Abstract:
An apparatus and method for determining the distance from a distance-measuring-device to an object by sending a light-pulse from a light-emitting component toward the object by passing a current pulse through the component. A value based on the current or voltage across the component is used to determine the reference-time T0 when the value exceeds a first threshold-value, and determines first-time T1 when the value is reduced to a second threshold-value THL2. T1 is used to define two time-windows when intensity of reflected light is integrated to provide two values (Q1/Q3 and Q2/Q4) to determine a time of flight for the light-pulse based on the two values (Q1/Q3 and Q2/Q4), the difference between T0 and T1 (T1-T0), and the speed of light, c.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit under 35 U.S.C. §119(a) of European Patent Application EP 14172114.2, filed 12 Jun. 2014, the entire disclosure of which is hereby incorporated herein by reference. 
       TECHNICAL FIELD OF INVENTION 
       [0002]    This disclosure relates to a distance-measuring-device determining a distance of an object from the distance-measuring-device and its associated method for doing the same. 
       BACKGROUND OF INVENTION 
       [0003]    One method of distance-measuring-devices is the measurement of the distance to an object by determining the time of flight of a light pulse. It is known in the art to provide distance-measuring-devices which send a precisely timed light pulse toward an object and to make gated measurements of the light reflected thereby. The time of flight of the light pulse is simply related to the range of the object through the relation d=c*Δtd/2, where d is the distance to the object, c stands for the velocity of light in the medium through which it propagates and Δtd is the time delay between the pulse emission and its detection. 
         [0004]    Several distance-measuring-devices that include time-gated detection schemes are known in the art. As a well-known solution, a distance-measuring-device sends a light pulse toward an object and records the reflected light from the object by a light sensor (typically which converts the incident light to voltage/current) using two gated time-intervals equal of time. The light pulse emitted from a light source is typically driven by a pulse generator providing a digital pulse. A first gated time-interval is typically synchronized with the digital pulse such that it starts at the same time the digital pulse starts and another gated time-interval starting at the end of the digital pulse. The durations of both gated time-intervals are generally set to be equal to the length of the digital pulse. The range of the object is in one type of method is determined by the ratio between the amount of reflected light received during the first gated time-interval and the sum of the two amounts of reflected light received respectively during the first gated time-interval and the second gated time-interval. 
         [0005]    The existing distance-measuring-devices are not accurate enough for security applications such as safety functions of automotive applications. Synchronization of digital pulses generates error. Furthermore the pulse length variation and delays are introduced which vary with environmental parameters such as supply voltage, temperature, or variation in electronic component parameters which drift from their nominal values. Solutions such as a calibration table stored in a memory of a distance-measuring-device exist. However, those solutions do not fully compensate the existing variations. Those solutions cost time, effort, and money for implementation. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the invention to provide an improved distance-measuring-device and associated method for doing the same in order to overcome such problems. It is an object of the invention to determine accurately the distance of an object by determining accurately the length of an emitted light pulse and its starting time. 
         [0007]    In one aspect is provided a method for determining the distance d from a unit to an object that includes sending a pulse of light from a light-emitting component to the object by passing a current pulse I1 sent through the component. The method also includes measuring the current I1 through or voltage across the light-emitting component and determining a reference-time T0 when the value exceeds or is equal to a pre-determined first threshold-value THL1. The method also includes determining the first-time T1 when the value is reduced to a pre-determined second threshold-value THL2. The method also includes using the time first-time T1 to define two time-windows. The method also includes receiving a reflected pulse of light from the object at the unit by a light sensor. The method also includes integrating a measure of the intensity of reflected light in the light sensor during the two time-windows to provide two values (Q1/Q3 and Q2/Q4). The method also includes determining the time of flight from the values), the difference between T0 and T1 (T1−T0) and the speed of light, c. 
         [0008]    In a preferred embodiment, a method for determining the distance d from a unit to an object may include a step of sending a pulse of light from a light-emitting component to the object by means of passing a current pulse I1 sent through the component; a step of measuring the current I1 through or voltage across the light-emitting component and determining the reference-time T0 when the value exceeds a pre-determined first threshold-value THL1 and determining the first-time T1 when the value is reduced to a pre-determined second threshold-value THL2; a step of receiving a reflected pulse of light from the object at the unit by a light sensor; a step of integrating a measure of the intensity of reflected light in the light sensor between time T0 and T1 to provide a value Q1; a step of integrating a measure of the intensity of reflected light in the light sensor between time T1 and T2, where T2 is the time-interval equal to T0+( 2 *(T1−T0)), to provide a value Q2; a step of determining the distance d from the formula d=½*c*(T1−T0)*(Q1/(Q1+Q2)). 
         [0009]    In another preferred embodiment a method for determining a distance d from a unit to an object may include a step of sending a pulse of light from a light-emitting component to the object by means of passing a current pulse I1 sent through the component; a step of measuring the current I1 through or voltage across the light-emitting component and determining the reference-time T0 when the value exceeds a pre-determined first threshold-value THL1 and determining the first-time T1 when the value is reduced to a pre-determined second threshold-value THL2; a step of receiving a reflected pulse of light from the object at the unit by a light sensor; a step of measuring the current Ipd through or voltage across the light sensor and determining the point TA when the value exceeds a pre-determined third threshold-value THL3 and determining the point TB when the value is reduced to a pre-determined fourth threshold-value THL4; a step of integrating a measure of the intensity of reflected light in the light sensor between time TA and T1 to provide a value Q3; a step of integrating a measure of the intensity of reflected light in the light sensor between time T1 and TB to provide a value Q4; a step of determining the distance d from the formula d=½*c*(T1−T0)*(Q3/(Q3+Q4)). 
         [0010]    The measure of the intensity of reflected light may be current Ipd through the light sensor. 
         [0011]    The value of (T1−T0) may be determined by charging a first capacitor of value C1 by a constant current Ic between times T0 and T1 to a voltage value VC1 and where T1−T0=(VC1*C1)/Ic. 
         [0012]    The thresholds THL1 and THL2 may be equal. 
         [0013]    In another embodiment, an apparatus for determining the distance d from a unit to an object is provided. The apparatus includes a means to send a pulse of light from a light-emitting component to the object by passing a current pulse I1 through the component; means to measure the current I1 through or voltage across the light-emitting component and to determine the reference-time T0 when the value exceeds a pre-determined first threshold-value THL1 and determining the first-time T1 when the value is reduced to a pre-determined second threshold-value THL2; a means to using the time first-time T1 to define two time-windows; means to receive a reflected pulse of light from the object at the unit by a light sensor; a means to integrate a measure of the intensity of reflected light in the light sensor during the two time-windows to provide two values (Q1/Q3 and Q2/Q4); means to determine the time of flight from the two values (Q1/Q3 and Q2/Q4), the difference between T0 and T1 (T1−T0) and the speed of light, c. 
         [0014]    In another preferred embodiment an apparatus for determining the distance from the unit to an object may further include means for sending a pulse of light from a light-emitting component to the object by means of passing a current pulse I1 sent through the component; means for measuring the current I1 through or voltage across the light-emitting component; means for determining the reference-time T0 when the values of the current I1 exceeds a pre-determined first threshold-value THL1 and means for determining the first-time T1 when the value is reduced to a pre-determined second threshold-value THL2; means for determining the point T2 where T2 is the time-interval equal to T0+(2*(T1−T0)); means for receiving a reflected pulse; means for integrating a measure of the intensity of reflected light in the light sensor between time T0 and T1 to provide a value Q1 and for integrating a measure of the intensity of reflected light in the light sensor between time T1 and T2, where T2 is the time-interval equal to T0+( 2 *(T1−T0)), to provide a value Q2; means for determining the distance d from the formula: d=½ *c*(T1−T0)*(Q1/(Q1+Q2)). 
         [0015]    In another preferred embodiment an apparatus for determining the distance d from the unit to an object may further include means for sending a pulse of light from a light-emitting component to the object by means of passing a current pulse I1 sent through the component; means for measuring the current I1 through or voltage across the light-emitting component and determining the reference-time T0 when the values of the current I1 exceeds a pre-determined first threshold-value THL1 and means for determining the first-time T1 when the value is reduced to a pre-determined second threshold-value THL2; light sensor means for receiving the reflected pulse of light from the object; means for measuring the current Ipd through or voltage across the light sensor means and means for determining the point TA when the values of the current Ipd through the light sensor exceeds a pre-determined third threshold-value THL3 and means for determining the point TB when the values of the current is reduced to a pre-determined fourth threshold-value THL4; means for integrating a measure of the intensity of reflected light in the light sensor between time TA and T1 to provide a value Q3 and means for integrating a measure of the intensity of reflected light in the light sensor between time T1 and TB; means for determining the distance from the formula: d=½*c*(T1−T0)*(Q3/(Q3+Q4)). 
         [0016]    The thresholds THL1 and THL2 may be equal. 
         [0017]    The means of measuring the intensity of reflected light may comprise means to measure current Ipd through the light sensor. 
         [0018]    The means of determining the value of (T1−T0) may comprise a first capacitor of value C1 charged by a constant current Ic between times T0 and T1 to a voltage value VC1 such that T1−T0=(VC1*C1)/Ic. 
         [0019]    The means for determining the reference-time T0 and for determining the first-time T1 may comprise a voltage comparator having an adjustable threshold voltage input. 
         [0020]    Further features and advantages will appear more clearly on a reading of the following detailed description of the preferred embodiment, which is given by way of non-limiting example only and with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0021]    The present invention will now be described, by way of example with reference to the accompanying drawings, in which: 
           [0022]      FIG. 1  shows a block diagram of a prior art distance-measuring-device in accordance with one embodiment; 
           [0023]      FIG. 2  shows timing charts of signals processed by the prior art distance-measuring-device in accordance with one embodiment; 
           [0024]      FIG. 3  shows a block diagram of a distance-measuring-device according to one example of the invention in accordance with one embodiment; 
           [0025]      FIG. 4  shows a simplified schematic diagram of a Dual Slope Time to Digital Converter according to one example of the invention in accordance with one embodiment; and 
           [0026]      FIG. 5  shows timing charts of signals processed by the distance-measuring-device according to one example of the invention. 
       
    
    
     DETAILED DESCRIPTION 
     Prior Art 
       [0027]      FIG. 1  is a block diagram of a prior art distance-measuring-device  10 . The device  10  determines a distance d from an object  24  from the device  10 . The device  10  includes a light driver  20  having an optical diode  30  such as a laser diode able to generate an light pulse  22 , towards the object  24  whose distance d is to be measured, and a controller  28  having a connection with the light driver  20  for providing control of the light driver  20  by way of a digital pulse. The device  10  further includes a light sensor  26 , connected to the controller  28 , able to detect a received light pulse  32  generated by the reflection of the emitted light pulse  22  by the object  24 , and converts into a current/voltage pulse. 
         [0028]    A signal generator  34  which is located in the controller  28  and managed by the controller  28 , controls generation of the digital pulse that is sent to the light driver  20  so that the length ΔTd of the digital pulse sent to the emitter is known by the controller  28 . 
         [0029]    The light sensor  26  includes a photosensitive element  27  such as a photo diode which produces an electrical current Iph as a function of (intensity of) the reflected light-pulse  32  and a current integrator which is connected to the photosensitive element  27  and converts the current Iph to an electrical charge. The output of the charge integrator is connected to the controller  28  via multiplexer internal of the photosensitive element. The light sensor can further includes a first and a second capacitors  21 ,  23  which terminals are the integrator&#39;s output. Functions of the light sensor  26  will be explained in details through the description of  FIG. 2 . 
         [0030]      FIG. 2  shows timing charts of signals processed by the prior art distance-measuring-device  10 . A first timing chart A 1  shows amplitude of the digital pulse that is sent to the light driver  20  by the signal generator  34  of the controller  28 . It is characterized by its length ΔTd known by the controller  28  between its rising edge and its falling edge. The rising edge occurs at a first time Tr and the falling edge occurs at a second time Tf. 
         [0031]    A second timing chart A 2  shows the amplitude of emitted light pulse  22  according to the setting of the prior art distance-measuring-device  10 . The controller  28  has no information about the emitted light pulse  22  as there is no feedback information from the light driver  20  to the controller  28 . There is a time delay ΔTde between the emitted light pulse  22  and the digital pulse sent to the optical diode  30  illustrated by the first timing chart A 1 . The time delay ΔTde is partially due to inherent hardware characteristics of the light driver  20 . 
         [0032]    A third timing chart A 3  shows the amplitude of light pulse  32  reflected by the object  24  and received by the light sensor  26 . At a time Ta, the light sensor  26  starts to receive the reflected light-pulse  32 . At a time Tb, the light sensor  26  ends to receive the reflected pulse  32 . 
         [0033]    A fourth timing chart A 4  shows a first and a second time slots TSa, TSb. The ‘Y’ axis is a representation of the activation of the time slots (TSa, TSb). The first and the second time slots TSa, TSb are under the control of the controller  28 , such that the light sensor  26  registers the current Iph through the light sensor  26  e.g. by integrating the current Iph during these time slots so as to get a measure of aggregate light falling on the light sensor  26  in these time slot periods. The duration of the first and the second time slot TSa, TSb is set equal to the length ΔTd of the digital pulse. The first time slot TSa starts at a time Tr the controller  28  initiates the digital pulse for the light sensor  26  and ends at a time Tf the controller  28  stops the digital pulse i.e. at the end of the digital pulse. The second time slot TSb starts at the time Tf and ends at a time Te such that the duration of the second time slot Tsb is equal of the duration of the first time slot TSa. The photosensitive element  27  starts to generate the current Iph at the time Ta and ends to generate it at the time Te i.e. the reflected light-pulse  32  starts to influence the photosensitive element  27  at the time Ta and stops at the time Tb. 
         [0034]    The current Iph is integrated only during the first and a second adjacent time slots TSa, TSb of equal time that results respectively to a first and a second amount of electrical charges Qa, Qb. 
         [0035]    The distance d to the object  24  can be calculated by using Eq. 1 where c stands for the velocity of light in the medium through which it propagates: 
         [0000]        d= ½* c*ΔTd *( Qa /( Qa+Qb ))  Eq. 1.
 
         [0036]    As an example, the first capacitor  21  is connected with the photosensitive element  27  only during the first time slot TSa such that it charges by the current Ipd from the time Ta until the time Tf. The charge of the first capacitor  21  results in the first amount of electrical charge Qa stored on it. The second capacitor  23  is connected with the photosensitive element  27  only during the second time slot TSb such that it charges by the current Iph from the time Tf until the time Te. The charge of the second capacitor  23  results in the second amount of electrical charge Qb stored on it. From the controller  28  perspective, the first and the second amounts of electrical charges Qa, Qb can be acquired through the terminals of the first capacitor  21  and the second capacitor  23 , in such a case the terminals constitute a first and a second integrator&#39;s output voltages value Va, Vb, those voltages output being simply dependent on the capacitors value. Considering that the first capacitor  21  and the second capacitor  23  have a same value, the distance d to the object  24  can be calculated by the controller  28  using the formula (2) where c stands for the velocity of light in the medium through which it propagates: 
         [0000]        d= ½* c*ΔTd *( Va /( Va+Vb ))  Eq. 2.
 
         [0037]    To summarize, the prior art distance-measuring-device  10  uses digital parameters for the distance d determination as the length of the digital pulse and its starting time. These digital parameters do not accurately represent what is really emitted in the air, because of the temperature dependency of the hardware, of the length of the emitted light pulse and of the time delay ΔTde. Following description relative to the invention provides an example of an improved distance-measuring-device. 
       Example of the Invention 
       [0038]      FIG. 3  is a block diagram example of a distance-measuring-device  50  according to one aspect of the invention. The distance-measuring-device  50  determines the distance d of the object  24  from the distance-measuring-device  50 . The distance-measuring-device  50  includes light-emitting component  60  having another optical diode  70  such as another laser diode able to generate another emitted light-pulse  62  towards the object  24  whose distance d is to be measured, controller  68  having another connection with the light-emitting component  60  for providing it a digital pulse, and the light sensor  26 , having its integrator output connected to the controller  68 , able to detect another received light pulse  72  generated by the reflection of the other emitted light-pulse  62  by the object  24 . 
         [0039]    A signal generator  84  which is located in the controller  68  and managed by the controller  68 , controls generation of the digital pulse that is sent to the light-emitting component  60 . Alternatively, the signal generator  84  can be located externally to the controller  68 . The controller  68  can be an integrated circuit such as a Field Programmable Gate Array (FPGA). 
         [0040]    While receiving the digital pulse from the signal generator  84 , the light-emitting component  60  biases its optical diode  70  for duration equal to the length of the digital pulse. A current I1 consequently flows through the optical diode  70  and thus the emitted light-pulse  62  emitted from the optical diode  70  is sent toward the object  24 . The light-emitting component  60  informs the controller  68  on the level of the emitted light-pulse  62  by a wired connection such that the controller  68  is able to assess the level of the emitted light-pulse  62 . In other words, as example, the current I1 flowing through the optical diode  70  flows through a resistor R connected in series with the optical diode  70 , such that the voltage over the resistor R, which is proportional to the current I1, can be determined by the controller  68  via a wire connection. 
         [0041]    Alternatively, the light sensor  26  may be used to detect the level of the emitted light-pulse  62 . 
         [0042]    The distance-measuring-device  50  further includes a first voltage comparator  80 , which is located in the controller  68 , having two inputs, one input being set to a reference level THL1 such as a first adjustable threshold voltage, the second input being connected to the light-emitting component  60  such that the level of the current I1 of the optical diode  70  can be compared with the first adjustable threshold voltage. The first voltage comparator  80  further includes an output that switches its logic state in function of the level of the current I1 compared with the first adjustable threshold voltage. In an example, in order to implement this, the current I1 flowing through the optical diode  70  flows through the resistor R connected in series with the optical diode  70 , such that the voltage over the resistor R, which is proportional to the current I1, can be effectively compared with the first adjustable threshold voltage. The first voltage comparator  80  can thus provide to the controller the information on the presence of the rising edge of the emitted light-pulse  62  if the current I1 provides a voltage over the resistor R exceeding the first adjustable threshold voltage value THL1 during the rising edge of the emitted light-pulse  62 . The first voltage comparator  80  can further provide to the controller the information on the falling edge of the emitted light-pulse  62  if the current I1 provides a voltage over the resistor R smaller than the first adjustable threshold voltage value THL1 during the falling edge of the emitted light-pulse  62 . 
         [0043]    Alternatively, the first voltage comparator  80  can be located externally to the controller  68 , as a stand-alone first voltage comparator  80 , or integrated in a device as in the light-emitting component  60 . 
         [0044]    Alternatively, the distance-measuring-device  50  can comprise other methods so the first voltage comparator  80  can monitor the level of the emitted light-pulse  62 . The level of the emitted light-pulse  62  can be compared to another reference THL1 than a threshold voltage by other means than the first voltage comparator  80 . Well known solution can be for instance the usage of a current comparator using current mirrors. 
         [0045]    The light sensor  26  includes the photosensitive element  27  such as a photo diode which produces an electrical current Ipd as a function of the reflected light-pulse  72  and a current integrator which is connected to the photosensitive element  27  and converts (e.g. sums/integrates) the current Ipd to an electrical charge. The output of the charge integrator is connected to the controller  68 . Functions of the light sensor  26  will be explained in details through the description of  FIG. 5 . 
         [0046]    As an option, the light sensor  26  is further comprises a second voltage comparator  82  capable of comparing the intensity of the reflected light-pulse  72  with a second reference level. The intensity may dictate the current Ipd through the photosensitive element  27 . In an example the voltage across the photosensitive element  27  is proportion to the generated current Ipd and thus provides a voltage to be compared with a second adjustable threshold voltage, the output of the second voltage comparator  82  being connected with the controller  68 . As example, the second voltage comparator  82  can provide to the controller  68  the information on the presence of the rising edge of the reflected light-pulse  72  when the level of the reflected light-pulse  72  exceeds the second adjustable threshold voltage value THL2. The second voltage comparator  82  can further provide to the controller  68  the information on the falling edge of the reflected light-pulse  72  when the level of the reflected light-pulse  72  is smaller than the second adjustable threshold voltage value THL2. Alternatively, the second voltage comparator  82  can be located externally to the light sensor  26 , as a stand-alone second voltage comparator, or integrated in a device as in the controller  68 . 
         [0047]    Functions of the option of the light sensor  26  will be explained in detail later with reference to  FIG. 5 . 
         [0048]    Alternatives can be provided; a point to note is that effectively a threshold of intensity is determined to recognize the start of the received reflected light-pulse. So alternatively the current through the light sensor may be compared with relevant thresholds. Alternatively, the distance-measuring-device  50  can comprise other solution than the second voltage comparator  82  to monitor the level of the reflected light-pulse  72 . The level of the reflected light-pulse  72  can be compared to another reference level than a threshold voltage by other means than the second voltage comparator  82 . Well-known solution can be for instance the usage of a current comparator using current mirrors. 
         [0049]    The distance-measuring-device  50  further includes an analog Dual Slope Time to Digital Converter  71  (DSTDC). The DSTDC  71 , including a first capacitor  73 , a second capacitor  75 , a first switch  74 , and a second switch  76 , controlled by the controller  68 , a current source  79  and a third voltage comparator  78 , communicates with the controller  68  and provides the charging level of at least one of the first capacitor  73  and the second capacitor  75  to the controller  68  via an Analog to Digital Converter  86 . Details are explained in more detail hereafter. 
         [0050]      FIG. 4  shows an example of a simplified schematic diagram of the Dual Slope Time to Digital Converter  71 . The constant current Ic from current source  79  is provided to two paths. Each path includes one of the two electronic switches  74 ,  76 , controlled by the controller  68  connecting the current source to one of the first capacitor  73  and the second capacitor  75 . Each of the first capacitor  73  and the second capacitor  75  is connected to one of the two inputs of the third voltage comparator  78  such that the charge level of the first capacitor  73  can be compared to the charge level of the second capacitor  75 . The output of the third voltage comparator  78  is connected to the controller  68 . The first capacitor  73  is connected to the Analog to Digital Converter  86  located in the controller  68 . The Analog to Digital Converter  86  is able to provide a voltage value of the first capacitor  73  to the controller  68 . Alternatively, the Analog to Digital converter  86  can be located externally to the controller  68  as a stand-alone Analog to Digital Converter or integrated in the DSTDC  71 . 
       Methodology 
     Example Method 1 
       [0051]      FIG. 5  shows an example of timing charts of signals processed by the distance-measuring-device  50  according to one implementation of the methodology. 
         [0052]    A first timing chart B 1  shows the characteristics of the emitted light-pulse  62  and specifically shows the current I1 flowing through the optical diode  70  during the emission of the emitted light-pulse  62 . The form of the current I1 includes a smooth rising edge RE and a smooth falling edge FE. The smooth rising edge RE corresponds to the transient during activation of the gate driver of the light-emitting component  60 , excited by the sharp rising edge of the digital pulse generated by the signal generator  84 . The smooth falling edge FE corresponds to the transient during deactivation of the gate driver of the light-emitting component  60  excited by the sharp falling edge of the digital pulse generated by the signal generator  84 . 
         [0053]    A reference time T0 is shown when the level of the current I1 of the optical diode  70  exceeds a first level, e.g. when the current I1 produces a voltage that exceeds a first adjustable threshold voltage THL1. A time T1 is shown when the level of the current I1 becomes smaller than the first adjustable threshold THL1. The controller  68  is informed by the state of the output of the first voltage comparator  80  of the reference time T0 and T1. As example, the first adjustable threshold voltage THL1 can be adjusted at a threshold-value permitting the detection of the height of the emitted light-pulse  62  having amplitude greater than the amplitude of the ambient light of the medium through which it propagates. 
         [0054]    A second timing chart B 2  shows the voltage Vc of the first capacitor  73  and the second capacitor  75  of the DSTDC  71  during charging of the first capacitor  73  and the second capacitor  75  by the constant current Ic. At the reference time T0, when the controller  68  is informed by the first voltage comparator  80  that the level of the current I1 exceeds the first adjustable threshold THL1, the controller  68  activates the closure of the first switch  74  of the DSTDC  71  enabling the charge of the first capacitor  73  by the constant current Ic. 
         [0055]    At the time T1, the controller  68  activates the opening of first switch  74  of the Dual Slope Time to Digital Converter  71  stopping the charging of the first capacitor  73  by the constant current Ic. During the first time-interval [T0-T1], the first capacitor  73  is charged by the constant current Ic of the current source  79 . As the Analog to Digital Converter  86  is able to provide the first voltage valueVC1 of the first capacitor  73  to the controller  68 , the controller is then able to accurately calculate the duration ΔT1 of the first time-interval [T0−T1] by Eq. 3: 
         [0000]      Δ T 1=( VC 1* C 1)/ Ic=T 1− T 0.  Eq. 3.
 
         [0056]    The time duration ΔT1 of the first time-interval [T0, T1] is considered as an accurate value of the length of the emitted light-pulse  62 . 
         [0057]    Alternatively, the length of the emitted light-pulse  62  determined from the charge of the first capacitor  71  performs between two events defined as the level of current I1 exceeding a first reference level THL1 and the level of the current I1 becoming smaller than the first reference level THL1. Thus could be determined by a counter based digital based Time-to-Digital converter. In such alternative, the length of the emitted light pulse would have been determined by a cumulative quantifiable number of digital clock cycles between the two identified events. 
         [0058]    To summarize the above steps determine accurately the length ΔT1 of the emitted light-pulse  62  by the following steps. At the time T1, when the level of the current I1 through the optical diode  70 , effectively measured by the voltage across it becomes smaller than the first reference level THL1, the controller  68  activates the closure of the second switch  76  of the DSTDC  71  enabling the charging of the second capacitor  75  by the constant current Ic. When the third voltage comparator  78  detects that the voltage value of the second capacitor  75  charged by the constant current Ic exceeds the voltage value VC1 stored on the first capacitor  73 , the logic state of the output of the third voltage comparator  78  switches from its previous logic state to the logic state so that the controller  68  identifies a time T2 as illustrated. At the time T2, the controller  68  activates the opening of the second switch  76  of the DSTDC  71  stopping the charge of the second capacitor  75  by the constant current Ic. 
         [0059]    Additionally, during the second time-interval [T1−T2], the second capacitor  75  charged by the constant current Ic of the current source  79 , is accumulating electrical charges that provides a second value Qn of the cumulative metric as a second voltage value VC2 so that the voltage value VC2 stored on the second capacitor  75  is considered as equal to the voltage value VC1 stored on the first capacitor  73 . The duration ΔT2 of the second time-interval [T1−T2] can be determined by Eq. 4: 
         [0000]      Δ T 2=( VC 2* C 2)/ Ic   Eq. 4.
 
         [0060]    In one example of the present invention, the value C2 of the second capacitor  75  being equal to the value C1 of the first capacitor  73  and the voltage value VC2 across the second capacitor  75  being considered as equal to the voltage value VC1 stored on the first capacitor  73 , the duration ΔT2 of the second time-interval [T1-T2] becomes equal to the duration ΔT1 of the first time-interval [T0-T1]. 
         [0061]    A third timing chart B 3  shows the characteristics of the reflected light-pulse  72  and specifically shows the current Ipd provided through the light sensor  26  during the reception of the reflected light-pulse  72 . The form of the current Ipd includes a smooth rising edge RE2 and a smooth falling edge FE2. 
         [0062]    At a time TA, the level of the current Ipd through the light sensor  26  exceeds a second level, e.g. when the current Ipd produces a voltage that exceeds a second adjustable threshold voltage THL2. 
         [0063]    A time TB is shown when the level of the current Ipd of the light sensor  26  becomes smaller than the second adjustable threshold voltage THL2. The controller  68  is informed by the second voltage comparator  82  of when voltage produced by the current exceeds the second threshold voltage THL2 such that the controller  68  is able to identify the time TA and TB. At a time T3, the light sensor  26  starts to receive the reflected light-pulse  72 . At a time T4, the light sensor  26  ends to receive the reflected light-pulse  72 . 
         [0064]    A fourth timing chart B 4  shows a first and a second time slots TS1 and TS2. The ‘Y’ axis is a representation of the activation of the time slots (TS1, TS2). The first and the second time slots TS1, TS2 are under the control of the controller  28 , such that the light sensor  26  is effectively enable by the controller  28  only during the time slots TS1, TS2 to register the current Ipd through the light sensor  26  e.g. by integrating the current Ipd during these time slots TS1, TS2 so as to get a measure of aggregate light falling on the light sensor  26  in these time slot periods. The duration of the first time slot TS1 is equal to the duration ΔT1 of the first time-interval [T0−T1]. The duration of the second time slot TS2 is equal to the duration ΔT2 of the second time-interval [T1−T2]. As stated above, both time-intervals duration are equal to the length of the emitted light-pulse  62 . The first time slot TS1 starts at the reference time T0. The second time slot TS2 starts at the time T1. The photosensitive element  27  starts to generate the current Ipd at the time T3 and ends to generate the current Ipd at the time T4, i.e. the reflected light-pulse  72  starts to influence the photosensitive element  27  at the time T3 and stops at the time T4. 
         [0065]    The current Ipd is integrated during the first and a second adjacent time slots TS1, TS2 of equal time that results respectively to a first and a second amount of electrical charges Q1, Q2. 
         [0066]    The distance d to the object  24  can be calculated by using Eq. 5 where c stands for the velocity of light in the medium through which it propagates: 
         [0000]        d= ½* c*ΔT 1*( Q 1/( Q 1+ Q 2)).  Eq. 5.
 
         [0067]    As an example, the first capacitor  21  is connected with the photosensitive element  27  only during the first time slot TS1 such that it charges by the current Ipd from the time T3 until the time T1. The charge of the first capacitor  21  results in the first amount of electrical charge Q1 stored on it. The second capacitor  23  is connected with the photosensitive element  27  only during the second time slot TS2 such that it charges by the current Ipd from the time T1 until the time T4. The charge of the second capacitor  23  results in the second amount of electrical charge Q2 stored on it. From the controller  68  perspective, the first and the second amounts of electrical charges Q1, Q2 can be acquired through the terminals of the first capacitor  21  and the second capacitor  23 , in such a case the terminals constitute a first and a second integrator&#39;s output voltages value V1, V2, those voltages output being simply dependent on the value of the first capacitor  21  and the second capacitor  23 . Considering that the first capacitor  21  and the second capacitor  23  have a same value, the distance d to the object  24  can be calculated by the controller  28  using Eq. 6 where c stands for the velocity of light in the medium through which it propagates: 
         [0000]        d= ½* c*ΔT 1*( V 1/( V 1+ V 2))  Eq. 6.
 
       Example Method 2 
       [0068]    A fifth timing chart B 5  shows an alternative method. The ‘Y’ axis is a representation of the activation of the time slots (TS1, TS2). The method integrates the current Ipd starting at the time TA that depends on the point the second adjustable threshold voltage THL2 is achieved and ends at the time TB. The photosensitive element  27  starts to generate the current Ipd at the time TA and ends to generate the current Ipd at the time TB, i.e. the reflected light-pulse  72  starts to influence the photosensitive element  27  at the time TA and stops at the time TB. The current Ipd is integrated during the first and the second time slots TS1, TS2 that results respectively to a third and a fourth amount of electrical charges Q3, Q4. 
         [0069]    The distance d to the object  24  can be calculated by using Eq. 7 where c stands for the velocity of light in the medium through which it propagates: 
         [0000]        d= ½* c*ΔT 1*( Q 3/( Q 3+ Q 4)).  Eq. 7.
 
         [0070]    In order to implement this with the apparatus of  FIG. 3 , the first capacitor  21  is connected with the photosensitive element  27  only from time TA until time T1 such that it charges by the current Ipd. The charge of the first capacitor  21  results in the third amount of electrical charge Q3 stored on it. The second capacitor  23  is connected with the photosensitive element  27  only from time T1 until time TB such that it charges by the current Ipd. The charge of the second capacitor  23  results in the second amount of electrical charge Q2 stored on it. From the controller  68  perspective, the third and the fourth amounts of electrical charges Q3, Q4 can be acquired through the terminals of the first capacitor  21  and the second capacitor  23 , in such a case the terminals constitute a third and a fourth integrator&#39;s output voltages value V3, V4, those voltages output being simply dependent on the value of the first capacitor  21  and the second capacitor  23 . Considering that the first capacitor  21  and the second capacitor  23  have a same value, the distance d to the object  24  can be calculated by the controller  28  using Eq. 8 where c stands for the velocity of light in the medium through which it propagates: 
         [0000]        d= ½* c*ΔT 1*( V 3/( V 3+ V 4))  Eq. 8.
 
         [0071]    This alternative option, where for instance the second threshold voltage THL2 is set at threshold-value greater than the amplitude of another current of the light sensor  26  induced by the ambient light, provides the advantage of refining the estimation of the distance d of the object  24  as the third integrator&#39;s output voltage V3 and the fourth integrator&#39;s output voltage V4 do not include part of the amount of charges that could have been accumulated by the first capacitor  21  and the second capacitor  23  charged by the other current of the light sensor  26  induced by the ambient light.