Distance measuring device and distance measuring method

Abstract

A distance measurement device according to one embodiment of the present disclosure comprises: a light-receiving part that receives reflected light from an object of distance measurement based on radiated light from a light-emitting part; a histogram-acquiring part that acquires a histogram showing how frequently reflected light is received by the light-receiving part; and a computing part that calculates the distance to the object of distance measurement on the basis of a time corresponding to the peak in the histogram acquired by the histogram-acquiring part. On the basis of the shape of the histogram acquired by the histogram-acquiring part, the computing part corrects the distance calculated on the basis of the time corresponding to the histogram peak.

Description

Technical Field

[0001] The present disclosure relates to a distance measuring device and a distance measuring method.

Background Art

[0002] As a distance measuring device (so-called distance meter) for measuring the distance to a distance measurement object (subject), there is a device (sensor) using the ToF (Time of Flight) method (see, for example, Patent Document 1). The ToF method irradiates light from a light emitting unit (light source) onto a distance measurement object (subject), and measures the distance to the distance measurement object by detecting the flight time of the light until the irradiated light is reflected by the distance measurement object and returns to the light receiving unit.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

[0004] By the way, among distance measurement objects, there are objects that cause a phenomenon called subsurface scattering, in which light incident from the surface of the object is scattered inside the object and then emitted to the outside. If the light irradiated from the light emitting unit is reflected on the surface of the distance measurement object, the distance to the distance measurement object can be correctly measured. However, in the case where the distance measurement object is an object that causes subsurface scattering, the time from the light emission time at the light emitting unit to the light reception time at the light receiving unit includes the time caused by subsurface scattering, so that the distance to the distance measurement object cannot be correctly measured.

[0005] In the prior art, the time caused by subsurface scattering in the distance measurement object has not been considered. Therefore, in the case where the distance measurement object is an object that causes subsurface scattering, the distance to the distance measurement object could not be correctly measured.

[0006] When the object to be measured is an object that causes sub-surface scattering, it is desirable to provide a distance measurement device and a distance measurement method that can correctly measure the distance to the object to be measured while considering the time caused by sub-surface scattering within the object to be measured.

[0007] A distance measurement device according to an embodiment of the present disclosure includes a light receiving unit that receives reflected light from an object to be measured based on irradiation light from a light emitting unit and , a histogram acquisition unit that acquires a histogram indicating the light reception frequency of the reflected light by the light receiving unit and , the histogram acquired by the histogram acquisition unit In [case name 1] and [case name 2], the standard deviation is obtained as the degree of spread in the time direction around the peak, a correction amount is obtained from the standard deviation, and the correction amount is subtracted from the average value of the histogram, and an arithmetic unit that calculates the distance to the object to be measured and . is .

[0008] A distance measurement method according to an embodiment of the present disclosure is a distance measurement device including a light receiving unit that receives reflected light from an object to be measured based on irradiation light from a light emitting unit, and a histogram acquisition unit that acquires a histogram indicating the light reception frequency of the reflected light by the light receiving unit. In the distance measurement device, the distance to the object to be measured is calculated In [case name 1] and [case name 2], the standard deviation is obtained as the degree of spread in the time direction around the peak, a correction amount is obtained from the standard deviation, and the correction amount is subtracted from the average value of the histogram, . to perform .

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Embodiments for Carrying Out the Invention

[0010] Hereinafter, embodiments for carrying out the technology of the present disclosure (hereinafter referred to as "embodiments") will be described in detail with reference to the drawings. The technology of the present disclosure is not limited to the embodiments. In the following description, the same reference numerals will be used for the same elements or elements having the same function, and redundant descriptions will be omitted. The description will be made in the following order. 1. General description of the distance measurement device and distance measurement method of the present disclosure 2. Distance measurement device according to the embodiment of the present disclosure 2-1. Configuration example of the distance measurement device 2-2. Regarding Histogram 2-3. Regarding Distance Measurement Error due to Sub-Surface Scattering 2-4. Functional Blocks of the Arithmetic Unit 2-5. Distance Measurement Method 2-5-1. Distance Measurement Processing by the Arithmetic Unit 2-5-2. Calculation Processing of Generation Rate 2-5-3. Calculation Processing of Ambient Light Incidence Probability 3. Modification Example 4. Application Example of the Technology of the Present Disclosure (Example of a Smartphone) 5. Configurations that the Present Disclosure Can Adopt

[0011] <General Explanation of the Distance Measurement Device and Distance Measurement Method of the Present Disclosure> In the distance measurement device and distance measurement method of the present disclosure, the light receiving element of the light receiving unit can be configured to be an avalanche photodiode operating in Geiger mode, preferably, a single photon avalanche diode.

[0012] In the distance measurement device and distance measurement method of the present disclosure including the above-described preferred configuration, the object to be distance-measured can be in a form that causes sub-surface scattering.

[0013] Further, in the distance measurement device and distance measurement method of the present disclosure including the above-described preferred configuration and form, regarding the histogram acquisition unit, it can be configured to include a time difference detection unit that detects the time from the time when the light emitting unit emits irradiation light to the time when the light receiving unit receives the reflected light from the object to be distance-measured, and an accumulation unit that forms a histogram based on the time detected by the time difference detection unit. Also, regarding the shape of the histogram, it can be configured to be the degree of spread around the peak in the reflected light distribution from the object to be distance-measured, which is obtained by performing predetermined statistical processing on the histogram acquired by the histogram acquisition unit.

[0014] In the distance measurement device and distance measurement method of the present disclosure including the above-described preferred configurations and forms, regarding the calculation unit, the degree of spread around the peak in the reflected light distribution from the object to be distance-measured is measured, a correction amount is calculated from the degree of spread around the peak, and correction is performed based on the calculated correction amount. Also, regarding the calculation unit, the distance can be corrected by subtracting the correction amount calculated from the degree of spread around the peak from the peak in the reflected light distribution.

[0015] In the distance measurement device and distance measurement method of the present disclosure including the above-described preferred configurations and forms, regarding the reflected light distribution, it is a distribution obtained by subtracting the ambient light component from the histogram acquired by the histogram acquisition unit, and the degree of spread around the peak in the reflected light distribution can be configured to be the standard deviation. Also, regarding the correction amount calculated from the degree of spread around the peak, it can be configured to be the reciprocal of the exponential distribution parameter obtained from the standard deviation.

[0016] <Distance Measurement Device According to Embodiment of the Present Disclosure> In the distance measurement device according to an embodiment of the present disclosure (hereinafter sometimes abbreviated as "the present embodiment"), as a measurement method for measuring the distance to the object to be distance-measured, a ToF method is adopted in which the flight time until the pulsed light (for example, laser light having a peak wavelength in the infrared wavelength range) irradiated toward the object to be distance-measured is reflected by the object to be distance-measured and returns is measured.

[0017] The ToF method includes an indirect ToF method and a direct ToF method. Indirect ToF is a method in which pulsed light with a predetermined period emitted from the light emitting unit is reflected by the object to be distance-measured, the period when the receiving unit receives the reflected light is detected, the flight time of light is measured from the phase difference between the emission period and the reception period, and the distance to the object to be distance-measured is measured. Direct ToF is a method of directly measuring the distance to the object to be distance-measured from the flight time difference of light. In the distance measurement device according to the present embodiment, as the ToF method, the latter method, that is, the direct ToF method is used.

[0018] [Configuration Example of Distance Measurement Device] FIG. 1 is a block diagram showing an example of the configuration of a distance measurement device according to an embodiment of the present disclosure. As shown in FIG. 1, the distance measurement device 10 according to the present embodiment includes a light emitting unit 11, a light receiving unit 12, a time difference detection unit 13, an integration unit 14, an arithmetic unit 15, a control unit 16, and an output terminal 17, and is configured to measure the distance to the measurement object 20 using the direct (direct) ToF method. Here, let the distance to the measurement object 20 be L.

[0019] The light emitting unit 11, the light receiving unit 12, the time difference detection unit 13, the integration unit 14, and the arithmetic unit 15 execute their respective processes under the control of the control unit 16 configured by an information processing device such as a CPU (Central Processing Unit). Hereinafter, each process of the light receiving unit 12, the time difference detection unit 13, the integration unit 14, and the arithmetic unit 15 executed under the control of the control unit 16 will be described.

[0020] In the light emitting unit 11, for example, a laser light source can be exemplified as the light source (light emitting element). Under the control of the control unit 16, in the light emitting unit 11, short-time light emission by the laser light source is performed, and the pulsed light 40 reaches the measurement object 20 as active light. The pulsed light 40 emitted from the light emitting unit 11 is reflected by the measurement object 20 and returns to the light receiving unit 12 as reflected light 41.

[0021] Of course, the pulsed light 40 irradiated from the light emitting unit 11 toward the measurement object 20 is not always reflected and returned by the measurement object 20. That is, probabilistically, there may be cases where it is reflected and returned by the measurement object 20, and there may also be cases where it does not return.

[0022] In addition to the light emission from the light emitting unit 11, the light 42 from the sun 30 also reaches the measurement object 20 as ambient light. The light 42 from the sun 30 is reflected by the measurement object 20 and reaches the light receiving unit 12 as light 43.

[0023] The light-receiving unit 12, similar to the pixel array unit in the imaging device, has pixels including light-receiving elements arranged in a two-dimensional array. The light-receiving unit 12 receives the reflected light 41 from the distance measurement target 20 based on the pulsed light 40 irradiated from the light-emitting unit 11 toward the distance measurement target 20. A lens (not shown) is attached to the incident portion of the light-receiving unit 12, and due to the condensing action of this lens, the reflected light 41 from the distance measurement target 20 can be efficiently received by the pixels including the light-receiving elements. Note that in this specification, the lens is not an important component, so its illustration is omitted.

[0024] In the distance measurement device 10 according to this embodiment, as the light-receiving element of the light-receiving unit 12, an element that generates a signal in response to the reception of photons, for example, a SPAD (Single Photon Avalanche Diode) element is used. The SPAD element is a type of avalanche photodiode that utilizes a phenomenon called avalanche multiplication to increase the light-receiving sensitivity, and operates in the Geiger mode in which the element is operated at a reverse voltage exceeding the breakdown voltage (yield voltage).

[0025] Here, the SPAD element is exemplified as the light-receiving element of the light-receiving unit 12, but it is not limited to the SPAD element. That is, as the light-receiving element of the light-receiving unit 12, in addition to the SPAD element, various elements that operate in the Geiger mode, such as APD (Avalanche Photodiode) and SiPM (Silicon Photomultiplier), can be used.

[0026] The time difference detection unit 13 adopts the ToF method as the distance measurement method, and more specifically, the direct ToF method. The time difference detection unit 13 is generally composed of a TDC (Time-to-Digital Converter). Under the control of the control unit 16, the time difference detection unit 13 measures the time from the time when the pulsed light 40 is emitted by the light emitting unit 11 (emission time) to the time when the reflected light 41 is received by the light receiving unit 12 (reception time), that is, the time from the emission time at the light emitting unit 11 to the reception time at the light receiving unit 12. Then, by multiplying the measured time by the speed of light c and dividing the result by 2, the distance to the distance measurement object 20 can be obtained. The reason for dividing by 2 is that the time for light to travel the round-trip distance to the distance measurement object 20 is measured.

[0027] By the way, there is a possibility of false determination in one emission by the light emitting unit 11 and one reception by the light receiving unit 12 based on that one emission. This is because ambient light may enter the light receiving unit 12 within the time from the emission by the light emitting unit 11 to the reflection by the distance measurement object 20 and return to the light receiving unit 12, causing the SPAD element (SPAD sensor) of the light receiving unit 12 to react. Also, the pulsed light 40 emitted from the light emitting unit 11 does not necessarily reflect off the distance measurement object 20 and return to the light receiving unit 12. These occur probabilistically.

[0028] For the reasons described above, under the control of the control unit 16, short-time emissions by the light emitting unit 11 are performed a plurality of times M (for example, several thousand to several tens of thousands of times). That is, the distance measurement device 10 according to this embodiment performs measurements by emitting and receiving light a plurality of times M, and detects dominant data from the measurement results of these plurality of times M.

[0029] The detection result of the time difference detection unit 13, that is, the value of the measured time, is sent to the accumulation unit 14. The accumulation unit 14 has a memory (not shown), and forms histogram data on that memory. Details of the histogram will be described later. In the accumulation unit 14, the histogram is updated by incrementing by "1" only the bin of the histogram corresponding to the value of time.

[0030] Still, the time difference detection unit 13 and the accumulation unit 14 are an example of the histogram acquisition unit described in the claims, that is, the histogram acquisition unit that acquires a histogram indicating the light reception frequency of the reflected light by the light reception unit 12. After M measurements, the data of the final histogram obtained by the accumulation unit 14 is sent to the calculation unit 15.

[0031] The calculation unit 15 calculates the distance L to the distance measurement target 20 based on the time corresponding to the peak of the final histogram obtained by the accumulation unit 14. The calculation unit 15 further corrects the information on the distance L calculated based on the time corresponding to the peak of the histogram based on the shape of the final histogram obtained by the accumulation unit 14. The information (distance information to the distance measurement target 20) obtained by the process of the calculation unit 15 is output from the output terminal 17.

[0032] By the way, the SPAD element used as the light receiving element of the light receiving unit 12 is a sensor that detects the first incident light (photon). Therefore, if the pulsed light 40 irradiated from the light emitting unit 11 arrives at the distance measurement target 20 and the reflected light 41 arrives earlier in time than the light 43 that is reflected by the distance measurement target 20 from the sun 30 and then incident, the distance measurement can be performed correctly.

[0033] Hereinafter, in some cases, the light 41 that is reflected by the distance measurement target 20 from the pulsed light 40 irradiated from the light emitting unit 11 may be described as active light, and the light 43 that is reflected by the distance measurement target 20 from the light 42 from the sun 30 may be described as ambient light.

[0034] The time detected by the time difference detection unit 13 is the round-trip time to the distance measurement target 20, and the distance L to the distance measurement target 20 can be calculated by multiplying by c / 2 (c is the speed of light). On the other hand, when the light 43 arrives earlier in time than the light 41, the correct time for distance measurement cannot be measured.

[0035] The reception of the light 43 which is ambient light (reception of ambient light) occurs at a constant probability regardless of time. On the other hand, the reception of the light 41 which is active light (reception of active light) occurs concentrated at a certain time (more specifically, the value obtained by dividing the value obtained by doubling the distance to the distance measurement target 20 by the speed of light c). Therefore, by detecting a peak on the histogram and obtaining the time corresponding to the peak, the distance L to the distance measurement target 20 can be obtained.

[0036] Thus, the process of detecting a peak from the histogram and the process of calculating the distance L by multiplying the time corresponding to the peak by c / 2 are executed in the arithmetic unit 15 under the control of the control unit 16. Details of the functions of the arithmetic unit 15 will be described later.

[0037] [Regarding the histogram] Here, a histogram showing the reception frequency of the reflected light by the light receiving unit 12 will be described. FIG. 2 is a diagram showing an example of a histogram generated by the accumulation unit 14 in the distance measurement device 10 according to the embodiment of the present disclosure.

[0038] In this histogram, time (horizontal axis) is expressed in units of width D. That is, if the value of the time detected by the time difference detection unit 13 is in the range from time 0 to time D, it is added to the frequency of the 0th bin. If it is in the range from time D to time 2D, it is added to the frequency of the 1st bin. If it is in the range from time 2D to time 3D, it is added to the frequency of the 2nd bin. Thereafter, similarly, if it is in the range from time (N - 1)×D to time N×D, it is added to the frequency of the (N - 1)th bin.

[0039] Here, D is the resolution of the TDC.

[0040] Note that in one measurement, the measurement time is limited to N×D. That is, if the light reception by the light receiving unit 12 does not occur even after N×D has elapsed since the light emission by the light emitting unit 11, the measurement is terminated there. In this case, the time difference detection unit 13 does not output the value of time, and the update of the histogram in the accumulation unit 14 is not performed. Here, N is a constant.

[0041] Therefore, for the final histogram obtained after M measurements, the sum of the frequencies of all bins may be less than M in some cases.

[0042] Let the histogram be h(n). Here, n represents the bin number and is an integer from 0 to N - 1.

[0043] [Regarding the ranging error due to subsurface scattering] By the way, among the ranging objects 20, there are objects that cause a phenomenon where light incident from the surface of the object is scattered inside the object and then emitted to the outside, that is, subsurface scattering. Examples of objects that cause subsurface scattering include, for example, human skin. If the light emitted from the light emitting unit 11 is reflected on the surface of the ranging object 20, the distance to the ranging object 20 can be correctly measured. However, when the ranging object 20 is an object that causes subsurface scattering, the time from the light emission time at the light emitting unit 11 to the light reception time at the light receiving unit 12 includes the time caused by subsurface scattering, so the distance to the ranging object 20 cannot be correctly measured, that is, a ranging error occurs.

[0044] Here, the ranging error caused by subsurface scattering in the ranging object 20 will be described with reference to FIG. 3. FIG. 3 shows an excerpt from FIG. 1 of the parts necessary for explaining the ranging error caused by subsurface scattering.

[0045] In FIG. 3, the object 20 to be distance-measured is an object that causes sub-surface scattering. In this case, the light 40 from the light-emitting unit 11 enters the object 20 to be distance-measured (40a in FIG. 3). This is sub-surface scattering. Then, it travels outside the object 20 to be distance-measured (41 in FIG. 3). Therefore, the optical path length is the total length of the optical path of the light 40 in FIG. 3, the optical path of the sub-surface scattering 40a, and the optical path of the light 41. That is, the optical path length becomes longer by the optical path of the sub-surface scattering 40a than the sum of the optical paths of the light 40 and the light 41 in FIG. 3, and the distance measurement result becomes a value larger than the actual distance L.

[0046] Note that in FIG. 3, only one (40a) optical path of the sub-surface scattering is drawn, but actually, there are a plurality of optical paths of the sub-surface scattering. That is, there is light that scatters inside the object 20 to be distance-measured for a long time (i.e., with a long optical path length) and then exits to the outside, and there is also light that scatters for a short time (i.e., with a short optical path length) and then exits to the outside.

[0047] The light that stays inside the object 20 to be distance-measured for a long time passes through a long optical path inside the object 20 to be distance-measured, and thus is absorbed accordingly. Let the residence time inside be ts. After ts time has elapsed since the light was incident on the object 20 to be distance-measured, the intensity of the light emitted from the object 20 to be distance-measured is, for example, an exponential distribution, that is,

Equation

[0048] This will be supplemented with reference to FIG. 4. FIG. 4 shows the intensity of the light emitted from the object 20 to be distance-measured at each time when light emitted for a short time is irradiated on the surface of the object 20 to be distance-measured. Here, the short-time light emission is instantaneous light emission and is light emission for a short time that can be approximated by a delta function.

[0049] In the above description, it was assumed that the emitted light is a delta function, but the light actually emitted by the light emitting unit 11 (that is, the light irradiated onto the distance measurement target 20) has a width in the time direction. For example, assuming that the light emission from the light emitting unit 11 can be approximated by a normal distribution, the intensity of the light from the light emitting unit 11 at time t is [Number] where σ is the standard deviation. Also, Equation (2) is normalized so that it becomes 1 when integrated in the time direction.

[0050] Note that the value of the standard deviation σ in Equation (2) can be made known by measuring it in advance. That is, in advance, in a dark room, an object made of a material such as metal and having no subsurface scattering is placed in front of the distance measuring device 10, and a distance measurement operation is performed on the object to create a histogram. Then, the standard deviation σ can be obtained from the shape of this histogram, more specifically, from the degree of spread around the peak in the reflected light distribution from the distance measurement target obtained by performing a predetermined statistical process on the histogram.

[0051] The waveform of the active light 40 irradiated by the light emitting unit 11 toward the distance measurement target 20 is shown in FIG. 5A. Here, the distance measurement target 20 at a distance L is an object made of a material such as metal and having no subsurface scattering. In this case, the light received by the light receiving unit 12 has the waveform shown in FIG. 5B.

[0052] Let the attenuation amount h be the total of the attenuation due to traveling a distance of 2×L and the attenuation due to the reflectivity of the distance measurement target 20. The intensity of the light when the light irradiated from the light emitting unit 11 is reflected by the distance measurement target 20 and received by the light receiving unit 12 is [Number] It becomes as follows. Equation (3) is a normal distribution, and the attenuation amount h and the time delay 2L / c are taken into account. Since sub-surface scattering does not occur, the standard deviation is σ as in the case of the light from the light-emitting unit 11.

[0053] Also, in FIG. 5B, the reception of light (ambient light) from the sun 30 is considered. Since the ambient light is always constant, if the intensity of the ambient light component is E, the intensity of the light received by the light-receiving unit 12 is

Number

[0054] In the case shown in FIG. 5B, since the peak position is the time 2L / c, by detecting the peak position and multiplying it by c / 2, the correct distance L can be obtained.

[0055] Next, consider the case where sub-surface scattering occurs at the distance measurement target 20 at the distance L. In this case, the light received by the light-receiving unit 12 has the waveform shown in FIG. 5C.

[0056] When the light irradiated from the light-emitting unit 11 is reflected by the distance measurement target 20 and received by the light-receiving unit 12, the intensity of the light is the intensity when sub-surface scattering occurs. Therefore, it is the intensity obtained by performing the convolution operation of Equation (1) and Equation (2). That is, it becomes a Gaussian distribution that is exponentially modified. Specifically,

Number

[0057] In addition, in Equation (5), the attenuation rate h is also taken into account. Furthermore, the intensity E of the light (ambient light) from the sun 30 is also taken into account. However, μ in Equation (5) is

Number

[0058] When the situation is as shown in FIG. 5C, that is, when sub-surface scattering occurs in the distance measurement object 20 at the distance L, since the peak position is behind the time 2L / c, even if the peak position is detected and multiplied by c / 2, the correct distance L cannot be obtained.

[0059] Therefore, in the distance measurement device 10 according to the present embodiment, in the arithmetic unit 15, for the distance calculated based on the time corresponding to the peak of the histogram obtained by the accumulation unit 14, correction is performed based on the shape of the histogram, more specifically, based on the degree of spread around the peak of the histogram. Thereby, even when the distance measurement object 20 is composed of an object in which sub-surface scattering occurs, the distance L to the distance measurement object 20 can be accurately calculated.

[0060] [Functional block diagram of the arithmetic unit] The arithmetic unit 15 calculates the distance to the distance measurement object based on the time corresponding to the peak of the histogram acquired by the histogram acquisition unit composed of the time difference detection unit 13 and the accumulation unit 14, and performs a process of correcting this calculated distance based on the shape of the histogram. The arithmetic unit 15 is composed of, for example, a CPU, and has the following functional units in order to perform the above processes. A functional block diagram of an example of the arithmetic unit 15 is shown in FIG. 6.

[0061] The arithmetic unit 15 has functional units of an occurrence rate calculation unit 151, an ambient light arrival rate calculation unit 152, an active light arrival rate calculation unit 153, an average value and standard deviation calculation unit 154, a λ calculation unit 155, a correction processing unit 156, and a c / 2 multiplication unit 157.

[0062] The occurrence rate calculation unit 151 calculates the occurrence rate based on the histogram h(n) created by the accumulation unit 14. The occurrence rate is the number of occurrences of a certain event per unit time. Therefore, the occurrence rate referred to here is the number of times photons fly to the SPAD element of the light receiving unit 12 per unit time. The occurrence rate in each bin is the probability that photons fly in the time corresponding to that bin. Let the occurrence rate in bin n be p(n).

[0063] The ambient light arrival rate calculation unit 152 calculates the probability that the light (ambient light) from the sun 30 arrives at the SPAD elements of the light receiving unit 12. The active light arrival rate calculation unit 153 calculates the probability that the active light from the light emitting unit 11 arrives at the SPAD elements of the light receiving unit 12 by subtracting the arrival rate of the ambient light obtained by the ambient light arrival rate calculation unit 152 from the generation rate p(n) obtained by the generation rate calculation unit 151.

[0064] Here, the process of calculating the probability that the active light arrives at the SPAD elements of the light receiving unit 12 will be described more specifically using the waveform diagrams of FIGS. 7A and 7B. Note that the waveform diagram of FIG. 7A is the same as the waveform diagram of FIG. 5C.

[0065] In the active light arrival rate calculation unit 153, by subtracting the arrival rate of the ambient light obtained by the ambient light arrival rate calculation unit 152 from the generation rate p(n) obtained by the generation rate calculation unit 151, data of the waveform shown in FIG. 7B in which the ambient light component E shown in FIG. 7A is subtracted from the generation rate p(n) is obtained. The waveform in FIG. 7B represents the time change of the probability that the active light from the light emitting unit 11 arrives at the SPAD elements of the light receiving unit 12. Note that in FIG. 7B, the average value of the exponentially modified Gaussian distribution is μ+(1 / λ), and the standard deviation is √{σ2+(1 / λ2)}.

[0066] The average value and standard deviation calculation unit 154 calculates the average value and standard deviation of the probability that the active light from the light emitting unit 11 arrives at the SPAD elements of the light receiving unit 12, which is obtained by the active light arrival rate calculation unit 153. The λ calculation unit 155 obtains the object-specific value λ from the width (standard deviation) σ of the active light irradiated from the light emitting unit 11 toward the distance measurement target object 20 and the standard deviation (=√{σ2+(1 / λ2)}) obtained by the average value and standard deviation calculation unit 154. Note that the object-specific value λ is known.

[0067] The correction processing unit 156 obtains the time obtained by subtracting 1 / λ, which is the correction amount, from the average value obtained by the average value and standard deviation calculation unit 154. This time is the round-trip time μ of light to the distance measurement object 20. That is, in the correction processing unit 156, as shown in FIG. 7C, the average value obtained by the average value and standard deviation calculation unit 154 (= μ + (1 / λ)), that is, the peak in the reflected light distribution the time corresponding to Distance correction processing is performed by subtracting 1 / λ as the correction amount therefrom.

[0068] The c / 2 multiplication unit 157 multiplies the time (round-trip time μ of light) obtained by the light round-trip time calculation unit 156 by c / 2 (c is the speed of light), and outputs the multiplied value as the distance measurement result. This distance measurement result is the correct distance L to the distance measurement object 20 considering the time caused by subsurface scattering within the distance measurement object 20.

[0069] [Distance Measurement Method] Subsequently, in the distance measurement device 10 according to the present embodiment shown in FIG. 1, the processing procedure of the distance measurement method executed by the arithmetic unit 15 under the control of the control unit 16 will be described using the flowchart of FIG. 8. The processing of this distance measurement method is executed under the control of the CPU constituting the arithmetic unit 15 in the case of a configuration in which the functions of the arithmetic unit 15 are realized by the CPU.

[0070] In the distance measurement device 10 according to the present embodiment shown in FIG. 1, in the accumulation unit 14, a histogram h(n) indicating the light reception frequency of the reflected light by the light reception unit 12 is created. The process of calculating the correct distance from this histogram h(n) is performed by the arithmetic unit 15.

[0071] (Distance Measurement Processing by Arithmetic Unit) The CPU (hereinafter simply described as "CPU") constituting the arithmetic unit 15 first acquires the histogram h(n) created in the accumulation unit 14 (step S1), and then calculates the generation rate p(n), which is the probability that photons fly to the SPAD element of the light reception unit 12, based on the histogram h(n) (step S2). The process of calculating the generation rate p(n) will be described later.

[0072] It still seems that it takes time to calculate the occurrence rate p(n). If you want to further speed it up, you may approximate H(n) = h(n). That is, the occurrence rate p(n) may be approximated to be the same as the histogram. By this approximation, the calculation processing time in step S2 can be substantially eliminated.

[0073] Next, the CPU calculates the probability that the light (ambient light) from the sun 30 reaches the SPAD element of the light receiving unit 12 (step S3). Then, by subtracting the arrival rate of the ambient light obtained in step S3 from the occurrence rate obtained in step S2, the CPU obtains the probability that the light (active light) from the light emitting unit 11 reaches the SPAD element of the light receiving unit 12 (step S4). The process of calculating the probability that the ambient light reaches the SPAD element of the light receiving unit 12 will be described later.

[0074] When subsurface scattering occurs in the ranging object 20, the waveform of the light received by the SPAD element of the light receiving unit 12 is shown in FIG. 7A, and the waveform representing the time change of the probability that the active light from the light emitting unit 11 reaches the SPAD element of the light receiving unit 12 is shown in FIG. 7B. The waveform shown in FIG. 7B is the waveform obtained by subtracting the ambient light component E shown in FIG. 7A from the occurrence rate p(n) obtained in step S2. In FIG. 7B, the average value of the exponentially modified Gaussian distribution is μ+(1 / λ), and the standard deviation is √{σ2+(1 / λ2)}.

[0075] Next, the CPU obtains the average value and the standard deviation of the probability that the active light from the light emitting unit 11 reaches the SPAD element of the light receiving unit 12 (FIG. 7B) obtained in step S4 (step S5). Then, the CPU obtains the object-specific value λ from the width (standard deviation) σ of the active light from the light emitting unit 11 and the standard deviation of the probability reaching the light receiving unit 12 obtained in step S5 (step S6). Specifically, the object-specific value λ is obtained from the standard deviation (=√{σ2+(1 / λ2)}) obtained in step S5. The object-specific value λ is known.

[0076] Next, as shown in FIG. 7C, the CPU obtains the time obtained by subtracting 1 / λ from the average value of the probability of light reaching the light receiving unit 12 obtained in step S5, that is, the round-trip time μ of light to the distance measurement object 20 (step S7). Next, the CPU multiplies the time obtained in step S7, that is, the round-trip time μ of light to the distance measurement object 20, by c / 2 and outputs the result as the distance measurement result (step S8). Then, a series of processes of this distance measurement method are terminated.

[0077] Here, a supplementary explanation of a series of processes of the above distance measurement method will be given with reference to FIGS. 7B and 7C.

[0078] The data shown in FIG. 7B represents the time change of the probability that the light (active light) from the light emitting unit 11 reaches the SPAD element of the light receiving unit 12. Therefore, its average value is μ+(1 / λ), and the standard deviation is √{σ2+(1 / λ2)}. Therefore, using the known width (standard deviation) σ of the active light, the object-specific value λ can be obtained, and further, the round-trip time μ of light to the distance measurement object 20 can be obtained. This is shown in FIG. 7C. By subtracting 1 / λ from the average value of the exponentially modified Gaussian distribution (FIG. 7B), the round-trip time μ of light to the distance measurement object 20 can be obtained.

[0079] Since the round-trip time μ of light to the distance measurement object 20 has the relationship of Equation 6, the accurate distance L to the distance measurement object 20 can be calculated in the process of step S8.

[0080] In this way, by applying the technology according to the present disclosure, the distance L to the distance measurement object 20 where subsurface scattering occurs can be accurately measured.

[0081] Now, in the above description, it was explained that "the value of σ is known by measuring it in advance." However, if it is not known, the following processing may be performed. That is, in step S5, although the average value and the standard deviation were obtained, the skewness is further obtained. Then, the value of σ may be calculated and obtained by the calculation shown in the literature (Olivier J., Norberg, M. M., (2010). Positively Skewed Data: Revisiting the Box-Cox Power Transformation. International Journal of Psychological Research, 3(1), 68-75.).

[0082] (Calculation process of occurrence rate) Subsequently, the process of step S2 described above, that is, the calculation process of the occurrence rate p(n) (the probability that photons fly to the SPAD element of the light receiving unit 12) will be described. FIG. 9 is a flowchart showing an example of the process of calculating the occurrence rate. This process is also executed under the control of the CPU constituting the arithmetic unit 15.

[0083] The CPU first inputs a numerical value M which is the number of measurement times performed for histogram creation (step S21), and then acquires the histogram data created by the accumulation unit 14 (step S22). In step S22, specifically, the frequency h(n) in each bin n of the histogram is acquired. Here, n = 0 to N - 1.

[0084] After the CPU acquires the frequency h(n) in each bin n of the histogram, it calculates M(n - 1) defined by the following formula (7) (step S23). [Number] Formula (7) is the probability that the frequency of bin n becomes h(n) although h(m) times occurred in bin m in M measurements. Here, n = 0 to N - 1.

[0085] Next, the CPU calculates H(n) = h(n) / M(n - 1) for n = 0 to N - 1 (step S24). Here, H(n) is the mean value in the normal distribution of the following equation (8), that is, the occurrence rate p(n).

[0086]

Number

[0087] Next, the CPU calculates σ(n) = √(1 / M(n - 1) × h(n) / M(n - 1) × {1 - (h(n) / M(n - 1))}) for n = 0 to N - 1 (step S25). Here, σ(n) is the standard deviation in the normal distribution of equation (8). Next, the CPU outputs H(n) as the occurrence rate p(n) and σ(n) as the standard deviation for n = 0 to N - 1, ends a series of processes for calculating the occurrence rate p(n), and returns to step S3 in FIG. 8.

[0088] (Calculation process of ambient light arrival probability) Subsequently, the process of step S3 described above, that is, the process of calculating the ambient light arrival probability (the probability that the ambient light from the sun 30 arrives at the SPAD element of the light receiving unit 12) will be described. FIG. 10 is a flowchart showing an example of the process of calculating the probability that the ambient light arrives at the SPAD element. This process is also executed under the control of the CPU that constitutes the arithmetic unit 15.

[0089] In the calculation process of the ambient light arrival probability, the occurrence rate p(n) obtained by the process of step S2 in FIG. 8, that is, the occurrence rate calculation process described in FIG. 9, is used. First, the CPU acquires the occurrence rate p(n) obtained by the occurrence rate calculation process described in FIG. 9 (step S31), and then obtains the probability (ambient light arrival probability) Iambient that the ambient light arrives at the SPAD element of the light receiving unit 12, which satisfies the following equation (9) (step S32).

[0090]

Number

[0091] Finally, the CPU outputs the ambient light arrival probability Iambient as the luminance value when the distance measurement object 20 is illuminated by the ambient light (step S33), and ends a series of processes for calculating the ambient light arrival probability Iambient.

[0092] As described above, in the distance measurement device or the distance measurement method according to the present embodiment, in the histogram indicating the light reception frequency of the reflected light by the light receiving unit 12 (or the occurrence rate calculated therefrom), the degree of spread in the time direction around the peak is measured. For example, when the light output from the distance measurement object 20 by the subsurface scattering can be approximated as an exponential distribution (Equation (1)), the standard deviation is obtained as the degree of spread.

[0093] Further, a correction amount regarding the component of the subsurface scattering is calculated from the degree of spread in the time direction around the peak. For example, when the light from the light emitting unit 11 (active light) can be approximated by a normal distribution (Equation (2)) and the light output from the distance measurement object 20 by the subsurface scattering can be approximated as an exponential distribution (Equation (1)), the exponential distribution parameter λ is obtained, and the reciprocal thereof is used as the correction amount.

[0094] Then, by subtracting the obtained correction amount from the distance measurement result, the correct distance L can be measured. For example, when the light from the light emitting unit 11 (active light) can be approximated by a normal distribution (Equation (2)) and the light output from the distance measurement object 20 by the subsurface scattering can be approximated as an exponential distribution (Equation (1)), the correct distance L can be measured by subtracting the correction amount (1 / λ) from the average value of the histogram (or the occurrence rate calculated therefrom).

[0095] <Modification Example> As described above, the technology of the present disclosure has been described based on the preferred embodiments, but the technology of the present disclosure is not limited to the embodiments. The configuration and structure of the distance measurement device described in the above embodiments are examples and can be changed as appropriate.

[0096] <Application Examples of the Technology of the Present Disclosure> The technology of the present disclosure described above can be used, for example, by being mounted on various electronic devices equipped with a face recognition function. Examples of electronic devices equipped with a face recognition function include mobile devices such as smartphones, digital cameras, tablets, and personal computers. However, the electronic devices to which the technology of the present disclosure can be applied are not limited to mobile devices.

[0097] Here, as a specific example of an electronic device to which the technology of the present disclosure can be applied, a smartphone equipped with a face recognition function is exemplified. FIG. 11 shows an external view of the smartphone according to the specific example of the electronic device to which the technology of the present disclosure can be applied, as viewed from the front side.

[0098] The smartphone 100 according to this specific example includes a display unit 120 on the front side of the housing 110. Further, the smartphone 100 includes a light emitting unit 11 and a light receiving unit 12 of the distance measuring device 10 according to the above-described embodiment, for example, in an upper part on the front side of the housing 110. However, the installation positions of the light emitting unit 11 and the light receiving unit 12 of the distance measuring device 10 are not limited to the upper part on the front side of the housing 110.

[0099] The smartphone 100 according to this specific example can have a function of recognizing the three-dimensional shape of a subject, for example, face recognition, by mounting the distance measuring device 10 according to the above-described embodiment. In particular, since the distance measuring device 10 can correctly measure the distance to the object to be measured while considering the time caused by subsurface scattering, for example, face recognition can be performed more reliably.

[0100] <Configurations that the Present Disclosure Can Adopt> In addition, the present disclosure can also adopt the following configurations.

[0101] ≪A. Distance Measuring Device≫ [A-01] A light receiving unit that receives reflected light from an object to be measured based on irradiation light from a light emitting unit A histogram acquisition unit that acquires a histogram indicating the light reception frequency of the reflected light by the light reception unit, and An arithmetic unit that calculates the distance to the object to be measured based on the time corresponding to the peak of the histogram acquired by the histogram acquisition unit, comprising: The arithmetic unit corrects the distance calculated based on the time corresponding to the peak of the histogram based on the shape of the histogram acquired by the histogram acquisition unit. A distance measurement device. [A-02] The light receiving element of the light receiving unit consists of an avalanche photodiode operating in Geiger mode. The distance measurement device according to [A-01] above. [A-03] The light receiving element of the light receiving unit consists of a single photon avalanche diode. The distance measurement device according to [A-02] above. [A-04] The object to be measured is an object that causes subsurface scattering. The distance measurement device according to any one of [A-01] to [A-03] above. [A-05] The histogram acquisition unit A time difference detection unit that detects the time from the time when the light emitting unit emits the irradiation light to the time when the light receiving unit receives the reflected light from the object to be measured, and A cumulative unit that forms a histogram based on the time detected by the time difference detection unit. The distance measurement device according to [A-04] above. [A-06] The shape of the histogram is the degree of spread around the peak in the reflected light distribution from the object to be measured, which is obtained by performing predetermined statistical processing on the histogram acquired by the histogram acquisition unit. The distance measurement device according to any one of [A-01] to [A-05] above. [A-07] The arithmetic unit measures the degree of spread around the peak in the reflected light distribution from the object to be measured, calculates a correction amount from the degree of spread around the peak, and performs correction based on the calculated correction amount. The distance measurement device according to [A-06] above. [A-08] The calculation unit corrects the distance by subtracting the correction amount calculated from the degree of spread around the peak from the peak in the reflected light distribution. The distance measuring device according to [A-07] above. [A-09] The reflected light distribution is a distribution obtained by subtracting the ambient light component from the histogram acquired by the histogram acquisition unit. The distance measuring device according to [A-08] above. [A-10] The degree of spread around the peak in the reflected light distribution is the standard deviation. The distance measuring device according to [A-01] above. [A-11] The correction amount calculated from the degree of spread around the peak is the reciprocal of the exponential distribution parameter obtained from the standard deviation. The distance measuring device according to [A-10] above.

[0102] ≪B. Distance measurement method≫ [B-01] A light receiving unit that receives reflected light from a distance measurement object based on irradiation light from a light emitting unit, and A histogram acquisition unit that acquires a histogram indicating the light reception frequency of the reflected light by the light receiving unit, In a distance measuring device including: Based on the time corresponding to the peak of the histogram acquired by the histogram acquisition unit, calculate the distance to the distance measurement object, Next, correct the distance calculated based on the time corresponding to the peak of the histogram based on the shape of the histogram acquired by the histogram acquisition unit. Distance measurement method. [B-02] The light receiving element of the light receiving unit consists of an avalanche photodiode operating in Geiger mode. The distance measurement method according to [B-01] above. [B-03] The light receiving element of the light receiving unit consists of a single photon avalanche diode. The distance measurement method according to [B-02] above. [B-04] The distance measurement object is an object that causes subsurface scattering. The distance measurement method according to any one of [B-01] to [B-03] above. [B-05] Detect the time from when the light emitting unit emits the irradiation light until the light receiving unit receives the reflected light from the object to be distance-measured, and form a histogram based on the detected time. The distance measurement method according to the above [B-04]. [B-06] The shape of the histogram is the degree of spread around the peak in the reflected light distribution from the object to be distance-measured, which is obtained by performing predetermined statistical processing on the histogram acquired by the histogram acquisition unit. The distance measurement method according to any one of the above [B-01] to the above [B-05]. [B-07] Measure the degree of spread around the peak in the reflected light distribution from the object to be distance-measured, calculate a correction amount from the degree of spread around the peak, and perform correction based on the calculated correction amount. The distance measurement method according to the above [B-06]. [B-08] Perform distance correction by subtracting the correction amount calculated from the degree of spread around the peak from the peak in the reflected light distribution. The distance measurement method according to the above [B-07]. [B-09] The reflected light distribution is a distribution obtained by subtracting the ambient light component from the histogram acquired by the histogram acquisition unit. The distance measurement method according to the above [B-08]. [B-10] The degree of spread around the peak in the reflected light distribution is the standard deviation. The distance measurement method according to the above [B-01]. [B-11] The correction amount calculated from the degree of spread around the peak is the reciprocal of the exponential distribution parameter obtained from the standard deviation. The distance measurement method according to the above [B-10].

[0103] This application claims priority based on Japanese Patent Application No. 2020-175913 filed with the Japan Patent Office on October 20, 2020, and incorporates all the contents of this application by reference.

[0104] Those skilled in the art can conceive of various modifications, combinations, sub - combinations, and changes according to the design requirements and other factors, and it is understood that they are included within the scope of the appended claims and their equivalents.

Claims

1. A light receiving unit that receives reflected light from a distance measurement target based on irradiation light from a light emitting unit, a histogram acquisition unit that acquires a histogram indicating the reception frequency of the reflected light by the light receiving unit, in the histogram acquired by the histogram acquisition unit, a standard deviation is obtained as the degree of spread in the time direction around the peak, a correction amount is obtained from the standard deviation, and the distance to the distance measurement target is calculated by subtracting the correction amount from the average value of the histogram. A distance measurement device.

2. The light receiving element of the light receiving unit is composed of an avalanche photodiode operating in Geiger mode. The distance measurement device according to Claim 1.

3. The light receiving element of the light receiving unit is composed of a single photon avalanche diode. The distance measurement device according to Claim 2.

4. The distance measurement target is an object that causes subsurface scattering. The distance measurement device according to Claim 1.

5. The histogram acquisition unit a time difference detection unit that detects the time from the time when the light emitting unit emits the irradiation light to the time when the light receiving unit receives the reflected light from the distance measurement target, an accumulation unit that forms the histogram based on the time detected by the time difference detection unit and has The distance measurement device according to Claim 4.

6. The correction amount is the reciprocal of the exponential distribution parameter obtained from the standard deviation. The distance measurement device according to Claim 1.

7. In a distance measurement device including a light receiving unit that receives reflected light from a distance measurement target based on irradiation light from a light emitting unit and a histogram acquisition unit that acquires a histogram indicating the reception frequency of the reflected light by the light receiving unit, in the histogram acquired by the histogram acquisition unit, a standard deviation is obtained as the degree of spread in the time direction around the peak, a correction amount is obtained from the standard deviation, and the distance to the distance measurement target is calculated by subtracting the correction amount from the average value of the histogram. Distance measurement method.

Images