Abstract:
The invention relates to a device comprising a photosensitive element producing an electric charge as a function of the radiation incident thereon and a charge integrator connected to the photosensitive element and converting the charge to a voltage. According to the invention, the device comprises a comparator capable of comparing the voltage delivered by the integrator with a threshold voltage, and a memory unit for storing the instant when the voltage delivered by the integrator exceeds the threshold voltage.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of 3D active imaging and, more especially, elementary detectors, or pixels, in arrays which form part of detectors used for this type of imaging. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    Systems used in the field of 3D active imaging such as, for example, Laser Detection and Ranging (LADAR) systems, emit brief bursts of electromagnetic radiation in the form of a pulse by using a laser, for instance, and use an imaging device which is sensitive in the emitted spectral band in order to detect the flux reflected by obstacles located on the path of the emitted radiation. 
         [0003]    These systems combine the detected reflected flux with temporal information such as, for example, sequencing or timestamping information, thereby making it possible to determine range information. This range information is classically produced by means of time-to-distance conversion depending on the pulse&#39;s round-trip delay (the propagation velocity of the pulse is known) by using the formula d=c t/2, where
       d is distance,   c is the pulse propagation velocity,   and t is the time which the pulse takes to complete the outward and inward trip.       
 
         [0007]    Usually, in order to obtain a 3D image of a scene, 3D active imaging systems employ a technique referred to as “image slicing” such as that described, for example, in the document by I. Baker, S. Duncan and J. Copley “ A low noise, laser - gated imaging system for long range target identification ”, Proceedings of SPIE, vol. 5406, pages 133-144. 
         [0008]    The principle of this technique is illustrated in  FIGS. 1 and 2 . The emitting part of a system  10  transmits a series of successive laser pulses  12 - 18  towards a scene which is to be observed  20 . After each pulse  12 - 18 , the detecting part of system  10  picks up the flux reflected by scene  20  during a time window  22 - 28  which is controlled by the system. 
         [0009]    The delay between generating a laser pulse  12 - 18  and the start of the corresponding time window  22 - 28  is adjusted by the system depending on the viewing distance of scene  20  and the duration of the window adjusts the observation depth. 
         [0010]    Thus, for example, first time window  22  is adjusted to observe scene  20  at a distance of 5 kilometres and over a depth of 1 metre, which corresponds here to one image slice T 1  of a cloud of mist  30 , whereas last time window  28  is adjusted to observe scene  20  at a distance of 6 kilometres over a depth of 1 metre which corresponds here to one image slice T 4  of building  32 . 
         [0011]    In such a 3D imaging system which uses image slicing, resolution is limited by the width of time windows  22 - 28 . A large number of pulse/window pairs must be generated in order to obtain high resolutions. However, the larger the number of pulses generated, the longer the time needed to reconstitute the 3D image becomes and this is not compatible with certain applications, especially in the field of real-time 3D imaging. In addition, multiplying the number of pulses has an adverse effect on the discretion which is sought-after in stealthy systems. 
         [0012]    Other 3D imaging systems commonly referred to as “3D flash” systems only use a single laser pulse. Such systems are, for example, described in document U.S. Pat. No. 6,133,989. In this type of system, the intensity of the flux reflected by the scene must be measured and timestamped independently, for each pixel which forms part of the flux detector, at the instant when the pulse returns. 
         [0013]    Several techniques to independently timestamp the instant when the pulse returns (more widely referred to by the expression “time-of-flight measurement”) have been developed. For example, time-of-flight measurement is realised by filtering in each pixel by measuring the maximum amplitude of the reflected flux detected by the pixel (see, for instance, the document by M. Browder et al. “3 D imaging laser radar ”, Proceedings of SPIE, vol. 4377, pages 73-83) or even by sampling the pulse multiple times (see, for example, document U.S. Pat. No. 7,206,062). 
         [0014]    However, measuring the maximum amplitude by filtering the reflected flux does not make it possible to perforin conventional flux measurement by current integration. The 2D performance of an imaging system which uses such a technique is therefore poor. In addition, such a system is relatively unsuitable for realising multimode detection which is capable of performing both passive 2D imaging (i.e. without emitting radiation), 2D active imaging (i.e. emitting radiation but without distance measurement) and 3D active imaging. 
         [0015]    Not only this, time-of-flight measurement by multiple sampling comes up against technical design problems. In fact, it is difficult to store a large number of samples over the surface area of a single pixel. Besides, the sampling frequency depends heavily on the technology used to implement the circuitry. 
         [0016]    Lastly, using this technique results in considerable power consumption which is not compatible with certain applications, especially portable imaging systems. Consequently, systems which use this technique usually have a time resolution which is limited to measuring the time of flight. 
         [0017]    Document US-A-2006/007422, for example, also discloses a 3D imaging system such as that shown in  FIG. 5  in that document wherein the charge produced by a detector is initially amplified (unit  152 ). The output of amplifier  152  is connected firstly to an integrator (unit  164 ) which integrates the amplified signal and secondly to a comparator (unit  154 ) which compares the amplified signal with a threshold value in order to determine the instant when a pulse reaches the detector. Charge measurement and time-of-flight measurement are thus performed in parallel, with the integrator and the comparator being connected in parallel. This circuit has several drawbacks such as the need for a large number of components and the need to have an amplifier  152  which is sufficiently linear over the entire range of input signals. In practice, at least three operational amplifiers are required in order to implement the input amplifier, comparator and integrator respectively. Realising operational amplifiers generally requires a considerable area of silicon and the overall dimensions of such a circuit are generally too large for it to be used in modern array architectures. In addition, the power consumption of such a circuit is generally high and this generates additional noise which degrades measurement quality. 
       SUMMARY OF THE INVENTION 
       [0018]    The object of the present invention is to solve the above-mentioned problems by proposing a device intended for 3D active imaging which allows simultaneous measurement of the reflected flux and time of flight while allowing multimode use and does not entail prejudicially high power consumption. 
         [0019]    To achieve this, the object of the invention is a photosensitive element which produces an electric charge as a function of the radiation incident thereon and a charge integrator which is connected to a photosensitive element and converts the charge to a voltage. Generally speaking, the photosensitive element supplies a current to the input of the integrator. This current is integrated across terminals of a capacitor of the integrator and the voltage across the terminals of this capacitor constitutes the integrator&#39;s output voltage. The output voltage of the integrator is an image of the electric charge produced by the photosensitive element. 
         [0020]    According to the invention, this device comprises a comparator capable of comparing the voltage delivered by the integrator with a threshold voltage, and a memory unit for storing the instant when the voltage delivered by the integrator exceeds the threshold voltage. 
         [0021]    In other words, the reflected flux is measured conventionally by using, for instance, a photodiode or a phototransistor coupled to an integrator. The comparator at the output of the integrator makes it possible to detect an increase in the voltage delivered by the integrator and this increase denotes incident radiation on the photosensitive element. The time during which the comparator&#39;s threshold value is exceeded thus makes it possible to determine the time of flight. 
         [0022]    In addition, the integrator and the comparator are connected in series. The integrator constitutes the input stage of the device, just after the photosensitive element, and performs current-to-voltage conversion (the signal output by the integrator is a voltage) which is required for the comparator. In other words, there is no need to provide a current-to-voltage conversion circuit, such as a dedicated amplifier, between the photosensitive element and the comparator. In practice, this obviates the need to implement an operational amplifier, in contrast to the device described in Document US-A-2006/007422. 
         [0023]    It should also be noted that it is not necessary to provide linear amplification of the charges produced by the photosensitive element and that this simplifies the design of the circuit. In fact, realising linear amplification is usually difficult; it demands a large surface area of silicon and considerable power. Furthermore, realising the current-to-voltage conversion function through the integrator makes it possible to overcome the bandwidth and noise sensitivity problems associated with a converter such as an amplifier. 
         [0024]    What is more, using a conventional integrator allows multimode use because the comparator does not disturb the overall operation of flux measurement. Furthermore, the dedicated components for measuring the time of flight are simple components (comparator and memory) which do not consume much energy. 
         [0025]    According to one particular embodiment of the invention, the integrator has variable amplification gain with the integrator&#39;s amplification gain before the instant when the threshold voltage is exceeded being larger than the amplification gain after the instant when said voltage is exceeded. More especially, the integrator comprises an operational amplifier mounted in inverse feedback by a first capacitor and a second switchable capacitor, with the first capacitor having a capacitance which is less than that of the second capacitor and the second capacitor being capable of being connected in parallel to the first capacitor after the instant when the threshold voltage is exceeded. 
         [0026]    The capacitance of the first capacitor is selected depending on the desired accuracy when determining the instant when the threshold voltage is exceeded. 
         [0027]    In other words, the charge on the input of the integrator is initially converted into a voltage with significant amplification gain. In fact, the integrator&#39;s output voltage rapidly reaches the threshold voltage, thus allowing accurate detection of the time of flight. Once the threshold voltage has been reached, the gain of the integrator diminishes so as to integrate all the charges produced by the photosensitive element. 
         [0028]    Thus, in the case of an integrator consisting of an amplifier which is feedback stabilised by a set of capacitors, the amplifier is feedback stabilised by a capacitance which initially has a low value (because the integrator&#39;s output voltage across the terminals of the set of capacitors increases faster, the smaller the overall capacitance of this set of capacitors is). The time constant of the integrator is therefore small and so the threshold value on its output is reached quickly. The time-of-flight measurement is therefore more accurate. 
         [0029]    Also, once this measurement has been made, a capacitance having a larger value is used to feedback stabilise the amplifier. This capacitance is sized so as to allow integration of all the charges produced by the incident radiation on the photosensitive element. It should be noted that the charges integrated by the first capacitor are not lost but are transferred to the final capacitance of the capacitors which are connected in parallel. 
         [0030]    According to one particular embodiment of the invention, the device comprises a module for estimating a detection delay which corresponds to the instant when the radiation on the photosensitive element starts and the threshold-exceeded instant as a function of the voltage delivered by the integrator and a module for correcting the instant when the threshold is exceeded by subtracting the determined detection delay from it. More especially, the module for estimating the detection delay is capable of estimating it in accordance with the following equation: 
         [0000]    
       
         
           
             
               T 
               det 
             
             = 
             
               
                 α 
                 
                   
                     
                       a 
                       · 
                       Δ 
                     
                      
                     
                         
                     
                      
                     
                       V 
                       int 
                     
                   
                   + 
                   b 
                 
               
               + 
               β 
             
           
         
       
     
         [0000]    where ΔV int  is a variation in voltage corresponding to the incident radiation on the photosensitive element and a, b, α, and β are predetermined parameters. 
         [0031]    In other words, there is time-of-flight detection latency. In fact, there is a certain delay between the start of the incident flux on the photosensitive element, which is the real time of flight, and measurement of the latter when the threshold value of the comparator is exceeded. 
         [0032]    It is possible to estimate this latency according to the characteristics of the integrator and of the voltage delivered by the latter. A time-of-flight measurement correction can thus be made in order to improve the temporal accuracy of the device. 
         [0033]    According to one particular embodiment of the invention, the threshold voltage of the comparator is adjusted to a voltage which is substantially equal to: 
         [0000]    
       
      
       V 
       comp 
       +V 
       offset 
       CTIA 
       +V 
       offset 
       comparator  
      
     
         [0000]    where V comp  is a predetermined voltage, V offset   CTIA  is an integrator offset voltage and V offset   comparator  is a comparator offset voltage. 
         [0034]    In other words, the integrator and the comparator are not perfect. In particular, the voltages which they deliver are contaminated by offset error which is a source of time-of-flight measurement inaccuracy. Incorporating the value of these offsets in the threshold voltage of the comparator eliminates their effects on time-of-flight measurement. 
         [0035]    More especially, the device comprises
       a first switch capable of connecting the output of the comparator to the latter&#39;s threshold voltage input,   a third capacitor connected between the threshold voltage input of the comparator and the first terminal of a second and a third switches, the second terminal of the third capacitor being connected to ground and to a predetermined voltage respectively,       
 
         [0038]    The threshold voltage of the comparator is adjusted by closing the first and second switches and by opening the third switch and then opening the first and second switches and closing the third switch. 
         [0039]    In other words, the real offsets of the integrator and the comparator are obtained in the threshold voltage thanks to a simple procedure to initialise the device. This does away with any factory calibration procedure while ensuring that real offset errors are compensated. 
         [0040]    The object of the invention is also an imaging array which comprises a plurality of devices of the above-mentioned type. 
         [0041]    The object of the invention is also a 3D active imaging system which comprises a controllable source of radiation and a detection array capable of detecting radiation from said source and reflected by obstacles. According to the invention, the detection array comprises a plurality of devices of the above-mentioned type. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]    The present invention will be made more readily understandable by the following description which is given merely by way of example and relates to the accompanying drawings in which identical references denote identical or analogous components and in which: 
           [0043]      FIGS. 1 and 2  illustrate 3D active imaging using the image slicing discussed in the preamble; 
           [0044]      FIG. 3  is a schematic view of a first embodiment of the pixel according to the invention; 
           [0045]      FIGS. 4 to 6  are timing diagrams of the signals in the pixel in accordance with the invention during one read frame; 
           [0046]      FIG. 7  is a schematic view of a second embodiment of the pixel according to the invention; and 
           [0047]      FIG. 8  is a schematic view of a third embodiment of the pixel according to the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0048]    In  FIG. 3 , a device or pixel  40  according to the invention comprises a photodiode  42  capable of receiving incident radiation  44  and producing, as a function of the latter, a current “I in ”. 
         [0049]    The anode of photodiode  42  is connected to ground and its cathode is connected to the inverting (−) terminal of operational amplifier  46  which forms an integrator with two capacitors  48 ,  50  which are feedback stabilised between the output of amplifier  46  and the inverting (−) terminal of the amplifier. 
         [0050]    Amplifier  46  also receives, on its non-inverting (+) terminal, a constant voltage V ref  and outputs a voltage V int  which is proportional to the electric charge produced by photodiode  42  due to the incident radiation  44  thereon. 
         [0051]    Pixel  40  also comprises a comparator  52 , the positive terminal (+) of which is connected to output V int  of amplifier  48  and the negative terminal (−) of which receives a constant predetermined threshold voltage V threshold . Comparator  52  therefore outputs the voltage difference between its positive terminal (+) and its negative terminal (−). 
         [0052]    Blocker  54  is also provided on the output of comparator  52 . The output of blocker  54  switches from a first voltage to a second voltage which exceeds the first voltage at the instant when voltage V int  delivered by amplifier  46  exceeds threshold voltage V threshold . Once this switchover has occurred, the output of blocker  54  remains latched at the second voltage until initialisation is subsequently performed. 
         [0053]    A memory unit  56  is connected to the output of blocker  54  and stores the instant at which the output of blocker  54  switched on the basis of a timebase received from a clock (not shown). 
         [0054]    Also, a first switch  58  is provided in series with second capacitor  58 . Switch  58  is driven by the output of blocker  54  and closes when the latter&#39;s output switches from the first voltage to the second voltage, i.e. at the instant when voltage V int  delivered by amplifier  46  exceeds threshold voltage V threshold . The second capacitor  50  is thus connected in parallel with the first capacitor  48 . 
         [0055]    Advantageously, first capacitor  48  has a low capacitance C 3D . The term “low” here is taken to mean capacitance values which allow the desired temporal accuracy for the time of flight and the minimal current level on the input of the integrator which the system must be able to handle. 
         [0056]    The current produced by photodiode  42  is thus integrated with high gain so that output V int  of amplifier  46  very quickly exceeds threshold voltage V threshold  when there is incident radiation on the photodiode. Once the threshold has been exceeded, i.e. once the time of flight has been timestamped, connection of the second capacitor is established, with the capacitance C 2D  of the latter being sized to allow integration of all the incident flux. 
         [0057]    Finally, the pixel comprises a second reset switch  60  which is connected in parallel with first and second capacitors  48  and  50  and is driven by a zero reset signal. This reset signal also controls initialisation of blocker  54 . During initialisation, first and second switches  58 ,  60  are closed, thus discharging first and second capacitors  48 ,  50 , and the output of blocker  54  is switched to the first voltage. 
         [0058]    Thus, the output of amplifier  46  provides the measured incident radiation on photodiode  42  and the output of memory unit  56  provides a timestamp TOF measured  for the start of the incident radiation on photodiode  42 . 
         [0059]      FIGS. 4 to 6  show an example of the signals in pixel  40  as a function of time. These Figures illustrate, for example, a time window  22 - 28  as described in relation to  FIGS. 1 and 2  during which flux reflected by the scene is incident on photodiode  42 . 
         [0060]    For the sake of clarity, it is assumed here that the incident radiation on photodiode  42  is in the form of a rectangular pulse. Photodiode  42  thus produces a rectangular pulse having an amplitude I p  and a duration T p , as illustrated in  FIG. 4 . Note that the duration T p  of the rectangular pulse is the same as that of the emitted laser pulses. 
         [0061]    The time window comprises an initialisation interval between T 0  and T 1  during which capacitors  48 ,  50  are discharged and the output of blocker  54 , shown in  FIG. 6 , is adjusted to the first voltage. 
         [0062]    Photodiode  42  starts producing current due to the effect of the incident radiation at instant T 2 , instant T 2  being the real time of flight TOF real  of the flux reflected by the scene. 
         [0063]    Integration of current I int  starts at this instant T 2 . Voltage V int  on the output of amplifier  46 , shown in  FIG. 5 , then increases rapidly because of the low capacitance value C 3D  of capacitor  48 . 
         [0064]    Voltage V int  on the output of amplifier  46  reaches the threshold voltage V threshold  of comparator  52  at instant T 2 +T det , the instant at which the output of blocker  54  switches to the second voltage, thereby timestamping the time of flight. 
         [0065]    At this same instant, switch  58  is closed, thus connecting second capacitor  50  in parallel with first capacitor  48 . The charge initially stored in the latter is then transferred to the total capacitance formed by both capacitors  48 ,  50  connected in parallel and integration of the current continues at a slower rate. 
         [0066]    The output voltage of amplifier  46  at the end of the time window therefore provides a measure of the total charge produced by the photodiode. 
         [0067]    Similarly, instant T 2 +T det  provides measured timestamping of the time of flight. 
         [0068]    Note that timestamping of the time of flight is affected by inaccuracy, namely delay T det . 
         [0069]    Assuming that the initial integrator (formed by amplifier  46  and first capacitor  48 ) is ideal, detection delay T det  depends on the difference between the threshold voltage V threshold  of the comparator, the initial value V ini  which output voltage V int  of the amplifier assumes before current integration, the capacitance C 3D  of first capacitor  48  and the amplitude I p  of the current generated by photodiode  47  in accordance with the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     T 
                     det 
                   
                   = 
                   
                     
                       
                         C 
                         
                           3 
                            
                           
                               
                           
                            
                           D 
                         
                       
                       × 
                       
                         ( 
                         
                           
                             V 
                             seuil 
                           
                           - 
                           
                             V 
                             ini 
                           
                         
                         ) 
                       
                     
                     
                       I 
                       p 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0070]    Advantageously, capacitance C 3D  and threshold voltage V threshold  are adjusted in order to minimise detection delay T det . Thus, capacitance C 3D  is chosen to be as small as possible and threshold voltage V threshold  is adjusted so that it is as close as possible to initial voltage V ini . 
         [0071]    However, minimising detection delay T det  is limited by the feasibility of an integrator which uses a low value capacitance as well as by the noise margin of comparator  52 . 
         [0072]    In fact, to ensure satisfactory operation of the pixel described above, there is always a delay T det  which limits the accuracy of time-of-flight timestamping and this can be a problem in certain applications. 
         [0073]      FIG. 7  is a schematic view of a second embodiment of the pixel according to the invention. This second embodiment differs from the first embodiment described in relation to  FIG. 3  in that it also comprises a module  70  for correcting the timestamp stored in memory unit  56 . This correction module  70  is, for example, implemented in logic circuitry by a processor or is realised in the integrated circuit which performs readout of photodiode  42 . 
         [0074]    Correction module  70  is connected to the output of amplifier  46  to measure the voltage V int  on the latter. Module  70  estimates detection delay T det  as a function of this voltage in accordance with the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     T 
                     det 
                   
                   = 
                   
                     
                       α 
                       
                         
                           
                             a 
                             · 
                             Δ 
                           
                            
                           
                               
                           
                            
                           
                             V 
                             int 
                           
                         
                         + 
                         b 
                       
                     
                     + 
                     β 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where ΔV int  is a variation in voltage V int  on the output of amplifier  46  between the end and the start of the time window and a, b, α, and β are predetermined parameters. 
         [0075]    A subtractor  72  is also provided on the output of memory unit  56  in order to subtract the detection delay estimated by unit  70  from the value stored in unit  56 . 
         [0076]    In fact, as described earlier, the measured time of flight, namely that stored in memory unit  56 , can be written as follows: 
         [0000]      TOF measured   =TOF   real   +T   det   (3) 
         [0077]    By approximation and assuming the incident flux on photodiode  42  is in the form of a rectangular pulse, the detection delay can be written according to equation (1) or, more generally, in the form: 
         [0000]        T   det   =α.I   p +β  (4) 
         [0078]    It has also been demonstrated that the amplitude I p  of the current pulse produced by photodiode  42  can be written, as a function of the output voltage V int  of amplifier  46 , in the following form: 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     p 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             C 
                             
                               3 
                                
                               
                                   
                               
                                
                               D 
                             
                           
                           + 
                           
                             C 
                             
                               2 
                                
                               
                                   
                               
                                
                               D 
                             
                           
                         
                         ) 
                       
                       · 
                       
                         ( 
                         
                           
                             
                               V 
                               int 
                             
                              
                             
                               ( 
                               
                                 
                                   TOF 
                                   real 
                                 
                                 + 
                                 
                                   T 
                                   p 
                                 
                               
                               ) 
                             
                           
                           - 
                           
                             
                               V 
                               int 
                             
                              
                             
                               ( 
                               
                                 TOF 
                                 real 
                               
                               ) 
                             
                           
                         
                         ) 
                       
                     
                     
                       T 
                       p 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0079]    However, the voltage difference V int  (TOF real +T p )−V int (TOF rea ), taken between the end of TOF real +T det  instant and the start of TOF real  instant of the current pulse is known and equals the voltage difference on the output of amplifier  46  between the start and the end of the time window. 
         [0080]    Amplitude I p  can therefore be written as follows: 
         [0000]        I   p =α( V   int ( TOF   real   +T   p )− V   int ( TOF   real ))+ b=a.ΔV   ini   +b   (6) 
         [0081]    Combining equations (4) and (6) gives equation (2). 
         [0082]    Parameters a and b in equation (6) are determined for the pixel during a prior calibration phase by using two unifoun radiation sources as a reference; these have a fixed pulse width T p , this pulse width also being that which will subsequently be used during operation of the 3D imaging system. 
         [0083]    Parameters α and β are also determined during the calibration phase after determining parameters a and b. 
         [0084]    To achieve this, two pulses having a different amplitude are emitted and reflected by the same obstacle which is located a known distance d from the pixel in order to illuminate the pixel uniformly. The time of flight is thus measured and therefore known. 
         [0085]    For each pulse, the voltage difference ΔV int , which corresponds to the reflected flux measured by the pixel, is measured. Knowing the real time of flight and the amplitude of the pulse from equation (6), it is possible to calculate parameters α and β. 
         [0086]    Alternatively, the time of flight measurement correction is made, without presuming any relationship between the incident flux and detection delay T det , by using a predetermined correction table and the flux measurement made by the pixel, i.e. ΔV int . 
         [0087]    In the embodiments which have just been described, the time of flight is timestamped by comparator  52  as a function of the voltage delivered by amplifier  46 . 
         [0088]    However, these elements are not ideal and produce offsets which have an adverse effect on time-of-flight timestamping accuracy. In particular, the output voltages of amplifier  46  and of comparator  52  are contaminated by offsets V offset   CTIA  and V offset   comparator  respectively. 
         [0089]    The third embodiment shown in  FIG. 8  compensates for such offsets. 
         [0090]    This third embodiment differs from the first embodiment described in relation to  FIG. 3  in that it comprises:
       a third switch  80  mounted between the output of comparator  52  and the negative (−) terminal of the latter which is used for the threshold voltage,   a third capacitor  82  connected, via one of its terminals, to the negative input (−) of said comparator  52 ,   a fourth switch  84  mounted between the other terminal of capacitor  82  and ground,   and a fifth switch  86  mounted between the other terminal of capacitor  82  and the means capable of producing a variation of potential ΔV for each of the terminals of the capacitor C ech  when switch  86  is closed (with opening of switch  84 ).       
 
         [0095]    The third, fourth and fifth switches  80 ,  84 ,  86  are also driven by a signal φ in accordance with an offset compensation strategy. 
         [0096]    After the reset phase, a first offset compensation phase involves closing the third and fourth switches  80 ,  84  and opening the fifth switch  86 . This establishes, on the negative terminal (−) of comparator  52 , a voltage V φ1  according to the following equation: 
         [0000]        V   φ1   =V   ref   +V   offset   CTIA   +V   offset   comparator   (7) 
         [0097]    Then, during a second offset compensation phase, the third and fourth switches  80 ,  84  are opened and the fifth switch  86  is closed. This establishes, on the negative terminal (−) of comparator  52 , a voltage V φ2  according to the following equation: 
         [0000]        V   φ2   =V   ref   +V   offset   CTIA   +V   offset   comparator   +ΔV   (8) 
         [0098]    Switches  80 ,  84 ,  86  then remain in this state in order to measure the incident flux and the time of flight. Voltage V φ2  is used as threshold voltage V threshold , as described earlier. 
         [0099]    It is apparent that, because the value of offsets V offset   CTIA  and V offset   comparator  is included in threshold voltage V threshold , this value compensates any offsets V offset   CTIA  and V offset   comparator  which occur on the output of comparator  52 . The voltage of amplifier  46 , directly obtained by integrating the current produced by photodiode  42 , is then compared with voltage V ref +ΔV. 
         [0100]    This offset compensation provides not only improved time-of-flight measurement accuracy, thanks to minimised detection delay, and also robustness in terms of the technological dispersion which affects pixels. In fact, the compensation used compensates the real offsets which are present in the pixel. 
         [0101]    This compensation also makes it possible to minimise the difference between threshold voltage V threshold  and the initial voltage on the positive input of comparator  52  which is connected to amplifier  46  by choosing an appropriate voltage ΔV, for instance by adjusting the value of voltage ΔV as close as possible to the value zero. It should be noted that voltage V ini  equals V ref +V offset   CTIA  and that comparator  52  switches when the voltage difference across its terminals is positive. Consequently, keeping voltage V φ1  on the inverting terminal (−) of comparator  52  poses a risk of the comparator being triggered inopportunely. Positive voltage ΔV avoids such a risk while minimising the triggering delay. 
         [0102]    Note that, in practice, voltage V ini , when considered during the compensation phase, also includes the amplifier offset and the effects of the charge injection by switches  58 ,  60 . Thus, the threshold voltage equals V comp +V offset   CTIA +V offset   comparator , with V comp +V offset   CTIA  being equal to V ini  at instant T 1  prior to the start of the integration phase. 
         [0103]    Finally, although the compensation phase is described above as being subsequent to the reset phase, it can, in contrast, be combined with it. 
         [0104]    A fourth embodiment of the pixel according to the invention comprises both detection delay compensation as described in relation to  FIG. 7  and offset compensation as described in relation to  FIG. 8 . 
         [0105]    The pixel according to the invention is especially suitable for multimode imaging. In fact, flux measurement and timestamping are performed simultaneously and, in particular, without timestamping measurement influencing flux measurement. 
         [0106]    To obtain passive 2D or 3D imaging, it is sufficient to simply disable the portion of the circuit located downstream from the integrator or not to take into account the timestamp value returned by the latter. 
         [0107]    An integrator formed by an operational amplifier which is feedback stabilised by switchable capacitors so as to obtain current-to-voltage conversion with variable-gain amplification is described above. It is understood that any type of variable-gain integrator that has an amplification gain before detecting that the threshold voltage of the comparator has been exceeded which exceeds its gain after said threshold is exceeded is suitable. Nevertheless, an integrator formed by an operational amplifier which is feedback stabilised by switchable capacitors is preferable because of its simple design and ease of control.