Photoelectric conversion device, photoelectric conversion device control method, and program

Abstract

To provide a photoelectric conversion device that suppresses deterioration of image quality due to miscounting.SOLUTION: A photoelectric conversion device includes a photoelectric conversion element having a pixel formed of an avalanche photodiode, a first control unit that controls a reverse bias voltage applied to the avalanche photodiode, and a second control unit that sets an exposure time of the photoelectric conversion element, and the first control unit controls the reverse bias voltage in accordance with the exposure time set by the second control unit.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to a photoelectric conversion device, a control method for a photoelectric conversion device, and a program. [Background technology]

[0002] 2. Description of the Related Art In recent years, a photoelectric conversion device has been proposed that digitally counts the number of photons arriving at an avalanche photodiode (APD) and outputs the counted value from a pixel as a photoelectrically converted digital signal.

[0003] It is known that a phenomenon called avalanche emission occurs in photoelectric conversion devices having APDs (Non-Patent Document 1). When avalanche emission occurs, the generated secondary electrons enter adjacent pixels, increasing the count number of the adjacent pixel values ​​and causing erroneous counting. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] I. Rech et al., “Optical crosstalk in single photon avalanche diode arrays: a new complete model”, OpEx 16(12), 2008 Summary of the Invention [Problem to be solved by the invention]

[0005] SUMMARY OF THE PRESENT EMBODIMENTS Accordingly, an object of the present invention is to provide a photoelectric conversion device that suppresses deterioration of image quality due to miscounting. [Means for solving the problem]

[0006] In order to achieve the above object, a photoelectric conversion device according to one aspect of the present invention includes a photoelectric conversion element having a pixel formed of an avalanche photodiode, a first control unit that controls a reverse bias voltage applied to the avalanche photodiode, and a second control unit that sets an exposure time of the photoelectric conversion element, and the first control unit controls the exposure time set by the second control unit. The shorter , The absolute value of the excess bias applied to the avalanche photodiode is reduced. The reverse bias voltage is controlled. Effect of the Invention

[0007] According to the present invention, it is possible to provide a photoelectric conversion device that suppresses deterioration of image quality due to miscounting. [Brief description of the drawings]

[0008] [Figure 1] 1 is a diagram showing a configuration example of a photoelectric conversion element according to a first embodiment. [Diagram 2] 3 is a diagram showing a configuration example of a sensor substrate according to the first embodiment. FIG. [Diagram 3] 2 is a diagram showing a configuration example of a circuit board according to the first embodiment. FIG. [Figure 4] 4 is a diagram showing a timing chart of the vertical scanning circuit of the first embodiment. [Diagram 5] 2 is a diagram showing an equivalent circuit of a pixel and a signal processing circuit corresponding to the pixel according to the first embodiment. FIG. [Figure 6] 3 is a diagram illustrating a relationship between the operation of the APD and an output signal according to the first embodiment. FIG. [Figure 7] FIG. 2 is a functional block diagram of the photoelectric conversion device according to the first embodiment. [Figure 8] 11A and 11B are diagrams illustrating light emission crosstalk when multiple rows are read simultaneously. [Figure 9] 11A and 11B are diagrams illustrating degradation of image quality due to erroneous counting caused by light emission crosstalk. [Figure 10] 11A and 11B are diagrams illustrating the effect of the length of exposure time on image quality degradation. [Figure 11]11A and 11B are diagrams illustrating the effect of the length of exposure time on image quality degradation. [Figure 12] 11A and 11B are diagrams illustrating the relationship between exposure time and the influence of light emission crosstalk. [Figure 13] 10 is a diagram illustrating the shutter linearity of the photoelectric conversion element according to the second embodiment. FIG. [Figure 14] FIG. 13 is a diagram showing a photoelectric conversion system according to a third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The following describes in detail the embodiments for carrying out the present invention. Note that the embodiments described below are examples for realizing the present invention, and should be appropriately modified or adjusted depending on the configuration of the device to which the present invention is applied and various conditions, and the present invention is not limited to the following embodiments. Also, in all drawings, parts having the same functions are given the same numbers, and repeated explanations are omitted.

[0010] <Embodiment 1> 1 is a diagram showing a configuration example of a photoelectric conversion element according to the first embodiment. In the following, the photoelectric conversion element 100 will be described as an example of a photoelectric conversion device having a so-called laminated structure, in which two substrates, a sensor substrate 11 and a circuit substrate 21, are laminated and electrically connected. The photoelectric conversion element 100 may have a so-called non-laminated structure in which the components included in the sensor substrate and the components included in the circuit substrate are arranged in a common semiconductor layer. The sensor substrate 11 includes a pixel region 12. The circuit substrate 21 includes a circuit region 22 that processes a signal detected in the pixel region 12.

[0011] 2 is a diagram showing a configuration example of the sensor substrate 11 of the first embodiment. The pixel region 12 of the sensor substrate 11 includes a plurality of pixels 101 arranged two-dimensionally across a plurality of rows and columns. The pixels 101 include a photoelectric conversion unit 102 including an avalanche photodiode (hereinafter, APD). The photoelectric conversion unit 102 converts light into an electric signal. Specifically, the photoelectric conversion unit 102 photoelectrically converts an optical image formed by an imaging optical system 301, which will be described later, into an electric signal. The number of rows and columns of the pixel array constituting the pixel region 12 is not particularly limited.

[0012] Fig. 3 is a diagram showing an example of the configuration of the circuit board 21 of the first embodiment. The circuit board 21 has a signal processing circuit 103 that processes charges photoelectrically converted by the photoelectric conversion unit 102 shown in Fig. 2, a readout circuit 112, a control pulse generation unit 115, a horizontal scanning circuit 111, a signal line 113, and a vertical scanning circuit 110. A signal output from the photoelectric conversion unit 102 of the pixel is processed by the signal processing circuit 103. The signal processing circuit 103 is provided with a counter, a memory, etc., and a digital value is held in the memory.

[0013] In order to read out the digital signal from the memory of each pixel, the horizontal scanning circuit 111 inputs a control pulse for sequentially selecting each column to the signal processing circuit 103. For the selected column, a signal is output from the signal processing circuit 103 of the pixel selected by the vertical scanning circuit 110 to a signal line 113. The signal output to the signal line 113 is output to the outside of the photoelectric conversion element 100 via an output circuit 114.

[0014] 2 and 3, a plurality of signal processing circuits 103 are arranged in a region overlapping the pixel region 12 in a planar view. A vertical scanning circuit 110, a horizontal scanning circuit 111, a readout circuit 112, an output circuit 114, and a control pulse generating unit 115 are arranged so as to overlap between an end of the sensor substrate 11 and an end of the pixel region 12 in a planar view. In other words, the sensor substrate 11 has the pixel region 12 and a non-pixel region arranged around the pixel region 12. A vertical scanning circuit 110, a horizontal scanning circuit 111, a readout circuit 112, an output circuit 114, and a control pulse generating unit 115 are arranged in a region overlapping the non-pixel region in a planar view.

[0015] The vertical scanning circuit 110 receives a control pulse supplied from the control pulse generating unit 115 and supplies a control pulse to each pixel. The vertical scanning circuit 110 is composed of a shift register and an address decoder in which multiple rows are connected as one unit, and realizes high-speed readout by reading multiple rows at once. In particular, in the case of a photoelectric conversion device that digitally counts the number of photons arriving at the APD and outputs the counted value from the pixel as a photoelectrically converted digital signal, it takes time for the counter circuit that digitally counts the number of photons to operate. Therefore, it is preferable to read multiple rows simultaneously for high-speed readout. That is, the vertical scanning circuit 110, which functions as a readout circuit that reads pixel signals from pixels, simultaneously reads pixel signals from pixels included in multiple rows in the pixel area 12.

[0016] FIG. 4 is a timing chart of the vertical scanning circuit 110 of the first embodiment. In the following, as shown in FIG. 4, when rows to be read out simultaneously are grouped into one group, the rows are distinguished according to the position of the row within the group and expressed as rows A to F. Note that FIG. 4 shows a case where the number of rows to be read out simultaneously is six, but this is merely an example and is not limited to six. Also, FIG. 4 shows a case where only six rows are simultaneously subjected to an exposure operation, but if the exposure time is long, the number of rows to be simultaneously subjected to an exposure operation will be an integer multiple of six. Differences due to exposure time will be described later.

[0017] The arrangement of the signal lines 113, the readout circuits 112, and the output circuits 114 is not limited to that shown in Fig. 3. For example, the signal lines 113 may be arranged extending in the row direction, and the readout circuits 112 may be arranged at the ends of the signal lines 113. The function of a signal processing unit does not necessarily need to be provided for each photoelectric conversion unit, and one signal processing unit may be shared by a plurality of photoelectric conversion units to perform signal processing sequentially.

[0018] FIG. 5 is a diagram showing an equivalent circuit of the pixel 101 and the signal processing circuit 103 corresponding to the pixel 101 in FIGS.

[0019] The APD 201 generates a pair of electric charges according to incident light by photoelectric conversion. One of the two nodes of the APD 201 is connected to a power supply line to which a driving voltage VL (first voltage) is supplied. The other of the two nodes of the APD 201 is connected to a power supply line to which a driving voltage VH (second voltage) higher than the voltage VL is supplied.

[0020] One node of the APD 201 shown in Fig. 4 is the anode, and the other node of the APD is the cathode. A reverse bias voltage VR is supplied to the anode and cathode of the APD 201 so that the APD 201 performs avalanche multiplication operation. Specifically, the reverse bias voltage VR is supplied between the anode, which is one node of the APD 201, and the cathode, which is the other node. Here, the reverse bias voltage VR is calculated by the following formula (1). VR = VL - VH (1) By supplying such a voltage, the charges generated by the incident light undergo avalanche multiplication, generating an avalanche current.

[0021] When a reverse bias voltage is supplied, there are two modes: Geiger mode, in which the voltage difference between the anode and cathode is greater than the breakdown voltage, and linear mode, in which the voltage difference between the anode and cathode is close to or less than the breakdown voltage. An APD that operates in Geiger mode is called a SPAD. In the case of a SPAD, for example, the voltage VL (first voltage) is -30V and the voltage VH (second voltage) is 1V.

[0022] The quench element 202 is connected to a power supply line to which a drive voltage VH is supplied and to one of the anode and cathode nodes of the APD 201. The quench element 202 functions as a load circuit (quench circuit) during signal multiplication by avalanche multiplication, and has a function of suppressing avalanche multiplication by suppressing the voltage supplied to the APD 201 (quench operation). The quench element 202 also has a function of returning the voltage supplied to the APD 201 to the drive voltage VH by flowing a current equivalent to the voltage drop caused by the quench operation (recharge operation).

[0023] 5 shows an example in which the signal processing circuit 103 has a waveform shaping section 210, a counter circuit 211, and a selection circuit 212 in addition to the quench element 202. However, it is sufficient for the signal processing circuit 103 to have at least one of the waveform shaping section 210, the counter circuit 211, and the selection circuit 212 in addition to the quench element 202.

[0024] The waveform shaping unit 210 shapes the voltage change of the cathode of the APD 201 obtained when a photon is detected, and outputs a pulse signal. For example, an inverter circuit is used as the waveform shaping unit 210. Although an example in which one inverter is used as the waveform shaping unit 210 is shown in Fig. 5, a circuit in which a plurality of inverters are connected in series may be used, or another circuit having a waveform shaping effect may be used.

[0025] The counter circuit 211 counts the pulse signal output from the waveform shaping unit 210 and holds the count value. When a control pulse RES is supplied via a drive line 213, the signal held in the counter circuit 211 is reset.

[0026] 3 through a drive line 214 (not shown in FIG. 3) in FIG. 5, and switches between electrical connection and non-connection between the counter circuit 211 and the signal line 113. The selection circuit 212 includes, for example, a buffer circuit for outputting a signal, and outputs an output signal from the counter circuit 211 of the pixel to the signal line 113.

[0027] Note that electrical connections may be switched by disposing a switch such as a transistor between the quench element 202 and the APD 201 or between the photoelectric conversion unit 102 and the signal processing circuit 103. Similarly, the supply of the voltage VH or the voltage VL to the photoelectric conversion unit 102 may be electrically switched using a switch such as a transistor.

[0028] FIG. 6 is a diagram showing a schematic diagram of the relationship between the operation of the APD 201 and the output signal of the first embodiment. The input side of the waveform shaping unit 210 is nodeA, and the output side is nodeB. Between time t0 and time t1, a potential difference of VR (VL-VH) is applied to the APD 201. When a photon is incident on the APD 201 at time t1, avalanche multiplication occurs in the APD 201, an avalanche multiplication current flows through the quench element 202, and the voltage of nodeA drops. When the voltage drop amount becomes larger and the potential difference applied to the APD 201 becomes smaller, the avalanche multiplication of the APD 201 stops as shown at time t2, and the voltage level of nodeA does not drop below a certain value. After that, between time t2 and time t3, a current that compensates for the voltage drop from the voltage VL flows through nodeA, and at time t3, nodeA is stabilized to the original potential level. At this time, the portion of the output waveform at node A that exceeds a certain threshold value is waveform shaped by the waveform shaping section 210 and is output as a pulse signal at node B. Hereinafter, the photoelectric conversion device 300 of this embodiment will be described in detail.

[0029] 7 is a functional block diagram of a photoelectric conversion device 300 in embodiment 1. The photoelectric conversion device 300 of this embodiment is configured to include a photoelectric conversion element 100, an imaging optical system 301, a signal processing unit 302, an exposure time control unit (second control unit) 303, a voltage control unit (first control unit) 304, and a system control unit 305. The photoelectric conversion element 100 is similar to the photoelectric conversion element 100 shown in FIGS. 1 to 6, and therefore a description thereof will be omitted.

[0030] The imaging optical system (image pickup optical system) 301 is a lens that forms an optical image of a subject, and includes a variable magnification lens (zoom lens) that moves in the optical axis direction when changing magnification, and a focus lens that moves in the optical axis direction when adjusting focus (focusing). The imaging optical system 301 may also include an aperture and ND that control the amount of incident light, and an IRCF that controls the amount of incident infrared light.

[0031] The signal processing unit 302 processes the signal acquired by the photoelectric conversion element 100 to generate an image. The processing (image processing) performed by the signal processing unit 302 will be described later. The image generated by the signal processing unit 302 through image processing is output to the outside of the photoelectric conversion device 300 via a communication unit (not shown). Note that the communication unit (not shown) has a function of receiving a signal from the outside in addition to outputting the generated image.

[0032] An exposure time control unit (second control unit) 303 sets (determines) an exposure time for integrating the number of photons incident on the photoelectric conversion element 100, thereby controlling the exposure time.

[0033] The voltage control unit (first control unit) 304 applies a reverse bias voltage to the APD 201 of each pixel of the photoelectric conversion element 100, and controls the reverse bias current. The voltage control unit 304 includes a control IC capable of controlling the reverse bias voltage, and can be configured to be capable of controlling the reverse bias voltage by rewriting a register of the control IC. Alternatively, the voltage control unit 304 may be configured to include a plurality of power sources, and to be able to select one of the plurality of power sources to be applied to the APD 201 of the photoelectric conversion element 100.

[0034] The system control unit 305 includes a processor 306, a memory 307 as a storage medium, drivers for each I / F, and the like, and is configured by at least one computer, and is connected via lines to each component of the photoelectric conversion device 300. Also, some or all of the functional blocks shown in Fig. 7 are realized by causing the computer included in the photoelectric conversion device 300 to execute a computer program stored in the memory 307 as a storage medium.

[0035] It is also possible to realize some or all of these functions using hardware. For the hardware, a dedicated circuit (ASIC) or a processor (reconfigurable processor, DSP) can be used. Moreover, the functional blocks shown in Fig. 7 do not have to be built into the same housing, and may be configured as separate devices connected to each other via signal paths.

[0036] The processor 306 is a central processing unit composed of one or more CPUs (Central Processing Units), MPUs (Micro Processing Units), etc. The memory 307 is configured to include a ROM (Read Only Memory) and a RAM (Random Access Memory). The ROM is a non-volatile memory, and stores the computer program in this embodiment and other programs and data necessary for control. The RAM is a volatile memory, and stores temporary data such as image data and processing results in this embodiment.

[0037] When the pixel is an avalanche photodiode (APD), avalanche light emission phenomenon causes erroneous counting in adjacent pixels, that is, crosstalk between adjacent pixels (hereinafter referred to as light emission crosstalk).

[0038] The effect that a photon incident on one pixel has on an adjacent pixel is determined by the probability of occurrence of light emission crosstalk. It is known that the probability of occurrence of light emission crosstalk depends on the reverse bias voltage applied to the APD 201. Specifically, when the breakdown voltage of the APD 201 is VBD, the greater the absolute value of the excess bias VEX determined by the following formula (2), the higher the probability of occurrence of light emission crosstalk. |VEX| = |VR| - |VBD| (2)

[0039] Therefore, in the photoelectric conversion device 300 of this embodiment, the reverse bias voltage applied to the APD 201 is controlled according to the exposure time set by the exposure time control unit 303. This makes it possible to suppress deterioration of image quality due to miscounting. The reason why the voltage control unit 304 in this embodiment can suppress deterioration of image quality due to miscounting by controlling the reverse bias current will be described below.

[0040] It is known that the higher the temperature of the photoelectric conversion element 100, the higher the absolute value of the breakdown voltage VBD in the above formula (2). Therefore, it is preferable to control the reverse bias voltage applied by the voltage control unit 304 not only by the exposure time set by the exposure time control unit 303 but also by the temperature of the photoelectric conversion element 100. Specifically, the photoelectric conversion device 300 has a temperature sensor that acquires the temperature of the photoelectric conversion element 100, and the reverse bias voltage applied by the voltage control unit 304 is controlled using the value acquired by the temperature sensor so that the absolute value of the excess bias VEX is constant regardless of the temperature. In this way, it is preferable to control the reverse bias voltage applied by the voltage control unit 304 using the value acquired by the temperature sensor so that the absolute value of the excess bias VEX is constant regardless of the temperature.

[0041] In the photoelectric conversion element 100 of this embodiment, the vertical scanning circuit 110 simultaneously reads out multiple rows as shown in Fig. 4, that is, simultaneously reads out signals from pixels included in multiple rows in the pixel region 12. Therefore, as shown in Fig. 8, the amount of light emission crosstalk received from surrounding pixels differs depending on the position in the multiple rows that are simultaneously read out. Fig. 8 is a diagram explaining light emission crosstalk when multiple rows are simultaneously read out. Specifically, row F receives light emission crosstalk only from the pixels in the upper half, while row C receives light emission crosstalk from the pixels above, below, left, and right.

[0042] Therefore, if the brightness of the subject is uniform, rows A and F are dark and rows C and D are brightest, as shown in Figure 9, causing horizontal stripes in the image and degrading image quality due to the effects of light emission crosstalk. Figure 9 is a diagram explaining degradation of image quality due to miscounting caused by light emission crosstalk.

[0043] Although details will be described later, the horizontal stripes described in Figs. 8 and 9 are more noticeable as the exposure time of the photoelectric conversion element 100 is shorter, and are less noticeable as the exposure time is longer. Therefore, the voltage control unit 304 of the photoelectric conversion device 300 of this embodiment controls the reverse bias voltage so that the absolute value of the excess bias applied to the APD 201 becomes smaller as the exposure time set by the exposure time control unit 303 becomes shorter. By controlling the reverse bias voltage applied to the APD 201 in this manner, it is possible to control the occurrence probability of light emission crosstalk to be lower as the exposure time becomes shorter. With this configuration, the photoelectric conversion device 300 of this embodiment can suppress deterioration of image quality due to erroneous counting caused by avalanche light emission.

[0044] Fig. 10 and Fig. 11 are diagrams showing timing charts of the photoelectric conversion element 100 when the exposure time is different. Fig. 10 is a diagram showing a timing chart of the photoelectric conversion element 100 when the exposure time is short. Fig. 11 is a diagram showing a timing chart of the photoelectric conversion element 100 when the exposure time is long.

[0045] In FIG. 10, six rows are exposed at the same time at all times during the exposure operation. In other words, pixels other than the multiple rows that are read simultaneously are not exposed, and are only affected by light emission crosstalk from pixels in the multiple rows that are read simultaneously. On the other hand, in FIG. 11, there is a period during the exposure operation during which pixels other than the multiple rows that are read simultaneously are also performing exposure. During that period, the pixels are also affected by light emission crosstalk from pixels other than the multiple rows that are read simultaneously. And, as can be seen from FIG. 11, the longer the exposure time, the longer the period during which the pixels are affected by light emission crosstalk from pixels other than the multiple rows that are read simultaneously.

[0046] Fig. 12 is a diagram showing the exposure time on the horizontal axis and the percentage of the period affected by light emission crosstalk from pixels other than the multiple rows that are read simultaneously on the vertical axis. As can be seen from Fig. 12, the percentage of the period affected by light emission crosstalk from pixels other than the multiple rows that are read simultaneously increases as the exposure time becomes longer, and once the exposure time becomes longer than a certain point, the percentage hardly changes at all.

[0047] Therefore, in the photoelectric conversion device 300 of this embodiment, the voltage control unit 304 controls the reverse bias voltage applied to the APD 201 so that the absolute value of the excess bias applied to the APD 201 decreases as the exposure time set by the exposure time control unit 303 decreases. This makes it possible to suppress deterioration of image quality due to erroneous counting caused by emission crosstalk.

[0048] As shown in Fig. 12, when the exposure time is long to a certain extent, the ratio of the period affected by the light emission crosstalk from pixels other than the multiple rows read simultaneously hardly changes. Therefore, it is preferable to set a predetermined (specific) threshold value in advance, and when the exposure time is equal to or longer than the predetermined threshold value, the voltage control unit 304 controls the value of the reverse bias voltage applied to the APD 201 so that the absolute value of the excess bias applied to the APD 201 is constant. Here, when the voltage control unit 304 controls the value of the excess bias, not only the light emission crosstalk but also the sensitivity of the APD 201 changes. Therefore, when the exposure time is equal to or longer than the predetermined threshold value and the degradation of image quality due to horizontal stripes caused by the light emission crosstalk can be ignored, it is preferable to control the value of the excess bias by placing importance on other conditions such as sensitivity.

[0049] Furthermore, the signal processing unit 302 in this embodiment can perform processing to apply a digital gain to the signal acquired by the photoelectric conversion element 100, i.e., the signal read out by the vertical scanning circuit 110. The signal processing unit 302 may adjust the brightness of the image by applying a digital gain to the signal acquired by the photoelectric conversion element 100. The higher the digital gain, the less visible the horizontal stripes caused by emission crosstalk due to the influence of random noise become. Therefore, it is preferable that the voltage control unit 304 controls the reverse bias voltage applied to the APD 201 so that the absolute value of the excess bias applied to the APD 201 becomes smaller as the digital gain applied by the signal processing unit 302 becomes lower.

[0050] Moreover, the signal processing unit 302 in this embodiment can perform processing to generate an image from the signal acquired by the photoelectric conversion element 100, i.e., the signal read by the vertical scanning circuit 110). When generating a captured image using the signal acquired by the photoelectric conversion element 100, the signal processing unit 302 can execute image processing such as black level correction, gamma curve adjustment, noise reduction, edge emphasis, and data compression. The signal processing unit 302 may generate a captured image by executing each of these processes. Since the visibility of horizontal stripes due to emission crosstalk also changes depending on the parameters of these image processes, it is preferable that the voltage control unit 304 controls the reverse bias voltage applied to the APD 201 depending on the presence or absence of the parameters of the image processing.

[0051] For example, when the noise reduction, which is the image processing described above, is strongly applied, random noise is reduced, and horizontal stripes due to emission crosstalk may become noticeable. Therefore, it is preferable that the voltage control unit 304 controls the reverse bias voltage applied to the APD 201 so that the absolute value of the excess bias applied to the APD 201 becomes smaller as the signal processing unit 302 applies stronger noise reduction.

[0052] Furthermore, when the above-mentioned image processing, such as increasing edge emphasis or applying a gamma curve that emphasizes the contrast of horizontal stripes, is performed, horizontal stripes may become noticeable. Therefore, when the signal processing unit 302 increases edge emphasis or applies a gamma curve that emphasizes the contrast of horizontal stripes, it is preferable to control the reverse bias voltage so that the absolute value of the excess bias applied to the APD 201 is reduced. The reverse bias voltage is similarly controlled by the voltage control unit 304.

[0053] In addition, the visibility of horizontal stripes caused by light emission crosstalk also varies depending on the subject. Specifically, the horizontal stripes are more noticeable in subjects without texture or with low spatial frequency components. Therefore, it is preferable for the voltage control unit 304 to control the reverse bias voltage applied to the APD 201 depending on the subject being photographed.

[0054] In addition, the photoelectric conversion element 100 of this embodiment may be a photoelectric conversion element equipped with an on-chip color filter and capable of acquiring color information. For example, when a photoelectric conversion element with an RGGB Bayer array is used, the more biased the color, the more likely the effect of light emission crosstalk will be observed. The reason for this will be explained below.

[0055] For example, when a green subject is photographed, the number of photons incident on the G pixel is greater than the number of photons incident on the R pixel or the B pixel. The more avalanche light emission occurs, the more avalanche light emission occurs. Therefore, when the number of photons incident on the G pixel is greater than that of the R pixel or the B pixel, the avalanche light emission occurs more frequently in the G pixel than in the R pixel or the B pixel. Therefore, when a green subject is photographed, the avalanche light emission generated in the G pixel causes many erroneous counts in the R pixel or the B pixel. That is, when a subject with a biased color is photographed, the horizontal stripes are more noticeable. Therefore, when a subject with a biased color is photographed, it is preferable that the voltage control unit 304 controls the reverse bias voltage applied to the APD 201 so that the absolute value of the excess bias applied to the APD 201 becomes smaller.

[0056] Furthermore, the photoelectric conversion device 300 of this embodiment can be configured to allow the voltage control unit 304 to control the reverse bias voltage applied to the APD 201 for each color channel, and this configuration is preferable. That is, the voltage control unit 304 controls the reverse bias voltage applied to the APD 201 so that the absolute value of the excess bias applied to the APD of a color channel in which the number of photons incident on the pixel is relatively large is smaller than that of a color channel in which the number of photons incident on the pixel is relatively small. This is preferable because it reduces the influence of horizontal stripes due to emission crosstalk.

[0057] The timing at which the voltage control unit 304 controls the reverse bias voltage applied to the APD 201 is preferably a timing that does not overlap with the exposure time of each pixel of the photoelectric conversion element 100, that is, a so-called blank period. The reason for this will be explained below.

[0058] As described above, when the voltage control unit 304 controls the value of the excess bias applied to the APD 201, the sensitivity of the APD 201 also changes. Therefore, if the voltage control unit 304 controls the reverse bias voltage applied to the APD 201 at a timing that overlaps with the exposure time of each pixel of the photoelectric conversion element 100, a brightness step occurs in the middle of the captured image. Therefore, in order to suppress the occurrence of the brightness step, it is preferable that the timing at which the voltage control unit 304 controls the reverse bias voltage applied to the APD 201 is the blank period.

[0059] As described above, in order to suppress deterioration of image quality due to horizontal stripes caused by light emission crosstalk, the reverse bias voltage applied to the APD 201 is controlled by the voltage control unit 304. However, a method other than the control of the reverse bias voltage applied to the APD 201 may be combined. For example, parameters in image processing may be changed according to the exposure time set by the exposure time control unit 303. That is, parameters of digital gain and noise reduction are changed according to the set exposure time. In the example shown in FIG. 9, since the positions of the relatively bright and dark lines among the multiple lines read simultaneously are fixed, it is sufficient to apply a negative digital gain to the bright lines and a positive digital gain to the dark lines. In addition, horizontal stripes may be made less noticeable by applying low-pass filter processing in the vertical direction.

[0060] <Embodiment 2> When the voltage control unit 304 controls the value of the excess bias, not only the emission crosstalk but also the sensitivity of the APD 201 changes. Therefore, when the absolute value of the reverse bias voltage applied to the APD 201 is reduced as the exposure time set by the exposure time control unit 303 becomes shorter, degradation of image quality due to horizontal stripes can be suppressed. On the other hand, the response characteristic with respect to the exposure time, so-called shutter linearity, may deviate.

[0061] Therefore, in the second embodiment, a photoelectric conversion device 300 is described that can suppress a decrease in shutter linearity that occurs when the absolute value of the reverse bias voltage applied to the APD 201 is reduced as the integral time set by the exposure time control unit 303 becomes shorter. Note that the configuration, circuit board, and each functional unit of the photoelectric conversion device 300 of the second embodiment are similar to those of the first embodiment, and therefore description thereof will be omitted.

[0062] FIG. 13 is a diagram for explaining the shutter linearity of the photoelectric conversion element 100 of the second embodiment. Specifically, the diagram shows characteristics when the horizontal axis is exposure time and the vertical axis is the magnitude (pixel value) of the signal output from the photoelectric conversion element divided by the exposure time. The dashed line in FIG. 13 shows a case where a constant reverse bias voltage is applied regardless of the exposure time. The solid line in FIG. 13 shows a case where the voltage control unit 304 controls the reverse bias voltage so that the absolute value of the excess bias VEX applied to the APD 201 becomes smaller as the exposure time becomes shorter. The sensitivity of the APD 201 becomes higher as the absolute value of the excess bias VEX becomes larger, so the characteristics shown by the solid line deviate from the dashed line showing the ideal shutter linearity characteristics, especially on the side where the exposure time is short.

[0063] Therefore, in the photoelectric conversion device 300 of this embodiment, the decrease in sensitivity that occurs when the exposure time is short is compensated for by lengthening the exposure time of the photoelectric conversion element 100 or applying digital gain. That is, the smaller the absolute value of the excess bias applied to the APD 201, the smaller the EV value applied to the photoelectric conversion device 300 by the voltage control unit 304 with respect to the set EV value of the exposure target. As a result, in the second embodiment, it is possible to provide a photoelectric conversion device that suppresses image quality degradation due to erroneous counting caused by avalanche light emission, as in the first embodiment.

[0064] When applying digital gain, the shorter the exposure time, the larger the digital gain applied to the signal value in the signal processing unit 302. This makes it possible to suppress image quality degradation caused by miscounting due to avalanche light emission, and also suppress deviations in shutter linearity.

[0065] Here, if the signal processing unit 302 increases the digital gain applied to the signal value as the exposure time becomes shorter, the random noise component is amplified by the digital gain, and the S / N ratio of the image decreases. Therefore, the signal processing unit 302 increases the digital gain applied to the signal value as the exposure time becomes shorter, and strengthens the noise reduction. In this way, it is more preferable that the signal processing unit 302 changes the image processing parameters according to the exposure time, i.e., the shorter the exposure time is.

[0066] As described above, when the exposure time is equal to or longer than a predetermined threshold, it is preferable that the voltage control unit 304 controls the value of the reverse bias voltage applied to the APD 201 so that the absolute value of the excess bias applied to the APD 201 is constant. At this time, when the exposure time is equal to or longer than a predetermined threshold, the signal processing unit 302 also keeps the gain applied to the signal value constant. In such a case, it is preferable to keep the gain applied to the signal value constant. The threshold in this case is set in advance.

[0067] Next, a case where the decrease in sensitivity is compensated for by the exposure time of the photoelectric conversion element 100 will be described. The exposure time set as an exposure target for photographing a subject with appropriate brightness is T0. If the sensitivity when the exposure time is sufficiently long is S, when the exposure time is short, the sensitivity decreases to S0 due to the reverse bias voltage applied to the APD 201. Therefore, the exposure time T actually set in the photoelectric conversion element 100 is calculated according to the following formula (3). T = T0 × (S / S0) (3)

[0068] That is, the shorter the exposure time, the longer the exposure time set in the photoelectric conversion element 100 is set by the exposure time control unit 303 than the exposure time determined from the exposure target. This makes it possible to suppress image quality degradation due to erroneous counting caused by avalanche light emission, and also suppress deviation in shutter linearity.

[0069] As described above, when the exposure time is equal to or greater than a predetermined threshold, it is preferable that the voltage control unit 304 controls the value of the reverse bias voltage applied to the APD 201 so that the absolute value of the excess bias applied to the APD 201 is constant. At this time, when the exposure time is equal to or greater than a predetermined threshold, the exposure time control unit 303 makes the exposure time determined from the exposure target equal to the exposure time set by the photoelectric conversion element. In such a case, it is preferable to make the exposure time determined from the exposure target equal to the exposure time set by the photoelectric conversion element. The threshold in this case is set in advance.

[0070] The loss of sensitivity that occurs when the exposure time is short may be compensated for by digital gain, by extending the exposure time, or by a combination of both. However, it is preferable to compensate by extending the exposure time. The reason for this is explained below.

[0071] 10 and 11, the longer the exposure time, the more noticeable the horizontal stripes caused by light emission crosstalk become. Therefore, by setting the exposure time set for the photoelectric conversion element 100 to be longer than the exposure time determined from the exposure target, it is possible to further suppress the degradation of image quality caused by the horizontal stripes.

[0072] For the above reasons, it is preferable to compensate for the decrease in sensitivity that occurs when the exposure time is short by lengthening the exposure time, since this can further suppress deterioration of image quality due to horizontal stripes.

[0073] <Embodiment 3> 14 is a system block diagram of a photoelectric conversion system using the photoelectric conversion device 300 of the embodiments 1 to 3. The photoelectric conversion system 800 in the embodiment 3 includes a photoelectric conversion device 300 having an imaging optical system 301 and a photoelectric conversion element 100, a control unit 801, a storage unit 802, and a communication unit 803. Note that the configuration, circuit board, and each functional unit of the photoelectric conversion device 300 included in the photoelectric conversion system of the embodiment 3 described below are the same as those of the embodiment 1, and therefore detailed description thereof will be omitted as appropriate.

[0074] The imaging optical system (image pickup optical system) 301 is a lens that forms an optical image of a subject. The photoelectric conversion element 100 captures the optical image formed by the imaging optical system 301. The signal read from the photoelectric conversion element 100 is subjected to image generation and correction processing in a signal processing unit 302, as well as processing (image processing) such as black level correction, gamma curve adjustment, noise reduction, and data compression, to generate a final image. When the photoelectric conversion element 100 has an on-chip RGB color filter, it is further preferable to perform processing such as white balance correction and color conversion.

[0075] The control unit 801 includes a processor (not shown), a memory as a storage medium, and drivers for each I / F, and is configured by at least one computer, and is connected to each component of the photoelectric conversion device 300 via a line. The control unit 801 functions as a control means that controls the operation of each part of the entire photoelectric conversion system 800 based on a computer program stored in the memory as a storage medium. In addition, some or all of the functional blocks shown in Fig. 14 are realized by causing the computer to execute a computer program stored in the memory as a storage medium.

[0076] It is also possible to realize some or all of these functions using hardware. For the hardware, a dedicated circuit (ASIC) or a processor (reconfigurable processor, DSP) can be used. Moreover, the functional blocks shown in Fig. 7 do not have to be built into the same housing, and may be configured as separate devices connected to each other via signal paths.

[0077] The processor (not shown) is a central processing unit composed of one or more CPUs, MPUs, etc. The memory (not shown) is configured to include a ROM and a RAM. The ROM is a non-volatile memory and stores the computer program in this embodiment and other programs and data necessary for control. The RAM is a volatile memory and stores temporary data such as image data and processing results in this embodiment.

[0078] The control unit 801 can execute each process performed by the exposure time control unit 303 and the voltage control unit 304 in the first and second embodiments. That is, in the third embodiment, the control unit 801 performs each process performed by the exposure time control unit 303 and the voltage control unit 304. Therefore, the third embodiment does not require the system control unit 305 described in the first embodiment. The control unit 801 also controls the length of the exposure period of each frame of the photoelectric conversion element 100 and the timing of the control signal CLK via the control pulse generation unit 115 of the photoelectric conversion element 100.

[0079] The storage unit 802 is, for example, a recording medium such as a memory card or a hard disk for storing electronic data and programs. An external storage device may be used as a device that performs a similar function. Here, the external storage device can be realized, for example, by a medium (recording medium) and an external storage drive for realizing access to the medium. Examples of such media include a flexible disk (FD), a CD-ROM, a DVD, a USB memory, an MO, and a flash memory. The external storage device may also be a server device connected via a network. The communication unit 803 includes a wireless or wired interface, and outputs the generated image to the outside of the photoelectric conversion system 800 and receives a signal from the outside.

[0080] As described above, in the photoelectric conversion system according to the third embodiment, it is possible to suppress deterioration in image quality caused by erroneous counting due to avalanche light emission, similarly to the first embodiment.

[0081] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and changes are possible within the scope of the gist of the present invention.

[0082] The disclosure of this embodiment includes the following configuration, method, and program.

[0083] (Configuration 1) a photoelectric conversion element having pixels each formed of an avalanche photodiode; A first control unit that controls a reverse bias voltage applied to the avalanche photodiode; A second control unit that sets an exposure time of the photoelectric conversion element, The photoelectric conversion device, wherein the first control unit controls the reverse bias voltage in accordance with an exposure time set by the second control unit.

[0084] (Configuration 2) 2. The photoelectric conversion device according to configuration 1, wherein the first control unit applies a reverse bias voltage between the anode and the cathode of the avalanche photodiode.

[0085] (Configuration 3) The photoelectric conversion device described in configuration 1 or 2, characterized in that the first control unit controls the reverse bias voltage so that the absolute value of the excess bias applied to the avalanche photodiode becomes smaller as the exposure time becomes shorter.

[0086] (Configuration 4) The photoelectric conversion device described in any one of configurations 1 to 3, characterized in that when the exposure time is equal to or longer than a predetermined threshold, the first control unit controls the reverse bias voltage so that an absolute value of an excess bias applied to the avalanche photodiode is constant.

[0087] (Configuration 5) a signal processing unit that applies a digital gain to the signal; The photoelectric conversion device described in any one of configurations 1 to 4, characterized in that the first control unit controls the reverse bias voltage so that the absolute value of the excess bias applied to the avalanche photodiode becomes smaller as the digital gain applied in the signal processing unit becomes lower.

[0088] (Configuration 6) a signal processing unit that performs image processing on the signal to generate an image, 5. The photoelectric conversion device according to any one of configurations 1 to 4, wherein the first control unit controls the reverse bias voltage according to a parameter of the image processing.

[0089] (Configuration 7) the image processing includes noise reduction; The photoelectric conversion device according to configuration 6, wherein the first control unit controls the reverse bias voltage so that the absolute value of the excess bias applied to the avalanche photodiode becomes smaller as the signal processing unit applies the noise reduction more strongly.

[0090] (Configuration 8) The image processing includes edge enhancement, The photoelectric conversion device according to configuration 6 or 7, wherein the first control unit controls the reverse bias voltage so that the absolute value of the excess bias applied to the avalanche photodiode becomes smaller as the signal processing unit applies the edge enhancement more strongly.

[0091] (Configuration 9) The image processing includes a gamma curve adjustment. 9. The photoelectric conversion device according to any one of configurations 6 to 8, wherein the first control unit controls the reverse bias voltage by a gamma curve.

[0092] (Configuration 10) 10. The photoelectric conversion device according to any one of configurations 1 to 9, wherein the first control unit controls the reverse bias voltage depending on a subject to be photographed.

[0093] (Configuration 11) 11. The photoelectric conversion device according to any one of configurations 1 to 10, wherein the photoelectric conversion element is a photoelectric conversion element that includes an on-chip color filter and is capable of acquiring color information.

[0094] (Configuration 12) The photoelectric conversion device described in configuration 11, characterized in that the first control unit controls the reverse bias voltage so that the absolute value of the excess bias applied to the avalanche photodiode becomes smaller when a subject with a biased color is being photographed.

[0095] (Configuration 13) 13. The photoelectric conversion device according to claim 11, wherein the first control unit controls the reverse bias voltage for each color channel.

[0096] (Configuration 14) 14. The photoelectric conversion device according to any one of configurations 1 to 13, wherein the first control unit controls the reverse bias voltage at a timing that does not overlap with an exposure time of each pixel of the photoelectric conversion element.

[0097] (Configuration 15) a signal processing unit that performs image processing on the signal to generate an image, 5. The photoelectric conversion device according to any one of configurations 1 to 4, wherein the first control unit changes a parameter of the image processing in accordance with an exposure time set by the second control unit.

[0098] (Configuration 16) The photoelectric conversion device described in any one of configurations 1 to 4, characterized in that the first control unit reduces the EV value applied to the photoelectric conversion device relative to the set exposure target EV value as the absolute value of the excess bias applied to the avalanche photodiode becomes smaller.

[0099] (Configuration 17) a signal processing unit that applies a digital gain to the signal; 17. The photoelectric conversion device according to configuration 16, wherein the signal processing unit increases the digital gain applied to the signal value as the exposure time becomes shorter.

[0100] (Configuration 18) 18. The photoelectric conversion device according to configuration 17, wherein the signal processing unit applies stronger noise reduction as the exposure time becomes shorter.

[0101] (Configuration 19) 19. The photoelectric conversion device according to configuration 17 or 18, wherein the signal processing unit keeps the digital gain applied to the signal value constant when the exposure time is equal to or longer than a predetermined threshold value.

[0102] (Configuration 20) The photoelectric conversion device described in configuration 16, characterized in that the second control unit sets the exposure time set for the photoelectric conversion element to be longer than the exposure time set based on the exposure target as the exposure time becomes shorter.

[0103] (Configuration 21) The photoelectric conversion device described in configuration 20, characterized in that when the exposure time is equal to or greater than a predetermined threshold value, the second control unit equalizes the exposure time determined from the exposure target and the exposure time set by the photoelectric conversion element.

[0104] (Configuration 22) a temperature sensor for acquiring a temperature of the photoelectric conversion element; The photoelectric conversion device described in any one of configurations 1 to 21, characterized in that the first control unit controls the reverse bias voltage based on the temperature acquired by the temperature sensor so that the absolute value of the excess bias applied to the avalanche photodiode is constant.

[0105] (Configuration 23) A method for controlling a photoelectric conversion device, comprising: Setting an exposure time of a photoelectric conversion element having a pixel constituted by an avalanche photodiode; A method for controlling a photoelectric conversion device, comprising controlling a reverse bias voltage applied to the avalanche photodiode in accordance with the exposure time that has been set.

[0106] (Configuration 24) A program for causing a computer to execute a control method for a photoelectric conversion device, Setting an exposure time of a photoelectric conversion element having a pixel constituted by an avalanche photodiode; a reverse bias voltage applied to the avalanche photodiode is controlled in accordance with the exposure time that is set.

[0107] A computer program that realizes all or part of the control in the above-described embodiment may be supplied to the photoelectric conversion device via a network or various storage media. Then, a computer (or a CPU, MPU, etc.) in the photoelectric conversion device may read and execute the program. In this case, the program and the storage medium storing the program constitute the present invention. It may also be realized by a circuit (e.g., ASIC) that realizes one or more functions. [Explanation of symbols]

[0108] 100 Photoelectric conversion element 300 Photoelectric conversion device 301 Imaging Optical System 302 Signal Processing Section 303 Exposure time control unit 304 Voltage control section

Claims

1. a photoelectric conversion element having pixels each formed of an avalanche photodiode; a first control unit that controls a reverse bias voltage applied to the avalanche photodiode; A second control unit that sets an exposure time of the photoelectric conversion element, The first control unit controls the reverse bias voltage so that the absolute value of the excess bias applied to the avalanche photodiode becomes smaller as the exposure time set by the second control unit becomes shorter.

2. 2. The photoelectric conversion device according to claim 1, wherein the first control unit applies a reverse bias voltage between the anode and the cathode of the avalanche photodiode.

3. The photoelectric conversion device according to claim 1, characterized in that, when the exposure time is equal to or longer than a predetermined threshold, the first control unit controls the reverse bias voltage so that an absolute value of an excess bias applied to the avalanche photodiode is constant.

4. a signal processing unit that applies a digital gain to a signal output from the photoelectric conversion element; 2. The photoelectric conversion device according to claim 1, wherein the first control unit controls the reverse bias voltage so that the absolute value of the excess bias applied to the avalanche photodiode becomes smaller as the digital gain applied in the signal processing unit becomes lower.

5. a signal processing unit that performs image processing on the signal output from the photoelectric conversion element to generate an image; 2. The photoelectric conversion device according to claim 1, wherein the first control unit controls the reverse bias voltage according to a parameter of the image processing.

6. The image processing includes noise reduction, The photoelectric conversion device according to claim 5, wherein the first control unit controls the reverse bias voltage so that the absolute value of the excess bias applied to the avalanche photodiode becomes smaller as the signal processing unit applies the noise reduction more strongly.

7. The image processing includes edge enhancement, 6. The photoelectric conversion device according to claim 5, wherein the first control unit controls the reverse bias voltage so that the absolute value of the excess bias applied to the avalanche photodiode becomes smaller as the signal processing unit applies the edge enhancement more strongly.

8. The image processing includes a gamma curve adjustment.

6. The photoelectric conversion device according to claim 5, wherein the first control unit controls the reverse bias voltage according to a gamma curve.

9. 2. The photoelectric conversion device according to claim 1, wherein the first control unit controls the reverse bias voltage depending on a subject to be photographed.

10. 2. The photoelectric conversion device according to claim 1, wherein the photoelectric conversion element is a photoelectric conversion element that includes an on-chip color filter and is capable of acquiring color information.

11. The photoelectric conversion device according to claim 10, characterized in that the first control unit controls the reverse bias voltage so that the absolute value of the excess bias applied to the avalanche photodiode becomes smaller when a subject with a biased color is being photographed.

12. The photoelectric conversion device according to claim 10 , wherein the first control unit controls the reverse bias voltage for each color channel.

13. 2 . The photoelectric conversion device according to claim 1 , wherein the first control unit controls the reverse bias voltage at a timing that does not overlap with an exposure time of each pixel of the photoelectric conversion element.

14. a signal processing unit that performs image processing on the signal output from the photoelectric conversion element to generate an image; 2. The photoelectric conversion device according to claim 1, wherein the first control unit changes a parameter of the image processing in accordance with an exposure time set by the second control unit.

15. The photoelectric conversion device according to claim 1, characterized in that the first control unit reduces the EV value applied to the photoelectric conversion device relative to the set exposure target EV value as the absolute value of the excess bias applied to the avalanche photodiode becomes smaller.

16. a signal processing unit that applies a digital gain to a signal output from the photoelectric conversion element; 16. The photoelectric conversion device according to claim 15, wherein the signal processing unit increases the digital gain applied to the signal value as the exposure time becomes shorter.

17. 17. The photoelectric conversion device according to claim 16, wherein the signal processing section applies stronger noise reduction as the exposure time becomes shorter.

18. 17. The photoelectric conversion device according to claim 16, wherein the signal processing unit keeps the digital gain applied to the signal value constant when an exposure time is equal to or longer than a predetermined threshold value.

19. 16. The photoelectric conversion device according to claim 15, wherein the second control unit sets the exposure time set for the photoelectric conversion element to be longer than the exposure time set based on an exposure target as the exposure time becomes shorter.

20. 20. The photoelectric conversion device according to claim 19, wherein the second control unit, when the exposure time is equal to or greater than a predetermined threshold value, makes the exposure time determined from an exposure target equal to the exposure time set by the photoelectric conversion element.

21. a temperature sensor for acquiring a temperature of the photoelectric conversion element; The photoelectric conversion device according to claim 1, characterized in that the first control unit controls the reverse bias voltage based on the temperature acquired by the temperature sensor so that the absolute value of the excess bias applied to the avalanche photodiode is constant.

22. A method for controlling a photoelectric conversion device, comprising: Setting an exposure time of a photoelectric conversion element having a pixel constituted by an avalanche photodiode; A method for controlling a photoelectric conversion device, comprising controlling a reverse bias voltage applied to the avalanche photodiode so that the absolute value of the excess bias applied to the avalanche photodiode becomes smaller as the set exposure time becomes shorter.

23. A program for causing a computer to execute a control method for a photoelectric conversion device, Setting an exposure time of a photoelectric conversion element having a pixel constituted by an avalanche photodiode; a reverse bias voltage applied to the avalanche photodiode such that the absolute value of the excess bias applied to the avalanche photodiode becomes smaller as the set exposure time becomes shorter.

Images