Patent Publication Number: US-10783760-B2

Title: Surveillance apparatus having an optical camera and a radar sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. application Ser. No. 14/889,081, filed Nov. 4, 2015 which is a National Stage Application based on PCT/EP2014/058755, filed Apr. 29, 2014, and claims priority to European Patent Application 13169006.7, filed in the European Patent Office on May 23, 2013, the entire contents of each of which being incorporated herein by reference. 
    
    
     BACKGROUND 
     Field of the Disclosure 
     The present disclosure relates to the field of surveillance cameras for safety and security applications. A surveillance apparatus, having an optical camera and an additional radar sensor, and a corresponding surveillance method are disclosed. Application scenarios include burglar, theft or intruder alarm as well as monitoring public and private areas. 
     Description of Related Art 
     Optical surveillance cameras are used in many public places such as train stations, stadiums, supermarkets and airports to prevent crimes or to identify criminals after they committed a crime. Optical surveillance cameras are widely used in retail stores for video surveillance. Other important applications are safety-related applications including the monitoring of hallways, doors, entrance areas and exits for example emergency exits. 
     While optical surveillance cameras show very good performance under regular operating conditions, these systems are prone to visual impairments. In particular, the images of optical surveillance cameras are impaired by smoke, dust, fog, fire and the like. Furthermore, a sufficient amount of ambient light or an additional artificial light source is required, for example at night. 
     An optical surveillance camera is also vulnerable to attacks of the optical system, for example paint from a spray attack, stickers glued to the optical system, cardboard or paper obstructing the field of view, or simply a photograph that pretends that the expected scene is monitored. Furthermore, the optical system can be attacked by laser pointers, by blinding the camera or by mechanical repositioning of the optical system. 
     In addition to imaging a scenery, it can be advantageous to obtain information about the distance to an object or position of an object or a person in the monitored scenery. A three-dimensional image of a scenery can be obtained, for example, with a stereoscopic camera system. However, this requires proper calibration of the optical surveillance cameras which is very complex, time consuming, and expensive. Furthermore a stereoscopic camera system typically is significantly larger and more expensive compared to a monocular, single camera setup. 
     In a completely different technological field, automotive driver assistance systems, US 2011/0163904 A1 discloses an integrated radar-camera sensor for enhanced vehicle safety. The radar sensor and the camera are rigidly fixed with respect to each other and have a substantially identical, limited field of view. 
     The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor(s), to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present disclosure. 
     SUMMARY 
     It is an object of the present disclosure to provide a surveillance apparatus and a corresponding surveillance method which overcome the above-mentioned drawbacks. It is a further object to provide a corresponding computer program and a non-transitory computer-readable recording medium for implementing said method. In particular, it is an object to expand the surveillance capabilities to measurement scenarios where a purely optical camera fails and to efficiently and flexibly monitor a desired field of view. 
     According to an aspect of the present disclosure there is provided a surveillance apparatus comprising
         an optical camera that captures images based on received light, said optical camera having a first field of view,   a radar sensor that emits and receives electromagnetic radiation, said radar sensor having a second field of view; and       

     wherein said first field of view is variable with respect to said second field of view. 
     According to a further aspect of the present disclosure there is provided a corresponding surveillance method comprising the steps of
         capturing images based on light received with an optical camera, said optical camera having a first field of view,   emitting and receiving electromagnetic radiation with a radar sensor, said radar sensor having a second field of view, and   wherein said first field of view is variable with respect to said second field of view.       

     According to a further aspect of the present disclosure there is provided a surveillance apparatus comprising
         an optical camera that captures images based on received light, said optical camera having a first field of view,   a radar sensor that emits and receives electromagnetic radiation, said radar sensor having a second field of view, and       

     wherein said second field differs from said first field of view. 
     According to a further aspect of the present disclosure there is provided a surveillance radar apparatus for retrofitting an optical surveillance camera, said surveillance radar apparatus comprising
         a housing for arrangement of the surveillance radar apparatus at the surveillance camera,   a radar sensor that emits and receives electromagnetic radiation, said radar sensor having a second field of view, and       

     wherein said first field of view is variable with respect to said second field of view. 
     According to still further aspects a computer program comprising program means for causing a computer to carry out the steps of the method disclosed herein, when said computer program is carried out on a computer, as well as a non-transitory computer-readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method disclosed herein to be performed are provided. 
     Preferred embodiments are defined in the dependent claims. It shall be understood that the claimed surveillance radar apparatus for retrofitting a surveillance camera, the claimed surveillance method, the claimed computer program and the claimed computer-readable recording medium have similar and/or identical preferred embodiments as the claimed surveillance apparatus and as defined in the dependent claims. 
     The present disclosure is based on the idea to provide additional sensing means, i.e., a radar sensor, that complements surveillance with an optical camera. A radar sensor can work in certain scenarios where an optical sensor has difficulties, such as adverse weather or visual conditions, for example, snowfall, fog, smoke, sandstorm, heavy rain or poor illumination or darkness. Moreover, a radar sensor can still operate after vandalism to the optical system. Synergy effects are provided by jointly evaluating the images captured by the (high-resolution) optical camera and the received electromagnetic radiation by the radar sensor. 
     The field of view of an optical camera that captures images based on received light is typically limited to a confined angular range. Attempts to widen the field of view of an optical camera exist, for example, in form of a fish-eye lens. While such optical elements significantly broaden the field of view of the optical camera, they also create a significantly distorted image of the observed scene. This makes image analyses difficult for an operator that monitors the images captured by the surveillance camera, if no additional correction and post-processing is applied. 
     The surveillance apparatus according to the present disclosure uses a different approach by combining an optical camera that captures images based on received light, and a radar sensor, that emits and receives electromagnetic radiation. The optical camera has a first field of view and the radar sensor has a second field of view. The first field of view is variable with respect to the second field of view. Alternatively, the second field of view differs from the first field of view. For example, the first field of view of the optical camera covers an angular range of about 50-80° to avoid substantial image distortions, whereas the second field of view of the radar sensor covers an angular range of at least 90°, preferably 180°, or even a full 360°. Thus, the field of view of the radar sensor is larger than the field of view of the optical camera and thereby monitors a wider field of view. However, the information gained from the radar sensor is often not sufficient for surveillance applications since often a high-resolution optical image is desired. Therefore, the field of view of the optical camera is variable with respect to the field of view of the radar sensor. In particular, the size and/or orientation of the first field of view are variable with respect to the second field of view. For example, an object can be identified with the radar sensor and the field of view of the optical camera is adjusted to cover said object. This is particularly beneficial if an object that is initially not covered by the field of view of the optical camera is now detected in the field of view of the radar sensor. 
     The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1A  shows a first embodiment of an optical surveillance camera, 
         FIG. 1B  shows a second embodiment of an optical surveillance camera, 
         FIG. 2  shows an application scenario of a surveillance apparatus according to the present disclosure, 
         FIG. 3  shows a first embodiment of a surveillance apparatus according to the present disclosure, 
         FIG. 4A  shows a second embodiment of a surveillance apparatus according to the present disclosure, 
         FIGS. 4B to 4D  illustrate examples of determining an angle of arrival, 
         FIGS. 5A and 5B  show a third embodiment of a surveillance apparatus according to the present disclosure. 
         FIGS. 6A and 6B  show a fourth embodiment of a surveillance apparatus according to the present disclosure, 
         FIGS. 7A and 7B  show a fifth embodiment of a surveillance apparatus according to the present disclosure, 
         FIG. 8  shows a sixth embodiment of a surveillance apparatus according to the present disclosure, 
         FIGS. 9A to 9C  show an embodiment of a surveillance radar apparatus for retrofitting a surveillance camera, 
         FIG. 10  shows a surveillance apparatus with a camera cover comprising a translucent antenna, 
         FIG. 11  shows a cross section of a camera cover comprising a translucent antenna, 
         FIG. 12  shows a cross section of a translucent antenna and feeding structure, and 
         FIG. 13  shows a perspective view of a housing incorporating an optical camera as well as conformal translucent antennas fed by printed RF circuit boards. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views,  FIG. 1  shows a surveillance apparatus  100  comprising an optical camera  101  and a mount  102  for mounting the camera, for example, to a wall, ceiling or pole. The optical camera is a security camera that comprises a housing  103  and a camera objective  104 . Optionally, the camera objective  104  is a zoom objective for magnifying a scenery. The front part of the optical camera  101  comprises a camera cover  105  for protecting the camera objective  104 . The housing  103  together with the camera cover  105  provide a certain degree of protection against vandalism. However, an optical camera is still vulnerable to attacks on the optical system. Such attacks include, but are not limited to, spray and paint attacks, gluing or sticking optically non-transparent materials on the camera cover  105  or blinding the camera by a laser. 
     The optical camera  101  of the surveillance apparatus  100  optionally features a light source for illuminating a region of interest in front of the camera. In this example, the camera  101  comprises a ring of infrared (IR) light emitting diodes (LEDs)  106  for illuminating the region of interest with non-visible light. To a certain extent, this enables unrecognized surveillance and surveillance in darkness over a limited distance. 
     Further optionally, the surveillance apparatus  100  comprises an actor  107  for moving the camera  101 . By moving the camera, a larger area can be monitored. However the movement speed is limited. Different areas cannot be monitored at the same time but have to be monitored sequentially. 
       FIG. 1B  shows a second embodiment of a surveillance apparatus  110  comprising an optical camera  111 . In this embodiment, the surveillance apparatus  110  has a housing  113  with a substantially circular outline. This housing  113  is typically mounted to or into a ceiling. The surveillance apparatus  110  comprises a translucent camera cover  115  wherein the optical camera  111  is arranged. In this embodiment, the camera cover  115  comprises a substantially hemispheric camera dome. However, the camera cover is not limited in this respect. 
     The field of view  118  of the optical camera  111  defines the region that is covered and thus imaged by the optical camera  111 . In order to increase the area that can be monitored with the surveillance apparatus  110 , the surveillance apparatus  110  can further comprise a first actor and a second actor to pan  119   a  and tilt  119   b  the optical camera  111 . 
       FIG. 2  shows an application scenario that illustrates the limitations of a surveillance apparatus  200  purely relying on an optical camera. The optical camera cannot see through smoke  201 , dust or fog, for example in case of a fire. Thus, a subject  202  is not detected and can, therefore, not be guided to the nearest safe emergency exit  203 . 
       FIG. 3  shows an embodiment of a surveillance apparatus  300  according to an aspect of the present disclosure comprising an optical camera  301  that captures images based on received light, and a radar sensor that emits and receives electromagnetic radiation. Advantageously, the radar sensor operates in the millimeter-wave frequency band. This embodiment shows a top view of a surveillance apparatus  300  having a housing  303  with a polygonal outline, in this example hexagonal outline. 
     The camera  301  is arranged at the center of the housing, for example, a dome-type camera as discussed with reference to  FIG. 1B . The optical camera  301  has a first field of view  308   a.  In this embodiment, the radar sensor comprises a plurality of antenna elements  304   a - 304   f  (in particular of single antennas) arranged on the periphery of the surveillance apparatus  300 . Individual antenna elements  304   a - 304   f  are provided on the sectored camera outline. Each antenna element  304   a - 304   f  is connected to a radar front end system  305  of the radar sensor. The field of view of the radar sensor with its antenna elements covers the entire surrounding of the surveillance apparatus  300 , i.e. a 360° field of view. Furthermore, the surveillance apparatus  300  can identify the sector of the radar sensor in which an object  306   a,    306   b  is located by evaluating the antenna elements  304   a,    306   b  corresponding to said sector. 
     In a first configuration, the field of view  308   a  of the optical camera  301  corresponds to the portion of the field of view of the radar sensor that is covered by the antenna element  304   a.  Even if the view of the optical camera  301  is obscured by smoke, the radar sensor can still detect the object  306   a,  since the frequency spectrum used for the electromagnetic radiation of the radar sensor penetrates through smoke. For example with reference to the application scenario in  FIG. 2 , the radar sensor of the surveillance apparatus indicates a trapped person and guides rescue personnel to primarily search for victims in rooms where the radar has indicated a trapped person. Furthermore, millimeter-waves can penetrate dust or fog, as well as thin layers of cardboard, wood, paint, cloth and the like. Hence, the surveillance apparatus remains operable after an attack on the optical camera  301 . 
     Using a radar sensor employing a frequency-modulated continuous wave (FMCW) modulation scheme or stepped CW allows ranging and relative speed detection. Measurement schemes, such as pulsed radar, can be used in the alternative. In principle, a single antenna is sufficient for ranging, such that in a most basic configuration, a single antenna  304   a  can be used. Thus, the range and speed of the target  306   a  can be determined. 
     The field of view of the radar sensor that emits and receives electromagnetic radiation comprises the field of view of the individual antenna elements  304   a - 304   f.  In the configuration shown in  FIG. 3 , each of the six antenna elements  304   a - 304   f  covers an angular range of 60°, such that the entire surrounding of the surveillance apparatus  300  can be monitored. The field of view  308   a  of the optical camera  301  that captures images based on received light in this example is limited to 60°. However, advantageously, the field of view of the optical camera  301  is variable with respect to the field of view of the radar sensor. In particular, the size and/or orientation of the field of view of the camera are variable with respect to the field of view of the radar sensor. This can be achieved by having an optical camera  301  that is movable with respect to the radar sensor. For example, the optical camera  301  is a dome-type camera as disclosed in  FIG. 1B  that further comprises an actuator that enables a pan and/or tilt movement. For example, the optical camera  301  can be oriented in a first position to cover the field of view  308   a  and can be moved to a second position to cover the field of view  308   b.    
     In a further scenario, the optical camera  301  is oriented to cover the field of view  308   a  with the object  306   a.  The radar sensor covering the entire 360° field of view detects an object  306   b  in the sector of antenna element  304   b.  The surveillance apparatus  300  can comprise a control unit  307  as part of the radar front end system  305  (as shown in  FIG. 3 ) or as a separate element for controlling the optical camera  301  based on radar information of the radar sensor. In this example, the direction of the optical camera is controlled based on the information from the radar that an object has been detected in the sector corresponding to antenna element  304   b.  Thus, the optical camera  301  is rotated towards the sector, wherein the second object  306   b  has been detected. Thereby, the second detected object  306   b  can be subject to a closer visual analysis, in particular with a high-resolution optical camera  301 . Further, this embodiment may be used to control the optical camera (based on information from the radar) to focus (or zoom) on a certain depth (range) where an object is expected or has been detected (by the radar). 
     Advantageously, this control of the optical camera  301  can be automated, such that a single optical camera  301  having a limited field of view  308   a,    308   b  can be used to cover an extended area, in this example the entire surrounding of the surveillance apparatus. Furthermore, the system cost can be lowered by combining the radar functionality for coarse monitoring of an entire area with a selective high-resolution monitoring of only limited parts of the area. The high resolution monitoring is triggered, if an object has been detected by the radar sensor. 
     The housing  303  accommodates the electronics of the surveillance apparatus  300 . In  FIG. 3  the electronics, in particular any printed circuit boards including the antenna elements  304   a - 304   f,  comprises planar elements which are arranged as a hexagonal structure corresponding to the housing  303 . As an alternative to 2-dimensional antenna elements, 3-dimensional antenna elements can also be used. Alternative structure types of the housing could also be envisaged, i.e., quadratic shape, octagonal shape, or also a cylindrical shape as currently employed for most security cameras. An arrangement of the electronics, in particular a shape of the printed circuit boards or antenna elements can correspond to a part of said housing. 
       FIG. 4A  shows a further embodiment of a surveillance apparatus  400  according to the present disclosure. In addition to having an antenna element  304  at each side of the hexagonal outline, as depicted in  FIG. 3 , the surveillance apparatus  400  features additional antenna elements, i.e. a plurality of antenna elements (that may form an antenna array) at each side of the outline. Using these additional antenna elements, the angle of an object  406   b  can be determined with respect to the antenna elements  404   a  and  404   b.  The angle of arrival can be determined, for example, by using the radar monopulse principles. For example, electromagnetic radiation is emitted by at least one of the antenna elements  404   a  and  404   b.  By applying the amplitude or phase monopulse principle to the reflected signal received by the two antenna elements, the direction of the object  406   b  can be determined. The distance of the target can be determined, for example, by evaluating a beat frequency (the difference of the sent and received signal) as known from FMCW radar systems. Alternatively, a pulse radar can be used for determining the distance. 
     The range and/or direction of the object  406   b  can be determined by use of the generally known principles of interferometry or phase monopulse. The principle of phase monopulse is sketched in  FIG. 4B . The object  406   b  is oriented at an angle φ with respect to the two antenna elements  404   a  and  404   b.  The distance from the object  406   b  to antenna element  404   a  differs from the object  406   b  to the distance from antenna element  404   b  by a path difference Δs. Because of this path difference, the antenna element  404   b  receives a signal reflected from the object  406   b  with a time delay corresponding to the path difference. If a modulated signal is emitted towards and reflected from the target, the phase difference of the signals received with antenna elements  404   b  and  404   a  represents the path difference and thus the angle of incidence of the received signal. Thus, modulated electromagnetic radiation, for example sinusoidal intensity modulated electromagnetic radiation, is emitted by at least one radar antenna, and the phase difference of electromagnetic radiation received with antenna elements  404   a  and  404   b  is evaluated. Based on the phase difference between the two signals detected with the two antenna elements  404   a  and  404   b,  the angle of arrival (AOA, φ) towards the object  406   b  can be determined. Alternatively, a pulse radar can be used for determining the path difference. 
       FIG. 4C  illustrates the principle of amplitude monopulse for determining the angle of arrival. At least two antenna elements  404   a,    404   b  with differently shaped antenna patterns  420   a,    420   b  are used. The amplitude of the signal received with antenna element  404   a  with antenna pattern  420   a  is denoted U 1 . The amplitude of the signal received with antenna element  404   b  with antenna pattern  420   b  is denoted U 2 . The ratio of the amplitudes of the received signals U 1 /U 2  is computed. Because of the different antenna patterns  420   a,    420   a,  the ratio of the amplitudes of the received signals U 1 /U 2  depends on the angle φ of the object  406   b  with respect to the two antenna elements  404   a  and  404   b.  In  FIG. 4D , the ratio U 1 /U 2  is plotted as a function of the angle of arrival φ. Preferably, the curve  421  is a monotonic function to avoid ambiguities in the estimated angle of arrival. Furthermore, ambiguity has to be taken into account with respect to the number of objects for which an angle of arrival can be determined. With N antenna elements, the angle of arrival for N−1 objects can be determined. In case of two antenna elements, the angle of arrival for one object  406   b  can be determined. 
     An alternative approach for determining the direction to an object is described with reference to  FIG. 5A . In this embodiment, a radar sensor with a single narrow beam antenna  504  having a narrow field of view  509  is used. The housing  503  comprises a rotatable portion  510  comprising the radar sensor with antenna  504 . The rotatable portion  510  rotates around an optical camera  501 . In general, the field of view  504  of the radar sensor is moved with respect to the field of view  508   a  of the camera. The optical camera  501  can be for example a dome-type camera as depicted in  FIG. 1B , a camera as depicted in  FIG. 1A , or any other type of movable or fixed camera. In this example, the camera is fixed. 
       FIG. 5B  illustrates beam scanning with the surveillance apparatus  500  of  FIG. 5A . The directive antenna  504  including a radio frequency (RF) front end is implemented on a printed circuit board (PCB) which rotates around a center axis  511  of the housing  503 . The rotation can be confined to a limited angular range, for example an angular range corresponding to the field of view  508   a  of the optical camera  501 . Alternatively, the angle of rotation can be +/−180° or continuously spinning. 
     In case of +/−180° scanning, a flexible cable interconnect can be used between the static housing  503  and the movable part  510  including the antenna element  504 . For the case of a continuously scanning system, a rotary joint is required that may optionally comprise a filter for radio frequency signals (RF), DC signals, intermediate frequency signals (IF), and the like. Alternatively, multiple slip rings for providing a connection between the static housing  503  and the moving parts  510  can be employed. 
       FIG. 5B  further illustrates a very important use case for practical surveillance applications. The surveillance apparatus  500  further comprises processing circuitry  512  for processing the captured images of the optical camera  511  and the received electromagnetic radiation of the radar sensor, received with the antenna element  504 , and providing an indication of the detection of the presence of one or more objects  506   a,    506   b.  In particular, the processing circuitry can verify the detection of an object  506   a,    506   b  in the captured images of the optical camera  501  or in the received electromagnetic radiation of the radar sensor based on the received electromagnetic radiation of the radar sensor or the captured images of the optical camera, respectively. In other words, the processing circuitry  512  may verify the detection of an object  506   a,    506   b  in the captured images of the optical camera  501  by making a plausibility check using the received electromagnetic radiation of the radar sensor and/or the processed radar information. Alternatively, the processing unit  512  may verify the detection of an object  506   a,    506   b  in the received electromagnetic radiation of the radar sensor based on the captured images of the optical camera  501 . Furthermore, the processing circuitry  512  may provide an indication of whether two persons  506   a,    506   b  identified in the captured images of the optical camera are actually two persons or one person and his or her shadow by evaluating distance information to the two persons based on the received electromagnetic radiation of the radar sensor. This use case is illustrated with respect to  FIG. 5B . 
     The processing circuitry  512  identifies a first object  506   a  and a second object  506   b  in the field of view  508   a  of the optical camera  501 . For example, the processing circuitry performs image analysis on the captured image and identifies two dark spots as objects  506   a  and  506   b.  More advanced image processing algorithms can of course be employed that identify the outline of a person in both objects  506   a  and  506   b.  In addition to this result from the optical analysis, information acquired using the radar sensor with narrow beam antenna  504  can be used. 
     For example, the distances corresponding to the directions of objects  506   a  and  506   b  are evaluated. In the optical image, a person and its shadow may be falsely identified as two persons. However, using the information from the radar sensor, it can be clearly identified whether there are actually two persons or whether there is one person (a short distance is measured) and his shadow. For the case of a shadow, the distance measured with the radar sensor does not correspond to the distance of the object expected from the image captured by the optical camera. This use case is very important for counting people, for example to ensure that all kids have left a fun park or that all customers have left a shop or that everybody has left a danger zone. 
       FIGS. 6A and 6B  show an alternative to a mechanically scanning system. The acquisition speed of a mechanical scanning system depends on the scanning speed, i.e. the scan time for one full 360° scan or for multiple, for example 10-100, full 360° scans for a rotating or spinning system.  FIGS. 6A and 6B  show full electronic scanning systems, preferably using analog beam forming like phased array or digital beam forming or any other type of beam forming based on multiple, individual antenna elements. Such an electronic scanning system can yield multiple thousands of different beams per second. In case of electronic beam forming, no more moving parts are needed. Thus, electronic beam forming can increase the reliability of the system. 
     The surveillance apparatus  600  in  FIG. 6A  comprises an optical camera  601  in the center of a hexagonal housing  603 . A plurality of antenna elements  604  are arranged on the periphery of the surveillance apparatus  600 . In the shown example, a narrow antenna beam of electromagnetic radiation is emitted at each side of the hexagonal housing  603 . A side of the hexagonal outline is referred to as a sector. Each sector can be scanned by the antennas, for example in the range of +/−30° for a hexagonal shape or +/−22.5° for an octagonal shape, which results in a full 360° field of view. Alternatively, different scanning angles, for example overlapping scanning angles to provide redundancy, are provided. 
       FIG. 6B  shows an alternative embodiment of the surveillance apparatus  600  according to the present disclosure wherein the antenna elements  604  are arranged on a circular outline of the surveillance apparatus  600 . 
     According to a further aspect of the disclosure, the beam forming, for example digital beam forming with MIMO antenna elements, can be used to generate different beam forms. For example, a wide antenna beam similar to  FIG. 3  is emitted in a first configuration. In case that an object is detected with said wide beam, the antenna array switches to a scanning mode wherein the narrow antenna beam scans the scenery to determine an exact position of the detected object. Furthermore multiple narrow beams can be generated at the same time. 
     The previous embodiments have illustrated scanning an antenna beam in one direction, i.e. in the azimuth plane. In order to monitor a room in three dimensions, however, the radar sensor can scan in the elevation plane in addition to the azimuth plane. 
     The azimuth and the elevation can be monitored with a mechanical scanning radar system, a hybrid mechanical/electronic scanning radar system, or a purely electronic scanning radar system.  FIGS. 7A and 7A  illustrate a hybrid mechanical/electronic scanner. In this example, the surveillance apparatus shown in  FIG. 5A  is modified by replacing the single antenna element  504  by a plurality of antenna elements  704 . The surveillance apparatus  700  comprises an optical camera  701 , a common housing  703  and a radar sensor with antennas  704 . The antenna elements  704  are arranged on a rotatable part  710  of the housing  703  adapted to rotate around the optical camera  701  or generally to perform a rotating movement for scanning in the azimuth plane. The elevation plane, in turn, is covered by the linear array of antenna elements  704  for electronically scanning the elevation plane. 
     In the example shown in  FIG. 7A , the antenna array is implemented on a printed circuit board which is mounted in the rotatable ring  710  at an angle of 45° with respect to the axis of rotation. The 1-dimensional array allows beam forming in a direction orthogonally oriented to a rotation direction. By rotating the ring, 2-dimensional scanning is achieved. In this example, the scanning range in the elevation is +/−45°. Thereby, the entire hemisphere below the surveillance apparatus  700  is covered by the combination of mechanical scanning in the azimuth plane and electronic scanning by beam steering in the elevation plane. The electronic beam forming can be implemented as a one-dimensional, sparse MIMO array. 
       FIG. 8  shows an alternative embodiment of the surveillance apparatus  800  according to the present disclosure that provides electronic beam scanning both in azimuth and elevation. The surveillance apparatus  800  comprises an optical camera  801  and a radar sensor comprising a two-dimensional array of antenna elements  804 . This arrangement enables angular scanning in two dimensions, i.e. in azimuth and elevation, as well as determining the range at each antenna position. The antenna elements can be distributed over the outline of the camera housing. 
     In an alternative embodiment, the outline of the surveillance apparatus is a polygonal shape. Thereby, the two-dimensional antenna arrays can be implemented, for example, as patch antenna arrays on individual printed circuit boards that are placed at the sides of the polygonal shape. This reduces fabrication costs. 
     A further aspect of the present disclosure relates to retrofitting an optical surveillance camera, as for example shown in  FIGS. 1A and 1B , having a first field of view with a surveillance radar apparatus. In other words, the radar modality can be supplied directly with the optical surveillance camera as disclosed in the previous embodiments, or can be supplied as an add-on. Thereby, an optical camera can be provided with the radar sensor having a second field of view at a later point in time. 
     Optionally, the surveillance radar apparatus includes further functionalities, such as a converter for converting analog video signals of an existing analog optical camera to digital video signals, for example for connecting the existing analog optical camera via the surveillance radar apparatus to an IP network. 
       FIGS. 9A to 9C  illustrate an embodiment of a surveillance radar apparatus  900  for retrofitting an optical camera  901 . The surveillance radar apparatus  900  in this example can be sort of a ‘jacket’ with a polygonal housing  902  which is put around the cylindrical housing  912  of the camera  901 . In this non-limiting example, the housing  902  of the surveillance radar apparatus encompasses the surveillance camera. The surveillance radar apparatus  900  for retrofitting the optical surveillance camera is illustrated separately in  FIG. 9B . Antenna elements  904  of the radar sensor for emitting and receiving electromagnetic radiation are arranged on the periphery of the housing  902  of the surveillance radar apparatus  900 . Thereby, and existing optical camera  901  is provided with a radar sensor having a second field of view. For example, the antenna elements  904  of the radar sensor cover the entire periphery of the surveillance radar apparatus. The field of view  908   a  of the optical camera  901  is variable with respect to the second field of view provided by the radar sensor, such that the field of view  908   a  can be moved towards an object that has been detected in the received electromagnetic radiation by the radar sensor. 
     To ensure proper alignment of the optical surveillance camera  901  and the surveillance radar apparatus  900 , the housing  902  of the surveillance radar apparatus  900  further comprises an alignment member  921  for aligning a position of the surveillance radar apparatus  900  with respect to the surveillance camera  901 . For this purpose, the housing  912  of the surveillance camera  901  comprises a second alignment member  922  for engagement with the alignment member  921  of the housing of the surveillance radar apparatus  900 . In this embodiment, the second alignment member  922  of the camera housing  912  is a type of slot or groove where a tapped structure  921  from the housing  902  of the surveillance radar apparatus  900  fits into. Of course, this form fit can also be implemented vice versa. There can also be other embodiments of alignment structures or multiple of them, respectively. 
       FIG. 10  illustrates a further embodiment of the surveillance apparatus  1000  according to the present disclosure. The optical camera  1001  is arranged inside a camera dome  1015  that serves as a camera cover. In contrast to the previous embodiments, the camera dome  1015  comprises the antenna elements  1004  as translucent antenna elements. In this embodiment, the translucent antenna with its translucent antenna elements  1004  comprises several patch antenna elements. In general, the translucent antenna comprises at least one electrically conductive layer which comprises at least one of a translucent electrically conductive material and an electrically conductive mesh structure. An example of an optically translucent and electrically conductive material is indium tin oxide (ITO), however, any other optically translucent and electrically conductive material could be used as well. 
     A conventional camera cover usually only comprises one translucent layer, for example a translucent dome made from glass or a transparent polymer. Optionally, the camera cover comprises an anti-reflective coating, a tinting, or a one-way mirror, in order to obscure the direction the camera is pointing at. 
       FIG. 11  shows a cross section of a camera cover  1115  comprising a translucent antenna. The translucent antenna according to an aspect of the present disclosure comprises several layers. The example shown in  FIG. 11  comprises an optional outer protection layer  1131 , for example made of glass or a transparent polymer. This protection layer  1131  may further optionally comprise a coating. The outer protection layer  1131  is followed by a second layer comprising several patch antenna elements  1132 , for example ITO patch antennas that are separated by spacers  1133 . The separation of the antennas is typically in the range of 0.4 to 1.5 times the wavelength lambda. The third layer in this example is a translucent dome  1134 , for example made from glass or a translucent polymer, that provides mechanical stability to the camera cover. This layer is made from a dielectric, isolating material. The fourth layer in this example is a ground plane, in particular a slotted ground plane comprising several conductive ground plane elements  1135  and slots  1136 . The slots  1136  are arranged underneath or in close proximity to the patch antenna elements  1132 . A fifth layer is a translucent spacer  1137 , which separates the slotted ground plane from the sixth layer comprising microstrip feed lines  1138  for feeding the patch antenna elements  1132  via the slots  1136  of the slotted ground plane  1135 . The microstrip feed lines  1138  are connected to a radar circuitry  1139  as illustrated in more detail with reference to  FIG. 12 . The sequence of layers in this example can optionally be changed and layers omitted. For example, the outer layer may provide mechanical stability to the camera dome instead of the third layer in the example above. Further alternatively, a different feed structure with or without a slotted ground plane layer may be used, for example a differential wiring of the individual patch antenna elements. 
     According to an embodiment of the translucent antenna, the patch antennas  1132  make up a conformal patch antenna array. The array can cover the entire hemispherical camera cover and can consist of multiple arrays of patch antenna elements that are arranged for observing different sectors. Alternatively, individually controlling the individual patch antenna elements is possible to form a hemispherical phased antenna array. A corresponding feeding network for routing to the radar circuitry  1139  for feeding the individual patch antenna elements is then provided with the corresponding individual microstrip feed lines  1138  and power dividers for individually feeding the antenna elements. The same holds true in the receiving path. 
       FIG. 12  illustrates the coupling of he translucent antenna of the camera  1215  cover to the base of the surveillance apparatus with the housing  1203  of the surveillance apparatus  1000 . The translucent camera cover  1215  including the patch antenna elements  1232  is illustrated to the right side of the dashed line, whereas the base of the surveillance apparatus is illustrated to the left side of the dashed line in  FIG. 12 . 
     In this embodiment, conductive layers of the translucent antenna are preferably implemented by electrically conductive ITO (Indium-Tin-Oxide) layers  1240 . As a further alternative, conductive layers of the translucent antenna elements comprise AgHT (silver coated polyester film). Alternatively, printed patch antennas, which are approximated by wire meshes, can be used. This methodology does not need any special type of material. Standard metallic conductors such as copper, gold, chrome, etc. can be employed. By perforating large metal areas of the antenna, a high optical transparency can be achieved. In a wire mesh the metal grid is typically space by 0.01 . . . 0.1 lambda (i.e. 0.01 . . . 0.1 times the used wavelength). The thickness of the metal strips can be as small as 0.01 lambda. 
     The conductive layers  1240  are separated by dielectric layers made from glass or, alternatively, a translucent polymer that is not electrically conductive but can serve as a dielectric. Of course, the translucent antenna can be implemented using different layer structures, however, the layer structure preferably comprises a first electrically conductive layer comprising a ground plane and a second electrically conductive layer comprising an antenna element. 
     For example, the base of the surveillance apparatus  1000  comprises radar circuitry, in particular, a printed circuit board (PCB)  1250  further comprising a ground plane  1251  and a microstrip line  1252 . The microstrip line  1252  feeds the patch antenna elements  1232  via the shown structure. The ground plane  1251  further comprises a slot  1254  for coupling a signal from the microstrip line  1252  of the PCB to the microstrip line  1253  which connects the printed circuit board  1250  with the translucent antenna cover  1215  comprising the patch antenna elements  1232 . The patch antenna element  1232  is fed by the microstrip line  1253  via further slots  1255  in the ground plane  1256  which is at least electrically connected to the ground plane  1251 . In other words, an interconnection between the printed circuit board of the radar circuitry and the microstrip feed lines  1253 ,  1138  of the translucent camera cover  1215  is realized by a coupling structure which interconnects a microstrip line  1252  on the printed circuit board with a microstrip line  1253  on the translucent camera dome. 
       FIG. 13  illustrates a further embodiment of the surveillance apparatus according to the present disclosure comprising a hexagonal base  1303  and a hemispherical optically translucent camera cover comprising antenna elements. The camera cover comprising the antenna elements is also referred to as a radome  1315 . The radome has a continuous outline from the hemisphere to the hexagonal shape of the camera base. A transition section  1317  connects the radome with the camera base. For this purpose, the transition section may comprise antenna feed lines for connecting the transparent antenna elements to RF circuitry. The RF circuitry may comprise planar PCBS that are hosted planar sections of the housing. In an alternative embodiment, the antenna elements of the radar sensor are arranged in the transition section  1317 . 
     Thus, the foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. As will be understood by those skilled in the art, the present disclosure may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present disclosure is intended to be illustrative, but not limiting of the scope of the disclosure, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, defines, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public. 
     In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 
     In so far embodiments of the disclosure have been described as being implemented, at least in part, by software-controlled data processing apparatus, it will be appreciated that a non-transitory machine-readable medium carrying such software, such as an optical disk, a magnetic disk, semiconductor memory or the like, is also considered to represent an embodiment of the present disclosure. Further, such a software may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems, including fixed-wired logic, for example an ASIC (application-specific integrated circuit) or FPGA (field-programmable gate array). 
     It follows a list of further embodiments of the disclosed subject matter: 
     1. A surveillance apparatus comprising
         an optical camera that captures images based on received light, said optical camera having a first field of view,   a radar sensor that emits and receives electromagnetic radiation, said radar sensor having a second field of view, and
 
wherein said first field of view is variable with respect to said second field of view.
       

     2. The surveillance apparatus according to embodiment 1, 
     wherein size and/or orientation of said first field of view are variable with respect to said second field of view. 
     3. The surveillance apparatus according to any preceding embodiment, 
     wherein said optical camera is movable with respect to the radar sensor. 
     4. The surveillance apparatus according to any preceding embodiment, 
     further comprising a control unit that controls the optical camera based on radar information obtained with the radar sensor. 
     5. The surveillance apparatus according to any preceding embodiment, 
     wherein the optical camera further comprises a translucent camera cover. 
     6. The surveillance apparatus according to embodiment 5, 
     wherein the camera cover comprises a substantially hemispheric camera done. 
     7. The surveillance apparatus according to any preceding embodiment, 
     having a polygonal, cylindrical or circular outline. 
     8. The surveillance apparatus according to any preceding embodiment, 
     wherein the radar sensor comprises an antenna element arranged on the periphery of the surveillance apparatus. 
     9. The surveillance apparatus according to any preceding embodiment, 
     wherein the radar sensor is adapted to provide at least one of a direction, range and speed of an object relative to the surveillance apparatus. 
     10. The surveillance apparatus according to embodiment 5, 
     wherein the camera cover further comprises a translucent antenna. 
     11. The surveillance apparatus according to embodiment 10, 
     wherein the translucent antenna comprises an electrically conductive layer comprising at least one of a translucent electrically conductive material and an electrically conductive mesh structure. 
     12. The surveillance apparatus according to embodiment 11, 
     wherein a first electrically conductive layer comprises a ground plane and a second electrically conductive layer comprises an antenna element. 
     13. The surveillance apparatus according to embodiment 12, 
     wherein the ground plane comprises a slot for feeding the antenna element. 
     14. The surveillance apparatus according to embodiment 11, 12 or 13, 
     wherein the camera cover comprises at least one dielectric layer and two electrically conductive layers. 
     15. The surveillance apparatus according to embodiment 14, 
     wherein said dielectric layer is made from at least one of glass or a translucent polymer. 
     16. The surveillance apparatus according to any one of embodiments 10 to 15, 
     further comprising a feed structure comprising a microstrip feed line. 
     17. The surveillance apparatus according to any preceding embodiment, 
     further comprising processing circuitry that processes the captured images of the optical camera and the received electromagnetic radiation of the radar sensor and providing an indication of the detection of the presence of one or more objects. 
     18. The surveillance apparatus according to embodiment 17, 
     wherein the processing circuitry verifies the detection an object in the captured images of the optical camera or in the received electromagnetic radiation of the radar sensor based on the received electromagnetic radiation of the radar sensor or the captured images of the optical camera respectively. 
     19. The surveillance apparatus according to embodiment 18, 
     wherein the processing circuitry provides an indication of whether two persons identified in the captured images of the optical camera are actually two persons or one person and their shadow by evaluating distance information to the two identified persons based on the received electromagnetic radiation of the radar sensor. 
     20. A surveillance apparatus comprising
         an optical camera that captures images based on received light, said optical camera having a first field of view,   a radar sensor that emits and receives electromagnetic radiation, said radar sensor having a second field of view, and
 
wherein said second field differs from said first field of view.
       

     21. The surveillance apparatus according to embodiment 20, 
     wherein the second field of view is larger than the first field of view. 
     22. The surveillance apparatus according to embodiment 20 or 21, 
     wherein the second field of view covers an angular range of at least 90°. 
     23. A surveillance radar apparatus for retrofitting an optical surveillance camera, having a first field of view, comprising
         a housing for arrangement of the surveillance radar apparatus at the surveillance camera,   a radar sensor that emits and receives electromagnetic radiation, said radar sensor having a second field of view, and
 
wherein said first field of view is variable with respect to said second field of view.
       

     24. The surveillance radar apparatus according to embodiment 23, 
     wherein the housing of the surveillance radar apparatus encompasses the surveillance camera. 
     25. The surveillance radar apparatus according to embodiment 23, 
     wherein said housing of the surveillance radar apparatus further comprises an alignment member for aligning a position of the surveillance radar apparatus with respect to the surveillance camera. 
     26. A surveillance method comprising the steps of
         capturing images based on received light with an optical camera, said optical camera having a first field of view,   emitting and receiving electromagnetic radiation with a radar sensor, said radar sensor having a second field of view, and
 
wherein said first field of view is variable with respect to said second field of view.
       

     27. A computer program comprising program code means for causing a computer to perform the steps of said method as claimed in embodiment 26 when said computer program is carried out on a computer. 
     28. A non-transitory computer-readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method according to embodiment 26 to be performed.