Crosstalk calibration for direct time-of-flight sensors

Abstract

The invention relates to crosstalk calibration for direct time-of-flight sensors. A method of calibrating a direct time of flight (dToF) sensor to compensate for crosstalk includes transmitting N photon pulses from a transmitter of the dToF sensor, wherein N is a number of photon pulses; receiving, at a receiver of the dToF sensor, N crosstalk signals including a first crosstalk signal and N-1 remaining crosstalk signals; averaging the (N-1) remaining crosstalk signals to generate an average crosstalk signal; subtracting the first crosstalk signal from the average crosstalk signal to generate an estimated surround signal; subtracting the estimated surround signal from each of the N-1 remaining crosstalk signals to obtain a corrected crosstalk signal; and calibrating the dToF sensor using the corrected crosstalk signal.

Description

Technical Field

[0001] The present invention relates generally to direct time-of-flight (dToF) sensors and, in particular embodiments, to systems and methods for calibrating dToF sensors to compensate for crosstalk. Background Art

[0002] Time of flight (ToF) technology measures the amount of time it takes for a signal (e.g., an object, particle, wave, etc.) to travel from a transmitter to a target and reflect back to a receiver. ToF sensors (also called ToF imagers) are used in a variety of applications including distance detection, object presence, identification and motion detection, measuring characteristics (such as composition and flow rate), and three-dimensional imaging. In general, there are two types of ToF sensing: direct ToF (dToF) sensing and indirect ToF (iToF) sensing. In the case of dToF sensing, the dToF system transmits a signal pulse and directly measures the time elapsed between the signal emission and the reflected signal leaving the target object and returning. This elapsed time (time of flight) can then be converted into a distance measurement. In contrast, the iToF method compares the phase difference between the modulated transmitted signal and the return signal to indirectly measure the distance.

[0003] Objects that are very close to the transmitter and receiver of a ToF sensor can cause interference from the transmitted signal that the receiver is going to measure. A common example of such an object is a cover glass that can be mounted over a ToF sensor to protect the sensor. The characteristics of the cover glass (e.g., transparency to the transmitted and reflected signals, position, thickness, consistency, etc.) can be carefully selected to minimize the negative impact on the ToF sensor. However, some portion of the transmitted signal is still reflected back toward the ToF sensor, resulting in crosstalk. A crosstalk calibration process is used to account for this crosstalk signal.

[0004] During the crosstalk calibration process, a ToF sensor (e.g., a device into which the ToF sensor is integrated) is removed from the production line. A single ToF sensor is placed in a dark area at a fixed position relative to a target at a known distance from the ToF sensor. The crosstalk calibration process is then run while the ToF sensor remains in the dark area and relatively slowly, with each crosstalk calibration lasting on the order of seconds (e.g., 2 seconds). The ToF sensor must then be pushed back onto the production line, and the next ToF sensor must be removed for calibration.

[0005] The crosstalk calibration process has some disadvantages. The calibration time is long compared to other production line processes, resulting in reduced yield. In addition, there are special hardware and space requirements (e.g., dark areas, known targets, etc.) to properly perform the crosstalk calibration process. Therefore, it is desirable to have a crosstalk calibration process that does not interrupt production while still maintaining high accuracy. Summary of the invention

[0006] According to an embodiment of the present invention, a method for calibrating a direct time-of-flight (dToF) sensor to compensate for crosstalk includes: transmitting N photon pulses from a transmitter of the dToF sensor, where N is the number of photon pulses; receiving N crosstalk signals including a first crosstalk signal and N-1 remaining crosstalk signals at a receiver of the dToF sensor; averaging the N-1 remaining crosstalk signals to generate an average crosstalk signal; subtracting the first crosstalk signal from the average crosstalk signal to generate an estimated surround signal; subtracting the estimated surround signal from each of the N-1 remaining crosstalk signals to obtain a corrected crosstalk signal; and calibrating the dToF sensor using the corrected crosstalk signal.

[0007] According to another embodiment of the present invention, a dToF sensing device includes: a dToF sensor, the dToF sensor including a transmitter configured to transmit N photon pulses and a receiver configured to receive N crosstalk signals; a processor coupled to the dToF sensor; and a memory storing program instructions coupled to the processor. The N crosstalk signals include a first crosstalk signal and N-1 remaining crosstalk signals, where N is the number of photon pulses. When executed by the processor, the program instructions enable the processor to average the N-1 remaining crosstalk signals to generate an average crosstalk signal, subtract the first crosstalk signal from the average crosstalk signal to generate an estimated surround signal, subtract the estimated surround signal from each of the N-1 remaining crosstalk signals to obtain a corrected crosstalk signal, and calibrate the dToF sensor using the corrected crosstalk signal.

[0008] According to another embodiment of the present invention, a dToF sensing device includes: a dToF sensor including a focal plane and a field of view disposed above the focal plane; a cover glass disposed above the focal plane; a transmitter disposed at the focal plane and configured to transmit N photon pulses toward the cover glass to generate N crosstalk signals from a portion of each of the N photon pulses reflected by the cover glass; a receiver disposed at the focal plane and configured to receive the N crosstalk signals; and a processor coupled to the dToF sensor. The N crosstalk signals include a first crosstalk signal and N-1 remaining crosstalk signals, where N is the number of photon pulses. The processor is configured to average the N-1 remaining crosstalk signals to generate an average crosstalk signal, subtract the first crosstalk signal from the average crosstalk signal to generate an estimated surround signal, subtract the estimated surround signal from each of the N-1 remaining crosstalk signals to obtain a corrected crosstalk signal, and calibrate the dToF sensor using the corrected crosstalk signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

[0010] Figure 1 illustrates an example graph associated with a direct time of flight (dToF) sensor in accordance with an embodiment of the present invention, wherein there is no covering and no target in the field of view (FOV) of the dToF sensor, the graph qualitatively showing a dToF pulse, a received background signal, and a corresponding histogram;

[0011] Figure 2 illustrates an example graph associated with a dToF sensor with an overlay but no target in the FOV of the dToF sensor, the graph qualitatively showing dToF pulses, received crosstalk signals (and background signals), and corresponding histograms in accordance with an embodiment of the present invention;

[0012] Figure 3 illustrates example graphs associated with a dToF sensor transmitting N dToF pulses and receiving N crosstalk signals in a first scenario where no target is present in the FOV of the dToF sensor in accordance with an embodiment of the present invention;

[0013] Figure 4 illustrates example graphs associated with a dToF sensor transmitting N dToF pulses and receiving N crosstalk signals in a second scenario where there is a target in the FOV of the dToF sensor but not in the surrounding range of the dToF sensor in accordance with an embodiment of the present invention;

[0014] Figure 5 illustrates example graphs associated with a dToF sensor transmitting N dToF pulses and receiving N crosstalk signals in a third situation where there is a FOV target of the dToF sensor within the surrounding range of the dToF sensor in accordance with an embodiment of the present invention;

[0015] Figure 6 illustrates example graphs showing histogram frames of average crosstalk signals, estimated surround signals, and corrected crosstalk signals associated with a dToF sensor in a first case where no target is present in the FOV of the dToF sensor in accordance with an embodiment of the present invention;

[0016] Figure 7 illustrates another example graph showing a histogram frame of an average crosstalk signal, an estimated surround signal, and a corrected crosstalk signal associated with a dToF sensor in a second scenario where a target is present in the FOV of the dToF sensor but not within a surround range of the dToF sensor in accordance with an embodiment of the present invention;

[0017] Figure 8 illustrates yet another example graph showing a histogram frame of an average crosstalk signal, an estimated surround signal, and a corrected crosstalk signal associated with a dToF sensor in a third case where a target of the FOV of the dToF sensor is present within the surround of the dToF sensor in accordance with an embodiment of the present invention;

[0018] Fig. 9 illustrates an example dToF sensing device including a dToF sensor and a cover according to an embodiment of the present invention;

[0019] Fig.10 illustrates another example dToF device including a dToF sensor and a cover according to an embodiment of the present invention, wherein the dToF device is on a production line; and

[0020] Fig.11 A method of calibrating a dToF sensor to compensate for crosstalk according to an embodiment of the present invention is illustrated.

[0021] Unless otherwise indicated, corresponding numbers and symbols in different figures generally refer to corresponding parts. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale. The edges of features drawn in the figures do not necessarily indicate the end of the feature limits. DETAILED DESCRIPTION

[0022] The making and use of various embodiments are discussed in detail below. However, it should be understood that the various embodiments described herein are applicable to a variety of specific environments. The specific embodiments discussed are merely illustrative of specific ways to make and use various embodiments and should not be construed as being in limited scope. Unless otherwise specified, the expressions "about", "approximately" and "substantially" mean within 10% and preferably within 5% of a given value, or, such as in the case of substantially zero, less than 10% and preferably less than 5% of a comparable amount.

[0023] When a ToF sensor is embedded in a device (e.g., a customer application), it is usually located behind a transparent cover (generally called a cover glass, although it does not need to be glass) for aesthetic or protective reasons. Adding a cover glass over the ToF sensor introduces crosstalk into the ToF system, which can be reduced through careful design choices, but is unavoidable. Therefore, all applications of ToF sensors with a cover glass (or any other structure that causes crosstalk signals) require a crosstalk calibration process.

[0024] Conventional crosstalk calibration is an in-factory calibration process with specific settings that must be performed under special conditions. That is, conventional crosstalk calibration imposes several undesirable constraints on production, including hardware and space requirements (in order to ensure that each ToF sensor device is in an area that is dark enough so that there are no uncontrollable target objects within the field of view (FOV) of the ToF sensor) and time requirements (the ToF sensor devices must be removed from the production line and the crosstalk calibration itself takes on the order of seconds). As a result, conventional crosstalk calibration is undesirable.

[0025] There are two alternatives to conventional crosstalk calibration. First, a universal crosstalk calibration can be installed in each of the ToF sensor devices, and the individualized crosstalk calibration process can be avoided entirely. Unfortunately, the details of the crosstalk calibration are highly dependent on the specific environment very close to the ToF sensor. For example, a universal crosstalk calibration would need to be created for the exact cover glass arrangement (e.g., cover glass distance, material, thickness, size, etc.) of each ToF sensor application, but such a universal crosstalk calibration would not be sufficient to correct for the specific structural variations of each ToF sensor device. In particular, the ToF sensor and each cover glass are unique, and the universal crosstalk calibration cannot correctly account for variations that result in undesirable losses in ToF sensor device performance.

[0026] A second alternative to conventional crosstalk calibration is to perform the crosstalk calibration process without complying with the hardware, spatial, and temporal constraints of conventional crosstalk calibration. For example, crosstalk calibration can be performed without removing the ToF sensor device from a production line (e.g., even while the device is moving). This has the advantage of individualizing the crosstalk calibration (and thus accounting for device-to-device variations in cover glass and ToF sensor), but has the significant disadvantage of being performed outside of an ideal environment. Specifically, without a dedicated crosstalk calibration area, background noise levels can vary, and unknown targets can be located in the FOV of the ToF sensor.

[0027] A particular problem with performing crosstalk calibration in an uncontrolled environment is that the target may generate undetectable surround signals. Surround signals appear when an unknown target is present within the FOV of the ToF sensor and the return signal caused by the reflection of the first transmit pulse overlaps with the second transmit pulse. In this case, in addition to the crosstalk signal (and of course the background noise), the surround signal is also received. Because it is not known whether there is a target in the FOV of the ToF sensor, it is impossible to know whether the crosstalk signal includes the surround signal. Therefore, there is a risk of overestimating the crosstalk level and incorrectly calibrating the ToF sensor device. This can be particularly problematic given the critical nature of many applications of ToF sensors (e.g., automotive, medical, etc.).

[0028] In view of the above deficiencies, what is desired is an improved crosstalk calibration system and method. The example systems and methods described herein provide an alternative to conventional crosstalk calibration processes that are both accurate and efficient. For example, the example systems and methods can advantageously be performed without removing the ToF sensor device from a production line and without the risk of incorrect crosstalk calibration (e.g., from surrounding signals or from environmental factors). For example, the example systems and methods described herein can result in accurate and efficient crosstalk estimates even when an unknown target is within the FOV of a ToF sensor.

[0029] In various embodiments, a method of calibrating a ToF sensor to compensate for crosstalk includes using a first crosstalk signal of N crosstalk signals as a baseline signal without surround components. The first crosstalk signal is subtracted from an average of the N-1 remaining crosstalk signals to generate an estimated surround signal. The estimated surround signal can then be subtracted from each of the N-1 remaining crosstalk signals to obtain a corrected crosstalk signal. With the surround contribution removed, the corrected crosstalk signal can be used to perform crosstalk calibration of the ToF sensor.

[0030] N crosstalk signals can be obtained by transmitting N pulses (e.g., photon pulses) and receiving the N crosstalk signals using one or more detectors (e.g., photon detectors such as avalanche photodiodes, PIN diodes, etc.). For example, each of the N crosstalk signals can be detected and stored as a histogram, which includes several bins, each bin storing a signal received during a specific time period within a frame (i.e., a frame represents the total time between transmitted pulses, and each bin is a time segment within a frame). Because the first frame cannot include surrounding contributions, the crosstalk signal of the first frame (i.e., the first crosstalk signal) can be used as a baseline to estimate the surrounding signal in the remaining frames and remove it to generate a differential histogram.

[0031] The embodiment crosstalk calibration systems and methods described herein can have various additional advantages over conventional systems and methods. For example, the embodiment crosstalk calibration systems and methods can advantageously avoid specific hardware or space requirements. For example, no specific target is required in order to perform crosstalk calibration of a ToF sensor (e.g., because the embodiment crosstalk calibration method is responsible for all possible scenarios regarding unknown targets in the FOV). In addition, compared to conventional techniques, the embodiments can have the benefit of reducing or eliminating noise requirements during crosstalk calibration (e.g., no dark areas are required).

[0032] Other potential advantages of the embodiment crosstalk calibration system and method relate to calibration efficiency. Crosstalk calibration can be advantageously performed while the ToF sensor device is moving (e.g., on a production line). This allows crosstalk calibration to be performed without being removed from the production line, thereby improving efficiency. Since the surround signal is not a problem, the pulse period can also be reduced, which can advantageously increase the signal-to-noise ratio while also reducing the time required to perform crosstalk calibration. Because the pulse period is shorter than conventional methods and the ToF sensor device is not removed from the production line, the flexibility of the number of samples collected during the crosstalk calibration can be increased. That is, greater latitude to obtain larger sample sizes can be used to increase the accuracy of the crosstalk calibration while still being more efficient than conventional processing.

[0033] The embodiment crosstalk calibration systems and methods described herein may also have the advantage of being performed at a stable pulse frequency (i.e., a constant pulse period). For example, the potentially tricky complexity of multi-frequency crosstalk calibration may be advantageously avoided. Another potential benefit of a constant pulse frequency is consistent power consumption. In addition, the crosstalk calibration may have the benefit of being performed at a pulse frequency that matches or exceeds the operating pulse frequency of the ToF sensor (e.g., in compliance with specifications). Since the time required to obtain a statistically sufficient sample size is lower with increasing pulse frequency (i.e., decreasing pulse period) and the power consumption is constant, the total power consumption of the crosstalk calibration may also be advantageously reduced.

[0034] The embodiments provided below describe various systems and methods related to dToF sensors, and in particular, describe systems and methods for calibrating dToF sensors to compensate for crosstalk.The following description describes embodiments. Figure 1 and Figure 2 Used to compare dToF sensors with and without covering. Figures 3 to 5 Describes three example scenarios of a dToF sensor that emits dToF pulses. Figures 6 to 8 Description for Figures 3 to 5 The intermediate signal and the corrected crosstalk signal are processed for the three cases of crosstalk calibration. Fig. 9 Describes an example dToF sensing device. Fig.10 Another example dToF sensing device on a production line is described. Fig.11 A method for calibrating a dToF sensor to compensate for crosstalk is described.

[0035] Figure 1 An example graph associated with a direct time-of-flight (dToF) sensor in accordance with an embodiment of the present invention is illustrated, wherein there is no covering and no target in the field of view (FOV) of the dToF sensor, the graph qualitatively showing the dToF pulses, the received background signal, and the corresponding histogram.

[0036] Reference Figure 1 , the qualitative graph 100 shows the signal received by the receiver 15 of the dToF sensor 10 when a dToF pulse 22 is transmitted from the transmitter 13 and there is no target within the FOV of the dToF sensor 10. In particular, the dToF pulse 22 is transmitted from the transmitter 13, and the receiver 15 receives the background signal 18. Because there is nothing obstructing the transmitter 13, the dToF pulse 22 is transmitted without deflection, and the receiver 15 does not detect the crosstalk signal. In addition, because there is no target in the FOV of the dToF sensor 10, the dToF pulse 22 is not reflected back and no target signal is received.

[0037] The dToF sensor 10 can be any type of dToF sensor. In various embodiments, the dToF sensor 10 is a photon dToF sensor such as a ToF camera. The dToF sensor 10 can be configured to operate in various modes including imaging, ranging, object detection, tracking and identification, and others. Although photon dToF sensors are used as examples throughout this description, other dToF sensors such as ion dToF sensors, neutron dToF sensors, etc. are possible. Any particle scattering near the receiver 15 may cause crosstalk that needs to be corrected.

[0038] The dToF pulses 22 may be electromagnetic pulses (i.e., photon pulses) such as near infrared (NIR) pulses (e.g., such that the dToF pulses 22 are not visible to the human eye). In various embodiments, the wavelength of the dToF pulses 22 is between about 800 nm and about 1000 nm. For example, the wavelength of the dToF pulses 22 may be selected to be in a solar notch (a gap in an otherwise fairly continuous spectrum of the sun). In one embodiment, the wavelength of the dToF pulses 22 is about 940 nm. In another embodiment, the wavelength of the dToF pulses 22 is about 850 nm. In yet another embodiment, the wavelength of the dToF pulses 22 is about 905 nm. The wavelength of the dToF pulses 22 may also be longer (e.g., in the short-wave infrared (SWIR) portion of the electromagnetic spectrum emitted by InGaAs laser diodes, Ge-on-Si laser diodes, etc.). In various embodiments, the wavelength of the dToF pulses 22 is between about 980 nm and about 1600 nm, and in some embodiments, the wavelength is between about 1300 nm and about 1550 nm.

[0039] As shown, the transmitter 13 and the receiver 15 can be supported by a substrate 11, which can be any type of suitable substrate (e.g., a semiconductor substrate, a printed circuit board (PCB), a glass substrate, etc.). In a specific example of a photon dToF sensor, the transmitter 13 can include one or more light sources such as light emitting diodes (LEDs) or laser diodes. In the context of a photon dToF sensor, the transmitter 13 can be referred to as an illumination unit. In one embodiment, the transmitter 13 includes a vertical cavity surface emitting laser (VCSEL).

[0040] The receiver 15 can be implemented as a detector (or an array of detectors) that generates an electrical signal when a reflected dToF pulse is received. In various embodiments, the receiver 15 includes a particle detector, and in some embodiments, the receiver 15 includes a photon detector. The receiver 15 may include an avalanche photodiode (APD), and in one embodiment, a single photon avalanche diode (SPAD). The receiver 15 may also include a PIN diode, as well as other detector types. Detectors other than photon detectors are of course possible (e.g., any type of dToF sensor can benefit from the crosstalk calibration system and method described herein).

[0041] The dToF sensor 10 stores the received signal in a histogram 12. For example, the receiver 15 may convert the received signal into an analog electrical signal within the dToF sensor 10. The receiver 15 may then use a threshold (e.g., a voltage) to record a count indicating whether a signal is received during a certain time period. For this purpose, the histogram 12 is divided into frames each containing several bins. For example, a frame may be a time period between each dToF pulse 22. Graph 100 shows a single frame 14 divided into bins 16. Each bin 16 corresponds to a time period in a frame 14 (corresponding to an object distance) so that the time information of the received signal relative to the dToF pulse 22 can be stored and processed. The bin size (e.g., the time period associated with each bin) may be constant (although this is not particularly required). In various embodiments, the bin size is on the order of nanoseconds, and in some embodiments, the bin size is less than about 1ns. For example, the bin size may be lower (such as on the order of hundreds of picoseconds or ten picoseconds). The specific size of the bin may depend on the specific implementation, and how small the bin may be may depend on the hardware capabilities.

[0042] As mentioned above, SPADs can be used as detectors of reflected light (as part of receiver 15). In some applications, an array of SPADs is provided as a detector (e.g., a SPAD array) to detect reflected light pulses. The reflected photons can generate carriers in the SPADs by the photoelectric effect. The carriers generated by the photons can trigger an avalanche current in one or more SPADs in the SPAD array. The avalanche current can signal an event, i.e., a photon has been detected. Information related to the reflection intensity, also referred to as a "signal count", is output as a histogram of the SPAD array (e.g., a histogram 12 including multiple bins 16).

[0043] As shown, the dToF sensor 10 can be configured to identify or calculate a background noise level 21, and the binned signals of the histogram 12 below the background noise level 21 are considered to be background noise (e.g., not indicating the presence of a target object in the FOV of the dToF pulse 22). Here, because the receiver 15 does not receive the target signal and there is no crosstalk contribution, all binned signals are below the background noise level 21. Of course, the exact location of the background noise level 21 can vary, and (although not shown) the background signal 18 can occasionally exceed the background noise level 21. The handling of this situation and the positioning of the background noise level 21 can vary depending on the specific details of a given application.

[0044] Figure 2 An example graph associated with a dToF sensor in accordance with an embodiment of the present invention is illustrated, wherein there is an overlay but no target in the FOV of the dToF sensor, the graph qualitatively showing dToF pulses, received crosstalk signals (and background signals), and corresponding histograms. Like labeled elements may be as previously described.

[0045] Reference Figure 2 , qualitative graph 200 illustrates the signal received by the receiver 15 of the dToF sensor 10 when the cover 19 is included. The cover 19 may be referred to as a cover glass (even though it need not be made of glass). The cover 19 may be included to protect the dToF sensor 10 or for aesthetic purposes. The cover 19 may be selected to be as transparent as possible to the dToF pulses 22. For photon pulses, this may mean being transparent to light of a particular wavelength or range of wavelengths. In various embodiments, the cover 19 comprises a glass material, and in one embodiment, the cover 19 comprises borosilicate glass. In other embodiments, the cover 19 comprises an acrylic material. In one embodiment, the cover 19 comprises polymethyl methacrylate (PMMA). In one embodiment, the cover comprises polycarbonate (PC).

[0046] When the transmitter 13 transmits the dToF pulse 22, the cover 19 transmits some (ideally, most) of the dToF pulse 22, but a portion of the dToF pulse 22 is reflected back and received at the receiver 15 as a crosstalk signal 26. This signal is stored in the frame 14 of the histogram 12 and may exceed the background noise level 21 (as shown). Although the extent of the received crosstalk signal 26 may not exactly match the extent of the dToF pulse 22 (as shown), the crosstalk signal 26 may be localized to the early portion of the frame 14. For example, a subset of the bins 17 may include crosstalk contributions that exceed the background noise level 21. In some cases, some or all of the crosstalk calibration process may use only this subset of the bins 17, rather than the entire frame 14.

[0047] Because the crosstalk signal 26 exceeds the background noise level 21, the dToF sensor 10 may erroneously interpret the crosstalk signal 26 as a target signal. Therefore, it is desirable to compensate for the crosstalk signal 26 (e.g., perform a crosstalk calibration process that defines the crosstalk contribution of a particular dToF sensor 10 and stores corrections to reduce or eliminate sensor errors due to the crosstalk signal 26).

[0048] The specific details of the crosstalk signal 26 depend on the properties of the cover 19 and its placement relative to the transmitter 13 and the receiver 15. The crosstalk signal 26 can be highly sensitive to various characteristics of the cover 19, such as thickness, smoothness, material consistency, distance from the transmitter 13 and / or receiver 15, lateral extent of the cover 19, parallelism of the cover 19 relative to the substrate 11 (e.g., to the focal plane), and many other factors. Although appropriate values ​​for these parameters can be selected, the crosstalk signal 26 is highly sensitive to manufacturing variations, and therefore individualized crosstalk calibration (calibration based on the specific details of each dToF sensor 10) should be performed to ensure that the dToF sensor 10 is accurate. In addition, the cover 19 can be part of the device in which the dToF sensor 10 is integrated. In this case, the manufacturing of the dToF sensor 10 may not control the details of the cover 19. For these reasons, generalized crosstalk calibration may not be feasible.

[0049] Figures 3 to 5 Used to describe three situations involving the presence and position of targets in the FOV of the dToF sensor during the crosstalk calibration process. Figure 3 Illustrated are example graphs associated with a dToF sensor transmitting N dToF pulses and receiving N crosstalk signals in a first scenario where no target is present in the FOV of the dToF sensor in accordance with an embodiment of the present invention. Figure 4Illustrated are example graphs associated with a dToF sensor transmitting N dToF pulses and receiving N crosstalk signals in a second scenario where there is a target in the FOV of the dToF sensor but not in the surrounding range of the dToF sensor in accordance with an embodiment of the present invention. Figure 5 Illustrated are example graphs associated with a dToF sensor transmitting N dToF pulses and receiving N crosstalk signals in a third scenario where there are FOV targets of the dToF sensor within the surrounding range of the dToF sensor in accordance with an embodiment of the present invention. Like labeled elements may be as previously described.

[0050] Reference Figure 3 , a qualitative graph 300 shows N dToF pulses 20 emitted by a transmitter of a dToF sensor and N crosstalk signals 24 corresponding to the N dToF pulses 20 received by a receiver of the dToF sensor. Each crosstalk signal 26 of the N crosstalk signals 24 is stored in a histogram 12. Since N is greater than 1, the N crosstalk signals 24 include a first crosstalk signal 25 and N-1 remaining crosstalk signals 327. Since there is no target in the FOV, the first crosstalk signal 25 and the N-1 remaining crosstalk signals 327 are similar and only represent the crosstalk contribution. It should be mentioned that background signals (such as previously discussed) may also be present, but for clarity, they have been omitted from the graph 300. For example, the signals shown may represent what they might be if the background noise was subtracted / filtered out (which of course may be part of the process).

[0051] Now refer to Figure 4 , the qualitative graph 400 shows N dToF pulses 20 emitted by the dToF sensor and N crosstalk signals 24 corresponding to the N dToF pulses 20 received by the dToF sensor. Similar to the first case, the N crosstalk signals 24 include the first crosstalk signal 25 and the N-1 remaining crosstalk signals 427. However, in the second case where there is a target in the FOV, there is also a target signal 23 between the crosstalk signals 26. Therefore, although the first crosstalk signal 25 and the N-1 remaining crosstalk signals 427 still only represent crosstalk contributions, there is also a signal (target signal 23) that is not related to the crosstalk contribution in each of the pulse periods 32. For the sake of brevity and clarity, the convention that the elements that follow the pattern [x27] in various embodiments (where "x" is the figure number) can be the relevant implementation of the remaining crosstalk signals has been adopted. For example, the N-1 remaining crosstalk signals 427 can be similar to the N-1 remaining crosstalk signals 327, unless otherwise specified (for example, due to different environments). By using similar terminology in conjunction with the above mentioned numbering system, it is clear that similar conventions may also be adopted for other elements.

[0052] Go to the third case, refer to Figure 5 , a qualitative graph 500 shows N dToF pulses 20 emitted by a dToF sensor and N crosstalk signals 24 corresponding to the N dToF pulses 20 received by the dToF sensor. As before, the N crosstalk signals 24 include a first crosstalk signal 25 and N-1 remaining crosstalk signals 527. However, in the third case, the target in the FOV is at a distance that causes the target signal to overlap with the next crosstalk signal 26. That is, the surround signal 50 is added to the N-1 remaining crosstalk signals 527. In the surround case, the first crosstalk signal 25 includes only the crosstalk contribution, while the N-1 remaining crosstalk signals 527 include both the crosstalk contribution and the surround signal 50 (which is not related to the crosstalk contribution). Because the presence of the target is unknown, the surround signal 50 is indistinguishable from the crosstalk signal 26, and if the first crosstalk signal 25 and the N-1 remaining crosstalk signals 527 are simply averaged together, the estimated crosstalk contribution will be too high.

[0053] In all cases, the first step in the crosstalk calibration process is to obtain N crosstalk signals 24. Specifically, receiving the N crosstalk signals 24 can include storing each of the N crosstalk signals 24 as a histogram 12 in a plurality of bins. Each crosstalk signal can occupy only a portion of the early bins of each frame, such as three to five of the first ten bins of each frame. This is because the time period experiencing the crosstalk contribution is related to the timing of each dToF pulse 22. As expected, knowing this can allow only a subset of the bins 17 to be used in the crosstalk calibration.

[0054] The number N of dToF pulses 22 may be referred to as a sample size, and may depend on various factors such as desired (or required) calibration accuracy and calibration efficiency. For example, N dToF pulses 20 are transmitted in a time period 30 equal to approximately N-1 times the pulse period 32. It may be desirable that N is high enough to achieve a certain sample size to increase the statistical accuracy of the crosstalk calibration process. However, for various reasons such as the efficiency of the calibration process or environmental changes during the calibration process, larger values ​​of N may be impractical (e.g., when N is high, the time period 30 is longer and may be so long that the assumption that the noise and unknown targets in the FOV of the dToF sensor are constant becomes increasingly unlikely).

[0055] In various embodiments, N is on the order of hundreds. For example, in some embodiments, N is greater than about 100. If the pulse period 32 is made shorter, the distance over which the surround signal 50 is generated is shorter, and the risk of generating the surround signal 50 becomes more likely (e.g., when performing crosstalk calibration in a production environment). However, as will be discussed in more detail below, the surround signal 50 can be corrected to advantageously allow the pulse period 32 to be reduced. As a result, the sample size N can be made higher, such as greater than about 250 or more.

[0056] The wraparound distance depends on the pulse period 32 (e.g., laser period). The pulse period 32 may be on the order of ten nanoseconds. For example, the pulse period 32 may be less than about 50 ns, such as about 36 ns (corresponding to a wraparound distance of about 10.8 m). However, in various embodiments, the pulse period 32 is reduced, such as reduced to less than about 20 ns (about 6 m wraparound distance). In one embodiment, the target signal 23 is about 16 ns (about 4.8 m wraparound distance), and may be made shorter, such as 12 ns, 8 ns, etc.

[0057] Each of the dToF pulses 22 has a pulse width 34, which can be made much shorter than the pulse period 32 (e.g., so as to be able to resolve targets in the FOV). For example, the pulse width 34 can be less than about 5ns, such as about 2ns. Of course, the pulse width 34 can be much shorter, such as in the sub-nanosecond range. While obtaining the N crosstalk signals 24, both the pulse width 34 and the pulse period 32 can be kept constant. Maintaining a constant pulse period 32 can advantageously simplify the crosstalk calibration process compared to alternative techniques.

[0058] Even when the sample size N is relatively high, the pulse period 32 can be reduced so that N dToF pulses 20 are transmitted within a sufficiently short time period 30. In some embodiments, the time period 30 is less than about 5 μs. The time period 30 can also be shorter, such as less than about 2 μs, and in one embodiment, about 1.6 μs (e.g., N is 100 and the pulse period 32 is about 16 ns).

[0059] Figures 6 to 8 Used to describe the Figures 3 to 5 The present invention provides a technique for correcting the crosstalk contribution of three situations. Conventional methods are prone to uncontrollable calibration errors when performed in areas where unknown targets may be present in the FOV of the dToF sensor. As will be described below, the systems and methods of crosstalk calibration described herein can reduce or eliminate this error.

[0060] Figure 6Illustrated are example graphs showing histogram frames of average crosstalk signals, estimated surround signals, and corrected crosstalk signals associated with a dToF sensor in a first situation where no target is present in the FOV of the dToF sensor in accordance with the present embodiment. Figure 7 Another example graph of histogram frames showing an average crosstalk signal, an estimated surround signal, and a corrected crosstalk signal associated with a dToF sensor in a second situation where a target is present in the FOV of the dToF sensor but not within the surround range of the dToF sensor in accordance with an embodiment of the present invention is illustrated. Figure 8 Another example graph is illustrated showing a histogram frame of an average crosstalk signal, an estimated surround signal, and a corrected crosstalk signal associated with a dToF sensor in a third case where there is a FOV target of the dToF sensor within the surround range of the dToF sensor in accordance with an embodiment of the present invention. Like labeled elements may be as previously described.

[0061] Reference Figure 6 , qualitative graph 600 shows the steps for obtaining a corrected crosstalk signal for a first case where there is no target in the FOV of the dToF sensor. The first frame including the first crosstalk signal 25 may have no surround signal because no previous pulse was transmitted. Using this information, the N-1 remaining crosstalk signals are averaged together to generate an average crosstalk signal 660. The first crosstalk signal 25 is then subtracted from the average crosstalk signal 660 to obtain an estimated surround signal 662 (because there is no target in the surround range, it is negligible here and not shown).

[0062] The estimated surround signal 662 is then subtracted from each of the N-1 remaining crosstalk signals to obtain the corrected crosstalk signal 64. Because the first crosstalk signal 25 has no surround contribution, the result is N accurate crosstalk signals that can be used to calibrate the dToF sensor to compensate for crosstalk.

[0063] For the second case, refer to Figure 7 , qualitative graph 700 shows the result when the same steps are applied in the second case where there is a target in the FOV of the dToF sensor. The N-1 remaining crosstalk signals are averaged together to generate an average crosstalk signal 760. The first crosstalk signal 25 is then subtracted from the average crosstalk signal 760 to obtain an estimated surround signal 762 (again, since there is a target in the FOV but not in the surround range, it is negligible here and not shown). Subtracting the estimated surround signal 762 still results in the corrected crosstalk signal 64.

[0064] Now refer to Figure 8, qualitative graph 800 shows the result when the same steps are applied in a third case (surround case) with a target in the FOV of the dToF sensor in the surround range. The N-1 remaining crosstalk signals are averaged together to generate an average crosstalk signal 860. In contrast to the first and second cases, the average crosstalk signal 860 has a surround signal 50 that needs to be removed. The first crosstalk signal 25 is subtracted from the average crosstalk signal 860 to obtain an estimated surround signal 862. Now, the first crosstalk signal 25 has been used as a baseline and does not include the surround signal 50. Therefore, the estimated surround signal 862 is now the average of all surround signals 50 in the N-1 remaining crosstalk signals. Subtracting the estimated surround signal 862 removes the surround signal 50 from each of the remaining crosstalk signals (i.e., corrected crosstalk contribution estimate) and again obtains the corrected crosstalk signal 64.

[0065] As demonstrated above, the crosstalk calibration process produces a desired corrected crosstalk signal 64 in all possible situations involving unknown targets in the FOV of the dToF sensor (e.g., in a production environment, such as on a production line). Thus, regardless of the conditions, the crosstalk calibration can advantageously be run even in an unknown setting. Optionally, averaging the N-1 remaining crosstalk signals, subtracting the first crosstalk signal, and subtracting the estimated surround signal can use only a subset of the multiple bins of each histogram, where each subset contains a corresponding one of the N crosstalk signals. As already discussed, the above method may also involve processing for removing or correcting background noise, such as subtracting (e.g., filtering out) a background signal.

[0066] The crosstalk calibration steps described herein can be implemented as a computer program stored in a dToF sensing device in which a dToF sensor is embedded. For example, in one embodiment, the computer program is firmware stored in the integrated memory of a microcontroller of a dToF sensor. In another embodiment, the computer program is software stored in the device memory of a dToF sensing device (e.g., a rangefinder, a ToF camera, a smart phone, a car collision detection module, or a 3D imaging system, etc.). A computer program implementing the above steps can be used prior to a typical crosstalk calibration process to ensure that samples of the crosstalk signal are corrected (e.g., without surround damage).

[0067] Fig. 9 An example dToF sensing device including a dToF sensor and a cover according to an embodiment of the present invention is illustrated. Figures 1 to 8 and Fig.11 Any of the methods described in the method for calibrating a dToF sensor to compensate for crosstalk can be accomplished by Fig. 9 The dToF sensing device performs the same as previously described.

[0068] Reference Fig. 9 , the dToF sensing device 900 includes a dToF sensor 10. The dToF sensor 10 may be a module that may be embedded in a system such as the dToF sensing device 900. For example, the dToF sensing device 900 may be a rangefinder, a ToF camera, a smart phone, a car collision detection module, or a 3D imaging system, among others. The dToF sensor 10 may include a microcontroller 98 having its own ROM 93 (read-only memory), RAM 94 (random access memory), and integrated memory 96. For example, program instructions that, when executed, enable the microcontroller 98 to perform the crosstalk calibration method described herein may be stored as firmware 91 in the integrated memory 96 of the microcontroller 98 of the dToF sensor.

[0069] The dToF sensing device 900 may also include its own memory 97 and processor 99. The dToF sensor 10 may be embedded in the dToF sensing device 900, and some or all of the program instructions may be stored as software 92 in the memory 97 of the dToF sensing device 900. The program instructions, when executed by the processor 99, may then enable the processor 99 to perform the method (e.g., directly or by causing the microcontroller 98 to perform steps). Of course, other configurations are possible and may be apparent to those skilled in the art.

[0070] As shown, the transmitter 13 of the dToF sensor 10 transmits a dToF pulse 22 that passes through the cover 19, which may be reflected by an object 70 in the FOV 90 of the dToF sensor 10. The resulting target signal 23 also passes through the cover 19 and is received by the receiver 15. When the dToF pulse 22 is reflected (e.g., does not pass through) the cover 19, a crosstalk signal 26 is generated. The receiver 15 also receives the crosstalk signal 26 (due to the distance difference, the signals do not arrive at the same time, but for clarity, both are shown).

[0071] As previously mentioned, the crosstalk calibration methods described herein may be advantageously performed even when an object 70 is within the FOV 90 of the dToF sensor 10 , when the dToF sensing device 900 is moving 72 , and / or in the presence of ambient light 74 .

[0072] Fig.10 Another example dToF device including a dToF sensor and a cover according to an embodiment of the present invention is illustrated, wherein the dToF device is on a production line. Any of the methods for calibrating a dToF sensor to compensate for crosstalk may be performed by Fig.10 The dToF sensing device performs, such as e.g. Figures 1 to 8 and Fig.11The methods described in . Like labeled elements may be as previously described.

[0073] Reference Fig.10 , conceptually illustrates a production environment 1001 that may include an object 70 and / or ambient light 74. A dToF sensing device 1000 is shown moving 72 on a production line 80. The dToF sensing device 1000 may advantageously be calibrated while on the production line 80. That is, the crosstalk calibration method described herein may be performed while the dToF sensor 10 in the dToF sensing device 1000 is moving 72 on the production line 80.

[0074] Fig.11 The method can be combined with other methods and performed using the systems and devices described herein. For example, Fig.11 The method can be used with Figures 1 to 10 Although shown in a logical order, Fig.11 The arrangement and numbering of the steps are not intended to be limiting. Fig.11 The method steps may be performed in any suitable order, or concurrently with each other, as would be apparent to a person skilled in the art.

[0075] Reference Fig.11 The method 1100 for calibrating a dToF sensor to compensate for crosstalk includes a step 1101 of transmitting N photon pulses from a transmitter of the dToF sensor and a step 1102 of receiving N crosstalk signals at a receiver of the dToF sensor, where N is the number of photon pulses. The N crosstalk signals include a first crosstalk signal and N-1 remaining crosstalk signals.

[0076] In step 1103, the N-1 remaining crosstalk signals are averaged to generate an average crosstalk signal. In step 1104, the first crosstalk signal is subtracted from the average crosstalk signal to generate an estimated surround signal. Step 1105 includes subtracting the estimated surround signal from each of the N-1 remaining crosstalk signals to obtain a corrected crosstalk signal. In step 1106, the dToF sensor is calibrated using the corrected crosstalk signal.

[0077] Optionally, one or more of the steps may be repeated in step 1107 (repeating surround correction). For example, emitting N photon pulses (1101), receiving N crosstalk signals (1102), averaging the N-1 remaining crosstalk signals (1103), subtracting the first crosstalk signal (1104), and subtracting the estimated surround signal (1105) may be performed multiple times when the dToF sensor is on a production line to generate multiple corrected crosstalk signals. Multiple corrected crosstalk signals may then be used to calibrate the dToF sensor. Alternatively, the entire calibration (step 1108) may be repeated and the results averaged.

[0078] Here, exemplary embodiments of the present invention are summarized. Other embodiments may also be understood from the entire specification and claims filed herein.

[0079] Example 1. A method for calibrating a direct time-of-flight (dToF) sensor to compensate for crosstalk, the method comprising: transmitting N photon pulses from a transmitter of the dToF sensor, N being the number of photon pulses; receiving N crosstalk signals including a first crosstalk signal and N-1 remaining crosstalk signals at a receiver of the dToF sensor; averaging the N-1 remaining crosstalk signals to generate an average crosstalk signal; subtracting the first crosstalk signal from the average crosstalk signal to generate an estimated surround signal; subtracting the estimated surround signal from each of the N-1 remaining crosstalk signals to obtain a corrected crosstalk signal; and calibrating the dToF sensor using the corrected crosstalk signal.

[0080] Example 2. The method of Example 1, wherein transmitting the N photon pulses and receiving the N crosstalk signals are performed within a time period of less than about 5 μs.

[0081] Example 3. The method of Example 2, wherein N is greater than about 100.

[0082] Example 4. A method according to one of Examples 1 to 3, wherein emitting the N photon pulses comprises emitting N photon pulses with a constant pulse period, the constant pulse period being a time period between consecutive photon pulses.

[0083] Example 5. The method of Example 4, wherein the constant pulse period is less than about 50 ns.

[0084] Example 6. The method of Example 5, wherein the constant pulse period is approximately 16 ns.

[0085] Example 7. The method of one of Examples 1 to 6, wherein each of the N photon pulses comprises a pulse width of less than about 5 ns.

[0086] Example 8. The method of Example 7, wherein the pulse width is about 2 ns.

[0087] Example 9. The method of one of Examples 1 to 8, wherein the method is performed while the dToF sensor is moving on a production line.

[0088] Example 10. The method of one of Examples 1 to 9, wherein the method is performed when an object is located in the field of view of the dToF sensor, the object generating a surround signal.

[0089] Example 11. A method according to one of Examples 1 to 10, wherein when the dToF sensor is on a production line, transmitting the N photon pulses, receiving the N crosstalk signals, averaging the N-1 remaining crosstalk signals, subtracting the first crosstalk signal, and subtracting the estimated surround signal are performed multiple times to generate multiple corrected crosstalk signals, and wherein calibrating the dToF sensor includes using the multiple corrected crosstalk signals.

[0090] Example 12. A method according to one of Examples 1 to 11, wherein receiving the N crosstalk signals includes storing each of the N crosstalk signals as a histogram comprising multiple bins, and wherein averaging the N-1 remaining crosstalk signals, subtracting the first crosstalk signal, and subtracting the estimated surround signal uses only a subset of the multiple bins of each histogram, each subset containing a corresponding one of the N crosstalk signals.

[0091] Example 13. The method of one of Examples 1 to 12, further comprising: subtracting a background signal before calibrating the dToF sensor.

[0092] Example 14. The method according to one of examples 1 to 13, wherein the method is performed in the presence of ambient light.

[0093] Example 15. The method according to one of Examples 1 to 14, wherein program instructions are stored as firmware in an integrated memory of a microcontroller of the dToF sensor, which program instructions, when executed by the microcontroller, enable the microcontroller to perform the method.

[0094] Example 16. A method according to one of Examples 1 to 15, wherein the dToF sensor is embedded in a device comprising a memory and a processor, and wherein the program instructions are stored as software in the memory of the device, which when executed by the microcontroller enables the processor to perform the method.

[0095] Example 17. A direct time-of-flight (dToF) sensing device, comprising: a dToF sensor, the dToF sensor including a transmitter configured to transmit N photon pulses and a receiver configured to receive N crosstalk signals, the N crosstalk signals including a first crosstalk signal and N-1 remaining crosstalk signals, N being the number of photon pulses; a processor, the processor coupled to the dToF sensor; and a memory, the memory storing program instructions, the memory coupled to the processor, the program instructions, when executed by the processor, enabling the processor to average the N-1 remaining crosstalk signals to generate an average crosstalk signal, subtract the first crosstalk signal from the average crosstalk signal to generate an estimated surround signal, subtract the estimated surround signal from each of the N-1 remaining crosstalk signals to obtain a corrected crosstalk signal, and calibrate the dToF sensor using the corrected crosstalk signal.

[0096] Example 18. The dToF sensing device of Example 17, wherein the processor is a microprocessor of the dToF sensing device, wherein the memory is an integrated memory of the microprocessor, and wherein the program instructions are stored as firmware in the integrated memory.

[0097] Example 19. The dToF sensing device according to one of Examples 17 and 18, wherein the dToF sensor is embedded in the dToF sensing device, wherein the memory is a device memory separate from the dToF sensor, and wherein the program instructions are stored as software in the device memory.

[0098] Example 20. The dToF sensing device of one of Examples 17 to 19, wherein the program instructions further enable the processor to cause the dToF sensor to transmit the N photon pulses and receive the N crosstalk signals within a time period of less than about 5 μs.

[0099] Example 21. A dToF sensing device according to Example 20, wherein N is greater than about 100.

[0100] Example 22. A dToF sensing device according to one of Examples 17 to 21, wherein the program instructions further enable the processor to cause the dToF sensor to emit the N photon pulses with a constant pulse period, the constant pulse period being a time period between consecutive photon pulses.

[0101] Example 23. A dToF sensing device according to one of Examples 17 to 22, wherein when the dToF sensing device is on a production line, the program instructions also enable the processor to cause the dToF sensor to transmit the N photon pulses, receive the N crosstalk signals, average the N-1 remaining crosstalk signals, subtract the first crosstalk signal, and subtract the estimated surround signal multiple times to generate multiple corrected crosstalk signals, and wherein calibrating the dToF sensor includes using the multiple corrected crosstalk signals.

[0102] Example 24. A direct time-of-flight (dToF) sensing device, comprising: a dToF sensor, the dToF sensor comprising a focal plane and a field of view disposed above the focal plane; a cover glass, the cover glass disposed above the focal plane; an emitter, the emitter disposed at the focal plane and configured to emit N photon pulses toward the cover glass to generate N crosstalk signals from a portion of each of the N photon pulses reflected by the cover glass, the N crosstalk signals comprising a first crosstalk signal and N-1 remaining crosstalk signals, N being the number of photon pulses; a receiver disposed at the focal plane and configured to receive the N crosstalk signals; a processor coupled to the dToF sensor, the processor configured to average the N-1 remaining crosstalk signals to generate an average crosstalk signal, subtract the first crosstalk signal from the average crosstalk signal to generate an estimated surround signal, subtract the estimated surround signal from each of the N-1 remaining crosstalk signals to obtain a corrected crosstalk signal, and calibrate the dToF sensor using the corrected crosstalk signal.

[0103] Example 25. The dToF sensing device of Example 24, wherein the transmitter is configured to transmit the N photon pulses while the dToF sensing device is moving on a production line.

[0104] Example 26. The dToF sensing device of one of Examples 24 and 25, wherein the transmitter is configured to transmit the N photon pulses when an object is located in the field of view of the dToF sensor, the object generating a surround signal.

[0105] Example 27. A dToF sensing device according to one of Examples 24 to 26, wherein the transmitter is configured to transmit the N photon pulses in the presence of ambient light.

[0106] Although the present invention has been described with reference to exemplary embodiments, this description is not intended to be interpreted as having a limiting meaning. By reference to the description, various modifications and combinations of the exemplary embodiments and other embodiments of the present invention will be apparent to those skilled in the art. Therefore, the appended claims are intended to cover any such modifications or embodiments.

Claims

1. A method of calibrating a direct time of flight (dToF) sensor to compensate for crosstalk, the method comprising: emitting N photon pulses from a transmitter of the dToF sensor, N being the number of the photon pulses; At a receiver of the dToF sensor, receiving N crosstalk signals, the N crosstalk signals including a first crosstalk signal and N-1 remaining crosstalk signals; Averaging the remaining N-1 crosstalk signals to generate an average crosstalk signal; subtracting the first crosstalk signal from the average crosstalk signal to generate an estimated surround signal; subtracting the estimated surround signal from each of the N-1 remaining crosstalk signals to obtain a corrected crosstalk signal; as well as The dToF sensor is calibrated using the corrected crosstalk signal. 2 . The method of claim 1 , wherein transmitting the N photon pulses and receiving the N crosstalk signals are performed within a time period of less than about 5 μs.

3. The method of claim 2, wherein N is greater than about 100. 4 . The method of claim 1 , wherein emitting the N photon pulses comprises emitting N photon pulses with a constant pulse period, the constant pulse period being a time period between consecutive photon pulses. The method of claim 1 , wherein the method is performed while the dToF sensor is moving on a production line. 6 . The method of claim 1 , wherein the method is performed when an object is located in the field of view of the dToF sensor, the object generating a surround signal.

7. The method of claim 1 , wherein when the dToF sensor is on a production line, the following operations are performed multiple times: emitting the N photon pulses, receiving the N crosstalk signals, averaging the N-1 remaining crosstalk signals, subtracting the first crosstalk signal, and subtracting the estimated surround signal to generate a plurality of corrected crosstalk signals, and wherein calibrating the dToF sensor includes using the plurality of corrected crosstalk signals.

8. The method of claim 1 , wherein receiving the N crosstalk signals comprises storing each of the N crosstalk signals as a histogram, the histogram comprising a plurality of bins, and wherein averaging the N-1 remaining crosstalk signals, subtracting the first crosstalk signal, and subtracting the estimated surround signal uses only a subset of the plurality of bins of each histogram, each subset containing a corresponding one of the N crosstalk signals.

9. The method of claim 1, wherein the method is performed in the presence of ambient light.

10. A direct time-of-flight (dToF) sensing device, comprising: A dToF sensor, the dToF sensor comprising a transmitter configured to transmit N photon pulses and a receiver configured to receive N crosstalk signals, the N crosstalk signals comprising a first crosstalk signal and N-1 remaining crosstalk signals, N being the number of photon pulses; a processor coupled to the dToF sensor; as well as A memory storing program instructions, the memory being coupled to the processor, the program instructions enabling the processor to execute the program instructions when executed by the processor. Averaging the remaining N-1 crosstalk signals to generate an average crosstalk signal, subtracting the first crosstalk signal from the average crosstalk signal to generate an estimated surround signal, subtracting the estimated surround signal from each of the N-1 remaining crosstalk signals to obtain a corrected crosstalk signal, and The dToF sensor is calibrated using the corrected crosstalk signal.

11. The dToF sensing device according to claim 10, wherein the processor is a microprocessor of the dToF sensing device, wherein the memory is an integrated memory of the microprocessor, and The program instructions are stored as firmware in the integrated memory.

12. The dToF sensing device according to claim 10, wherein the dToF sensor is embedded in the dToF sensing device, wherein the memory is a device memory separate from the dToF sensor, and The program instructions are stored as software in the device memory.

13. The dToF sensing device of claim 10, wherein the program instructions further enable the processor to cause the dToF sensor to transmit the N photon pulses and receive the N crosstalk signals within a time period of less than approximately 5 μs. The dToF sensing device of claim 13 , wherein N is greater than about 100.

15. The dToF sensing device of claim 10, wherein the program instructions further enable the processor to cause the dToF sensor to emit the N photon pulses with a constant pulse period, the constant pulse period being a time period between consecutive photon pulses.

16. The dToF sensing device of claim 10, wherein when the dToF sensing device is on a production line, the program instructions further enable the processor to cause the dToF sensor to perform the following operations multiple times: emitting the N photon pulses, receiving the N crosstalk signals, averaging the N-1 remaining crosstalk signals, subtracting the first crosstalk signal, and subtracting the estimated surround signal to generate a plurality of corrected crosstalk signals, and wherein calibrating the dToF sensor includes using the plurality of corrected crosstalk signals.

17. A direct time-of-flight (dToF) sensing device comprising: a dToF sensor comprising a focal plane and a field of view disposed above the focal plane; a cover glass disposed above the focal plane; an emitter disposed at the focal plane and configured to emit N photon pulses toward the cover glass to generate N crosstalk signals from a portion of each of the N photon pulses reflected by the cover glass, the N crosstalk signals including a first crosstalk signal and N-1 remaining crosstalk signals, where N is the number of the photon pulses; a receiver disposed at the focal plane and configured to receive the N crosstalk signals; as well as a processor coupled to the dToF sensor, the processor being configured to Averaging the remaining N-1 crosstalk signals to generate an average crosstalk signal, subtracting the first crosstalk signal from the average crosstalk signal to generate an estimated surround signal, subtracting the estimated surround signal from each of the N-1 remaining crosstalk signals to obtain a corrected crosstalk signal, and The dToF sensor is calibrated using the corrected crosstalk signal.

18. The dToF sensing device of claim 17, wherein the transmitter is configured to transmit the N photon pulses while the dToF sensing device moves on a production line.

19. The dToF sensing device of claim 17, wherein the transmitter is configured to transmit the N photon pulses when an object is located in a field of view of the dToF sensor, the object generating a surround signal.

20. The dToF sensing device of claim 17, wherein the transmitter is configured to transmit the N photon pulses in the presence of ambient light.

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