Distance measuring device and distance measuring method

Abstract

A distance measuring apparatus according to one embodiment of the present disclosure includes a light receiving section that receives reflected light from a distance measurement target based on irradiation light from a light emitting section, a histogram acquisition section that acquires a histogram that indicates a frequency with which the light receiving section receives the reflected light, and a calculation section that calculates a distance to the distance measurement target based on a time corresponding to a peak in the histogram acquired by the histogram acquisition section. The calculation section corrects the distance calculated based on the time corresponding to the histogram peak based on a shape of the histogram acquired by the histogram acquisition section.

Description

Technical Field

[0001] This disclosure relates to distance measuring devices and distance measuring methods. Background Technology

[0002] A device (sensor) using a time-of-flight (ToF) system is called a distance measuring device (so-called a ranging device) that measures the distance to a target (object) (see, for example, Patent Document 1). A ToF system measures the distance to a target by detecting the time of flight of light from a light-emitting unit (light source) emitting illumination light towards the target to a light-receiving unit receiving the illumination light reflected from the target.

[0003] Reference List

[0004] Patent documents

[0005] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2016-211881. Summary of the Invention

[0006] Meanwhile, some distance measurement targets cause so-called subsurface scattering, which is the phenomenon where light incident on the surface of an object is scattered inside the object and then expelled to the outside. If the light emitted from the light-emitting unit is reflected from the surface of the distance measurement target, the distance to the target can be measured accurately. However, in the case where the distance measurement target is an object that causes subsurface scattering, the time from the emission of light by the light-emitting unit to the reception of light at the light-receiving unit includes the time attributable to subsurface scattering. Therefore, the distance to the target cannot be measured accurately.

[0007] Existing techniques fail to account for the time attributable to sub-surface scattering within the target of distance measurement. Therefore, when the target of distance measurement is an object that causes sub-surface scattering, existing techniques cannot accurately measure the distance to the target of distance measurement.

[0008] It is desirable to provide a distance measuring device and a distance measuring method, both of which take into account the time caused by sub-surface scattering within the distance measuring target, and enable accurate measurement of the distance to the distance measuring target when the distance measuring target is an object that causes sub-surface scattering.

[0009] The distance measuring apparatus according to an embodiment of the present disclosure includes a light receiving unit, a histogram acquisition unit, and a calculation unit. The light receiving unit receives reflected light from a distance measuring target. The reflected light is based on illumination light from a light emitting unit. The histogram acquisition unit acquires a histogram indicating the frequency of the reflected light received at the light receiving unit. The calculation unit calculates the distance to the distance measuring target based on the time corresponding to the peak value of the histogram acquired by the histogram acquisition unit. The calculation unit corrects the distance calculated based on the time corresponding to the peak value of the histogram based on the shape of the histogram acquired by the histogram acquisition unit.

[0010] The distance measurement method according to embodiments of the present disclosure enables a distance measuring device to calculate the distance to a distance measuring target based on the time corresponding to the peak value of a histogram acquired by a histogram acquisition unit, and to correct the distance calculated based on the time corresponding to the peak value of the histogram based on the shape of the histogram acquired by the histogram acquisition unit. The distance measuring device includes: a light receiving unit for receiving reflected light from the distance measuring target based on illumination light from a light emitting unit; and a histogram acquisition unit for acquiring a histogram indicating the frequency of the reflected light received at the light receiving unit. Attached Figure Description

[0011] Figure 1 This is a block diagram illustrating an example configuration of a distance measuring device according to an embodiment of the present disclosure.

[0012] Figure 2 This is a diagram illustrating an example of a histogram generated by an accumulator of a distance measuring device according to an embodiment of the present disclosure.

[0013] Figure 3 It is a diagram illustrating the distance measurement error caused by scattering from sub-surfaces inside the target.

[0014] Figure 4 It is a diagram showing the intensity of light emitted from the distance measuring target at each time interval when light emitted for a short period of time is incident on the surface of the distance measuring target.

[0015] Figure 5A This is a waveform diagram of the active light emitted from the light-emitting unit to the distance measurement target. Figure 5B It is a waveform diagram of the light received at the light receiving unit without causing scattering from the subsurface of the distance measurement target, and Figure 5C This is a waveform diagram of the light received at the light receiving unit under the condition that the target for distance measurement causes scattering on the subsurface.

[0016] Figure 6 This is a block diagram illustrating an example of the function of a computation unit in a distance measuring device according to an embodiment of the present disclosure.

[0017] Figure 7A This is a waveform diagram of the light received at the light receiving unit under the condition that the target for distance measurement causes scattering from the subsurface. Figure 7B It is a waveform diagram showing the change in the probability of active light emitted from the light-emitting unit reaching the SPAD element of the light-receiving unit over time, and Figure 7C This is a diagram showing the results of the distance correction process.

[0018] Figure 8 This is a flowchart illustrating an example of the processing procedure of a distance measurement method executed by the computing unit.

[0019] Figure 9 This is a flowchart illustrating an example of the process for calculating the occurrence rate.

[0020] Figure 10 This is a flowchart illustrating an example of the process for calculating the probability of ambient light reaching a SPAD element.

[0021] Figure 11 This is an exterior view of a smartphone, as seen from the front, as a specific example of an electronic device to which the technology according to this disclosure can be applied. Detailed Implementation

[0022] In the following, some embodiments (hereinafter referred to as "embodiments") for implementing the technology of this disclosure are described in detail with reference to the accompanying drawings. However, the technology of this disclosure should not be limited to these embodiments. In the following description, components that include the same elements or have the same function are indicated by the same reference numerals to omit redundant descriptions. It should be noted that the description is given in the following order.

[0023] 1. General description of the distance measuring device and distance measuring method disclosed herein

[0024] 2. Distance measuring device according to embodiments of the present disclosure

[0025] 2-1. Example of Distance Measuring Device Configuration

[0026] 2-2. Histogram

[0027] 2-3. Distance measurement error caused by subsurface scattering

[0028] 2-4. Functional block diagram of the arithmetic unit

[0029] 2-5. Distance Measurement Methods

[0030] 2-5-1. Distance measurement processing performed by the processing unit

[0031] 2-5-2. Processing for calculating the incidence rate

[0032] 2-5-3. Processing for calculating the probability of ambient light arrival

[0033] 3. Modify the example

[0034] 4. Application example of the technology disclosed herein (smartphone)

[0035] 5. Possible configurations of this disclosure

[0036] <1. General description of the distance measuring device and distance measuring method disclosed herein>

[0037] According to the distance measuring device and distance measuring method disclosed herein, the light receiving element of the light receiving unit can be an avalanche photodiode operating in Geiger mode, preferably a single-photon avalanche diode.

[0038] According to the distance measuring device and distance measuring method of this disclosure, which both include the above-described preferred configuration, the distance measuring target can be an object that causes sub-surface scattering.

[0039] Furthermore, according to the distance measuring apparatus and distance measuring method of this disclosure, which include the above-described preferred configurations and embodiments, the histogram acquisition unit may include a time difference detector and an accumulator. The time difference detector detects the time from the point in time when the emitting unit emits illumination light to the point in time when the reflected light from the distance measuring target is received at the light receiving unit. The accumulator generates a histogram based on the time detected by the time difference detector. Furthermore, the shape of the histogram represents the diffusion state of the reflected light from the distance measuring target around a peak. This diffusion state is obtained by performing predetermined statistical processing on the histogram acquired by the histogram acquisition unit.

[0040] Furthermore, according to the distance measuring apparatus and distance measuring method of this disclosure, which include the above-described preferred configurations and embodiments, the arithmetic unit can measure the diffusion state of the distribution of reflected light from the distance measuring target around a peak, calculate a correction amount based on the diffusion state around the peak, and perform correction based on the calculated correction amount. Additionally, the arithmetic unit can perform distance correction by subtracting the correction amount calculated based on the diffusion state around the peak from the peak of the reflected light distribution.

[0041] Furthermore, according to the distance measuring apparatus and distance measuring method of this disclosure, which all include the above-described preferred configurations and embodiments, the distribution of reflected light can be obtained by subtracting the ambient light component from the histogram obtained by the histogram acquisition unit, and the diffusion state of the reflected light distribution around the peak is the standard deviation. Moreover, the correction amount calculated based on the diffusion state around the peak is the reciprocal of the exponential distribution parameter determined based on the standard deviation.

[0042] <2. Distance measuring device according to embodiments of the present disclosure>

[0043] The distance measuring device according to an embodiment of the present disclosure (hereinafter, as needed, simply referred to as "this embodiment") employs a Time-of-Flight (ToF) system as a system for measuring the distance to a distance measuring target. In the ToF system, the time of flight is measured from emitting a pulse of light (e.g., a laser with a peak wavelength in the infrared wavelength region) toward the distance measuring target to receiving the light reflected from the distance measuring target.

[0044] Time-of-Flight (ToF) systems are classified into two types: indirect ToF systems and direct ToF systems. Indirect ToF systems measure the distance to the target by detecting the period of reflected light obtained at a light receiving unit due to the reflection of pulsed light of a predetermined period emitted from a light emitting unit from the target, and by measuring the time of flight of the light based on the phase difference between the emission period and the reception period. Direct ToF systems are used to directly measure the distance to the target from the time-of-flight difference of the light. In the distance measuring device according to this embodiment, the latter system (i.e., the direct ToF system) is used as the ToF system.

[0045] [2-1. Example of Distance Measuring Device Configuration]

[0046] Figure 1 This is a block diagram illustrating an example configuration of a distance measuring device according to an embodiment of the present disclosure. Figure 1 As shown, the distance measuring device 10 according to this embodiment includes a light-emitting unit 11, a light-receiving unit 12, a time difference detector 13, an accumulator 14, a calculation unit 15, a control unit 16, and an output terminal 17. The distance measuring device 10 is configured to measure the distance to the distance measuring target 20 using a direct ToF system. Here, the distance to the distance measuring target 20 is referred to as distance L.

[0047] The light-emitting unit 11, the light-receiving unit 12, the time difference detector 13, the accumulator 14, and the arithmetic unit 15 perform corresponding processes under the control of a control unit 16, which includes an information processing device such as a central processing unit (CPU). The corresponding processes performed by the light-receiving unit 12, the time difference detector 13, the accumulator 14, and the arithmetic unit 15 under the control of the control unit 16 are described below.

[0048] The light-emitting unit 11 may include, for example, a laser source as a light source (light-emitting element). Under the control of the control unit 16, the light-emitting unit 11 causes the laser source to emit light for a short time, and the pulsed light 40, as active light, reaches the distance measuring target 20. The pulsed light 40 emitted from the light-emitting unit 11 is reflected from the distance measuring target 20 and returns to the light receiving unit 12 as reflected light 41.

[0049] Of course, the pulsed light 40 emitted from the light-emitting unit 11 toward the distance measuring target 20 is not always reflected back from the distance measuring target 20. That is, in a random sense, the pulsed light 40 is reflected back from the distance measuring target 20 in some cases and not in others.

[0050] In addition to the light emitted from the light-emitting unit 11, light 42 from the sun 30 reaches the distance measuring target 20 as ambient light. The light 42 from the sun 30 is reflected from the distance measuring target 20 and reaches the light receiving unit 12 as light 43.

[0051] Similar to the pixel array unit of an imaging device, the light receiving unit 12 includes a two-dimensional array of pixels, each pixel including a light receiving element. The light receiving unit 12 receives reflected light 41 from the distance measuring target 20 based on pulsed light 40 emitted from the light emitting unit 11 toward the distance measuring target 20. The light receiving unit 12 includes an incident portion with a lens (not shown). The focusing operation of the lens enables the pixel including the light receiving element to effectively receive the reflected light 41 from the distance measuring target 20. Note that since the lens is not a major component, its description is omitted herein.

[0052] In the distance measuring device 10 according to this embodiment, the light receiving element of the light receiving unit 12 is an element that generates a signal in response to the reception of photons. For example, the light receiving element of the light receiving unit 12 is a single-photon avalanche diode (SPAD) element. The SPAD element is an avalanche photodiode with increased light receiving sensitivity through the use of a phenomenon known as avalanche multiplication. The SPAD element operates in Geiger mode, in which the element operates at a reverse voltage greater than the breakdown voltage.

[0053] It should be noted that although the SPAD element is illustrated as the light receiving element of the light receiving unit 12 in this embodiment, the light receiving element of the light receiving unit 12 is not limited to the SPAD element. That is, various elements operating in Geiger mode, such as avalanche photodiodes (APDs) or silicon photomultiplier tubes (SiPMs) and SPAD elements can be used as the light receiving element of the light receiving unit 12.

[0054] The time difference detector 13 employs a Time-of-Flight (ToF) system, more specifically, a direct ToF system as the distance measurement system. Typically, the time difference detector 13 is a time-to-digital converter (TDC). Under the control of the control unit 16, the time difference detector 13 measures the time from the point in time when the light-emitting unit 11 emits pulsed light 40 (i.e., the emission time) to the point in time when the light-receiving unit 12 receives reflected light 41 (i.e., the light reception time). That is, the time difference detector 13 measures the time from the emission time of the light-emitting unit 11 to the light reception time of the light-receiving unit 12. The measured time is then multiplied by the speed of light c, and the resulting value is divided by 2. The distance to the distance measurement target 20 is determined as a result. The reason for dividing the resulting value by 2 is that the measurement time is the time it takes for light to travel to the distance measurement target 20 and return.

[0055] Meanwhile, distance measurements based on a single emission of light from the light-emitting unit 11 and a single reception of light at the light-receiving unit 12 may lead to erroneous determinations. One reason is that during the time between the emission of light from the light-emitting unit 11 and the reception of the light reflected from the distance-measuring target 20 at the light-receiving unit 12, the SPAD element (SPAD sensor) of the light-receiving unit 12 can react to external light (ambient light) incident on the light-receiving unit 12. Another reason is that the pulsed light 40 emitted from the light-emitting unit 11 does not always return to the light-receiving unit 12 after being reflected from the distance-measuring target 20. These are random events.

[0056] For the reasons described above, under the control of the control unit 16, light emission M is emitted from the light-emitting unit 11 multiple times (e.g., several thousand to tens of thousands of times) within a short period of time. That is, according to this embodiment, the distance measuring device 10 performs multiple measurements of light emission and light reception of M, and detects advantage data based on the results of multiple M measurements.

[0057] The detection result (i.e., the value of the measured time) of the time difference detector 13 is sent to the accumulator 14. The accumulator 14 includes a memory (not shown) in which histogram data is formed. Details of the histogram will be described later. The accumulator 14 increments the histogram bin (BIN) corresponding to the time value by "1" to update the histogram.

[0058] It should be noted that the time difference detector 13 and the accumulator 14 are examples of the histogram acquisition unit described in the claims (i.e., the histogram acquisition unit that acquires a histogram indicating the frequency of the reflected light received at the light receiving unit 12). After multiple M measurements, the data on the final histogram acquired by the accumulator 14 is sent to the arithmetic unit 15.

[0059] The arithmetic unit 15 calculates the distance L to the distance measurement target 20 based on the time corresponding to the peak value of the final histogram acquired by the accumulator 14. Furthermore, the arithmetic unit 15 corrects the distance L calculated based on the time corresponding to the peak value of the final histogram acquired by the accumulator 14 based on its shape. The information obtained through the processing of the arithmetic unit 15 (i.e., information about the distance to the distance measurement target 20) is output from the output terminal 17.

[0060] Meanwhile, the SPAD element used as the light receiving element of the light receiving unit 12 is a sensor that detects the first arriving light (photon). Therefore, if the light 41 of the incident beam generated by the reflection of the pulse light 40 emitted from the light emitting unit 11 from the distance measuring target 20 arrives earlier than the light 43 of the incident beam generated by the reflection of the light 42 emitted from the sun 30 from the distance measuring target 20, accurate distance measurement can be achieved.

[0061] In the following text, light 41, which is an incident beam generated by the reflection of pulsed light 40 emitted from light-emitting unit 11 from distance measuring target 20, is referred to as active light, and light 43, which is an incident beam generated by the reflection of light 42 emitted from sun 30 from distance measuring target 20, is referred to as ambient light.

[0062] The time detected by the time difference detector 13 is the round-trip time to the distance measurement target 20. The time detected by the time difference detector 13 is multiplied by c / 2 (where c is the speed of light) to calculate the distance L to the distance measurement target 20. Conversely, if light 43 arrives earlier than light 41, it is impossible to measure the accurate time used for distance measurement.

[0063] Regardless of the time, the reception of ambient light 43 (i.e., reception of ambient light) always occurs with a fixed probability. On the other hand, the reception of active light 41 (i.e., reception of active light) occurs concentratedly at a certain moment (more specifically, the value obtained by dividing twice the distance to the measurement target 20 by the speed of light c). Therefore, by detecting peaks in the histogram and determining the time corresponding to the peaks, the distance L to the measurement target 20 is determined.

[0064] As described above, the arithmetic unit 15, under the control of the control unit 16, performs the processing of detecting peaks from the histogram and the processing of calculating the distance L by multiplying the time corresponding to the peak by c / 2. Details of the function of the arithmetic unit 15 will be described later.

[0065] [2-2. Histogram]

[0066] Now, a histogram indicating the frequency of reflected light received at the light receiving unit 12 will be described. Figure 2This is a diagram illustrating an example of a histogram generated by the accumulator 14 of the distance measuring device 10 according to an embodiment of the present disclosure.

[0067] In this histogram, time (horizontal axis) is represented in units of width D. That is, when the time value detected by time difference detector 13 falls within the range of time 0 to time D, the frequency is added to compartment 0. When the time value detected by time difference detector 13 falls within the range of time D to time 2D, the frequency is added to compartment 1. When the time value detected by time difference detector 13 falls within the range of time 2D to time 3D, the frequency is added to compartment 2. This also applies to subsequent ranges: when the time value detected by time difference detector 13 falls within the range of time (N-1)×D to time N×D, the frequency is added to compartment (N-1).

[0068] Here, D is the resolution of the TDC.

[0069] It should be noted that the measurement time for a single measurement is limited to N×D. That is, if the light receiving unit 12 does not receive light even though time N×D has elapsed since the light emission from the light-emitting unit 11, the measurement ends after time N×D. In this case, the time difference detector 13 does not output a time value, and the accumulator 14 does not update the histogram. Here, N is a constant.

[0070] Therefore, if all frequencies in the final histogram obtained after M measurements are summed, the sum will be less than M in some cases.

[0071] The histogram is represented by h(n), where n represents the number of warehouses and is an integer greater than or equal to 0 and less than or equal to N-1.

[0072] [2-3. Distance measurement error due to subsurface scattering]

[0073] Meanwhile, some distance measurement targets 20 cause so-called subsurface scattering, which is the phenomenon where light incident on the surface of an object is scattered inside the object and then expelled to the outside. Examples of objects causing subsurface scattering include human skin. If the light emitted from the light-emitting unit 11 is reflected from the surface of the distance measurement target 20, the distance to the distance measurement target 20 can be measured accurately. However, if the distance measurement target 20 is an object that causes subsurface scattering, the time from the time the light-emitting unit 11 emits light to the time the light is received at the light-receiving unit 12 includes the time attributed to subsurface scattering. Therefore, the distance to the distance measurement target 20 cannot be measured accurately. That is, a distance measurement error is generated.

[0074] Now, for reference Figure 3This describes the distance measurement error caused by scattering from the sub-surface inside the distance measurement target 20. Figure 3 This illustrates the necessary explanation for distance measurement errors caused by subsurface scattering. Figure 1 Extracted components.

[0075] exist Figure 3 In this context, the distance measurement target 20 is the object that causes subsurface scattering. In this case, light 40 emitted from the light-emitting unit 11 enters the distance measurement target 20 and causes scattering as described above. Figure 3 The subsurface scattering shown is 40a. Subsequently, as... Figure 3 As shown, light 41 travels to the outside of the distance measuring target 20. Therefore, the optical path length is Figure 3 The light path shown is the sum of the light path of light 40, the light path of sub-surface scattering 40a, and the light path of light 41. That is, the light path through sub-surface scattering 40a has a longer path length than... Figure 3 The sum of the optical paths of light 40 and light 41 is the length. Therefore, the distance measurement result has a value greater than the actual distance L.

[0076] It should be noted that, although in Figure 3 Only a single optical path of subsurface scattering 40a is shown, but in reality, there are multiple optical paths of subsurface scattering. That is, some beams are scattered inside the distance measuring target 20 for a long time (i.e., with a long optical path length) before being emitted outward, while other beams are scattered inside the distance measuring target 20 for a short time (i.e., with a short optical path length) before being emitted outward.

[0077] Light that remains inside the range measuring target 20 for a longer period of time has a longer optical path and is therefore absorbed in greater quantities. The time that the light remains inside the range measuring target 20 is represented by time ts. For example, the intensity of the light emitted from the range measuring target 20 after time ts from when the light is incident on it can be approximated by the following exponential distribution.

[0078] [Expression 1]

[0079]

[0080] Here, λ represents an object-specific value. The object-specific value λ is small because the object is more susceptible to subsurface scattering.

[0081] refer to Figure 4 Further explanation is provided. Figure 4 The diagram shows the intensity of light emitted from the distance measuring target 20 at each time interval when the surface of the target 20 is illuminated by light emitted for a short time. Here, the short-time emission is instantaneous emission, which can be approximated as a function of delta.

[0082] In the above description, the emitted light is a delta function; however, in reality, the light emitted from the light-emitting unit 11 (i.e., the light incident on the distance measuring target 20) has a width in the time direction. For example, if the light emitted from the light-emitting unit 11 can be approximated as a normal distribution, the intensity of the light emitted from the light-emitting unit 11 at time t is represented by the following expression.

[0083] [Expression 2]

[0084]

[0085] Here, σ is the standard deviation. Furthermore, expression (2) is normalized so that it becomes 1 when integrated in the time direction.

[0086] It should be noted that the value of the standard deviation σ in expression (2) is measured in advance and is therefore known. For example, in a dark room, an object comprising materials such as metal and not causing subsurface scattering is placed in front of the distance measuring device 10. The distance to the object is then measured in advance, and a histogram is generated. Subsequently, based on the shape of the histogram, and more specifically, based on the diffusion state around the peak of the distribution of reflected light from the distance measuring target, the standard deviation σ is determined. The diffusion state is obtained by performing a predetermined statistical processing on the histogram.

[0087] Figure 5A The waveform of the active light 40 emitted from the light-emitting unit 11 to the distance measuring target 20 is shown. Here, the distance measuring target 20, placed at a distance L, comprises a material such as metal and does not cause sub-surface scattering. In this case, the light received at the light receiving unit 12 has Figure 5B The waveform shown.

[0088] The sum of the attenuation caused by the distance traveled by 2×L and the attenuation caused by the reflectivity of the distance measuring target 20 is expressed as attenuation h. When the light emitted from the light-emitting unit 11 is reflected from the distance measuring target 20 and received by the light receiving unit 12, the intensity of the light is expressed by the following expression.

[0089] [Expression 3]

[0090]

[0091] Expression (3) is a normal distribution considering attenuation h and time delay 2L / c. It should be noted that since no sub-surface scattering occurs, the standard deviation is σ, which is the same as in the case of light from emitting unit 11.

[0092] In addition, Figure 5BThe reception of light from the sun 30 (ambient light) is also considered. Ambient light is always constant. Therefore, when the ambient light component has an intensity E, the light received by the light receiving unit 12 has an intensity expressed by the following expression.

[0093] [Expression 4]

[0094]

[0095] exist Figure 5B In the case shown, the peak position is located at time 2L / c. Therefore, the distance L can be accurately determined by detecting the peak position and multiplying it by c / 2.

[0096] Next, consider the case where sub-surface scattering occurs at the distance measurement target 20 placed at a distance L. In this case, the light received at the light receiving unit 12 has Figure 5C The waveform shown.

[0097] When light emitted from the light-emitting unit 11 is reflected from the distance-measuring target 20 and received by the light-receiving unit 12, the intensity of the light is the light intensity obtained under the condition of sub-surface scattering. Therefore, the intensity is determined by using the convolution operation (convolution) of expressions (1) and (2). That is, the intensity is a Gaussian distribution that changes exponentially. Specifically, the intensity is represented by the following expression.

[0098] [Expression 5]

[0099]

[0100] It should be noted that expression (5) also considers the attenuation h. Furthermore, expression (5) considers the intensity E of the light (ambient light) from the sun at 30°. Here, μ in expression (5) is represented by the following expression.

[0101] [Expression 6]

[0102]

[0103] That is, μ is the round-trip time of light to the target at a distance of 20.

[0104] exist Figure 5C In the case shown, where subsurface scattering occurs at the distance measurement target 20 placed at a distance L, the peak position is after time 2L / c. Therefore, the distance L cannot be accurately determined by detecting the peak position and multiplying it by c / 2.

[0105] To address this issue, in the distance measuring device 10 according to this embodiment, the arithmetic unit 15 corrects the distance calculated based on the time corresponding to the peak of the histogram obtained by the accumulator 14, based on the shape of the histogram, and more specifically, the diffusion state around the peak of the histogram. This allows the distance L to the distance measuring target 20 to be accurately calculated even if the distance measuring target 20 is an object that generates subsurface scattering.

[0106] [2-4. Functional Block of the Operation Unit]

[0107] The calculation unit 15 calculates the distance to the distance measurement target based on the time corresponding to the peak value of the histogram acquired by the histogram acquisition unit, which includes a time difference detector 13 and an accumulator 14. Furthermore, the calculation unit 15 performs a process to correct the calculated distance based on the shape of the histogram. The calculation unit 15 can be configured, for example, by a CPU and has the functional units described below to perform the above-described processes. Figure 6 An example of a functional block diagram of the arithmetic unit 15 is shown in the figure.

[0108] The arithmetic unit 15 includes various functional units, including an occurrence rate calculator 151, an ambient light arrival rate calculator 152, an active light arrival rate calculator 153, an average and standard deviation calculator 154, a λ calculator 155, a correction processor 156, and a c / 2 multiplier 157.

[0109] The occurrence rate calculator 151 calculates the occurrence rate based on the histogram h(n) generated at accumulator 14. The occurrence rate is the number of times a specific event is generated per unit time. Therefore, the term "occurrence rate" as used herein refers to the number of times a photon arrives at the SPAD element of the optical receiving unit 12 per unit time. The occurrence rate in each compartment is the probability of a photon arriving within the time corresponding to that compartment. The occurrence rate in compartment n is represented by p(n).

[0110] The ambient light arrival rate calculator 152 calculates the probability that light (ambient light) from the sun 30 reaches the SPAD element of the light receiving unit 12. The active light arrival rate calculator 153 calculates the probability that active light from the light emitting unit 11 reaches the SPAD element of the light receiving unit 12 by subtracting the ambient light arrival rate determined by the ambient light arrival rate calculator 152 from the occurrence rate p(n) determined by the occurrence rate calculator 151.

[0111] Now, for reference Figure 7A and Figure 7B The waveform diagram more specifically describes the process of calculating the probability of active light arriving at the SPAD element of the optical receiver unit 12. It should be noted that... Figure 7A waveform diagram and Figure 5C The waveforms are the same.

[0112] The active light arrival rate calculator 153 subtracts the ambient light arrival rate determined by the ambient light arrival rate calculator 152 from the occurrence rate p(n) determined by the occurrence rate calculator 151, thereby obtaining the arrival rate of the ambient light. Figure 7B The waveform data shown is obtained by subtracting the ambient light component E from the occurrence rate p(n). Figure 7B The waveform data shown. Figure 7B The waveform shown represents the change in the probability of active light from the light-emitting unit 11 reaching the SPAD element of the light-receiving unit 12 over time. It should be noted that... Figure 7B In this context, the mean of the exponentially changing Gaussian distribution is denoted as μ+(1 / λ), and the standard deviation is denoted as...

[0113] The mean and standard deviation calculator 154 calculates the mean and standard deviation of the probability of active light from the light-emitting unit 11 arriving at the SPAD element of the light-receiving unit 12, as determined by the active light arrival rate calculator 153. The λ calculator 155 is based on the width (standard deviation) σ of the active light emitted from the light-emitting unit 11 to the distance measurement target 20 and the standard deviation determined by the mean and standard deviation calculator 154. To calculate the object-specific value λ. It should be noted that the object-specific value λ is a known value.

[0114] The correction processor 156 subtracts a correction amount 1 / λ from the average value determined by the average value and standard deviation calculator 154, thereby determining the light round-trip time μ to the distance measurement target 20. That is, as Figure 7C As shown, the correction processor 156 performs distance correction processing by subtracting the correction amount 1 / λ from the average value (=μ+(1 / λ)) (i.e., the peak value of the reflected light distribution) determined by the average value and standard deviation calculator 154.

[0115] The c / 2 multiplier 157 multiplies the time (light round-trip time μ) determined by the correction processor 156 by c / 2 (where c is the speed of light) and outputs the multiplied value as the distance measurement result. The distance measurement result is the distance L to the distance measurement target 20 accurately determined, taking into account the time attributable to scattering from the sub-surfaces inside the distance measurement target 20.

[0116] [2-5. Distance Measurement Methods]

[0117] Next, refer to Figure 8 The flowchart describes the process under the control of control unit 16. Figure 1 The process of the distance measurement method executed by the arithmetic unit 15 in the distance measuring device 10 according to this embodiment is shown. When the function of the arithmetic unit 15 is implemented by a CPU, the distance measurement method is executed under the control of the CPU of the arithmetic unit 15.

[0118] exist Figure 1 In the distance measuring device 10 according to this embodiment, the accumulator 14 generates a histogram h(n) that indicates the frequency of reflected light received at the light receiving unit 12. The arithmetic unit 15 performs processing to calculate the precise distance based on the histogram h(n).

[0119] (2-5-1. Distance measurement processing performed by the processing unit)

[0120] First, the CPU of the arithmetic unit 15 (hereinafter referred to as "CPU") acquires the histogram h(n) generated by the accumulator 14 (step S1), and then calculates the occurrence rate p(n) as the probability of a photon arriving at the SPAD element of the optical receiving unit 12 based on the histogram h(n) (step S2). The process of calculating the occurrence rate p(n) will be described later.

[0121] It should be noted that when calculating the incidence rate p(n) is time-consuming and faster calculation is required, an approximation H(n) = h(n) can be made. That is, the incidence rate p(n) can be approximated as being the same as the histogram. This approximation allows the computation time in step S2 to be essentially negligible.

[0122] Subsequently, the CPU calculates the probability that light from the sun 30 (ambient light) reaches the SPAD element of the light receiving unit 12 (step S3), and then calculates the probability that light from the light emitting unit 11 (active light) reaches the SPAD element of the light receiving unit 12 by subtracting the arrival rate of ambient light determined in step S3 from the occurrence rate determined in step S2 (step S4). The process of calculating the probability of ambient light reaching the SPAD element of the light receiving unit 12 will be described later.

[0123] Figure 7A The waveform of the light received at the SPAD element of the light receiving unit 12 is shown when subsurface scattering occurs at the distance measuring target 20. Figure 7B The waveform shown indicates the change over time of the probability that active light from the light-emitting unit 11 reaches the SPAD element of the light-receiving unit 12. Figure 7B The waveform shown is obtained by subtracting the occurrence rate p(n) determined in step S2. Figure 7A The waveform obtained is based on the ambient light component E shown. It should be noted that... Figure 7B In the exponentially changing Gaussian distribution, the mean is μ+(1 / λ) and the standard deviation is .

[0124] Subsequently, the CPU calculates the probability, determined in step S4, that the active light from the light-emitting unit 11 reaches the SPAD element of the light-receiving unit 12. Figure 7BThe CPU calculates the average and standard deviation of the light source width (standard deviation) σ from the light-emitting unit 11 and the standard deviation of the probability of reaching the light-receiving unit 12 determined in step S5 (step S6). Specifically, the CPU calculates the object-specific value λ based on the standard deviation determined in step S5. Calculate the object-specific value λ. Note that the object-specific value λ is a known value.

[0125] After that, as Figure 7C As shown, the CPU subtracts 1 / λ from the average probability of reaching the light receiving unit 12 determined in step S5 to determine the light round-trip time μ to the distance measurement target 20 (step S7). Thereafter, the CPU multiplies the time determined in step S7 (i.e., the light round-trip time μ to the distance measurement target 20 by c / 2) and outputs the result as the distance measurement result (step S8). Then, the CPU terminates the series of processes in the distance measurement method.

[0126] Now, for reference Figure 7B and Figure 7C The following supplementary explanations are provided regarding the series of processes involved in the aforementioned distance measurement method.

[0127] Figure 7B The data shown represents the change over time in the probability of light (active light) from the light-emitting unit 11 reaching the SPAD element of the light-receiving unit 12. Its average value is μ+(1 / λ), and its standard deviation is... Therefore, the object-specific value λ can be determined using the known width (standard deviation) σ of the active light, and the round-trip time μ of the light to the target at a distance of 20 can be determined. This calculation is performed in... Figure 7C As shown in the figure. By using a Gaussian distribution that changes exponentially ( Figure 7B Subtracting 1 / λ from the average value of ) allows us to calculate the light round-trip time μ at a distance of 20 from the target.

[0128] The round-trip time μ of light to the distance measurement target 20 has a relationship represented by expression 6. Therefore, in the processing of step S8, the distance L to the distance measurement target 20 can be accurately calculated.

[0129] The application of the technique according to this disclosure as described above enables the accurate measurement of the distance L of the distance measurement target 20 in which sub-surface scattering occurs.

[0130] Although the description above states that "the value of σ is pre-measured and therefore known," if the value of σ is not known, the following procedure can be performed. That is, the skewness can be further determined in step S5, which determines the mean and standard deviation. Alternatively, the value of σ can be determined using the calculations indicated in the literature: Olivier J., Norberg, MM, (2010). Positively Skewed Data: Revisiting the Box-Cox Power Transformation. International Journal of Psychological Research, 3(1), 68-75.

[0131] (2-5-2. Processing for calculating the incidence rate)

[0132] Next, the processing in step S2 above, namely the processing of calculating the occurrence rate p(n) (the probability of a photon arriving at the SPAD element of the optical receiving unit 12), will be described. Figure 9 This is a flowchart illustrating an example of the process for calculating the occurrence rate. This process is also executed under the control of the CPU in the arithmetic unit 15.

[0133] First, the CPU receives a numerical value M representing the number of measurements performed to generate the histogram (step S21), and then acquires data about the histogram generated by accumulator 14 (step S22). Specifically, in step S22, the CPU acquires the frequency h(n) in each bin n of the histogram. Here, n is from 0 to N-1.

[0134] After obtaining the frequency h(n) in each bin n of the histogram, the CPU calculates M(n-1) as defined by the following expression (7) (step S23).

[0135] [Expression 7]

[0136]

[0137] Expression (7) represents the probability that the frequency in warehouse n changes to h(n) after M measurements are performed, regardless of the number of times h(m) occur in warehouse m. Here, n is from 0 to N-1.

[0138] Subsequently, the CPU performs the calculation H(n) = h(n) / M(n-1), where n is from 0 to N-1 (step S24). It should be noted that H(n) is the mean of the normal distribution represented by the following expression (8), i.e., the occurrence rate p(n).

[0139] [Expression 8]

[0140]

[0141] After that, the CPU performs calculations. Where n is 0 to N-1 (step S25). It should be noted that σ(n) is the standard deviation of the normal distribution in expression (8). Afterwards, the CPU outputs H(n) as the occurrence rate p(n) and σ(n) as the standard deviation, where n is 0 to N-1 (step S26). Then, the CPU ends the series of processes for calculating the occurrence rate p(n), and the flow returns to... Figure 8 Step S3 in the process.

[0142] (2-5-3. Processing for calculating the arrival probability of ambient light)

[0143] Next, the processing at step S3 above, namely the processing of calculating the ambient light arrival probability (the probability that ambient light from the sun 30 will reach the SPAD element of the light receiving unit 12), will be described. Figure 10 This is a flowchart illustrating an example of the process for calculating the probability of ambient light reaching the SPAD element. This process is also executed under the control of the CPU in the arithmetic unit 15.

[0144] In the process of calculating the probability of ambient light arrival, it is used in Figure 8 The occurrence rate p(n) determined in step S2, i.e., refer to Figure 9 The description describes the processing of the calculated occurrence rate. First, the CPU obtains the reference... Figure 9 The occurrence rate p(n) is determined in the process of calculating the occurrence rate described (step S31). Thereafter, the CPU determines the ambient light arrival probability Iambient, which is the probability that ambient light will arrive at the SPAD element of the light receiving unit 12 (step S32). The ambient light arrival probability Iambient satisfies the following expression (9).

[0145] [Expression 9]

[0146]

[0147] Finally, the CPU outputs the ambient light arrival probability Iambient as the brightness value obtained when the target 20 is illuminated by ambient light (step S33), and ends the series of processes for calculating the ambient light arrival probability Iambient.

[0148] As described above, in the distance measuring apparatus or distance measuring method according to this embodiment, the diffusion state around the peak in the time direction is measured based on a histogram indicating the frequency (or the occurrence rate calculated therefrom) of the reflected light received at the light receiving unit 12. For example, when it is possible to approximate the light output from the distance measuring target 20 due to sub-surface scattering as an exponential distribution (expression (1)), the standard deviation is determined as the diffusion state.

[0149] Furthermore, a correction amount for the subsurface scattering component is calculated based on the diffusion state around the peak in the time direction. For example, if the light from the light-emitting unit 11 (active light) can be approximated as a normal distribution (expression (2)) and the light output from the distance measuring target 20 due to subsurface scattering can be approximated as an exponential distribution (expression (1)), the exponential distribution parameter λ is determined, and its reciprocal is set as the correction amount.

[0150] Furthermore, subtracting the calculated correction from the distance measurement result allows for accurate measurement of distance L. For example, if the light from the light-emitting unit 11 (active light) can be approximated as a normal distribution (expression (2)) and the light output from the distance measurement target 20 due to sub-surface scattering can be approximated as an exponential distribution (expression (1)), the correction (1 / λ) is subtracted from the average value of the histogram (or the occurrence rate calculated from it). This allows for accurate measurement of distance L.

[0151] <3. Modified Example>

[0152] Although the above description is based on some preferred embodiments of the technology disclosed herein, the technology disclosed herein should not be limited to these embodiments. The configuration and structure of the distance measuring device described in the above embodiments are merely examples and can be modified as appropriate.

[0153] <4. Examples of the application of the technology disclosed herein>

[0154] For example, the technologies described above can be installed on various electronic devices with facial authentication capabilities. Examples of electronic devices with facial authentication capabilities may include mobile devices such as smartphones, digital cameras, tablets, and personal computers. However, electronic devices to which the technologies of this disclosure can be applied are not limited to these mobile devices.

[0155] Here, a smartphone with facial recognition functionality is exemplified as a specific example of an electronic device to which the technologies disclosed herein can be applied. Figure 11 This is an exterior view of a smartphone, as seen from the front, as a specific example of an electronic device to which the technology according to this disclosure can be applied.

[0156] According to a specific example, the smartphone 100 includes a main body 110 and a display 120 disposed on the front of the main body 110. The smartphone 100 also includes a light-emitting unit 11 and a light-receiving unit 12 of the distance measuring device 10 according to the above embodiment. The light-emitting unit 11 and the light-receiving unit 12 are disposed, for example, on the upper part of the front of the main body 110. It should be noted that the position of the light-emitting unit 11 and the light-receiving unit 12 of the distance measuring device 10 is not limited to the upper part of the front of the main body 110.

[0157] The distance measuring device 10 according to the above embodiment can be installed on a smartphone 100 according to a specific example. This enables the smartphone 100 to recognize the three-dimensional shape of an object, for example, facial authentication. Specifically, the distance measuring device 10 allows for accurate measurement of the distance to the target while taking into account the time attributable to subsurface scattering. Therefore, the distance measuring device 10 is able to perform facial authentication with higher certainty, for example.

[0158] <5. Possible Configurations of this Disclosure>

[0159] It should be noted that this disclosure may also have the following configurations.

[0160] A. Distance Measuring Device

[0161] [A-01] A distance measuring device, comprising:

[0162] A light receiving unit receives reflected light from a distance measuring target based on illumination light from a light emitting unit;

[0163] The histogram acquisition unit acquires a histogram indicating the frequency of reflected light received at the light receiving unit; and

[0164] The calculation unit calculates the distance to the distance measurement target based on the time corresponding to the peak value of the histogram obtained by the histogram acquisition unit.

[0165] The processing unit corrects the distance calculated based on the time corresponding to the peak value of the histogram based on the shape of the histogram obtained by the histogram acquisition unit.

[0166] [A-02] In the distance measuring device according to [A-01] above, the light receiving element of the light receiving unit is an avalanche photodiode operating in Geiger mode.

[0167] [A-03] In the distance measuring device according to [A-02] above, the light receiving element of the light receiving unit is a single-photon avalanche diode.

[0168] [A-04] The distance measuring device according to any one of [A-01] to [A-03] above, wherein the distance measuring target is the object that causes sub-surface scattering.

[0169] [A-05] According to the distance measuring device of [A-04] above, the histogram acquisition unit includes:

[0170] A time difference detector detects the time from the emission of illumination light by the light-emitting unit to the reception of reflected light from the target at the light-receiving unit; and

[0171] The accumulator generates a histogram based on the time detected by the time difference detector.

[0172] [A-06] According to any one of [A-01] to [A-05] above, the shape of the histogram is the diffusion state of the reflected light distribution from the distance measurement target around the peak obtained by performing predetermined statistical processing on the histogram acquired by the histogram acquisition unit.

[0173] [A-07] In the distance measuring device according to [A-06] above, the arithmetic unit measures the diffusion state of the reflected light distribution from the distance measuring target around the peak, calculates a correction amount based on the diffusion state around the peak, and performs correction based on the calculated correction amount.

[0174] [A-08] In the distance measuring device according to [A-07] above, the arithmetic unit performs distance correction by subtracting a correction amount calculated based on the diffusion state around the peak from the peak of the reflected light distribution.

[0175] [A-09] In the distance measuring device according to [A-08] above, the distribution of reflected light is obtained by subtracting the ambient light component from the histogram obtained by the histogram acquisition unit.

[0176] [A-10] According to the distance measuring device of [A-01] above, the diffusion state of the reflected light distribution around the peak is the standard deviation.

[0177] [A-11] According to the distance measuring device of [A-10] above, the correction amount calculated based on the diffusion state around the peak is the reciprocal of the exponential distribution parameter determined based on the standard deviation.

[0178] B. Distance Measurement Methods

[0179] [B-01] A distance measurement method, wherein the distance measuring device:

[0180] The distance to the target is calculated based on the time corresponding to the peak value of the histogram acquired by the histogram acquisition unit; and

[0181] The distance calculated based on the time corresponding to the peak value of the histogram is corrected based on the shape of the histogram obtained by the histogram acquisition unit.

[0182] The distance measuring device includes:

[0183] A light receiving unit receives reflected light from a distance measuring target based on illumination light from a light emitting unit; and

[0184] The histogram acquisition unit acquires a histogram indicating the frequency of reflected light received at the light receiving unit.

[0185] [B-02] According to the distance measurement method of [B-01] above, the light receiving element of the light receiving unit is an avalanche photodiode operating in Geiger mode.

[0186] [B-03] According to the distance measurement method of [B-02] above, the optical receiving element of the optical receiving unit is a single-photon avalanche diode.

[0187] [B-04] According to any one of [B-01] to [B-03] above, the distance measurement target is the object that causes scattering from the sub-surface.

[0188] [B-05] According to the distance measurement method of [B-04] above, the histogram acquisition unit includes:

[0189] A time difference detector detects the time from the emission of illumination light by the light-emitting unit to the reception of reflected light from the target at the light-receiving unit; and

[0190] The accumulator generates a histogram based on the time detected by the time difference detector.

[0191] [B-06] According to any one of [B-01] to [B-05] above, the shape of the histogram is the diffusion state of the reflected light distribution from the distance measurement target near the peak obtained by performing predetermined statistical processing on the histogram acquired by the histogram acquisition unit.

[0192] [B-07] According to the distance measurement method of [B-06] above, the arithmetic unit measures the diffusion state of the reflected light distribution from the distance measurement target around the peak, calculates a correction amount based on the diffusion state around the peak, and performs correction based on the calculated correction amount.

[0193] [B-08] According to the distance measurement method of [B-07] above, the arithmetic unit performs distance correction by subtracting a correction amount calculated based on the diffusion state around the peak from the peak of the reflected light distribution.

[0194] [B-09] According to the distance measurement method of [B-08] above, the distribution of reflected light is obtained by subtracting the ambient light component from the histogram obtained by the histogram acquisition unit.

[0195] [B-10] According to the distance measurement method of [B-01] above, the diffusion state of the reflected light distribution around the peak is the standard deviation.

[0196] [B-11] According to the distance measurement method of [B-10] above, the correction amount calculated based on the diffusion state around the peak is the reciprocal of the exponential distribution parameter determined based on the standard deviation.

[0197] This application claims the benefit of Japanese patent application JP2020-175913, filed on October 20, 2020, with the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0198] Those skilled in the art will understand that various modifications, combinations, sub-combinations and alterations can be made according to design requirements and other factors, as long as such modifications, combinations, sub-combinations and alterations are within the scope of the appended claims or their equivalents.

Claims

1. A distance measuring device, comprising: A light receiving unit receives reflected light from a distance measuring target based on illumination light from a light emitting unit; The histogram acquisition unit acquires a histogram indicating the frequency of the reflected light received at the light receiving unit; as well as The calculation unit calculates the distance to the distance measurement target based on the time corresponding to the peak value of the histogram obtained by the histogram acquisition unit. The calculation unit corrects the distance calculated based on the time corresponding to the peak value of the histogram based on the shape of the histogram obtained by the histogram acquisition unit. The shape of the histogram includes the diffusion state of the reflected light distribution from the distance measurement target around the peak, obtained by performing predetermined statistical processing on the histogram acquired by the histogram acquisition unit. The processing unit measures the diffusion state of the reflected light from the distance measurement target around the peak value, calculates a correction amount based on the diffusion state around the peak value, and performs correction based on the calculated correction amount. The arithmetic unit performs distance correction by subtracting a correction amount calculated based on the diffusion state around the peak from the peak of the reflected light distribution. The diffusion state of the reflected light distribution around the peak includes a standard deviation.

2. The distance measuring device according to claim 1, wherein The optical receiving element of the optical receiving unit includes an avalanche photodiode operating in Geiger mode.

3. The distance measuring device according to claim 2, wherein The optical receiving element of the optical receiving unit includes a single-photon avalanche diode.

4. The distance measuring device according to claim 1, wherein, The distance measurement target is the object that causes scattering from the subsurface.

5. The distance measuring device according to claim 4, wherein, The histogram acquisition unit includes: A time difference detector detects the time from the point in time when the emitting unit emits the illumination light to the point in time when the light receiving unit receives the reflected light from the distance measuring target; and An accumulator generates a histogram based on the time detected by the time difference detector.

6. The distance measuring device according to claim 1, wherein, The distribution of the reflected light is obtained by subtracting the ambient light component from the histogram obtained by the histogram acquisition unit.

7. The distance measuring device according to claim 1, wherein, The correction amount, calculated based on the diffusion state around the peak, includes the reciprocal of the exponential distribution parameter determined based on the standard deviation.

8. A distance measurement method, wherein the distance measuring device: The distance to the target is calculated based on the time corresponding to the peak value of the histogram acquired by the histogram acquisition unit; and The distance calculated based on the time corresponding to the peak value of the histogram is corrected based on the shape of the histogram obtained by the histogram acquisition unit. The distance measuring device includes: A light receiving unit receives reflected light from the distance measuring target based on the illumination light from the light emitting unit; as well as The histogram acquisition unit acquires a histogram indicating the frequency of the reflected light received at the light receiving unit. The shape of the histogram includes the diffusion state of the reflected light distribution from the distance measurement target around the peak, obtained by performing predetermined statistical processing on the histogram acquired by the histogram acquisition unit. The distance measuring device measures the diffusion state of the reflected light from the distance measuring target around the peak value, calculates a correction amount based on the diffusion state around the peak value, and performs correction based on the calculated correction amount. The distance measuring device performs distance correction by subtracting a correction amount calculated based on the diffusion state around the peak from the peak of the reflected light distribution. The diffusion state of the reflected light distribution around the peak includes a standard deviation.

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