EP1972133A1

EP1972133A1 - Camera system

Info

Publication number: EP1972133A1
Application number: EP06835661A
Authority: EP
Inventors: Johannes Petrus Marinus Cornelis Van Schijndel
Original assignee: Internova Holding bvba
Current assignee: Internova Holding bvba
Priority date: 2005-12-15
Filing date: 2006-12-15
Publication date: 2008-09-24
Also published as: NL1030678C1; WO2007069892A1

Abstract

A camera system (1) is described, comprising: a first camera (10) for providing first image signals; a second camera (20) for providing second image signals; an output (3) for outputting image signals; and at least one light level sensor (70) for providing a light level signal that is representative for an ambient light level. The camera system is adapted, depending on the light level signal of the light level sensor, to output either the first image signals of the first camera (10) or the second image signals of the second camera (20) at the output (3) .

Description

Camera system

The present invention relates in general to a camera system, suitable for guarding and/or monitoring objects. More particularly, the present invention relates to a camera system for placement ^' outside, for guarding and/or monitoring buildings, particularly houses and business premises.

For guarding and/or monitoring buildings, it is known to use a camera system that produces image signals, particularly video images, of the guarded object, wherein the camera system is arranged outside this building. The camera system may be directed to entrances of this building, such as doors or windows, but the camera may also be directed to a part of the terrain surrounding the building concerned, such as a garden. The video images provided by the camera system may be stored locally, or may be transferred to a central storage location. It is also possible that the camera system is provided with image processing software for detecting movement, so that the camera can be used as a movement detector in a guarding system.

Both for the purpose of guarding and for the purpose of observation, it is desirable to obtain images of high quality, both during the day and during the evening and the night . It is then a problem that the illumination conditions during the day deviate strongly from the illumination conditions during the evening or the night . During the day, the light level of the surroundings is usually high enough for providing good video images with relatively simple cameras. For use during the evening or the night, however, a camera must have a very high light sensitivity. Special digital cameras have been developed for this purpose, which will hereinafter for sake of convenience be indicated with the phrase "night view camera" , which cameras are always monochromatic cameras with a high light sensitivity, while also the signal/noise ratio must be very good in order to be able to provide qualitatively good video images. A good night view camera is very costly. On the other hand, a night view camera is not well useable during the day: the light level is much too high for the image pickup (CCD- chip) . Further, during the day, it is rather desirable to obtain colour images .

On the other hand, digital cameras have been developed specifically for daylight circumstances. In that case, the light level is high enough for providing qualitatively good colour images with a relatively simple camera and the signal/noise ratio of the CCD-chip is less critical. Such cameras, which in the following for the sake of convenience will be indicated by the phrase "day view camera", are however not suitable for use in the dark, because then the light level is much too low. Cameras have also been developed, intended for 24 hours use, i.e. use during the day as well as during the evening and the night. Such cameras, which in the following will be indicated by the phrase "combi-cameras, are always a compromise between the good and bad properties of day view cameras and night view cameras. During the day, combi-cameras provide better images then a night view camera, but the quality is less then the quality of a day view camera. In dark conditions, combi-cameras provide better images then a day view camera, but the quality is less then the quality of a night view camera.

An important problem in the case of combi-cameras is that during the day use must be made of an IR-filter, but not during the night. To this end, a combi-camera has a displacement mechanism for the IR-filter, which mechanism is controlled by a light sensor. A problem with such mechanism is that it is susceptible to wear: preferably, a camera system should not have movable parts. Moveable parts are susceptible to wear and can fail .

Further, it is a problem that the optical path is influenced by positioning or removing the IR-filter, which leads to problems with an autofocus mechanism. This leads to more noise in the images obtained, which, in the case of use as image detector, is disadvantages because this noise is interpreted as movement. Further, this apparent movement has the consequence that a processor, when encoding/compressing the video data, requires more computational power and that the digital video output signal comprises more data.

An other problem of known combi-cameras is that they are provided with an automatically operating diaphragm. In the case of use in dark conditions, the diaphragm is opened as much as possible in order to ensure that the CCD-chip receives as much light as possible. In the case of use during the day, the diaphragm is reduced in order to prevent damage to the CCD-chip, and to prevent the CCD-chip from getting saturated. Such automatic diaphragm is again a moving part, which is disadvantaging. If the diaphragm brakes down, it is possible that too much light arrives on the CCD-chip, so that it fails. It is a general objective of the present invention to provide a camera system in which the above-mentioned disadvantages are eliminated or at least reduced.

More particularly, the present invention aims to provide a camera system capable of providing image signals of very high quality, both during the night and during the day, without the necessity of moving parts.

According to an important aspect of the present invention, a camera system comprises two digital cameras, the one camera being a day view camera and the other being a night view camera. Further, the camera system comprises at least one light sensor. When there is sufficient light (during the day) the day view camera is used. When it is dark (during the evening or the night) the night view camera is used. Thus, in all light conditions this system always has the good properties of day view cameras and night view cameras and the bad properties of day view cameras and night view cameras are eliminated.

The two cameras may be on continuously, and only one is used depending on the light conditions. Preferably, however, the camera which is not in use is switched off. Thereby, the useful life of the individual cameras is increased.

When a camera is switched on from its OFF-state, it takes some time before it is operational. For a stable functioning it is of particular importance that the camera has a certain operational temperature . In order to avoid the delay caused by this, it is preferred that a camera is first switched STANDBY before it is switched on.

In the morning, at daybreak, the system therefore distinguishes two different light intensity thresholds. When the detected light intensity reaches the first threshold, the day view camera is switched to STANDBY. On reaching the second, higher threshold, the day view camera is switched ON and the image signals of the day view camera are used. Now, the night view camera may be switched off. During the night, at nightfall, the system again distinguishes two different light intensity thresholds. When the detected light intensity reaches the first threshold, the night view camera is switched to STANDBY. On reaching the second, lower threshold, the night view camera is switched ON and the image signals of the night view camera are used. Now, the day view camera can by switched off.

In a further elaboration, it is further possible that an IR-light source is switched on in the case of dark conditions.

These and other aspects, features and advantages of the present invention will be clarified in more detail by the following description with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: Figures IA and IB show schematic perspective views of a camera system according to the present invention;

Figure 2 schematically shows a block diagram of the. camera system; Figure 3 is a graphical representation illustrating the operation of a camera system according to the present invention;

Figure 4 is a graphical representation illustrating the operation of an other embodiment of a camera system according to the present invention.

Figures IA and IB show schematic perspective views of a camera system 1 according to the present invention. The system 1 comprises a housing 100, with a front wall 101, a rear wall 102, side walls 103 and 104, a floor 105, and a shielding cap 106. Figure IB shows the housing 100 with the shielding cap 106 removed, so that it can be seen that two digital video cameras are arranged in the interior of the housing 100, a first camera 10 and a second camera 20. Each camera 10, 20 has a corresponding lens system 111, 121. The front wall 101 is provided with two viewing openings 110, 120 arranged next to each other, for the respective cameras 10, 20. The cameras 10, 20 are arranged within the housing 100 in such a way that their respective viewing lines are substantially mutually parallel, wherein the first camera 10 is arranged to "look" outside through the first viewing opening 110 while the second camera 20 is arranged to "look" outside through the second viewing opening 120. In each viewing opening 110, 120, a transparent plate is accommodated, for instance glass or plastic, on the one hand closing the viewing opening 110, 120 concerned and on the other hand being transparent to the relevant part of the light spectrum.

The figures IA and IB also show that two viewing openings 171, 172 are arranged in the front wall 101 for in this case two light sensors. In other embodiments, there may be more then two light sensors present, but it is also possible that there is only one light sensor present. Further, it is not necessary that a light sensor is arranged in the front wall 101, although this is preferred.

Figure 2 schematically shows a block diagram of a preferred embodiment of the camera system 1. The first camera 10 has three operative states, i.e. a first operative state (OFF) in which the camera is out of operation, a second operative state (ON) in which the camera is fully operational, and a third operative state (STANDBY) in which the camera is ready to become fully operational but consumes less energy then in the ON-state. The first camera 10 is a controllable camera, and has a control input 10a for receiving a first control signal SCl. The first camera 10 is arranged, in response to the control signal SCl, to operate in either its ON-state, or its OFF-state, or its STANDBY-state . Further, the first camera 10 has a signal output 10b for providing a video output signal SVl. The camera system 1 further has a control device 2 , with a first control output 11 for providing the first control signal SCl. The control device 2 may for instance be a suitably programmed microprocessor, and will in the following also be indicated by the phrase "controller".

In a comparable manner, the second camera 20 has three possible operative states OFF, ON and STANDBY, a control input 20a for receiving a control signal SC2, and an output terminal 20b for providing a video signal SV2, and the controller 2 has a second control output 12 for providing the second control signal SC2.

The camera system 1 has a system output 3 for outputting an image signal SV, provided by either the first camera 10 or the second camera 20. For selecting the first video output signal SVl of the first camera 10 or the second video output signal SV2 of the second camera 20, the camera system 1 comprises a controllable switch 30, of which a first input 31 is coupled to the signal output 10b of the first camera 10, of which a second input 32 is coupled to the signal output 20b of the second camera 20, and of which an output 33 is coupled to the system output 3. The switch 30 is of a type that is switchable between a first operative state in which its output 33 is connected to the first input 31, and a second operative state in which its output 33 is connected to its second input 32. The controllable switch 30 as a control input 34, and is adapted to set either its first operative state or its second operative state in response to receiving a third control signal SC3 at its control input 34. The controller 2 has a third control output 13 for providing the third control signal SC3.

It is noted that controllable switches are known per se and that a more detailed explanation of the construction and operation of the controllable switch 30 is not necessary here. In the embodiment shown, the camera system 1 further has a switchable infrared light source 40, which is switchable on or off in response to fourth control signal SC4, which is provided by the controller 2 at a fourth control output 14. It is noted that suitable controllable infrared light sources are known per se, and that a more detailed explanation of the construction and operation of the infrared light source 40 is not necessary here. The two cameras 10 and 20 are mutually not identical, but are specially designed for mutually different light conditions. In the following, it will be assumed that the first camera 10 is specially designed for circumstances with sufficient ambient light, and for that reason will be indicated by the phrase "day view camera", while the second camera 20 is specially designed for dark circumstances, i.e. circumstances with only very little or no ambient light, and for that reason will be indicated by the phrase "night view camera" . It is noted that such specially designed cameras are known per se, so that a more detailed explanation of the design and operation of those cameras is superfluous here. It suffices to note that the day view camera 10 typically is a colour camera provided with an infrared filter, and that the night view camera typically is a monochromatic camera without an infrared filter. It is further noted that the cameras typically may have a CCD-chip as light-sensitive element, but cameras with other light-sensitive elements are also suitable for use.

The controller 2 is adapted to generate its control signals SCl, SC2, SC3 , SC4, for the cameras 10, 20, the switch 30 and the infrared light source 40 on the basis of the amount of ambient light. To this end, the camera system 1 comprises at least one light sensor 70, which provides a light signal SL, and the controller 2 has a sensor input 17 for receiving the measuring signal SL of the sensor 70

As will be explained in more detail in the following, the controller 2 is adapted to provide its output signals depending on whether the ambient light level is above or below predetermined threshold levels. In the exemplary embodiment shown, the camera system 1 has a signal light sensor 70, of which the output signal is representative for the detected light intensity, and the controller 2 is adapted to compare the measuring signal received with predetermined reference values. Alternatively, it is also possible that the light sensor is a sensor with a build-in threshold level, in which case the sensor provides an output signal with a first value if the detected light level is less then the threshold value concerned and has a second value if the detected level is higher then the threshold value concerned. The comparison of the ambient light level with the predetermined threshold value in that case is therefore not executed by the controller but by the light sensor. Then, for each threshold value a separate light sensor is required, and for each light sensor the controller must have a signal input.

Figure 3 is a graphical representation illustrating the operation of an example of the camera system 1. The horizontal axis represents time. The ambient light intensity LN is set along the vertical axis in arbitrary units . Above that are the operative states of the two cameras 10 and 20 and of the controllable switch 30.

First, the operation will be explained for a scenario in the morning in which case the surroundings change from dark to light. The increasing ambient light level is indicated with a curve 71 at the left hand side in the figure. When it is dark, the ambient light level LN is lower than a first threshold level Ll. In that case, the night view camera 20 is ON, the day view camera 10 is OUT, and the switch 30 is in its second switching state, so that the output signal SV2 of the second camera 20 is provided as output signal SV at the system output 3.

On time tl, the increasing ambient light level LN passes the first threshold level. At that moment, the day view camera 10 is switched STANDBY.

At time t2, the increasing ambient light level LN passes a second threshold value L2 higher than the first threshold value Ll. Now, the controller 2 switches the day view camera 10 on, and switches the switch 30 to its first switch state, so that the system output 3 outputs the image signal SVl of the first camera 10.

The night view camera 20 can now be switched off. The controller can switch off the night view camera 30 simultaneously with switching the day view camera 10 on, but it is also possible that the night view camera 20 is switched off a brief time later. In this case, the controller 2 may implement a predetermined time delay, but it is also possible that the night view camera is switched off when the ambient light level LN passes a third threshold level L3 higher than the second threshold value L2 as illustrated in figure 4 at time t3.

It is noted that it is possible that the controller 2 executes the switching of the switch 30 somewhat later than the switching on of the day view camera 10, for instance by implementing a predetermined time delay.

At the right hand in figure 3, the actions of the controller 2 during a scenario at the beginning of the evening are illustrated, in which case curve 72 illustrates the decreasing ambient light level LN. During the day the ambient light level LN is relatively high, the day view camera 10 is ON, the night view camera 20 is OFF, and the switch 30 is in its first switch state. When the decreasing ambient light level LN passes a sixth threshold level L6 on a time t6, the controller 2 switches the night view camera 20 STANDBY.

At a later time t7, when the decreasing ambient light level 72 passes a seventh threshold level L7 lower than the sixth level L6, the controller 2 switches the night view camera 20 ON and the controller 2 switches the switch 30 to its second switching state.

Then, the day view camera 10 can be switched off, either simultaneously with the switching of the switch 30, or at a somewhat later time t8. In this case, again, a constant time delay may be implemented, or the day view camera 10 is only switched off when the ambient light level LN decreases below a threshold level L8 lower then the seventh threshold level L7.

In this case, too, it holds that the switch 30 may be switched to its second switching state simultaneously with the switching of the night view camera 20 or after a short time delay.

From the foregoing it appears that both during the increasing of the ambient light level and during the decreasing of the ambient light level, there are at least two different threshold levels. On passing the first threshold level, the camera being off until then is switched over to its standby state, and on passing the next threshold level this camera is switched on, and the camera used until then is switched off. In that case, it is possible that the controller 2 gives a switch over command to the cameras 10 and 20 and to the switch 30 only at the moment of passing of a threshold level. It is also possible that the controller 2, continuously or regularly, compares the momentaneous value of the light level LN to the threshold levels and generates the control signals on the basis of this.

It is possible that the threshold levels Ll and L2 used in the case of increasing light intensity are equal to the light levels L7 and L6 used in the case of decreasing light levels. Thus, in that case, it is not univocally determined which camera is active during twilight, i.e. at a light intensity between the first threshold level L1/L7 and the second threshold level L2/L6, because the selection of the active camera now depends on the history: in the case of increasing light intensity between time tl and t2 the night view camera 20 is active, and in the case of decreasing light intensity between t6 and t7 the day view camera 10 is active.

It is also possible that the second light level L2 is chosen equal to the seventh threshold level L7. In that case, switching from the night view camera 20 to the day view camera 10 in the case of increasing light intensity takes place at the same light level L2/L7 as the switching from the day view camera 10 to the night view camera 20 in the case of decreasing light intensity. Then, however, it is a disadvantage that it is possible that the light intensity fluctuates somewhat, in which case a switching 'in one direction and back can occur from the night view camera to the day view camera and vice-versa. In order to prevent this, it is preferred that the seventh threshold level L7 is lower than the second threshold level L2.

The controller 2 may switch ON the infrared light source 40 simultaneously with switching the night view camera 20 ON, i.e. at the time t7 when the decreasing light level LN passes the seventh threshold level L7. Then, switching OFF of the infrared light source 40 can take place simultaneously with switching the night view camera 20 OFF, i.e. at the time t2 when the increasing light level LN passes the second threshold level L2 , or at somewhat later time t3. Switching the infrared light source 40 is then coupled to the ON switching of the night view camera 20. However, it is also possible that the infrared light source 40 is already switched on during the twilight period, i.e. when the decreasing light level LN passes the sixth threshold value L6. Then, switching of the infrared light source 40 may be coupled to the switching of the night view camera 20 STANDBY/OFF. Then, as long as the night view camera 20 is STANDBY, the day view camera 10 already profits from the light emitted by the infrared light source 40. With respect to the switching by the controller 2 it is further noted that the controller 2 is provided with a hysteresis facility to prevent undesirable switching behaviour. For instance, when the increasing light level LN passes the first threshold level Ll at time tl and the day view camera 10 is switched to its STANDBY state, it is possible that the ambient light level LN fluctuates, so that a short time later the ambient light level LN again decreases to below the first threshold level Ll. In that case, the day view camera 10 could be switched again to its OFF state. Such frequently switching hither and back, however, is undesirable. This can be counteracted because the controller 2 has a build- in time delay, which stops a switching back from the standby state to the off state during a predetermined time, and also the controller 2 may be provided with a build-in delay which stops the switching back from the ON state to the STANDBY state during a predetermined time. It is also possible that the signal of the light level sensor 70 is averaged over a longer time, for instance an averaging time of 5 minutes, causing fast fluctuations to have little or no influence. It is also possible that the eighth light level at which the day view camera 10 in the case of decreasing light level is switched to its OFF state is lower than the first threshold level Ll. Comparable remarks apply of course, mutatis mutandis, in relation to the STAND BY switching or OFF switching of the night view camera 20 around the sixth threshold level L6.

Figure 4 is a graphical illustration comparable to figure 3 of the operation of another embodiment of the camera system according to the present invention. In this case, during the increase or decrease of the ambient light level LN, the controller 2 takes three successive threshold levels into account . An increasing light level indicating the scenario in the morning is shown with curve 73 at the left hand side in figure 4, while a decreasing light level illustrating a scenario in the evening is shown with curve 74 at the right hand side in figure 4.

During dark circumstances, the day view camera 10 is OFF, the night view camera 20 is ON, and the switch 30 is in its second switching state. When the light level Ll increases and passes the -first threshold value Ll, the day view camera 10 is switched STANDBY. When the increasing light level LN subsequently passes the second threshold level L2 higher than the first threshold level Ll at a second time t2, the day view camera 10 is switched ON and the switch 30 is switched to its first switching state. In this case, as well, the switching of the switch 30 to its first switching state may happen somewhat later than switching the day view camera 10 on. In deviation from the embodiment discussed with reference to figure 3, the night view camera 20 is now first switched back to its STANDBY state. Only when the increasing light level LN passes the third threshold value L3 higher then the second threshold value L2, the night view camera 20 is switched OFF.

It is noted that switching the night view camera 20 from the ON state to the STANDBY state may take place simultaneously with switching the day view camera 10 from the STANDBY state to the ON state at time t2 , but it is also possible that switching the night view camera 20 occurs somewhat later. To this end, the controller 2 may be provided with a timer, switching the night view camera 20 with a predetermined time delay after time t2, but it is also possible that switching the night view camera 20 STANDBY occurs on a time t4 when the increasing light level LN passes a fourth threshold level L4 higher than the second threshold level L2, as illustrated.

When at the end of the day the ambient light level LN decreases, the operation is as follows. As long as the ambient light level LN is sufficiently high, the day view camera 10 is ON, the night view camera 20 is OFF, and the switch 30 is in its first switching state. When the ambient light level LN decreases to below a sixth threshold value L6, the night view camera 20 is switched STANDBY. When the decreasing light level LN subsequently passes a seventh threshold value L7 lower than the sixth threshold value L6, the night view camera 20 is switched ON and the switch 30 is switched to its second switching state. In this case, too, switching the switch 30 may take place with some delay.

The day view camera 10 may now be switched STANDBY. It is possible that this also takes place simultaneously with switching the night view camera 20 ON, or with switching the switch 30 to its second switching state if this happens later, but more in the illustrated variation the day view camera 10 is switched standby when the decreasing light level LN passes a ninth threshold value L9 lower than the seventh threshold value L7. In the border case, the ninth threshold level L9 thus is equal to the seventh threshold level L7. When subsequently the decreasing light level LN passes a tenth threshold level LlO lower than the ninth threshold level L9 at time tlO, the day view camera 10 is switched OFF.

In a possible embodiment, the second threshold level L2, the fourth threshold level L4, the seventh threshold level L7, and the ninth threshold level L9 are mutually equal to each other, the fifth and sixth threshold level L5 and L6 are mutually equal to each other, and the first and tenth threshold level Ll and LlO are mutually equal to each other. Then, the system distinguishes four ambient light conditions: a first condition "night" when the ambient light level LN is less than the first threshold value L1/L10; a second condition "twilight" when the ambient light level LN is between the first threshold value L1/L10 and the second threshold value L2/L4/L7/L9; a third condition "clouded" when -the ambient light level LN is between the second threshold value L2/L4/L7/L9 and the highest threshold value L5/L6; and the condition "daylight" when the ambient light level LN is above this highest threshold value L5/L6.

In order to prevent undesirable switching behaviour, however, it is preferred that the switching moments in the case of decreasing light intensity do not coincide with the switching moments in the case of increasing light intensity. Therefore, it is preferred that the tenth threshold value LlO is somewhat lower than the first threshold value Ll, that the sixth threshold level L6 is somewhat lower than the fifth threshold level L5 , that the ninth threshold level L9 is somewhat lower than the second threshold level L2 , and that the seventh threshold level L7 is somewhat lower than the fourth threshold level L4.

The controller 2 may switch on the infrared light source 40 simultaneously with switching on the night view camera 20, i.e. at time t7. Preferably, however, the infrared light source 40 is already switched on earlier, simultaneously with switching the night view camera 20 STANDBY, i.e. on the sixth time t6. Switching OFF of the infrared light source 40 may then take place simultaneously with the switching OFF of the night view camera 20, i.e. at the fifth time L5 , in which case undesirable switching behaviour of the infrared light source 40 is avoided if the fifth light level L5 is somewhat higher then the sixth light level L6.

It should be clear to a person skilled in the art that the invention is not limited to the exemplary embodiments discussed in the above, but that several variations and modifications are possible within the protective scope of the invention as defined in the attached claims.

In the above, the present invention is explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It may be clear that one or more of these functional blocks may be implemented in hardware, in which case the function of such functional blocks is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or by a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.

Claims

1. Camera system (1), comprising:

- a first camera (10) for providing first image signals (SVl) _;

- a second camera (20) for providing second image signals (SV2);

- an output (3) for outputting image signals; and at least one light level sensor (70) for providing a light level signal (SL) representative for an ambient light level (SN) ; wherein the camera system is arranged, depending on the light level signal of the light level sensor, to output at the output (3) either the first image signals of the first camera (10) or the second image signals of the second camera (20) .

2. Camera system according to claim 1, wherein the first camera (10) is a camera suitable for high light intensities, wherein the second camera (20) is a camera suitable for low light intensities, and wherein the camera system is adapted to output the first image signals of the first camera (10) if the light level signal of the light level sensor is indicative for a relatively high ambient light intensity, and to output the second image signals of the second camera (20) if the light level signal of the light level sensor is indicative for a relatively low ambient light intensity.

3. Camera system according to claim 1 or 2, further comprising a controllable switch (30) , of which a first input (31) is coupled to an image signal output (10b) of the first camera (10) , of which a second input (32) is coupled to an image signal output (20b) of the second camera (20) , and of which an output (33) is coupled to a system output (3) .

4. Camera system according to claim 3, further comprising a control device (2) , of which an input (17) is coupled to an output of the light level sensor (70) , and of which a control signal output (13) is coupled to a control input (34) of the switch (30) .

5. Camera system according to claim 1 of 2, wherein each camera (10; 20) has a first operative state (OFF) and a second operative state (ON) ; and wherein the camera system is adapted, depending on the light level signal of the light level sensor, to switch a camera to its first operative state (OFF) or to its second operative state (ON) .

6. Camera system according to claim 5, wherein the camera system further comprises a control device (2) , of which an input (17) is coupled to an output of the light level sensor (70) , of which a first control signal output (11) is coupled to a control input (10a) of the first camera (10) and of which a second control signal output (12) is coupled to a control input (20a) of the second camera (20) .

7. Camera system according to claim 6, wherein each camera (10; 20) has a third operative state (STANDBY) ; and wherein the control device (2) is adapted, depending on the light level signal of the light level sensor, to switch a camera to its third operative state (STANDBY) .

8. Camera system according to claim 7, wherein the system is adapted to : a) when the ambient light level is less than a first threshold level (Ll) , to operate the first camera (10) in its first operative state (OFF) and to operate the second camera (20) in its second operative state (ON) , and to output the second image signals (SV2) of the second camera (20) at the system output (3) ; b) when the ambient light level exceeds the first threshold level (Ll) , to switch the first camera (10) to its third operative state (STANDBY) ; c) when the ambient light level exceeds a second threshold level (L2) higher than the first threshold level (Ll) , to output the first image signals (SVl) of the first camera (10) at the system output (3) .

9. Camera system according to claim 8, wherein the system is adapted, when the ambient light level exceeds a third threshold level (L3) , to switch the second camera (20) to its first operative state (OFF) .

10. Camera system according to claim 9, wherein the third threshold level (L3) is equal to the second threshold level

(L2) or is higher than the second threshold level (L2) .

11. Camera system according to claim 8, wherein the system is adapted, when the ambient light level exceeds a fourth threshold level (L4) , to switch the second camera (20) to its third operative state (STANDBY) , and, when the ambient light level exceeds a fifth threshold level (L5) higher than the fourth threshold level (L4) , to switch the second camera (20) to its first operative state (OFF) .

12. Camera system according to claim 11, wherein the fourth threshold level (L4) is equal to the second threshold level (L2) or is higher then the second threshold level (L2) .

13. Camera system according to any of the claims 8-12, wherein the system is adapted: d) when the ambient light level is higher than a sixth threshold level (L6) , to operate the second camera (20) in its first operative state (OFF) and to operate the first camera (10) in its second operative state (ON) , and to output the first image signals (SVl) of the first camera (10) at the system output (3) ; e) when the ambient light level understeps the sixth threshold level (L6) , to switch the second camera (20) to its third operative state (STANDBY) ; f) when the ambient light level understeps a seventh threshold level (L7) lower than the sixth threshold level (L6) , to output the second image signals (SV2) of the second camera (20) at the system output 3.

14. Camera system according to claim 13 , wherein the system is adapted, when the ambient light level exceeds an eighth threshold level (L8) , to switch the first camera (10) to its first operative state (OFF) .

15. Camera system according to claim 14 , wherein the eighth threshold level (L8) is equal to the seventh threshold level (L7) or is lower than the seventh threshold level (L7) .

16. Camera system according to any of the claims 13-15, wherein the sixth threshold level (L6) is equal to the second threshold level (L2) or is lower than the second threshold level (L2) , and wherein the seventh^' threshold level (L7) is equal to the first threshold level (Ll) or is lower than the first threshold level (Ll)

17. Camera system according to any of the claims 13-15, wherein the seventh threshold level (L7) is equal to the second threshold level (L2) or is lower than the second threshold level (L2) .

18. Camera system according to claim 13, wherein the system is adapted, when the ambient light level understeps a ninth threshold level (L9) , to switch the first camera (10) to its third operative state (STANDBY) , and, when the ambient light level understeps a tenth threshold level (LlO) lower than the ninth threshold level (L9) , to switch the first camera (10) to its first operative state (OFF) .

19. Camera system according to claim 18, wherein the ninth threshold level (L9) is equal to the seventh threshold level (L7) or is lower than the seventh threshold level (L7) .

20. Camera system according to claim 18 or 19, wherein the ninth threshold level (L9) is equal to the second threshold level (L2) or is lower than the second threshold level (L2) .

21. Camera system according to any of the previous claims, further comprising a switchable infrared light source (40) , wherein the system is adapted to switch the infrared light source on or off depending on the light level signal of the light level sensor.

22. Camera system according to claim 21, wherein the system is adapted to switch the infrared light source simultaneously with switching the second camera (20) to the second operative state (ON) .

23. Camera system according to claim 21, wherein the system is adapted to switch the infrared light source on simultaneously with switching the second camera (20) from the first operative state (OFF) to the third operative state (STANDBY) .

24. Camera system according to any of the previous claims, wherein the system comprises a single light sensor (70) providing a measuring signal (SL) and wherein a controller (2) is adapted to generate control signals (SCl, SC2, SC3, SC4) on the basis of this measuring signal (SL) .

25. Camera system according to any of the previous claims 1- 23, wherein the system comprises multiple light sensors, each adapted to generate an output signal indicating whether the momentary light level (LN) is higher or lower than a predetermined threshold value .