Patent Publication Number: US-2020284909-A1

Title: Distance measuring sensor and distance measuring method

Description:
TECHNICAL FIELD 
     The disclosure relates to a distance measuring sensor and a distance measuring method. 
     BACKGROUND ART 
     A method using a time-of-flight system (a TOF system) distance measuring sensor is available as a method for measuring a distance to a target by using light. The TOF-system distance measuring sensor measures a distance to a target by measuring a propagation time of light from when irradiation light is radiated to the target until reflection light is received. In general, in order to facilitate the measurement of a distance to a target, the distance measuring sensor radiates modulated light to the target as irradiation light and calculates the distance to the target by using a phase difference between the irradiation light and reflection light reflected by the target. 
     For a modulation cycle T, an irradiation-light modulation wave (an irradiation modulation wave), which is expressed by equation (1) below, and a reflection-light modulation wave (a reflection modulation wave), which is expressed by equation (2) below, are described below. Although the light is modulated in this case so as to have a sine wave indicated by equation (1) below, the light may also be modulated so as to have a pulse wave. 
         y ( t )= a  sin(ω t )+ b   (1)
 
         Y ( t )= A  sin(ω t −θ)+ B   (2)
 
     Here, a in equation (1) is the amplitude of the irradiation modulation wave, and b is offset of the irradiation modulation wave. A in equation (2) is the amplitude of the reflection modulation wave, B is offset of the reflection modulation wave, and θ is a phase difference. Also, ω in equations (1) and (2) is 2π/T. 
     The modulation cycle T needs to be set according to a largest measurement distance to the target. Equation (3) below indicates the relationship between a largest measurement distance L max  and the modulation cycle T. 
         L   max   =cT/ 2  (3)
 
     Here, c is the speed of light and is about 3×10 8  m/s. 
     The distance measuring sensor receives the reflection light by sampling the reflection modulation wave every T/4. 
       FIG. 1  shows a graph showing the relationship between the modulation cycle T of an irradiation modulation wave α and a reflection modulation wave β and light intensities (signal intensities) A0, A1, A2, and A3. Equations (4) to (7) below indicate the light intensities A0, A1, A2, and A3 of the reflection modulation wave β at T=0, T=π/4, T=π/2, and T=3π/4, respectively. 
         A 0= Y (0)=− A  sin(θ)+ B   (4)
 
         A 1= Y ( T/ 4)= A  sin(π/2−θ)+ B=A  cos(θ)+ B   (5)
 
         A 2= Y ( T/ 2)= A  sin(π−θ)+ B=A  sin(θ)+ B   (6)
 
         A 3= Y (3 T/ 4)= A  sin(3π/2−θ)+ B=−A  cos(θ)+ B   (7)
 
     When the amplitude A of the reflection modulation wave β and the offset B of the reflection modulation wave β, the amplitude A and the offset B being constants, are deleted from equations (4) to (7), equation (8) below is obtained. 
         A 2− A 0/ A 1− A 3=2 A  sin(θ)/2 A  cos(θ)  (8)
 
     From equation (8), the phase difference θ can be determined using A0, A1, A2, and A3, as in equation (9) below. 
       θ=arctan( A 2− A 0/ A 1− A 2)  (9)
 
     Hence, using the phase difference θ in equation (9), a distance L to the target can be calculated as in equation (10) below. 
         L=L   max ×θ/2 π=cTθ/ 4π  (10)
 
     As is apparent from equations (9) and (10), the distance L to the target is dependent on a light intensity at a measurement point and is vulnerable to influences of disturbance light. Although increasing the light intensity of the light source unit in the distance measuring sensor is conceivable in order to reduce the influences of the disturbance light, there are problems in that the sensitivity of the light receiving unit in the distance measuring sensor is saturated, the eyes of the human are adversely affected, and so on. 
     As technologies for solving the above-described problems, for example, there are technologies as in PTLs 1 and 2. 
     PTL 1 discloses a technology in which light of colors having mutually different wavelengths is used as measurement light during measurement of a distance to a distance-measurement target. Also, PTL 1 discloses a technology in which a combination of colors that become white when they are mixed together is used as the measurement light. 
     PTL 2 discloses a technology in which reflection light from a subject, the reflection light being based on first light modulated in a first cycle, and second light modulated in a second cycle are mixed together. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] Japanese Unexamined Patent Application Publication No. 2010-169405 
         [PTL 2] Japanese Unexamined Patent Application Publication No. 2016-53566 
       
    
     SUMMARY 
     Technical Problem 
     However, in the technology disclosed in PTL 1, when other distance measuring sensors, such as distance measuring sensors  3  and  4 , that are the same type as a distance measuring sensor  1  exist right beside or along an extension of the distance measuring sensor  1 , as in  FIG. 2  showing one example of the positional relationship between the plurality of distance measuring sensors  1 ,  3 , and  4  and a target  2 , the technology becomes vulnerable to influences of disturbance light due to irradiation light from the other distance measuring sensors. 
     Also, the technology disclosed in PTL 2 becomes less vulnerable to influences of disturbance light due to light having a different wavelength, but cannot be made less vulnerable to influences of disturbance light when a similar sensor uses a similar wavelength. 
     An object of the disclosure is to provide a distance measuring sensor and a distance measuring method that reduce influences of ambient light and disturbance light, such as light of another distance measuring sensor, and that can measure highly accurate distance information. 
     Solution to Problem 
     (1) One embodiment of the disclosure is a TOF-system distance measuring sensor that measures a distance to a target by measuring a time from when a light source unit radiates irradiation light to the target until a light receiving unit receives reflection light reflected by the target. The distance measuring sensor comprises: a light source unit that radiates light to the target as the irradiation light, the light being subjected to primary modulation so that the distance to the target can be measured and being subjected to secondary modulation so that influences of disturbance light are reduced; and a light receiving unit that receives the reflection light subjected to the secondary modulation and that subjects the reflection light subjected to the secondary modulation to secondary demodulation so that influences of disturbance light are reduced.
 
(2) Also, an embodiment of the disclosure is a distance measuring sensor in which, in addition to the configuration in (1) described above, the light source subjects at least one of polarization, an amplitude, a frequency, and a phase of the light to the primary modulation so that the distance to the target can be measured.
 
(3) Also, an embodiment of the disclosure is a distance measuring sensor in which, in addition to the configuration in (1) described above, based on a direct spreading system using a PN code in an m-sequence, the light source unit subjects the light to the secondary modulation so that the influences of the disturbance light are reduced.
 
(4) Also, an embodiment of the disclosure is a distance measuring sensor in which, in addition to the configuration in (1) described above, the light source unit radiates light to the target as the irradiation light, the light being subjected to the secondary modulation by externally supplying a modulation signal.
 
(5) Also, an embodiment of the disclosure is a distance measuring sensor in which, in addition to the configuration in (1) described above, the light source unit radiates light to the target as the irradiation light, the light being subjected to the secondary modulation by externally supplying a modulation signal to the light subjected to the primary modulation.
 
(6) An embodiment of the disclosure is a distance measuring sensor that comprises a plurality of light receiving units that is arranged in an array form, in addition to the configuration in (1), (2), (3), (4), or (5) described above.
 
(7) Also, an embodiment of the disclosure is a TOF-system distance measuring method that measures a distance to a target by measuring a time from when a light source unit radiates irradiation light to the target until a light receiving unit receives reflection light reflected by the target. The distance measuring method includes: a step of radiating light to the target as the irradiation light, the light being subjected to primary modulation so that the distance to the target can be measured and being subjected to secondary modulation so that influences of disturbance light are reduced; and a step of receiving the reflection light subjected to the secondary modulation and subjecting the reflection light subjected to the secondary modulation to secondary demodulation so that influences of disturbance light are reduced.
 
(8) Also, an embodiment of the disclosure is a TOF-system distance measuring camera that measures a distance to a target by measuring a time from when a light source unit radiates irradiation light to the target until a light receiving unit receives reflection light reflected by the target. The distance measuring camera comprises: a light source unit that radiates light to the target as the irradiation light, the light being subjected to primary modulation so that the distance to the target can be measured and being subjected to secondary modulation so that influences of disturbance light are reduced; and a plurality of light receiving unit that receives the reflection light subjected to the secondary modulation and that subjects the reflection light subjected to the secondary modulation to secondary demodulation so that influences of disturbance light are reduced. The plurality of light receiving units is arranged in an array form.
 
     Advantageous Effects of Disclosure 
     According to the disclosure, it is possible to provide a distance measuring sensor and a distance measuring method that reduce influences of ambient light and disturbance light, such as light of another distance measuring sensor, and that can measure highly accurate distance information. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a graph showing a relationship between a modulation cycle of an irradiation modulation wave and a reflection modulation wave and light intensities. 
         FIG. 2  is a diagram showing one example of a positional relationship between a plurality of distance measuring sensors and a target  2 . 
         FIG. 3  is a diagram showing the configuration of a distance measuring sensor according to a first embodiment of the disclosure. 
         FIG. 4  is a diagram showing the configuration of a light source unit in the distance measuring sensor according to the first embodiment of the disclosure. 
         FIG. 5  is a diagram for describing secondary modulation based on a direct spreading system using a PN code in an m-sequence. 
         FIG. 6  shows one example of an m-sequence generating circuit. 
         FIG. 7  is a diagram showing the configuration of a light receiving unit in the distance measuring sensor according to the first embodiment of the disclosure. 
         FIG. 8  is diagram for describing secondary modulation and secondary demodulation based on the direct spreading system using a PN code in an m-sequence. 
         FIG. 9  is a diagram showing the configuration of a distance measuring sensor according to a second embodiment of the disclosure. 
         FIG. 10  is a diagram showing the configuration of a light source unit in the distance measuring sensor according to the second embodiment of the disclosure. 
         FIG. 11  is a diagram showing the configuration of a light receiving unit in the distance measuring sensor according to the second embodiment of the disclosure. 
         FIG. 12  is a diagram showing the configuration of a distance measuring sensor according to a third embodiment of the disclosure. 
         FIG. 13  is a diagram showing the configuration of a light source unit in the distance measuring sensor according to the third embodiment of the disclosure. 
         FIG. 14  is diagram showing the configuration of a light receiving unit in the distance measuring sensor according to the third embodiment of the disclosure. 
         FIG. 15  is a diagram showing the configuration of a distance measuring sensor according to a fourth embodiment of the disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     A distance measuring sensor  1  according to a first embodiment of the disclosure will be described below using  FIGS. 3 to 8 . 
     [Distance Measuring Sensor  1 ] 
       FIG. 3  is a diagram showing the configuration of the distance measuring sensor  1  according to the first embodiment of the disclosure. As shown in  FIG. 3 , the distance measuring sensor  1  is a TOF-system distance measuring sensor that comprises a light source unit  10  and a light receiving unit  20 . 
     In the distance measuring sensor  1 , the light source unit  10  radiates irradiation light to a target  2 , and the light receiving unit  20  receives reflection light reflected by the target  2 . The distance measuring sensor  1  measures a time from when the light source unit  10  radiates the irradiation light to the target  2  until the light receiving unit  20  receives the reflection light reflected by the target  2 . That is, the distance measuring sensor  1  measures a round-trip time (a time of flight) of light. Based on the measured round-trip time of the light, the distance measuring sensor  1  calculates a distance to the target  2 . 
     [Light Source Unit  10 ] 
     The light source unit  10  radiates light to the target  2  as the irradiation light, the light being subjected to primary modulation so that the distance to the target  2  can be measured and being subjected to secondary modulation by a unique spreading system so that influences of disturbance light are reduced. This reduces influences of ambient light and disturbance light, such as light of another distance measuring sensor and makes it possible to measure highly accurate distance information. 
     The configuration of the light source unit  10  will be described below in more detail by using  FIG. 4 .  FIG. 4  is a diagram showing the configuration of the light source unit  10  in the distance measuring sensor  1  according to the first embodiment of the disclosure. As shown in  FIG. 4 , the light source unit  10  modulates light, generated from a light emitting element  11 , such as a light-emitting diode (LED) or a semiconductor laser (LD), with a modulation signal. The modulation signal is generated by subjecting a signal of the primary modulation to the secondary modulation. The light emitting element  11  in the light source unit  10  radiates light (modulation light) subjected to the primary modulation and the secondary modulation to the target  2  as the irradiation light. A system in which the light emitting element  11 , such as a light-emitting diode (LED) or a semiconductor laser (LD), is used to directly convert changes in the modulation signal into changes in the intensity of irradiation light radiated from the light emitting element  11  is referred to as a direct modulation system. 
     According to the above-described direct modulation system, it is possible to simplify the configuration of the light source unit  10 . Thus, unlike the invention described in PTL 2, the light source unit  10  does not require a relatively large optical system elements, such as two light sources and a dichroic mirror and a half mirror for aligning optical axes of the two light sources, in order to reduce influences of disturbance light. Accordingly, the distance measuring sensor  1  that comprises the light source unit  10  can be preferably utilized for small-size distance measuring sensors that are incorporated into smartphones. Also, since the light emitting element  11 , such as a light-emitting diode and a semiconductor laser, allows the light intensity of irradiation light to be changed according to electrical current that is applied, it is possible to easily modulate the amplitude of an irradiation modulation wave. 
     The light source unit  10  may subject at least one of the polarization, the amplitude, the frequency, and the phase of light to the primary modulation so that the distance to the target  2  can be measured. This makes it possible to subject the light to the modulation so that the distance to the target  2  can be more preferably measured. 
     Also, based on a direct spreading system using an m-sequence in a PN code, the light source unit  10  may subject the light to the secondary modulation so that influences of disturbance light are reduced. This makes it possible to more easily reduce influences of disturbance light. 
     The secondary modulation based on the direct spreading system using a PN code in an m-sequence, which is a pseudorandom number sequence, will be described below using  FIG. 5 .  FIG. 5  is a diagram for describing the secondary modulation based on the direct spreading system using a PN code in an m-sequence. As shown in  FIG. 5 , the light source unit  10  generates a spreading signal by combining a PN code in an m-sequence with a signal of light subjected to the primary modulation. By doing this, the light source unit  10  performs the secondary modulation for modulating the light subjected to the primary modulation into light of the spreading signal. 
     The m-sequence will be described below in more detail by using  FIG. 6 .  FIG. 6  is a diagram showing one example of an m-sequence generating circuit  30 . As shown in  FIG. 6 , the m-sequence generating circuit  30  comprises a shift register  31  and an XOR circuit (exclusive disjunction)  32 . The m-sequence generating circuit  30  causes the shift register  31  to shift the signal of the primary modulation bit by bit and performs arithmetic operation by using the XOR circuit  32 . By doing this, the m-sequence generating circuit  30  generates an m-sequence whose, autocorrelation becomes an impulse. 
     [Light Receiving Unit  20 ] 
     The light receiving unit  20  receives the reflection light subjected to the secondary modulation and subjects the reflection light subjected to the secondary modulation to secondary demodulation by using a unique spreading system so that influences of disturbance light are reduced. Thereafter, by using a signal of the primary modulation with respect to the reflection light subjected to the secondary demodulation, the light receiving unit  20  calculates a distance to the target  2  which is based on a TOF system. 
     The reflection light subjected to the secondary modulation is subjected to the secondary demodulation so that influences of disturbance light are reduced, as described above, to thereby reduce influences of ambient light and disturbance light, such as light of another distance measuring sensor, and make it possible to measure highly accurate distance information. 
     The configuration of the light receiving unit  20  will be described below in more detail by using  FIG. 7 .  FIG. 7  is a diagram showing the configuration of the light receiving unit  20  in the distance measuring sensor  1  according to the first embodiment of the disclosure. As shown in  FIG. 7 , the light receiving unit  20  receives the reflection light by using a light receiving element  21 , such as a photodiode (PD) or a CCD, and converts the reflection light into an electronic signal. By using a demodulation circuit  22 , the light receiving unit  20  can retrieve a demodulation signal by demodulating the electrical signal. Here, the demodulation circuit  22  may perform digital demodulation by performing digital signal processing digitized by an AD converter or may perform analog demodulation through diode wave detection. 
     The secondary modulation and the secondary demodulation based on the direct spreading system using a PN code in an m-sequence will be described below in more detail by using  FIG. 8 .  FIG. 8  is a diagram for describing the secondary modulation and the secondary demodulation based on the direct spreading system using a PN code in an m-sequence. The light source unit  10  spreads a narrow-width frequency spectrum of a signal of the light subjected to the primary modulation, as shown in (a)  FIG. 8 , in a frequency direction by combining a PN code in an m-sequence and subjecting the signal to the secondary modulation, as shown in (b) in  FIG. 8 . As shown (c) in  FIG. 8 , noise is superimposed on the irradiation light radiated from the light source unit  10  in a state in which the frequency spectrum extends in the frequency direction in the manner described above. The light receiving unit  20  then receives reflection light based on the irradiation light and reflected by the target  2 . Next, the light receiving unit  20  combines a PN code that is analogous to the PN code in the m-sequence used during the secondary modulation with the reflection light, as shown in (d) in  FIG. 8 . Thus, as shown in (d) in  FIG. 8 , the reflection light subjected to the secondary modulation is demodulated into a signal that is analogous to the original signal of the light subjected to the primary modulation, because of the property that autocorrelation of a PN code in an m-sequence becomes an impulse. Also, as a result of the above-described secondary demodulation, the light spread with an m-sequence that is different from the m-sequence used during the secondary demodulation by the light receiving unit  20  and uncorrelated disturbance noise are widely spread to thereby reduce the signal level. This makes it possible to reduce influences of light of another distance measuring sensor and disturbance light. 
     [Distance Measuring Method] 
     A distance measuring method according to the first embodiment of the disclosure is A TOF-system distance measuring method that measures a distance to a target  2  by measuring a time from when a light source unit  10  radiates irradiation light to the target  2  until a light receiving unit  20  receives reflection light reflected by the target  2 . The distance measuring method includes: a step of radiating light to the target  2  as the irradiation light, the light being subjected to primary modulation so that the distance to the target  2  can be measured and being subjected to secondary modulation so that influences of disturbance light are reduced; and a step of receiving the reflection light subjected to the secondary modulation and subjecting the reflection light subjected to the secondary modulation to secondary demodulation so that influences of disturbance light are reduced. 
     Second Embodiment 
     Next, a distance measuring sensor  1   a  according to a second embodiment of the disclosure will be described using  FIGS. 9 to 11 . For convenience of description, members having the same functions as the members described in the first embodiment are denoted by the same reference numerals, and descriptions thereof are omitted. 
     [Distance Measuring Sensor  1   a ] 
       FIG. 9  is a diagram showing the configuration of the distance measuring sensor  1   a  according to the second embodiment of the disclosure. As shown in  FIG. 9 , the distance measuring sensor  1   a  comprises a light source unit  10   a  and a light receiving unit  20   a , instead of the light source unit  10  and the light receiving unit  20  in the distance measuring sensor  1  according to the first embodiment. Except for this point, the distance measuring sensor  1   a  has a configuration that is analogous to that of the distance measuring sensor  1  according to the first embodiment. 
     [Light Source Unit  10   a ] 
     The configuration of the light source unit  10   a  will be described below using  FIG. 10 .  FIG. 10  is a diagram showing the configuration of the light source unit  10   a  in the distance measuring sensor  1   a  according to the second embodiment of the disclosure. As shown in  FIG. 10 , the light source unit  10   a  further comprises a modulation element  12  in addition to a light emitting element  11   a  instead of the light emitting element  11  in the first embodiment. Except for this point, the light source unit  10   a  has a configuration that is analogous to that of the light source unit  10  in the first embodiment. 
     The light source unit  10   a  controls the modulation element  12  by causing unmodulated light (non-modulation light) to be generated from the light emitting element  11   a , such as a light-emitting diode (LED) or a semiconductor laser (LD), and using a modulation signal. The modulation signal can be generated by subjecting the signal of the primary modulation to the secondary modulation. The light source unit  10   a  radiates modulation light (light) to the target  2  as irradiation light, the modulation light being subjected to the secondary modulation by externally supplying the modulation signal. A system in which a modulation signal is externally supplied to non-modulation light generated from the light emitting element  11   a  to thereby modulate the non-modulation light supplied to the modulation element  12  into modulation light, as described above, is referred to as “external modulation”. The external modulation is suitable for high-speed switching. 
     (Modulation Element  12 ) 
     Elements of lithium niobate (LiNbO 3 ) and so on which utilize an electro-optic effect in which modulation is performed using an electric field are available as the modulation element  12 . Elements of gallium phosphorous, germanium, and so on which utilize an acousto-optic effect in which modulation is performed using ultrasonic waves are also available as the modulation element  12 . In addition, various elements utilizing a magneto-optical effect, a thermo-optical effect, and a nonlinear optical effect are available as the modulation element  12 . By changing the refractive index of light, the modulation element  12  can easily subject at least one of the polarization, the amplitude, the frequency, and the phase of light to the primary modulation so that a distance to a target can be measured. 
     [Light Receiving Unit  20   a ] 
     The configuration of the light receiving unit  20   a  will be described below using  FIG. 11 .  FIG. 11  is a diagram showing the configuration of the light receiving unit  20   a  in the distance measuring sensor  1   a  according to the second embodiment of the disclosure. As shown in  FIG. 11 , the light receiving unit  20   a  comprises a light receiving element  21   a  and a demodulation circuit  22   a , instead of the light receiving element  21  and the demodulation circuit  22  in the first embodiment, and further comprises a demodulation element  23 . Except for this point, the light receiving unit  20   a  has a configuration that is analogous to that of the light receiving unit  20  in the first embodiment. 
     The demodulation element  23  subjects the reflection light to the secondary demodulation while it is in a reflection light state. The light receiving element  21   a , such as a photodiode (PD) or a CCD, receives the reflection light subjected to the secondary demodulation and converts the reflection light into an electrical signal. The demodulation circuit  22   a  can retrieve the demodulation signal from the electrical signal. Here, the demodulation circuit  22   a  may perform digital demodulation by performing digital signal processing digitized by an AD converter or may perform analog demodulation through diode wave detection. 
     Third Embodiment 
     Next, a distance measuring sensor  1   b  according to a third embodiment of the disclosure will be described using  FIGS. 12 to 14 . For convenience of description, members having the same functions as the members described in the above embodiments are denoted by the same reference numerals, and descriptions thereof are omitted. 
     [Distance Measuring Sensor  1   b ] 
       FIG. 12  is a diagram showing the configuration of the distance measuring sensor  1   b  according to the third embodiment of the disclosure. As shown in  FIG. 12 , the distance measuring sensor  1   b  comprises a light source unit  10   b  and a light receiving unit  20   b , instead of the light source unit  10   a  and the light receiving unit  20   a  in the distance measuring sensor  1   a  according to the second embodiment. Except for this point, the distance measuring sensor  1   b  has a configuration that is analogous to that of the distance measuring sensor  1   a  according to the second embodiment. 
     [Light Source Unit  10   b ] 
     The configuration of the light source unit  10   b  will be described below using  FIG. 13 .  FIG. 13  is a diagram showing the configuration of the light source unit  10   b  in the distance measuring sensor  1   b  according to the third embodiment of the disclosure. As shown in  FIG. 13 , the light source unit  10   b  comprises a light emitting element  11   b  and a modulation element  12   b , instead of the light emitting element  11   a  and the modulation element  12 . Except for this point, the light source unit  10   b  has a configuration that is analogous to that of the light source unit  10   a  in the second embodiment. 
     The light source unit  10   b  supplies a primary modulation signal to the light emitting element  11   b , such as a light-emitting diode (LED) or a semiconductor laser (LD), to thereby subject light generated from the light emitting element  11   b  to the primary modulation. The light emitting element  11   b  generates primary modulation light, and the light source unit  10   b  supplying a secondary modulation signal to the modulation element  12   b  to thereby subject the primary modulation light to the secondary modulation. Thus, the light source unit  10   b  externally supplies the secondary modulation signal to the primary modulation light generated from the light emitting element  11   b . By doing this, the light source unit  10   b  performs external modulation for modulating the primary modulation light, supplied to the modulation element  12   b , into modulation light subjected to the secondary modulation. The light source unit  10   b  radiates the modulation light (light) to the target  2  as the irradiation light, the modulation light being subjected to the secondary modulation by externally supplying the modulation signal to the primary modulation light (the light subjected to the primary modulation). 
     [Light Receiving Unit  20   b ] 
     The configuration of the light receiving unit  20   b  will be described below using  FIG. 14 .  FIG. 14  is a diagram showing the configuration of the light receiving unit  20   b  in the distance measuring sensor  1   b  according to the third embodiment of the disclosure. As shown in  FIG. 14 , the light receiving unit  20   b  comprises a light receiving element  21   b , a demodulation circuit  22   b , and a demodulation element  23   b , instead of the light receiving element  21 , the demodulation circuit  22 , and the demodulation element  23 . Except for this point, the light receiving unit  20   b  has a configuration that is analogous to that of the light receiving unit  20   a  in the second embodiment. 
     The demodulation element  23   b  subjects the reflection light to the primary demodulation while it is in a reflection light state. The light receiving element  21   b , such as a photodiode (PD) or a CCD, receives primary demodulation light, subjected to the primary demodulation, and converts the primary demodulation light into an electrical signal. The demodulation circuit  22   b  can retrieve a demodulation signal by subjecting the electrical signal to the secondary demodulation. Here, the demodulation circuit  22   b  may perform digital demodulation by performing digital signal processing digitized by an AD converter or may perform analog demodulation through diode wave detection. 
     Although combinations of the direct and external modulation/demodulation, such as a combination of the direct modulation and the direct demodulation, a combination of the external modulation and the external demodulation, have been described above in the embodiments described above, the disclosure is not limited to those combinations. In the disclosure, for example, the external modulation and the direct demodulation may be combined together, or the direct modulation and the external demodulation may be combined together. 
     Fourth Embodiment 
     Next, a distance measuring sensor  1   c  according to a fourth embodiment of the disclosure will be described below using  FIG. 15 . For convenience of description, members having the same functions as the members described in the above embodiments are denoted by the same reference numerals, and descriptions thereof are omitted. 
     [Distance Measuring Sensor  1   c ] 
       FIG. 15  is a diagram showing the configuration of the distance measuring sensor  1   c  according to the fourth embodiment of the disclosure. As shown in  FIG. 15 , the distance measuring sensor  1   c  comprises a light receiving unit  20   c , instead of the light receiving unit  20  in the distance measuring sensor  1  according to the first embodiment. Except for this point, the distance measuring sensor  1   c  has a configuration that is analogous to that of the distance measuring sensor  1  according to the first embodiment. 
     [Light Receiving Unit  20   c ] 
     As shown in  FIG. 15 , the light receiving unit  20   c  comprises a plurality of light receiving units  20  arranged in an array form. The light receiving unit  20   c  in which the plurality of light receiving units  20  (for example, CCDs, CMOS imagers, and so on) is arranged in an array form receives reflection light reflected by a target  2  after the light source unit  10  radiates irradiation light to the target  2 . The light receiving unit  20   c  can create a distance map by calculating distances to the target  2  for the respective light receiving units  20 . 
     [Modification] 
     Since a distance map can be created as in the examples described above, the distance measuring sensor  1   c  can also function as a distance measuring camera. That is, the distance measuring camera is a TOF-system distance measuring camera that measures a distance to a target  2  by measuring a time from when a light source unit  10  radiates irradiation light to the target  2  until a light receiving unit  20   c  receives reflection light reflected by the target  2 . The distance measuring camera comprises: a light source unit  10  that radiates light to the target  2  as the irradiation light, the light being subjected to primary modulation so that the distance to the target  2  can be measured and being subjected to secondary modulation so that influences of disturbance light are reduced; and a plurality of light receiving units  20  that receives the reflection light subjected to the secondary modulation and that subjects the reflection light subjected to the secondary modulation to secondary demodulation so that influences of disturbance light are reduced. The plurality of light receiving units  20  is arranged in an array form. 
     The disclosure is not limited to each embodiment described above, various changes are possible within the scope recited in the claims, and embodiments obtained by appropriately combining the technical means respectively disclosed in the different embodiments are also encompassed by the technical scope of the disclosure. In addition, new technical features can be formed by combining the technical means respectively disclosed in the embodiments.