Abstract:
A distance measuring system and a distance measuring method which use a time-of-flight (TOF) method. The distance measuring system obtains a reference light quantity of reflected light which is a cumulative light quantity of the reflected light during a reference period, obtains a measured light quantity of the reflected light which is a cumulative light quantity of the reflected light during a measurement period, and calculates, on the basis of a ratio of the measured light quantity of the reflected light to the reference light quantity of the reflected light and a ratio of the reflected light incident period to the reference period, a reflected light incident period that is a period which is included in the measurement period and during which the reflected light is incident upon photoelectric conversion elements of a light-receiving device. Then, the distance measuring system calculates the distance between the distance measuring system and an object on the basis of the reflected light incident period.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a rangefinding (distance measuring) system and to a rangefinding (distance measuring) method, which employ a time-of-flight (TOF) process. 
       BACKGROUND ART 
       [0002]    One known rangefinding system for measuring a distance up to an object in a contactless manner employs a time-of-flight (TOF) process. According to such a TOF process, light that is emitted toward an object, and a period of time consumed after the light is emitted toward the object and until the light bounces off the object and returns, are measured, whereby the distance up to the object can be measured based on the period of time and the velocity of light (see Japanese Laid-Open Patent Publication No. 2001-281336, U.S. Pat. No. 5,754,280, and Ryohei Miyagawa and Takeo Kanade “CCD-Based Range-Finding Sensor,” IEEE Transactions on Electron Devices, Vol. 44, No. 10, October 1997, pp. 1648-1652 (hereinafter referred to as the “Miyagawa paper”). 
         [0003]    The Miyagawa paper contains a detailed explanation concerning the timing at which pulsed light is emitted, as well as the timing of operations of a photosensitive device in a rangefinding system. More specifically, pulsed light is emitted and emission of the pulsed light is stopped repeatedly for identical periods (by a light-emitting device, which is energized at a duty ratio of 50%). The photosensitive device transfers photoelectrons alternately in two directions in synchronism with the emitted and non-emitted pulsed light (see  FIG. 1  of the Miyagawa paper). A period of time consumed until the pulsed light bounces off the object and then returns is determined based on a difference between two output voltages of the photosensitive device. 
       SUMMARY OF INVENTION 
       [0004]    The rangefinding system described in the Miyagawa paper still requires improvement as to the accuracy of measurement thereof. For example, the photosensitive device of the rangefinding system detects ambient light such as sunlight, etc., in addition to reflected light from the object. However, the rangefinding system does not perform processing in view of the effects of ambient light. When the photosensitive device detects ambient light as well as reflected light, photoelectrons output from the photosensitive device are representative both of reflected light and ambient light. Therefore, photoelectrons, which are representative of reflected light, tend to be relatively small, resulting in a reduction in the signal-to-noise ratio (S/N ratio). Furthermore, the intensity of light is proportional to the square of the distance. According to the rangefinding system of the Miyagawa paper, if the measuring range (the range of measurable distances) is to be increased, then it is necessary to increase the detecting sensitivity (dynamic range) of the photosensitive device depending on the square of the distance. 
         [0005]    The present invention has been made in view of the aforementioned problems. It is an object of the present invention to provide a rangefinding system and a rangefinding method, which are capable of measuring distances with increased accuracy. 
         [0006]    A rangefinding system according to the present invention comprises a light-emitting device for emitting pulsed light toward an object, a light-detecting device for detecting reflected light from the pulsed light and producing an output signal depending on the energy of reflected light that is detected, a control device for controlling the light-emitting device and the light-detecting device, and an arithmetic device for calculating a distance up to the object according to a time-of-flight process using the output signal from the light-detecting device. The light-detecting device further comprises a photodetector for detecting the reflected light and generating photoelectrons by the detected reflected light, first through fourth capacitors for storing the photoelectrons from the photodetector, a photoelectron discharger for discharging the photoelectrons from the photodetector, first through fourth gate electrodes disposed between the photodetector and the first through fourth capacitors, for allotting the photoelectrons with respect to the first through fourth capacitors in synchronism with emission of the pulsed light, and a fifth gate electrode disposed between the photodetector and the photoelectron discharger, for controlling the supply of photoelectrons from the photodetector and the photoelectron discharger. If it is assumed that a time at which the pulsed light starts to be emitted is referred to as time Teu, a time at which the pulsed light stops being emitted is referred to as time Ted, a time at which the reflected light stops being exposed to the photodetector is referred to as time Trd, respective times at which the first through fourth gate electrodes are opened are referred to as times Tg 1   u , Tg 2   u , Tg 3   u , and Tg 4   u , respective times at which the first through fourth gate electrodes are closed are referred to as times Tg 1   d , Tg 2   d , Tg 3   d , and Tg 4   d , a period from time Tg 1   u  to time Tg 1   d  is referred to as period P 1 , a period from time Tg 2   u  to time Tg 2   d  is referred to as period P 2 , a period from time Tg 3   u  to time Tg 3   d  is referred to as period P 3 , a period from time Tg 4   u  to time Tg 4   d  is referred to as period P 4 , a period from time Tg 4   d  to time Trd is referred to as period Psr, a photoelectron quantity stored in the first capacitor during period P 1  is referred to as photoelectron quantity Q 1 , a photoelectron quantity stored in the second capacitor during period P 2  is referred to as photoelectron quantity Q 2 , a photoelectron quantity stored in the third capacitor during period P 3  is referred to as photoelectron quantity Q 3 , a photoelectron quantity stored in the fourth capacitor during period P 4  is referred to as photoelectron quantity Q 4 , a period during which the pulsed light is emitted, reflected by the object, and returned as reflected light is referred to as round trip time ΔP, and a distance between the rangefinding system and the object is referred to as distance D, then the control device controls emission of pulsed light from the light-emitting device and opening and closing of the first through fourth gate electrodes so as to satisfy the relationships (1) P 1 =P 3 , (2) P 2 =P 4 , and (3) Tg 1   u &lt;Tg 1   d ≦Tg 2   u &lt;Tg 2   d ≦Teu&lt;Tg 3   u &lt;Tg 3   d ≦Tg 4   u ≦Ted&lt;Tg 4   d , or Teu&lt;Tg 3   u &lt;Tg 3   d ≦Tg 4   u ≦Ted&lt;Tg 4   d &lt;Tg 1   u &lt;Tg 1   d ≦Tg 2   u &lt;Tg 2   d , and the control device opens the fifth gate electrode to discharge the photoelectrons when all of the first through fourth gate electrodes are closed. Further, the arithmetic device acquires light energy information of the reflected light during the period P 3  based on the difference between the photoelectron quantity Q 3  stored in the third capacitor corresponding to ambient light, and the reflected light and the photoelectron quantity Q 1  stored in the first capacitor corresponding to the ambient light, the arithmetic device acquires light energy information of the reflected light during the period Psr based on the difference between the photoelectron quantity Q 4  stored in the fourth capacitor corresponding to the ambient light and the reflected light, and the photoelectron quantity Q 2  stored in the second capacitor corresponding to the ambient light, the arithmetic device calculates the round trip time ΔP based on a ratio of the light energy information of the reflected light during the period P 3  and the light energy information of the reflected light during the period Psr, and a ratio of the period P 3  and the period Psr, and the arithmetic device measures the distance D based on the round trip time ΔP. 
         [0007]    According to the present invention, the rangefinding system has an increased dynamic range and is capable of reducing or eliminating the effect of ambient light. As a consequence, the rangefinding system increases measurement accuracy. 
         [0008]    According to the present invention, more specifically, the rangefinding system determines the photoelectron quantity Q 1  stored during the period P 1 , in which only ambient light is exposed to the photodetector, and the photoelectron quantity Q 3  stored during the period P 3 , during which both ambient light and reflected light are exposed to the photodetector. Since the period P 1  and the period P 3  are set to the same length, the rangefinding system can determine the photoelectron quantity corresponding to the reflected light during the period P 3  (corresponding to light energy [J] of the reflected light in the period P 3 ) from the difference between the photoelectron quantity Q 3  and the photoelectron quantity Q 1  (Q 3 −Q 1 ). 
         [0009]    The rangefinding system also determines the photoelectron quantity Q 2  stored during the period P 2 , in which only ambient light Ls is exposed to the photodetector, and the photoelectron quantity Q 4  stored during the period P 4 . The period P 4  comprises a period (period Psr), during which both ambient light and reflected light are exposed to the photodetector, and a period (period Ps) during which only ambient light is exposed to the photodetector. Since the period P 2  and the period P 4  are set to the same length, the rangefinding system can determine the photoelectron quantity corresponding to the period Psr that occurs within the period P 4  (corresponding to the energy of reflected light in the period Psr) from the difference between the photoelectron quantity Q 4  and the photoelectron quantity Q 2  (Q 4 −Q 2 ). 
         [0010]    If the intensity of reflected light (light energy per unit time [W]) is constant while the reflected light is being exposed to the photodetector, then a ratio of the difference between the photoelectron quantity Q 4  and the photoelectron quantity Q 2  to the difference between the photoelectron quantity Q 3  and the photoelectron quantity Q 1  (Q 4 −Q 2 :Q 3 −Q 1 ) becomes equal to a ratio of the period Psr to the period P 3  (Psr:P 3 ). Therefore, the period Psr can be determined by the following equation: 
         [0000]        Psr ={( Q 4 −Q 2)/( Q 3 −Q 1)}× P 3
 
         [0000]    When time Ted and time Tg 4   u  are equal to each other, the period Psr becomes equal to the round trip time ΔP. When time Ted is subsequent to time Tg 4   u , the period Psr becomes equal to the difference between the round trip time ΔP and the period from time Ted to time Tg 4   u  (ΔP−(Ted−Tg 4   u )}. Since time Ted and time Tg 4   u  can be set to predetermined values, the round trip time ΔP can also be determined in this case. Consequently, the round trip time ΔP can be determined in either case, and hence the distance D can be determined based on the round trip time ΔP and the velocity of light (about 300,000 kilometers per second). Since the rangefinding system removes the photoelectron quantity generated due to ambient light, the rangefinding system is capable of eliminating or reducing the effect of ambient light. 
         [0011]    As the distance D becomes smaller, the period during which reflected light is exposed (round trip time ΔP) is made shorter, whereas, as the distance D becomes greater, the period during which reflected light is exposed is made longer. Generally, assuming the same object is involved, as the distance D becomes smaller, the intensity [W] of light reflected from the object becomes greater, whereas, as the distance D becomes greater, the intensity of light reflected from the object becomes smaller. Consequently, if the distance D is comparatively small, then reflected light of a greater intensity is exposed over a shorter period. On the other hand, if the distance D is comparatively great, then reflected light of a smaller intensity is exposed over a longer period. As a result, a change in the cumulative energy of reflected light, which is exposed during the period Psr, is small compared with the change in distance D. This implies that it is possible to narrow the dynamic range of the rangefinding system, and therefore, the dynamic range of the rangefinding system can be increased. 
         [0012]    The control device may control the light-emitting device so as to emit pulsed light a plurality of times during each measuring cycle, and the arithmetic device may calculate the round trip time ΔP using the photoelectron quantities Q 1  through Q 4 , after the photoelectrons have been stored a plurality of times in each of the first through fourth capacitors. Generally, the intensity of ambient light (e.g., sunlight) varies at all times. By emitting pulsed light a plurality of times during each measuring cycle, and calculating the round trip time ΔP using the photoelectron quantities Q 1  through Q 4 , which are stored as many times as the number of times that the pulsed light is emitted, the intensity of the ambient light can be averaged. As a consequence, the accuracy at which the photoelectron quantity generated by ambient light is removed can be increased, thereby increasing measurement accuracy. 
         [0013]    The light-emitting device may set a period during which pulsed light is emitted to at most 1 percent of each measuring cycle, while the light-detecting device may set a period during which the first through fourth gate electrodes are opened to at most 1 percent of each measuring cycle. As a result, the possibility of interference with another rangefinding system (i.e., such that pulsed light from another rangefinding system is falsely recognized as pulsed light from the actual rangefinding system) is low. In addition, the effect that ambient light has as a noise component is reduced, thus resulting in an increased signal-to-noise ratio (S/N). Furthermore, it is possible to prevent aliasing (i.e., a phenomenon in which pulsed light emitted in a previous measuring cycle is detected in a current measuring cycle) from occurring. 
         [0014]    The light-detecting device may further comprise first through fourth amplifiers having respective gates connected respectively to the first through fourth capacitors, for outputting voltages depending on potentials across the first through fourth capacitors. 
         [0015]    The light-detecting device may further comprise a power supply and sixth gate electrodes for resetting potentials across the first through fourth capacitors. 
         [0016]    The photodetector may comprise one of a photodiode, a pinned photodiode, and a photogate. Each of the first through fourth capacitors may comprise one of an MIM capacitor, a MOS capacitor, a pinned photodiode, and a PN junction. 
         [0017]    The light-detecting device may further comprise a light shield for blocking light from entering into the first through fourth gate electrodes and the first through fourth capacitors. 
         [0018]    The light-emitting device may include a light emitter comprising one of a light-emitting diode, a laser diode, and a semiconductor laser bar. The light-emitting device may include a light emitter comprising an array of semiconductor laser bars. 
         [0019]    The light-detecting device may include one of a line sensor and an image sensor having a plurality of pixels each comprising the photodetector, the first through fourth capacitors, the photoelectron discharger, and the first through fifth gate electrodes. 
         [0020]    According to the present invention, there is also provided a rangefinding method carried out by a rangefinding system including a light-emitting device for emitting pulsed light toward an object, a light-detecting device for detecting reflected light from the pulsed light and producing an output signal depending on the energy of reflected light that is detected, a control device for controlling the light-emitting device and the light-detecting device, and an arithmetic device for calculating a distance up to the object according to a time-of-flight process using the output signal from the light-detecting device. The rangefinding method comprises the steps of determining reflected light reference energy, which represents cumulative light energy of the reflected light during a reference period, determining measured reflected light energy, which represents cumulative light energy of the reflected light during a measuring cycle, calculating a reflected light incident period, which represents a period during which the reflected light is exposed to a photodetector of the light-detecting device within the measuring cycle, based on a ratio of the measured reflected light energy and the reflected light reference energy, and a ratio of the reflected light incident period and the reference period, and calculating a distance between the rangefinding system and the object based on the reflected light incident period. 
         [0021]    According to the present invention, the dynamic range of the rangefinding system is increased together with increasing measurement accuracy. More specifically, according to the present invention, a reflected light incident period, which represents a period during which reflected light is exposed to a photodetector of the light-detecting device within the measuring cycle, is calculated based on a ratio of the measured reflected light energy and the reflected light reference energy, and a ratio of the reflected light incident period and the reference period. As the distance between the rangefinding system and the object become smaller, the reflected light incident period is made shorter, whereas, as the distance becomes greater, the reflected light incident period is made longer. Generally, assuming the same object is involved, as the distance becomes smaller, the intensity of light reflected from the object becomes greater, whereas, as the distance becomes greater, the intensity light reflected by the object becomes smaller. Consequently, if the distance is comparatively small, then reflected light of a greater intensity is exposed over a shorter period. On the other hand, if the distance is comparatively great, then reflected light of a smaller intensity is exposed over a longer period. As a result, a change in the cumulative energy of reflected light, which is exposed during the reflected light incident period, is small compared with the change in distance. This implies that it is possible to narrow the dynamic range of the rangefinding system, and therefore, the dynamic range of the rangefinding system can be increased. 
         [0022]    The step of determining the reflected light reference energy may further comprise the steps of determining ambient light reference energy, which represents cumulative light energy of ambient light during a first reference period, determining combined light reference energy, which represents cumulative combined light energy of the ambient light and the reflected light during a second reference period, which has the same length as the first reference period, and calculating the reflected light reference energy during the second reference period by subtracting the ambient light reference energy from the combined light reference energy. The step of determining the measured reflected light energy may further comprise the steps of determining measured ambient light energy, which represents cumulative light energy of the ambient light during a first measuring cycle, determining measured combined light energy, which represents cumulative combined light energy of the ambient light and the reflected light during a second measuring cycle, which has the same length as the first measuring cycle, and calculating the measured reflected light energy during the second reference period by subtracting the measured ambient light energy from the measured combined light energy. Since the cumulative energy of ambient light is removed, the effect of such ambient light can be reduced or eliminated. 
         [0023]    The rangefinding method may further comprise the steps of setting a plurality of first reference periods, a plurality of second reference periods, a plurality of first measuring cycles, and a plurality of second measuring cycles in each measuring cycle, determining the ambient light reference energy as a sum of cumulative light energy of the ambient light respectively during the first reference periods, determining the combined light reference energy as a sum of cumulative combined light energy respectively during the second reference periods, determining the measured ambient light energy as a sum of cumulative light energy of the ambient light respectively during the first measuring cycles, and determining the measured combined light energy as a sum of cumulative combined light energy respectively during the second measuring cycles. Generally, the intensity of ambient light (e.g., sunlight) as a noise component varies at all times. The intensity of ambient light can be averaged by setting a plurality of first reference periods, a plurality of second reference periods, a plurality of first measuring cycles, and a plurality of second measuring cycles during each measuring cycle, determining the ambient light reference energy as a sum of cumulative light energy of the ambient light respectively during the first reference periods, determining the combined light reference energy as a sum of cumulative combined light energy respectively during the second reference periods, determining the measured ambient light energy as a sum of cumulative light energy of the ambient light respectively during the first measuring cycles, and determining the measured combined light energy as a sum of cumulative combined light energy respectively during the second measuring cycles. As a consequence, the accuracy at which photoelectron quantities generated by ambient light are removed can be increased, thereby increasing measurement accuracy. 
         [0024]    Each of a period during which pulsed light is emitted, the reference period, and the measuring period may be set to at most 1 percent of each measuring cycle. As a result, the possibility of interference with another rangefinding system (i.e., such that pulsed light from another rangefinding system is falsely recognized as pulsed light from the actual rangefinding system) is low. In addition, the effect that ambient light has as a noise component is reduced, thus resulting in an increased signal-to-noise ratio (S/N). Furthermore, it is possible to prevent aliasing (i.e., a phenomenon in which pulsed light emitted in a previous measuring cycle is detected in a current measuring cycle) from occurring. 
         [0025]    According to the present invention, the dynamic range of the rangefinding system is increased, while the effect of ambient light is reduced or eliminated. As a result, the measurement accuracy of the rangefinding system can be increased. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0026]      FIG. 1  is a block diagram of a rangefinding system according to an embodiment of the present invention; 
           [0027]      FIG. 2  is a block diagram of a photodetector according to the embodiment; 
           [0028]      FIG. 3  is a circuit diagram of a pixel in an image sensor of the photodetector; 
           [0029]      FIG. 4  is a partial vertical cross-sectional view of the pixel; 
           [0030]      FIG. 5  is a timing chart of a measuring cycle according to the embodiment; 
           [0031]      FIG. 6  is a timing chart of a photoelectron storage period within the measuring cycle; 
           [0032]      FIG. 7  is a timing chart, which is a modification of the timing chart shown in  FIG. 6 ; 
           [0033]      FIG. 8  is a partial vertical cross-sectional view of a modified pixel; and 
           [0034]      FIG. 9  is a timing chart of emitted light, reflected light, a plurality of gates, and a photogate of the pixel shown in  FIG. 8 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     A. Embodiment 
       [0035]    A rangefinding system according to an embodiment of the present invention will be described below with reference to the drawings. 
       1. Arrangement of the Rangefinding System  10 : 
     (1) Overall Arrangement of the Rangefinding System  10 : 
       [0036]      FIG. 1  is a block diagram of a rangefinding system  10  according to an embodiment of the present invention. The rangefinding system  10  serves to acquire a three-dimensional image using distances that are measured based on output signals from pixels  32  of an image sensor  30 , to be described later. The rangefinding system  10  includes a light-emitting device  12 , a light-detecting device  14 , a control device  16 , and an arithmetic device  17 . The rangefinding system  10  operates such that, in response to a command from the control device  16 , a light emitter  18  of the light-emitting device  12  emits pulsed light Lp, and the pulsed light Lp is reflected by an object W and exposed to the light-detecting device  14 . Based on a command from the control device  16 , the light-detecting device  14  outputs a signal representative of photoelectrons depending on the amount of detected light (stored photoelectron signal Sc) to the arithmetic device  17 . The arithmetic device  17  calculates a period (round trip time ΔP) [s] that the pulsed light Lp has taken in order to travel from the light-emitting device  12  to the light-detecting device  14 , and then calculates a distance D [m] between the rangefinding system  10  and the object W based on the round trip time ΔP. The calculated result from the arithmetic device  17  is output to a display device, not shown, for example. For illustrative purposes, the pulsed light Lp that travels from the light-emitting device  12  to the object W will be referred to as “emitted light Le,” whereas the pulsed light Lp that travels from the object W to the light-detecting device  14  will be referred to as “reflected light Lr”. 
       (2) Light-Emitting Device  12 : 
       [0037]    The light-emitting device  12  comprises the light emitter  18 , which outputs pulsed light Lp based on a command from the control device  16 . According to the present embodiment, the light emitter  18  of the light-emitting device  12  comprises a stack of (series-connected) surface emission semiconductor laser bars, each having a linear array of light-emitting spots (emitters). 
         [0038]    According to the present embodiment, the light emitter  18  is capable of emitting 100 watts (W) of infrared radiation at a wavelength of 870 nanometers. Further, according to the present embodiment, a plurality of exposure processes (photoelectron storing processes) are performed during each measuring cycle Cm (i.e., a cycle during which a measured value is determined) (see  FIG. 5 ). Whereas an exposure process has a cycle (exposing cycle or photoelectron storage cycle Tca 2 ) of 100 microseconds (μs), the light emitter  18  outputs pulsed light Lp for an output time (pulse duration) of 100 nanoseconds (ns). Stated otherwise, the light emitter  18  has a duty ratio of 0.1 percent (%). 
         [0039]    The light emitter  18  may comprise a linear array of light-emitting spots or a matrix of light-emitting spots. The light emitter  18  may employ light-emitting elements such as laser diodes, light-emitting diodes (LEDs), or other types of light-emitting elements. Pulsed light Lp emitted by the light emitter  18  may have other wavelengths in a range from 700 nm to 1050 nm, for example. The light emitter  18  may emit other output levels in a range from 20 W to 10 kW. The pulsed light Lp may have other pulse durations in a range from 10 nanoseconds to 1 millisecond. In addition, the light emitter  18  may have other duty ratios in a range from 0.01% to 1%. 
       (3) Light-Detecting Device  14 : 
     (a) Overall Arrangement of the Light-Detecting Device  14 : 
       [0040]    As shown in  FIG. 1 , the light-detecting device  14  comprises a lens  20  and a light detector  22 . Reflected light Lr and ambient light Ls that have passed through the lens  20  are focused onto the light detector  22 . The lens  20  may comprise a linear array of lenses or a matrix of lenses. 
         [0041]    As shown in  FIG. 2 , the light detector  22  comprises an image sensor  30  made up from a matrix of pixels  32 , a first power supply  34 , a second power supply  36 , a gate drive circuit  38 , a vertical selecting circuit  40 , a sample and hold circuit  41 , a horizontal selecting circuit  42 , an output buffer  43 , and an A/D converter  44 . 
         [0042]    The image sensor  30  outputs a stored photoelectron signal Sc depending on the amount of light detected by each of the pixels  32 . The first power supply  34  applies a positive power supply voltage VDD [V] (see  FIG. 3 ) to the image sensor  30 , and the second power supply  36  applies a positive reference voltage Vref [V] to the image sensor  30 . The gate drive circuit  38  outputs a gate drive signal Sdg (see  FIG. 3 ), which collectively refers to gate drive signals Sdg 1  through Sdg 4 , resetting signals Sreset 1  through Sreset 4 , and a photoelectron discharge signal Sde, to thereby selectively drive the gates of first switching elements  60   a  through  60   d , third switching elements  68   a  through  68   d , and a fourth switching element  70  of the image sensor  30 . The vertical selecting circuit  40  includes a multiplexer (not shown), which selectively outputs read signals Sread 1  through Sread 4  (see  FIG. 3 ) to a row in which a pixel  32  to be read belongs, so as to enable the pixel  32  to output a stored photoelectron signal Sc. The horizontal selecting circuit  42  includes another multiplexer (not shown), which selects a column in which the pixel  32  to be read belongs. The stored photoelectron signal Sc read from the pixel  32  is converted by respective constant-current circuits  58   a ,  58   b  into output voltages Vout 1 , Vout 2  (see  FIG. 3 ). Thereafter, the stored photoelectron signal Sc is held by the sample and hold circuit  41 , and then the stored photoelectron signal Sc is output from the horizontal selecting circuit  42 . The stored photoelectron signal Sc, having been output from the horizontal selecting circuit  42 , is sent via the output buffer  43  and the A/D converter  44  to the arithmetic device  17 . When the arithmetic device  17  receives the stored photoelectron signal Sc, the arithmetic device  17  determines an amount Ar of reflected light Lr from the stored photoelectron signal Sc, and then, as described in detail below, the arithmetic device  17  calculates a distance D between the rangefinding system  10  and the object W. 
       (b) Pixel  32 : 
       [0043]      FIG. 3  is a circuit diagram of one pixel  32  in the image sensor  30 . As shown in  FIG. 3 , the pixel  32  includes a photodetector  50 , first through fourth photoelectron storage units  52   a  through  52   d , a photoelectron discharger  54 , and output lines  56   a ,  56   b . The constant-current circuits  58   a ,  58   b  are associated respectively with each column of the pixel  32 . 
       (i) Photodetector  50 : 
       [0044]    According to the present embodiment, the photodetector  50  comprises a pinned photodiode, which generates photoelectrons depending on the amount Ar of reflected light Lr. Alternatively, the photodetector  50  may comprise another type of photodetector, such as a photodiode, a photogate, or the like, rather than the pinned photodiode. (ii) First through Fourth Photoelectron Storage Units  52   a  through  52   d:    
         [0045]    The first photoelectron storage unit  52   a  comprises a first switching element  60   a , a capacitor  62   a , an amplifier  64   a , a second switching element  66   a , and a third switching element  68   a . Similarly, the second through fourth photoelectron storage units  52   b  through  52   d  comprise, respectively, first switching elements  60   b  through  60   d , capacitors  62   b  through  62   d , amplifiers  64   b  through  64   d , second switching elements  66   b  through  66   d , and third switching elements  68   b  through  68   d . In the present embodiment, each of the first switching elements  60   a  through  60   d , the amplifiers  64   a  through  64   d , the second switching elements  66   a  through  66   d , and the third switching elements  68   a  through  68   d  are constituted by NMOS transistors. 
         [0046]    The first switching elements  60   a  through  60   d  select which of the first through fourth photoelectron storage units  52   a  through  52   d  the photoelectrons generated by the photodetector  50  are supplied to. More specifically, the first switching elements  60   a  through  60   d  have respective sources S 11  through S 14 , which are connected respectively to the photodetector  50 , respective drains D 11  through D 14 , which are connected respectively to the first through fourth capacitors  62   a  through  62   d , and respective gates G 11  through G 14 , which are connected to the gate drive circuit  38 . In response to the gate drive signals Sdg 1  through Sdg 4 , which are supplied from the gate drive circuit  38  to the gates G 11  through G 14 , the first switching elements  60   a  through  60   d  are selectively turned on and off, so as to allocate the photoelectrons generated by the photodetector  50  to any one of the first through fourth photoelectron storage units  52   a  through  52   d . For example, when the first switching element  60   a  is turned on, photoelectrons generated by the photodetector  50  are supplied to the first photoelectron storage unit  52   a . As described later, in the event that all the first switching elements  60   a  through  60   d  are turned off, the photoelectrons generated by the photodetector  50  are discharged from the photoelectron discharger  54 . 
         [0047]    When the first switching element  60   a  is turned on, the capacitor  62   a  stores the photoelectrons generated by the photodetector  50 . Similarly, when the second through fourth switching elements  60   b  through  60   d  are turned on, the capacitors  62   b  through  62   d  store the photoelectrons generated by the photodetector  50 . 
         [0048]    When the gate G 31  of the second switching element  66   a  is turned on, the amplifier  64   a  outputs to the output line  56   a  a stored photoelectron signal Sc depending on the quantity of photoelectrons (photoelectron quantity Q 1 ) [C] stored in the capacitor  62   a . The amplifier  64   a  has a drain D 21 , which is connected to the positive power supply voltage VDD [V] of the first power supply  34 , a gate G 21  connected to the capacitor  62   a , and a source S 21 , which is connected to the output line  56   a  through the second switching element  66   a . The amplifier  64   a  outputs the stored photoelectron signal Sc having a voltage (voltage V 1 ) [V] that depends on the photoelectron quantity Q 1 . 
         [0049]    Similarly, when gates G 32  through G 34  of the second switching elements  66   b  through  66   d  are turned on, the amplifiers  64   b  through  64   d  output to the output lines  56   a ,  56   b  stored photoelectron signals Sc, which depend on quantities of photoelectrons (photoelectron quantities Q 2  through Q 4 ) [C] stored respectively in the capacitors  62   b  through  62   d . The amplifiers  64   b  through  64   d  have respective sources S 22  through S 24 , which are connected to a negative power supply voltage VSS [V] of the first power supply  34  through the second switching elements  66   b  through  66   d  and the output lines  56   a ,  56   b , respective drains D 22  through d 24 , which are connected to the positive power supply voltage VDD [V] of the first power supply  34 , and respective gates G 22  through G 24  connected to the capacitor  62   a . The amplifiers  64   b  through  64   d  output the stored photoelectron signals Sc having voltages (voltages V 2  through V 4 ) [V] that depend on the photoelectron quantities Q 2  through Q 4 . 
         [0050]    The second switching elements  66   a  through  66   d  selectively supply the voltages V 1  through V 4  from the amplifiers  64   a  through  64   d  to the output lines  56   a ,  56   b . More specifically, the second switching elements  66   a  through  66   d  comprise, respectively, sources S 31  through S 34  connected to the output lines  56   a ,  56   b , drains D 31  through D 34  connected respectively to the sources S 21  through S 24  of the amplifiers  64   a  through  64   d , and gates G 31  through G 34  connected to the vertical selecting circuit  40 . When the vertical selecting circuit  40  selectively supplies gate drive signals (read signals Sread 1  through Sread 4 ) respectively to the gates G 31  through G 34 , i.e., when the vertical selecting circuit  40  selectively applies high-level voltages to the gates G 31  through G 34 , the gates G 31  through G 34  are selectively turned on, thereby outputting selectively the stored photoelectron signals Sc from the capacitors  62   a  through  62   d  to the output lines  56   a ,  56   b . The stored photoelectron signals Sc supplied to the output lines  56   a ,  56   b  are then supplied to the arithmetic device  17  via the sample and hold circuit  41 , the horizontal selecting circuit  42 , the output buffer  43 , and the A/D converter  44 . 
         [0051]    Conversely, when the vertical selecting circuit  40  does not supply the read signals Sread 1  through Sread 4  to the respective gates G 31  through G 34  (i.e., when the vertical selecting circuit  40  applies low-level voltages to the gates G 31  through G 34 ), the gates G 31  through G 34  are not turned on (i.e., current does not flow from the drains D 21  through D 24  to the sources S 21  through S 24  of the amplifiers  64   a  through  64   d ) and the stored photoelectron signals Sc are not output from the capacitors  62   a  through  62   d  to the output lines  56   a ,  56   b.    
         [0052]    The third switching elements  68   a  through  68   d  reset the photoelectron quantities Q 1  through Q 4  that are stored in the capacitors  62   a  through  62   d . More specifically, the third switching elements  68   a  through  68   d  include respective sources S 41  through S 44 , which are connected respectively to the capacitors  62   a  through  62   d , respective drains D 41  through D 44 , which are connected respectively to the positive reference voltage Vref [V] of the second power supply  36 , and respective gates G 41  through G 44  connected to the gate drive circuit  38 . When the gate drive circuit  38  selectively or simultaneously applies gate drive signals (resetting signals Sreset 1  through Sreset 4 ) to the respective gates G 41  through G 44 , the third switching elements  68   a  through  68   d  are selectively or simultaneously turned on, thereby transferring photoelectrons stored in the capacitors  62   a  through  62   d  to the second power supply  36  and resetting the photoelectron quantities Q 1  through Q 4  stored in the capacitors  62   a  through  62   d.    
         [0000]    (iii) Photoelectron Discharger  54 : 
         [0053]    The photoelectron discharger  54  comprises a fourth switching element  70  for discharging photoelectrons. The fourth switching element  70  discharges photoelectrons when all of the first switching elements  60   a  through  60   d  are turned off, i.e., when the photoelectrons generated by the photodetector  50  are not allocated to the first through fourth photoelectron storage units  52   a  through  52   d . The fourth switching element  70  includes a source S 5 , which is connected to the photodetector  50 , a drain D 5 , which is connected to the positive power supply voltage VDD [V] of the first power supply  34 , and a gate G 5  connected to the gate drive circuit  38 . When the gate drive circuit  38  applies a gate drive signal (photoelectron discharge signal Sde) to the gate G 5 , (i.e., when the gate drive circuit  38  applies a high-level voltage to the gate G 5 ), the gate G 5  is turned on, thereby discharging the photoelectrons generated by the photodetector  50  without allocated the photoelectrons with respect to the first through fourth photoelectron storage units  52   a  through  52   d . Therefore, the first through fourth photoelectron storage units  52   a  through  52   d  are supplied selectively with the photoelectrons, which are generated by the photodetector  50 , only during a period in which the gates G 11  through G 14  are turned on. As a result, the distance D between the rangefinding system  10  and the object W can be measured according to the process described hereinbelow. 
       (iv) Structural Example of Pixel  32 : 
       [0054]      FIG. 4  is a partial vertical cross-sectional view of the pixel  32 . As shown in  FIG. 4 , the pixel  32  comprises a P-type substrate  80 , a P-type first semiconductor region  82 , an N-type second semiconductor region  84 , an N-type third semiconductor region  86 , a transfer gate  88  in the form of a polysilicon layer, and a light shield  90 . 
         [0055]    The entire upper surface of the P-type first semiconductor region  82  is exposed outwardly for receiving reflected light Lr and ambient light Ls. 
         [0056]    According to the present invention, the P-type first semiconductor region  82  and the N-type second semiconductor region  84  jointly make up the photodetector  50 . As described above, the photodetector  50  comprises a pinned photodiode. The substrate  80 , the second semiconductor region  84 , the third semiconductor region  86 , and the transfer gate  88  jointly make up the first switching element  60   a . The third semiconductor region  86  also functions as a floating diffusion and constitutes the capacitor  62   a.    
         [0057]    The light shield  90  is disposed in covering relation to the upper surface of the third semiconductor region  86  and the transfer gate  88 , for thereby blocking and preventing reflected light Lr and ambient light Ls from entering into the third semiconductor region  86  and the transfer gate  88 . 
       2. Process of Measuring the Distance D Between the Rangefinding System  10  and the Object W: 
       [0058]    A process for measuring the distance D between the rangefinding system  10  and the object W will be described below. 
       (1) Measuring Cycle Cm: 
       [0059]    As shown in  FIG. 5 , while the rangefinding system  10  is in operation, each measuring cycle Cm (i.e., a period for determining a measured value) [count/s] comprises a cumulative photoelectron storage period Tca 1  [s], during which photoelectrons are stored cumulatively in the first through fourth capacitors  62   a  through  62   d , and a readout period Tr [s], during which the photoelectrons stored in the first through fourth capacitors  62   a  through  62   d  are read out. The cumulative photoelectron storage period Tca 1  comprises a plurality of photoelectron storage periods Tca 2 , each of which represents a period during which a process (i.e., a photoelectron storing process) of exposing the pixel  32  to pulsed light Lp is carried out, in order to store photoelectrons in the first through fourth capacitors  62   a  through  62   d . In the present embodiment, each of the cumulative photoelectron storage period Tca 1  and the readout period Tr is 10 milliseconds, whereas each of the photoelectron storage periods Tca 2  is 100 microseconds. In each of the photoelectron storage periods Tca 2 , pulsed light Lp is emitted for an output time (pulse duration) of 100 nanoseconds. Therefore, the light emitter  18  is energized at a duty ratio of 0.1 percent during each of the photoelectron storage periods Tca 2 . 
         [0060]    Since the rangefinding system  10  is capable of outputting measured results as a three-dimensional image, each measuring cycle Cm can also be defined as a frame rate [frames/s] of such a three-dimensional image. 
         [0061]    In the present embodiment, the rangefinding system  10  performs 100 photoelectron storing processes during the cumulative photoelectron storage period Tca 1 , and measures the round trip time ΔP and the distance D based on the photoelectron quantities Q 1  through Q 4 , which as a result of the photoelectron storing processes, are stored in the capacitors  62   a  through  62   d.    
       (2) Summary of the Measuring Process (One Photoelectron Storage Period Tca 2 ): 
       [0062]    In the present embodiment, the rangefinding system  10  measures the round trip time ΔP and the distance D based on the photoelectron quantities Q 1  through Q 4 , which are stored in the capacitors  62   a  through  62   d  during the entire cumulative photoelectron storage period Tca 1 . To facilitate understanding of the present invention, however, an explanation shall be made in which the rangefinding system  10  measures the round trip time ΔP and the distance D based on the photoelectron quantities Q 1  through Q 4 , which are stored in the capacitors  62   a  through  62   d  during one photoelectron storage period Tca 2 . 
         [0063]      FIG. 6  is a timing chart of the emitted light Le, the reflected light Lr, the gates G 11  through G 14  of the first switching elements  60   a  through  60   d , and the gate G 5  of the fourth switching element  70 , during the photoelectron storage period Tca 2 . 
         [0064]    According to the present embodiment, as shall be described in detail later, the rangefinding system  10  measures the distance D based on the fact that, if the reflected light Lr has a constant intensity Ir [W], then a period during which reflected light Lr is exposed to the photodetector  50  (a reflected light incident period Pri [s]) is proportional to the cumulative light energy of reflected light Lr during the reflected light incident period Pri (measured reflected light energy Amr [J]). 
         [0065]    More specifically, the rangefinding system  10  determines cumulative light energy (ambient light reference energy Ars) [J] (equivalent to the photoelectron quantity Q 1  of the capacitor  62   a ) during a period P 1 , as a first reference period during which only ambient light Ls is exposed to the photodetector  50 . Further, the rangefinding system  10  determines cumulative light energy (combined light reference energy Ari) [J] (equivalent to the photoelectron quantity Q 3  of the capacitor  62   c ) during a period P 3  (=P 1 ), as a second reference period during which both ambient light Ls and reflected light Lr are exposed to the photodetector  50 . Additionally, the rangefinding system  10  determines cumulative light energy (measured ambient light energy Ams) [J] (equivalent to the photoelectron quantity Q 2  of the capacitor  62   a ) during a period P 2 , as a first measurement period during which only ambient light Ls is exposed to the photodetector  50 . Further, the rangefinding system  10  determines cumulative light energy (measured combined light energy Ami) [J] (equivalent to the photoelectron quantity Q 3  of the capacitor  62   c ) during a period P 4  (=P 2 ), as a second measurement period during which both ambient light Ls and reflected light Lr are exposed to the photodetector  50 . The period P 4  comprises a period (period Psr [s]) during which both ambient light Ls and reflected light Lr are exposed, together with a period (period Ps [s]) during which only ambient light Ls is exposed. The period Psr is proportional to the distance D up to the object W. 
         [0066]    A ratio of the difference between the combined light reference energy An and the ambient light reference energy Ars (reflected light reference energy Arr [J]) to the difference between the measured combined light energy Ami and the measured ambient light energy Ams (measured reflected light energy Amr [J]) (Ari−Ars:Ami−Ams) is equal to a ratio of the period P 3  (=P 1 ) to the reflected light incident period Pri (P 3 :Pri). Based on such equal ratios, the rangefinding system  10  determines a period (round trip time ΔP) during which pulsed light Lp emitted from the rangefinding system  10  impinges upon the object W and then travels back to the rangefinding system  10 . Then, based on the round trip time ΔP, the rangefinding system  10  determines the distance D between the rangefinding system  10  and the object W. 
       (3) Details of the Measuring Process (One Photoelectron Storage Period Tca 2 ): 
     (a) Explanation of Timing Chart: 
       [0067]    In  FIG. 6 , time Teu represents a time at which emission of emitted light Le is started, whereas time Ted represents a time at which emission of the emitted light Le is stopped. The period Pe represents a period from time Teu to time Ted. Time Tru represents a time at which exposure of reflected light Lr to the photodetector  50  is started, whereas time Trd represents a time at which exposure of the reflected light Lr to the photodetector  50  is stopped. The period Pr represents a period from time Tru to time Trd. 
         [0068]    Times Tg 1   u , Tg 2   u , Tg 3   u , and Tg 4   u  represent times at which the gates G 11  through G 14  of the first switching elements  60   a  through  60   d  are opened, whereas times Tg 1   d , Tg 2   d , Tg 3   d , and Tg 4   d  represent times at which the gates G 11  through G 14  are closed. The period P 1  represents a period from time Tg 1   u  to time Tg 1   d . The period P 2  represents a period from time Tg 2   u  to time Tg 2   d . The period P 3  represents a period from time Tg 3   u  to time Tg 3   d . The period P 4  represents a period from time Tg 4   u  to time Tg 4   d . The period Psr represents a period from time Tg 4   u  to time Trd, and the period Ps represents a period from time Trd to time Tg 4   d.    
         [0069]    Times Td 1   u  and Td 2   u  represent times at which the gate G 5  of the fourth switching element  70  is opened, whereas times Td 1   d  and Td 2   d  represent times at which the gate G 5  is closed. The period P 5  represents a period from time Td 1   u  to time Td 1   d . The period P 6  represents a period from time Td 2   u  to time Td 2   d.    
         [0070]    The period Pr, during which reflected light Lr is exposed to the photodetector  50 , is equal to the period Pe (Pe=Pr), although there is a delay from time Teu to time Tru or from time Ted to time Trd (round trip time ΔP). The period Pr may be set to a value in a range from  10  nanoseconds to 1 microsecond. In the present embodiment, the period Pr is 100 nanoseconds. In the control device  16 , the period P 1  and the period P 3  are set equal to each other, and the period P 2  and the period P 4  also are set equal to each other (P 1 =P 3 , P 2 =P 4 ). The periods P 1  and P 3  can be set to values in a range from 10 nanoseconds to 90 nanoseconds, for example. In the present embodiment, the periods P 1  and P 3  are 30 nanoseconds. The period P 2  can be set to a value in a range from 10 nanoseconds to 90 nanoseconds, for example. In the present embodiment, the period P 2  is 70 nanoseconds. The period P 5  can be set to a value in a range from 0 seconds to 90 nanoseconds, for example. In the present embodiment, the period P 5  is 70 nanoseconds. The period P 6  can be set to a value in a range from 10 microseconds to 1 millisecond. In the present embodiment, the period P 6  is about 100 microseconds. Therefore, among the periods P 1  through P 6 , the period P 6  is considerably long. 
         [0071]    As can be seen from  FIG. 6 , during the photoelectron storage period Tca 2  [s] of the rangefinding system  10 , the gate G 11  is opened for the period P 1 , and simultaneously with closing of the gate G 11 , the gate g 12  is opened and remains open for the period P 2 . Simultaneously with closing of the gate G 12 , emitted light Le is emitted toward the object W, and the gate G 5  remains open for the period P 5 . While emitted light Le is output during the period Pe, reflected light Lr starts to be exposed to the photodetector  50  at time Tru. After elapse of the period Pe from start of emission of the emitted light Le at time Teu, the gate G 5  is closed and the gate G 13  is opened. The gate G 13  remains open for the period P 3 . After elapse of the period Pe, emission of the emitted light L 3  is stopped, the gate G 13  is closed, and the gate G 14  is opened, whereupon the gate G 14  remains open for the period P 4 . While the gate G 14  is open during the period P 4 , reflected light Lr stops being exposed to the photodetector  50  at time Trd. Stated otherwise, the period P 4  determines the measurement range [m] (range of distances that can be measured) of the rangefinding system  10 . Simultaneously with closing of the gate G 14 , the gate G 5  is opened and remains open for the period P 6 . After elapse of the period P 6 , the gate G 5  is closed, whereupon one photoelectron storage period Tca 2  is completed at time Td 2   d . At the same time, a subsequent photoelectron storage period Tca 2  is initiated, whereupon the gate G 11  is opened at time Tg 1   u . The control device  16  controls the respective components of the light-emitting device  12  and the light-detecting device  14 . Preferably, the control device  16  is fabricated by a CMOS process on the same silicon substrate as the light detector  22 . 
       (b) Explanation of Measurement Principles: 
     (i) Calculation of Reflected Light Reference Energy Arr: 
       [0072]    If the rangefinding system  10  and the object W are fixed in respective positions, then reflective light Lr, which is reflected by the object W and returned to the rangefinding system  10 , can be regarded as having a constant intensity (light energy per unit time). Since the period P 1  is set to a period during which only ambient light Ls is exposed to the photodetector  50 , the capacitor  62   a  of the first photoelectron storage unit  52   a  stores photoelectrons that are generated only due to ambient light Ls. Since the period P 3  is set to a period during which both ambient light Ls and reflected light Lr are exposed to the photodetector  50 , the capacitor  62   c  of the third photoelectron storage unit  52   c  stores photoelectrons that are generated due to both ambient light Ls and reflected light Lr. The lengths of the period P 1  and the period P 3  are the same. 
         [0073]    Therefore, the difference between the photoelectron quantity Q 3  stored in the capacitor  62   c  and the photoelectron quantity Q 1  stored in the capacitor  62   a  is representative of the cumulative energy (reflected light reference energy Arr) of reflected light Lr during the period P 3  (=period P 1 ). 
       (ii) Calculation of Measured Reflected Light Energy Amr and Round Trip Time ΔP: 
       [0074]    If the rangefinding system  10  and the object W are located in respective fixed positions, then reflective light Lr, which is reflected by the object W and returned to the rangefinding system  10 , can be regarded as having a constant intensity. Since the period P 1  is set to a period during which only ambient light Ls is exposed to the photodetector  50 , the capacitor  62   b  of the second photoelectron storage unit  52   b  stores photoelectrons that are generated only from ambient light Ls. Since the period P 4  is set to a period including both a period (period Psr) during which ambient light Ls and reflected light Lr are exposed to the photodetector  50 , together with a period (Ps) during which only ambient light Ls is exposed to the photodetector  50 , the capacitor  62   d  of the fourth photoelectron storage unit  52   d  stores photoelectrons generated both from ambient light Ls and from reflected light Lr. The lengths of the period P 2  and the period P 4  are the same. 
         [0075]    Therefore, the difference between the photoelectron quantity Q 4  stored in the capacitor  62   d  and the photoelectron quantity Q 2  stored in the capacitor  62   b  represents cumulative energy (measured reflected light energy Amr) of reflected light Lr during the period P 3  (=period P 2 ). In the present embodiment, the period P 4  starts at time Ted, at which emission of the emitted light Le is stopped. Consequently, during the period P 4 , reflected light Lr corresponding to the pulse round trip time ΔP is exposed to the photodetector  50 , whereupon photoelectrons are stored in the capacitor  62   d . Therefore, the photoelectron quantity Q 4  stored in the capacitor  62   d  corresponds to the sum of the cumulative energy (measured ambient light energy Ams) of ambient light Ls during the entire period P 4 , together with the cumulative energy (measured reflected light energy Amr) of reflected light Lr during the round trip time ΔP. Thus, the difference between the photoelectron quantity Q 4  and the photoelectron quantity Q 2  represents a photoelectron quantity that corresponds to the measured reflected light energy Amr. The round trip time ΔP depends on the distance D between the rangefinding system  10  and the object W. Accordingly, a ratio of the measured reflected light energy Amr (corresponding to the difference between the photoelectron quantity Q 4  and the photoelectron quantity Q 2 ) to the reflected light reference energy Arr (corresponding to the difference between the photoelectron quantity Q 3  and the photoelectron quantity Q 1 ) is equal to a ratio of the round trip time ΔP to the period P 3  (=period P 1 ) (Amr:Arr=Q 4 −Q 2 :Q 3 −Q 1 =ΔP:P 3 ). The round trip time ΔP can therefore be calculated by the following equation (1): 
         [0000]      Δ P={ ( Q 4 −Q 2)/( Q 3 −Q 1))× P 3
 
       (III) Calculation of Distance D: 
       [0076]    Once the round trip time ΔP is known, the distance D between the rangefinding system  10  and the object W can be calculated by equation (2) below. In equation (2), c represents a constant indicative of the velocity of light (about 300,000 kilometers per second). The product c×ΔP is divided by 2 due to the fact that the pulsed light Lp reciprocates between the rangefinding system  10  and the object W, and hence travels a distance that is twice the distance D. 
         [0000]        D=c×ΔP/ 2 
       (iv) Others: 
       [0077]    The pixel  32  is initialized (reset) by the following process. In response to the resetting signals Sreset 1  through Sreset 4  being applied to the gates G 41  through G 44  (i.e., in response to high-level voltages being applied to the gates G 41  through G 44 ), the third switching elements  68   a  through  68   d  are turned on simultaneously. Further, simultaneously therewith, in response to the photoelectron discharge signal Sde being applied to the gate G 5  (i.e., in response to a high-level voltage applied to the gate G 5 ), the fourth switching element  70  is turned on. At this time, the gate drive signals Sdg 1  through Sdg 4  are not applied to the gates G 11  through G 14  (i.e., low-level voltages are applied to the gates G 11  through G 14 ), so that the first switching elements  60   a  through  60   d  remain turned off. The capacitors  62   a  through  62   d  are thus set to the reference voltage Vref. Thereafter, the resetting signals Sreset 1  through Sreset 4  stop being applied to the gates G 41  through G 44  (i.e., low-level voltages are applied to the gates G 41  through G 44 ), thereby setting the capacitors  62   a  through  62   d  to the reference voltage Vref. Subsequently, the measuring process shown in  FIG. 6  is performed. 
       (4) Details of the Measuring Process (Cumulative Photoelectron Storage Period Tca 1 ). 
       [0078]    In items (2) and (3) described above, a measuring process has been described for one photoelectron storage period Tca 2 . According to the present embodiment, the rangefinding system  10  calculates the round trip time ΔP in the same manner described above, using the photoelectron quantities Q 1  through Q 4  (hereinafter referred to as “photoelectron quantities Q 1   a  through Q 4   a ”), which are stored in the capacitors  62   a  through  62   d  during 100 photoelectron storage periods Tca 2  (cumulative photoelectron storage period Tca 1 ). 
         [0079]    The photoelectron quantity Q 1   a  represents the sum of the photoelectron quantities Q 1  that are stored in the capacitor  62   a  during the first through 100th photoelectron storage periods Tca 2 . Similarly, the photoelectron quantities Q 2   a  through Q 4   a  represent the sums of the photoelectron quantities Q 2  through Q 4 , which are stored in the capacitors  62   b ,  62   c ,  62   d  during the first through 100th photoelectron storage periods Tca 2 . 
       (5) Others: 
       [0080]    According to the present embodiment, the rangefinding system  10  measures the distance D using the photoelectron quantities Q 1  through Q 4  (photoelectron information) stored in each of the pixels  32 . The rangefinding system  10  can obtain a three-dimensional image by combining the distance information from the pixels  32 . 
       3. Advantages of the Present Embodiment: 
       [0081]    According to the present embodiment, the rangefinding system  10  exhibits an increased dynamic range and is capable of reducing or eliminating the effect of ambient light Ls. As a consequence, the measurement accuracy of the rangefinding system  10  is increased. 
         [0082]    More specifically, according to the present embodiment, the rangefinding system  10  determines the photoelectron quantity Q 1  stored during the period P 1 , in which only ambient light Ls is exposed to the photodetector  50 , and the photoelectron quantity Q 3  stored during the period P 3 , during which both ambient light Ls and reflected light Lr are exposed to the photodetector  50 . Since the period P 1  and the period P 3  are set to the same length, the rangefinding system  10  is capable of determining a photoelectron quantity corresponding to the reflected light Lr during the period P 3  (i.e., corresponding to the reference energy Arr of the reflected light Lr during the period P 3 ) from the difference between the photoelectron quantity Q 3  and the photoelectron quantity Q 1  (Q 3 −Q 1 ). 
         [0083]    The rangefinding system  10  also determines the photoelectron quantity Q 2  stored during the period P 2 , in which only ambient light Ls is exposed to the photodetector  50 , and the photoelectron quantity Q 4  stored during the period P 4 . Period P 4  comprises a period (period Psr) during which both ambient light Ls and reflected light Lr are exposed to the photodetector  50 , as well as a period (period Ps) during which only ambient light Ls is exposed to the photodetector  50 . Since the period P 2  and the period P 4  are set to the same length, the rangefinding system  10  can determine the photoelectron quantity corresponding to the period Psr that occurs within the period P 4  (corresponding to the energy of reflected light Lr in the period Psr) from the difference between the photoelectron quantity Q 4  and the photoelectron quantity Q 2  (Q 4 −Q 2 ). 
         [0084]    If the intensity Ir of reflected light Lr is constant while the reflected light Lr is exposed to the photodetector  50 , then a ratio of the difference between the photoelectron quantity Q 4  and the photoelectron quantity Q 2  to the difference between the photoelectron quantity Q 3  and the photoelectron quantity Q 1  (Q 4 −Q 2 :Q 3 −Q 1 ) becomes equal to a ratio of the period Psr to the period P 3  (Psr:P 3 ). Therefore, the period Psr can be determined by the following equation (3): 
         [0000]        Psr ={( Q 4 −Q 2)/( Q 3 −Q 1)}× P 3   (3)
 
         [0085]    Since time Ted and time Tg 4   u  are equal to each other, the period Psr is equal to the round trip time ΔP. Therefore, the period Psr can be determined by the above equation (3), and hence, the distance D can be determined based on the round trip time ΔP and the velocity of light. 
         [0086]    Since the rangefinding system  10  removes the photoelectron quantity Q 2 , which is generated by the ambient light Ls, the rangefinding system  10  is capable of eliminating or reducing the effect of ambient light Ls. 
         [0087]    As the distance D becomes smaller, the period during which reflected light Lr is exposed (i.e., the period Psr) is made shorter, whereas, as the distance D becomes greater, the period Psr is made longer. Generally, assuming the same object W is involved, as the distance D becomes smaller, the intensity Ir of the reflected light Lr reflected from the object becomes greater, whereas, as the distance D becomes greater, the intensity Ir of the reflected light Lr reflected from the object becomes smaller. Consequently, if the distance D is comparatively small, then reflected light Lr of a greater intensity Ir is exposed over a shorter period. On the other hand, if the distance D is comparatively great, then reflected light Lr of a smaller intensity Ir is exposed over a longer period. As a result, a change in the energy Ar of reflected light Lr, which is exposed during the period Psr, is small compared with the change in the distance D. This implies that it is possible to narrow the dynamic range of the rangefinding system  10 , and therefore, the dynamic range of the rangefinding system  10  can be increased. 
         [0088]    According to the present embodiment, pulsed light Lp is emitted 100 times during each measuring cycle Cm, and the round trip time ΔP is calculated using the photoelectron quantities Q 1  through Q 4 , which are stored 100 times in the capacitors  62   a  through  62   d . Generally, the intensity of ambient light Ls (e.g., sunlight) varies at all times. By emitting the pulsed light Lp 100 times during each measuring cycle Cm, and calculating the round trip time ΔP using the photoelectron quantities Q 1  through Q 4 , which are stored as many times as the number of times that the pulsed light Lp is emitted, the intensity of the ambient light Ls can be averaged. As a consequence, the accuracy at which the photoelectron quantity generated by ambient light Ls is removed can be increased, thereby increasing measurement accuracy. 
         [0089]    According to the present embodiment, the pulsed light Lp has a pulse duration (output period) of 10 microseconds (=100 nanometers×100), which is 0.05 percent of each measuring cycle Cm (20 milliseconds). Commensurate with such a pulse duration, the periods P 1  through P 4  during which the gates G 11  through G 14  are opened also are shortened. Therefore, even if another rangefinding system, which uses pulsed light having the same frequency, is present in the vicinity of the rangefinding system  10 , the possibility that the timing at which the other rangefinding system outputs pulsed light and the timing at which the rangefinding system  10  outputs pulsed light Lp will overlap with each other is low. As a result, the possibility of interference with the other rangefinding system (i.e., that pulsed light from the other rangefinding system will be falsely recognized as constituting pulsed light Lp from the rangefinding system  10 ) is low. 
         [0090]    In addition, since the periods P 1  through P 4 , during which times the gates G 11  through G 14  are opened, also are shortened, the lengths of time during which ambient light Ls is exposed to the photodetector  50  within the periods P 1  through P 4  are shortened. Thus, the effect that ambient light Ls has as a noise component is reduced, thereby increasing the signal-to-noise ratio (S/N). More particularly, if such ambient light Ls is sunlight, then shot noise caused by the sunlight is reduced. 
         [0091]    According to the present embodiment, since the periods P 1  through P 4 , during which the gates G 11  through G 14  are opened, are very short compared with the measuring cycle Cm, the possibility of a phenomenon (i.e., aliasing) in which pulsed light Lp emitted in a previous measuring cycle Cm is detected in the current measuring cycle Cm can be reduced. More specifically, according to the present embodiment, inasmuch as each photoelectron storage period Tca 2  is 100 microseconds, and the period Pe during which pulsed light Lp is emitted is short, such pulsed light Lp is emitted at intervals of about 100 microseconds. Since the velocity c of light is about 300,000 kilometers per second, a possibility for aliasing to occur exists only if the actual position of the object W is 15 kilometers (=100 μs]×30 [Mm/s]/2) farther than the distance D output by the rangefinding system  10 . However, because the intensity of pulsed light Lp, which is exposed to the object W by the light emitter  18 , is reduced in proportion to the square of the distance D, the intensity Ir of reflected light Lr from a position that is 15 kilometers farther than the distance D is very small compared to the intensity Ir of reflected light Lr from the object W at the distance D, thus making it almost impossible for the photodetector  50  to detect the reflected light Lr. Accordingly, the rangefinding system  10  according to the present embodiment can prevent aliasing from occurring. 
       B. Modifications 
       [0092]    The present invention is not limited to the above embodiment, but various modified arrangements may be adopted based on the details of the above description. For example, the present invention may adopt the following arrangements described below. 
         [0093]    In the above embodiment, the gates G 11  through G 14  are controlled according to the timing chart shown in  FIG. 6 . However, the present invention is not limited to the timing chart shown in  FIG. 6 . For example, the periods P 3 , P 4  in  FIG. 6  may be positioned ahead of the periods P 1 , P 2 . Furthermore, although time Tg 1   d  and time Tg 2   u  occur simultaneously in  FIG. 6 , time Tg 2   u  may occur subsequently to time Tg 1   d . The relationship between time Tg 2   d  and time Teu, as well as the relationship between time Tg 3   d  and time Tg 4   u , may be changed in a similar manner. Moreover, time Tg 4   u  may occur non-simultaneously with time Ted, if the correlation thereof with time Ted is known. 
         [0094]      FIG. 7  is a timing chart in which time Ted occurs subsequently to time Tg 4   u . In this case, the round trip time ΔP is calculated by the following equation (4): 
         [0000]      Δ P=[ ( Q 4 −Q 2)/( Q 3 −Q 1)]× P 3−( Ted−Tg 4 u )   (4)
 
         [0095]    Alternatively, time Ted may occur in advance of time Tg 4   u . In this case, the round trip time ΔP is calculated by the following equation (5): 
         [0000]      Δ P=[ ( Q 4 −Q 2)/( Q 3 −Q 1)]× P 3+( Tg 4 u−Ted )   (5)
 
         [0096]    In the above embodiment, the periods P 1  and P 2  are provided in order to eliminate or reduce the effect of ambient light Ls. However, if there is no ambient light Ls, for example in a dark room or the like, or if the ambient light Ls is small compared with the reflected light Lr and thus has little effect, then the round trip time ΔP can be determined only from the periods P 3  and P 4 . More specifically, the round trip time ΔP is calculated by the following equation (6): 
         [0000]      Δ P =( Q 4 /Q 3)× P 3   (6)
 
         [0097]    In the above embodiment, the photodetector  50  comprises a pinned photodiode ( FIG. 4 ). Alternatively, the photodetector  50  may comprise another type of photodetector, such as a photodiode, a photogate, or the like, rather than the pinned photodiode. 
         [0098]      FIG. 8  is a partial vertical cross-sectional view of a pixel  32   a , which incorporates therein a photogate, which serves as a photodetector. As shown in  FIG. 8 , the pixel  32   a  comprises a P-type substrate  100 , a photogate  102 , an N-type semiconductor region  104 , a transfer gate  106  in the form of a polysilicon layer, and a light shield  108 . 
         [0099]    In the pixel  32   a , the photogate  102  makes up a photodetector  50   a . The substrate  100 , the photogate  102 , the semiconductor region  104 , and the transfer gate  106  jointly make up a first switching element  60   a   1 . The semiconductor region  104  also functions as a floating diffusion, and constitutes a capacitor  62   a   1 . The pixel  32   a  includes first switching elements  60   b   1  through  60   d   1 , which are identical in structure to the first switching element  60   a   1 , and capacitors  62   b   1  through  62   d   1 , which are identical in structure to the capacitor  62   a   1 . 
         [0100]    The light shield  108  is disposed in covering relation to the semiconductor region  104  and the transfer gate  106 , for thereby blocking the reflected light Lr and the ambient light Ls from entering into the semiconductor region  104  and the transfer gate  106 . 
         [0101]      FIG. 9  is a timing chart of the emitted light Le, the reflected light Lr, the gates G 11  through G 14  of the first switching elements  60   a   1  through  60   d   1 , the gate G 5  of the fourth switching element  70 , and the photogate  102 , for measuring the distance D using the pixel  32   a.    
         [0102]    The timing chart shown in  FIG. 9  is the same as the timing chart shown in  FIG. 6 , except for the timing of the photogate  102 . In  FIG. 9 , the photogate  102  is opened at times Tp 1   u  through Tp 6   u , while the photogate  102  is closed at times Tp 1   d  through Tp 6   d  within the periods P 1  through P 6 . The photogate  102  is closed by a period Pp earlier than the times Tg 1   d  through Tg 4   d , Td 1   d , and Td 2   d , at which the periods P 1  through P 6  terminate. Therefore, photoelectrons, which are photoelectrically generated by the photogate  102 , can quickly be transferred to the semiconductor region  104  without remaining inside the photogate  102 .