Imaging method, device and system based on SPAD single photon avalanche diode

Abstract

The invention discloses an SPAD single photon avalanche diode-based imaging method, imaging device and system, and a readable storage medium. The method comprises the steps of obtaining an electric signal generated by an SPAD; converting the electric signal into a digital signal formed by a plurality of digital pulses; sampling the digital signals based on the rising edge and / or the falling edge of the clock signal to obtain the pulse widths of the digital signals, and accumulating the pulse widths of the plurality of digital signals to obtain the total pulse width of the plurality of digital signals; determining the number of photons received by the SPAD according to the total pulse width of the plurality of digital signals; and determining a pixel gray value corresponding to the SPAD based on the photon number, and generating a corresponding gray image. According to the method, in the SPAD-based imaging process, even if a plurality of digital pulses are adhered in a scene with relatively high light intensity, an accurate and clear grayscale image can be generated, and the grayscale image is relatively high in consistency with an actual scene.

Description

Technical Field

[0001] This application relates to the field of photonic measurement technology, and in particular to an imaging method, apparatus and system based on SPAD single-photon avalanche diode. Background Technology

[0002] A SPAD (Single Photon Avalanche Diode) is a highly sensitive photodetector that achieves high-sensitivity detection at the single-photon level through the avalanche multiplication effect. The SPAD generates a detectable electrical signal through the avalanche multiplication process, which can be amplified to a high voltage without additional circuitry, enabling the SPAD to detect single photons. This operating mode makes SPADs perform exceptionally well in low-light detection scenarios.

[0003] After each photon is detected and the avalanche multiplication effect occurs, a SPAD needs to be quenched to return to its initial state and detect the next photon. SPADs include active quenching and passive quenching. Active quenching refers to actively reducing the reverse bias voltage of the SPAD through a feedback circuit, so that the SPAD exits the avalanche state. Passive quenching refers to using a resistor voltage divider to reduce the voltage of the SPAD and exit the avalanche state.

[0004] In two-dimensional imaging, each time a SPAD detects a photon, it undergoes an avalanche multiplication effect, generating an electrical signal. This signal is then converted into a digital pulse signal by an inverter. The number of photons detected by the SPAD is typically determined by counting these digital pulses, and the light intensity is estimated based on this count. In some scenarios with strong ambient light, passive quenching may prevent the SPAD from completely exiting the avalanche state, such as... Figure 1 As shown, the digital pulse signals output from the inverter may become stuck together. In this case, the number of digital pulse signals counted is less than the number of photons actually detected by the SPAD. Figure 2 As shown, if the light intensity increases further, the adhesion of digital pulse signals will become more severe.

[0005] In scenes with strong ambient light, the accuracy of light intensity estimation based on SPAD is low, resulting in a deviation between the final image and the actual scene. In the final image, the brighter parts appear darker. Summary of the Invention

[0006] This application provides an imaging method, apparatus, and system based on SPAD single-photon avalanche diode to solve the problem that the accuracy of light intensity estimation based on SPAD is low in scenes with strong ambient light, resulting in a deviation between the final imaging effect and the actual scene.

[0007] In a first aspect, embodiments of this application provide an imaging method based on a SPAD single-photon avalanche diode, the method comprising:

[0008] Acquire the electrical signal generated by the SPAD; convert the electrical signal into a digital signal formed by multiple digital pulses; sample the digital signal based on the rising edge and / or falling edge of the clock signal to obtain the pulse width of the digital signal, and sum the pulse widths of multiple digital signals to obtain the total pulse width of the multiple digital signals; determine the number of photons received by the SPAD based on the total pulse width of the multiple digital signals; determine the pixel grayscale value corresponding to the SPAD based on the number of photons, and generate the corresponding grayscale image.

[0009] In some embodiments, sampling the digital signal based on the rising edge and / or falling edge of the clock signal to obtain the pulse width of the digital signal includes: sampling the digital signal at the rising edge of each clock signal; if the digital signal corresponding to the rising edge of a clock signal is high, then outputting a sampling result characterizing the pulse width of the digital pulse; and accumulating multiple sampling results to obtain the pulse width of a single digital signal.

[0010] In some alternative embodiments, the frequency of the clock signal is greater than the frequency of the digital pulse.

[0011] In some optional embodiments, sampling a digital signal based on the rising edge and / or falling edge of a clock signal to obtain the pulse width of the digital signal includes: sampling the rising edge of a digital signal to obtain a start signal characterizing a time measurement based on the rising edge of the digital signal; sampling the falling edge of the same digital signal to obtain a stop signal characterizing a time measurement based on the falling edge of the digital signal; and obtaining the pulse width of the digital signal based on the time difference between the stop signal and the start signal.

[0012] In some optional embodiments, sampling the digital signal based on the rising edge and / or falling edge of the clock signal to obtain the pulse width of the digital signal includes: sampling the rising edge of the digital signal based on the TDC time-to-digital converter circuit to obtain a start signal for starting the TDC circuit; sampling the falling edge of the same digital signal based on the TDC time-to-digital converter circuit to obtain a stop signal for stopping the TDC circuit; and obtaining the pulse width of the digital signal based on the time difference between the stop signal and the start signal.

[0013] In some optional embodiments, the sampling results are summed to obtain the total pulse width of multiple digital signals, including: acquiring the pulse width of each digital signal in the multiple digital signals; summing the pulse widths of each digital signal to obtain the total pulse width of the multiple digital signals, wherein the larger the total pulse width of the digital signals, the stronger the light intensity.

[0014] In some embodiments, determining the number of photons received by the SPAD based on the total pulse width of multiple digital signals includes: determining the dead time of the SPAD during the quenching process, obtaining the pulse width of a single digital pulse based on the dead time, and determining the number of photons received by the SPAD based on the ratio of the total pulse width of the digital signals to the pulse width of a single digital pulse.

[0015] Secondly, embodiments of this application provide an imaging device based on a SPAD single-photon avalanche diode, comprising: a SPAD, a shaping circuit, a sampling circuit, and a calculation circuit; the SPAD generates an electrical signal; the shaping circuit converts the electrical signal into a digital signal formed by multiple digital pulses; the sampling circuit samples the pulse width of the digital signal based on the rising and / or falling edges of a clock signal when a digital signal contains multiple digital pulses, and accumulates the sampling results to obtain the total pulse width of the multiple digital signals; the calculation circuit determines the number of photons received by the SPAD based on the ratio of the total pulse width of the digital signal to the pulse width of a single digital pulse; and determines the pixel grayscale value corresponding to the SPAD based on the number of photons, and generates a corresponding grayscale image.

[0016] Thirdly, embodiments of this application provide an imaging system based on a SPAD single-photon avalanche diode, which includes the aforementioned imaging device and a hard disk recorder.

[0017] Fourthly, embodiments of this application provide a readable storage medium storing computer-readable instructions, which are executed by a processor to implement the above-described method.

[0018] The method provided in this application embodiment obtains the total pulse width of multiple digital signals by sampling the clock signal, and determines the number of photons detected by the SPAD based on the total pulse width of the multiple digital signals. Even if multiple digital pulses are stuck together in a scene with high light intensity, the actual number of photons detected by the SPAD can be accurately reflected, making the final image clearer and more consistent with the actual scene. Attached Figure Description

[0019] Figure 1 This is a first schematic diagram of digital pulse signal adhesion in some embodiments;

[0020] Figure 2 This is a second schematic diagram of digital pulse signal adhesion in some embodiments;

[0021] Figure 3 This is a flowchart of the imaging method based on SPAD single-photon avalanche diode in the embodiments of this application;

[0022] Figure 4This is the first embodiment of the statistical digital signal pulse width in this application;

[0023] Figure 5 This is a schematic diagram of the principle of statistical digital signal pulse width based on clock signal in the embodiments of this application;

[0024] Figure 6 This is a second embodiment of the statistical digital signal pulse width in this application;

[0025] Figure 7 This is a schematic diagram of the digital signal pulse width statistical method based on TDC (Time-to-Digital Converter) in an embodiment of this application.

[0026] Figure 8 This is a schematic diagram of the imaging device in an embodiment of this application. Detailed Implementation

[0027] The present application will be described in detail below with reference to the specific embodiments shown in the accompanying drawings. However, these embodiments do not limit the present application. Any structural, methodological, or functional modifications made by those skilled in the art based on these embodiments are included within the protection scope of the present application.

[0028] This application provides an imaging method based on SPAD single-photon avalanche diode. In some embodiments, the method can be applied in an imaging device, which is equipped with an optical element that receives and focuses light. The imaging device is used to receive light focused by the optical element and perform the method steps described in the following embodiments.

[0029] In another embodiment, the method can also be applied to an imaging device and a hard disk video recorder (HDV), where some steps of the imaging method are performed by the imaging device, and subsequent processing steps are performed by the HDVV. The HDVV may include a DVR (Digital Video Recorder) and an NVR (Network Video Recorder).

[0030] like Figure 3 As shown, the imaging method based on SPAD single-photon avalanche diode specifically includes the following steps:

[0031] Step S301: Acquire the electrical signal generated by SPAD.

[0032] SPAD is a highly sensitive photodetector. When a SPAD detects a photon, an avalanche multiplication effect occurs, generating a detectable electrical signal through the avalanche multiplication process.

[0033] It should be noted that after each photon is detected and the avalanche multiplication effect occurs, the SPAD needs to be quenched to return to a detectable state and continue detecting the next photon. Quenching includes active quenching and passive quenching. Active quenching refers to actively reducing the reverse bias voltage of the SPAD through a feedback circuit, causing the SPAD to exit the avalanche state. Passive quenching refers to using a resistor voltage divider to reduce the voltage of the SPAD and exit the avalanche state.

[0034] Step S302: Convert the electrical signal into a digital signal formed by multiple digital pulses.

[0035] In some embodiments, the electrical signal generated by the SPAD can be converted into a digital pulse by an inverter. When the SPAD triggers the avalanche multiplication effect, its cathode outputs an electrical signal, which is a negative current pulse. The negative current pulse is converted into a positive current pulse by an inverter, making the electrical signal suitable for subsequent digital circuits, and then output as a digital pulse after passing through the inverter.

[0036] In environments with strong ambient light, SPADs cannot be completely quenched using passive quenching; that is, the voltage across the SPAD cannot drop below its breakdown voltage. If the SPAD detects the next photon and generates a new electrical signal, the digital pulse, after being converted by the inverter, will stick together with the previous digital pulse, forming a digital signal composed of at least two digital pulses.

[0037] In one sampling period, there are multiple digital signals, each formed by at least one digital pulse. That is, there are digital signals formed by one digital pulse and digital signals formed by at least two digital pulses. A digital signal formed by one digital pulse indicates that the SPAD has been completely quenched and the next photon has been detected. A digital signal formed by at least two digital pulses indicates that the SPAD has detected the next photon before it has been completely quenched, causing two adjacent digital pulses to stick together.

[0038] Step S303: Sample the digital signal based on the rising edge and / or falling edge of the clock signal to obtain the pulse width of the digital signal, and sum the pulse widths of multiple digital signals to obtain the total pulse width of multiple digital signals.

[0039] like Figure 4 As shown, in some embodiments, the digital signal is sampled based on the rising edge and / or falling edge of the clock signal to obtain the pulse width of the digital signal, including the following steps:

[0040] Step S401: Sample the digital signal at the rising edge of each clock signal.

[0041] Step S402: If the digital signal corresponding to the rising edge of a clock signal is high, the accumulator is incremented by one.

[0042] Step S403: Accumulate multiple sampling results to obtain the pulse width of a single digital signal.

[0043] In one sampling period, the frequency of the clock signal is greater than the frequency of the digital pulse. For example, the clock signal is a high-frequency clock signal with a frequency of not less than 200MHz. The higher the frequency of the clock signal, the higher the accuracy of the pulse width of the digital signal determined based on the method steps of the above embodiments.

[0044] like Figure 5 As shown, further, the pulse width of each digital signal in the multiple digital signals is obtained. The pulse widths of the various digital signals are accumulated to obtain the total pulse width of the multiple digital signals. The larger the total pulse width of the digital signals, the stronger the light intensity. The level of the digital signal is detected at the rising edge of each clock signal. If the digital signal is high, the value of the accumulator is incremented by 1; if the digital signal is low, the value of the accumulator remains unchanged. The total pulse width of the digital signals is directly proportional to the value of the accumulator, that is, the total pulse width of the digital signals and the value of the accumulator satisfy the following relationship:

[0045] T total =N high *T clk ;

[0046] In the formula, T total N represents the total pulse width of the digital signal. high T represents the value of the accumulator. clk This indicates the sampling period of the clock signal.

[0047] For example, Figure 5 If the rising edge of the clock signal samples the high level of the digital signal 5 times, and the clock signal frequency is 500MHz with a sampling period of 2ns, then the total pulse width T of the digital signal is... total =5*2ns=10ns.

[0048] Based on the above method steps, discrete sampling counts are converted into continuous time quantities (total pulse width of digital signals), which solves the problem of counting distortion caused by digital pulse adhesion under strong light with low hardware complexity. Moreover, the total pulse width of the output digital signal is monotonically related to the light intensity, providing a reliable input for subsequent imaging algorithms.

[0049] like Figure 6 As shown, in some other embodiments, sampling the digital signal based on the rising edge and / or falling edge of the clock signal to obtain the pulse width of the digital signal further includes the following steps:

[0050] Step S601: Sample the rising edge of a digital signal to obtain the start signal characterizing the time measurement based on the rising edge of the digital signal.

[0051] Step S602: Sample the falling edge of the same digital signal and obtain the termination signal characterizing the time measurement based on the falling edge of the digital signal.

[0052] Step S603: Obtain the pulse width of the digital signal based on the time difference between the termination signal and the start signal.

[0053] like Figure 7 As shown, the TDC circuit measures the width of each pulse. The rising edge of the digital signal is used as the start signal of the TDC circuit, and the falling edge is used as the stop signal. After the TDC circuit starts timing, it stops at the falling edge and outputs the time measurement value, such as t1, t2, t3, etc.

[0054] Further, the pulse width of each digital signal is acquired. The pulse widths of each digital signal are summed to obtain the total pulse width; the larger the total pulse width, the stronger the light intensity. After each frame of measurement, all measured values ​​are summed. The total pulse width T of the digital signal is... total = t1 + t2 + t3. This method directly measures the time difference, avoiding sampling errors.

[0055] Step S304: Determine the number of photons received by the SPAD based on the total pulse width of multiple digital signals, determine the pixel grayscale value corresponding to the SPAD based on the number of photons, and generate the corresponding grayscale image.

[0056] In some embodiments, the pulse width of a single digital pulse is obtained based on the dead time of the SPAD during the quenching process. The number of photons received by the SPAD is determined based on the ratio of the total pulse width of the digital signal to the pulse width of a single digital pulse.

[0057] It should be noted that dead time refers to the time it takes for the SPAD to return to a detectable state after detecting a photon and triggering the avalanche multiplication effect, through the quenching circuit. During this period, the SPAD cannot respond to new photons.

[0058] In this embodiment, the total pulse width of the digital signal is directly proportional to the number of photons, that is, the total pulse width of the digital signal satisfies the following relationship:

[0059] T total =N detected *t p ;

[0060] Among them, T total N represents the total pulse width of the digital signal. detected t represents the number of photons detected by SPAD. pThe pulse width, representing a single digital pulse, is fixed by hardware circuitry. However, due to the existence of dead time, the relationship between the number of photons detected by SPAD and the actual number of photons satisfies the following equation:

[0061]

[0062] Where, N detected N represents the number of photons detected by SPAD. photon T represents the actual number of photons. frame t represents the time it takes for one frame to be imaged. d This represents the dead time, which depends on the design of the quenching resistor in the SPAD.

[0063] Based on the above formula, the true number of photons can be obtained by inversely calculating the total pulse width of the digital signal. The true number of photons satisfies the following relationship:

[0064]

[0065] Among them, T total N represents the total pulse width of the digital signal. detected N represents the number of photons detected by SPAD. photon t represents the actual number of photons. p The pulse width t represents a single digital pulse. d This represents the dead time, which depends on the design of the quenching resistor in the SPAD.

[0066] In some embodiments, each SPAD records the number of photons hitting it and outputs a numerical value counts_ij. This numerical value counts_ij is then mapped to a grayscale value displayed in the image. The grayscale value typically ranges from 0 to 255. Here, counts represents the number of photons recorded by the SPAD, and i and j represent the row and column coordinates of the SPAD. Each pixel position (i, j) of each SPAD corresponds to a grayscale value, and these grayscale values ​​are organized into a two-dimensional matrix.

[0067] For example, 1000*2000 SPAD arrays form a 1000*2000 pixel array. Based on the above method, the two-dimensional matrix of the pixel array under spatial mapping is determined to obtain a grayscale image corresponding to the SPAD array.

[0068] An 800*800 SPAD array forms an 800*800 pixel array. Based on the above method, the two-dimensional matrix of the pixel array under spatial mapping is determined to obtain a grayscale image corresponding to the SPAD array.

[0069] Using the above method, even in scenes with high light intensity where multiple digital pulses are stuck together, the actual number of photons detected by SPAD can be determined relatively accurately, resulting in clearer final imaging, more distinct boundaries between bright and dark areas, and higher consistency with the actual scene.

[0070] Based on the same inventive concept, such as Figure 8 As shown, this application embodiment also provides an imaging device 10, which includes:

[0071] SPAD101 is used to generate electrical signals;

[0072] Shaping circuit 102 is used to convert the electrical signal into a digital signal formed by a plurality of digital pulses;

[0073] The sampling circuit 103 is used to sample the pulse width of the digital signal based on the rising edge and / or falling edge of the clock signal when the digital signal contains multiple digital pulses, and to accumulate the sampling results to obtain the total pulse width of the multiple digital signals.

[0074] The calculation circuit 104 is used to determine the number of photons received by the SPAD based on the ratio of the total pulse width of the digital signal to the pulse width of a single digital pulse, determine the pixel grayscale value corresponding to the SPAD based on the number of photons, and generate the corresponding grayscale image.

[0075] This application provides an imaging system based on a SPAD single-photon avalanche diode, which includes the imaging device and a hard disk recorder described above.

[0076] This application provides a readable storage medium storing computer-readable instructions. When the computer-readable instructions are executed by a processor, the above-described method is implemented.

[0077] In the above embodiments, implementation can be achieved entirely or partially through software, hardware, firmware, or any combination thereof. When implemented using software, it can be implemented entirely or partially in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, all or part of the processes or functions described in the embodiments of this application are generated. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted from one computer-readable storage medium to another. For example, the computer instructions can be transmitted from one website, computer, server, or data center to another website, computer, server, or data center via wired (e.g., coaxial cable, fiber optic, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) means. The computer-readable storage medium can be any available medium that a computer can access or a data storage device such as a server or data center that integrates one or more available media. The available medium can be a magnetic medium (e.g., floppy disk, hard disk, magnetic tape), an optical medium (e.g., DVD), or a solid-state drive (SSD), etc.

[0078] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0079] The various embodiments in this specification are described in a related manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on its differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments. The above descriptions are merely preferred embodiments of this application and are not intended to limit the scope of protection of this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application are included within the scope of protection of this application.

Claims

1. An imaging method based on SPAD single-photon avalanche diode, characterized in that, The method includes: Acquire the electrical signal generated by the SPAD; The electrical signal is converted into a digital signal formed by multiple digital pulses; The digital signal is sampled based on the rising edge and / or falling edge of the clock signal to obtain the pulse width of the digital signal, and the pulse widths of multiple digital signals are accumulated to obtain the total pulse width of the multiple digital signals; The number of photons received by the SPAD is determined based on the total pulse width of the multiple digital signals, the pixel grayscale value corresponding to the SPAD is determined based on the number of photons, and a corresponding grayscale image is generated.

2. The method according to claim 1, characterized in that, The sampling of the digital signal based on the rising and / or falling edges of the clock signal to obtain the pulse width of the digital signal includes: The digital signal is sampled at each rising edge of the clock signal; If the digital signal is high at the rising edge of a clock signal, a sampling result representing the pulse width of the digital pulse is output. The multiple sampling results are summed to obtain the pulse width of a single digital signal.

3. The method according to claim 2, characterized in that, The frequency of the clock signal is greater than the frequency of the digital pulse.

4. The method according to claim 1, characterized in that, The sampling of the digital signal based on the rising and / or falling edges of the clock signal to obtain the pulse width of the digital signal includes: The rising edge of the digital signal is sampled, and a start signal characterizing the time measurement is obtained based on the rising edge of the digital signal. Sampling is performed on the falling edge of the same digital signal, and a termination signal characterizing the time measurement is obtained based on the falling edge of the digital signal; The pulse width of the digital signal is obtained based on the time difference between the termination signal and the start signal.

5. The method according to claim 1, characterized in that, The sampling of the digital signal based on the rising and / or falling edges of the clock signal to obtain the pulse width of the digital signal includes: The rising edge of the digital signal is sampled based on the TDC time-to-digital converter circuit to obtain the start signal for activating the TDC circuit; Based on the falling edge sampling of the same digital signal by the TDC time-to-digital converter circuit, a termination signal for stopping the TDC circuit is obtained; The pulse width of the digital signal is obtained based on the time difference between the termination signal and the start signal.

6. The method according to any one of claims 2 to 5, characterized in that, The step of accumulating the sampling results to obtain the total pulse width of the multiple digital signals includes: Obtain the pulse width of each of the multiple digital signals; The pulse widths of the individual digital signals are summed to obtain the total pulse width of the multiple digital signals. The larger the total pulse width of the digital signals, the stronger the light intensity.

7. The method according to claim 1, characterized in that, Determining the number of photons received by the SPAD based on the total pulse width of the plurality of digital signals includes: Determine the dead time of the SPAD during the quenching process, and obtain the pulse width of a single digital pulse based on the dead time; The number of photons received by the SPAD is determined based on the ratio of the total pulse width of the digital signal to the pulse width of a single digital pulse.

8. An imaging device based on a SPAD single-photon avalanche diode, characterized in that, include: SPAD, used to generate electrical signals; A shaping circuit is used to convert the electrical signal into a digital signal formed by multiple digital pulses; A sampling circuit is used to sample the pulse width of a digital signal based on the rising edge and / or falling edge of a clock signal when a digital signal contains multiple digital pulses, and to accumulate the sampling results to obtain the total pulse width of the multiple digital signals; The computing circuit is used to determine the number of photons received by the SPAD based on the ratio of the total pulse width of the digital signal to the pulse width of a single digital pulse, determine the pixel grayscale value corresponding to the SPAD based on the number of photons, and generate a corresponding grayscale image.

9. An imaging system based on a SPAD single-photon avalanche diode, characterized in that, It includes a hard disk recorder and the imaging device as described in claim 8.

10. A readable storage medium, characterized in that, The readable storage medium stores computer-readable instructions that are executed by a processor to implement the method as described in any one of claims 1 to 7.

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