Abstract:
The invention is related to a camera device ( 10 ) for CCTV applications. The camera device ( 10 ) comprises a sensor ( 12 ) comprising a lens ( 14 ) for monitoring the scene in front of the camera device ( 10 ). At least one polarizer element ( 16 ) is assigned to said sensor ( 12 ), the at least one polarizer element ( 16 ) being rotatably mounted with respect to said sensor ( 12 ), or said polarizer element ( 16 ) uses opto-electric effects.

Description:
CROSS-REFERENCE 
     The invention described and claimed hereinbelow is also described in PCT/EP2005/057119, filed on Dec. 22, 2005. This Patent Application, whose subject matter is incorporated here by reference, provides the basis for a claim of priority of invention under 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The use of polarizer filters is known from photography applications in order to improve contrast. In photography applications, the polarizing angle needs to be adjusted in order to get a maximum suppression of the reflections, which on a photo camera is done manually. The adjustment of the polarizing angle is dependent on the position of the sun and environmental circumstances, to mention but a few such as reflections on a water, which tend to blind the observer and prevent seeing what is in the water. To improve the observation conditions in photography, the polarizers used with photographic cameras are adjusted accordingly. Since the user of the photographic camera is close to the camera, the adjustment of the polarization angle by adjusting the polarizer can be done manually and does not constitute a problem. 
     For video systems used as CCTV systems, there exists the problem that for video systems using a sensor (either CCD or CMOS)-camera, in the following referred to as CCTV applications, the sensor-camera is mounted in places that in general are not nearby or not easy to access. Since the user of a sensor-camera, i.e. the user of a CCTV application, is not near to the sensor-camera, the change of a polarization angle cannot be realized manually. However, the polarizing angle needs to be adjusted in order to get the maximum suppression of the reflections. In order to realize an adjustment of the polarizing angle, a motor is used. Video cameras used as CCTV-cameras in general have a video detector to establish a measure for the video level. Many forms of detectors are known, some just averaging the video contents over a frame time, or summing all pixel values in a frame which is a representation of the video, or detecting the peak value in a frame of the video. Here, the form of the video detector is of less importance. The assumption made is that the output of the video detector is a representation of the video level. 
     Further, there exists the requirement to maintain a sensor-camera&#39;s sensitivity under minimum light conditions to realize a use of the CCTV application even during night times. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a sensor-camera of a CCTV video system is disclosed, in which the polarizing angle can be adjusted automatically. By means of a driven polarizer, the polarizing angle is adjustable over a portion of at least 180° with an accuracy of a few degrees. In order to achieve this accuracy, the drive of the polarizer of the sensor-camera can be realized as a stepper-motor, or as DC motor using a gearbox to achieve low revelation speeds. 
     Preferably, the control of the polarizer which is used to vary the polarization angle is achieved by remote control or even more preferably via automatic control. For the determination of the optimum position of the polarizer, a control algorithm is applied. In order to determine the optimum position of the polarizer when varying the polarizing angle, information concerning the environmental conditions is required. This information is provided by the video detector of the camera, which determines the video level and in which information is stored concerning the light conditions and information concerning the threshold of the minimum light conditions. The criteria to determine the optimum position of the polarizer is to obtain the lowest video detector output which is equivalent to the maximum suppression of reflection. 
     This algorithm follows the strategy to find the minimum of all video detector outputs, i.e. the lowest video detector output. During the determination of the lowest video detector output, the polarization angle is continuously changed. The camera level control loop as sketched allows for finding the lowest video detector output which corresponds to the optimum position of the polarizer. 
     The polarizer, according to the present invention assigned to a sensor-camera, can be realized as a disk-shaped element, to which a drive is assigned, by the means of which the rotation position of the disk-shaped element can be varied. A further alternative is to realize the polarizers using opto-electric effects. 
     In the embodiment in which the polarizer is of a disk-shaped shape, the polarizer could be combined with an IR-filter which is required to obtain colour images. The polarizer filter, i.e. a combination of polarizer with an IR-filter covers the sensor of the camera over at least 180° of rotation. During low light conditions the disk-shaped polarizer filter is in a position in which the IR-filter and the polarizer are absent from the sensor of the camera. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention now is further described by the accompanying drawings, in which: 
         FIG. 1  shows a polarizer filter, assigned to a sensor-camera, the polarizer filter being a combination of a polarizer and an IR-filter, being not in front of the sensor, 
         FIG. 2  shows the polarizer filter in a first position, the polarizing angle being 45° and 
         FIG. 3  shows the polarizer filter according to  FIG. 1  in a second position, the polarizing being 135°, 
         FIG. 4  shows a block diagram of the camera, 
         FIG. 5  shows a polarizer filter assigned to a sensor-camera, the polarizer filter being a combination of a partial polarizer and an air filter being not in front of the sensor, 
         FIG. 6  shows a first position where the polarizer is not and the IR-filter is in front of the sensor, 
         FIG. 7  shows the polarizer filter and the IR-filter in front of the sensor, the polarizing angle being 90° and 
         FIG. 8  shows the polarizer filter and the IR-filter in front of the sensor, the polarizing angle being an arbitrary angle. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a first embodiment of the present invention in which a polarizer in disk-shaped form is assigned to a camera of a CCTV-application. 
     According to this embodiment, a camera device  10  comprises a sensor  12  having a lens, the exit pupil of which is labelled with reference number  14 . The camera device  10  further comprises a polarizer element  16  in which an IR-filter  18  and a polarizer area  20  are combined. The polarizer element  16  is rotatable about an axis of rotation labelled  22  in a first direction indicated by arrow  24  and in a second direction indicated by arrow labelled  26 . The polarizer element  16  has an outer circumference  28  and a base  30  and is shown in its inactive position  32 . In the inactive position  32 , best shown in  FIG. 1 , the sensor  12  of the camera device  10  is not covered by the polarizer area  20  but is instead withdrawn into a position where the integrated IR-filter  18  and the polarizer-area  20  are offset from the sensor  12  which is important during low light conditions. 
     By means of a drive, for example a stepper-motor, the polarizer element  16  is rotatable about its axis of rotation  22 . The rotation angle of the polarizer area can be varied within a 180°-angle of rotation. The accuracy obtained concerning the rotational position of the polarizer element  16  is within a few degrees, for instance between 5° and 10°-rotation angles. Due to the shape of the polarizer element  16 , it is ensured that in the inactive position  32  given in  FIG. 1 , the complete transparent mode is available in order to maintain the sensor&#39;s  12  sensitivity, even under minimum light conditions, for instance during night times. 
     This addresses the circumstance that all polarizing filters by definition attenuate 50%, but in practice even 60% to 70%. By the shape of the polarizer element  16  in the first embodiment according to the present invention, this advantage inherent to polarizer element  16  is ruled out, since in the inactive position  32 , the complete transparent mode is realized concerning the sensor&#39;s  12  sensitivity. 
     In  FIG. 2 , a first position of the polarizer element  16  is given, the polarizer element having an integrated IR-filter  18  and a polarizer area  20 . 
     In the first position  38  of the polarizer element  16 , the polarizer element  16  has performed a first 90°-rotation with respect to its inactive position  32  given in  FIG. 1 . The rotation about 90°, indicated by reference number  44 , results in the disk-shaped polarizer element  16  being turned about its axis of rotation  22 . Due to the rotation of the polarizer element  16 , said polarizer area  20  now covers the exit pupil of lens  14  (shown in  FIG. 1 ) and a first polarization angle  34  is realized. Due to the rotation of the polarizer element  16 , said base  30  now adopts a substantially vertically extending position. The polarizer area  20  now is orientated into a first polarization angle  34 . For adopting the first position  38  of the polarizer element  16 , said disk-shaped polarizer element  16  is rotated by its drive, either a step motor or either a continuous drive motor, into the first direction  24  of rotation. 
     In  FIG. 3 , said polarizer element  16  comprising an integrated IR-filter  18  and the polarizer area  20  has adopted a second position  40 . In this orientation, the polarizer element  16  is oriented upside down with respect to its inactive position  32  shown in  FIG. 1 . With respect to  FIG. 2 , the polarizer element  16  has been rotated about its axis of rotation  22  in another 90°-quarter rotation. With respect to the inactive position  32  of the polarizer element  16  shown in  FIG. 1 , said polarizer element  16  has performed a 180°-rotation, as indicated by arrow  46 . In the second position  40  of the polarizer element  16 , a second polarization angle  36  of 135° is realized. 
       FIG. 4  shows a block diagram of the camera. Light  50  is falling through the lens  14  and the polarizer/IR-filter  18 ,  20  on the sensor  12 . The signal of the sensor  12  may or may not be processed further to generate a video signal  52 . A video detector  54  takes the video signal  52  and generates a number which is representative for the video level. The number is used by a search algorithm  60  that may be hardware or software. The algorithm  60  makes the drive control  58  via motor  56  to turn the IR/polarizer disk  18 ,  20  in a certain direction and tracks if the video output increases or decreases. If it decreases, it continues in that direction, if it increases, the direction of rotation is reversed. For the case that the video output is not changed and it is certain to be at a minimum, the drive control  58  for turning the IR/polarizer disk  20  is stopped. 
     The control of the disk-shaped polarizer element  16 , shown in greater details in  FIGS. 1 ,  2  and  3 , respectively, the rotation of which is realized by a drive to reach the optimum position, is preferably performed automatically. To find the optimum position for the polarizer element  16 , a search and a control algorithm  60  is provided. Said sensor  12  comprises a video detector  54 . The video detector  54  sums up the values of all pixels available from the sensor  10 . The criterion to be met is to obtain the lowest video detector output which corresponds to a maximum suppression of reflection. Variations of the video detector  54  as function of the polarizer rotation are in general in the magnitude of about 30%. During the search algorithm  60 , the minimum value for the video detector output is searched, while driving the polarization element  16  continuously. During the search for the minimum value of the video detector output, an interaction between the camera level control loops and the polarizer element  16  are interrupted. By having implemented the search algorithm  60 , the polarizer element  16  is adjusted at intervals to its optimum position. 
     In the embodiments shown in  FIGS. 1 ,  2  and  3 , the polarizer element  16  is shown as a combined polarizer-filter, comprising a polarizer area  20  and an integrated IR-filter  18 . The integrated IR-filter  18  is required to take coloured images. When it is in position according to  FIG. 1 , the image should be switched to black and white, as colour performance cannot be guaranteed. 
     The camera device  10  according to the present invention is continuously looking for the lowest value for the video detector output. The determination of the lowest video detector output is coupled to a driving of the polarizer element  16  which is a challenge for the mechanics involved. To avoid undue wear of the mechanics, the search algorithm  60  can be stopped after a few cycles and the polarizer element  16  consequently is to be kept in the established optimum position. If, however, the sensor  12  of the camera device  10  adopts a new position or if the video contents, i.e. the pictures being taken by the pixels, completely changes, then a new run for obtaining an optimum position of polarizer element  16  assigned to the sensor  12  can be initiated. Alternatively, the search algorithm  60  can be started by the operator using a remote control facility. Further, to reduce the wear of the mechanical components involved to drive the polarizer element  16  upon finding an optimum position for the polarizer element  16  when the search algorithm  60  is initiated, a timed update can be performed, since the polarization effect is dependent on the daylight conditions and particularly on the position of the sun. Thus, the search for the optimum position of the polarizer element  16 , can be coupled to certain fixed times of the day, for instance every hour. This means that per day the optimum position is determined only twelve times or even less. 
     The polarizer element  16  according to the embodiments given in  FIGS. 1 ,  2  and  3  comprises said integrated IR-filter  18  and the polarization area  20 . Depending on the lines of the polarization element  16 , the first polarization angle  34  of about 45° can be reached in the first position  38  of the polarizer element  16  as shown in  FIG. 2 . In the second position  40  of the polarizer element  16 , the second polarization angle  36 , i.e. about 135°, is realized. Concerning the geometry of the polarizer element  16 , this area is calculated dependent on the size of the sensor  12 . If the sensor  12  has a rectangular cross section defined by base and height, then the minimum radius of the polarizer element  16  with respect to its axis of rotation  22  is given by
 
 r =sqrt(½ b   2 +( h+ ½ b ) 2 )
 
where r=radius, b=base and h=height.
 
     In general, b equals 4/3 h which results in sqrt (53/9 h 2 )=˜2.45 h. 
     For common geometries using a ⅓″, height h=3.64 mm, consequently, r=8.92 mm. In practice, the radius of the polarizer element  16  is somewhat larger. The limitation for the radius of the polarizer element  16  according to the present invention is the building space which is present in a standard sized camera. The polarizer element  16  is preferably to be integrated into a standard sized camera to avoid modifications of existing designs. 
     In an alternative embodiment, the polarizer element  16  can be made as a component, having a polarizer area  20 , an integrated IR-filter  18  partially covered by the polarizer area  20  and a segment  48  without polarizer or IR-filter. 
     According to the alternative embodiment given in  FIGS. 5 to 8 , respectively, the polarizer element  16  is in general of disk shape. In the embodiment given in  FIGS. 5 to 8 , said polarizer element  16  comprises the polarizer area  20 , in integrated IR-filter as well as a segment  48  without IR-filter and polarizer. 
     According to  FIG. 5 , the polarizer element  16  assigned to said sensor  12  is given in a position in which the polarizer area  20  as well as the IR-filter  18  are absent from the lens  14 . The position given in  FIG. 5  corresponds to the position of the polarizer element  16  given in  FIG. 4  according to which light  50  penetrates said lens  14  in front of the sensor  12 . 
     In  FIG. 6 , said polarizer element  16  is turned about its axis of rotation  22 , being driven by said drive  56  according to  FIG. 4  into a position in which the integrated IR-filter  18  only is present in front of said sensor  12 . The segment  48  without IR-filter  18  and without polarizer has turned away from said sensor  12  into counterclockwise direction. 
     Starting from the rotation position given in  FIG. 6 , according to  FIG. 7 , the polarizer element  16  is further turned in counterclockwise direction about its axis of rotation  22  into a position in which the polarizing angle corresponds to 90°. In the position given in  FIG. 7 , the polarizer area  20  of said disk-shaped polarizer element  16  covers the front of said sensor  12  entirely. The base  30  of said polarizer area  20  in this position adopts a substantially vertical position. Upon further rotation of said polarizer element  16  into counterclockwise direction, the polarization area  20  moves further into counterclockwise direction. 
     In the stage given in  FIG. 8 , the rotational position of the polarizer area  20  of said disk-shaped polarizer element  16  results in an arbitrary polarizing angle. A stepwise rotation of the polarizer area  20  between the positions given in  FIG. 7  and  FIG. 8 , respectively, allows for adjustment of selective polarizing angles which may depend on the application of the camera device  10  having said sensor  12 . 
     In the position according to  FIG. 6  according to the present invention, a color image can be taken, since there is an IR-filter  18  present in front of said sensor  12 , however, the polarizing area  20  of said disk-shaped polarizer element  16  is absent, so that the attenuation by the polarization is not applicable in this special position according to  FIG. 6 .