Patent Publication Number: US-11391824-B2

Title: Distance measuring device and distance measuring method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2018-173543, filed on Sep. 18, 2018 the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments of the present invention relate to a distance measuring device and a distance measuring method. 
     BACKGROUND 
     There is known a distance measuring device called LIDAR (Light Detection and Ranging, Laser Imaging Detection and Ranging). The distance measuring device irradiates laser light on a measurement target object and converts the intensity of reflected light reflected by the measurement target object into a time-series measurement signal on the basis of an output of a sensor. Consequently, the distance to the measurement target object is measured on the basis of a time difference between a point in time of emission of the laser light and a point in time corresponding to a peak of a signal value of the measurement signal. 
     However, when the number of input photons per unit time to a sensor increases, an output signal value of the sensor is sometimes saturated. Measurement accuracy is likely to be deteriorated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a schematic overall configuration of a distance measuring device according to a first embodiment; 
         FIG. 2  is a diagram showing a configuration example of the distance measuring device according to the first embodiment; 
         FIG. 3  is a diagram schematically showing an emission pattern of a light source; 
         FIG. 4  is a schematic diagram enlarging and showing irradiation positions of respective laser beams on a measurement target object; 
         FIG. 5  is a block diagram showing a detailed configuration of a time acquirer and a distance measurer; 
         FIG. 6  is a diagram showing an example of a rising time and a falling time of a measurement signal by a time detector; 
         FIG. 7  is a diagram schematically showing a state in which a measurement signal of a sensor piles up; 
         FIG. 8  is a diagram for explaining a temporal relation between an emission point in time of the light source and conversion into an electric signal by the sensor; 
         FIG. 9  is a diagram showing an output waveform example during one photon detection of an SPAD cell; 
         FIG. 10  is a diagram showing a simulation example of a measurement signal value of the SPAD cell; 
         FIG. 11  is a diagram showing a relation between calculated timing and a rising time and a falling time; 
         FIG. 12  is a diagram showing input and output characteristics of a second amplifier; 
         FIG. 13  is a diagram showing signal values based on different input and output characteristics; 
         FIG. 14  is a diagram showing a measuring method of a CFD as a comparative example; 
         FIG. 15  is a block diagram showing a detailed configuration example of a distance measurement processor according to a second embodiment; 
         FIG. 16  is a diagram showing a maximum value of measurement signal values obtained by changing the number of photons per unit time; 
         FIG. 17  is a diagram of a first weight coefficient calculated for each measurement signal value corresponding to the maximum value; 
         FIG. 18  is a diagram of the first weight coefficient corresponding to the maximum value calculated by changing an intensity value of environment light; 
         FIG. 19  is a diagram of the first weight coefficient corresponding to a corrected maximum value calculated by changing the intensity value of the environment light; 
         FIG. 20  is a block diagram showing a detailed configuration example of a distance measurement processor according to a third embodiment; 
         FIG. 21  is a conceptual diagram for explaining characteristics of a measurement signal according to pulse widths; and 
         FIG. 22  is a diagram for explaining characteristics of functions used in arithmetic expressions corresponding to C and D. 
     
    
    
     DETAILED DESCRIPTION 
     A distance measuring device according to an embodiment includes a time acquisition circuit and a distance measurement circuit. The time acquisition circuit acquires a rising time in which a measurement signal obtained by converting reflected light of a laser beam from an object into a signal reaches a first threshold and a falling time in which the measurement signal reaches a second threshold after reaching the first threshold. The distance measurement circuit measures the distance to a target object on the basis of a time difference between timing based on the rising time and the falling time and irradiation timing of the laser beam. 
     Distance measuring devices and distance measuring methods according to embodiments of the present invention are explained in detail below with reference to the drawings. Note that the embodiments explained below are examples of embodiments of the present invention. The present invention is not interpreted to be limited to the embodiments. In the drawings referred to in the embodiments, the same portions and portions having the same functions are denoted by the same or similar reference numerals and signs. Repeated explanation of the portions is sometimes omitted. Dimension ratios of the drawings are sometimes different from actual ratios for convenience of explanation. A part of components is sometimes omitted from the drawings. 
     First Embodiment 
       FIG. 1  is a diagram showing a schematic overall configuration of a distance measuring device  1  according to an embodiment. As shown in  FIG. 1 , the distance measuring device  1  generates a distance image of a measurement target object  10  using a scanning scheme or a TOF (Time Of Flight) scheme. More specifically, the distance measuring device  1  includes an emitter  100 , an optical mechanism system  200 , a measurer  300 , and an image processor  400 . 
     The emitter  100  intermittently emits laser light L 1 . The optical mechanism system  200  irradiates the laser light L 1  emitted by the emitter  100  on the measurement target object  10  and makes reflected light L 2  of the laser light L 1  reflected on the measurement target object  10  incident on the measurer  300 . The laser light means light having an aligned phase and an aligned frequency. 
     The measurer  300  measures the distance to the measurement target object  10  on the basis of the reflected light L 2  received via the optical mechanism system  200 . That is, the measurer  300  measures the distance to the measurement target object  10  on the basis of a time difference between a point in time when the emitter  100  irradiates the laser light L 1  on the measurement target object  10  and a point in time when the reflected light L 2  is measured. 
     The image processor  400  performs removal of noise, distortion correction, and interpolation processing and outputs final distance image data on the basis of distances to a plurality of measurement points on the measurement target object  10 . The image processor  400  may be incorporated in a housing of the distance measuring device  1 . 
     More detailed configuration examples of the emitter  100 , the mechanism optical mechanism system  200 , and the measurer  300  of the distance measuring device  1  according to the first embodiment are explained with reference to  FIG. 2 .  FIG. 2  is a diagram showing a configuration example of the distance measuring device  1  according to the first embodiment. As shown in  FIG. 2 , the distance measuring device  1  includes the emitter  100 , the optical mechanism system  200 , the measurer  300 , and the image processor  400 . Among scattered lights L 3 , scattered light in a predetermined direction is referred to as reflected light L 2 . 
     The emitter  100  includes a light source  11 , an oscillator  11   a , a first driving circuit  11   b , a controller  16 , and a second driving circuit  16   a.    
     The optical mechanism system  200  includes an irradiation optical system  202  and a light-receiving optical system  204 . The irradiation optical system  202  includes a lens  12 , a first optical element  13 , a lens  13   a , and a mirror (a reflection device)  15 . 
     The light-receiving optical system  204  includes a second optical element  14  and the mirror  15 . That is, the irradiation optical system  202  and the light-receiving optical system  204  share the mirror  15 . 
     The measurer  300  includes a photodetector  17 , a sensor  18 , a lens  18   a , a first amplifier  19 , a second amplifier  20 , a time acquirer  21 , and a distance measurer  22 . Note that, as an existing method for scanning light, there is a method of rotating the distance measuring device  1  to scan light (hereinafter referred to as rotating method). As another existing method for scanning light, there is an OPA method (Optical Phased Array). This embodiment does not rely on a method of scanning light. Therefore, light may be scanned by the rotating method or the OPA method. 
     The oscillator  11   a  of the emitter  100  generates a pulse signal on the basis of control by the controller  16 . The first driving circuit  11   b  drives the light source  11  on the basis of the pulse signal generated by the oscillator  11   a . The light source  11  is a laser light source such as a laser diode. The light source  11  intermittently emits the laser light L 1  according to driving by the first driving circuit  11   b.    
       FIG. 3  is a diagram schematically showing an emission pattern of the light source  11 . In  FIG. 3 , the horizontal axis indicates time and the vertical axis indicates emission timing of the light source  11 . A figure on the lower side is a partially enlarged view in a figure on the upper side. As shown in  FIG. 3 , the light source  11  intermittently repeatedly emits laser light L 1 ( n ) (0≤n&lt;N), for example, at an interval of T=several microseconds to several ten microseconds. The laser light L 1  emitted n-th is represented as L 1 ( n ). For example, “N” indicates the number of times of irradiation of the laser light L 1 ( n ) irradiated to measure the measurement target object  10 . 
     As shown in  FIG. 2 , the light source  11 , the lens  12 , the first optical element  13 , the second optical element  14 , and the mirror  15  are disposed in this order on an optical axis O 1  of the irradiation optical system  202 . Consequently, the lens  12  condenses the intermittently emitted laser light L 1  and guides the laser light L 1  to the first optical element  13 . 
     The first optical element  13  transmits the laser light L 1  and makes a part of the laser light L 1  incident on the photodetector  17  along an optical axis O 3 . The first optical element  13  is, for example, a beam splitter. 
     The second optical element  14  further transmits the laser light L 1  transmitted through the first optical element  13  and makes the laser light L 1  incident on the mirror  15 . The second optical element  14  is, for example, a half mirror. 
     The mirror  15  includes a reflection surface  15   a  that reflects the laser light L 1  intermittently emitted from the light source  11 . The reflection surface  15   a  is capable of rotating around, for example, two rotation axes RA 1  and RA 2  crossing each other. Consequently, the mirror  15  cyclically changes an irradiation direction of the laser light L 1 . 
     The controller  16  includes, for example, a CPU (Central Processing Unit) and a storage that stores a program and executes control by executing the program. The controller  16  performs, on the second driving circuit  16   a , control for continuously changing an inclination angle of the reflection surface  15   a . The second driving circuit  16   a  drives the mirror  15  according to a driving signal supplied from the controller  16 . That is, the controller  16  controls the second driving circuit  16   a  to change the irradiation direction of the laser light L 1 . 
       FIG. 4  is a schematic diagram enlarging and showing irradiation positions of the laser light L 1  on the measurement target object  10 . As shown in  FIG. 4 , the reflection surface  15   a  changes the irradiation direction for each laser light L 1  and discretely irradiates the laser light L 1  along a substantially parallel plurality of linear paths P 1  to Pm (m is a natural number equal to or larger than 2) on the measurement target object  10 . In this way, the distance measuring device  1  according to this embodiment irradiates the laser light L 1 ( n ) (0≤n&lt;N) toward the measurement target object  10  once at a time while changing an irradiation direction O(n) (0≤n&lt;N) of the laser light L 1 ( n ). The irradiation direction of the laser light L 1 ( n ) is represented as  0 ( n ). That is, in the distance measuring device  1  according to this embodiment, the laser light L 1 ( n ) is irradiated once in the irradiation direction O(n). 
     An interval of irradiation positions of laser lights L 1 ( n ) and L 1 ( n +1) on the measurement target object  10  corresponds to the irradiation interval T=several microseconds to several ten microseconds ( FIG. 3 ) between the laser lights L 1 . In this way, the laser lights L 1  having different irradiation directions are discretely irradiated on the linear paths P 1  to Pm. Note that the number of linear paths and a scanning direction are not particularly limited. 
     As shown in  FIG. 2 , on an optical axis O 2  of the light receiving optical system  204 , the reflection surface  15   a  of the mirror  15 , the second optical element  14 , the lens  18   a , and the sensor  18  are disposed in the order of incidence of the reflected light L 2 . The optical axis O 1  is a focal axis of the lens  12  that passes the center position of the lens  12 . The optical axis O 2  is a focal axis of the lens  18   a  that passes the center position of the lens  18   a.    
     The reflection surface  15   a  makes the reflected light L 2  traveling along the optical axis O 2  among the scattered lights L 3  scattered on the measurement target object  10  incident on the second optical element  14 . The second optical element  14  changes a traveling direction of the reflected light L 2  reflected on the reflection surface  15   a  and makes the reflected light L 2  incident on the lens  18   a  of the measurer  300  along the optical axis O 2 . The lens  18   a  condenses the reflected light L 2  made incident along the optical axis O 2  to the sensor  18 . 
     On the other hand, a traveling direction of light reflected in a direction different from the direction of the laser light L 1  among the scattered lights L 3  deviates from the optical axis O 2  of the light-receiving optical system  204 . Therefore, even if the light reflected in the direction different from the direction of the optical axis O 2  among the scattered lights L 3  is made incident in the light-receiving optical system  204 , the light is absorbed by a black body in a housing in which the light-receiving optical system  204  is disposed or is made incident on a position deviating from an incident surface of the sensor  18 . On the other hand, among environment lights such as sunlight scattered by some object, there are lights traveling along the optical axis O 2 . These lights are made incident on the incident surface of the sensor  18  at random and become random noise. 
     Note that, in  FIG. 2 , optical paths of the laser light L 1  and the reflected light L 2  are separately shown for clarification. However, actually, the laser light L 1  and the reflected light L 2  overlap. An optical path in the center of a light beam of the laser light L 1  is shown as the optical axis O 1 . Similarly, an optical path of the center of a light beam of the reflected light L 2  is shown as the optical axis O 2 . 
     The sensor  18  detects the reflected light L 2  made incident from the lens  18   a . The sensor  18  is composed of, for example, a silicon photomultiplier (SiPM). The silicon photomultiplier is a photocounting device obtained by converting an avalanche photo diode (APD) in a Geiger mode into multiple pixels. The silicon photomultiplier is capable of detecting feeble light in a photocounting level. That is, each of light receiving elements configuring the sensor  18  outputs an output signal corresponding to the intensity of light received via the optical mechanism system  200 . The avalanche photodiode used in the Geiger mode is sometimes called SPAD (single-photon avalanche diode). 
     The sensor  18  according to this embodiment is composed of the silicon photomultiplier but is not limited to this. For example, the sensor  18  may be configured by disposing a plurality of photodiodes, avalanche breakdown diodes (ABDs), or the like. The photodiode is composed of, for example, a semiconductor functioning as a photodetector. The avalanche diode is a diode that causes avalanche breakdown at a specific inverse voltage to thereby increase light reception sensitivity. 
     The second amplifier  20  is, for example, a transimpedance amplifier. The second amplifier  20  amplifies an electric signal based on the reflected light L 2 . The second amplifier  20  amplifies and converts, for example, a current signal of the sensor  18  into a voltage signal serving as a measurement signal. 
     The time acquirer  21  acquires a rising time in which a measurement signal obtained by converting reflected light of the laser beam L 1  into a signal reaches a first threshold and a falling time in which the measurement signal reaches a second threshold after reaching the first threshold. 
     The distance measurer  22  measures the distance to the measurement target object  10  on the basis of a time difference between timing based on a first time obtained by weighting the rising time acquired by the time acquirer  21  with a first weight coefficient and a second time obtained by weighting the falling time acquired by the time acquirer  21  with a second weight coefficient and irradiation timing of the laser beam L 1 . 
       FIG. 5  is a block diagram showing a detailed configuration of the time acquirer  21  and the distance measurer  22 . As shown in  FIG. 5 , the time acquirer  21  includes a rising detector  21   a , a first TDC  21   b , a falling detector  21   c , and a second TDC  21   d . The distance measurer  22  includes a distance measurement processor  22   a . The block diagram of  FIG. 5  shows an example of signals. Order of the signals and wiring for the signals are not limited to order and wiring shown in  FIG. 5 . 
       FIG. 6  is a diagram showing an example of a rising time and a falling time of a measurement signal by the time detector  21 . The horizontal axis of  FIG. 6  indicates an elapsed time from light emission time of the laser beam L 1 . The vertical axis of  FIG. 6  indicates a signal value of the measurement signal. Two kinds of signals having different values at peak timing TL 2  of the measurement signal are shown. A rising time Tup in which the measurement signal reaches a first threshold Th 1  and a falling time Tdn in which the measurement signal falls and reaches a second threshold Th 2  after reaching the first threshold are respectively shown with respect to the two kinds of measurement signals. 
     As shown in  FIG. 5 , the rising detector  21   a  is, for example, a comparator. The rising detector  21   a  compares a signal value of a measurement signal output by the second amplifier  20  and the first threshold and outputs a rising signal at time when the signal value of the measurement signal exceeds the first threshold. That is, when the measurement signal reaches the first threshold according to the positive theory, the rising detector  21   a  outputs the rising signal. 
     The first TDC  21   b  is, for example, a time to digital converter (TDC). The first TDB  21   b  measures the rising time Tup from when the laser beam L 1  is emitted until the rising detector  21   a  outputs the rising signal. That is, the first TDB  21   b  acquires the rising time Tup in which a measurement signal obtained by converting reflected light of the laser beam into a signal reaches the first threshold. 
     The falling detector  21   c  is, for example, a comparator. The falling detector  21   c  compares the signal value of the measurement signal output by the second amplifier  20  and the second threshold and outputs a falling signal at time when the signal value of the measurement signal exceeds the second threshold. That is, when the measurement signal reaches the second threshold according to the negative theory, the falling detector  21   c  outputs the falling signal. For example, when the signal value of the measurement signal decreases and reaches the second threshold after reaching the first threshold, the falling detector  21   c  outputs the falling signal. That is, a time when the rising signal is output corresponds to a time when the measurement signal reaches the second threshold after reaching the first threshold after light emission time of the laser beam L 1 . 
     The second TDC  21   d  is, for example, a time to digital converter (TDC). The second TDC  21   d  measures the falling time Tdn from when the laser beam L 1  is emitted until the falling detector  21   c  outputs the falling signal. That is, the second TDC  21   d  acquires the falling time Tdn in which the measurement signal obtained by converting the reflected light of the laser beam into a signal reaches the second threshold. 
     The measurement signal may be negatively reversed and subjected to threshold processing. In this case, the rising detector  21   a  compares the signal value of the measurement signal output by the second amplifier  20  and the first threshold and outputs the rising signal when the signal value of the measurement signal changing in time series decreases as time elapses and exceeds the first threshold. The falling detector  21   c  compares the signal value of the measurement signal output by the second amplifier  20  and the second threshold and outputs the falling signal when the signal value of the measurement signal changing in time series increases as time elapses and exceeds the second threshold after reaching the first threshold. 
     The distance measurer  22  includes, for example, an adder, a subtractor, a multiplier, and a divider. The distance measurer  22  measures the distance to a target object on the basis of the rising time Tup ( FIG. 6 ) and the falling time Tdn ( FIG. 6 ) acquired by the time acquirer  21 . That is, the distance measurement processor  22   a  measures the distance to the target object on the basis of a time difference between timing based on a first time obtained by weighting the rising time Tup ( FIG. 6 ) with a first weight coefficient W 1  and a second time obtained by weighting the falling time Tdn ( FIG. 6 ) with a second weight coefficient W 2  and irradiation timing of the laser beam. For example, the distance measurement processor  22   a  acquires timing TL 3  corresponding to the peak timing TL 2  of the measurement signal on the basis of the first time obtained by weighting the rising time Tup ( FIG. 6 ) with the first weight coefficient W 1  and the second time obtained by weighting the falling time Tdn ( FIG. 6 ) with the second weight coefficient W 2 . That is, the timing TL 3  can be indicated by the timing TL 3 =the first weight coefficient W 1 ×the rising time Tup+the second weight coefficient W 2 ×the falling time Tdn. The first threshold Th 1  ( FIG. 6 ) according to this embodiment is equal to the second threshold Th 2  ( FIG. 6 ). Therefore, there is a relation of the second weight coefficient W 2 =(1−the first weight coefficient W 1 ). 
     In this way, the distance measurement processor  22   a  measures the distance to the target object on the basis of an equation; a measurement distance=light speed×(the timing TL 3 −the timing TL 1  when the photodetector  17  detects the laser beam L 1 )/2. In this way, the measurement distance can be indicated as Expression (1). Also, the measurement distance may be calculated based on the power of Tup.
 
[Expression 1]
 
Measurement distance=light speed×(( W 1× Tup +(1− W 1)× Tdn )− TL 1)/2  Expression (1)
 
     Each of the time acquirer  21  and the distance measurer  22  is composed of hardware. For example, each of the time acquirer  21  and the distance measurer  22  is composed of a circuit. As explained above, the second threshold Th 2  according to this embodiment is set to the same value as the first threshold Th 1  in order to simplify calculation of a weight coefficient. However, the second threshold Th 2  is not limited to this. 
       FIG. 7  is a diagram schematically showing a state in which a measurement signal of the sensor  18  ( FIG. 1 ) piles up. The horizontal axis indicates an elapsed time from light emission time of the laser beam L 1 . The vertical axis indicates a signal value of the measurement signal. Two kinds of signals having different peak values of the measurement signal are shown. The sensor  18  ( FIG. 1 ) according to this embodiment is composed of an SPAD cell. A relation between the number of received photons per unit time and an output value of the SPAD cell has a linear characteristic when the number of received photons per unit time is small. On the other hand, when the number of received photons per unit time increases, the output value of the SPAD cell is saturated. The relation between the number of received photons per unit time and the output value of the SPAD cell has a nonlinear characteristic. Therefore, as shown in  FIG. 7 , a peak of the measurement signal sometimes becomes gentle. This is called pile-up. 
       FIGS. 8 to 11  are diagrams for explaining a simulation example of the timing TL 3  at the time when the measurement signal piles up. It is explained below that, even if the measurement signal piles up, the timing TL 3  based on the first time obtained by weighting the rising time Tup with the first weight coefficient W 1  and the second time obtained by weighting the falling time Tdn with the second weight coefficient W 2  indicates a value substantially equal to a value at the peak timing TL 2  ( FIG. 6 ). 
       FIG. 8  is a diagram for explaining a temporal relation between an emission point in time of the light source  11  and conversion into an electric signal by the sensor  18 . The horizontal axis indicates time. P 10  indicates an example of an irradiation time of laser pulse light irradiated by the light source  11 . T=0 indicates an irradiation start time. 10 ns indicates a pulse width. P 12  indicates a time range in which the laser pulse light irradiated at P 10  is reflected and returns from the measurement target object  10  present 10 meters ahead. That is, since round-trip speed of light is 6.7 ns/m, when the distance to the measurement target object  10  is 10 m, the time range is 67 ns. Therefore, when the measurement target object  10  is present 10 meters ahead, photons received in a time range of T=67 to T=77 ns in a measurement environment such as the outdoor are reflected light of the laser pulse light and environment light. Photons received in the other time are the environment light. 
       FIG. 9  is a diagram showing an output waveform example during one photon detection of the SPAD cell. When an output waveform is represented as I(t), the output waveform I(t) is approximately indicated by Expression (2). An output time constant of the SPAD cell is set to 10 ns. I0 indicates an output signal value at t=0.
 
[Expression 2]
 
 I ( t )∝exp(− t/ 10 ns), or
 
 I ( t )= I 0×exp(− t/ 10 ns)  Expression (2)
 
       FIG. 10  is a diagram showing a simulation example of a measurement signal value of the SPAD cell at the time when the pile-up is not considered. The vertical axis indicates a measurement signal value. The horizontal axis indicates time “t”. A PDE (the number of detected photons/the number of input photons) of the SPAD cell is set to 10 percent. The number of SPAD cells per one pixel is set to 10. The output waveform I(t) indicated by Expression (2) is used. Under this condition, as the number of excited photons before pile-up, for example, during input of ten photons, one photon can be detected in average. During input of ten thousand photons, as the number of excited photons before pile-up, one thousand photons (=10000×10%) can be detected in average. In  FIG. 10 , a simulation is performed with the number of photons per unit time, that is, photons/input time set to 10.0, 14.4, 20.7, 29.8, 42.8, 61.6, 88.6, 127.4, 183.3, 263.7, 379.3, 545.6, 784.8, 1128.8, 1623.8, 2335.7, 3359.8, 4832.9, 6951.9, and 10000.0. Environment light is set to 0. An output band of the second amplifier  20  ( FIG. 1 ) is set to 100 MHZ. 
     In the simulation, the rising time Tup and the falling time Tdn at the time when the first threshold Th 1 =the second threshold Th 2 ¬=0.2 are calculated. The first weight coefficient W 1  with which a distance measurement result does not change with respect to the number of excited photons before pile-up is calculated. That is, the first weight coefficient W 1  with which the timing TL 3  and the peak timing TL 2  of the measurement signal shown in  FIG. 10  coincide most is calculated. More specifically, the first weight coefficient W 1  with which TL 3 =(W 1 ×Tup+(1−W 1 )×Tdn) coincides most with the peak timing TL 2  of the measurement signal shown in  FIG. 10  is calculated. With such calculation, in this parameter example, the first weight coefficient W 1 =0.71 is calculated. 
       FIG. 11  is a diagram showing a relation between the timing TL 3  calculated using the first weight coefficient W 1 =0.71 and the rising time Tup and the falling time Tdn. The vertical axis indicates time. The horizontal axis indicates the number of excited photons before pile-up. A first line  130   a  indicates the rising time Tup. A second line  130   b  indicates the falling time Tdn. A third line  130   c  indicates the timing TL 3 . As shown in  FIG. 12 , the rising time Tup and the falling time Tdn fluctuate according to the number of excited photons before pile-up. On the other hand, even if the number of excited photons before pile-up is changed, the timing TL 3  calculated using the first weight coefficient W 1 =0.71 indicates a substantially fixed value, that is, 8.9 ns. 
     As it is seen from the above, even if the number of excited photons before pile-up is changed, it is possible to calculate the timing TL 3  substantially the same as the peak timing TL 2  of the measurement signal shown in  FIG. 12  using one first weight coefficient W 1 =0.71. In this way, even if the number of excited photons before pile-up is changed, by setting the first threshold Th 1  and the second threshold Th 2  in a range in which linearity of an input and output characteristic of the SPAD cell is maintained, it is possible to calculate the timing TL 3  substantially the same as the peak timing TL 2  of the measurement signal shown in  FIG. 12  even if the measurement signal piles up. According to the same calculation, it is possible to calculate the first weight coefficient W 1  in advance for the sensor  18  having different characteristics as well. Furthermore, the sensor  18  according to this embodiment is composed of a silicon photomultiplier tube but is not limited to this. The first weight coefficient W 1  can be calculated for other types of imaging elements. 
     A reduction effect for waveform distortion caused by the second amplifier  20  is explained with reference to  FIGS. 12 and 13 .  FIG. 12  is a diagram showing input and output characteristics of the second amplifier  20 . The vertical axis indicates an output signal value. The horizontal axis indicates an input signal value. An input and output characteristic  140   a  is linear and indicates an ideal input and output characteristic. An input and output characteristic  140   b  is nonlinear and indicates an input and output characteristic with distortion. 
       FIG. 13  is a diagram showing a signal value based on the ideal input and output characteristic and a signal value based on the input and output characteristic with distortion. The vertical axis indicates a signal value. The horizontal axis indicates time. A signal value  150   a  is a signal value obtained when the second amplifier  20  having the ideal input and output characteristic is used. A signal value  150   b  is a signal value obtained when the second amplifier  20  having the input and output characteristic with distortion is used. The signal values  150   a  and  150   b  with the first weight coefficient W 1 =0.5 are data in a range in which pile-up of the sensor  18  ( FIG. 1 ) does not occur. 
     A measurement distance by the signal value  150   a  is (0.5*T 1 +0.5*T 4 )/6.7. A measurement distance by the signal value  150   b  is (0.5*T 2 +0.5*T 3 )/6.7. As a premise, T 2 −T 1 =T 4 −T 3 . Therefore, the measurement distance by the signal value  150   a  is equal to the measurement distance by the signal value  150   b . As it is seen from this, when the calculation method according to this embodiment indicated by Expression (1) is used, it is possible to prevent deterioration in measurement accuracy even if the second amplifier  20  having the input and output characteristic with distortion is used. By setting a threshold in the range in which the linearity of the input and output characteristic of the sensor  18  ( FIG. 1 ) is maintained as explained above, it is possible to prevent deterioration in measurement accuracy even if the measurement signal is saturated or piles up and even if the second amplifier  20  having the input and output characteristic with distortion is used. 
     In this embodiment, the current signal is converted into the measurement signal by the second amplifier  20 . However, the conversion of the current signal into the measurement signal is not limited to this. The current signal may be converted into the measurement signal by the second amplifier  20  and an AD converter or the like. In this case, it is possible to prevent deterioration in measurement accuracy even if the AD converter has the input and output characteristic with distortion. 
       FIG. 14  is a diagram showing a measurement method for a CFD as a comparative example. As shown in  FIG. 14 , the CFD (constant fraction discriminator) acquires attenuation signals  70   b  and  72   b  obtained by attenuating measurement signals  70   a  and  72   a . The CFC reverses and delays the measurement signals  70   a  and  72   a  to acquire signals  70   c  and  72   c . The CFD acquires signals  70   d  and  72   d  obtained by adding the reversed and delayed signals  70   c  and  72   c  to the attenuation signals  70   b  and  72   b . The CFD uses zero-cross points of the signals  70   d  and  72   d  as reference times of distance measurement. As shown in  FIG. 14 , when saturation and pile-up do not occur, the influence of peak values of the measurement signals  70   a  and  72   a  can be prevented. However, in the CFD, when saturation or pile-up occurs, the zero-cross points deviate in the measurement signals  70   a  and  72   a . Accuracy of distance measurement is deteriorated. On the other hand, according to this embodiment, the reference time of the distance measurement is acquired on the basis of the rising time Tup in which the measurement signal reaches the first threshold and the falling time Tdn in which the measurement signal falls and reaches the second threshold after reaching the first threshold. Therefore, it is possible to prevent the influence on measurement accuracy even if saturation or pile-up occurs in the measurement signals  70   a  and  72   a.    
     As explained above, according to this embodiment, the distance to the target object is measured on the basis of the rising time Tup in which the measurement signal reaches the first threshold and the falling time Tdn in which the measurement signa falls and reaches the second threshold after reaching the first threshold. Consequently, it is possible to reduce the influence of a measurement signal having a value equal to or larger than the first threshold on the distance measurement. It is possible to accurately and stably measure the distance to the target object even if the measurement signal, for example, piles up or is saturated. 
     Second Embodiment 
     In a second embodiment, a measurement distance is more highly accurately acquired by acquiring a weight coefficient referring to at least one of an intensity value of a measurement signal and an intensity value of environment light. In the following explanation, differences from the first embodiment are explained. 
       FIG. 15  is a block diagram showing a detailed configuration example of the distance measurer  22  according to the second embodiment. As shown in  FIG. 15 , the distance measurer  22  includes a signal light intensity detector  22   b , an environment light intensity detector  22   c , and a weight coefficient acquirer  22   d . Each of the signal light intensity detector  22   b , the environment light intensity detector  22   c , and the weight coefficient acquirer  22   d  is composed of hardware. For example, each of the signal light intensity detector  22   b , the environment light intensity detector  22   c , and the weight coefficient acquirer  22   d  is composed of a circuit. The block diagram of  FIG. 15  shows an example of signals. Order of the signals and wiring for the signals are not limited to order and wiring shown in  FIG. 15 . 
     The signal light intensity detector  22   b  acquires, for example, a maximum value as a first representative value of a measurement signal in a time T from when the light source  11  ( FIG. 1 ) irradiates a laser beam L 1 ( n ) until the light source  11  irradiates the next laser beam L 1 ( n +1). The signal light intensity detector  22   b  may acquire a maximum value of a signal value after reducing noise with filtering processing. 
     The environment light intensity detector  22   c  detects the intensity of environment light. More specifically, the environment light intensity detector  22   c  acquires, for example, an average value as a second representative value of the measurement signal in the time T in a period in which the light source  11  ( FIG. 1 ) stops irradiation of a laser beam. The environment light intensity detector  22   c  may acquire a representative value after reducing noise with filtering processing. The second representative value may be a maximum value, an intermediate value, or the like of the measurement signal in the time T. 
     The weight coefficient acquirer  22   d  acquires the weight coefficients W 1  and W 2  on the basis of at least one of the first representative value detected by the signal light intensity detector  22   b  and the second representative value detected by the environment light intensity detector  22   c . As explained above, the second weight coefficient W 2  according to this embodiment has the relation of W 2 =(1−W 1 ). Consequently, the distance measurement processor  22   a  substitutes the first weight coefficient W 1  acquired by the weight coefficient acquirer  22   d  in Expression (1) described above and measures the distance to the measurement target object  10 . In this way, the distance measurement processor  22   a  changes the first weight coefficient W 1  according to at least one of the signal intensity of the measurement signal and the environment light intensity. 
     The weight coefficient acquirer  22   d  is explained more in detail with reference to  FIGS. 16 to 20 . 
       FIG. 16  is a diagram showing a maximum value of a measurement signal value obtained by changing the number of photons per unit time shown in  FIG. 10 . The vertical axis indicates the maximum value of the measurement signal value. The horizontal axis indicates the number of photons per unit time, that is, photons/input time. As shown in  FIG. 16 , the maximum value of the measurement signal value corresponds to the number of photons per unit time. The maximum value has a tendency that the maximum value monotonously increases as the number of photons per unit time increases and an increase rate decreases as the number of photons increases. 
       FIG. 17  is a diagram of the first weight coefficient W 1  calculated for each measurement signal value corresponding to the maximum value shown in  FIG. 16 . The vertical axis indicates the first weight coefficient W 1 . The horizontal axis indicates the maximum value of the measurement signal value shown in  FIG. 16 . 
     The first weight coefficient W 1  is a value calculated on the basis of the measurement signal for each number of photons per unit time shown in  FIG. 10 . That is, the rising time Tup and the falling time Tdn at the time when the first threshold Th 1 =the second threshold Th 2 ¬=0.2 are calculated for each measurement signal. The weight coefficient W 1  with which the timing TL 3  and the peak timing TL 2  of the measurement signal shown in  FIG. 12  coincide most is calculated. As explained above, the maximum value of the measurement signal value corresponds to the number of photons per unit time. Therefore, the weight coefficient W 1  with which the timing TL 3  and the peak timing TL 2  of the measurement signal coincide mode can be obtained for each maximum value of the measurement signal value. 
     As shown in  FIG. 17 , The first weighting factor W  1  monotonously decreases as the maximum value of the measured signal value increases and the first weighting factor W  1  increases monotonically with the predetermined value as the boundary as the maximum value of the measured signal value increases. In this way, the first weighting coefficient W  1  monotonically increases as the maximum value of the measured signal value increases when the maximum value of the measured signal value is equal to or larger than the predetermined value. 
     The weight coefficient acquirer  22   d  has stored therein, as a lookup table, for example, a relation between the maximum value and the first weight coefficient W 1  shown in  FIG. 17 . Consequently, the weight coefficient acquirer  22   d  acquires the first weight coefficient W 1  using, as an argument, the first representative value, that is, the maximum value detected by the signal light intensity detector  22   b . In this way, the weight coefficient acquirer  22   d  acquires the first weight coefficient W 1  on the basis of the first representative value, that is, the maximum value detected by the signal light intensity detector  22   b . Therefore, the weight coefficient acquirer  22   d  can obtain the first weight coefficient W 1  that is more highly accurately calculated. Consequently, the distance measurement processor  22   a  is capable of substituting the first weight coefficient W 1  acquired by the weight coefficient acquirer  22   d  in Expression (1) described above and more highly accurately measuring the distance to the measurement target object  10 . 
       FIG. 18  is a diagram of the first weight coefficient W 1  corresponding to the maximum value of the measurement signal calculated by changing an intensity value of environment light. The vertical axis indicates the first weight coefficient W 1 . The horizontal axis indicates the maximum value of the measurement signal value. The environment light is photons steadily made incident on the sensor  18  ( FIG. 1 ). Therefore, the environment light excites an electric signal in a DC (direct current) manner. That is, as the environment light increases, an effect equivalent to the effect achieved when the number of SPADs is reduced is generated in average. Therefore, in  FIG. 18 , the intensity of the environment light is indicated as a percentage of a reduction of the number of SPADs. For example, 10% is equivalent to a reduction of 10% of the number of SPADs. 20% is equivalent to a reduction of 20% of the number of SPADs. 30% is equivalent to a reduction of 30% of the number of SPADs. That is, values of the first weight coefficient W 1  for each intensity value of the environment light shown in  FIG. 18  are obtained by performing calculation equivalent to the calculation in  FIG. 17  while changing the number of SPAD cells. 
     As shown in  FIG. 18 , the weight coefficient acquirer  22   d  increases an increase rate of the first weight coefficient respect to an increase of a maximum value of the measured signal value as the intensity value of the environment light increases when the maximum value of the measurement signal is equal to or larger than a predetermined value. 
     The weight coefficient acquirer  22   d  has stored therein, as a lookup table, for example, a relation between the maximum value and the intensity value of the environment light and the first weight coefficient W 1  shown in  FIG. 18 . Consequently, the weight coefficient acquirer  22   d  acquires the first weight coefficient W 1  using, as arguments, the first representative value, that is, the maximum value detected by the signal light intensity detector  22   b  and the second representative value, that is, the average value of the measurement signal by the environment light detected by the environment light intensity detector  22   c.    
       FIG. 18  is a diagram of the first weight coefficient W 1  corresponding to the maximum value of the measurement signal calculated by changing an intensity value of environment light. Signal intensity by the environment light can be considered as a DC component of a measurement signal value. 
       FIG. 19  is a diagram of the first weight coefficient W 1  corresponding to a corrected maximum value of a measurement signal calculated by changing the intensity value of the environment light W 1 . The vertical axis indicates the first weight coefficient W 1 . The horizontal axis indicates a corrected maximum value of the measurement signal value. A corrected maximum value CMa is indicated by Expression (3).
 
[Expression 3]
 
 CMa =Max(1+ ES )  Expression (3)
 
     A maximum value Ma is the first representative value detected by the signal light intensity detector  22   b . An intensity value ES of the environment light is the second representative value detected by the environment light intensity detector  22   c.    
     An approximate line  200   a  indicates an approximate line of the first weight coefficient W 1  with respect to the corrected maximum value CMa. 
     The weight coefficient acquirer  22   d  has stored therein, as a lookup table, the approximate line  200   a  of the first weight coefficient W 1  with respect to the corrected maximum value CMa. Alternatively, the weight coefficient acquirer  22   d  has stored therein, as a linear equation, the approximate line  200   a  of the first weight coefficient W 1  with respect to the corrected maximum value CMa. The linear equation is a general primary regression equation. Therefore, description of the linear equation is omitted. 
     In this way, the approximate line  200   a  is stored as the lookup table. Therefore, a memory amount can be further reduced than when a lookup table is stored for each intensity value of the environment light. When the approximate line  200   a  is stored as the linear equation, the memory amount can be further reduced than when the lookup table is stored. 
     In this way, the weight coefficient acquirer  22   d  adds 1 to the intensity value ES of the environment light, which is the second representative value, detected by the environment light intensity detector  22   c , multiplies the maximum value Ma, which is the first representative value, detected by the signal light intensity detector  22   b , and acquires the corrected maximum value CMa. The weight coefficient acquirer  22   d  acquires the first weight coefficient W 1  corresponding to the corrected maximum value CMa. When the corrected maximum value CMa is small, accuracy is low. However, in that case, the distance to the measurement target object  10  is considered to be long. Very high accuracy is considered to be unnecessary. The distance measurement processor  22   a  changes the first weight coefficient W 1  according to the signal intensity of the measurement signal and the environment light intensity. Consequently, the measurement distance to the measurement target object  10  can be more highly accurately acquired. 
     As explained above, according to this embodiment, the first weight coefficient W 1  used for weighting the rising time Tup and the falling time Tdn is acquired with reference to at least one of the intensity value of the measurement signal and the intensity value of the environment light. Consequently, even if the input and output characteristic of the sensor  18  changes according to the intensity value of the measurement signal and the intensity value of the environment light, it is possible to acquire the first weight coefficient W 1  corresponding to the input and output characteristic. Even if the intensity value of the measurement signal and the intensity value of the environment light change, it is possible to accurately and stably measure the distance to a target object. 
     Third Embodiment 
     In a third embodiment, a measurement distance is more highly accurately acquired by changing the first weight coefficient according to a pulse width of a measurement signal. In the following explanation, differences from the second embodiment are explained. 
       FIG. 20  is a block diagram showing a detailed configuration example of the distance measurer  22  according to the third embodiment. As shown in  FIG. 20 , the distance measurer  22  further includes a pulse width acquirer  22   e . The pulse width acquirer  22   e  is composed of a circuit. The block diagram of  FIG. 20  shows an example of signals. Order of the signals and wiring for the signals are not limited to order and wiring shown in  FIG. 20 . 
     The pulse width acquirer  22   e  acquires a pulse width on the basis of the rising time Tup detected by the rising detector  21   a  and the falling time Tdn detected by the falling detector  21   c . For example, the pulse width acquirer  22   e  acquires a difference between the falling time Tdn and the rising time Tup as the pulse width. The falling detector  21   c  according to this embodiment is configured to output an error signal including information indicating a detection error when the falling detector  21   c  cannot detect the falling time Tdn even if the irradiation interval T of the light source  11  ( FIG. 1 ) is exceeded. 
       FIG. 21  is a conceptual diagram for explaining characteristics of a measurement signal according to pulse widths. The vertical axis indicates a measurement signal value. The horizontal axis indicates time. As shown in  FIG. 21 , the pulse widths increase in the order of A, B, C, and D. 
     In A, the number of photons per unit time is the smallest. In B, the number of photons per unit time is larger than the number of photons in A and the measurement signal piles up. In C, the number of photons per unit time is larger than the number of photons in B and a time constant of falling changes from the time constant in B. In D, the number of photons per unit time is larger than the number of photons in C and the falling detector  21   c  cannot detect the falling time Tdn. In this way, the characteristics of the measurement signal change according to the pulse widths. 
     Therefore, the distance measurement processor  22   a  changes an acquisition method for the first weight coefficient W 1  on the basis of the pulse width acquired by the pulse width acquirer  22   e . More specifically, when the pulse width corresponds to A, the distance measurement processor  22   a  integrates, for several laser irradiation, measurement signals output by the time division processor  21  and measures the distance to the measurement target object  10  on the basis of a peak value of an integrated signal. When the pulse width corresponds to B, the distance measurement processor  22   a  measures the distance to the measurement target object  10  on the basis of Expression (1) as explained above. 
       FIG. 22  is a diagram for explaining characteristics of functions used in arithmetic expressions corresponding to C and D. The vertical axis indicates a value of a function. The horizontal axis indicates a pulse width P. P=Tup-Tdn and P 1  and P 2  are constants serving as fitting parameters. As shown in  FIG. 22 , a function f 1 ( p ) is a function that is 0 up to a pulse width P 1 , linearly increases according to a linear equation of f 1 ( p )=k×(P−P 1 ) when the pulse width P exceeds a pulse width P 1 , and becomes a constant CONST 1  when the pulse width reaches a pulse width Pd. 
     A function f 2 ( p ) is a function that monotonously increases according to an equation of f 2 ( p )=k/(P+P 2 ) and becomes a constant CONST 2  when the pulse width P reaches the pulse width Pd. 
     In a state of C in which the pulse width P is in a range of the pulse widths Pc to Pd, the distance measurement processor  22   a  acquires a measurement distance according to, for example, Expression (4). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         
                           Measurement 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           distance 
                         
                         = 
                           
                         ⁢ 
                         
                           light 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           speed 
                           × 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             ( 
                             
                               Tup 
                               + 
                               
                                 f 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
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                                 ⁢ 
                                 
                                   ( 
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                                   ) 
                                 
                               
                             
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                           ⁢ 
                           
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                           ⁢ 
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                         = 
                           
                         ⁢ 
                         
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                           ⁢ 
                           
                               
                           
                           ⁢ 
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                         ⁢ 
                         
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                             Tup 
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                                 ( 
                                 
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                                     P 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                 
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                               ⁢ 
                               
                                 / 
                               
                               ⁢ 
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                         = 
                           
                         ⁢ 
                         
                           light 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           speed 
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                             ( 
                             
                               Tup 
                               + 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           k 
                           × 
                           
                             ( 
                             
                               Tdn 
                               - 
                               Tup 
                               - 
                               
                                 P 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                             ) 
                           
                           ⁢ 
                           
                             / 
                           
                           ⁢ 
                           2 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           light 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           speed 
                           × 
                           
                             ( 
                             
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   k 
                                 
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                               × 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         
                             
                           ⁢ 
                           
                             Tup 
                             + 
                             
                               k 
                               × 
                               Tdn 
                             
                             - 
                             
                               k 
                               × 
                               P 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           / 
                         
                         ⁢ 
                         2 
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           light 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           speed 
                           × 
                           
                             ( 
                             
                               W 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                               × 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           Tup 
                           + 
                           
                             
                               ( 
                               
                                 1 
                                 - 
                                 
                                   W 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                 
                               
                               ) 
                             
                             × 
                             Tdn 
                           
                           - 
                         
                       
                     
                   
                   
                     
                       
                         
                             
                           ⁢ 
                           
                             k 
                             × 
                             P 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           ) 
                         
                         ⁢ 
                         
                           / 
                         
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                         2 
                       
                     
                   
                 
               
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
           
         
       
     
     In Expression (4), (1-k) is represented as W 1 . As it is seen from this, Expression (4) is an expression equivalent to Expression (1) excluding k×P 1 , which is an offset component. 
     In a state of D in which the pulse width P is equal to or larger than Pd, the distance measurement processor  22   a  acquires a measurement distance according to, for example, Expression (5).
 
[Expression 5]
 
Measurement distance=light speed×( Tup+CONST 1)/2  Expression (5)
 
     In the state of C in which the pulse width P is in the range of the pulse widths Pc to Pd, the distance measurement processor  22   a  acquires a measurement distance according to, for example, Expression (6). In the state of C, since f 2 (P) monotonously increases, the weight of the rising time Tup becomes larger than the weight of the falling time Tdn as the pulse width P increases.
 
[Expression 6]
 
Measurement distance=light speed×( Tup+f 2( P )× Tdn )/2  Expression (6)
 
     In the state of D in which the pulse width P is equal to or larger than Pd, f 2 (P) is treated as CONST 2 . 
     In this way, in the state of C in which the pulse width P is in the range of the pulse widths Pc to Pd, the rising time Tup is steep and the falling time Tdn is gentle. The rising time Tup has small input light amount dependency. The falling time Tdn has large input light amount dependency. By acquiring the measurement distance according to Expression (4) or Expression (6), it is possible to reduce the light amount dependency and reduce a measurement error. 
     In the state of D in which the pulse width is in a range of the pulse width Pd or more, the falling time Tdn sometimes cannot be acquired. However, in the state of D, the distance measurement processor  22   a  according to this embodiment does not use the falling time Tdn. Therefore, it is possible to measure a measurement distance. By introducing the pulse width P into the distance measurement in this way, a degree of freedom further increases and fitting accuracy of a measurement signal is further improved. 
     According to this embodiment, the weight of the rising time Tup is set larger than the weight of the falling time Tdn as the pulse width P increases. The rising time Tup has small input light amount dependency and the falling time Tdn has large input light amount dependency. Therefore, even if the input light amount dependency is reduced and the intensity value of the measurement signal further increases, it is possible to accurately and stably measure the distance to the target object. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.