LIGHT DETECTION DEVICE, LIGHT DETECTION METHOD AND OPTICAL DISTANCE SENSOR

Abstract

A light detection device for detecting incident light according to a detection start time, the light detection device comprising: a plurality of photosensors configured to receive light to generate output signals, each indicating the results of the light detection; a signal combination circuit configured to sum the plurality of output signals from the respective photosensors to generate a combined signal; a detection circuit configured to detect a time at which the combined signal reaches a first threshold or higher to generate a detection signal; a timing circuit configured to measure a counting period between the detection start time and the detected time based on the detection signal;and a time extraction circuit configured to extract timing information from a predetermined period defined by the detected time as a reference, wherein one end of the predetermined period corresponds to the time detected by the detection circuit, and wherein the timing information indicates a time at which the combined signal increases.

Description

TECHNICAL AREA The present disclosure relates to a light detection device, a light detection method and an optical distance sensor including the light detection device. TECHNICAL BACKGROUND An optical distance sensor is known that uses the time-of-flight (TOF) of light. The optical distance sensor illuminates an object with light and detects the light reflected from the object, thereby measuring a distance corresponding to the time it takes for the light to travel back and forth to the object. A technique is proposed for the optical distance sensor that uses a single-photon avalanche photodiode (SPAD) for light detection (e.g., patent documents 1 and 2). Patent document 1 discloses a light detector with multiple SPADs in an optical distance measuring device. The light detector of patent document 1 sums rectangular pulses emitted by the plurality of SPADs, compares a summed output value with a predetermined reference value, and outputs a trigger signal according to the comparison result. Patent document 2 discloses a distance measuring device with several SPADs in a receiver unit. The distance measuring device of patent document 2 determines that a measuring pulse is detected when a sum signal, which indicates the sum of the electrical pulses emitted by the several SPADs, exceeds a predetermined threshold value and a rising edge of the sum signal exceeds a predetermined edge threshold value. QUOTE LIST Patent document Patent document 1: JP 5 644 294 B2Patent document 2: US 2015 / 0177369 A1 SUMMARY Technical problem In the prior art, as e.g., in patent document 1, the detection goal is a time point for each light detection using the plurality of SPADs, with the aim of knowing the total number of photons simultaneously detected by the plurality of SPADs. Since the response of the SPAD to the photon is stochastic, it is desirable to obtain a plurality of time points when the plurality of SPADs are caused to simultaneously receive light in order to perform the light detection accurately, e.g., through statistical processing. One objective of the present disclosure is to provide a light detection device, a light detection method and an optical distance sensor that are capable of enabling accurate light detection in an optical distance sensor. Solution to the problem A light detection device according to the present disclosure detects incident light after a detection start time. The light detection device comprises a plurality of photosensors, a signal combining circuit, a detection circuit, a timing circuit, and a time extraction circuit. The plurality of photosensors receive light to generate output signals, each indicating the results of the light reception. The signal combining circuit sums a plurality of output signals from the respective photosensors to generate a combined signal. The detection circuit detects a time at which the combined signal reaches a first threshold or a higher threshold to generate a detection signal indicating the detected time. The timing circuit measures a counting period between the detection start time and the detected time based on the detection signal.The time extraction circuit extracts timing information from a predetermined period, defined by the detected time as a reference, where the timing information indicates a time at which the combined signal increases. A light detection method according to the present disclosure provides a method in which a light detection device detects incident light according to a detection start time. An optical distance sensor according to the present disclosure comprises a light projector that projects light and a light detection device. The timing circuit in the light detection device measures the counting period using a time at which the light projector projects light as the detection start time. Beneficial effect With the light detection device, the light detection method and the optical distance sensor according to the present disclosure, it is possible to enable accurate light detection with the optical distance sensor. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view illustrating an application example for a light detection device according to the present disclosure. Fig. 2 is a block diagram illustrating a configuration of an optical distance sensor according to a first embodiment. Fig. 3 is a block diagram illustrating a configuration of a light detection device according to the first embodiment. Fig. 4 is a circuit diagram showing a configuration example for a holding circuit in the light detection device. Figs. 5A to 5D are timing diagrams describing a method for combining a combined signal in the light detection device. Figs. 6A to 6F are timing diagrams illustrating an operation of the light detection device according to the first embodiment. Fig. 7 is a block diagram showing a configuration of a light detection device according to a second embodiment.Figures 8A to 8E are timing diagrams illustrating the operation of the light detection device according to the second embodiment. Figure 9 is a block diagram illustrating a configuration of a light detection device according to a third embodiment. Figure 10 is a circuit diagram illustrating a configuration example for an integration circuit in the light detection device. Figures 11A to 11F are timing diagrams illustrating the operation of the light detection device according to the third embodiment. Figure 12 is a circuit diagram illustrating a modification of a circuit for adjusting the detection threshold in the light detection device. DETAILED DESCRIPTION The following describes exemplary embodiments of a light detection device, a light detection method, and an optical distance sensor according to the present disclosure with reference to the accompanying drawings. Note that the same components are designated by the same reference numerals in each of the following exemplary embodiments. (Application example) An example in which a light detection device according to the present disclosure can be used is described with reference to Fig. 1. Fig. 1 is a view illustrating an application example for a light detection device 1 according to the present disclosure. The light detection device 1 according to the present disclosure is applied to an optical distance sensor 2 of the TOF type. The optical distance sensor 2 includes a light projector or light emitter 20 that projects pulsed light outwards, as shown, for example, in Fig. 1. The light detection device 1 is a receiver unit that receives light from outside the optical distance sensor 2. The optical distance sensor 2 according to the present disclosure can, for example, be based on a photoelectric sensor for applications in industrial automation. The optical distance sensor 2 detects reflected light of the pulsed light projected by the light projector 20 using the light detection device 1 in order to measure a distance to an object that reflects the light based on the time of flight of the light. The optical distance sensor 2 can detect whether the object is located at a specific position. In the present application example, a SPAD (photoelectric sensor) is used as a sensor element in the optical distance sensor 1 to improve the sensitivity of light detection. The SPAD is highly sensitive enough to respond to the incident photon. However, the detection can exhibit fluctuations because the response is stochastic. To suppress these SPAD detection fluctuations, the present application example records multiple time points when the light detection device 1 performs light detection using several SPADs simultaneously, thus enabling highly accurate light detection. (Configuration example) The following are examples of configurations of the light detection device 1 and the optical distance sensor 2. (First embodiment) In a first embodiment, the optical distance sensor 2 and the light detection device 1 are described, which detect local time points near a time point at which the number of SPADs that have received light has reached a detection threshold or more. 1. Configuration The following describes the configurations of the optical distance sensor 2 and the light detection device 1 according to the first embodiment. 1-1. Configuration of the optical distance sensor The configuration of the optical distance sensor 2 according to the present embodiment is described with reference to Fig. 2. Fig. 2 is a block diagram illustrating the configuration of the optical distance sensor 2. The optical distance sensor 2 comprises, for example, the light projector 20, a controller 25, and the light detection device 1, as shown in Fig. 2. The light projector 20 includes, for example, a light source 21 and a light source driver 22. In the light projector 20, the light source 21 comprises, for example, an LD (laser diode) or an LED. The light source 21 emits light, such as infrared light. The light source driver 22 is a circuit that controls the light emission of the light source 21. The light source driver 22 causes the light source 21 to emit light in the form of a pulse, i.e., pulsed light, at a time controlled by the controller 25. The pulsed light has a pulse width of, for example, a few nanoseconds up to several tens of nanoseconds. The controller 25 includes, for example, a CPU, RAM, ROM, and similar components, and controls each component. For example, the controller 25 generates various control signals to control the entire operation of the optical distance sensor 2. As shown in Fig. 2, the light detection device 1 comprises, for example, a SPAD array 10, a signal processor 11, and a distance sensor 12. The light detection device 1 includes, for example, an amplifier that amplifies an electrical signal generated by the SPAD in response to incident light, a driver circuit for the SPAD, and similar components in the SPAD array 10 or the signal processor 11. The SPAD array 10 is configured by arranging a multitude of SPADs in an array form. Each SPAD of the SPAD array 10 is implemented by operating an avalanche photodiode (APD) in Geiger mode. The signal processor 11 performs signal processing to determine, based on the output signal of the SPAD array 10, the time at which light arrives as the detection target of the light detection device 1. The distance measuring device 12 calculates a distance value based on a signal processing result from the signal processor 11, which indicates a distance corresponding to the time of flight of light. Details of the configuration of the light detection device 1 are described below. 1-2. Configuration of the light detection device A configuration example of the light detection device 1 according to the first embodiment is described with reference to Fig. 3 and Fig. 4. Fig. 3 is a block diagram illustrating the configuration of the light detection device 1 according to the present embodiment. As shown in Fig. 3, the light detection device 1 of the present embodiment includes a plurality of SPADs 10a to 10c, which form the SPAD array 10, as well as a signal combination circuit 13, a detection circuit 14 and a time extraction circuit 3, which form a signal processor 11. In addition, the light detection device 1 includes, for example, a TDC (time-to-digital converter) 4 and a computer 5, which form the distance measuring device 12. SPADs 10a to 10c are examples of photosensors that respond stochastically to photons incident on the light detection device 1. An example is described below in which the number of SPADs 10a to 10c in the SPAD arrangement 10 is three. Each of the SPADs 10a, 10b, and 10c receives light and generates the output signals Sa, Sb, and Sc, respectively, each indicating a light reception result. For example, a waveform shaping circuit, which shapes the signal waveforms of the output signals Sa to Sc of SPADs 10a to 10c into rectangular pulse shapes, is suitably integrated into the light detection device 1. The respective output signals Sa to Sc of the SPAD matrix 10 are input into the signal combination circuit 13 of the signal processor 11. The signal combination circuit 13 sums the multitude of output signals Sa to Sc that are input to it in order to generate a combined signal S1. The signal combination circuit 13 outputs the combined signal S1 thus generated to the detection circuit 14 and the time extraction circuit 3. The signal combination circuit 13 can be configured by applying a known technique (e.g., see patent document 1). Based on the combined signal S1 from the signal combination circuit 13, the detection circuit 14 detects the time of the light received as the detection target of the light detection device 1 in order to generate a detection signal S2 that indicates a detection result. As shown in Fig. 3, the detection circuit 14 includes, for example, a detection threshold setting circuit 15 and a determination circuit 16. The detection threshold setting circuit 15 sets a predefined detection threshold V1 at the determination circuit 16. The detection threshold V1 provides a reference for determining a time of simultaneous detection in the majority of SPADs 10a to 10. The detection threshold V1 is an example of a first threshold in the present embodiment. The detection circuit 16 comprises, for example, a comparator and a logic circuit. The detection circuit 16 compares and determines the combined signal S1 and the detection threshold V1 and generates the detection signal S2 according to the result. In the present embodiment, the detection circuit 16 generates the detection signal S2, which indicates whether the combined signal S1 reaches or exceeds the detection threshold V1. The detection signal S2 is input to the TDC 4 and the time-of-time extraction circuit 3. The time-of-time extraction circuit 3 of the present embodiment comprises one or more binarization threshold setting circuits 31a and 31b (generally referred to as "binarization threshold setting circuit 31"), one or more comparator circuits 32a and 32b (generally referred to as "comparator circuit 32"), and one or more hold circuits 33a and 33b (generally referred to as "hold circuit 33"). An example is described below in which the time-of-time extraction circuit 3 comprises two sets of the binarization threshold setting circuit 31, the comparator circuit 32, and the hold circuit 33. Regarding the binarization threshold setting circuit 31, the first binarization threshold setting circuit 31a, for example, sets a first binarization threshold V3a, which is smaller than the detection threshold V1, on the first comparator circuit 32a. Additionally, the second binarization threshold setting circuit 31b sets a second binarization threshold V3b on the second comparator circuit 32b, where the second binarization threshold V3b is, for example, larger than the first binarization threshold V3a and smaller than the detection threshold V1. Each of the binarization thresholds V3a and V3b provides a reference point for determining a time at which the response of SPADs 10a to 10c changes near the time captured, for example, by the detection threshold V1. Each of the binarization thresholds V3a and V3b (generally referred to as "binarization threshold V3") is an example of a second threshold in the present embodiment. The combined signal S1 from the signal combination circuit 13 is fed into the comparator circuit 32 of the time-of-time extraction circuit 3. The first comparator circuit 32a compares the combined signal S1 with the first binarization threshold V3a and generates, for example, a first binary signal S3a, which indicates whether the combined signal S1 is equal to or greater than the first binarization threshold V3a. The first comparator circuit 32a outputs the generated first binary signal S3a to the first hold circuit 33a. Similar to the first binary signal S3a, the second comparator circuit 32b generates a second binary signal S3b based on the result of comparing the combined signal S1 with the second binarization threshold V3b and outputs the second binary signal S3b to the second holding circuit 33b. Hereafter, the first and second binary signals S3a and S3b are generally referred to as "binary signal S3". The detection signal S2 from the detection circuit 14 is fed into the hold circuit 33 of the time extraction circuit 3. The first and second hold circuits 33a and 33b each hold a first and a second time information D3a and D3b (generally referred to as "time information D3"), which indicate the signal states of the respective binary signals S3a and S3b for a predetermined hold period with the detection signal S2 as a reference. The hold duration is, for example, a few nanoseconds up to several tens of nanoseconds. A configuration example of the hold circuit 33 is shown in Fig. 4. In the configuration example of Fig. 4, the holding circuit 33 contains a plurality of storage elements 34 and a plurality of delay elements 35 connected in series. Each of the storage elements 34 is connected to a delay element 35 at an input terminal and holds a binary value of "0" or "1". It is desirable that each of the delay elements 35 has a common delay period (e.g., a few tens of picoseconds up to several hundred picoseconds). In the holding circuit 33 in the example of Fig. 4, the binary signal S3 from the comparator circuit 32 is fed into a series connection of delay elements 35. The detection signal S2 from the detection circuit 14 is fed into a control terminal of each of the storage elements 34. Each of the storage elements 34 receives and holds the binary signal S3, which is output to the corresponding delay element 35, in accordance with the timing specified by the detection signal S2. Consequently, the holding circuit 33 can hold the timing information D3, which indicates a signal state of the binary signal S3 for each delay period within the holding period, ending at the time indicated by the detection signal S2. Returning to Fig. 3, a detection start time signal S0 is input from the controller 25 into the TDC 4. The detection start clock signal S0 is an example of a control signal that specifies a time at which the operation of the TDC 4 is started. The TDC 4 is an example of a timing circuit that generates time information as a digital value (time-to-digital conversion). The TDC 4 measures a counting period, ranging from the time indicated by the detection start clock signal S0 to the time indicated by the detection signal S2, based on the detection start clock signal S0 and the detection signal S2, and generates a time value D1 that displays the counting period as the measurement result. Computer 5 contains, for example, a CPU that, in conjunction with software, performs various arithmetic processes, RAM, ROM, and similar components. Computer 5 operates as a distance sensor 12 in conjunction with the TDC 4. Computer 5 receives the time information D1 from the TDC 4 and also receives the respective time information D3a and D3b from the respective holding circuits 33a and 33b of the time extraction circuit 3. Computer 5 performs arithmetic processing to calculate a distance based on the acquired time information D1 and the time information D3a and D3b, corresponding to the travel time of light. This arithmetic processing includes, for example, various types of statistical processing and similar operations. It should be noted that hardware resources, such as the CPU that forms computer 5, can be shared by controller 25 of optical distance sensor 2 or provided separately. Furthermore, computer 5, controller 25, and the like can be configured using various hardware circuits, such as an ASIC and an FPGA. 2. Operation The operation of the optical distance sensor 2 and the light detection device 1 configured as described above is described below. In the optical distance sensor 2, the controller 25 (Fig. 2) controls the light source driver 22 of the light projector 20 so that the light source 21 emits pulsed light, for example, at predefined time intervals. If the projected pulsed light is reflected by an object that is a distance measurement target of the optical distance sensor 2, the projected pulsed light can reach the optical distance sensor 2 as reflected light. At the time of controlling the light projector 20, the controller 25 generates the detection start clock signal S0, which indicates the time for the light projection, and outputs the detection start clock signal S0 to the TDC 4 ( Fig. 3 ) of the distance meter 12. In synchronization with the light projection or emission of the light projector 20, the light detection device 1 in the optical distance sensor 2 performs light detection to detect the reflected light of the pulsed light during a predetermined light reception period from the moment the light is projected. The light reception period is set to a period that is shorter than, for example, the time interval of the pulsed light, and can be adjusted with respect to the light travel time according to an upper limit of a distance to be measured (e.g., a light reception period of 200 ns for a distance limit of 30 m). During light detection by the light detection device 1, the SPAD array 10 receives light, and the signal processor 11 performs signal processing on a signal resulting from the light reception, thereby generating the detection signal S2, which indicates the time at which the reflected light arrives. Based on the detection signal S2, the distance measuring device 12 measures the time it takes for the projected pulsed light to be reflected by the object and received by the TDC 4, as the counting period. The distance measuring device 12 can calculate a distance value by, for example, multiplying half of the measured counting period by the speed of light. By using SPADs 10a to 10c in the light detection device 1 in the optical distance sensor 2 above, it is possible to increase the sensitivity of the light detection and improve the accuracy of the distance measurement. However, since the detection of photons by SPADs 10a to 10c is stochastic, stochastic detection variations are conceivable in the situation where the multitude of SPADs 10a to 10c simultaneously detect photons for a light projection. To solve this, the light detection device 1 of the present embodiment records several time points for a detection start time by comparison and determination using the detection threshold V1 of the detection circuit 14 and each of the binarization thresholds V3a and V3b of the time extraction circuit 3, and performs the statistical processing in the rangefinder 12. The operation of the light detection device 1 of the present embodiment is described in detail below. 2-1. Operation of the light detection device Details of the operation of the light detection device 1 according to the present embodiment are described with reference to Fig. 5 and Fig. 6. Figures 5A to 5D are timing diagrams describing a method for combining the combined signal S1 in the light detection device 1. Figures 6A to 6F are timing diagrams illustrating the operation of the light detection device 1. In the light detection device 1 (Fig. 3) of the present embodiment, the SPADs 10a to 10c receive light in the respective stochastic operations and generate the output signals Sa, Sb and Sc respectively. The signal waveforms of the output signals Sa, Sb and Sc are shown in Fig. 5A, Fig. 5B and 5C respectively. In the example shown in Figures 5A to 5C, each of the output signals Sa to Sc is a rectangular pulse P1 with a predefined pulse width. Each of the SPADs 10a to 10c responds stochastically to incident photons, so that the rectangular pulse P1 is generated in each of the output signals Sa to Sc. In the example shown in Figures 5A to 5C, the output signal Sa of the first SPAD 10a increases at time t1 (Figure 5A), and the output signal Sb of the second SPAD 10b increases at time t3 after time t1 (Figure 5B). Furthermore, the output signal Sa of the third SPAD 10a increases at time t2 between time t1 and time t3 (Figure 5C). The signal combination circuit 13 sums the output signals Sa to Sc from the SPADs 10a to 10c to generate the combined signal S1. The combined signal S1 is based on the output signals Sa to Sc in Fig. 5A to 5C and is shown in Fig. 5D. The combined signal S1 shown in Fig. 5D is the sum of the three output signals Sa to Sc (Figs. 5A to 5C) at the same time. For example, the sum of the combined signal S1 corresponds to the output signal Sa from Fig. 5A from time t1 to time t2. Additionally, in the example shown in Fig. 5D, the combined signal S1 increases from "1" to "2" at time t2 by the sum of the two rectangular pulses P1 (Fig. 5A and Fig. 5C). The combined signal S1 then increases further from "1" to "3" at time t3 by the sum of the three rectangular pulses P1 (Fig. 5A to 5C). In this way, the signal level of the combined signal S1 changes according to the number of SPADs 10a to 10c with the received light. The relationship between the combined signal S1, as described above, and the various threshold values ​​V1, V3a, and V3b is shown in Fig. 6A. An example is described below in which the light projector 20 projects light at time t0, and the light detection device 1 performs light detection during a light reception period T1, starting from time t0, as shown in Figs. 6A to 6D. In this case, the detection start clock signal S0, which indicates time t0, is input or supplied to the TDC 4 by the controller 25. The combined signal S1 in the example of Fig. 6A increases after time t0, reaches the first binarization threshold V3a at time t11, reaches the second binarization threshold V3b at time t12, and reaches the detection threshold V1 at time t13. In the light detection device 1, the combined signal S1 is input into the detection circuit 14 and into the time extraction circuit 3. The detection circuit 14 compares the combined signal S1 and the detection threshold V1 to generate the detection signal S2. Simultaneously, the comparison circuits 32a and 32b of the time-of-time extraction circuit 3 compare the combined signal S1 and the binarization thresholds V3a and V3b, respectively, and generate the respective binary signals S3a and S3b. Figures 6B, 6C, and 6D illustrate the detection signal S2, the first binary signal S3a, and the second binary signal S3b, respectively, based on the combined signal S1 of the example in Figure 6A. At time t11, the combined signal S1 reaches the first binarization threshold V3a, so that the first binary signal S3a changes from "0" to "1", as shown in Fig. 6C. Similarly, at time t12, the second binary signal S3b changes from "0" to "1", as shown in Fig. 6D. The binary signals S3a and S3b are sequentially fed into the hold circuits 33a and 33b, respectively. At time t13 after time t12, the combined signal S1 reaches the detection threshold V1, causing the detection signal S2 to increase, as shown in Fig. 6B. The TDC 4 then performs the measurement (time / digital conversion) of one period T2 from time t0, indicated by the detection start time signal S0, until time t13, when the detection signal S2 increases as the counting period. The TDC 4 contains the time information D1, which specifies the measured counting period T2. Furthermore, in response to the rise of the detection signal S2 at time t13, the respective holding circuits 33a and 33b of the time extraction circuit 3 hold the timing information D3a and D3b on the binary signals S3a and S3b, which are input after time t10, which is one holding period T3 before time t13. The timing information D3a and D3b, which holds the binary signals S3a and S3b in Fig. 6C and Fig. 6D, are shown in Fig. 6E and 6F, respectively. In Figures 6E and 6F, a more recent signal state is shown on the left side of the drawing, corresponding to the arrangement of the storage elements 34 from Figure 4. In Figure 6E, "1" is recorded on the left side of the drawing in accordance with the binary signal S3a from Figure 6C, which is "1" at the end from time t11 to time t13. In Figure 6F, a section in which "1" is recorded is shorter than in Figure 6E, in accordance with the binary signal S3b in Figure 6D, which is "1" from time t12 onwards, which is after time t11. Based on the time values ​​D3a and D3b in Figures 6E and 6F, it is possible to identify the times t11 and t12 at which the combined signal S1 reaches the respective binarization thresholds V3a and V3b, or higher, relative to time t13 at the end. Each piece of time information D3a and D3b is entered into computer 5. Computer 5 performs a distance measurement operation based on the time information D1, which specifies the counting period T2, and the time information D3a and D3b. For example, computer 5 first converts the relative time information based on time information D3a and D3b into absolute time information using the detection start time t0 as a reference. Specifically, computer 5 calculates the periods T31 and T32 from time t0 to times t11 and t12, respectively, as shown in Fig. 6C and Fig. 6D, for example, based on the counting period T2 and the length of segment "1" in the time information D3a and D3b. Furthermore, computer 5 performs a predefined statistical processing based on the counting period T2 of TDC 4 and the periods T31 and T32 calculated from the time information D3. For example, computer 5 calculates an average from the multitude of periods T2, T31, and T32 for the single projection or emission and reception of light in order to calculate the light travel time or a corresponding distance value. Furthermore, the computer 5 can perform statistical processing using a histogram or similar by accumulating the time information D1 and the time information D3, which are obtained by repeatedly projecting or emitting and receiving light in a RAM or similar device, and can, for example, calculate a distance value corresponding to a peak position in the histogram. Using the time information D1 and the timing information D3, it is possible to efficiently increase the number of histogram samples. According to the operating principle of the light detection device 1 described above, when the time t13, at which the simultaneous light reception by the multiple SPADs 10a to 10b occurs, is detected based on the combined signal S1 obtained by combining the output signals Sa to Sc of the SPADs 10a to 10c, the time information D31 and D32, which indicate the times t11 and t12 at which the combined signal S1 increased immediately before time t13, can be further recorded. As a result, the multiple times t11 to t13 at which the multiple SPADs 10a to 10c react to a single projection or single emission and reception of light are obtained. In this way, it is possible to enable precise light detection in the optical distance sensor 2. 3. Summary As described above, the light detection device 1, according to the present embodiment, detects incident light according to the predetermined detection start time. The light detection device 1 comprises the plurality of SPADs 10a to 10c, the signal combination circuit 13, the detection circuit 14, the TDC 4, and the time extraction circuit 3. The plurality of SPADs 10a to 10c receive light and generate the respective output signals Sa to Sc, which indicate the results of the light reception. The signal combination circuit 13 sums the plurality of output signals Sa to Sc from the SPADs 10a to 10c to generate the combined signal S1. The detection circuit 14 detects the time at which the combined signal S1 is equal to or greater than the detection threshold V1, which is an example of the first threshold, and generates the detection signal S2, which indicates the detected time. The TDC 4 measures the counting period T2, i.e.a period between the detection start time and the detected time based on the detection signal S2. The timing extraction circuit 3 extracts the timing information D3a and D3b, which indicates whether the combined signal S1 is equal to or greater than the binarization thresholds V3a and V3b, which are examples of the second threshold, within the predetermined holding period T3, defined by the detected timing as a reference. According to the light detection device 1 described above, it is possible to facilitate light detection in the optical distance sensor 2 by recording the time information D3a and D3b in addition to the counting period T2. In the present embodiment, the photosensors of the light detection device 1 are the SPADs 10a to 10c, which respond stochastically to the incident photons. Even if each of the SPADs 10a to 10c operates stochastically, it is possible to obtain accurate light detection using the time information D3. In the present embodiment, the light detection device 1 additionally includes the computer 5. Based on the extracted time information D3a and D3b and the measured counting period T2, the computer 5 calculates the periods T31 and T32 from the time of the start of detection until the time specified by the time information. The computer 5 performs statistical processing based on the calculated periods T31 and T32 and the counting period T2. This statistical processing enables the implementation of highly accurate distance measurement. In the present embodiment, the timing extraction circuit 3 comprises the comparator circuit 32 and the hold circuit 33. The comparator circuit 32 is provided for each binarization threshold V3 and generates the binary signal S3, which indicates whether the combined signal S2 is equal to or greater than the binarization threshold V3. The hold circuit 33 is provided for each comparator circuit 32 and holds the timing information D3, which is based on the binary signal S3 received from the comparator circuit 32, for the hold period T3 until the timing indicated by the detection signal S2. Consequently, the timing information D3, which is based on each of the binary signals S3, can be acquired. Furthermore, according to the present embodiment, the optical distance sensor 2 comprises the light projector or light emitter 20, which projects or emits light, and the light detection device 1. The TDC 4 of the light detection device 1 measures the counting period T2 using the time at which the light projector 20 projects light as the detection start time. With the optical distance sensor 2 of the present embodiment, it is possible to perform the light detection in the light detection device 1 accurately and to improve the accuracy of the distance measurement. Furthermore, the light detection method according to the present embodiment is a method in which the light detection device 1, including the plurality of SPADs 10a to 10c, detects incident light according to a predetermined detection start time. The present method comprises: receiving light with the plurality of SPADs 10a to 10c to generate each of the output signals Sa to Sc, which indicate the light reception result; and summing the plurality of output signals Sa to Sc to generate the combined signal S1. The present method further comprises: detecting a time at which the combined signal S1 is equal to or greater than a predetermined first threshold value to generate the detection signal S2, which indicates the detected time; and measuring the counting period T2 between the detection start time and the detected time based on the detection signal S2.Furthermore, the present method includes the acquisition of the timing control information D3, which indicates whether the combined signal S1 is equal to or greater than at least one second threshold value during the holding period T3, based on the acquired timing control. According to the present method, it is possible to obtain accurate light detection in the optical distance sensor 2. The description above describes an example where the number of SAPDs 10a to 10c contained in the light detection device 1 is three. The number of SAPDs 10a to 10c contained in the light detection device 1 can be four or more, or two. Furthermore, the above description described an example in which the time-extraction circuit 3 of the light detection device 1 contains two sets of the respective circuits 31, 32, and 33. The time-extraction circuit 3 of the light detection device 1 can contain three or more sets of the respective circuits 31 to 33, or one set of the respective circuits 31 to 33. Additionally, a plurality of sets of the detection circuit and the time-extraction circuit can be provided. (Second example) In the first embodiment, the time information D3 indicates a point in time within the holding period, which ends at the time indicated by the detection signal S2, but the time information acquired by a light detection device is not limited to this. A second embodiment describes a configuration example of the above light detection device with reference to Figures 7 and 8. Fig. 7 is a block diagram showing a configuration of a light detection device 1A according to the second embodiment. In a light detection device 1A according to the present embodiment, a detection circuit 14A, in addition to a configuration similar to that of the light detection device 1 of the first embodiment (Fig. 3), includes a delay circuit 17, as shown in Fig. 7. In the detection circuit 14A of the present embodiment, the delay circuit 17 delays the detection signal S2 output by the determining circuit 16 by a predetermined delay time (e.g., several hundred picoseconds up to several nanoseconds) in order to generate a delayed detection signal S2A. In the present embodiment, instead of the detection signal S2 from the determining circuit 16, the delayed detection signal S2A is output by the delay circuit 17 to the TDC 4 and the hold circuits 33a and 33b of the time-of-time extraction circuit 3. Figures 8A to 8E are timing diagrams illustrating the operation of the light detection device 1A according to the second embodiment. Figure 8A is an example of a timing diagram of the combined signal S1. Figure 8B illustrates the detection signal S2 based on the combined signal S1 from Figure 8A. Figure 8C illustrates the delayed detection signal S2A based on the detection signal S2 from Figure 8B. Figures 8D and 8E illustrate the first and second binarization signals S3a and S3b, respectively, based on the combined signal S1 from Figure 8A. In the present configuration example, the first binarization threshold V3a is set to a value greater than the detection threshold V1, as shown in Fig. 8A. In other words, the detection threshold V1 is set to a value less than any of the multiple binarization thresholds V3a and V3b. For example, setting the detection threshold V1 to a small value may be advantageous to enable detection even when the number of photons received by the light detection device 1A is low. The combined signal S1 in the example of Fig. 8A increases after time t0, reaches the detection threshold V1 at time t21, and then reaches the first binarization threshold V3a at time t22. Therefore, the detection signal S2 in Fig. 8B increases at time t21 before time t22, when the binary signal S3a in Fig. 8D increases. In contrast, the delayed detection signal S2A, which is delayed from time t21 by a delay time T4 of the delay circuit 17, increases at time t23 after time t22, as shown in Fig. 8C. According to the light detection device 1A of the present embodiment, the respective holding circuits 33a and 33b of the time extraction circuit 3 hold the time information D3a and D3b for the holding time T3, which ends at time t23 and is indicated by the delayed detection signal S2A. Consequently, it is possible to obtain the time information D3a, which indicates the time at which the combined signal S1 reaches the binarization threshold V3a or higher, after time t21, at which the combined signal S1 reaches the detection threshold V1 or higher. As described above, in the light detection device 1A according to the present embodiment, the detection circuit 14A delays the detection signal S2 by the predetermined delay period in order to output the delayed signal to the holding circuit 33 as the delayed detection signal S2A. One of the binarization thresholds V3 is greater than the detection threshold V1. Consequently, it is possible to obtain the time information D3, which indicates the time at which the signal increases after the time specified by the detection signal S2. The number of binarization thresholds V3 that are greater than the detection threshold V1 is not limited to one and there can be several. Furthermore, each binarization threshold value V3 can be greater than V1. (Third embodiment) In the first embodiment, the configuration example was described in which the light detection device 1 is provided with the holding circuits 33a and 33b for each binarization threshold V3 in order to hold the plurality of time information D3a and D3b. In the present embodiment, a light detection device that integrally holds the time information of the plurality of binarization thresholds V3a and V3b is described with reference to Figs. 9, 10 to 11. Fig. 9 is a block diagram showing a configuration of a light detection device 1B according to the third embodiment. The light detection device 1B according to the present embodiment is obtained by modifying the configuration of a time-of-moment extraction circuit 3A, as shown in Fig. 9, from a configuration similar to that of the light detection device 1 of the first embodiment (Fig. 3). The time-of-moment extraction circuit 3A of the present embodiment includes an integration circuit 37 configured to integrate the plurality of binary signals S3a and S3b, and a hold circuit 33 instead of the plurality of hold circuits 33a and 33b of a configuration similar to the first embodiment. The integration circuit 37 generates an integrated signal S30 that indicates the timing of each comparison result in an integrated manner, based on the plurality of binary signals S3a and S3b from the comparison circuits 32a and 32b. A configuration example for the integration circuit 37 is shown in Fig. 10. In the configuration example of Fig. 10, the integration circuit 37 contains a plurality of logic gates 71a and 71b and a plurality of delay elements 72a and 72b, which are provided for each of the comparator circuits 32, as well as an OR gate 70. It is desirable that each of the delay elements 72a and 72b has a common delay period or delay time. The first logic gate 71a calculates a logical product between the first binary signal S3a and an inverted signal of a delayed result of the same binary signal S3a by the delay element 72a to generate a first logic signal S31a. Similarly, the second logic gate 71b generates a second logic signal S31b based on the second binary signal S3b. The logic signals S31a and S31b are input to the OR gate 70. The OR gate 70 calculates a logical sum of the multiple logic signals S31a and S31b to generate the integrated signal S30, which indicates a calculation result. Figures 11A to 11F are timing diagrams illustrating the operation of the light detection device 1B according to the third embodiment. Figure 11A is an example of a timing diagram of the combined signal S1. Figure 11B shows the detection signal S2 based on the combined signal S1 from Figure 11A. Figures 11C and 11D illustrate the first and second logic signals S31a and S31b, respectively, based on the combined signal S1 from Figure 11A. Figure 11E illustrates the integrated signal S30 based on the logic signals S31a and S31b from Figures 11C and 11D. Figure 11F illustrates the timing information D3 based on the integrated signal S30 from Figure 11E. According to the integration circuit 37, the logic signals S31a and S31b form rectangular pulses, as shown in Fig. 11C and Fig. 11D, in response to times t11 and t12, when the combined signal S1 reaches or exceeds the binarization thresholds V3a and V3b, with the binary signals S3a and S3b increasing. The rectangular pulse of each of the logic signals S31a and S31b has a pulse width corresponding to the delay period of the delay elements 72a and 72b. The integration circuit 37 takes the logical sum of the logic signals S31a and S31b of Figs. 11C and 11D, based on the binary signals S3a and S3b, to generate the integrated signal S30. Each square wave pulse of the logic signals S31a and S31b from Figs. 11C and 11D is incorporated into the integrated signal S30, as shown in Fig. 11E. Since the integrated signal S30 is held as time information D3, "1" is recorded in accordance with times t11 and t12, at which the logic signals S31a and S31b increase, as shown in Fig. 11F. Consequently, the amount of information corresponding to the multiple time control information D3a and D3b in the first embodiment can be obtained using only a portion of the time control information D3. Furthermore, the circuit area corresponding to the hold circuit 33 can be reduced. As described above, the timing extraction circuit 3A contains the majority of the comparator circuits 32, the integrator circuit 37, and the hold circuit 33 in the light detection device 1B according to the present embodiment. The comparator circuit 32 is provided for every second threshold and generates the binary signal S3, which indicates whether the combined signal S1 is equal to or greater than the second threshold. The integrator circuit 37 generates the integrated signal S30, which indicates the timing of the respective comparator circuit results in an integrated manner based on the plurality of binary signals S3 from the respective comparator circuits 32. The hold circuit 33 holds the timing information D3 for the hold period T3 until the time indicated by the detection signal S2 is based on the integrated signal S30.Consequently, it is possible to obtain the time information D3 by integrating the multitude of comparison results. (Other examples) The example in which the detection threshold V1 is a constant value has been described in each of the embodiments above. In the light detection device of the present embodiment, the detection threshold V1 can be set based on the maximum value of the combined signal S1. A modification of circuit 15 for setting the detection threshold when the detection threshold V1 is set based on the maximum value of the combined signal S1 is described with reference to Fig. 12. As shown in Fig. 12, the circuit 15 for setting the detection threshold of the present modification includes, for example, a comparator 61 and two multiplexers 60 and 62. The detection threshold setting circuit 15 holds the maximum value of the combined input signal S1 as the detection threshold V1 and outputs the held detection threshold V1. The combined signal S1 from the signal combination circuit 13 is input to the comparator 61 and the multiplexer 62 in the detection threshold setting circuit 15. The multiplexer 60 outputs an initial value signal Si, indicating an initial value or the detection threshold V1, to the comparator 61 and the multiplexer 62. The comparator 61 compares the combined signal S1 with the signal output of the multiplexer 60. The comparator 61 outputs a signal indicating a comparison result to a control terminal of the multiplexer 62. The multiplexer 62 switches a signal to be output at an input terminal of the multiplexer 60 between the combined signal S1 and the initial value signal Si or the detection threshold V1 output by the multiplexer 60 according to the comparison result of the comparator 61. According to circuit 15 for setting the detection threshold of the configuration example above, the updated detection threshold V1 can be generated each time the combined signal S1 updates to its maximum value. The detection circuit 14 in the light detection device 1 of the present modification compares and determines, for example, the detection threshold V1 output by the detection threshold setting circuit 15 of Fig. 12 with the combined signal S1, which is delayed accordingly by means of the determination circuit 16 to generate the detection signal S2. The controller 25 can, for example, output a reset signal Sr immediately before the light projection or similar event to set the detection threshold V1 to its initial value. The configuration example where the photosensors of the light detection devices 1 to 1B are SPADs 10a to 10c has been described in each of the above embodiments. In the present embodiment, the photosensor of the light detection device is not necessarily a SPAD. Furthermore, the configuration examples of the light detection devices 1 to 1B, which extract the time information D3 by binarization using the binarization threshold V2, were described in each of the above embodiments. The light detection device of the present embodiment can obtain time information without using the binarization threshold V2 by an increase in the combined signal S1 during the hold time or similarly. Furthermore, the above description presented an application example of the optical distance sensor 2 for applications in industrial automation. The application of the optical distance sensor 2 and the light detection devices 1 to 1B according to the present disclosure is not limited to this and can, for example, be an application in a vehicle. The optical distance sensor 2 can, for example, be a LiDAR or a distance imaging sensor. (Attachment) As described above, various embodiments of the present disclosure have been described, but the present disclosure is not limited to the above-mentioned content, and various modifications can be made within a range where the technical idea is essentially the same. Several aspects of the present disclosure are further described below. A first aspect according to the present disclosure is a light detection device (1) for detecting incident light after a detection start time. The detection device 1 comprises a plurality of photosensors (10a to 10c), a signal combining circuit (13), a detection circuit (14), a timing circuit (4), and a time-of-time extraction circuit (3). The multiple photosensors receive light to generate output signals (Sa to Sc), each indicating the results of the light reception. The signal combining circuit sums a plurality of output signals from the respective photosensors to generate a combined signal (S1). The detection circuit detects a time at which the combined signal reaches a first threshold value or greater (V1) to generate a detection signal (S2) indicating the detected time.The timing circuit measures a counting period between the detection start time and the detected time based on the detection signal. The time extraction circuit extracts timing information (D3) from a predetermined period (T3), defined by the detected time as a reference, where the timing information indicates a time at which the combined signal rises. In the light detection device of the second aspect, the photosensor is a single-photon avalanche photodiode (SPAD) formed by an avalanche photodiode operating in Geiger mode. As a third aspect, the light detection device, in addition to the first or second aspect, also includes a computer (5). Based on the extracted timing information and the measured counting period, the computer calculates a period or duration from the detection start time until the time specified by the timing information. As a fourth aspect, the computer in the light detection device performs statistical processing based on the calculated period and the counting period, in accordance with the third aspect. As the fifth aspect of the light detection device, following any of the first four aspects, the timing extraction circuit includes at least one comparator circuit (32) and one hold circuit (33). The comparator circuit corresponds to a second threshold and generates a binary signal (S3) indicating whether the combined signal is equal to or greater than the second threshold. The hold circuit corresponds to each comparator circuit and holds timing information, based on the binary signal obtained from a corresponding comparator circuit, for a predetermined duration until the timing signal indicated by the detection signal is reached. As the sixth aspect of the light detection device, following any one of the first four aspects, the timing extraction circuit (3A) includes a plurality of comparison circuits (32), an integration circuit (37), and a holding circuit (33). The comparison circuit corresponds to each of the second thresholds and generates a binary signal (S3) indicating whether the combined signal is one or more of the corresponding second thresholds. Based on a plurality of binary signals from the respective comparison circuits, the integration circuit generates an integrated signal (S30) that displays the timings of the respective comparison results in an integrated manner. The holding circuit retains the timing information (D3) for a predetermined period until the timing indicated by the detection signal based on the integrated signal. As the seventh aspect in the light detection device of the fifth or sixth aspect, the detection circuit includes a delay circuit (17) that delays the detection signal by a predetermined delay period in order to output the delayed detection signal to the hold circuit. At least one of the second thresholds is greater than the first threshold. As the eighth aspect of the light detection device, after one of the first to seventh aspects, the first threshold is set based on a maximum value of the combined signal. A ninth aspect comprises a light projector (20) that projects light, and the light detection device according to one of the first through eighth aspects. The timing circuit in the light detection device measures the counting period using a time at which the light projector projects light as the detection start time. A tenth aspect is a light detection method in which a light detection device (1) comprising a plurality of photosensors (10a to 10c) detects incident light according to a detection start time. The present method comprises: receiving light at the plurality of photosensors to generate each of the output signals (Sa to Sc) indicating light reception results; and summing the plurality of output signals (Sa to Sc) from the respective photosensors to generate a combined signal (S1). The present method comprises: detecting a time at which the combined signal reaches a first threshold (V1) or greater in order to generate a detection signal (S2) indicating the detected time; and measuring a counting period (T2) between the detection start time and the detected time based on the detection signal.The present method comprises the acquisition of timing control information indicating a timing control in which the combined signal increases as a reference over a predetermined period defined by the acquired timing control. List of reference signs 1, 1A, 1B Light detection device 10a to 10c SPAD 13 Signal logic circuit 14, 14A Detection circuit 2 Optical distance sensor 20 Light projector 3, 3A Time extraction circuit 32 Comparator circuit 33 Hold circuit 37 Integration circuit 4 TDC 5 Computer

Claims

A light detection device for detecting incident light according to a detection start time, the light detection device comprising: a plurality of photosensors configured to receive light to generate output signals, each indicating the results of the light detection; a signal combination circuit configured to sum the plurality of output signals from the respective photosensors to generate a combined signal; a detection circuit configured to detect a time at which the combined signal reaches a first threshold or higher to generate a detection signal; a timing circuit configured to measure a counting period between the detection start time and the detected time based on the detection signal;and a time extraction circuit configured to extract timing information from a predetermined period defined by the detected time as a reference, wherein one end of the predetermined period corresponds to the time detected by the detection circuit, and wherein the timing information indicates a time at which the combined signal increases. Light detection device according to claim 1, wherein the photosensors are single-photon avalanche photodiodes, each configured by an avalanche photodiode operated in Geiger mode. Light detection device according to claim 1 or 2, further comprising a computer configured to calculate a period from the detection start time to the time specified by the time information, based on the extracted timing information and the measured counting period. Light detection device according to claim 3, wherein the computer is configured to perform statistical processing based on the calculated period and the counting period. The light detection device according to any one of claims 1 to 4, wherein the circuit for temporal extraction comprises: at least one comparator circuit corresponding to each second threshold and configured to generate a binary signal indicating whether the combined signal equals or exceeds the second threshold; and a holding circuit corresponding to each comparator circuit and configured to hold timing information based on the binary signal obtained from a corresponding comparator circuit for a predetermined duration until the time indicated by the detection signal. The light detection device according to any one of claims 1 to 4 comprises the timing extraction circuit: a plurality of comparison circuits, each corresponding to each of the second thresholds and configured to generate a binary signal indicating whether the combined signal equals or exceeds the second threshold; the integration circuit configured to generate an integrated signal based on a plurality of binary signals from respective comparison circuits, indicating the timings of the respective comparison results in an integrated manner; and the holding circuit configured to hold the timing information for a predetermined duration until the time indicated by the detection signal based on the integrated signal. Light detection device according to claim 5 or 6, wherein the detection circuit comprises a delay circuit configured to delay the detection signal by a predetermined delay period in order to output the delayed detection signal to the holding circuit, wherein at least a second threshold is greater than the first threshold. Light detection device according to one of claims 1 to 7, wherein the first threshold is set on the basis of a maximum value of the combined signal. An optical distance sensor comprising: a light projector configured to emit light; and the light detection device configured according to any one of claims 1 to 8, wherein the timing circuit in the light detection device is configured to measure the counting period using a time point at which the light projector emits light as the detection start time. Light detection method with a light detection device comprising a plurality of photosensors for detecting incident light according to a detection start time, the method comprising: receiving light at the plurality of photosensors to each generate an output signal indicating the result of the light reception; summing the plurality of output signals of the respective photosensors to generate a combined signal; detecting a time at which the combined signal reaches a first threshold or more in order to generate a detection signal indicating the detected time; measuring a counting period between the detection start time and the detection time based on the detection signal;and acquisition of timing information that specifies a time at which the combined signal increases as a reference in a predetermined period defined by the detected time, with one end of the predetermined period corresponding to the detected time.

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