JP6020547B2 - Image acquisition apparatus and method - Google Patents

Description

  The present invention relates to an image acquisition apparatus and method, and more particularly to an image acquisition apparatus and method for acquiring an image of an object by repeatedly irradiating the object with pulsed light and receiving light from the object.

  2. Description of the Related Art Conventionally, an apparatus for acquiring an image of an object by repeatedly irradiating the object with pulsed light and receiving light from the object has been proposed. For example, Patent Document 1 discloses an integrated circuit that includes an array of SPADs (Single Photon Avalanche Diodes) and acquires an image of an object illuminated with pulsed light. The first SPAD group connected to the 2D readout circuit for obtaining the intensity of the pulsed light reflected by the object, and the pulsed light irradiated on the object to measure the distance to the object is reflected on the object. An integrated circuit is disclosed that includes a second SPAD group connected to a 3D readout circuit that determines a time of flight (TOF).

US Pat. No. 7,262,402

  However, the above-described technique has a problem that an image of a nearby object has a high intensity, and an image of a distant object has a low intensity. That is, the power density of the irradiated pulsed light is attenuated in inverse proportion to the square of the distance. In the technique of the above-mentioned patent document 1, since the image of the object is acquired by the pulsed light reflected by the object, there is a problem that the intensity of the image of the object depends on the distance from the object.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an image acquisition apparatus and method capable of acquiring an image of an object independent of the distance to the object. .

  The present invention includes a light projecting unit that repeatedly irradiates a target object with pulsed light during a light projection period, and pauses the irradiation of the pulsed light onto the target object during a light projection pause period, and a light receiving unit that receives light from the target object. The light receiving means includes a first light receiving element group connected to a 2D readout circuit for obtaining the intensity of light received from the object during the light projection pause period, and light projecting means from the object during the light projection period. 3D for calculating the distance to the object based on the time difference between the time when the light projecting means irradiated the pulsed light and the time when the reflected light was received. And a second light receiving element group connected to the readout circuit.

  According to this configuration, the light projecting unit repeatedly irradiates the object with the pulsed light during the light projecting period, pauses the irradiation of the pulsed light onto the object during the light projection period, and the light receiving unit performs the light projecting pause. Time difference between the first light receiving element group connected to the 2D readout circuit for obtaining the intensity of light received from the object during the period, and the time when the light projecting means radiates the pulsed light and the time when the reflected pulsed light is received And a second light receiving element group connected to a 3D readout circuit for calculating a distance to the object based on the first light receiving element group connected to the 2D read circuit In this case, the intensity of the received light does not include the reflected light component of the pulsed light emitted by the light projecting means, and an image of the object that does not depend on the distance to the object can be acquired.

  In this case, it is preferable that the first light receiving element group is connected to a 2D readout circuit for obtaining the intensity of light received during a period including both the light projection pause period and the light projection period.

  According to this configuration, the first light receiving element group connected to the 2D readout circuit obtains the intensity of the light received in the period including the light projecting period, and thus the sensitivity is high even in a situation where the brightness of the object is insufficient. It is possible to acquire an image of the object without being insufficient.

  In addition, it is preferable that the first light receiving element group is connected to a 2D readout circuit for obtaining the intensity of light received from the object during the entire light emission suspension period.

  According to this configuration, the first light receiving element group is connected to the 2D readout circuit for obtaining the intensity of the light received from the object during the entire projection suspension period. The readout circuit and the 3D readout circuit for obtaining the distance make it easy to classify the measurement data to be used, and it becomes easy to share measurement data such as a histogram.

  The present invention also provides a light projecting unit that repeatedly irradiates a target with pulsed light, a light receiving unit that repeatedly receives reflected pulsed light of the pulsed light emitted from the target by the light projecting unit, and a light projecting unit that emits pulsed light. The distance calculating means calculates the distance to the object based on the time difference between the irradiation time and the time when the light receiving means receives the reflected pulse light, and the distance calculating means calculates the intensity of the pulsed light received by the light receiving means. An image acquisition device comprising: conversion means for converting according to the distance to the object.

  According to this configuration, the distance calculating means calculates the distance to the object based on the time difference between the time when the light projecting means radiates the pulsed light and the time when the light receiving means receives the reflected pulsed light, and the converting means Since the intensity of the pulsed light received by the light receiving means is converted according to the distance to the object calculated by the distance calculating means, the intensity of the pulsed light received by the light receiving means is corrected according to the distance to the object. Thus, an image of the object that does not depend on the distance to the object can be acquired.

  In this case, it is preferable that the conversion unit performs conversion so that the correction amount that increases the intensity of the pulsed light received by the light receiving unit increases as the distance to the object calculated by the distance calculating unit increases.

  According to this configuration, the conversion unit converts the correction amount to increase the intensity of the pulsed light received by the light receiving unit as the distance to the target calculated by the distance calculation unit increases. Even if the distance is long, it can be received as pulsed light having a high intensity by correction by the conversion means, and it becomes easier to obtain an image of the object independent of the distance to the object.

  On the other hand, the present invention relates to a light projecting unit that repeatedly irradiates a target with pulsed light, a light receiving unit that receives light from the target, and a pulsed light projecting time that is a time when the light projecting unit radiates pulsed light. , A pulsed light projecting time that is the time when the light projecting unit irradiates the pulsed light, and a pulsed light receiving time that is the time when the light receiving unit receives the reflected pulsed light of the pulsed light irradiated by the projecting unit from the object Histogram creation means for repeatedly measuring the time difference and creating a histogram of the time difference, distance calculation means for calculating the distance to the object based on the maximum value of the histogram created by the histogram creation means, and histogram creation means An image acquisition apparatus comprising: density calculation means for calculating density based on a value obtained by subtracting the maximum value of the histogram from the total sum of the histogram.

  According to this configuration, the histogram creating means repeats the time difference between the pulse light projecting time that is the time when the light projecting means radiates the pulse light and the pulse light receiving time that is the time when the light receiving means receives the reflected pulse light. The distance calculation means calculates the distance to the object based on the time when the histogram created by the histogram creation means shows the maximum value, and the density calculation means The density is calculated based on the value obtained by subtracting the maximum value of the histogram from the total sum of the histograms created. The sum of the histograms representing the intensity of the reflected pulse light received by the light receiving means is calculated based on the reflected light component from the light projecting means and the background. It can be separated into light components.

  Since the component of the reflected light forms a maximum value (peak) on the histogram, the background light component can be specified by subtracting the value of the maximum value (or the vicinity of the maximum value) from the sum total of the histogram. The reflected light component attenuates in inverse proportion to the square of the distance, but the background light component does not depend on the distance. The density calculating means outputs the background light component as a density value, thereby obtaining an image of the object independent of the distance. Further, according to this configuration, since the distance measurement and the concentration measurement are simultaneously performed, the measurement time can be shortened.

  The density calculation means preferably calculates the density based on a value obtained by interpolating a bin indicating the maximum value for a value obtained by subtracting the maximum value of the histogram from the total sum of the histograms created by the histogram creation means.

  According to this configuration, the density calculating unit calculates the density based on the value obtained by interpolating the bin indicating the maximum value for the value obtained by subtracting the maximum value of the histogram from the total sum of the histograms generated by the histogram generating unit. Even when the background light component is larger than the reflected light component, or even when the time width of the pulsed light is large and the number of bins showing the maximum value is large, the reflected light component is accurately removed, The background light component can be obtained.

  On the other hand, the present invention repeatedly irradiates the object with pulsed light during the light projection period, stops the irradiation of the pulsed light onto the object during the light emission pause period, and receives the light from the object, A step of obtaining the intensity of light received from the object during the light projection pause period, and by repeatedly receiving the reflected pulse light of the pulse light emitted from the object during the light projection period, and the time and reflection of the pulse light irradiation. And a step of calculating a distance to the object based on a time difference from the time when the pulsed light is received.

  In this case, it is preferable to obtain the intensity of the light received in the period including both the light projection suspension period and the light projection period.

  In addition, it is preferable to obtain the intensity of light received from the object during the entire period of the light projection suspension period.

  The present invention also includes a step of repeatedly irradiating an object with pulsed light, a step of repeatedly receiving reflected pulsed light of the pulsed light emitted from the object, a time of irradiating the pulsed light, and a time of receiving the reflected pulsed light. An image acquisition method comprising a step of calculating a distance to an object based on a time difference between and a step of converting the intensity of received pulsed light according to the calculated distance to the object.

  In this case, it is preferable to perform conversion so that the greater the calculated distance to the object, the greater the correction amount that increases the intensity of the received pulsed light.

  Furthermore, the present invention includes a step of repeatedly irradiating an object with pulsed light, a step of receiving light from the object, a pulsed light projecting time that is a time when the pulsed light is irradiated, and a pulse irradiated from the object. Repeatedly measures the time difference from the pulsed light reception time, which is the time when the reflected reflected light of the light is received, and creates a histogram of the time difference, and calculates the distance to the object based on the maximum value of the created histogram An image acquisition method comprising: a step; and a step of calculating a density based on a value obtained by subtracting a maximum value of the histogram from the total sum of the created histograms.

  In this case, for the value obtained by subtracting the maximum value of the histogram from the total sum of the created histograms, it is preferable to calculate the density based on the value obtained by interpolating the bin indicating the maximum value.

  According to the image acquisition apparatus and method of the present invention, it is possible to acquire an image of an object that does not depend on the distance to the object.

It is a block diagram which shows the structure of the 3D camera which concerns on 1st Embodiment. It is a figure which shows the structure of the two-dimensional light receiving element array which concerns on 1st Embodiment. It is a figure which shows the structure of the circuit of each pixel of the two-dimensional light receiving element array in 1st Embodiment. It is a figure which shows the principle of TOF detection by the histogram process which concerns on 1st Embodiment. It is a figure which shows the example of the histogram in 1st Embodiment. It is a figure which shows the example of the histogram in 1st Embodiment. (A) and (b) are timing charts showing pulse emission timing and intensity acquisition timing. It is a block diagram which shows the structure of the 3D camera which concerns on 3rd Embodiment. It is a figure which shows the structure of the two-dimensional light receiving element array which concerns on 3rd Embodiment. It is a figure which shows the structure of the circuit of each pixel of the two-dimensional light receiving element array in 3rd Embodiment. It is a figure which shows the example of the histogram in 3rd Embodiment. It is a figure which shows the example of the histogram in 3rd Embodiment.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. First, the types of the image acquisition device and the optical distance measuring device according to the present invention will be described. Laser range finders (laser ranging devices) that measure the distance to an object with a laser beam include a scan type and a non-scan type.

  The scan type repeatedly irradiates an object while scanning a laser beam, and the reflected light is received by one light receiving element. Since the scan type emits collimated light, the power density on the object is relatively large, and the optical SN ratio can be increased. However, in order to obtain the spatial distribution of distance with high resolution, the scan type requires high-density scanning, and the scanning time increases. Therefore, in an automobile application where immediacy is required, the scan type generally scans with a high density in the horizontal direction and only a few lines in the vertical direction in order to shorten the scan time.

  On the other hand, the non-scan type irradiates light having a spread covering the measurement object, forms an image of the reflected light on a two-dimensional array of light receiving elements, and receives the light. The non-scan type has an advantage that a high-resolution spatial distribution can be obtained at a high speed because the distances of multiple points can be measured simultaneously with a single light irradiation. On the other hand, in the non-scan type, it is important to remove noise components such as irradiation light.

  FIG. 1 is a block diagram illustrating a configuration of a 3D camera 10a that is a non-scan type laser range finder according to the embodiment. The non-scanning laser range finder described above is also called a 3D camera, and as shown in FIG. 1, a light projecting unit 20a that irradiates the object O with irradiation light L1 and a light receiving unit that receives the reflected light L2 from the object O. 30a.

  The light projecting unit 20a includes a clock generation circuit 21a, a circuit board 22, a drive circuit 23, a laser diode array including a plurality of laser diodes 24, a Fresnel lens 25, and a diffusion plate 26. The clock generation circuit 21a generates clock signals CLK1, CLK2, and CLK3 for operating the light projecting unit 20a and the light receiving unit 30a. Each laser diode 24 is arranged on a circuit board 22 and includes a drive circuit 23 and a Fresnel lens 25. The laser light generated by each laser diode 24 is diffused by the diffusion plate 26 and applied to the object O.

  The light receiving unit 30a includes an interference filter 31, a lens 32, and a two-dimensional light receiving element array 33a. As shown in FIG. 2, the two-dimensional light receiving element array 33a is configured such that each pixel 330a including an avalanche photodiode 332 having a light receiving portion 331 and its peripheral circuit 333a is arranged in a grid pattern. FIG. 3 shows an example of the circuit configuration of each pixel 330a. As shown in FIG. 3, the peripheral circuit 333a includes a resistor 334, a buffer 335, a TDC (time digital converter) 336, a histogram circuit 337, and a signal processing circuit 338a.

  The clock generation circuit 21a of the light projecting unit 20a generates a clock signal CLK1 having a period T, a clock signal CLK2 having a period MT (M is an arbitrary natural number equal to or greater than 1), and a clock signal CLK3 indicating timing for acquiring a density value. To do. The clock signal CLK1 is a clock signal that defines the pulse emission timing of the laser diode 24 of the light projecting unit 20a. The clock signal CLK2 is the distance of the signal processing circuit 338a of the light receiving unit 30a after the laser diode 24 has emitted M times. It is a clock signal that defines the timing for calculation. The clock signal CLK1 is input to the laser diode array of the light projecting unit 20a, and the clock signal CLK1, the clock signal CLK2, and the clock signal CLK3 are input to the two-dimensional light receiving element array 33a of the light receiving unit 30a.

  The clock signal CLK1 input to the laser diode array is distributed to each drive circuit 23 and generates a current signal for driving each laser diode 24 in synchronization. The laser diode 24 is driven by the current signal generated by the drive circuit 23, and emits pulses in synchronization with the clock signal CLK1. The advantage of using the laser diode 24 as a light-emitting element is due to high-speed response and single wavelength, but the laser diode 24 can be replaced with other light-emitting elements such as LEDs. The output light from the laser diode 24 is converted into parallel light by the Fresnel lens 25, is diffused by the diffusion plate 26, and is irradiated toward the object O. The diffusion plate 26 has a light distribution characteristic unique to the light emitting element, and the beam spreading angle is determined by the light distribution characteristic. The output light from each laser diode 24 is diffused by the diffusion plate 26 and added together to form one conical beam.

  As described above, in the present embodiment, the entire object O to be measured is illuminated at once with the irradiation light L1, which is a conical beam, and the reflected light L2 is simultaneously received by the two-dimensional light receiving element array 33a of the light receiving unit 30a. Therefore, it is possible to acquire distance data with high spatial resolution. On the other hand, in this embodiment, since the irradiation range of the irradiation light L1 is wide, the power density of the irradiation light L1 is reduced, so the optical SN ratio is reduced.

  Another advantage of using the diffuser 26 is that the size of the apparent light source can be increased. “Aperent light source” is a term defined by the laser safety standard of JIS (Japanese Industrial Standards). It is a real or virtual object that connects the smallest retinal images. If the output is constant, the size of the apparent light source is large. Is safer for the eyes. In the present embodiment, by increasing the apparent light source size with the diffusion plate 26, the safety of the eyes can be made more reliable.

  The reflected light L2 reflected on the object O passes through the interference filter 31 of the light receiving unit 30a and is imaged on the two-dimensional light receiving element array 33a by the lens 32. The imaged light is received by each avalanche photodiode 332 on the two-dimensional light receiving element array 33a. The interference filter 31 is a filter that transmits only light in a specific narrow wavelength range. By making the center wavelength of the laser light equal to the center wavelength of the interference filter 31, most of disturbance light such as sunlight is removed. can do. The bandwidth of laser light is generally several nm, but the center wavelength varies with temperature. In order to guarantee operation in a wide temperature range, the bandwidth of the interference filter 31 is set to about several tens to one hundred nm. For this reason, all the disturbance light cannot be removed by the interference filter 31, and noise removal by signal processing is required.

  The avalanche photodiode is a kind of photodiode and has a function related to application of a high electric field. Avalanche photodiodes include a linear mode in which a reverse bias voltage is operated at a breakdown voltage or lower and a Geiger mode in which the reverse bias voltage is operated at a breakdown voltage or higher. An avalanche photodiode generates an electron-hole pair when a photon is incident, and each electron and hole is accelerated by a high electric field to generate new electron-hole pairs one after another like an avalanche. ).

  In the linear mode, the proportion of electron-hole pairs that disappear (exit from a high electric field region) is larger than the proportion of generated electron-hole pairs, and the avalanche phenomenon stops naturally. Since the output current is substantially proportional to the amount of incident light, it can be used to measure the amount of incident light. While the photon count in the Geiger mode to be described later is digital, the light amount measurement in the linear mode is analog and is sometimes referred to as analog measurement.

  In the Geiger mode, an avalanche phenomenon can be caused even by the incidence of a single photon, and the avalanche phenomenon can be stopped by lowering the applied voltage to the breakdown voltage. Stopping the avalanche phenomenon by lowering the applied voltage is called quenching. The simplest quenching circuit can be realized by connecting a resistor 334 in series with an avalanche photodiode 332 as shown in FIG. 3, and the bias voltage decreases due to the voltage increase between the terminals of the resistor 334 due to the avalanche current. Then the avalanche current stops. With this quenching circuit, the peripheral circuit 333a can take out and count the incidence of photons as a voltage pulse. For this reason, Geiger mode is also called photon count mode.

The sensitivity of a light receiving element for analog measurement is generally expressed by quantum efficiency and dark current. Quantum efficiency is the ratio of the number of electrons generated per incident photon. The dark current is a current that flows due to electrons excited by heat in a state where no light is incident, and is the lowest level of thermal noise. The minimum input required to obtain an output of noise level (dark current) can be calculated from the quantum efficiency. If the dark current is 100 nA and the quantum efficiency is 50%, the charge of one electron is 1.6 × 10 ⁻¹⁹ C. Therefore, the minimum input required to obtain a noise level output is (100 [nA ] /1.6×10 ⁻¹⁹ [C]) / 0.5 = 1.25 × 10 ¹² [photons / second].

  On the other hand, the sensitivity of the photon counter is expressed by detection efficiency and dark count rate. In an avalanche photodiode in the Geiger mode, electron-hole pairs continue to be generated one after another unless the avalanche current is stopped by a quenching circuit, so that the quantum efficiency becomes infinite. The generation of an avalanche current with respect to the incidence of photons is a stochastic phenomenon, and this probability is called detection efficiency. Similar to the dark current in the linear mode, an avalanche current may be generated due to the thermal motion of electrons in the Geiger mode, and the amount of thermal noise is expressed by the number of avalanche generation (dark count rate) per hour.

  In this embodiment, an avalanche photodiode 332 that operates in Geiger mode is used. C. Niclass, A. Rochas, PABesse, and E. Charbon, “Design and Characterization of a CMOS 3-D Image Sensor Based on Single Photon Avalanche Diodes”, IEEE Journal of Solid-State Circuits, vol.40, n. 9, Sep. 2005. In recent years, a technology to realize Geiger mode avalanche photodiodes using a standard CMOS process has been developed, which makes it possible to form a two-dimensional array of avalanche photodiodes at low cost. It was.

  In this embodiment, the TDC 336, the histogram circuit 337, and the signal processing circuit 338a included in each pixel 330a can all be mounted on the same LSI by a CMOS process. As shown in FIG. 3, among the clock signals CLK1, CLK2, and CLK3 input to the two-dimensional light receiving element array mounted on the LSI, the clock signal CLK1 is distributed to the TDC 336 of each pixel 330a, and the clock signal CLK2 , CLK3 are distributed to the signal processing circuit 338a of each pixel 330a.

  Hereinafter, the operation of each pixel when light, that is, photons are incident on the avalanche photodiode will be described with reference to FIG. When photons enter the avalanche photodiode 332, an avalanche current flows, and the voltage at the terminal A increases. Due to the voltage rise, a pulse PLS is generated via the buffer 335 and input to the TDC 336. The TDC 336 converts the time from the input time point of the clock signal CLK1 to the input time point of the pulse PLS into a digital value and outputs the digital value to the histogram circuit 337. When the pulse PLS is not input, the TDC 336 outputs a predetermined maximum value.

  The histogram circuit 337 increments the memory value of the address corresponding to the output value of the TDC 336 by 1 at the timing of the clock signal CLK1. The histogram circuit 337 does not increment the memory value when the value of the TDC 336 is a value indicating that the pulse PLS has not been input. After a series of operations from the generation of the clock CLK1 to the increment of the histogram memory is repeated M times, the signal processing circuit 338a reads the value of the histogram memory at the timing of the clock CLK2, and corresponds to the address where the maximum value is stored. The distance is calculated and output from the time to light and the speed of light. The distance value calculated in each pixel is sequentially read out of the two-dimensional light receiving element array 33a through the reading circuit.

  As described above, since all the disturbance light cannot be removed by the interference filter 31, the light received by the avalanche photodiode 332 includes the irradiation light L1 from the laser diode 24 of the light projecting unit 20a and the disturbance light. . Hereinafter, it will be described with reference to FIG. 4 that TOF can be detected correctly even when disturbance light is included in the received light.

  The pulse PLS output from the avalanche photodiode 332 includes one that reacts to photons of the reflected light L2 and one that reacts to photons of disturbance light. If it is assumed that the object O and the 3D camera 10 do not move relative to each other during the measurement, the light reception timing of the reflected light L2 by the irradiation light L1 is constant. Therefore, the detection timing of the pulse PLS is repeatedly measured many times to obtain the maximum frequency. By selecting the measured value, the TOF of the reflected light L2 can be obtained. When the histogram circuit 337 measures the detection timing of the repetitive pulse PLS with respect to many (M) times of pulse emission and creates a histogram, the measurement corresponding to the reflected light L2 forms a peak (maximum value) in the histogram. .

  On the other hand, the light reception timing of disturbance light such as sunlight is uncorrelated with the timing of the clock signal CLK1, which is the pulse emission timing of the irradiation light L1, and is therefore uniformly distributed on the histogram. By detecting the peak position of the histogram by the signal processing circuit 338a, it is possible to remove the influence of disturbance light and detect the correct TOF, that is, the distance. The histogram of FIG. 4 is an actual measurement example when the optical SN ratio is 0.01, and it can be seen that a peak can be correctly extracted even when the disturbance light component is large. In the two-dimensional light receiving element array 33a, since the distance to the object O corresponding to each pixel 330a is different, the light receiving timing of the reflected light L2 corresponding to the irradiation light L1 is different for each pixel 330a. In this embodiment, since a histogram is created independently for each pixel 330a and a peak is extracted, a distribution of different distances for each pixel 330a can be obtained.

  In the present embodiment, the simplest example of detecting the peak position of the histogram has been shown, but various modifications can be made to the method of detecting the peak position. For example, the peak position can be detected by obtaining the maximum value after smoothing the bin values of the histogram. When each bin time interval is short, the relative fluctuation amount of each bin value increases, but the peak position can be detected robustly against the fluctuation by smoothing. After obtaining the peak position, the TOF can be obtained with higher accuracy by obtaining the position of the center of gravity in the vicinity of the peak position. This TOF detection is based on the fact that the frequency of light reception increases in the vicinity of the TOF on the histogram, and the TOF can also be detected by other histogram processing methods that take into account the peak shape.

  Next, output of density values in the present embodiment will be described using the histograms of FIGS. In the histograms of FIGS. 5 and 6, the dashed curve graph indicates the distribution of the true light reception time, and the solid bar graph indicates the detected histogram. FIG. 5 is an example of a histogram generated during the pulse emission period of the light projecting unit 20a, and FIG. 6 is an example of a histogram generated during the pulse emission pause period of the light projecting unit 20a.

  The density value is obtained by the signal processing circuit 338a calculating the sum of the histograms, that is, the received light intensity. In the histogram generated during the pulse emission period, a peak corresponding to the component of the reflected light L2 of the pulse light is formed. In the case of the spread irradiation light L1, since the power density of the reflected light L2 is inversely proportional to the square of the distance, the peak of the near object O is large and the peak of the distant object O is small. Therefore, the sum of the histograms, that is, the received light intensity depends on the distance, and there is a problem that the received light intensity for the nearby object O increases and the received light intensity for the distant object O decreases. On the other hand, since the histogram generated during the pulse light emission suspension period does not include the pulse light component, the received light intensity does not depend on the distance.

  In the present embodiment, as shown in the timing chart of FIG. 7, since the density value is acquired during the pulse emission suspension period, a histogram that does not form a peak as shown in FIG. 6 is obtained, and the received light intensity does not depend on the distance. .

  In the present embodiment, the idle period is set as a state in which the clock signal CLK3 is at a high level. In the example of FIG. 7A, a predetermined pause period is provided after M pulses of light that are the timing of the clock signal CLK2. In the example of FIG. 7B, the pulse emission cycle is doubled to 2T, and the latter half time T of each cycle is set as a pause period. In the example of FIG. 7A, the histogram for detecting TOF and the histogram for detecting density can be shared, and in the example of FIG. 7B, the histogram for detecting TOF and the histogram for detecting density are used. It is necessary to prepare it separately. 7A and 7B, a histogram is created in a state where the clock signal CLK3 is at a high level, that is, a pulse emission pause period, and the sum is calculated, so that a density value independent of distance is acquired. Can do.

  When the received light intensity of the reflected light L2 is below a predetermined threshold and is low, the light emission pause period and the light emission period are partially overlapped with the pulse light emission period of the clock signal CLK1 and the intensity detection period of the clock signal CLK3. You may make it obtain | require the intensity | strength of the light received in the period including both a period. By doing in this way, even in a situation where the brightness of the object O is insufficient, the sensitivity is not insufficient, and an image of the object O can be acquired.

  Further, the avalanche photodiode 332 of each pixel 330b of the two-dimensional light receiving element array 33a may be configured such that the avalanche photodiode 332 of all the pixels 330b performs both distance measurement and image acquisition as described above. As in the first patent document, each avalanche photodiode 332 of each pixel 330b is connected to a 3D readout circuit that reads a distance value, and is connected to a 2D readout circuit that reads a density value. And may be distinguished.

  Hereinafter, a second embodiment of the present invention will be described. The apparatus configuration of the present embodiment has the same apparatus configuration as that of the first embodiment, but the signal processing circuit 338a corrects the received light intensity of the avalanche photodiode 332 based on the calculated distance value, and calculates the calculated distance value. Is corrected so that the correction amount for increasing the received light intensity increases as the distance increases, and the correction amount for increasing the received light intensity decreases as the calculated distance value decreases. In a conical beam, the power density decreases in proportion to the square of the distance. The signal processing circuit 338a performs correction by multiplying the received light intensity by the square of the distance.

  According to the present embodiment, the signal processing circuit 338a determines the distance to the object O based on the time difference between the time when the light projecting unit 20 emits the pulsed light and the time when the avalanche photodiode 332 receives the reflected pulsed light. In order to calculate and convert the intensity of the pulsed light received by the avalanche photodiode 332 according to the calculated distance to the object O, the intensity of the pulsed light received by the avalanche photodiode 332 is the intensity of the irradiation light on the object O. The correction is made according to the power density, and an image of the object that does not depend on the distance to the object O can be acquired.

  Hereinafter, a third embodiment of the present invention will be described. FIG. 8 is a block diagram showing the configuration of the 3D camera 10b according to the third embodiment, FIG. 9 is a diagram showing the configuration of the two-dimensional light receiving element array 33b, and FIG. 10 shows each pixel of the two-dimensional light receiving element array 33b. It is a figure which shows the structure of the circuit of 330b.

  As shown in FIG. 8, the 3D camera 10b according to the present embodiment has the same configuration as that of the first embodiment. However, the clock generation circuit 21b of the light projecting unit 20b includes a clock signal CLK1 having a cycle T and a cycle MT. Only the clock signal CLK2 is generated (M is an arbitrary natural number equal to or greater than 1). The clock signal CLK1 is a clock signal that defines the timing of pulse emission of the laser diode 24 of the light projecting unit 20b, and the clock signal CLK2 is the distance of the signal processing circuit 338b of the light receiving unit 30b after the laser diode 24 has emitted M times. It is a clock signal that defines the timing for calculation. The clock signal CLK1 is input to the laser diode array of the light projecting unit 20b, and the clock signal CLK1 and the clock signal CLK2 are input to the two-dimensional light receiving element array 33b of the light receiving unit 30b.

  As shown in FIG. 7, as in the first embodiment, the two-dimensional light receiving element array 33b of the present embodiment includes each pixel 330b including an avalanche photodiode 332 having a light receiving portion 331 and its peripheral circuit 333b. They are arranged in a grid pattern. As shown in FIG. 8, among the clock signals CLK1 and CLK2 input to the two-dimensional light receiving element array mounted on the LSI, the clock signal CLK1 is distributed to the TDC 336 of each pixel 330b, and the clock signal CLK2 is The signal is distributed to the signal processing circuit 338b of the pixel 330b.

  Next, output of density values in the present embodiment will be described using the histograms of FIGS. In the histograms of FIGS. 11 and 12, the dashed curve graph indicates the true light reception time distribution, and the solid bar graph indicates the detected histogram. In the present embodiment, the signal processing circuit 338b separates the received light intensity obtained as the sum of the histograms into a component of the pulsed light P reflected from the object O and a component of the background light B, and the component of the background light B is a density value. Output as. 11 and 12 show two methods for separating the received light intensity into two components, respectively. In either method, the signal processing circuit 338b performs TOF detection before calculating the concentration value, and the bin number i having the maximum value is obtained when the TOF is detected.

  In the method of FIG. 11, the signal processing circuit 338 b obtains the sum of the bin values indicated by hatching in the drawing excluding the vicinity of the i-th bin, for example, ± 1 to 5 bins, Extract ingredients. In the method of FIG. 12, the signal processing circuit 338b excludes the bin values near the i-th bin, for example, ± 1 to 5th bins, and further interpolates the excluded bin values to obtain the bin values indicated by diagonal lines in the figure. The component of the background light B is extracted by obtaining the sum. In the method of FIG. 12, even if the background light component is larger than the reflected light component or the time width of the pulsed light P is larger by interpolating the excluded bin values, The component of the background light B can be obtained accurately.

  In the present embodiment, since only the background light B component not including the pulse light P component is obtained, a density value independent of the distance is obtained. Furthermore, since the TOF detection and the density value detection are performed with the same histogram, the measurement time can be shortened. Conversely, in applications where it is desired to obtain a density value corresponding to the distance, it is also possible to remove the background light component and obtain only the reflected light component. In the present embodiment, an example in which there is one peak in the histogram has been shown, but even when there are two or more peaks, the bins near the peaks are excluded and excluded when necessary, as described above. The density value is obtained by interpolating the bin values and calculating the sum of the histograms. As an example in which two or more peaks exist, when an object moves during measurement, a case where reflected light of a plurality of objects is received by the same pixel due to multiple reflection can be considered.

  It should be noted that the present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

10a, 10b ... 3D camera, 20a, 20b ... projector, 21a, 21b ... clock generation circuit, 22 ... circuit board, 23 ... drive circuit, 24 ... laser diode, 25 ... Fresnel lens, 26 ... diffuser plate, 30a, 30b ... light receiving unit, 31 ... interference filter, 32 ... lens, 33a, 33b ... two-dimensional light receiving element array, 330a, 330b ... pixel, 331 ... light receiving unit, 332 ... avalanche photodiode, 333a, 333b ... peripheral circuit, 334 ... Resistance, 335... Buffer, 336... TDC, 337... Histogram circuit, 338 a, 338 b.

Claims (4)

A light projecting means for repeatedly irradiating an object with pulsed light;
A light receiving means for receiving light from the object;
A pulse light projecting time that is a time when the light projecting unit irradiates the pulse light, and a pulse that is a time when the light receiving unit receives the reflected pulse light of the pulse light emitted by the light projecting unit from the object. Histogram creation means for repeatedly measuring the time difference with the light reception time and creating a histogram showing the frequency distribution with respect to the time difference;
Distance calculating means for calculating a distance to the object based on the time difference at which the frequency of the histogram created by the histogram creating means is maximized ;
The distribution of the frequency at which the light receiving unit receives disturbance light with respect to the time difference is calculated from the histogram created by the histogram creating unit by removing the bin of the time difference at which the frequency of the histogram is maximized. A calculation means ;
An image acquisition apparatus comprising: Said calculation means, from said histogram the histogram producing means, for the histogram excluding the bin of said time difference the frequency of the histogram is maximized, the frequency of the bin of the time difference becomes maximum, except The image acquisition apparatus according to claim 1, wherein the distribution of the frequency at which the light receiving unit with respect to the time difference receives disturbance light is calculated by interpolating the frequency . A step of repeatedly irradiating an object with pulsed light;
Receiving light from the object;
The time difference is measured repeatedly by measuring the time difference between the pulsed light projecting time, which is the time when the pulsed light is irradiated, and the pulsed light receiving time, which is the time when the reflected pulsed light of the pulsed light irradiated from the object is received. Creating a histogram showing the frequency distribution for
Calculating a distance to the object based on the time difference at which the frequency of the created histogram is maximized ;
Calculating the distribution of the frequency at which the disturbance light with respect to the time difference is received by the histogram excluding the bin of the time difference from which the frequency of the histogram is maximized from the created histogram;
An image acquisition method comprising: The time difference is obtained by interpolating the frequency of the bin of the time difference at which the removed frequency is maximized from the created histogram, except for the bin of the time difference at which the frequency of the histogram is maximized. The image acquisition method according to claim 3, wherein a distribution of the frequency at which disturbance light is received is calculated.

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