Abstract:
A recording device for distance images becomes multi-target-enabled by means of the arrival of light pulses reflected at object regions at different distances being temporally resolved. This is done using extrema of the gradient of a correlation function between the received light pulses and a time window during which sensor elements of a camera are activated.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2006/066855, filed Sep. 28, 2006 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2005 046 950.7 DE filed Sep. 30, 2005, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a device for recording distance-measuring images, comprising a light source which emits light pulses and a photodetector which integrates the light stream of a light pulse, reflected on an object region, via a time window whose time interval can be varied in relation to the process of emission of a light pulse, into a multiplicity of light quantity values, and comprising an evaluation unit connected downstream of the photodetector, which evaluation unit determines, on the basis of the correlation function formed by the light quantity values, the distance of the object region. 
         [0003]    The invention also relates to a method for recording distance-measuring images, in which, with the aid of a light-propagation-time-measuring device with short-term integration, a correlation function is determined between a light pulse reflected on an object region and a time window of the light-propagation-time-measuring device. 
       BACKGROUND OF INVENTION 
       [0004]    Such a device and such a method are known from WO 03/016944 A2. In the case of the known device and the known method, light pulses are transmitted from a light source to an object surface and reflected there. The light reflected on the object surface goes to an image converter manufactured using CMOS technology, which image converter has a multiplicity of photodetectors in which the incident light stream can generate charge carriers when the photodetector is activated. The photodetectors can be activated for a variable time window. The quantity of charge present in the photodetector at the end of the time window is a measure of the light quantity which has been detected by the photodetector during the time window. By means of the photodetector, the incident light stream is therefore integrated during the time window. 
         [0005]    By varying the time interval between the process of emission of the light pulse and the time window used for integration, a correlation function between the light pulse and the time window can be determined. If the duration of the light pulses is equal to the duration of the time window, the correlation function has a pronounced maximum. The maximum of the correlation function lies at a time value which corresponds to the light propagation time of the light pulse from the light source to the photodetector. The light propagation time can therefore be determined directly from the maximum of the correlation function. The distance between object surface and device can then be calculated from the light propagation time. 
         [0006]    The known device and the known method can be used in the fields of automotive engineering, navigation or building engineering, safety engineering and automation. 
         [0007]    One disadvantage of the known device and of the known method is that objects lying partially concealed, for example on a vehicle behind a fence or a bush, are not recognized correctly. Likewise, dust or other contamination on the optical system can lead to incorrect measurements. In the known method and the known device, an average distance value is displayed for objects lying partially concealed, said average distance value being produced from the averaging of various distance values. 
       SUMMARY OF INVENTION 
       [0008]    Taking this prior art as a starting point, an object underlying the invention is therefore to create a device and a method for recording distance-measuring images which are multi-target-capable. 
         [0009]    This object is achieved by a device and a method having the features of the independent claims. Advantageous embodiments and further developments are indicated in the claims dependent thereon. 
         [0010]    In the device and the method, extreme values of the gradient are determined in the correlation function and from these the distances of differently distant object regions are determined. This is because light pulses which are reflected on differently distant object regions, arrive at the respective photodetector in a delayed manner. They therefore contribute to the light quantity signal only if the respective light pulse lies within the time window used in each case. A stepped rise in the correlation function can therefore be attributed to the detection of additional light pulses. The light propagation time of the light pulses reflected on different object regions can then be determined from the gradient maxima. The device and the method are to this extent multi-target-capable. 
         [0011]    In a preferred embodiment, the duration of the light pulses emitted by the light source is chosen so as to be less than or equal to the light propagation time for a distance difference to be resolved. The duration of the light pulses is to be understood here to be the half-amplitude duration of the light pulses. With light pulses of this duration it is possible to resolve the required distance interval. 
         [0012]    Furthermore, the gradient maxima are preferably determined on the basis of a derivation function of the correlation function. In this case, the determination of the gradient maxima can be traced to a search for local extreme values. 
         [0013]    The gradient function is preferably determined by determining the difference between two light quantity values which have a time interval in the range of the pulse duration. This reduces susceptibility to interferences and noises typical in the computation of a derivation function. 
         [0014]    The light quantity values recorded by a photodetector can be temporarily stored in an intermediate memory. The computation of the derivation can then be carried out on the basis of the light quantity values stored in the intermediate memory. In such an embodiment, all the photodetectors can be directed at different object regions and thereby contribute to full spatial resolution. 
         [0015]    In a further modified embodiment, two adjacent photodetectors are respectively operated with offset time windows and the difference value read out in each case. This embodiment offers the advantage that the variation of the light pulses has no influence on the difference signal. Also, the reset noise typical of photodetectors manufactured using CMOS technology is largely eliminated. 
         [0016]    In a further preferred embodiment, the additional locational information is used to enhance the spatial resolution of the distance-measuring image. With unknown objects, the distance values of adjacent image elements of the distance-measuring image can be used for this purpose, different distance values being assigned to partial regions of an image element according to the distance values of the adjacent image elements. 
         [0017]    With known objects, the distance-measuring image can be generated through adaptation to the known object. This is particularly advantageous in the case of monitoring tasks, where the aim is to detect moving known objects and the objects to be detected are partially concealed by other structures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0018]    Further advantages and characteristics of the invention will emerge from the description below in which exemplary embodiments of the invention will be explained in detail with reference to the attached drawings, in which: 
           [0019]      FIG. 1  shows a view of a device for generating distance-measuring images; 
           [0020]      FIG. 2  shows timing diagrams of the light pulses, the time windows used for integration and the reflected light pulses, including the portions detected by the sensor, and a representation of a correlation function; 
           [0021]      FIG. 3  shows timing diagrams with the emitted light pulse, two reflected light pulses and an integration window which captures one of the two reflected light pulses; 
           [0022]      FIG. 4  shows timing diagrams as shown in  FIG. 3 , in which the integration window captures the second reflected light pulse at least partially; 
           [0023]      FIG. 5  shows timing diagrams as shown in  FIG. 3 , in which the integration window captures both light pulses; 
           [0024]      FIG. 6  shows a representation of a correlation function where two reflected light pulses are present; 
           [0025]      FIG. 7  shows the derivation of the correlation function from  FIG. 6 ; 
           [0026]      FIG. 8  shows a block diagram of a device for recording distance-measuring images; 
           [0027]      FIG. 9  shows a block diagram of a further device for recording distance-measuring images; and 
           [0028]      FIG. 10  shows a representation of the use of the additional distance information for enhancing the resolution. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0029]      FIG. 1  shows a recording device  1  for distance-measuring images. The recording device  1  comprises a laser light source  2  which transmits light pulses  3  onto a foreground object  4  and a background object  5 . The light pulses  6  reflected by the foreground object  4  and by the background object  5  are detected by a camera  7  which comprises a light sensor  8 , manufactured using CMOS technology, comprising a multiplicity of sensor elements  9 . 
         [0030]    It should be noted that the reflected light pulses  6  may be both reflected and scattered back. 
         [0031]    The foreground object  4  is located at the distance d 1  from the camera  7 , while the background object  5  is located at a distance d 2  from the camera. Through an optical system  10  of the camera  7 , the foreground object  4  and the background object  5  are mapped onto the sensor elements  9 . Due to the low spatial resolution of the light sensor  8 , an image of the foreground object  4  covers only a partial area of the sensor element  9 . The rest of the sensor element  9  is occupied by an image  12  of the background element  5 . The sensor element  9  concerned therefore receives both light pulses which have been reflected by the foreground object  4  and light pulses which have been reflected by the background object  5 . Light pulses  6  therefore arrive in a sensor element  9  concerned, delayed by different lengths of time. 
         [0032]      FIG. 2  represents in timing diagrams a sequence of light pulses  3  emitted by the laser light source  2 , of reflected light pulses  6  and a sequence of integration windows  13 , as well as a correlation function  14  emerging from the measurement. 
         [0033]    In the timing diagram containing the light pulses  3 , the light streams Φ S  are shown. In the timing diagram containing the reflected light pulses  6 , the light streams Φ R  of the reflected light pulses  6  are shown. The reflected light pulses  6  are delayed by a time span T d  according to the time needed for the path to the object and back to the camera  7 . 
         [0034]    The sensor elements  9  are activated for the duration of the integration windows  13 . The integration windows  13  have a time duration T 1  and are each offset in the individual measurement processes by a time span T V0  to T VN . Integration of the light stream of the reflected light pulse  6  takes place only within the integration window  13 . The light quantity Q of the reflected light pulses  6  which strikes the respective sensor element  9  in the integration window  13  is determined. The light quantity values  15  are proportional to the quantity of charge or voltage U read out from the sensor elements. The light quantity values  15  determined in this manner form a correlation curve  14 . Since the light quantity value  15  is at its maximum when the reflected light pulse  6  falls fully within the integration window  13 , the propagation-time-dependent delay T d  of the light pulse can be determined, for example, from the maximum of the correlation curve  14 . A prerequisite for this, however, is that the integration duration T 1  of the integration window  13  is approximately equal to the pulse duration T PW  of the light pulse  3 . 
         [0035]      FIG. 3  now represents the situation in which the emitted light pulse  3  is reflected firstly on the foreground object  4  and secondly on the background object  5 . Two time-shifted light pulses  6  therefore strike the sensor element  9 . In  FIG. 3 , only the second light pulse  6 , which has been reflected on the background object  5 , is captured, as the sensor element  9  concerned is activated at the time of arrival of the light pulse  6  reflected on the background object  5 . The timing diagrams shown in  FIG. 3  relate to the delay time T V5 . 
         [0036]    In  FIG. 4 , the situation is represented where the delay period is T V7 . In this case, the time interval between the emitted light pulse  3  and the integration window  13  has been shortened further. As a consequence, the integration window captures not only the light pulse  6  reflected on the background object  5 , but at least in part also the light pulse  6  reflected on the foreground object  4 . 
         [0037]      FIG. 5  represents the conditions where the delay time is T V9 . The time interval between the emitted light pulse  3  and the integration window  13  is now so small that both reflected light pulses  6  lie within the integration window  13  and are captured. 
         [0038]      FIG. 6  shows the correlation curve  16  which is produced by resolving the individual reflected light pulses  6 . At the rising end, the correlation curve  16  has a step  17  between 0 and the delay time T V5  and a further step  18  between the delay time T V5  and the delay time T V9 . At the falling end, the correlation curve  16  has falling steps  19  and  20  corresponding to the rising steps  17  and  18 . By detecting the inversive points of steps  17  and  18  and also of  19  and  20 , the light propagation time of the light pulses  6  reflected on the foreground object  4  and background object  5  can be determined. 
         [0039]    The correlation curve  16  represented in  FIG. 6  was recorded on the basis of a multiple scene comprising a foreground object  4  and background object  5 , which were at a distance of 150 cm from one another, with a pulse width T PW  of 2 ns and an integration time T 1  of 30 ns. 
         [0040]    With a pulse width of 0.1 ns, distance differences of as little as around 1.5 cm can be resolved. 
         [0041]    Determination of the inversive points is advantageously carried out on the basis of a difference curve  21 , represented in  FIG. 7 , in which the inversive points become local extreme values which can be determined more simply and reliably than the inversive points of the correlation curve  16 . In the difference curve  21  as per  FIG. 7 , the rising steps  17  and  18  are each associated with local maxima  22  and  23 , while the falling steps  19  and  20  in the differential curve  21  correspond to a minimum  24  and  25  respectively. The number and distance of the extreme values of the differential curve  21  determine the number of and the distance of the different object layers. 
         [0042]    The difference curve  21  is advantageously formed by computing the difference of a defined value of the correlation curve with a value of the correlation curve  16  shifted by a time interval Δt. For the value of the difference curve  21 , U DIFF =U(t)−U(t-Δt) then applies, where the time interval Δt is advantageously chosen so as to be equal to the pulse width T PW . This reduces the susceptibility to signal interference and signal noise typical when computing the difference curve  21 . 
         [0043]      FIG. 8  shows a block diagram of a circuit  26  provided for operating the recording device  1  from  FIG. 1 . The circuit  26  comprises a control unit  27  which sends a trigger signal  28  to the laser light source  2 . The trigger signal  28  is also sent by a delay unit  29  and controls the light sensor  8 . The rising edge of the trigger signal  28  can be used, for example, to trigger in the laser light source  2  a light pulse  3  which is emitted to a scene  30  to be investigated, is reflected there and arrives at the light sensor  8  as a reflected light pulse  6 . While the trigger signal  28  occupies a high level, the light sensor  8  remains activated. The quantity of charge generated in the individual sensor elements  9  is read out, digitalized and stored in an intermediate memory  31 . An evaluation unit  32  connected downstream of the intermediate memory  31  then computes on the basis of the correlation curve  16  stored in the intermediate memory  31  the difference curve  21 . From the difference curve  21  the evaluation unit  32  can then determine the number and distance of the image layers. 
         [0044]    In accordance with  FIG. 8 , the intermediate memory  31  and the evaluation unit  32  also receive control signals  33  from the control unit  27 . 
         [0045]      FIG. 9  shows a block diagram of a further circuit  34  which can be used for the recording device  1 . 
         [0046]    The trigger signal  28  generated by the control unit  27  of the circuit  34  is routed in the circuit  34  on the path to the laser light source  2  via a delay unit  35 . This is readily possible, since what ultimately counts is only that the time interval between the emission of the light pulses  3  by the laser light source  2  and the integration window  13  can be varied. The trigger signal  28  is received by the sensor elements  9 , pairs of adjacent sensor elements  9  operating with time-shifted integration windows  13  due to a delaying of the trigger signal  28  in a delay unit  36 . Consequently, as soon as the sensor elements  9  are triggered, it is possible to compute the values of the difference curve  21  in a difference-forming unit  37  and to store these in the intermediate memory  31 . The evaluation unit  32  can then determine the number and distance of the object layers on the basis of the position and the number of the local extreme values. 
         [0047]    Through parallel capture in the sensor elements  9 , the same light pulse  6  is always integrated in both integration windows  13 . This increases the measurement accuracy since the reset noise typical of light sensors  8  manufactured using CMOS technology is largely eliminated. In addition, the variation in the pulse duration T PW  of the light pulses  3  has no influence on the values of the difference curve  21 . 
         [0048]    It should be pointed out that the recording device  1  is also capable of detecting contamination and moisture on the optical equipment. To this end, a protective glass cover is advantageously mounted in front of the laser light source  2  and the camera  7 , through which protective glass cover both the light of the emitted light pulses  3  and the light pulses  6  reflected by the scene  30  pass. Dirt or moisture on the outside of the protective glass cover lead to part of the emitted light pulse  3  being reflected in the direction of the camera  7 . The other part of the emitted light pulse, however, goes to the scene  30  and is reflected by the scene  30 . The successively arriving light pulses  6  can be detected on the basis of the correlation curve  16 . In this way it is possible to respond early to contamination or wetting of the optical equipment, for example by activating a cleaning device. Such a cleaning device may, for example, be a windscreen wiper or a blowing device, by means of which the function of the recording device  1  can be maintained even under adverse environmental conditions. 
         [0049]    By means of the recording device  1 , it is also possible to achieve a lateral spatial resolution that is not restricted by the lateral extension of the sensor elements  9 . Normally, structures of the scene  30  to be investigated whose representation on the light sensor  8  falls below the size of a sensor element  9  cannot be detected. However, by resolving the distance based on the basis of the multi-target capability of the recording device  1 , a distance-measuring image can be generated with enhanced resolution. 
         [0050]    If the scene  30  to be investigated is unknown, the distance values of adjacent sensor elements  9  are employed for generating the enhanced-resolution distance-measuring image. In this way, in accordance with  FIG. 10  a low-resolution distance-measuring image  38  can be converted into an enhanced-resolution distance-measuring image  39 , whereby image elements  40  which reproduce multiple targets with multiple object layers and to which no unique distance value can therefore be assigned are subdivided and the partial areas are assigned corresponding distance values by adjacent image elements  41 . 
         [0051]    In a modified embodiment, the distance-measuring image is generated through adaptation to the known model. This is particularly advantageous for monitoring tasks where known moving objects are to be detected in three-dimensional space and the objects to be detected are partially concealed by other structures. The objects to be monitored can be detected by the multi-target-capable recording device  1  despite being partially concealed. This is because, using model knowledge, the distance value from the actual object can be assigned to an image element. 
         [0052]    Monitoring tasks of this type include, for example, the recognition, counting and tracking of persons and vehicles. Railway platform monitoring can also constitute a monitoring task of this type.