Abstract:
A distance measuring device and a method for determining a distance are provided. The method includes: illuminating an object with a sequence of the light pulses, capturing one arriving light pulse corresponding to an intensity T e,l  within a first integration gate, and outputting a signal value U 1 , capturing another arriving light pulse corresponding to the intensity T e,l  within a second integration gate, and outputting a signal value U 2 , capturing one arriving light pulse corresponding to an intensity I e,h  within the first integration gate and outputting a signal value U 3 , capturing the other arriving light pulse corresponding to the intensity I e,h  within the second integration gate and outputting a signal value U 4 , and calculating the distance between the distance measuring device and the object based on U 1 , U 2 , U 3 , and U 4 .

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of international patent application PCT/EP2015/076794, filed Nov. 17, 2015, designating the United States and claiming priority from German application 10 2014 117 097.0, filed Nov. 21, 2014, and the entire content of both applications is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a distance measuring device and a method for determining a distance with the distance measuring device. 
       BACKGROUND 
       [0003]    Distances can be measured between a measuring device and an object without a physical contact between the device and the object by optical methods. In these methods, the object is illuminated by a light source of the device and the light back reflected from the object is then captured by a light detector of the device. 
         [0004]    Distances can for example be determined by periodically modulating the light intensity which is emitted from the light source and by measuring the phase difference between the emitted light and the back reflected light arriving on the detector. However, due to the periodicity of the light intensity, this method results in an ambiguous distance measurement. Distances can be unambiguously determined by measuring the time of flight between the emission of a light pulse and the arrival of a back reflected light pulse on the detector. 
         [0005]    Ambient light, for example sun-light, can interfere with the distance measurement and therefore result in a reduction of the precision for the distance measurement. Conventionally, a background measurement is carried out without illuminating the object with the light pulse. The background measurement leads to an irregular operation of the light source. The irregular operation is disadvantageous because it results in a reduction of the life time of the light source and in fluctuations of the parameters of the light pulses, in particular intensity, pulse width, rise times and/or fall times. These fluctuations cause a reduction of the precision for the distance measurement. 
       SUMMARY 
       [0006]    It is an object of the invention to provide a distance measuring device and a method for measuring a distance with the distance measuring device, wherein the distance can be measured with a high precision. 
         [0007]    The distance measuring device according to an aspect of the invention includes a light source configured to illuminate an object with light pulses having a duration T p , at least one photo element configured to capture the light pulses after being back reflected from the object, a trigger generator configured for controlling the emission of the light pulses and for activating the photo element during temporal integration gates, wherein the photo element is adapted to output a signal value U at the end of each integration gate with the signal value U being proportional to the energy of the light arriving on the photo element during its activation. The trigger generator stores a trigger scheme to control the emission of the light pulses such that a sequence of the light pulses including four consecutive light pulses consisting of two light pulses having an intensity I e,l  and two light pulses having an intensity T e,h  being higher than I e,l  is emitted and that the repetition rate 1/Δ rep  of the light pulses is constant, and to activate the photo element such that the delays between the integration gates and the emission start points in time of the four light pulses are such that the light pulses arriving on the photo element are captured such that one arriving light pulse corresponding to the intensity I e,l  and one arriving light pulse corresponding to the intensity I e,h  are captured by the photo element within first integration gates with an integration start point in time T 1,s  and an integration end point in time T 1,e  as well as the other arriving light pulse corresponding to the intensity T e,l  and the other arriving light pulse corresponding to the intensity I e,h  are captured by the photo element within second integration gates with an integration start point in time T 2,s  and an integration end point in time T 2,e , wherein the delay for the first integration gates is chosen such that either T 1,s  or T 1,e  is between Δ tof  and Δ tof +T p  and the delay for the second integration gates is chosen such the respective light pulses are at least partially within the second integration gates, wherein T 1,s , T 1,e , T 2,s , T 2,e  are the delays from the emission start point in time and Δ tof  is the first point in time the arriving light pulses arrive on the photo element, and a processing unit adapted to calculate the distance between the distance measuring device and the object by using the difference of the signal values U being output at the end of the first integration gates and the difference of the signal values U being output at the end of the second integration gates. The duration T 1,e -T 1,s  of the first integration gates can be equal to or can be different from the duration T 2,e -T 2,s  of the second integration gates. 
         [0008]    The method according to an aspect of the invention for determining a distance with the distance measuring device includes the steps of: a) illuminating the object with the sequence of the light pulses; b) capturing one arriving light pulse corresponding to the intensity I e,l  within one of the first integration gates, and outputting a signal value U 1  at the end of the first integration gate; c) capturing the other arriving light pulse corresponding to the intensity I e,l  within one of the second integration gates, and outputting a signal value U 2  at the end of the second integration gate; d) capturing one arriving light pulse corresponding to the intensity I e,h  within the other first integration gate and outputting a signal value U 3  at the end of the first integration gate; e) capturing the other arriving light pulse corresponding to the intensity I e,h  within the other second integration gate and outputting a signal value U 4  at the end of the second integration gate; f) calculating the distance between the distance measuring device and the object by using the difference of the signal values U 2  and U 1  and the difference of the signal values U 4  and U 3 . 
         [0009]    In order to arrange the integration gates with respect to the emission start point in time, a distance range in which the object can be located is predetermined. From the distance range T p , T 1,s  and T 1,e  can be chosen such that T 1,s  or T 1,e  is between Δ tof  and Δ tof +T p  for all possible distances of the distance range. T 2,s  and T 2,e  can then be chosen such that the respective light pulses are at least partially within the second integration gates for all possible distances of the distance range. 
         [0010]    According to an aspect of the invention, Δ tof  and Δ tof +T p  are between T 2,s  and T 2,e . For the case that T 1,s  is between Δ tof  and Δ tof +T p , the time of flight Δ tof  from the emission of the light pulses to the arrival of the light pulses on the photo element is calculated by: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                     tof 
                   
                   = 
                   
                     
                       T 
                       
                         1 
                         , 
                         s 
                       
                     
                     + 
                     
                       
                         
                           T 
                           p 
                         
                          
                         
                           ( 
                           
                             
                               
                                 
                                   U 
                                   3 
                                 
                                 - 
                                 
                                   U 
                                   1 
                                 
                               
                               
                                 
                                   U 
                                   4 
                                 
                                 - 
                                 
                                   U 
                                   2 
                                 
                               
                             
                             - 
                             1 
                           
                           ) 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   
                     equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0011]    For the case that T 1,e  is between Δ tof  and Δ tof +T p , Δ tof  is calculated by: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                     tof 
                   
                   = 
                   
                     
                       T 
                       
                         1 
                         , 
                         e 
                       
                     
                     - 
                     
                       
                         T 
                         p 
                       
                        
                       
                         
                           
                             
                               U 
                               3 
                             
                             - 
                             
                               U 
                               1 
                             
                           
                           
                             
                               U 
                               4 
                             
                             - 
                             
                               U 
                               2 
                             
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
         [0012]    Also, according to the aspect of the invention, the durations of the first and second integration gates can be equal or can be different. If the durations are different, the duration T 2,e -T 2,s  can be longer than the duration T 1,e -T 1,s  to ensure that the complete light pulses are within the second integration gate. 
         [0013]    Alternatively, according to another aspect of the invention, in case T 1,s  is between Δ tof  and Δ tof +T p , T 2,s  is between Δ tof  and Δ tof +T p , T 2,e  is later than Δ tof +T p  and T 2,s  is different from T 1,s , and in the case T 1,e  is between Δ tof  and Δ tof +T p , T 2,e  is between Δ tof  and Δ tof +T p , T 2,s  is earlier than Δ tof  and T 2,e  is different from T 1,e . 
         [0014]    For the case that T 1,e  is between Δ tof  and Δ tof +T p , Δ tof  is calculated by 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                     tof 
                   
                   = 
                   
                     
                       T 
                       
                         1 
                         , 
                         e 
                       
                     
                     - 
                     
                       
                         ( 
                         
                           
                             T 
                             
                               2 
                               , 
                               e 
                             
                           
                           - 
                           
                             T 
                             
                               1 
                               , 
                               e 
                             
                           
                         
                         ) 
                       
                        
                       
                         
                           
                             
                               U 
                               2 
                             
                             - 
                             
                               U 
                               1 
                             
                           
                           
                             
                               ( 
                               
                                 
                                   U 
                                   4 
                                 
                                 - 
                                 
                                   U 
                                   3 
                                 
                               
                               ) 
                             
                             - 
                             
                               ( 
                               
                                 
                                   U 
                                   2 
                                 
                                 - 
                                 
                                   U 
                                   1 
                                 
                               
                               ) 
                             
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     equation 
                      
                     
                         
                     
                      
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
         [0015]    For the case that T 2,e  is between Δ tof  and Δ tof +T p , Δ tof  is calculated by: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                     tof 
                   
                   = 
                   
                     
                       T 
                       
                         1 
                         , 
                         s 
                       
                     
                     - 
                     
                       T 
                       p 
                     
                     - 
                     
                       
                         ( 
                         
                           
                             T 
                             
                               2 
                               , 
                               s 
                             
                           
                           - 
                           
                             T 
                             
                               1 
                               , 
                               s 
                             
                           
                         
                         ) 
                       
                        
                       
                         
                           
                             
                               U 
                               2 
                             
                             - 
                             
                               U 
                               1 
                             
                           
                           
                             
                               ( 
                               
                                 
                                   U 
                                   4 
                                 
                                 - 
                                 
                                   U 
                                   3 
                                 
                               
                               ) 
                             
                             - 
                             
                               ( 
                               
                                 
                                   U 
                                   2 
                                 
                                 - 
                                 
                                   U 
                                   1 
                                 
                               
                               ) 
                             
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     equation 
                      
                     
                         
                     
                      
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
         [0016]    For all cases, the distance r between the distance measuring device and the object is then calculated by 
         [0000]        r= 0.5 *c*Δ   tof   (equation 5),
 
         [0000]    wherein c is the speed of light in the medium in which the distance measurement is carried out. 
         [0017]    With the distance measuring device and the method according to an aspect of the invention, it is possible to eliminate the influence of background light, for example sun light, without taking a background measurement. The background measurement would include outputting a signal value U at the end of an integration gate without illuminating the object with a light pulse. Since it is not necessary to take the background measurement, it is possible to operate the light source with the constant repetition rate 1/Δ rep , which results in a long life time of the light source. Δ rep  denotes the duration between two consecutive emission start points in time. Another advantage of the constant repetition rate is that the intensity fluctuations of the light pulses are reduced. Both, the elimination of the influence of the background light and the reduction of the intensity fluctuations result in a high precision for the distance measurement. 
         [0018]    According to an aspect of the invention, the trigger scheme controls the emission of the sequence such that single light pulses having the intensity T e,l  are emitted alternating with single light pulses having the intensity I e,h . This results in a particular regular operation of the light source which results in a particular long life time of the light source and in particular small intensity fluctuations of the light pulses. According to an aspect of the invention, the trigger scheme controls the emission of the light pulses such that the sequence includes a dummy light pulse preceding the four light pulses. Since the dummy light pulse is not used for the distance measurement, it is advantageously achieved that only light pulses with small intensity fluctuations are used for the distance measurement. According to another aspect of the invention, the trigger scheme controls the emission of the light pulses such that the sequence includes a plurality of dummy light pulses, wherein at least one dummy light pulse precedes each of the four light pulses. By using the multitude of dummy light pulses, it is possible to maintain a stable and constant repetition rate 1/Δ rep  and to carry out the measurements of the signal values U at a measurement frequency, even if the measurement frequency of the photo element is lower than repetition rate 1/Δ rep  for a stable operation of the light source. 
         [0019]    According to an aspect of the invention, the light source includes light emitting diodes, VCSELs (vertical-cavity surface-emitting laser) and lasers or any combination thereof that are in particular configured to emit in the visible and/or infrared spectral region. According to another aspect of the invention, the distance measuring device includes a CCD chip with an image intensifier and/or a CMOS chip that includes the at least one photo element. 
         [0020]    According to a further aspect of the invention, the light source includes a first group with at least one light emitting diode, VCSEL and/or laser and a second group with at least one light emitting diode, VCSEL and/or laser, wherein the trigger scheme controls the emission of the light pulses such that the first group emits light pulses with the repetition rate 1/Δ rep  and with the intensity I e,l  and such that the second group emits light pulses with the repetition rate 0.5/Δ rep  and with the intensity I e,h -I e,l  so that the overlap of the emission of the first group and second group results in the light pulses with the intensity I e,h . Here, it is advantageously achieved that the first group and the second group are operated perfectly regularly so that the life times of the first group and the second group are particularly increased. 
         [0021]    According to an aspect of the invention, Δ tof  and Δ tof +T p  are between T 2,s  and T 2,e . Alternatively, according to another aspect of the invention, in case T 1,s  is between Δ tof  and Δ tof +T p , T 2,s  is between Δ tof  and Δ tof +T p , T 2,e  is later than Δ tof +T p  and T 2,s  is different from T 1,s , and in case T 1,e  is between Δ tof  and Δ tof +T p , T 2,e  is between Δ tof  and Δ tof +T p , T 2,s  is earlier than Δ tof  and T 2,e  is different from T 1,e . 
         [0022]    According to an aspect of the invention, in step a) the sequence is such that single light pulses having the intensity I e,l  are emitted alternating with single light pulses having the intensity I e,h . The sequence preferably includes a dummy light pulse preceding the four light pulses, wherein the dummy light pulse is not used for the determination of the distance. According to an aspect of the invention, the sequence includes a plurality of dummy light pulses, wherein at least one dummy light pulse precedes each of the four light pulses, wherein the dummy light pulses are not used for the determination of the distance. According to another aspect of the invention, in step a) the sequence includes a multitude of sets of the four light pulses and a respective distance is determined for each set by repeating steps b) to f). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The invention will now be described with reference to the drawings wherein: 
           [0024]      FIG. 1  shows temporal profile diagrams with integration gates and intensities of light pulses, and 
           [0025]      FIG. 2  shows a schematic cross section through a distance measuring device. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0026]      FIG. 2  shows a distance measuring device  14  including a light source  15 , a photo element  16 , a trigger generator  17 , a memory unit  18  and a processing unit  19 . The light source  15  includes light emitting diodes, VCSEL (vertical-cavity surface-emitting laser) and/or lasers, wherein the light emitting diodes, the VCSELs and/or the lasers are configured to emit in the visible and/or infrared spectral region. The distance measuring device  14  includes a CCD chip with an image intensifier and/or a CMOS chip that includes the at least one photo element  16 . The CMOS chip includes at least one condenser that can be discharged via a photodiode. The trigger generator  17  provides an activation signal  25  for controlling the emission of the light source  15  and an activation signal  26  for activating the photo element  16  during a temporal integration gate  6 . The CCD chip is activated by switching on the image intensifier and the CMOS chip is activated by closing a switch in the circuit of the condenser and the photodiode, which allows that the condenser is discharged via the photodiode. The photo element  16  is configured to output a signal value U at the end of the integration gate  6 , wherein the signal value U is proportional to the energy of the light arriving on the photo element during its activation. The signal value U is read out in a readout operation  27  and stored in the memory unit  18 . The memory unit  18  is configured to store a multitude of signal values U. The multitude of the signal values U is then processed by the processing unit  19  in a processing operation  28  in order to determine a distance between the distance measuring device  14  and an object  22 . 
         [0027]    The signal value U can be measured directly, for example if a CCD chip or CMOS image sensor is used. The charge measured at the end of the integration gate is proportional to the energy of the light arriving on the photo element during its activation and therefore the signal value U, which is proportional to the charge, is proportional to the energy of the light. On the other hand, the signal value U can be determined indirectly if the relation between a measured value and the energy of the light arriving on the photo element during its activation is known. For example, if the photo element includes a condenser that is discharged via a photodiode during the activation of the photo element, the measured value is a voltage that is approximately inversely proportional to the energy of the light arriving on the photo element during its activation. 
         [0028]    Detection optics  21  are arranged in front of the photo element  16  in order to image a field of view  24  onto the photo element  16 . Illumination optics  20  are arranged in front of the light source  15  in order to shape the light emitted by the light source  15  such that an illumination area  23  can be illuminated by the light source  15 . The illumination area  23  and the field of view  24  are shaped such that the field of view  24  is substantially completely covered by the illumination area  23 . The distance measuring device  14  is adapted such that the light emitted by the light source  15  impinges onto the object  22  located within the field of view  24 , and arrives on the photo element  16  after being back reflected from the object  22 . The illumination optics  20  and the detection optics  21  are preferably respective lenses. It is also possible to use a single lens for both the illumination optics  20  and the detection optics  21 . 
         [0029]    In  FIG. 1 , three temporal profile diagrams are shown, wherein an intensity  1  and an integration gate  2  are plotted versus time  3 . The first temporal profile diagram is a plot of the intensity  4  of the emitted light pulses  7 ,  8  versus the time  3 , the second temporal profile diagram is a plot of the intensity  5  of the light pulses  9 ,  10  arriving on the photo element  16  after being back reflected from the object  22  versus the time  3 , and the third temporal profile diagram is a plot of the integration gates  6  versus the time  3 . 
         [0030]    The first temporal profile diagram shows that the light source  15  emits a sequence of consecutive light pulses  7 ,  8 . The light pulses  7 ,  8  have a preferably rectangular temporal profile so that the light source  15  switches the intensity of the light pulses  7 ,  8  at an emission start point in time  13  from a lower intensity to a higher intensity and after a pulse duration T p  from the emission start point in time  13  back to the lower intensity. The pulse duration T p  is preferably the same for all the light pulses  7 ,  8  and is in the order of picoseconds or nanoseconds. The repetition rate 1/Δ rep  for all the light pulses in the sequence is constant, wherein Δ rep  is the duration between two consecutive emission start points in time  13 . The repetition rate 1/Δ rep  for the light pulses  7 ,  8  is from 1 Hz to 20 kHz. 
         [0031]    In the following it is assumed that the lower intensity is zero. The sequence includes a set of four consecutive light pulses  7 ,  8 , wherein two light pulses  7  of the four light pulses  7 ,  8  have the intensity I e,l  and the other two light pulses  8  the four light pulses  7 ,  8  have the intensity I e,h , wherein I e,h &gt;I e,l . In the sequence, a single light pulse  7  with the intensity I e,l  and a single light pulse  8  with the intensity I e,h  are always emitted alternatingly. After the emission, the light pulses  7 ,  8  impinge on the object  22  located within the field of view  24  and are back reflected from the object  22 . Afterwards the light pulses  9 ,  10  arrive on the photo element  16 , wherein Δ tof  is the first point in time from the emission start point in time  13 , when the light pulses  9 ,  10  arrive on the photo element  16 . The two light pulses  9  arriving on the photo element  16  and corresponding to the light pulses  7  with the intensity I e,l  have the intensity I a,l , wherein I a,l &lt;I e,l . The two light pulses  10  arriving on the photo element  16  and corresponding to the light pulses  8  with the intensity I e,h  have the intensity I a,h , wherein I a,h &lt;I e,h . 
         [0032]    The third temporal profile diagram shows that the set of the four light pulses  9 ,  10  arriving on the photo element  16  are captured within two first integration gates  11  and two second integration gates  12 . The first integration gates  11  have an integration start point in time T 1,s  and an end integration end point in time T 1,e , wherein T 1,s  and T 1,e  are the delays from the emission start point in time  13 . The second integration gates  12  have an integration start point in time T 2,s  and an integration end point in time T 2,e , wherein T 2,s  and T 2,e  are the delays from the emission start point in time  13 . One of the four light pulses  9  having the intensity I a,l  and one of the four light pulses  10  having the intensity I a,h  are captured by the photo element  16  within a respective first integration gate  11  such that T 1,e  is between Δ tof  and Δ tof +T p  and that T 1,s  is earlier than Δ tof . Alternatively, it is possible that the respective first integration gate  11  is such that T 1,s  is between Δ tof  and Δ tof +T p  and that T 1,e  is later than Δ tof +T p . The other of the four light pulses  9  having the intensity I a,l  and the other of the four light pulses  10  having the intensity I a,h  are captured by the photo element  16  within a respective second integration gate  12  such that Δ tof  and Δ tof +T p  are between T 2,s  and T 2,e . 
         [0033]    The hatched areas in the second temporal profile diagram are proportional to the energy of the light arriving on the photo element  16  during its activation. Since the signal value U is proportional to the energy of light arriving on the photo element during its activation, the signal value U is also proportional to the hatched areas. A signal value U 1  is put out at the end of the first integration gate  11  that captures one of the light pulses  9  with the intensity I a,l . A signal value U 3  is output at the end of the first integration gate  11  that captures one of the light pulses  10  with the intensity I a,h . A signal value U 2  is put out at the end of the second integration gate  12  that captures the other of the light pulses  9  with the intensity I a,l . A signal value U 4  is put out at the end of the second integration gate  12  that captures the other of the light pulses  10  with the intensity I a,h . 
         [0034]      FIG. 1  shows that Δ tof +U 1 /I a,l =T 1,e  and Δ tof +U 3 /I a,h =T 1,e . These two equations are equivalent to: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       I 
                       
                         a 
                         , 
                         l 
                       
                     
                     = 
                     
                       
                         U 
                         1 
                       
                       
                         
                           T 
                           
                             1 
                             , 
                             e 
                           
                         
                         - 
                         
                           Δ 
                           tof 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   and 
                 
               
               
                 
                   ( 
                   
                     equation 
                      
                     
                         
                     
                      
                     6 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     I 
                     
                       a 
                       , 
                       h 
                     
                   
                   = 
                   
                     
                       
                         U 
                         3 
                       
                       
                         
                           T 
                           
                             1 
                             , 
                             e 
                           
                         
                         - 
                         
                           Δ 
                           tof 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   
                     equation 
                      
                     
                         
                     
                      
                     7 
                   
                   ) 
                 
               
             
           
         
       
     
         [0035]    Furthermore,  FIG. 1  shows that U 2 =T p *I a,l  and U 4 =T p *I a,h . These two equations are equivalent to: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       I 
                       
                         a 
                         , 
                         l 
                       
                     
                     = 
                     
                       
                         U 
                         2 
                       
                       
                         T 
                         p 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   and 
                 
               
               
                 
                   ( 
                   
                     equation 
                      
                     
                         
                     
                      
                     8 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     I 
                     
                       a 
                       , 
                       h 
                     
                   
                   = 
                   
                     
                       
                         U 
                         4 
                       
                       
                         T 
                         p 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   
                     equation 
                      
                     
                         
                     
                      
                     9 
                   
                   ) 
                 
               
             
           
         
       
     
         [0036]    By subtracting equation 6 from equation 7 and equation 8 from equation 9 it follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         I 
                         
                           a 
                           , 
                           h 
                         
                       
                       - 
                       
                         I 
                         
                           a 
                           , 
                           l 
                         
                       
                     
                     = 
                     
                       
                         1 
                         
                           
                             T 
                             
                               1 
                               , 
                               e 
                             
                           
                           - 
                           
                             Δ 
                             tof 
                           
                         
                       
                        
                       
                         ( 
                         
                           
                             U 
                             3 
                           
                           - 
                           
                             U 
                             1 
                           
                         
                         ) 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   and 
                 
               
               
                 
                   ( 
                   
                     equation 
                      
                     
                         
                     
                      
                     10 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     
                       I 
                       
                         a 
                         , 
                         h 
                       
                     
                     - 
                     
                       I 
                       
                         a 
                         , 
                         l 
                       
                     
                   
                   = 
                   
                     
                       1 
                       
                         T 
                         p 
                       
                     
                      
                     
                       
                         ( 
                         
                           
                             U 
                             4 
                           
                           - 
                           
                             U 
                             2 
                           
                         
                         ) 
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   
                     equation 
                      
                     
                         
                     
                      
                     11 
                   
                   ) 
                 
               
             
           
         
       
     
         [0037]    By equalizing the right hand sides of equation 10 and equation 11, it is then possible to derive equation 2. Equation 3 can be derived in an analogous manner. By subtracting the equations 6 to 9, the influence of background light is eliminated. 
         [0038]    The sequence can include a dummy light pulse preceding the set of the four light pulses  7 ,  8 , wherein the dummy light pulse is not used for the determination of a distance. In this case, the dummy light pulse has an emission start point in time that is a duration Δ rep  earlier than the emission start point in time  13  of the earliest of the four light pulses  7 ,  8 . 
         [0039]    The sequence can also include a multitude of dummy pulses preceding each of the four light pulses  7 ,  8  wherein the dummy light pulses are not used for the determination of a distance. In these cases, the dummy light pulses have emission start points in time that are earlier than the emission start point of the respective measurement light pulse  7 ,  8 . 
         [0040]    Furthermore, the sequence can include a multitude of the sets of the four light pulses  7 ,  8  and a respective distance is calculated for each of the sets. If the sequence includes the multitude of the sets, the repetition rate of all the light pulses is maintained constant with the repetition rate 1/Δ rep . The distance measuring device  14  can include a plurality of photo elements  16  and a respective distance is determined for each of the photo elements  16 . 
         [0041]    In an exemplary embodiment, the light source includes a first group with at least one light emitting diode, VCSEL and/or laser and a second group with at least one light emitting diode, VCSEL and/or laser, wherein the emission of the light pulses is controlled such that the first group emits light pulses with the repetition rate 1/Δ rep  and with the intensity I e,l  and such that the second group emits light pulses with the repetition rate 0.5/Δ rep  and with the intensity I e,h -I e,l  so that the overlap of the emission of the first group and second group results in the light pulses with the intensity I e,h . 
         [0042]    It is understood that the foregoing description is that of the exemplary embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           1  intensity 
           2  integration gate 
           3  time 
           4  intensity of emitted light pulses 
           5  intensity of arriving light pulses 
           6  integration gates 
           7  emitted light pulse with low intensity 
           8  emitted light pulse with high intensity 
           9  arriving light pulse with low intensity 
           10  arriving light pulse with high intensity 
           11  first integration gate 
           12  second integration gate 
           13  emission start point in time 
           14  distance measuring device 
           15  light source 
           16  photo element 
           17  trigger generator 
           18  memory unit 
           19  processing unit 
           20  illumination optics 
           21  detection optics 
           22  object 
           23  illumination area 
           24  field of view 
           25  activation signal for light source 
           26  activation signal for photo element 
           27  readout operation 
           28  processing operation 
         Δ rep  repetition duration 
         T p  pulse duration 
         Δ tof  time of flight 
         T 1,s  integration start point in time of first integration gate 
         T 1,e  integration end point in time of first integration gate 
         T 2,s  integration start point in time of second integration gate 
         T 2,e  integration end point in time of second integration gate 
         I e,l  intensity of emitted light pulse 
         I e,h  intensity of emitted light pulse 
         I a,l  intensity of arriving light pulse 
         I a,h  intensity of arriving light pulse