Abstract:
A monitoring device arrangement is disclosed comprising a mounting rail, and at least two fixed monitoring devices, said fixed monitoring devices each being mounted on a respective holder, said holders being arranged on said mounting rail, wherein each said holder comprises a lock device locking said holder in a determined position on said mounting rail, and wherein said lock device is releasable to allow sliding repositioning of said holder on said mounting rail.

Description:
FIELD OF INVENTION 
       [0001]    The present invention concerns a monitoring device arrangement comprising a mounting rail and at least two fixed monitoring devices mounted on holders arranged on said mounting rail. 
       BACKGROUND 
       [0002]    Monitoring device arrangements may include a number of monitoring devices, such as cameras and IR detectors. The monitoring devices may be mounted on a support and may be used for monitoring an area around the monitoring device arrangement. The monitoring devices in such a monitoring device arrangement may all be of the same type, or they may be of different types. For instance, one monitoring device may be a fish-eye camera and another monitoring device may be a fix focus lens camera. 
         [0003]    It may be desirable to monitor, e.g., a 360° field of view around the monitoring device arrangement. However, in some locations, there may be objects, such as walls, blocking the view of one or more of the monitoring devices, reducing the usable field of view of the monitoring device arrangement. Further, the most important parts of the scene to monitor may be located such that they are near the edge of the field of view of one of the monitoring devices, whereas it would be desirable to have these parts at the centre of the field of view. It would therefore be desirable to have a monitoring device arrangement allowing flexibility in the positioning of the monitoring devices. 
         [0004]    WO 2004/095386 discloses a surveillance apparatus incorporating a plurality of CCTV cameras individually adjusted and fixed to survey the required areas of a site. The cameras are mounted from a camera support ring by a hinged bracket arrangement. The cameras are manually adjustable in azimuth by sliding the bracket arrangements around the support ring and clamping them in the required positions. 
       SUMMARY 
       [0005]    It is an object of the present invention to provide a monitoring device arrangement which alleviates the problems associated with prior art monitoring device arrangements. 
         [0006]    A particular object is to provide a monitoring device arrangement which enables flexible, yet well-defined positioning of monitoring devices. 
         [0007]    According to the invention, these and other objects are achieved, in full or at least in part, by a monitoring device arrangement comprising a mounting rail, and at least two fixed monitoring devices, said fixed monitoring devices each being mounted on a respective holder, said holders being arranged on said mounting rail, wherein each said holder comprises a lock device locking said holder in a determined position on said mounting rail, and wherein said lock device is releasable to allow sliding repositioning of said holder on said mounting rail. It may be noted that the term “fixed monitoring device” refers to a monitoring device which is not in itself capable of panning or tilting, but which may be capable of zooming. With a monitoring device arrangement according to the invention, it is possible to mount fixed monitoring devices on the mounting rail with a possibility of adjusting the positions of the fixed monitoring device by sliding them along the mounting rail with the lock device released. Once the fixed monitoring devices have been placed in the desired position, the lock device of the holder may be locked, and the fixed monitoring devices may be securely held on the mounting rail. In this manner, the monitoring device arrangement is flexible in that the positions of the fixed monitoring devices in relation to each other or in relation to objects in the surroundings may be chosen in accordance with current needs. Thus, for instance, fixed monitoring devices may be positioned to cover a desired field of view and avoiding positions where the field of view of one or more of the fixed monitoring devices would be blocked by a wall or other object. Further, parts of a potential field of view which for other reasons should not be monitored may also be avoided. This may, e.g., be the case if a monitoring device arrangement is placed indoors, and it is not allowable to monitor persons outside, through a window. By adjusting the positions of the fixed monitoring devices on the mounting rail, their usable combined field of view may thus be utilised, instead of having to shut off an inappropriately directed fixed monitoring device. Conversely, there may be particularly important parts of the scene to monitor, and by adjusting the positions of the fixed monitoring devices on the mounting rail, the field of view may be centred on the most important parts of the scene. 
         [0008]    In some embodiments, the mounting rail is arcuate or angled. Thereby, a wide range of positions for the fixed monitoring devices are available in order to cover a desired field of view. If the mounting rail is arcuate and is free from sharp angles, the holders may easily be slidingly repositioned along the entire arc portion. 
         [0009]    The mounting rail may be circular. In this manner, the fixed monitoring devices may be positioned in a large number of positions along the circular mounting rail, thereby allowing a large combined field of view. 
         [0010]    In an embodiment, the lock device of the holder comprises a magnet, and the mounting rail comprises ferromagnetic material. This is a practical way of achieving a releasable lock, which does not require tools for releasing and locking the holder. 
         [0011]    The mounting rail may comprise at least one magnet, and said holder may comprise ferromagnetic material. This is also a practical way of achieving a releasable lock, which does not require tools for releasing and locking the holder. 
         [0012]    In one embodiment, the holder comprises a biasing member biasing said holder against said mounting rail. In this manner, it may be ensured that the holder is securely held in position on the mounting rail. 
         [0013]    The biasing member may comprise a leaf spring. 
         [0014]    The mounting rail may have at least one recess or protrusion in a predetermined position along said mounting rail, and the biasing member may be adapted to engage said recess or protrusion for locking said holder in said predetermined position. This makes it easy for a person installing the monitoring device arrangement to place the fixed monitoring devices correctly in case one of the predetermined positions is desired. Similarly, this may assist in adjusting the position of the fixed monitoring devices in an already mounted monitoring device arrangement. For instance, the mounting rail may be provided with four recesses or protrusions in positions 90° apart along the mounting rail. This may simplify equidistant positioning of four fixed monitoring devices for a 360° field of view. 
         [0015]    The holder may comprise a rotation prevention member preventing said holder from rotating around said mounting rail. This may ensure a stable positioning of the fixed monitoring device once the holder is locked in the desired position. 
         [0016]    In one embodiment, the holder comprises a retaining member arranged to retain said holder on said mounting rail also when said lock device is released. Hereby, it may be ensured that the holder is retained on the rail even during a repositioning movement. It may also be ensured that the holder is retained on the mounting rail even if the locking function of the lock device were to fail for some reason. 
         [0017]    The retaining member may comprise a hook retaining said holder on said mounting rail. This is a practical way of ensuring that the holder is kept on the mounting rail. 
         [0018]    At least one of said fixed monitoring devices may be a fix focus lens camera. Further, at least one of said fixed monitoring devices may be a non-pannable, non-tiltable camera capable of zooming. 
         [0019]    In an embodiment, the monitoring device arrangement further comprises a pannable and/or tiltable monitoring device, wherein the mounting rail is arranged surrounding said pannable and/or tiltable monitoring device. For instance, the fixed monitoring devices may be used for getting an overview and the pannable and/or tiltable monitoring device may be used for targeting objects found using the overview monitoring devices. 
         [0020]    The monitoring device arrangement may further comprise a shielding portion arranged to shield said pannable and/or tiltable monitoring device from environmental factors, such as sunlight or rain. In this manner, the pannable and/or tiltable monitoring device may be protected from harmful environmental factors, such as ingress of rain. If the pannable and/or tiltable monitoring device is a camera, it may be advantageous to shield it from the sun in order to obtain good images. 
         [0021]    The pannable and/or tiltable monitoring device may be a PTZ camera, i.e. a camera capable of panning, tilting and zooming. If the fixed monitoring devices are cameras, these may be used for getting an overview image of an area surrounding the monitoring device arrangement. The PTZ camera may be used for tracking and zooming in on objects found in the overview image. The overview image may be stitched together from the images captured by the respective fixed cameras to form a panoramic image. The overview may also be shown as separate images, e.g., on a video wall. 
         [0022]    The monitoring device arrangement may comprise a motor for sliding repositioning of said holder on said mounting rail. In this manner, the positions of the fixed monitoring devices may be adjusted even if the monitoring device is difficult to reach, or if adjustment is needed also after installation of the monitoring device, e.g., for different monitoring needs during the day and at night. A further scope of applicability of the present invention will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. 
         [0023]    Hence, it is to be understood that this invention is not limited to the particular component parts of the device described or steps of the methods described as such device and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It must be noted that, as used in the specification and the appended claim, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements unless the context clearly dictates otherwise. Thus, for example, a reference to “an object” or “the object” may include several objects, and the like. Furthermore, the word “comprising” does not exclude other elements or steps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The invention will now be described in more detail by way of example and with reference to the accompanying schematic drawings, in which: 
           [0025]      FIG. 1  is a perspective view of a monitoring device arrangement according to a first embodiment, 
           [0026]      FIG. 2  is a perspective view of a detail in the monitoring device arrangement in  FIG. 1  showing a monitoring device in a holder arranged on a mounting rail, 
           [0027]      FIG. 3  is a perspective view of the monitoring device and the holder shown in  FIG. 2 , 
           [0028]      FIG. 4  is a sectional view of the monitoring device and holder of  FIG. 3 , 
           [0029]      FIG. 5  is a perspective view of a monitoring device arrangement according to a second embodiment, 
           [0030]      FIG. 6  is an exploded view of the monitoring device arrangement shown in  FIG. 5 , 
           [0031]      FIG. 7  is a perspective view of part of a monitoring device arrangement with a holder according to a third embodiment, 
           [0032]      FIG. 8  is a perspective view of a holder according to a fourth embodiment, and 
           [0033]      FIG. 9  is a perspective view of part of a monitoring device arrangement with a holder according to a fifth embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    In  FIG. 1  a monitoring device arrangement  1  is shown comprising a number of monitoring devices  2 . In this embodiment, the arrangement is a camera arrangement comprising a number of fixed cameras, i.e. cameras that are in themselves incapable of panning and tilting. The camera arrangement  1  has a shielding portion  3  for shielding the fixed cameras  2  against environmental factors, such as sunlight, rain, and snow. Further, the camera arrangement  1  has an attachment portion  4  for attaching the camera arrangement to, e.g., a mounting bracket (not shown in  FIG. 1 ) for mounting on a surface such as a wall, a ceiling, or a pole. The fixed cameras  2  are arranged inside a protective window  5 . At an end, which in  FIG. 1  is a lower end, of the camera arrangement, there is an opening  6  surrounded by a trim ring  7 . The shielding portion  3  may be made of aluminium, making it allowable to arrange the camera arrangement recessed in a ceiling while fulfilling fire safety requirements. 
         [0035]    Turning to  FIG. 2 , a fixed camera  2  of the camera arrangement  1  may be seen. The fixed camera  2  is arranged in a holder  8 , which is mounted on a mounting rail  9 . In the embodiment shown, the mounting rail  9  is of circular shape, such that it surrounds the opening  6 . 
         [0036]    As may be more clearly seen in  FIG. 3 , the holder  8  has a number of retaining members in the form of hooks  10  and bosses  11 . The hooks  10  are arranged at the front of the holder  8 , such that when the holder  8  is mounted on the mounting rail  9 , the hooks  10  are engaged around a radially outer rim  12  of the mounting rail  9  (see  FIGS. 2 and 4 ). The bosses  11  are arranged at the back of the holder  8 , such that when the holder  8  is mounted on the mounting rail  9 , the bosses  11  abut a radially inner rim  13  of the mounting rail  9 , thereby preventing the holder  8  from tilting or rotating forwards. Further, the holder  8  has a biasing member in the form of a leaf spring  14  arranged at the back of the holder  8 . The leaf spring  14  is slightly hook-shaped. When the holder  8  is mounted on the mounting rail  9 , the leaf spring  14  urges the holder  8  radially outwardly against the inner rim  13  of the mounting rail  9  and the hook-shape engages the inner rim  13 . 
         [0037]    With reference to the cross-section shown in  FIG. 4 , the holder  8  has a locking device in the form of a magnet  15 . In this embodiment, the mounting rail  9  is made of a ferromagnetic material, and the magnet may therefore securely lock the holder  8  in position on the mounting rail. By slightly lifting the holder  8  off the mounting rail  9 , or by pressing back the leaf spring  14  and tilting the holder  8  slightly forward, the magnetic force of the magnet  15  may be overcome, such that the holder can be moved slidingly along the mounting rail  9  to a desired position. The hooks  10 , bosses  11 , and leaf spring  14  ensure that the holder is retained on the mounting rail  9  during this movement. Once the holder  8  has been moved to the desired position, the holder is released, such that the force of the magnet  15  locks the holder  8  in place. Thus, the holder  8  may be unlocked, moved, and locked without the need for tools. 
         [0038]    As may be seen in  FIG. 2 , recesses or notches  16  are formed in the mounting rail  9  at predetermined positions. The holder  8  has a protrusion in the form of the leaf spring  14 , which may engage the notch  16  when the holder is positioned in one of the predetermined positions. When a holder  8  is moved to one of the predetermined positions, the leaf spring  14  of the holder snaps into the corresponding notch  16 . This snapping action of the leaf spring may be heard and/or felt by the person moving the holder  8 , indicating that the fixed camera  2  and the holder  8  have reached the predetermined position. For instance, the mounting rail  9  may have four notches  16  spaced 90° apart, thereby simplifying mounting of four fixed cameras  2  at right angles for creating a combined 360° view. 
         [0039]    Now turning to  FIG. 5 , the camera arrangement  1  may be used as an accessory for a pannable and/or tiltable camera  17 . In such case, the pannable and/or tiltable camera  17  may be mounted inside the camera arrangement  1  with a dome  18  of the pannable and/or tiltable camera  17  protruding through the opening  6  of the camera arrangement  1 . The combination of the camera arrangement  1  and the pannable and/or tiltable camera  18  may be mounted to, e.g., a wall using a mounting bracket  19 , which may be attached via the attachment portion  4  of the camera arrangement  1 . When the accessory  1  and the pannable and/or tiltable camera  17  are assembled to a combined camera arrangement  101 , the shielding portion  3  may shield the pannable and/or tiltable camera  17  from sun, rain, snow, and other potentially harmful environmental factors. 
         [0040]    The fixed cameras  2  may be fix focus lens cameras, and the pannable and/or tiltable camera  17  may be a PTZ camera, i.e. a camera capable of panning, tilting and zooming. The fix focus lens cameras  2  may be used for creating an overview image of an area surrounding the combined camera arrangement  101 . If, for instance, four fixed cameras  2  are placed approximately 90° apart on the mounting rail  9 , a 360° view may be achieved. Depending on the viewing angle of the fixed cameras, a 360° view may be achieved with fewer cameras, for instance three fixed cameras  2  placed approximately 120° apart. When using the combined camera arrangement  101 , the PTZ camera  17  may be directed to points of interest identified in the overview image created by the fixed cameras  2 . Hereby, objects or persons may be tracked throughout the area covered by the cameras  2 ,  17  of the combined camera arrangement  101 , while still maintaining an overview of the area. 
         [0041]    When mounting the combined camera arrangement  101 , the environment surrounding the combined camera arrangement  101  may be such that one or more of the fixed cameras would have all or parts of the field of view blocked by objects, such as building walls. In such case, the predetermined positions of the fixed cameras  2  may not be suitable. However, the holders  2  allow the fixed cameras  2  to be moved to more suitable positions along the mounting rail  9 . Should, for instance, one of the predetermined positions on the mounting rail  9  entail that one of the fixed cameras be pointing into a wall, the fixed cameras may be moved closer together along the remaining portion of the mounting rail, such that they provide more overlap in the useable field of view surrounding the combined camera arrangement. Similarly, should the camera arrangement  101  be placed indoors, it might not be allowable to monitor persons walking past outside a window. Also in this type of situation, it may be desirable to adjust the positions of the fixed cameras  2  so as not to monitor a certain part of the surrounding area, but to use the fixed cameras  2  to get good coverage of the remaining surrounding area. It will be appreciated that a person skilled in the art can modify the above described embodiments in many ways and still use the advantages of the invention as shown in the embodiments above. As an example, although reference has here been made to monitoring devices in the form of cameras, other monitoring devices may be used instead. Examples of such monitoring devices include IR detectors and microphones. 
         [0042]    Instead of, or in combination with, cameras employing visible light, the cameras of the combined camera arrangement may comprise IR cameras, or thermal cameras. 
         [0043]    The fixed monitoring devices need not necessarily each contain their own electronics. Instead, the electronics necessary for the two or more fixed monitoring devices may be arranged at one, common location. 
         [0044]    In the embodiment described above, there are four fixed monitoring devices. However, another number of fixed monitoring devices may be arranged in the camera arrangement. Depending on the local conditions in the place where the monitoring device arrangement is mounted, it may be possible to create a desired overview with three, or as few as two fixed monitoring devices. In other instances, it may be desirable to have more than four fixed monitoring devices. For instance, a combination of a number of fixed cameras and other monitoring devices may be arranged on the mounting rail. 
         [0045]    When the camera arrangement has been mounted, it may be possible to supplement it with more monitoring devices. 
         [0046]    The monitoring devices may be removable from the holders, allowing exchange of monitoring devices for maintenance or for changing from one type of monitoring device to another. 
         [0047]    In the embodiment described above, the fixed cameras are fix focus lens cameras, but cameras capable of zooming may be used instead. 
         [0048]    The number of recesses or notches in the mounting rail need not necessarily be three or four. A smaller or larger number of recesses may be provided depending on the number of predetermined positions to mark. Further, the predetermined positions may be marked by recesses of other shapes than the notch shown in  FIG. 2 , such as through holes and blind holes. 
         [0049]    The mounting rail and the holders, respectively, may be formed such that locking of the holders is only possible in the predetermined positions. They may be achieved, e.g., by suitable shapes of the recesses and protrusions. However, generally, it is preferable to allow an infinitely variable positioning of the holders on the mounting rail. 
         [0050]    Instead of the recesses, the mounting rail may be provided with protrusions marking the predetermined positions. In such case, the holder may be provided with a corresponding recess for engagement with the protrusions. 
         [0051]    In the embodiment shown in  FIG. 5 , a PTZ camera is placed in the opening of the camera arrangement. However, the pannable and/or tiltable monitoring device may be another kind of camera or monitoring device. The camera arrangement may in some instances be mounted without a central, pannable and/or tiltable monitoring device. For example, a spotlight or loudspeaker may be mounted in the opening. In some instances it may even be desirable to have no other device in the opening of the camera arrangement. In such case, the opening may be covered by a lid. 
         [0052]    The magnet may be a permanent magnet, e.g., a neodymium magnet. In one embodiment, the magnet may have two magnetic poles and be pivotable, such that it locks the holder in place when oriented in one direction, and releases the holder oriented in the opposite direction. 
         [0053]    Instead of permanent magnets, electro-magnets may be used. In such case, it is preferable if the action of the electro-magnet be arranged such that when the monitoring device is supplied with electric current, the electro-magnet is turned off, and such that it is turned on when the electric power supply is interrupted. Thereby, the fixed monitoring devices are locked securely in position in the case of a power outage. 
         [0054]    The arrangement of the magnet and the ferromagnetic material may be reversed, such that each holder comprises ferromagnetic material, and the mounting rail comprises magnets. The ferromagnetic material used in the mounting rail or in the holder may be martensitic stainless steel, but other ferromagnetic materials are also possible. 
         [0055]    Instead of lock devices employing magnets, the holders in the monitoring device arrangement may have lock devices employing other means. For instance, each holder may be provided with a spring or a snap lock. Examples of additional embodiments of holders are shown in  FIGS. 7-9 . 
         [0056]    In  FIG. 7 , a lock device  215  of a holder  208  employing spring function is shown. The lock device  215  is essentially made up of bent sheet metal. At the front of the lock device  215 , i.e. the part of the lock device  215  which is arranged at the radially outer rim of the mounting rail, the lock device  215  has two release tabs  220 . By pressing the release tabs  220  towards each other, the lock device  215  is released, such that the holder  208  may be moved slidingly along the mounting rail to a desired position. Similar to the embodiment described above in connection with, e.g.,  FIG. 3 , this holder  208  has hooks  210  for retaining the holder  208  on the mounting rail even when the lock device  215  is released. 
         [0057]    In  FIG. 8 , another embodiment of a holder  308  having a lock device  315  employing a snap lock is shown. This holder  308  is essentially made of plastic, and the lock device  315  comprises a curved metallic bar  319  which is connected to a lock lever  320 . Similar to the embodiments already described, the holder  308  in  FIG. 8  has hooks  310  for retaining the holder  308  on the mounting rail, even when the lock device  315  is released. In the locked position, as shown in  FIG. 8 , the lock lever  320  positions the curved bar in a position where it presses against the underside of the mounting rail, thereby locking the holder  308  in position on the mounting rail. When the lock lever  320  is pivoted forwards, i.e. in a direction radially outwardly, the curved bar  319  is rotated such that it no longer presses against the mounting rail. The holder  308  may then be moved slidingly along the mounting rail. 
         [0058]    As may be seen in  FIG. 9 , combinations of features of the different embodiments of holders that have been described are also possible. Here, the holder  408  is of a type similar to the one shown in  FIG. 7 , and may be made essentially of sheet metal. However, the lock device  415  is of the type shown in  FIG. 8 , comprising a curved metallic bar  419  and a lock lever  420 . In  FIG. 9 , the holder  408  is shown with the lock lever  420  in the locking position, in which the curved bar presses against the mounting rail  409 . Thus, the holder  408  and the fixed camera  402  are held securely in position on the mounting rail. If the lock lever  420  is pivoted forwards, i.e. radially outwardly, the lock device  415  is released in the same way as described in connection with  FIG. 8 . With the lock device  415  released, the holder  408  with the fixed camera  402  may be moved along the mounting rail  409  to a desired position in a sliding motion. The lock lever  420  may be made long enough to protrude radially outside the outer rim of the mounting rail  409  such that it prevents replacing the protective window (not shown in  FIG. 9 ) when in the unlocked position. In this manner, it may be ensured that a person installing the monitoring device arrangement does not accidentally forget to lock the locking device  415  before closing the monitoring device arrangement. Once the arrangement is locked and closed, the fixed cameras  402  are held securely in place on the mounting rail  409 , protected by the shielding portion  403  and the protective window. 
         [0059]    Other types of locking devices are also possible, such as locking devices employing rubber bands, clamping screws or tension bolts. Advantageously, locking devices not requiring tools for locking and releasing are used, thereby simplifying installation and adjustment of the monitoring device arrangement. 
         [0060]    In the embodiments shown, the holders are intended to be moved manually along the mounting rail. However, there may be situations where it is useful to be able to move the holders by remote control. This could be of interest if the monitoring device arrangement is mounted in a location that is difficult to reach, e.g., high up on a wall or pole. A remote-controlled movement of the holders may be achieved using a toothed rim on the mounting rail. The inner rim or the outer rim or both rims of the mounting rail could be provided with teeth, and the holder be provided with corresponding teeth. A motor, such as a piezoelectric motor, could be provided in the holder for moving the holder along the toothed mounting rail. It should be noted that the manual as well as the remote-controlled movement of the holders are intended for installation and adjustment of the fixed monitoring devices, and not for any movement during the actual monitoring. Thus, once the fixed monitoring devices have been placed in the desired positions, they are intended to be securely held in position. Remote-controlled or motorised movement of the fixed monitoring devices may also be useful for setting personal preferences, such as the preferences of different security guards. For instance, the monitoring needs may be different during the day and at night, such that a security guard on a day shift wishes to centre the field of view of one of the fixed monitoring devices on one part of the surrounding area, while a security guard working on a night shift may wish to centre the field of view on another part. 
         [0061]    The holders may be arranged such that the monitoring devices are fixedly attached in the holders, allowing removal only in a workshop. 
         [0062]    A monitoring device arrangement, such as the one shown in  FIGS. 1-4 , may be manufactured and sold separately as an accessory to a pannable and/or tiltable monitoring device. Hereby, an existing pannable and/or tiltable monitoring device may be upgraded by providing it with the accessory, and without any real modification of the pannable and/or tiltable monitoring device. The monitoring device arrangement may also be manufactured already combined, such that it includes the fixed monitoring devices arranged on the mounting rail, and the pannable and/or tiltable monitoring device arranged in the opening of the monitoring device arrangement. 
         [0063]    In the embodiments shown, the mounting rail is circular. However, other shapes are also possible. In some embodiments, the mounting rail may be elliptical. The mounting rail may have an arcuate shape, although not forming a full circle or ellipse. In other embodiments, the mounting rail may be angled, having straight portions along which the holders are movable. It is also possible to combine arcuate and straight portions along the mounting rail. For mounting close to a wall, it may for instance be sufficient to have a semi-circular mounting rail, allowing positioning of fixed monitoring devices only in positions not facing the wall. It may also be useful to have a mounting rail resembling a semicircle, but made up of three straight portions placed at angles of, e.g., 120°. 
         [0064]    Thus, the invention should not be limited to the shown embodiments but should only be defined by the appended claims.