Patent Description:
Existing and emerging consumer applications have created an increasing need for real-time three-dimensional (3D) imagers. These imaging devices, also known as depth sensors, depth mappers, or light detection and ranging (LiDAR) sensors, enable the remote measurement of distance (and often intensity) to each point in a target scene - referred to as target scene depth - by illuminating the target scene with an optical beam and analyzing the reflected optical signal. A commonly-used technique to determine the distance to each point on the target scene involves transmitting one or more pulsed optical beams towards the target scene, followed by the measurement of the round-trip time, i.e. time-of-flight (ToF), taken by the optical beams as they travel from the source to the target scene and back to a detector array adjacent to the source.

Some ToF systems use single-photon avalanche diodes (SPADs), also known as Geiger-mode avalanche photodiodes (GAPDs), in measuring photon arrival time. For example, <CIT>, describes a sensing device that includes an array of SPAD sensing elements. Each sensing element includes a photodiode, including a p-n junction, and a local biasing circuit, which is coupled to reverse-bias the p-n junction at a bias voltage greater than the breakdown voltage of the p-n junction by a margin sufficient so that a single photon incident on the p-n junction triggers an avalanche pulse output from the sensing element. A bias control circuit is coupled to set the bias voltage in different ones of the sensing elements to different, respective values.

<CIT>, describes the use of this sort of variable biasing capability in selectively actuating individual sensing elements or groups of sensing elements in a SPAD array. For this purpose, an electro-optical device includes a laser light source, which emits at least one beam of light pulses, a beam steering device, which transmits and scans the at least one beam across a target scene, and an array of sensing elements. Each sensing element outputs a signal indicative of a time of incidence of a single photon on the sensing element. (Each sensing element in such an array is also referred to as a "pixel. ") Light collection optics image the target scene scanned by the transmitted beam onto the array. Circuitry is coupled to actuate the sensing elements only in a selected region of the array and to sweep the selected region over the array in synchronization with scanning of the at least one beam.

Embodiments of the present invention that are described hereinbelow provide improved depth mapping systems and methods for operating such systems.

There is therefore provided, in accordance with the invention, depth sensing apparatus, including a radiation source, which includes a first array of emitters arranged in multiple banks, which are configured to emit a first plurality of pulsed beams of optical radiation toward a target scene. A second plurality of sensing elements are arranged in a second array and are configured to output signals indicative of respective times of incidence of photons on the sensing elements, wherein the second plurality exceeds the first plurality. Objective optics are configured to form an image of the target scene on the array of sensing elements. Processing and control circuitry is coupled to actuate the multiple banks in alternation to emit the pulsed beams and is coupled to receive the signals from the sensing elements, and is configured to identify, responsively to the signals, areas of the second array on which the pulses of optical radiation reflected from corresponding regions of the target scene are incident, and to process the signals from the sensing elements in the identified areas in order measure depth coordinates of the corresponding regions of the target scene based on the times of incidence.

In a disclosed embodiment, the emitters in the array include vertical-cavity surface-emitting lasers (VCSELs).

In some embodiments, the multiple banks include at least four banks, each bank containing at least four of the emitters. In one embodiment, the radiation source includes a diffractive optical element (DOE) which is configured to split the optical radiation emitted by each of the emitters into multiple ones of the pulsed beams. Additionally or alternatively, the at least four banks include at least eight banks. Each bank may contain at least twenty of the emitters. In a disclosed embodiment, the banks of emitters are interleaved on a substrate.

Additionally or alternatively, the sensing elements include single-photon avalanche diodes (SPADs).

In some embodiments, the processing and control circuitry is configured to group the sensing elements in each of the identified areas together to define super-pixels, and to process together the signals from the sensing elements in each of the super-pixels in order to measure the depth coordinates. In a disclosed embodiment, the processing and control circuitry includes multiple processing units, wherein each of the processing units is coupled to process the signals from a respective one of the super-pixels. Additionally or alternatively, each of the processing units is configured to construct a histogram of the times of incidence of the photons on the sensing elements in each of the super-pixels.

In some embodiments, the pulses of the optical radiation emitted from the multiple banks of the emitters are incident, after reflection from the target scene, on different, respective sets of the identified areas of the second array, and the processing and control circuitry is configured to receive and process the signals from the sensing elements in the respective sets in synchronization with actuation of the multiple banks. In the disclosed embodiments, when any given bank of the emitters is actuated, the processing and control circuitry is configured to read and process the signals only from the sensing elements in a corresponding set of the identified areas of the second array, while the remaining sensing elements in the array are inactive.

There is also provided, in accordance with an embodiment of the invention, a method for depth sensing, which includes driving a first array of emitters arranged in multiple banks to emit a first plurality of pulsed beams of optical radiation toward a target scene, while actuating the multiple banks in alternation to emit the pulsed beams. An image of the target scene is formed on a second plurality of sensing elements, which are arranged in a second array and are configured to output signals indicative of respective times of incidence of photons on the sensing elements, wherein the second plurality exceeds the first plurality. Responsively to the signals from the sensing elements, areas of the second array are identified on which the pulses of optical radiation reflected from corresponding regions of the target scene are incident. The signals from the sensing elements in the identified areas are processed in order measure depth coordinates of the corresponding regions of the target scene based on the times of incidence.

In some of the embodiments described in the above-mentioned <CIT>, SPADs are grouped together into "super-pixels," wherein the term "super-pixel" refers to a group of mutually-adjacent pixels along with data processing elements that are coupled directly to these pixels. At any instant during operation of the system, only the sensing elements in the area or areas of the array that are to receive reflected illumination from a beam are actuated, for example by appropriate biasing of the SPADs in selected super-pixels, while the remaining sensing elements are inactive. The sensing elements are thus actuated only when their signals provide useful information. This approach reduces the background signal, thus enhancing the signal-to-background ratio, and lowers both the electrical power needs of the detector array and the number of data processing units that must be attached to the SPAD array.

One issue to be resolved in a depth mapping system of this sort is the sizes and locations of the super-pixels to be used. For accurate depth mapping, with high signal/background ratio, it is important that the super-pixels contain the detector elements onto which most of the energy of the reflected beams is imaged, while the sensing elements that do not receive reflected beams remain inactive. Even when a static array of emitters is used, however, the locations of the reflected beams on the detector array can change, for example due to thermal and mechanical changes over time, as well as optical effects, such as parallax.

In response to this problem, some embodiments of the present invention provide methods for calibrating the locations of the laser spots on the SPAD array. For this purpose, processing and control circuitry receives timing signals from the array and searches over the sensing elements in order to identify the respective regions of the array on which the light pulses reflected from the target scene are incident. Detailed knowledge of the depth mapping system is used in order to pre-compute likely regions of the reflected laser spots to be imaged onto the SPAD array. A random search in these regions will converge rapidly to the correct locations of the laser spots on the array. Alternatively or additionally, a small subset of the locations of laser spots can be identified in an initialization stage. These locations can be used in subsequent iterative stages to predict and verify the positions of further laser spots until a sufficient number of laser spots have been located.

Even following meticulous calibration, it can occur in operation of the depth mapping system that some of the pixels or super-pixels on which laser spots are expected to be imaged fail to output usable timing signals. In some cases, ancillary image data can be used to identify areas of the scene that are problematic in terms of depth mapping, and to recalibrate the super-pixel locations when necessary. This ancillary image data can be provided, for example, by a color image sensor, which captures two-dimensional (2D) images in registration with the SPAD array.

The emitter arrays used in the embodiments described below are "sparse," in the sense that the number of pulsed beams of optical radiation that are emitted toward a target scene is substantially less than the number of pixels (i.e., SPADs or other sensing elements) in the array that receives the radiation reflected from the scene. The illumination power available from the emitter array is projected onto a correspondingly sparse grid of spots in the scene. The processing and control circuitry in the apparatus then receives and processes signals only from the pixels onto which these spots are imaged in order to measure depth coordinates.

The pixels onto which the spots are imaged are referred to herein as the "active pixels," and the "super-pixels" are made up of groups of adjacent active pixels, for example 2x2 groups. The pixels in the array that fall between the active pixels are ignored, and need not be actuated or read out at all, as they do not contribute to the depth measurement and only increase the background level and noise. Alternatively, a different number, such as one, two, three or more pixels, may be included in a super-pixel. Furthermore, although the embodiments described herein relate specifically to rectangular super-pixels, the group of SPAD pixels in a super-pixel may have a different shape, such as, for example, diamond shape, triangular, circular, or irregular. The exact location of the spot within the SPAD pixels varies slightly depending on the distance to the scene due to a small amount of parallax. At any given time, the signals from the SPAD pixels of the super-pixel are processed together in measuring for a given laser spot both its strength (intensity) and its time of flight. Additionally, the signals from the SPAD pixels may be processed as individual signals for determining the location of the laser spot within the super-pixel.

An advantage of using a sparse emitter array is that the available power budget can be concentrated in the small number of projected spots, rather than being spread over the entire field of view of the sensing array. As a result of this concentration of optical power in a small number of spots, the signal levels from the corresponding active pixels - and thus the accuracy of ToF measurement by these pixels - are enhanced. This signal enhancement is particularly beneficial for long-range depth measurements and for depth mapping in conditions of strong ambient light, such as outdoors.

The concentration of optical power in a sparse array of spots can be further enhanced by arranging the emitters in multiple banks, and actuating these banks in alternation. The laser beams generated by the emitters are typically collimated by a collimating lens and may be replicated by a diffractive optical element (DOE) in order to increase the number of projected spots. The pulses of optical radiation emitted from the different banks of the emitters are incident, after reflection from the target scene, on different, respective sets of the active pixels. The processing and control circuitry can then receive and process the signals from the active pixels in these respective sets in synchronization with actuation of the corresponding banks of emitters. Thus, during any given period in the operation of the apparatus, the processing and control circuitry need receive and process the signals only from one active set of sensing elements, while all other sets remain inactive. This sort of multi-bank, synchronized operation makes it possible to time-multiplex processing resources among the different sets of sensing elements, and thus reduce circuit complexity and power consumption.

Because the spots reflected from the target scene are imaged sparsely onto the SPAD array, the number of possible super-pixels is much larger than the number of laser spots, and only a small fraction of the total number of pixels in the SPAD array should be active at any given time and coupled to a processing unit for the purpose of measuring time-of-flight. Therefore, information is required as to which of the SPAD super-pixels to activate at any given time.

A mapping of SPAD pixels to processing units, i.e., the assignment of SPAD pixels to super-pixels, may be determined initially during a factory calibration. However, temperature changes during operation, as well as mechanical shocks, may alter the mechanical parameters of the mapping, thus modifying the positions of the laser spots on the SPAD array and necessitating recalibration during operation in the field. An exhaustive search could be used to determine which of the SPAD pixels to connect to the processing units, wherein all pixels are searched to detect laser spots; but this approach suffers from at least two basic problems:.

The embodiments of the present invention that are described herein address these problems by providing improved methods for calibrating the locations of the laser spots on the SPAD array. These methods can be applied not only in the sorts of arrays that are shown in the figures and described hereinbelow, but also in other SPAD-based systems, such as systems comprising multiple banks of SPADs, as well as SPADs of various sizes, and systems using various sorts of emitters and emitter arrays, including emitters whose beams are replicated by a DOE. The present methods can then be extended, mutatis mutandis, to multi-bank systems, by activating the SPAD pixels and performing the calibration bank by bank.

According to the invention, detailed knowledge of the depth mapping system is utilized to pre-compute likely regions of the reflected laser spots to be imaged onto the SPAD array. A search in these regions, for example a random search, will converge rapidly to the correct locations of the laser spots on the array.

Another aspect of the invention uses a two-stage solution: in an initialization stage, a small subset of the locations of laser spots is identified, and in a subsequent iterative stage, the positions of further laser spots are predicted by a model and verified. Iterative steps of spot detection are utilized to refine the model and add locations, until a sufficient number of laser spots have been located.

<FIG> is a schematic side view of a depth mapping system <NUM>, in accordance with an embodiment of the invention. Depth mapping system <NUM> comprises a radiation source <NUM>, which emits M individual beams (for example, M may be on the order of <NUM>). The radiation source comprises multiple banks of emitters arranged in a two-dimensional array <NUM> (as shown in detail in <FIG>), together with beam optics <NUM>. The emitters typically comprises solid-state devices, such as vertical-cavity surface-emission lasers (VCSELs) or other sorts of lasers or light-emitting diodes (LEDs). The beam optics typically comprise a collimating lens and may comprise a diffractive optical element (DOE, not shown), which replicates the actual beams emitted by array <NUM> to create the M beams that are projected onto the scene <NUM>. (For example, an array of four banks of pixels with <NUM> VCSELs in a <NUM> X <NUM> arrangement in each bank may be used to create <NUM> X <NUM> beams, and a DOE may split each beam into <NUM> X <NUM> replicas to give a total of <NUM> X <NUM> beams. ) For the sake of simplicity, these internal elements of beam optics <NUM> are not shown.

A receiver <NUM> in system <NUM> comprises a two-dimensional SPAD array <NUM>, together with J processing units <NUM> and select lines <NUM> for coupling the processing units to the SPADs, along with a combining unit <NUM> and a controller <NUM>. SPAD array <NUM> comprises a number of detector elements N that is much larger than M, for example, 100X100 pixels or 200X200 pixels. The number J of processing units <NUM> depends on the number of pixels of SPAD array <NUM> to which each processing unit is coupled, as will be further described with reference to <FIG>.

Array <NUM>, together with beam optics <NUM>, emits M pulsed beams of light <NUM> towards a target scene <NUM>. Although beams <NUM> are depicted in <FIG> as parallel beams of constant width, each beam diverges as dictated by diffraction. Furthermore, beams <NUM> diverge from each other so as to cover a required area of scene <NUM>. Scene <NUM> reflects or otherwise scatters those beams <NUM> that impinge on the scene. The reflected and scattered beams are collected by objective optics <NUM>, represented by a lens in <FIG>, which form an image of scene <NUM> on array <NUM>. Thus, for example, a small region <NUM> on scene <NUM>, on which a beam 30a has impinged, is imaged onto a small area <NUM> on SPAD array <NUM>.

A Cartesian coordinate system <NUM> defines the orientation of depth mapping system <NUM> and scene <NUM>. The x-axis and the y-axis are oriented in the plane of SPAD array <NUM>. The z-axis is perpendicular to the array and points to scene <NUM> that is imaged onto SPAD array <NUM>.

For clarity, processing units <NUM> are shown as if separate from SPAD array <NUM>, but they are commonly integrated with the SPAD array. Similarly, combining unit <NUM> is commonly integrated with SPAD array <NUM>. Processing units <NUM>, together with combining unit <NUM>, comprise hardware amplification and logic circuits, which sense and record pulses output by the SPADs in respective super-pixels, and thus measure the times of arrival of the photons that gave rise to the pulses, as well as the strengths of the optical pulses impinging on SPAD array <NUM>.

As further described below in reference to <FIG>, processing units <NUM> together with combining unit <NUM> may assemble histograms of the times of arrival of multiple pulses emitted by array <NUM>, and thus output signals that are indicative of the distance to respective points in scene <NUM>, as well as of signal strength. Circuitry that can be used for this purpose is described, for example, in the above-mentioned <CIT>. Alternatively or additionally, some or all of the components of processing units <NUM> and combining unit <NUM> may be separate from SPAD array <NUM> and may, for example, be integrated with controller <NUM>. For the sake of generality, controller <NUM>, processing units <NUM> and combining unit <NUM> are collectively referred to herein as "processing and control circuitry.

Controller <NUM> is coupled to both radiation source <NUM> and receiver <NUM>. Controller <NUM> actuates the banks of emitters in array <NUM> in alternation to emit the pulsed beams. The controller also provides to the processing and combining units in receiver <NUM> an external control signal <NUM>, and receives output signals from the processing and combining units. The output signals may comprise histogram data, and may be used by controller <NUM> to derive both times of incidence and signal strengths, as well as a precise location of each laser spot that is imaged onto SPAD array <NUM>.

To make optimal use of the available sensing and processing resources, controller <NUM> identifies the respective areas of SPAD array <NUM> on which the pulses of optical radiation reflected from corresponding regions of target scene <NUM> are imaged by lens <NUM>, and chooses the super-pixels to correspond to these areas. The signals output by sensing elements outside these areas are not used, and these sensing elements may thus be deactivated, for example by reducing or turning off the bias voltage to these sensing elements. Methods for choosing the super-pixels initially and for verifying and updating the selection of super-pixels are described, for example, in the above-mentioned provisional patent applications.

External control signal <NUM> controls select lines <NUM> so that each processing unit <NUM> is coupled to a respective super-pixel, comprising four SPAD pixels, for example. The control signal selects the super-pixels from which the output signals are to be received in synchronization with the actuation of the corresponding banks of emitters. Thus, at any given time, processing units <NUM> and combining unit <NUM> read and process the signals only from the sensing elements in the areas of SPAD array <NUM> that receive the reflected pulses from scene <NUM>, while the remaining sensing elements in the array are inactive. The processing of the signals from SPAD array <NUM> is further described in reference to <FIG>. For the sake of simplicity, the detailed structures of emitter array <NUM> and SPAD array <NUM> are not shown in <FIG>.

For clarity, the dimensions of emitter array <NUM> and SPAD array <NUM> have been exaggerated in <FIG> relative to scene <NUM>. The lateral separation of emitter array <NUM> and SPAD array <NUM>, referred to as the "baseline," is in reality much smaller than the distance from emitter array <NUM> to scene <NUM>. Consequently a chief ray <NUM> (a ray passing through the center of objective optics <NUM>) from scene <NUM> to SPAD array <NUM> is nearly parallel to rays <NUM>, leading to only a small amount of parallax.

Of those M super-pixels that are activated and coupled to the J processing units <NUM>, either all of them or a subset of m super-pixels, wherein m ≤ M, will receive a reflected laser beam <NUM>. The magnitude of m depends on two factors:.

Even if all M laser beams <NUM> were to be reflected from scene <NUM>, m will be less than M if SPAD array <NUM> is not properly calibrated. (Calibration procedures described in the above-mentioned provisional patent applications can be used to maximize m. ) Consequently, controller <NUM> will receive signals indicating times of arrival and signal strengths from only m processing units <NUM>. Controller <NUM> calculates from the timing of the emission of beams <NUM> by VCSEL array <NUM> and from the times of arrival measured by the m processing units <NUM> the time-of-flight of the m beams, and thus maps the distance to the corresponding m points on scene <NUM>.

Controller <NUM> typically comprises a programmable processor, which is programmed in software and/or firmware to carry out the functions that are described herein. Alternatively or additionally, controller <NUM> comprises hard-wired and/or programmable hardware logic circuits, which carry out at least some of the functions of the controller. Although controller <NUM> is shown in the figure, for the sake of simplicity, as a single, monolithic functional block, in practice the controller may comprise a single chip or a set of two or more chips, with suitable interfaces for receiving and outputting the signals that are illustrated in the figure and are described in the text.

One of the functional units of controller <NUM> is a depth processing unit (DPU) <NUM>, which processes signals from both processing units <NUM> and combining unit <NUM>, as will be further described below. DPU <NUM> calculates the times of flight of the photons in each of beams <NUM>, and thus maps the distance to the corresponding points in target scene <NUM>. This mapping is based on the timing of the emission of beams <NUM> by emitter array <NUM> and from the times of arrival (i.e., times of incidence of reflected photons) measured by processing units <NUM>. Controller <NUM> typically stores the depth coordinates in a memory, and may output the corresponding depth map for display and/or further processing.

<FIG> is a schematic side view of radiation source <NUM>, in accordance with an embodiment of the invention. VCSEL array <NUM> comprises an integrated circuit chip on which multiple banks of VCSELs are formed (as shown in <FIG>, for example). The VCSELs emit respective beams <NUM> toward optics <NUM>, which collimate and project the beams toward the target scene. Optics <NUM> optionally comprise a diffractive optical element (DOE), which splits the optical radiation emitted by each of the VCSELs into multiple beams <NUM>, for example a 3x3 array of beams.

To enable selection and switching among the different banks, array <NUM> is mounted on a driver chip <NUM>, for example, a silicon chip with CMOS circuits for selecting and driving the individual VCSELs or banks of VCSELs. The banks of VCSELS in this case may be physically separated, for ease of fabrication and control, or they may be interleaved on the VCSEL chip, with suitable connections to driver chip <NUM> to enable actuating the banks in alternation. Thus, beams <NUM> likewise irradiate the target scene in a time-multiplexed pattern, with different sets of the beams impinging on the respective regions of the scene at different times.

<FIG> is a schematic frontal view of array <NUM> used in beam source <NUM>, in accordance with an embodiment of the invention. Array <NUM> in this example comprises eight banks <NUM>, with seventy-two emitters <NUM>, such as VCSELs, in each bank. In this case, array <NUM> generates <NUM> beams.

<FIG> is a schematic frontal view of an array <NUM> of vertical emitters <NUM> that can be used in beam source <NUM> in place of array <NUM>, in accordance with another embodiment of the invention. In this case four banks 62a, 62b, 62c and 62d of emitters <NUM> are interleaved as alternating vertical stripes on a substrate <NUM>, such as a semiconductor chip: Each bank comprises a number of stripes that alternate on the substrate with the stripes in the other banks. Alternatively, other interleaving schemes may be used.

As further alternatives to the pictured embodiments, array <NUM> may comprise a larger or smaller number of banks and emitters. Typically, for sufficient coverage of the target scenes with static (non-scanned) beams, array <NUM> comprises at least four banks <NUM> or <NUM>, with at least four emitters <NUM> in each bank, and possibly with a DOE for splitting the radiation emitted by each of the emitters. For denser coverage, array <NUM> comprises at least eight banks <NUM> or <NUM>, with twenty emitters <NUM> or more in each bank. These options enhance the flexibility of system <NUM> in terms of time-multiplexing of the optical and electrical power budgets, as well as processing resources.

<FIG> is a schematic representation of a pattern of spots <NUM> of optical radiation that are projected onto target scene <NUM>, in accordance with an embodiment of the invention. Each spot <NUM> is cast by a corresponding beam <NUM> (<FIG>). In the present embodiments, different groups of spots <NUM> are projected onto scene <NUM> in alternation, corresponding to the alternating actuation of the corresponding banks <NUM> of emitters <NUM> (<FIG>).

<FIG> is a schematic frontal view of SPAD array <NUM> onto which target scene <NUM> is imaged, in accordance with an embodiment of the invention. The sensing elements, such as SPADs, in array <NUM> are too small to be seen in this figure. Rather, <FIG> shows the locations of spots <NUM> that are reflected from target scene <NUM> and imaged onto array <NUM> by lens <NUM>. In other words, each spot <NUM> is the image on array <NUM> of a corresponding spot <NUM> that is projected onto scene <NUM> by emitter array <NUM>. Lens <NUM> images a region <NUM> of target scene <NUM> (<FIG>), including spots <NUM> that the area contains, onto a corresponding area <NUM> on array <NUM>.

<FIG> is a schematic detail view of area <NUM> of array <NUM>, showing the locations of spots <NUM> that are imaged onto the array, in accordance with an embodiment of the invention. These spots <NUM> may be imaged at the same time, if they originate from the same bank of emitters, or at different, alternating times if they are from different banks. In this view shown in <FIG>, it can be seen that array <NUM> comprises a matrix of sensing elements <NUM>, such as SPADs. (As noted earlier, sensing elements <NUM> in an array are also referred to as "pixels. ") Controller <NUM> assigns each processing unit <NUM> to a super-pixel <NUM> comprising a 2x2 group of the sensing elements <NUM>. In this example, it is assumed that during an initial calibration stage, spots <NUM> were imaged onto array <NUM> at locations 72a. Controller <NUM> thus selected the sensing elements <NUM> to assign to each super-pixel <NUM> so as to maximize the overlap between the corresponding spot <NUM> and the super-pixel, and thus maximize the signal received from each super-pixel.

At some later stage, however, spots <NUM> shifted to new locations 72b on array <NUM>. This shift may have occurred, for example, due to mechanical shock or thermal effects in imaging device <NUM>, or due to other causes. Spots <NUM> at locations 72b no longer overlap with super-pixels <NUM> in area <NUM>, or overlap only minimally with the super-pixels. Sensing elements <NUM> on which the spots are now imaged, however, are inactive and are not connected to any of processing units <NUM>. To rectify this situation, controller <NUM> may recalibrate the locations of super-pixels <NUM>, as described in the above-mentioned provisional patent applications.

<FIG> are schematic frontal views of SPAD array <NUM> showing sets of super-pixels 80a and 80b that are selected for activation and readout in two different time periods, in accordance with an embodiment of the invention. In the embodiment, the selection of the set of super-pixels is synchronized with the selection of banks of emitters. Specifically, assuming array <NUM> (<FIG>) is used for generating the spots on the target scene, super-pixels 80a will be used when bank 62a is actuated, and super-pixels 80b will be used when bank 62b is actuated (and so forth for banks 62c and 62d). Thus, during any given period in the operation of the system <NUM>, processing units <NUM> serve only the one active set of super-pixels <NUM>, while all other sets remain inactive, in an integrated time-multiplexing scheme.

<FIG> is a block diagram that schematically illustrates the processing of signals from a super-pixel <NUM>, in accordance with an embodiment of the invention. In the pictured embodiment, super-pixel <NUM> comprises four sensing elements <NUM> of SPAD array <NUM>. Each processing unit <NUM> comprises one or more time-to-digital converters (TDC) <NUM>, wherein the TDCs are hardware elements translating the avalanche events (signals from a SPAD pixel due to a detected photon) from each sensing element <NUM> to time-of-arrival information. Each processing unit <NUM> further comprises, for each TDC <NUM>, a weight <NUM>, and may comprise a histogram unit (not shown), wherein the time-of-arrival information is aggregated into histograms, generally over thousands of pulses from VCSEL array <NUM>. In the present embodiment, however, the histograms are aggregated centrally for super-pixel, without individual histogram units in processing units <NUM>.

In <FIG>, each processing unit <NUM> is coupled to a single sensing element <NUM>, thus requiring one TDC <NUM>. Alternatively, processing unit <NUM> may be coupled to two or more sensing elements <NUM>, and will then comprise a number of TDC's <NUM> equal to the number of pixels. For example, if each processing unit <NUM> is coupled to four SPAD pixels <NUM>, the number of TDCs <NUM> per processing unit will be four, and J=<NUM>*M. Additionally or alternatively, as noted earlier, processing units <NUM> may be switched among different sensing elements <NUM>, which are activated at different, alternating times, in synchronization with the alternating actuation of the corresponding banks <NUM> of emitters <NUM> (<FIG>).

The time-of-arrival information from the four processing units <NUM> is aggregated by combining unit <NUM>, using weights <NUM>, to yield a single histogram <NUM> for super-pixel <NUM>. This combined histogram <NUM> is sent to DPU <NUM>, which in turn detects, based on histogram <NUM>, whether any object or structure was detected in scene <NUM> by super-pixel <NUM> and, if so, reports its depth information based on time-of-flight data.

Additionally, the respective numbers of events reported by the four processing units <NUM> may be separately summed in combining unit <NUM> over a predefined interval of arrival times to yield an indication of the received signal strength for that interval for each sensing element <NUM>. Typically the interval is configured to start after the end of a so-called "stray pulse" and continue to the end of the histogram. A stray pulse is a pulse that is generated within system <NUM> as a result of, for example, an imperfect coating of an optical surface, which causes a reflection of the pulses emitted by VCSEL array <NUM> directly back into the optical path to SPAD array <NUM>. It is typically an undesired pulse, but one that is very difficult to eliminate altogether. The stray pulse may be utilized for calibrating the timing signals as follows: A time of arrival of a stray pulse is recorded and subtracted from a subsequent timing signal due to a laser pulse that has been reflected by scene <NUM>. This subtraction yields a relative time-of-flight for the received laser pulse, and compensates for any random firing delays of VCSEL array <NUM>, as well as for most of the VCSEL and SPAD drifts related to temperature changes.

These four indicators of signal strength are also transferred to DPU <NUM> (in conjunction with combined histogram <NUM>). The indicators may be used by DPU <NUM> to determine a precise location of the spot on sensing elements <NUM>.

In some embodiments, the four units of TDC <NUM>, as well as combining unit <NUM>, reside in the same chip as SPAD array <NUM>, while the rest of signal processing, including DPU <NUM>, resides in separate controller <NUM>. A major reason for generating single combined histogram <NUM> for super-pixel <NUM> is to reduce the information that is transferred from SPAD array <NUM> to DPU <NUM> and to controller <NUM>. The partitioning into two separate units reflects the fact that SPAD array <NUM> and the associated units perform primarily optical and analog functions, while controller <NUM> performs mostly digital and software-driven operations.

<FIG> is a flowchart that schematically illustrates a method for identifying the pixels in a sensing array that receive laser spots, in accordance with the invention. This method is described, for the sake of convenience and clarity, with reference to SPAD array <NUM> and the other elements of system <NUM> (<FIG>). The method can be performed, for example, each time system <NUM> is turned on. Alternatively, the method can be carried out prior to an initial use of system <NUM>, and the results can be stored for future use. The method can then be repeated periodically and/or when system performance indicates that recalibration may be required.

Alternatively, however, the principles of this method may be applied, mutatis mutandis, in other depth mapping systems of similar configuration. For example, VCSEL array <NUM> could be replaced by a single laser (or a small number of lasers), with a beamsplitting element, such as a diffractive optical element (DOE), to split the laser output into multiple beams. As another example, other types of sensing arrays, comprising other sorts of detector elements, could be used in place of SPADs. The method of <FIG>, as described below, is similarly applicable not only to system <NUM>, but to other depth mapping systems, as well.

In the method of <FIG>, three input steps: a nominal design value step <NUM>, an assembly tolerance step <NUM>, and an operational tolerance step <NUM>, provide inputs for a pre-computation step <NUM>. Design value step <NUM> provides the nominal system design values for depth mapping system <NUM> (<FIG>); assembly tolerance step <NUM> provides the assembly tolerances of the depth mapping system; and operational tolerance step <NUM> provides expected operational tolerances, such as variations of ambient temperature and the effects of a mechanical shock on the depth mapping system.

The above inputs include multiple parameters. For example, a typical focal length of collection lens <NUM> has a nominal value of <NUM>, an assembly tolerance of <NUM> and an operational tolerance of <NUM>. Each tolerance is normally distributed around zero, with a standard deviation equal to the above tolerance. The probability distribution of the focal length is a normal distribution combined from the two normal distributions and centered at the nominal value of <NUM>. An additional example of a parameter is the baseline between VCSEL array <NUM> and SPAD array <NUM>. The multiple parameters, such as the two examples described above, allow controller <NUM> to model accurately the optical path taken by the laser pulses and thus calculate the locations where the spots impinge on SPAD array <NUM>.

Based on these inputs, controller <NUM> calculates a search region for each of the M laser spots expected on SPAD array <NUM> (<FIG>), in a pre-computation step <NUM>. Each search region includes a group of pixels for which a probability of receiving a laser beam reflected from scene <NUM> is estimated to be higher than a preset threshold, such as <NUM>%. As an example of the calculations performed by controller <NUM>, an increase of <NUM>% in the focal length of collection lens <NUM> magnifies the image on SPAD array <NUM> by <NUM>%, thus shifting the spots in an outward radial direction. The probability distribution of this parameter and all other parameters of the input translates to a region around each of the nominal spot locations on SPAD array <NUM> in which there is a probability higher than <NUM>% to find the spot.

Once the search regions have been chosen in pre-computation step <NUM>, controller <NUM>, in a random iterative search step <NUM>, fires a succession of pulses of beams <NUM> from VCSEL array <NUM> (<FIG>), and at the same time performs random searches within the search regions to identify the M super-pixels that receive the pulsed beams. Alternatively, controller <NUM> may apply other search strategies, not necessarily random, within the search regions. During step <NUM>, each processing unit <NUM> is coupled to receive signals from a different pixel following each laser pulse or sequence of multiple pulses, and controller <NUM> checks, using DPU <NUM>, which pixels have output signals due to an incident photon, and which have not. Based on the results, controller <NUM> selects the pixels to include in each super-pixel as those on which the photons were found to be incident. In simulations, the search was found to converge within a succession of <NUM>-<NUM> repeated sequences of pulsed beams <NUM> and thus identify the M super-pixels of SPAD array <NUM> that receive the M beams.

Once controller <NUM> has found the M super-pixels, it finishes the search and assigns, in an assignment step <NUM>, these super-pixels for use in 3D mapping of scene <NUM> by depth mapping system <NUM>.

<FIG> is a flowchart that schematically illustrates a two-stage method for identifying the super-pixels in SPAD array <NUM> (<FIG>) that receive laser spots, in accordance with the invention. The first stage starts by providing, in an input step <NUM>, a small number m<NUM> of potential process candidates, wherein, in this context, the term "process candidate" is used for those SPAD super-pixels likely to receive laser spots. A typical number for potential process candidates is either a fixed number, such as m<NUM> = <NUM>, or a percentage, such as <NUM>%, of the number M. These potential process candidates may be obtained, for example, from a previous use of depth mapping system <NUM>.

These potential candidates are coupled to respective processing units <NUM>, in a candidate processing step <NUM>. In a first detection step <NUM>, controller <NUM> fires a sequence of pulses of beams <NUM> from VCSEL array <NUM> and queries processing units <NUM> and combining unit <NUM> to find out how many of the m<NUM> process candidates on SPAD array <NUM> reported "hits," i.e., output signals indicating that they had received photons. In a first comparison step <NUM>, controller <NUM> checks whether the number of reported hits in first detection step <NUM> exceeds a first preset threshold, for example <NUM>% of M (if initially <NUM>% of M were selected as process candidates).

If the number of hits was below the threshold, controller <NUM> searches, in a search step <NUM>, for hits in the areas around the process candidates by firing successive pulsed beams <NUM> from VCSEL array <NUM> and performing a single pixel search around the candidates. After new hits have been identified, the previous process candidates in process candidate step <NUM> are replaced by the new hits, and steps <NUM> and <NUM> are repeated until the number of detected hits in first comparison step <NUM> exceeds the first preset threshold.

The detected hits in first comparison step <NUM> are used by controller <NUM> to build a model in a modeling step <NUM>. The model expresses the deviation of the locations of the hits in SPAD array <NUM> relative to their nominal locations, i.e., the locations where the reflected laser beams were expected to be incident on SPAD array <NUM> according to the design geometry of system <NUM>, for example. The model may be a quadratic model, a simplified pinhole camera model, or a homographic model, for example, and it may take into account system tolerances as previously described.

A homographic model h is an eight-parameter transformation (h<NUM>,. , h<NUM>), mapping a point p = (x, y) to another point p' = (x', y') through the relation: <MAT> The coordinates x and y refer to Cartesian coordinate system <NUM> of <FIG>. Such a model represents the correct spot locations on SPAD array <NUM> in a case where scene <NUM> comprises a plane (for instance, a wall).

A quadratic model is given by: <MAT> <MAT>.

The equations of a simplified pinhole camera model are as follows: Given a point (x, y, z) in Cartesian coordinate system <NUM>, we first compute the undistorted image coordinates: <MAT> We then apply a distortion operation to obtain the final image coordinates: <MAT> where <MAT> and p(r) = <NUM> +k<NUM>r<NUM> + k<NUM>r<NUM> + k<NUM>r<NUM> is a distortion polynomial. The parameters of the model are, therefore, the following constants: f, cx, cy, k<NUM>, k<NUM>, k<NUM> (see <NPL>).

Additionally or alternatively, other models, such as splines or more elaborate models that describe the optics of system <NUM> to a higher degree of complexity, may be employed.

Based on one of the above models, controller <NUM> predicts the locations of a number of new process candidates, by applying the model to the nominal locations of a number of additional pixels where other reflected laser beams were expected to be incident, in a candidate addition step <NUM>, making now up a total of m<NUM> process candidates. Typically, m<NUM> increases in each iteration at candidate addition step <NUM>. In a second detection step <NUM> controller <NUM> fires an additional sequence of pulses of beams <NUM> from VCSEL array <NUM> and queries how many of the m<NUM> process candidates on SPAD array <NUM> have reported hits.

In a second comparison step <NUM>, controller <NUM> compares the relative number of hits (the ratio between the hits and the total number M of pulsed beams <NUM>) to a second preset threshold. This latter threshold is typically set to a high value, corresponding to a situation in which the large majority of beams <NUM> are successfully received by corresponding super-pixels. If the relative number of hits is less than the second preset threshold, controller <NUM> adjusts the model in modeling step <NUM> based on the detected hits. The adjustment of the model includes recalculating the model coefficients, as well as, where required, an increase in the complexity of the model. In an iterative process, of <NUM>-<NUM> loops, for example, controller <NUM> adds new process candidates based on the model in candidate addition step <NUM>, queries hits in second detection step <NUM>, and compares their relative number to the second preset threshold in second comparison step <NUM>. As long as the relative number of hits does not exceed the second preset threshold, controller <NUM> keeps looping back to model step <NUM>, improving the model based on the new hits.

Once the relative number of detected hits exceeds the second preset threshold, controller <NUM> finishes the search and assigns, in an assignment step <NUM>, the detected hits for use in 3D mapping of scene <NUM> by depth mapping system <NUM>.

Claim 1:
Depth sensing apparatus (<NUM>), comprising:
a radiation source (<NUM>), comprising a first array of emitters (<NUM>) arranged in multiple banks (<NUM>, <NUM>), which are configured to emit a first plurality of pulsed beams (<NUM>) of optical radiation toward a target scene (<NUM>);
a second plurality of sensing elements (<NUM>), which are arranged in a second array (<NUM>) and are configured to output signals indicative of respective times of incidence of photons on the sensing elements, wherein the second plurality exceeds the first plurality;
objective optics (<NUM>) configured to form an image of the target scene on the array of sensing elements, whereby the beams of optical radiation reflected from the target scene form spots on the second array; and
processing and control circuitry (<NUM>, <NUM>, <NUM>), which is coupled to actuate the multiple banks in alternation to emit the pulsed beams and is coupled to receive the signals from the sensing elements, and which is configured to calibrate locations of the spots on the second array by searching over the sensing elements, in pre-computed regions based on knowledge of the depth sensing apparatus or by identifying in an initialization stage, a small subset of the locations of laser spots, and in a subsequent iterative stage, predicting by a model and verifying the positions of further laser spots, in order to identify, responsively to the signals, areas of the second array on which the pulses of optical radiation reflected from corresponding regions of the target scene are incident, and to process the signals from the sensing elements in the identified areas in order measure depth coordinates of the corresponding regions of the target scene based on the times of incidence.