectral components of the display device weighted by the CIE photopic luminous efficiency function V( ). By keeping this integrated modulation transfer function MTF(o) above the claimed threshold, achromatic effects are low in the exit region.

The viewing device may comprise a color display device with at least one display in it. The left and right display areas are formed by the color display device. Advantageously, the color display device comprises two displays, one for the left display area and the other for the right display area.

In this context, a "color" display advantageously comprises at least two, in particular at least three, types of spectrally different light emitters in the visible spectrum for distinct colors, such light emitters for R-, G-, and B-components. with each display pixel being provided with at least one of each of the different types of light emitters.

For easy manufacturing, the display device is, in each display area, flat. To compensate for the non-spherical shape of the display device and to improve imaging quality, each of the left and right eyepiece lens systems comprises, at an end closest to the display area, a lens having negative focal length, in particular a concave lens, in particular a plano-concave lens having a flat surface facing the display device.

The ophthalmic microscope device may further comprise left and a right peripheral areas located, on said color display device, adjacent to the left and right display areas, respectively. In addition, the control unit of the ophthalmic microscope device is adapted to display: - in said left and right display areas, color images from the stereoscopic camera and

- in said left and right peripheral areas, monochromatic image elements only.

In other words, the stereoscopic color images from the camera are displayed in the left and right display areas. In addition, monochromatic image elements, e.g., image elements generated by only one of the color components of the color display device, are displayed in the peripheral areas. This reduces chromatic errors perceivable by the user while keeping the eyepiece lens systems simple.

The left and right eyepiece lens systems are adapted and structured to afocally project the left and right peripheral area into the exit region. In this context, "to afocally project" is to be understood such that the light emitted by a given pixel is converted to a substantially collinear light field in the exit region such that the user placing their pupil in the exit region will be able to see the pixels without much strain to their eyes, i.e., the eyes are adapted to view the image at infinity or at least at a distance of at least 30 cm, in particular of at least 100 cm.

The eyepiece lens system may be structured to be tuned to deviate from a perfectly afocal projection to allow a viewing of the display areas at subjective distance between 30 cm and infinity.

In this case, a viewer placing their pupil in the exit region will see the stereoscopic image of the target at good resolution. In addition, they will see the monochromatic information. Since this monochromatic information originates from an area outside the display areas, it may not have such a good resolution as the stereoscopic image, and in particular it may suffer from pronounced dispersion. However, since it is monochromatic, dispersion effects will be less evident or absent to the user. Hence, the usable area of the display device can be expanded to display additional information without having to resort to more expensive and complex optics.

Advantageously, the stereoscopic microscope unit is movably mounted to the stand and movable in respect to the viewing device. Hence, the microscope unit is supported by the stand but, in operation, the relative position of the microscope unit in respect to the stand and in respect to the viewing device can be adjusted by the user to adapt to them their needs and to more comfortable viewing.

The viewing device is advantageously a compact device having a length, along its viewing axis Z, of less than 20 cm, in particular of less than 10 cm.

In a particularly advantageous embodiment, the viewing device comprises a guiding elements for the user's head for assisting the user to keep their eyes in the cuboids. For this purpose, the viewing device may comprise the following elements:

- A front plane defined to extend perpendicularly to the viewing axis Z of the viewing device, wherein the user-facing ends of the eyepiece lens systems lie in the front plane: This front plane is to be understood as a geometric plane, but the viewing device may also comprise a window located in the front plane.

- Left and right lateral guides having inner surfaces that project by at least 40 mm over the front plane towards a user-side of the viewing device: These guides are used to laterally guide the user's head. The distance between the inner surfaces of the lateral guides may be adapted to the typical width of a human's head at the eye region and to the width of the cuboids. To do so, a horizontal line extending in a direction parallel to the front plane, with the line being at a distance of 17 mm from the front plane and intersecting the optical axes of the left and right eyepiece lens systems, intersects the inner surfaces of the guides at two points that have a distance from each other between 10 cm and 18 cm.

If the cuboids, as defined above, of the left and the right eyepiece lens systems are located, at least in part, in the space between the left and the right lateral guides, the lateral guides will assist to position the user's head such that the pupils come to lie within the area of good optical resolution.

Advantageously, the lateral guides are non-transparent in order to shield the user from the environment as they are using the viewing device.

The viewing device may further comprise a forehead rest extending horizontally between at least a part of the distance between the lateral guides: This forehead rest can be used, by the user, as an axial stop for placing their head. It projects by at least 10 mm but by less than 30 mm over the front plane towards a userside of the viewing device to maintain a sufficient relief distance between the eyes and the user-facing ends of the eyepiece lens systems. In the direction parallel to the front plane, the distance between the forehead rest to optical axes of the left and right eyepiece lens systems is at least 10 mm in order not to obstruct the user's view.

Brief Description of the Drawings

The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein: Fig. l is a schematic drawing of an embodiment of the ophthalmic microscope device,

Fig. 2 shows some parts of the device of Fig. 1 in more detail,

Fig. 3 is a schematic component diagram of the optical elements of the device,

Fig. 4 shows an embodiment of an eyepiece lens system for a small display area,

Fig. 5 shows an embodiment of an eyepiece lens system for a large display area,

Fig. 6 illustrates some of the parameters of the viewing device,

Fig. 7 is a schematic sectional view of the viewing device of Fig. 6 along the plane of the optical axes of the left and right eyepiece lens systems,

Fig. 8 is a schematic sectional view of the viewing device of Fig. 6 along a vertical plane, with the optical axes of one of the eyepiece lens systems lying in said vertical plane,

Fig. 9 is an example of the contents of one of the display areas in an embodiment that reduces visible chromatic errors at the periphery,

Fig. 10 illustrates two means to position the user's head along direction Y.

Modes for Carrying Out the Invention

Definitions

The "modulation transfer function" MTF describes the spatial (in focal space) or angular (in afocal space) transmission of an imaging system. In afocal space, such as in the exit pupil, it is expressed as a function of angular frequency and it is typically calculated for a given observer pupil diameter. In the present context, the observer pupil diameter is 2 mm. Advantageously, MTF is averaged over all spectral components of the display device, optionally weighted by the CIE photopic luminous efficiency function V(+) (see e.g. https://en.wikipedia.org/wiki/Luminous_effi- ciency function).

In the present context, we compute the MTF for nine field points on a central cross of the display area (on the center, at X=1.0, +0.7, -0.7, and -1.0 times the maximum X-field coordinate of the display area, and Y=-1.0, -0.7, +0.7, and +1.0 times the maximum Y-field coordinate of the display area). The central point has a weight of 4.0 while the other points have a weight of 1.0. At each of these points, we take the average of the tangential and sagittal value for each RGB-color and finally the average of these colors.

It should be noted that the image quality towards the periphery of the display area, even outside the display area, usually does not drop immediately. Therefore, it is not impossible to obtain a good image of an area that has a linear extension e.g. 10 - 20% larger than the display area — but with a slightly lower image quality beyond the actual display area.

The "intensity transfer function" ITF(i, j, p) describes the transmission of light from a given pixel i, j into its respective angular coordinates at a given point p in the exit region. If all pixels emit light at equal intensity, ITF(i, j, p) describes the brightness distribution of the display area as measured at the given point p of the exit region over the pixels i, j. Again, advantageously, ITF is averaged over all spectral components of the display device weighted by the CIE photopic luminous efficiency function V( ).

The "R/number" of a lens having a spherical surface is given by the radius of curvature of the spherical surface divided by the clear aperture of the spherical surface.

The term "transversal" is to be understood such that if two planes or lines extend transversally to each other they are non-parallel to each other.

The "viewing axis" of the viewing device is parallel to the optical axes of the left and right eyepiece lens systems. If the axes of the left and right eyepiece lens systems are not exactly parallel (but optimized for a slightly convergent view), the viewing axis is the angle bisector of the axes of the left and right eyepiece lens systems.

The "stand" of the device may be a classical support designed to support the microscope device from below, e.g., by resting on the floor of an operating theater. It may, however, also be designed to suspend the microscope device from above and/or from the side, e.g., by being mounted to a ceiling or a wall.

Mechanical setup

Figs. 1 and 2 show the mechanical components of an embodiment of the ophthalmic microscope device. In this embodiment, the device is an ophthalmic surgical microscope as used for eye surgery.

The device comprises a stand 2, which is e.g., designed to rest on the floor of an operating theater, e.g., by means of a plurality of rollers 4, which form part of a foot section 6 of stand 2. As mentioned above, it may, however, also be designed to suspend the microscope device from a ceiling or wall. An arm 8 is mounted to foot section 6 and connects foot section 6 to a head section 10 of stand 2. Arm 8 is movable such that head section 10 can be displaced, in respect to foot section 6, in three linear dimensions. In the shown embodiment, this is implemented by four pivotal joints 12a, 12b, 12c, 12d allowing for pivotal displacement about two vertical and two horizontal axes.

The skilled person is familiar with various stand designs adapted to hold head section 10 such that it is movable in three linear dimensions. They may, e.g., use springs and or one or more counterweights for reducing the forces due to gravity.

Head section 10 is pivotal, in respect to arm 8, about a vertical axis 14.

The device of Figs. 1 and 2 further comprises a microscope unit 16 and a viewing device 18. As it will be described in more detail below, microscope unit 16 is stereoscopic and comprises variable magnification optics and a stereoscopic camera device. Viewing device 18 comprises a stereoscopic display device showing the images recorded by the stereoscopic camera device, and it further comprises left and right eyepiece lens systems that allow the user to view the images on the stereoscopic display device.

Stereoscopic microscope unit 16 is movably mounted to stand 2 and movable in respect to viewing device 18. As described above, this allows to adjust the locations of the two components according to the user's current needs.

In particular, the ophthalmic microscope device comprises a connecting member 20 (see Fig. 2) that connects stereoscopic microscope unit 16 to head section 10 of stand 2.

Stereoscopic microscope unit 16 is pivotal, about a horizontal pivot axis 22, in respect to connecting member 20, thereby allowing the operator to tilt unit 16 into the desired viewing direction.

Further, connecting member 20 is horizontally displaceable, in two horizontal directions 24a, 24b, in respect to head section 10. This displacement, which may in particular extend over at least, e.g., 5 cm along each direction 24a, 24b, allows to easily fine-tune the position of microscope unit 16 in respect to the target.

Viewing device 18 is also connected to head section 10. Advantageously, viewing device 18 is pivotal about a horizontal axis 26 and vertically displaceable in respect to head section 10. This allows to adjust its height and viewing angle to match the user's needs.

Further, viewing device 18 is horizontally displaceable in respect to head section 10. Advantageously, viewing device 18 is also pivotal about horizontal axis 26, vertically displaceable and horizontally displaceable in respect to microscope unit 16, which makes the position of viewing device 18 independent, at least partially, from the position of microscope unit 16.

In the shown embodiment, viewing device 18 is connected to a carrier member 28 and pivotal in respect thereto around horizontal axis 26 and vertically displaceable in respect thereto along a vertical guide 30. Carrier member 28 may be mounted to connecting member 20 and horizontally displaceable in respect thereto along a horizontal guide 32. It must be noted, that, in alternative embodiments, none, only one, or only two of these degrees of freedom may be implemented. In another embodiment, carrier member 28 may also be connected to head section 10 instead of being connected to connecting member 20.

The displacements about the various degrees of freedom of the device of Figs. 1 and 2 are, advantageously, self-locking, in the sense that the user can actively displace two components in respect to each other, whereafter they maintain the new position, e.g., against gravitational forces. This may, e.g., be implemented by frictional self-locking, i.e., the friction between the components is sufficient to maintain, after displacement, the mutual position of the two components. It may, e.g., also be implemented by means of electrical brakes and/or motors with self-locking gearing.

Optical Functionality

Fig. 3 schematically shows the primary optical components and the control unit of the device.

As can be seen, microscope unit 16 comprises an objective lens 34, variable magnification optics 36, imaging optics 38, and a stereoscopic camera device 40.

Advantageously, objective lens 34 is a "common main objective" (CMO), i.e., both stereographic viewing channels extend through the same objective lens 34.

Objective lens 34, variable magnification optics 36, and imaging optics 38 project the image of a target 42 (i.e., of the eye of the patient) onto left and right camera areas 40L, 40R of camera device 40.

Advantageously, microscope unit 16 has an operating distance DI between 100 and 280 mm. Variable magnification optics 36 is advantageously an afocal system, in particular a Galileo optical system, e.g., with swappable lens pairs for the different magnifications or an afocal zoom-system.

Imaging optics 38 comprises, e.g., a pair of achromatic lenses for the left and right channels, with each of them projecting an image of target 42 onto the respective camera area 40L, 40R.

The camera areas 40L, 40R may be formed by separate cameras or by different regions on a single camera.

Advantageously, each camera area 40L, 40R has a resolution of at least 1000 x 1000 pixels, in particular of at least 2000 x 2000 pixels.

Microscope unit 16 further comprises an illumination system 41a, 41b, 41c for generating at least two light fields that propagate, substantially coaxially or exact coaxially with the viewing channels, through objective lens 34 and, in addition thereto, an additional illumination propagating through objective lens 34 or not. The angle between this additional illumination and the observation channels may be e.g. 6°.

Objective lens 34, variable magnification optics 36, and camera device 40 are advantageously arranged in a common housing 35. At least part of the illumination system 41a, 41b, 41c may also be arranged on or in housing 35.

The stereographic images of the camera areas 40L, 40R are electronically transferred to a control unit 42, which may optionally perform image processing, image storage, and/or other functions as known to the skilled persons.

When stereographic images are to be observed by the user, control unit 42 forwards them to viewing device 18.

Viewing device 18 comprises a color display device 43 with left and right display areas 44L, 44R. The display areas 44L, 44R may be formed by separate displays or by separate areas of a single display. In operation, the display areas 44L, 44R are connected to the camera areas 40L, 40R via control unit 42 to display the recorded images. As mentioned, this connection may include image processing carried out, e.g., by control unit 42.

Viewing device 18 further comprises left and right eyepiece lens systems 46L, 46R. Left eyepiece lens system 46L is attributed to the left display area 44L and to a left exit region 48L and afocally projects the left display area 44L into the left exit region. Right eyepiece lens system 46R is attributed to the right display area 44R and to a right exit region 48R and afocally projects the right display area 44R into the right exit region 48R. Viewing device 18 further comprises left and right lateral guides 50L, 50R and a forehead rest 80 to help the user to properly position their head.

In operation, the stereoscopic image recorded by microscope unit 16 is displayed in viewing device 18, where the user may view it stereoscopically.

Details of the optical and mechanical design of viewing device 18 are described in more detail in the following sections.

Viewing Device Design

As mentioned above, viewing device 18 is designed to give the user freedom for placing their eyes by not only extending the exit pupil over a large volume but also be guaranteeing a good resolution and field-of-view over all of said volume.

An example of the implementation of this functionality is illustrated, for a small display area 44, in Fig. 4. (In the following, the suffixes L and R for reference numbers are omitted because the description applies equally to both of the channels of viewing device 18 and because the left and right channels advantageously have the same design.)

For describing the properties of the viewing device, a coordinate system based on orthogonal directions X, Y, and Z is used. Z is the direction along the viewing axis of viewing device 18, which is the direction along which the user sees the center of display area 44. X is perpendicular to Z and extends horizontally. Y is perpendicular to X and Z, i.e., it is vertical when viewing device 18 is pivoted such that Z extends horizontally.

As mentioned, the image in display area 44 is projected, by means of a lens system 46, into an exit region 48. This projection is an afocal projection, which is in the present context defined such that the light of a given point (pixel) of display area 44 is converted, at exit region 48, into a substantially parallel light as illustrated in Fig. 4, with the light from different pixels having different angles of incidence into exit region 48. Hence, when the user places their eyes into exit region 48 and adjust them to view far-away objects, they will see the image that is displayed on the display areas 44L, 44R.

As also mentioned above, eyepiece lens system 46 is designed to generate a large exit region 48 where the image in display area 44 can be viewed under a wide field-of-view angle a with a good resolution.

In this context, the (full) field-of-view angle a is defined as follows: When the user views the display area from exit region 48, he sees an image of the pixels of display area 44. The light from the outmost two pixels Pl, P2 of display area 44 arrives at a mutual angle a at exit region 48. This angle a is shown in Fig. 4 (for the assumption that it lies in the plane of Fig. 4). For a square or rectangular display area 44, the pixels Pl, P2 are located at opposite comers of the display area. For a round display area 44, the pixels Pl, P2 are two pixels on opposite sides at the edge of the display area.

Since display area 44 should appear to be large for the user when they use viewing device 18, the field-of-view angle a is advantageously at least 35°, in particular at least 45°, in particular at least 50°.

In order to see fine details, however, a large field-of-view angle a alone is not enough. Rather, the modulation transfer function MTF(o) between display area 44 and exit region 48 should be sufficiently large to distinguish the details that may be displayed by the pixels in display area 44.

Since exit region 48 is in afocal space, the modulation transfer function MTF(o) is expressed as a function of angular frequency o. For the users to see fine details, a good imaging quality for angular frequencies at and below 11.1 cy- cles/deg is considered to be a prerequisite. If the field-of-view is at least 35°, this corresponds to a resolution of almost 400 stripes or 800 adjacent black and white pixels. For a field-of-view of 45°, it corresponds to roughly 500 stripes or 1000 pixels. Further, the angular frequency of 11.1 cycles/deg (corresponding to an angular spacing of 0.09°) is still well below the angular resolution limit of the human eye of roughly 1/60° (= 0.017°) and therefore well visible.

If these conditions for the field-of-view angle a and the modulation transfer function MTF are maintained, there is good visibility of display area 44 for a user placing their eyes in exit region 48. As mentioned above, however, the exit region 48, which is defined as the region where these conditions are maintained, should cover a large volume of space, and in particular it should span a cuboid 54 that fulfills the following conditions (see also Figs. 6, 7):

- Its extension Ex along the X-direction is at least 4 mm.

- Its extension Ey along the Y-direction is at least 4 mm.

- Its extension Ez along the Z-direction is at least 4 mm.

In addition, cuboid 54 should not be too close to eyepiece lens system 46 in order to provide sufficient eye relief, in particular for users wearing glasses. Hence, the distance ER of cuboid 54 to the user-facing end 56 of eyepiece lens system 46 should be at least 17 mm or the cuboid should extend at least by dz beyond ER in direction to the user. Advantageously, the extension of cuboid 54 is even larger than the numbers given above. In particular, at least one of the following conditions is fulfilled: a) Cuboid 54 extends over at least 6 mm, in particular over at least 10 mm along the X-direction b) Cuboid 54 extends over at least 6 mm, in particular over at least 10 mm along the Y-direction c) Cuboid 54 extends over at least 6 mm, in particular over at least 10 mm along the Z-direction,

In particular, at least the conditions a) and b) are fulfilled for the user to move their head up and down as well as left and right over a conveniently large range. In particular, cuboid 54 extends over at least 10 mm along the X-direction and the Y-direction.

Eyepiece lens system 46 should be optimized to provide a large cuboid 54 for a given diameter of display area 44, i.e., for a given ratio of the effective focal length EFFL as compared to the field-of-view angle a. This is the case if the extensions Ex, Ey of cuboid 54 along directions X and Y are at least K EFFL/a, with K being 6°, in particular 12°.

For direction Z, a large extension of cuboid 54 is easier to implement in view of the basically afocal light field there, i.e., advantageously, the extension Ez of cuboid 54 along direction Z is at least 3 K EFFL/a, with K as defined above.

Hence, advantageously, if EFFL is the effective focal length of eyepiece lens system 46, at least one of the following conditions is fulfilled: a) Cuboid 54 extends over at least K EFFL/a along the X-direction b) Cuboid 54 extends over at least K EFFL/a along the Y-direction c) Cuboid 54 extends over at least 3 K EFFL/a along the Z-direction, wherein K = 6°, in particular K = 12°.

Again, advantageously, at least the conditions a) and b) should be met in combination.

The above conditions a), b), and/or c) are particularly advantageous if combined with display areas having small diameters of, e.g., less than 3.5 inch or less than 1.5 inch as described in the section "Notes".

As mentioned, the modulation transfer function MTF(o) should, in the exit region, be at least 0.5 for any angular frequencies o < 11.1 cycles/deg. Further, advantageously, it should not vary too much over the whole exit region. Hence, advantageously, exit region 48 is further restricted to the points p where the following condition is met for all angular frequencies o < 11.1 cy- cles/deg

MTF(o, p) > MTF(o, p max) • k, wherein p max is the point p for which MTF(o, p) has its maximum value for angular frequency o, and wherein k > 0.8, in particular k > 0.9.

As mentioned, display area 44 should extend over at least 1000 x 1000 pixels. For rich detail, however, it should extend over at least 2000 x 2000 pixels.

In addition, for display areas having that many pixels, the field-of- view angle a is advantageously at least 45°.

Further, as mentioned above, vignetting within exit region 48 should be low, i.e., for all points within exit region 48, all pixels i, j within display area 44 should have similar brightness.

In particular, if ITF(i, j, p) is the intensity transfer function as defined in the section "Definitions" above, the variation of ITF(i, j, p) over all pixels i, j should, at every point p in exit region 48, be small. Quantitatively, this can be expressed as follows: At any point p in exit region 48, the value of the intensity transfer function ITF(i, j, p) for the pixels i, j of the display area, should be, for all pixels i, j, at least 10%, in particular at least 20%, of the largest value of the intensity transfer function ITF(i, j, p).

Hence, the following condition should be met for every point p the exit region 48 and for all pixels i, j of display area 44

ITF(i, j, p) > ITF(i_max, j max, p) • kl, wherein i max, j max is the pixel for which ITF(i, j, p) has its maximum value for point p, and wherein kl > 0.1, in particular kl > 0.2.

Similarly, when the user displaces the eye within exit region 48, the perceived brightness of any given pixel i, j of display area 44 should advantageously not change too much. Hence, advantageously, the intensity transfer function ITF(i, j, p) is, for all points p in the exit region, at least 10%, in particular at least 20%, of the largest value that the intensity transfer function ITF(i, j) for the pixel i, j reaches within the exit region. Hence, the following condition should be met for every point p the exit region 48 and for all pixels i, j of display area 44

ITF(i, j, p) > ITF(i, j, p max) • k2, wherein p max is the point for which ITF(i, j, p) has its maximum value for the pixel i, j, and wherein k2 > 0.1, in particular k2 > 0.2.

In summary, the above describes some mandatory as well as some optional but advantageous conditions to be met in exit region 48, thereby defining the extent of exit region 48.

As mentioned, exit region 48 should extend at least over a cuboid 54 having a minimum volume and a sufficient eye relief as specified above. It must be noted, however, that parts of exit region 48 may extend beyond said cuboid, i.e., there may be points outside cuboid 54 having the large field-of-view, good modulation transfer, and, optionally, the low degree of vignetting as defined above.

Lens Design

In order to obtain the desired viewing quality at least all over cuboid 54, eyepiece lens system 46 needs to be designed properly.

One specific embodiment of eyepiece lens system 6, optimized for a comparatively small display area 44, is shown in Fig. 4. The lens system 46 comprises, along a path from display area 44 to exit region 48, a plano-concave field lens 60 with its flat side facing display area 44, a first achromat 62, and a second achromat 64.

For a display area 44 having a diameter of 18.4 mm (corresponding to the width/height of a square 1.03 inch display), an eye relief ER = 17 mm, a field- of-view angle a = 50°, and an exit region spanning a cuboid of Ex = Ey = Ez = 4.4 x 4.4 x 32, the following parameters (all lengths in mm) may, e.g., be used:

- The distance from display area 44 to field lens 60 is 0.3, the distance between field lens 60 to first achromat 62 is 4.429 (0.3 at the edge), and the distance between first achromat 62 and second achromat 64 is 0.3 mm.

- Field lens 60 is glass (H-7F72AGT, Nd = 1.922860, Vd = 18.90), left surface planar with 0e = 21.1037, right surface concave with R = 14.8215 and 0e = 18.5282.

- The left singlet of first achromat 62 is glass (H-BAK7GT, Nd = 1.568830, Vd = 56.04), left surface planar with 0e = 22.6089, right surface convex with R = 18.0857 and 0e = 22.6089. - The right singlet of first achromat 62 is glass (H-ZF72AGT, Nd = 1.922860, Vd = 18.90), left surface concave with R = 18.0857, 0e = 22.6089, right surface convex with R = 26.5333 and 0e = 26.2147.

- The left singlet of second achromat 64 is glass (H-ZF52TT, Nd = 1.846660, Vd = 23.78), left surface convex with R = 49.2331, 0e = 27.0039, right surface concave with R = 21.8205 and 0e = 26.0665.

- The right singlet of second achromat 64 is glass (H-ZBAF1, Nd = 1.622300, Vd = 53.17), left surface convex with R = 21.8205, 0e = 26.0665, right surface convex with R = 26.3943 and 0e = 26.0665.

The design of Fig. 4 can, for example, also be used for a display area having a diameter of 53.9 mm (3 inch display), an eye relief ER = 19 mm, a field-of-view angle a = 60°, an exit region spanning a cuboid of Ex = Ey = Ez = 12.5 x 12.5 x 20 mm. In this case, the following parameters may, e.g., be used (again, all lengths in mm):

- The distance from display area 44 to field lens 60 is 0.284, the distance between field lens 60 to first achromat 62 is 13.0, and the distance between first achromat 62 and second achromat 64 is 7.36 mm.

- Field lens 60 is glass (N-LASF9, Nd = 1.850249, Vd = 32.17), left surface planar with 0e = 59.4229, right surface is concave with R = 32.7688 and 0e = 46.8464.

- The left singlet of first achromat 62 is glass (N-SF66, Nd = 1.922860, Vd = 20.88), left surface concave with R = 168.98 and 0e = 46.8561, right surface concave with R = 69.6039 and 0e = 50.588.

- The right singlet of first achromat 62 is glass (N-SSK2, Nd = 1.622294, Vd = 53.27), left surface convex with R = 69.6039 and 0e = 50.588, right surface convex with R = 62.2616 and 0e = 50.558.

- The left singlet of second achromat 64 is glass (N-LASF9, Nd = 1.840249, Vd = 32.17), left surface convex with R = 63.9432, 0e = 49.7947, right surface concave with R = 32.8568 and 0e = 41.0747.

- The right singlet of second achromat 64 is glass (N-SSK2, Nd = 1.622294, Vd = 53.27), left surface convex with R = 32.8568, 0e = 41.0747, right surface convex with R = 52.2647 and 0e = 41.0747.

Hence, in some advantageous examples, eyepiece lens system 46 comprises a plano-convex field lens 60 as well as two achromats 62, 64.

For larger display areas, though, a somewhat simpler eyepiece lens system 46 can be used. For example, and as shown in Fig. 5, lens system 46 may comprise, along a path from display area 44 to exit region 48, a plano-concave field lens 60 with its flat side facing display area 44, an achromat 62, and a meniscus lens 64, e.g., as shown in Fig. 5.

For a display area 44 having a diameter of 135 mm (5.5-inch nonsquare display), an eye relief ER = 25 mm, a field-of-view angle a = 60°, and an exit region spanning a cuboid of Ex = Ey = Ez = 8 x 6 x 15, the following parameters (all lengths in mm) may, e.g., be used:

- The distance from display area 44 to field lens 60 is approximately 0, the distance between field lens 60 an achromat 62 is 115, and the distance between achromat 62 and meniscus is approximately 0.

- Field lens 60 is glass (N-BK7 (Schott) or H-K9L (CDGM), Nd = 1.5168, Vd = 64.167), rectangular, left surface planar with 0e > 115 x 64, right surface is concave with R = 120 and 0e > 115.

- Achromat 62 has a focal length of f = 750 mm.

- Meniscus 66 is a positive meniscus lens, with f = 150 mm.

As the skilled person will understand, eyepiece lens systems different from those shown in Figs. 4 and 5 can be used to achieve similar results, but the lenses must be calculated such that the desired extension and quality of exit region 48 are achieved.

All the embodiments of eyepiece lens systems shown here are designed to directly project display area 44 onto the user's retina, without generating an intermediate image. Even though classic ocular systems using intermediate images could — if dimensioned correctly — be used as well, such systems would be much less compact.

Hence, in an advantageous embodiment, the left and right eyepiece lens systems 46L, 46R are adapted to afocally project the left and right display areas 44L, 44R into the left and right exit regions 48L, 48R without forming an intermediate image.

In order to have a large field-of-view, the ratio between the half-diameter R of the left and right display areas 44L, 44R and the effective focal length EFFL of the eyepiece lens system 46 is advantageously high, with R/EFFL being at least tan(a/2), i.e. at least 0.32, in particular at least 0.41 for a > 35° or 45°, respectively.

Advantageously, the effective focal length EFFL should be no more than 250 mm, in particular no more than 80 mm. If the eyepiece lens system comprises at least one lens with a spherical surface having an R/number (as defined above) of less than 1, it becomes possible to design a more compact system.

The eyepiece lens system 46 may be structured to be tuned to deviate from a perfectly afocal projection of display area 44 into exit region 48 to allow a viewing of the display area at a subjective distance between 30 cm and infinity. For this purpose, for example, the distance between eyepiece lens system 46 and display area 44 may be variable.

Head Guidance

As mentioned, one advantageous aspect of the present viewing device relates to providing it with means for guiding the user's head, in particular for limiting its motion such that the user's pupils remain within exit region 48.

In particular, and as best seen from the example of Figs. 6 - 8, viewing device 18 may comprise left and right lateral guides 70L, 70R. Examples of the geometry of these guides 70L, 70R are described in the following.

This description makes reference to a "front plane" 72 of viewing device 18, which is defined as the plane perpendicular to the viewing axis Z of the viewing device, with the user-facing ends 56 lying in front plane 72.

In one embodiment, viewing device 18 may comprise a front plate 74 having a front surface coinciding with front plane 72 and being transparent at least in front of the left and right eyepiece lens systems 46L, 46R, e.g., protecting the viewing device 18 and making it easier to clean. Advantageously, front plate 74 has a continuous, smooth surface covering, from the user's side, both eyepiece lens systems 46L, 46R.

The left and right lateral guides 70L, 70R project by an extension Eb of at least 40 mm over front plane 72 towards the user-facing side of viewing device 18, with Eb being at least 40 mm, in order to laterally guide the user's head.

The guides 70L, 70R extend along direction Y and have inner surfaces 76 extending substantially vertically and facing each other. The spacing between these inner surfaces 76 is chosen to receive the user's head and limiting its horizontal position by abutting against its temples and/or cheekbones.

A suitable spacing of the inner surfaces 76 can be determined by looking at a horizontal line 78 extending parallel to front plane 72 (i.e., a line parallel to direction X) and being at a distance of 17 mm (the minimum eye relief) from front plane 72. By definition, the position of this line along direction Y is such that it intersects the optical axes A of the left and right eyepiece lens systems 46L, 46R. This line 72 intersects the inner surfaces 76 at points BL and BR as shown, e.g., in Figs. 6 and 7. In order to give good lateral guidance to an adult user's head, the distance Db between points BL and BR is advantageously between 10 and 14 cm.

The cuboids 54L, 54R are located at least in part in the volume of space extending between the left and right guides 70L, 70R.

The inner surfaces 76 of the lateral guides 70 may be elastically deformable to accommodate, in some degree, for differently sized heads and or lateral movements of the user's head. Advantageously, at the points BL, BR, the inner surfaces 76 of the guides 70L, 70R are each elastically deformable, along direction X, by at least 5 mm.

As mentioned, the lateral guides 70L, 70R are advantageously nontransparent and block the light in order to shield the user from environmental light.

Viewing device 18 further comprises a forehead rest 80 extending horizontally (i.e., along direction X) over at least part of the distance between the left and right lateral guides 70L, 70R). It is provided to make it easier for the user to position their eyes along direction Z.

Forehead rest 80 projects over front plane 72 by a distance EH towards the user-side of viewing device 16. At the center 82 of forehead rest 80, i.e., at the center plane between the optical axes A of the left and right eyepiece lens systems 46L, 46R, EH is between 10 and 30 mm. When an adult user places their forehead against forehead rest 80, this choice of EH places the user's pupils, at least approximately, within the exit regions 48L, 48R.

Forehead rest 80 may be curved, with its length of projection over front plane 72 being at a minimum in center 82, to better center the user's head.

In addition, forehead rest 80 should be located not too close to the optics in order not to interfere with the user's view. Hence, in a direction parallel to front plane 72, in particular along direction Y, the distance DH between forehead rest 80 and the optical axes A of the left and right eyepiece lens system is at least 20 mm. Advantageously, though, it is no more than 50 mm.

Alternatively to, or in addition to, the lateral guides, other means for laterally centering the user's head may be provided. As mentioned, for example, forehead rest 80 may be curved for supporting a lateral centering.

Another means for centering the user's head laterally as well as along direction Y is a nose recess 81 adapted and structured to receive the user's nose, as shown in Fig. 6, formed in viewing device 18 in a plane of symmetry between the optical axes A of the eyepiece lens systems 46L, 46R. The means for guiding the user's head may also be adjustable.

In particular, the lateral guides 70L, 70R may be adjustable between at least two, in particular more than two stable positions wherein, in said positions, the distances between the left and right lateral guides 70L, 70R differ. In particular, the distances Db between points BL and BR differ, in said positions. For example, and as illustrated under reference number 70R', the lateral guides 70L, 70R may be pivotal about an axis parallel to direction Y.

In that case, advantageously, the microscope device comprises a drive (as shown at reference number 71 in Fig. 7) for changing the distance Db by means of control unit 42 automatically, e.g., in view of individual user settings stored in control unit 42.

Fig. 10 illustrates two further means 90, 92 for guiding the user's head in direction Y. Even though the figure shows both these means provided on a viewing device 18, in an advantageous embodiment, the viewing device 18 includes only one of them.

A first one of these means is a top head stop 90 arranged, along direction Y, above the optical axes A, at a distance Rl, and projecting, parallel to the viewing axis Z, over plane 72 toward the user over a length of SI .

Rl and SI are selected such that top head stop 90 comes to lie on the top of the user's head when the user has placed their eyes properly in cuboid 54. Hence, advantageously SI is at least 5 cm, and Rl is between 5 cm and 20 cm.

Advantageously, top head stop 90 is displaceable to adjust Rl, in particular over a range of at least 5 cm. For example, and as shown, viewing device 18 may comprise a top support 94 forming a linear guide 96 for top head stop 90 to move along, and top head stop 90 may comprise a lock 98 for fixing its position along direction Y.

A first one of these means is a chin stop 92 arranged, along direction Y, below the optical axes A, at a distance R2, and projecting, parallel to the viewing axis Z, over plane 72 toward the user over a length of S2.

R2 and S2 are selected such that chin stop 92 comes to lie beneath the chin of the user's head when the user has placed their eyes properly in cuboid 54. Hence, advantageously S2 is between 3 cm and 15 cm, and R2 is between 10 cm and 25 cm.

Advantageously, chin stop 92 is displaceable to adjust R2, in particular over a range of at least 5 cm. For example, and as shown, viewing device 18 may comprise a bottom support 100 forming a linear guide 102 for chin stop 92 to move along, and chin stop 92 may comprise a lock 104 for fixing its position along direction Y.

Hence, and in more general terms viewing device 18 advantageously comprises:

- A front plane 72 perpendicular to the viewing axis Z of viewing device 18. The user-facing ends 56 of the eyepiece lens systems 46L, 46R lie, by definition, in the front plane 72.

- A top head stop 90 located along the direction Y at a distance R1 above the optical axes A of the left and right eyepiece lens systems 46L, 46R. Top head stop 90 projects by a distance SI over the front plane 72 towards a user-side of viewing device 18.

51 is at least 5 cm and R1 is between 5 cm and 20 cm. In particular, top head stop 90 is displaceable between positions having different distances Rl.

In addition thereto or alternatively, viewing device 18 advantageously comprises:

- A chin stop 92 located along the direction Y at a distance R2 below the optical axes A of the left and right eyepiece lens systems 46L, 46R. Chin stop 92 projects by a distance S2 over the front plane 72 towards a user-side of viewing device 18.

52 is between 3 cm and 15 cm, and R2 is between 10 cm and 25 cm. In particular, chin stop 92 is displaceable between positions having different distances R2.

In this context, the terms "above" and "below" are defined to be the vertical directions when viewing device 18 is pivoted such that directions X and Z are horizontal.

Reducing Dispersion Effects

As mentioned, the modulation transfer function MTF between display area 44 and exit region 48 should, within the exit region 48, exceed the claimed threshold when it is integrated over the whole visible range of the display device. Thus, achromatic effects in the image generated from display area 44 are low.

However, and as shown in Fig. 9 for one channel of viewing device 18, color display device 43 may comprise further pixels located outside display area 44. Eyepiece lens system 46 is designed project display area 44 with low dispersion, high resolution and a large field-of-view into exit region 48. In addition, it may also be able to project pixels outside display area 44, e.g., located within a peripheral area 84, into exit region 48. This peripheral area 84 is located adjacent to display area 44, radially outside (as seen from optical axis A) from display area 44.

The image within peripheral area 84 will typically be projected with inferior quality, as compared to the image within display area 44. In particular, it will suffer stronger dispersion, i.e., the user would perceive a multi-color image to be chromatically distorted. However, if control unit 42 is adapted to display, in the left and right peripheral areas 84, monochromatic image elements only, the negative consequences of chromatic distortions can be suppressed.

"Monochromatic image elements" in this context are understood consist of one or more spatially separated image elements, with each image element extending over a FWHM spectral range much smaller than the visible spectral range, in particular over a FWHM spectral range of no more than 100 nm, in particular no more than 60 nm. In particular, if color display device 43 has light emitters of three spectral types, e.g., RGB light emitters, each image element may be generated by light emitters of only one type.

For example, as shown in Fig. 9, such image elements 86a, 86b, 86c... may comprise writing and/or graphical symbols that add information relevant for the user.

It must be noted that the individual image elements 86a, 86b, 86c may have different colors: as long as they are spatially separated (in particular at a mutual distance from each other by more than 3 pixels, in particular by more than 10 pixels), dispersion effects are unlikely to be noticed by the user. For example, some of the image elements may be in red color, some of the image elements may be in green color, and some of the image elements may be in blue color.

In the embodiment of Fig. 9, peripheral area 84 surrounds display area 44. However, it may, e.g., be arranged on only one side of image area 44 or on only two opposite sides of display area 44. In particular if an elongate display is used for each channel, display area 44 may cover the whole width of the display while peripheral area 84 is arranged at one side or two opposite sides of display area 44 along the longer direction of the display.

Positioning Feedback

For systems with such large exit regions and such large field of view, it can be difficult for the user to see if they are still inside the high-quality exit region or not. This is particularly true for the compact eyepiece lens systems shown here, where it is impossible to provide an aperture that limits the light, at the location of the user's eyes, to the high-quality exit region 48 only.

Hence, advantageously, viewing device 18 is equipped to detect the user's eye position in at least one dimension and to give feedback if the eyes are positioned properly (i.e., with the pupils within exit region 48).

Hence, viewing device 18 advantageously comprises at least one sensor adapted to detect a user position, and control unit 42 is adapted to provide feedback indicating if the user's pupils are properly positioned (at least along one dimension) in exit region 48.

For example, viewing device 18 may comprise distance sensors 120R, 120L, as shown in Fig. 3, measuring the distances of the user's temples from the lateral guides 70L, 70R. If the difference between these two distances is too large, it can be determined that the user's head is not properly centered. This may be done using, for example, by means of two time-of-flight sensors pointing on the patient's temples.

In another embodiment, sensors 122 may be located in viewing device to measure specular reflections from the user's cornea in order to determine the position of the pupils. This may be done, for example, by the use of near-infrared light. Eye-tracking systems of this type are well-known in the art.

Feedback may be generated, by control unit 42, e.g., on the images displayed on the display areas 44L, 44R, e.g., by means of arrows displayed there, or it may be provided by other means, such as acoustically or by lamps located on the microscope device.

Notes

In the embodiment of Figs. 1 and 2, the ophthalmic microscope device is shown to be a surgical ophthalmic microscope. The same techniques, however, can also be applied to slit lamp microscopes, where similar problems arise.

The distance Q between the optical axes A at the user end 56 of the eyepiece lens systems 46L, 46R is advantageously adjustable in order to accommodate for the interpupillary distance of the user. Even though the present concept provides a large exit region 48 where the user can place their pupils, such a variable distance Q allows to reduce the requirements on the optics.

Advantageously, distance Q is at least adjustable between 55 and 72 mm.

As, e.g., shown in Fig. 7, this can, e.g., be implemented by placing the left and right eyepiece lens systems on different mounts 110L, 110R, the distance of which is adjustable along direction X. Advantageously, a drive 112 is provided for electrically displaying the mounts 110L, 110R, thereby changing distance Q by means of control unit 42 automatically, e.g., in view of individual user settings stored in control unit 42.

Hence, advantageously, the distance Q between the optical axes A of the left and right eyepiece lens systems 46L, 46R (with Q being defined at the user-facing ends (56) of the eyepiece lens systems 46L, 46R if the axes are non-paral- lel) is adjustable. In particular, the microscope device comprises a drive 112 for automatically adjusting the distance Q.

The display areas 44L, 44R are, in the example of Fig. 9, square. It must be noted, though, that they may also have other shapes, e.g., they may be rectangular or round.

Advantageously, the display areas 44L, 44R have small diameters for a compact design. In particular, the maximum diameters of the display areas 44L, 44R are no more than 89 mm (3.5 inch), in particular no more than 38 mm (1.5 inch).

For similar reasons, the effective focal lengths EFFL of the eyepiece lens systems 46L, 46R are advantageously small, with EFFL < 90 mm, in particular EFFL < 40 mm.

While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

1. An ophthalmic microscope device comprising a stand (2), a stereoscopic microscope unit (16) comprising variable magnification optics (36) and a stereoscopic camera device (40), a stereographic viewing device (18) movably mounted to the stand (2) and comprising

- a left display area (44L) of at least 1000 x 1000 pixels connected to a left channel of the stereoscopic camera device (40),

- a left eyepiece lens system (46L) attributed to the left display area (44L) and to a left exit region (48L) and projecting the left display area (44L) into the left exit region (48L),

- a right display area (44R) of at least 1000 x 1000 pixels connected to a right channel of the stereoscopic camera device (40),

- a right eyepiece lens system (46R) attributed to the right display area (44R) and to a right exit region (48R) and projecting the right display area (44R) into the right exit region (48R), wherein, for each eyepiece lens system (46L, 46R):

- a field-of-view angle a of the associated display area (44L, 44R) in the exit region (48L, 48R) is at least 35°

- a modulation transfer function MTF(o) between the associated display area (44L, 44R) and the associated exit region (48L, 48R) is at least 0.5 for angular frequencies below 11.1 cycles/deg at any point in the associated exit region (48L, 48R), wherein the exit region (48L, 48R) extends at least over a cuboid (54) that spans

- at least 4 mm along a Z-direction corresponding to a viewing axis of the viewing device (18),

- at least 4 mm along a horizontal X-direction,

- at least 4 mm along a Y-direction extending perpendicularly to the X- and Z-directions, and wherein the cuboid (54) is at a distance of at least 17 mm from a user-facing end (56) of the eyepiece lens system (46L, 46R).

2. The device of claim 1 wherein the field-of-view angle a for any point in the exit region (48L, 48R) is at least 45°, in particular at least 50°.

3. The device of any of the preceding claims wherein, assuming that ITF(i, j, p) is an intensity transfer function from a pixel i, j of the display area (44L, 44R) to a point p in the exit region (48L, 48R), the following condition is met for every point p of the exit region (48L, 48R) and for all pixels i, j

4. The device of any of the preceding claims wherein, assuming that ITF(i, j, p) is an intensity transfer function from a pixel i, j of the display area (44L, 44R) to a point p in the exit region (48L, 48R), the following condition is met for every point p of the exit region (48L, 48R) and for all pixels i, j

5. The device of any of the preceding claims wherein each of the left and right display areas (44L, 44R) extends over at least 2000 x 2000 pixels.

6. The device of any of the preceding claims wherein the viewing device (18) comprises at a color display device (43) with at least one display, and wherein the left and right display areas (44L, 44R) are formed by the color display device (43).

7. The device of claim 6 wherein the display device (43) is, in each display area (44L, 44R), flat, and wherein the left and right eyepiece lens systems (46L, 46R) comprise, at an end closest to the display area (44L, 44R), a lens (60) having negative focal length, in particular a, in particular a plano-concave lens (60) having a flat surface facing the display device (43).

8. The device of any of the claims 6 or 7 further comprising left and right peripheral areas (84) located, on said color display device (43), adjacent to the left and right display areas (44L, 44R), and a control unit (42) adapted to display,

- in said left and right display areas (44L, 44R), color images from the stereoscopic camera and wherein the left and right, respectively, eyepiece lens systems (46L, 46R) are adapted and structured to afocally project the left and right peripheral area into the exit region (48L, 48R).

9. The device of any of the preceding claims wherein a ratio R/EFFL between a half-diameter R of each of the left and right display areas (44L, 44R), respectively, and an effective focal length EFFL of the left and right, respectively, eyepiece lens system (46L, 46R) is at least 0.32, in particular 0.41.

10. The device of any of the preceding claims wherein an effective focal length EFFL of the left and right eyepiece lens systems (46L, 46R) is no more than 250 mm, in particular no more than 80 mm.

11. The device of any of the preceding claims wherein each of the left and right eyepiece lens systems (46L, 46R) comprises at least one lens with a spherical surface having an R/number of less than 1.

12. The device of any of the preceding claims wherein the viewing device (18) comprises a front plane (72) perpendicular to the viewing axis of the viewing device (18), wherein the user-facing ends (56) of the eyepiece lens systems (46L, 46R) lie in the front plane (72), left and right lateral guides (70L, 70R) having inner surfaces (76) that project by at least 40 mm over the front plane (72) towards a user-side of the viewing device (18), wherein said cuboids (54) of the left and the right eyepiece lens systems (46L, 46R) are located, at least in part, in a space between the left and the right lateral guides (70L, 70R).

13. The device of claim 12 wherein the viewing device (18) further comprises a forehead rest (80) extending horizontally between at least a part of a distance between the lateral guides (70L, 70R), projecting over the front plane (72) towards the user-side of the viewing device (18) by a length EH, wherein, at a center plane between the optical axes (A) of the left and right eyepiece lens systems (46L, 46R), EH is between 10 and 30 mm, wherein a distance DH, in a direction parallel to the front plane (72), between the forehead rest (80) and optical axes (A) of the left and right eyepiece lens systems (46L, 46R) is at least 20 mm, and in particular wherein the distance DH is no more than 50 mm, and in particular wherein the forehead rest (80) is curved, with its length of projection over the front plane (72) being at a minimum at a center plane between optical axes (A) of the left and right eyepiece lens systems (46L, 46R).

14. The device of any of the claims 12 or 13 wherein, at said points (BL, BR) of intersection with the horizontal line (78), the inner surfaces (76) of the guides (70L, 70R) are each elastically deformable by at least 5 mm.

15. The device of any of the claims 12 to 14 wherein a horizontal line (78) located at a distance of 17 mm from said front plane (72), extending parallel to the front plane (72), and intersecting optical axes (A) of the left and right eyepiece lens systems (46L, 46R), intersects the inner surfaces (76) at two points (BL, BR) having a distance (Db) between 10 cm and 18 cm.

16. The device of any of the claims 12 to 18 wherein the left and right guides (70L, 70R) are adjustable between at least two, in particular more than two stable positions, wherein the distances, in said positions, between the left and right guides (70L, 70R) differ, and in particular wherein the left and right guides (70L, 70R) are pivotal.

17. The device of claim 16 further comprising a drive (71) for automatically adjusting the distance between the left and right guides.

18. The device of any of the preceding claims wherein the viewing device (18) comprises a front plane (72) perpendicular to the viewing axis of the viewing device (18), wherein the user-facing ends (56) of the eyepiece lens systems (46L, 46R) lie in the front plane (72), a top head stop (90) located along the direction Y at a distance R1 above optical axes (A) of the left and right eyepiece lens systems (46L, 46R), wherein the top head stop (90) projects by a distance SI over the front plane (72) towards a user-side of the viewing device (18), and wherein SI is at least 5 cm and R1 is between 5 cm and 20 cm.

19. The device of claim 18 wherein the top head stop (90) is displaceable between positions having different distances Rl.

20. The device of any of the preceding claims wherein the viewing device (18) comprises a front plane (72) perpendicular to the viewing axis of the viewing device (18), wherein the user-facing ends (56) of the eyepiece lens systems (46L, 46R) lie in the front plane (72), a chin stop (92) located along the direction Y at a distance R2 below optical axes (A) of the left and right eyepiece lens systems (46L, 46R), wherein the chin stop (92) projects by a distance S2 over the front plane (72) towards a userside of the viewing device (18), and wherein S2 is between 3 cm and 15 cm, and R2 is between 10 cm and 25 cm.

21. The device of claim 20 wherein the chin stop (92) is displaceable between positions having different distances R2.

22. The device of any of the preceding claims wherein the stereoscopic microscope unit (16) is movably mounted to the stand (2) and movable in respect to the viewing device (18).

23. The device of claim 22 further comprising a connecting member (20) connecting the stereoscopic microscope unit (16) to a head section (10) of the stand (2), wherein the stereoscopic microscope unit (16) is pivotal, about a horizontal pivot axis (22), in respect to the connecting member (20), and/or wherein the connecting member (20) is displaceable, in two non-parallel horizontal directions (24a, 24b), in respect to the head section (10), and in particular wherein the viewing device (18) is mounted to the connecting member (20).

24. The device of claim 23 wherein the stand (2) comprises a foot section (6) and a movable arm (8) connecting the foot section (6) to the head section (10) and wherein the position of the head section (10) is displaceable, in respect to the foot section (6), in three linear dimensions.

25. The device of claim 24 wherein the head section (10) is pivotal, in respect to the arm (8), about a vertical axis (14).

26. The device of any of the claims 22 to 25 wherein the viewing device (18) is vertically displaceable and is pivotal about a horizontal axis (26) in respect to the stereoscopic microscope unit (16).

27. The device of any of the claims 23 to 25 and of claim 26 wherein the viewing device (18) is vertically displaceable and is pivotal about the horizontal axis (16) in respect to the connecting member (20).

28. The device of any of the preceding claims wherein the cuboid (54) fulfills at least one of the following conditions: a) it extends over at least 6 mm, in particular over at least 10 mm along the X-direction b) it extends over at least 6 mm, in particular over at least 10 mm along the Y-direction c) it extends over at least 6 mm, in particular over at least 10 mm along the Z-direction, and in particular wherein it fulfills at least the conditions a) and b).

29. The device of any of the preceding claims wherein the left and right eyepiece lens systems (46L, 46R) are adapted to afocally project the left and right display areas (44L, 44R) into the left and right exit regions (48L, 48R) without forming an intermediate image.

30. The device of any of the preceding claims wherein the modulation transfer function MTF(o) is integrated over a visible range of a display device (43) forming the left and right display area (44L, 44R), in particular averaged over all spectral components of the display device (43) weighted by CIE photopic luminous efficiency function V( ).

31. The device of any of the preceding claims wherein the viewing device (18) further comprises a nose recess (81) arranged in a plane of symmetry between optical axes (A) of the left and right eyepiece lens systems (46L, 46R).

32. The device of any of the preceding claims wherein a distance (Q) between optical axes (A) of the left and right eyepiece lens systems (46L, 46R) at the user-facing ends (56) of the eyepiece lens systems (46L, 46R) is adjustable.

33. The device of claim 32 further comprising a drive (112) for automatically adjusting said distance (Q).

34. The device of any of the preceding claims wherein the viewing device (18) comprises at least one sensor (120, 122) adapted to detect a user position, and control unit (42) is adapted to provide feedback indicating if the user's pupils are properly positioned.

35. The device of claim 34 and of any of the claims 12 to 17 comprising distance sensors (120) located in lateral guides (70L, 70R).

36. The device of any of the preceding claims wherein, for any point p in the exit region (48), the following condition is met for all angular frequencies o < 11.1 cycles/deg

37. The device of any of the preceding claims wherein, if EFFL is an effective focal length of the left and right eyepiece lens system (46), at least one of the following conditions is fulfilled: a) the cuboid (54) extends over at least K EFFL/a along the X-di- recti on b) the cuboid (54) extends over at least K EFFL/a along the Y-di- rection c) the cuboid (54) extends over at least 3 K EFFL/a along the -direction, wherein K = 6°, in particular K = 12°. and in particular at least the conditions a) and b) are met in combination.

38. The device of any of the preceding claims wherein maximum diameters of the display areas (44L, 44R) are no more than 89 mm, in particular no more than 38 mm.

39. The device of any of the preceding claims wherein effective focal lengths (EFFL) of the eyepiece lens systems (46L, 46R) are smaller than 90mm, in particular smaller than 40 mm.

40. The device of any of the preceding claims wherein the viewing device (18) further comprises a front plate (74) having a continuous surface covering both eyepiece lens systems (46L, 46R).