Patent Publication Number: US-11029391-B2

Title: System for determining a distance to an object

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Phase filing under 35 U.S.C. § 371 of PCT/EP2017/0075095, entitled “SYSTEM FOR DETERMINING A DISTANCE TO AN OBJECT” filed on Oct. 3, 2017, and claims priority to European Patent Application No. EP 16192102.8, entitled “SYSTEM FOR DETERMINING A DISTANCE TO AN OBJECT” filed on Oct. 3, 2016. The entire contents of each application are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention pertains to the field of systems for determining a distance to an object, in particular to time-of-flight based sensing systems to be used for the characterization of a scene or a part thereof. 
     BACKGROUND 
     In the field of remote sensing technology, mainly in the usage of making high-resolution maps of the surroundings, to be used in many control and navigation applications such as but not limited to the automotive and industrial environment, gaming applications, and mapping applications, it is known to use time-of-flight based sensing to determine the distance of objects from a sensor. Time-of-flight based techniques include the use of RF modulated sources, range gated imagers, and direct time-of-flight (DToF) imagers. For the use of RF modulated sources and range gated imagers, it is necessary to illuminate the entire scene of interest with a modulated or pulsed source. Direct time-of-flight systems, such as most LIDARs, mechanically scan the area of interest with a pulsed beam, the reflection of which is sensed with a pulse detector. 
     In order to be able to correlate an emitted RF modulated signal with the detected reflected signal, the emitted signal must meet a number of constraints. In practice, these constraints turn out to make the RF modulated systems highly impractical for use in vehicular systems: the attainable range of detection is very limited for signal intensities that are within conventional safety limits and within the power budget of regular vehicles. 
     A direct TOF (DToF) imager, as used in most LIDAR systems, comprises a powerful pulsed laser (operating in a nanosecond pulse regime), a mechanical scanning system to acquire from the 1D point measurement a 3D map, and a pulse detector. Systems of this type are presently available from vendors including Velodyne Lidar of Morgan Hill, Calif. The Velodyne HDL-64E, as an example of state-of-the-art systems, uses 64 high-power lasers and 64 detectors (avalanche diodes) in a mechanically rotating structure at 5 to 15 rotations per second. The optical power required by these DToF LIDAR systems is too high to be obtained with semiconductor lasers, whose power is in the range of five to six orders of magnitude lower. In addition, the use of mechanically rotating elements for scanning purposes limits the prospects for miniaturization, reliability, and cost reduction of this type of system. 
     United States Patent application publication no. 2015/0063387 in the name of Trilumina discloses a VCSEL delivering a total energy of 50 mW in a pulse having a pulse width of 20 ns. The commercially available Optek OPV310 VCSEL delivers a total energy of 60 mW in a pulse having a duration of 10 ns and it can be estimated by extrapolation to have a maximum optical output power of 100 mW. This value is only realized under very stringent operating conditions, meaning optimal duty cycle and short pulse width so as to avoid instability due to thermal problems. Both the Trilumina disclosure and the Optek system illustrate that continuous-wave VCSEL systems are reaching their physical limits with respect to optical peak power output, due to thermal constraints inherently linked to the VCSEL design. At these pulse energy levels, and using ns pulses as presently used in DToF applications, the mere number of photons that can be expected to be usefully reflected by an object at a distance of 120 m is so low that it defeats detection by means of conventional semiconductor sensors such as CMOS or CCD or SPAD array. Thus, increasing the VCSEL power outputs by 5 or 6 orders of magnitude, as would be required to extend the range of the known DToF systems, is physically impossible. 
     Even the use of avalanche diodes (AD or SPAD), which are theoretically sufficiently sensitive to capture the few returning photons, cannot be usefully deployed in the known LIDAR system architectures. A solid state implementation of an array of SPADs must be read out serially. A high number of SPADs is required to achieve the desired accuracy. The serial read-out constraints of the solid state implementation limit the bandwidth of the system turning it inappropriate for the desired accuracy. For accuracies such as that of the Velodyne system (0.02 m to 0.04 m, independent of distance), the required read-out data rate exceeds the practically achievable bandwidth in case of today&#39;s IC implementation. For operation at 120 m, a SPAD array of 500×500 pixels is required, which, in an IC-based implementation, must be read-out serially. For the same precision as the aforementioned Velodyne system, it would require 1000 pulses per millisecond and hence 1000 frames per millisecond, translating into a readout rate of 250 Gigapixels per second. This is believed to be technically unfeasible in the context of current SPAD IC technology. 
     The paper by Neil E. Newman et al., “High Peak Power VCSELs in Short Range LIDAR Applications”,  Journal of Undergraduate Research in Physics,  2013, http://www.jurp.org/2013/12017EXR.pdf, describes a VCSEL-based LIDAR application. The paper states that the maximum output power of the described prototype system was not great enough to do wide-field LIDAR at a range greater than 0.75 m. With a relatively focused beam (0.02 m spot size at 1 m distance), the authors were able to range a target object at a distance of up to 1 m. 
     The above examples clearly indicate that the optical power emitted by present semiconductor lasers cannot meet the power requirements necessary for operations in the known LIDAR systems to be of practical use in automotive applications (e.g. for ranges up to 120 m). 
     U.S. Pat. No. 7,544,945 in the name of Avago Technologies General IP (Singapore) Pte. Ltd., discloses vehicle-based LIDAR systems and methods using multiple lasers to provide more compact and cost-effective LIDAR functionality. Each laser in an array of lasers can be sequentially activated so that a corresponding optical element mounted with respect to the array of lasers produces respective interrogation beams in substantially different directions. Light from these beams is reflected by objects in a vehicle&#39;s environment, and detected so as to provide information about the objects to vehicle operators and/or passengers. The patent provides a solid state projector in which the individual lasers are consecutively activated in order to replace the known mechanical scanning in the known DToF LIDAR systems. 
     A high-accuracy medium-range surround sensing system for vehicles that does not use time-of-flight detection, is known from international patent application publication WO 2015/004213 A1 in the name of the present applicant. In that publication, the localization of objects is based on the projection of pulsed radiation spots and the analysis of the displacement of detected spots with reference to predetermined reference spot positions. More in particular, the system of the cited publication uses triangulation. However, the accuracy that can be achieved correlates with the triangulation base, which limits the further miniaturization that can be achieved. 
     US patent application publication no. US 2012/0038903 A1 discloses methods and systems for adaptively controlling the illumination of a scene. In particular, a scene is illuminated, and light reflected from the scene is detected. Information regarding levels of light intensity received by different pixels of a multiple pixel detector, corresponding to different areas within a scene, and/or information regarding a range to an area within a scene, is received. That information is then used as a feedback signal to control levels of illumination within the scene. More particularly, different areas of the scene can be provided with different levels of illumination in response to the feedback signal. 
     European patent application publication no. EP 2 322 953 A1 discloses a distance image sensor capable of enlarging the distance measurement range without reducing the distance resolution. A radiation source provides first to fifth pulse trains which are irradiated to the object as radiation pulses in the first to fifth frames arranged in order on a time axis. In each of the frames, imaging times are prescribed at points of predetermined time from the start point of each frame, also the pulses are shifted respectively by shift amounts different from each other from the start point of the first to fifth frames. A pixel array generates element image signals each of which has distance information of an object in distance ranges different from each other using imaging windows A and B in each of five frames. A processing unit generates an image signal by combining the element image signals. Since five times-of-flight measurement are used, the width of the radiation pulse does not have to be increased to obtain distance information of the object in a wide distance range, and the distance resolution is not reduced. 
     European patent application publication no. EP 2 290 402 A1 discloses a range image sensor which is provided on a semiconductor substrate with an imaging region composed of a plurality of two-dimensionally arranged units, thereby obtaining a range image on the basis of charge quantities output from the units. One of the units is provided with a charge generating region (region outside a transfer electrode) where charges are generated in response to incident light, at least two semiconductor regions which are arranged spatially apart to collect charges from the charge generating region, and a transfer electrode which is installed at each periphery of the semiconductor region, given a charge transfer signal different in phase, and surrounding the semiconductor region. 
     The article by Shoji Kawahito et al., “A CMOS Time-of-Flight Range Image Sensor With Gates-on-Field-Oxide Structure”,  IEEE Sensors Journal , Vol. 7, no. 12, p. 1578-1586, discloses a type of CMOS time-of-flight (TOS) range image sensor using single-layer gates on field oxide structure for photo conversion and charge transfer. This structure allows the realization of a dense TOF range imaging array with 15×15 μm 2  pixels in a standard CMOS process. Only an additional process step to create an n-type buried layer which is necessary for high-speed charge transfer is added to the fabrication process. The sensor operates based on time-delay dependent modulation of photocharge induced by back reflected infrared light pulses from an active illumination light source. To reduce the influence of background light, a small duty cycle light pulse is used and charge draining structures are included in the pixel. The TOF sensor chip fabricated measures a range resolution of 2.35 cm at 30 frames per second an improvement to 0.74 cm at three frames per second with a pulse width of 100 ns. 
     European patent application no. EP15191288.8 in the name of the present applicant, which has not been published at the filing date of the present application, describes some aspects of a system and method for determining a distance to an object. 
     There is a continuing need to obtain extreme miniaturization and/or longer-range in complex vehicular surround sensing applications, such as ADAS (autonomous driving assistance system) applications and autonomous driving applications, and this at a reasonable cost and in a compact, semiconductor-integrated form factor. There is a particular need for systems that combine a wide field of view with a very high detection sensitivity. 
     SUMMARY OF THE INVENTION 
     It is an objective of embodiments of the present invention to provide a further miniaturized and longer-range alternative for displacement-based vehicular surround sensing systems. Furthermore, it is an objective of embodiments of the present invention to provide a full solid-state alternative for the known LIDAR systems. 
     According to an aspect of the present invention, there is provided a system for determining a distance to an object comprising: a solid-state light source arranged for projecting a pattern of discrete spots of laser light towards the object in a sequence of pulses; a detector comprising a plurality of picture elements, the detector being configured for detecting light representing the pattern of discrete spots as reflected by the object in synchronization with the sequence of pulses; and processing means configured to calculate the distance to the object as a function of exposure values generated by the picture elements in response to the detected light; wherein the picture elements are configured to generate the exposure values by accumulating, for all of the pulses of the sequence, a first amount of electrical charge representative of a first amount of light reflected by the object during a first predetermined time window and a second electrical charge representative of a second amount of light reflected by the object during a second predetermined time window, the second predetermined time window occurring after the first predetermined time window; and wherein the solid-state radiation source emits substantially monochromatic light having a wavelength spread of less than ±20 nm and the detector is equipped with a corresponding narrow bandpass filter. 
     The present invention relies on the same physical principles as direct time-of-flight based ranging systems, viz. the fact that light always takes a certain amount of time to travel a given distance. However, the present invention uses range gating to determine the distance travelled by a light pulse that has been transmitted and subsequently reflected by a target object. The present invention is inter alia based on the insight of the inventors that by combining range gating, an at least partially simultaneous spot pattern projection (based on a novel illumination scheme) and a low-power semiconductor light source, a substantially miniaturized, full solid state and energy-efficient long-range distance detection method can be obtained. The term “pattern” as used herein refers to a spatial distribution of simultaneously projected spots. In order to determine the position of the detected spot reflection in three-dimensional space, it is necessary to combine the distance information obtained from the ranging step with angular information to fix the remaining two spatial coordinates. A camera comprising a pixel array and suitably arranged optics can be used to provide the additional angular information, by identifying the pixel in which the reflection is detected. 
     Embodiments of the present invention are based on the further insight of the inventors that in order to be able to use spot patterns generated by solid-state light sources in a LIDAR system at the desired ranges, a way to circumvent the optical power limitations is needed. The inventors have found that by prolonging the pulse duration and by integrating the reflected energy of multiple VCSEL-generated light pulses within at least two semiconductor sensor wells or within at least two pixels, followed by a single read-out of the integrated charge, a solid-state LIDAR system can be obtained with a significantly greater operating range than is currently possible with solid-state implementations. Hereinafter, the term “storage” will be used to designate the well or the pixel in which charge is accumulated in response to the detection of photons. 
     It is an advantage of the present invention that the solid-state light source and the solid-state sensor (such as a CMOS sensor, a CCD sensor, SPAD array or the like) may be integrated on the same semiconductor substrate. The solid-state light source may comprise a VCSEL array. 
     Moreover, by assessing the reflected light energy detected in two consecutive time windows, and normalizing for the total accumulated charge in the two consecutive windows, the impact of varying reflectivity of the object under study and the contribution of ambient light can adequately be accounted for in the distance calculation algorithm. 
     In the picture elements, charge representative of the impinging light can be accumulated at well level or at pixel level. An advantage of charge accumulation at the well level is that read-out noise is minimized, leading to a better signal-to-noise ratio. 
     The transmission and detection of the sequence of pulses may be repeated periodically. 
     Vehicle-mounted object detection systems have to be able to detect the reflections of the projected pattern, i.e. the projected spots, over a potentially intense “background radiation” consisting of sunlight in the order of 500 W/m 2 . The present invention relies on a further feature to overcome that problem, which consists of projecting spots as monochromatic light, and filtering the light to be detected to a narrow band around the wavelength of the projected spots, thus reducing the amount of sunlight to be dealt with to the light present in the relevant band. 
     As used herein, monochromatic light is understood to include laser light as produced by common semiconductor laser devices. The monochromatic light has a wavelength spread of less than ±20 nm, preferably less than ±10 nm, more preferably less than ±5 nm, most preferably less than ±3 nm or less than ±1 nm. Accordingly, the narrow bandpass filter preferably has a passband bandwidth in the range of 1-3 nm The radiation source may be adapted to generate laser light in the spectrum of 700-1500 nm, preferably 800-1000 nm. In a specific embodiment, the optics further comprise a minilens/microlens array arranged between the narrow bandpass filter and the at least one detector, such that individual minilenses of the minilens array focus incident light on respective light-sensitive areas of individual pixels of the at least one detector. It is an advantage of this one-minilens-per-pixel arrangement that the loss due to the fill factor of the underlying sensor can be reduced, by optically guiding all incident light to the light-sensitive portion of the pixels. 
     In an embodiment of the system according to the present invention, the solid-state light source is adapted to emit the pattern of discrete spots such that different spots have different respective wavelengths, the different respective wavelengths corresponding to the maximum of the passband of the narrow bandpass filter under different angles of incidence. The latter is called the blue shift due to the angle of incidence. 
     It is an advantage of this embodiment that the optics at the detector side, which are arranged to focus the reflection of the projected spots onto the picture elements, can be provided after (considered in the direction of the optical path of the light rays) the narrow bandpass filter. This allows to work with standard filters making the design cheaper and more compact. In this embodiment, the fact that such an optical arrangement allows different light rays to pass through the narrow bandpass filters under different angles, resulting in different effective filter characteristics for different light rays, can be adequately compensated by ensuring that the different spots have wavelengths that are adapted to the effective filter characteristic that applies to the physical path that the light ray for that spot will take. 
     In a particular embodiment, the solid-state light source comprises a plurality of tiles emitting light at the different respective wavelengths. 
     It is an advantage of this embodiment, that the different tiles can easily be configured to emit light at the desired wavelength. It is a further advantage of this embodiment, that the yield of the light source production process increases, as the natural variation in effective wavelengths of the produced sources becomes an advantage instead of a detriment: produced sources with effective wavelengths that deviate from the nominal wavelength can be used in different positions in the array of tiles. 
     In another particular embodiment, the solid-state light source comprises a VCSEL array equipped with micro-electro-mechanical systems arranged to adapt an emission wavelength of individual VCSELs among the VCSEL array so as to obtain the different respective wavelengths. 
     It is an advantage of this embodiment that the optical characteristics of the individual VCSELs in the array can be modulated according to the desired wavelength. 
     In another particular embodiment, the solid-state light source comprises a VCSEL array equipped with piezo-electrical transducers arranged to adapt an emission wavelength of individual VCSELs among the VCSEL array so as to obtain the different respective wavelengths. 
     It is an advantage of this embodiment that the optical characteristics of the individual VCSELs in the array can be modulated according to the desired wavelength. 
     In an embodiment, the system according to the present invention further comprises optics arranged so as to modify an angle of incidence onto the narrow bandpass filter, to confine the angle of incidence to a predetermined range around a normal of a main surface of the narrow bandpass filter, wherein the plurality of picture elements is arranged on a substrate having a curvature corresponding to the focal plane. 
     Known optical systems do not deliver the incoming radiation at a sufficient perpendicular angle onto the narrowband filter to obtain satisfactory results (i.e., constant filtering characteristics for the entire image). While this can be corrected with appropriate optics, such as telecentric optics, such optics tend to present a non-planar focal plane; this effect becomes more pronounced when a wide field-of-view is desired. Accordingly, the image projected onto a purely planar detector would be out-of-focus in a certain portion of the detector. It is an advantage of the present invention that it allows for extremely accurate filtering by combining the narrowband filter with a specific optical arrangement to render the incoming radiation perpendicular to the narrow passband filter and a detector whose shape follows the shape of the focal plane of the optics. 
     In a particular embodiment, the optics comprise an image-space telecentric lens. 
     In a more particular embodiment, the optics comprise two lenses with approximately the same focal length arranged in an image space telecentric configuration. 
     In a more particular embodiment, the optics further comprise a first minilens array arranged between the two lenses and the narrow bandpass filter so as to direct substantially all light beams incident on the first minilens array from the two lenses into the predetermined range. 
     In a more particular embodiment, the optics further comprise a second minilens array arranged between the narrow bandpass filter and the detector, wherein individual minilenses of the minilens array focus incident light on respective light-sensitive areas of individual pixels of the detector. 
     In an embodiment of the system according to the present invention, the first predetermined time window and the second predetermined time window are of substantially equal duration and occur back-to-back. 
     It is an advantage of this embodiment that the contribution of the ambient light in the distance calculation formula can easily be cancelled out by carrying out a subtraction of the accumulated ambient light averaged from surrounding pixels. 
     In a specific embodiment, each of the plurality of picture elements comprises at least two charge storage wells, and the detecting of the first amount of light and said detecting of said second amount of light occurs at respective ones of said at least two charge storage wells. 
     The term “charge storage well” designates a storage provided in the semiconductor substrate, e.g. a capacitor, that stores electrical charges generated by the conversion of photons impinging on the pixel. The objective of this specific embodiment is to realize a better signal-to-noise ratio improving the overall range of the sensor. 
     According to an aspect of the present invention, there is provided a vehicle, comprising: a system as described above arranged to operatively cover at least a part of an area surrounding said vehicle. 
     The system according to the present invention is particularly advantageous in a vehicle with ADAS or autonomous driving control unit such as but not limited to ECU (electrical control unit). The vehicle may further comprise a vehicle control unit, adapted for receiving measurement information from the system and for using the information for ADAS control or autonomous driving decision taking. The part of an area surrounding the vehicle may include a road surface ahead of, beside, or behind the vehicle. Accordingly, the system may provide road profile information of the surface ahead of the car, to be used for active suspension or semi-active suspension. 
     According to an aspect of the present invention, there is provided a camera, the camera comprising a system as described above, whereby the system is adapted to add 3D information to the camera image based on information obtained from the system, making it possible to create a 3D image. 
     The technical effects and advantages of embodiments of the camera and the vehicle according to the present invention correspond, mutatis mutandis, to those of the corresponding embodiments of the system according to the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       These and other aspects and advantages of the present invention will now be described in more detail with reference to the accompanying drawings, in which: 
         FIG. 1  represents a flow chart of an embodiment of the method according to the present invention; 
         FIG. 2  schematically represents an embodiment of the system according to the present invention; 
         FIG. 3  represents a timing diagram for light projection and detection in embodiments of the present invention; 
         FIG. 4  provides diagrams of exemplary pixel output in function of incident light power as obtained by logarithmic tone mapping (top) and multilinear tone mapping (bottom); 
         FIG. 5  provides a diagram of exemplary pixel outputs in function of incident light power as obtained by a high dynamic range multiple output pixel; 
         FIG. 6  schematically illustrates the structure of a high-dynamic range pixel for use in embodiments of the present invention; 
         FIG. 7  schematically illustrates an embodiment of a pixel architecture with two charge wells (bins) with each a separate transfer gate for use in embodiments of the present invention; 
         FIG. 8  schematically illustrates a first exemplary optical arrangement for use in embodiments of the present invention; 
         FIG. 9  schematically illustrates a second exemplary optical arrangement for use in embodiments of the present invention; 
         FIG. 10  schematically illustrates a third exemplary optical arrangement for use in embodiments of the present invention; 
         FIG. 11  schematically illustrates a fourth exemplary optical arrangement for use in embodiments of the present invention; and 
         FIG. 12  schematically illustrates a fifth exemplary optical arrangement for use in embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The surround sensing systems of the type disclosed in international patent application publication WO 2015/004213 A1, in the name of the present applicant, has the advantage of observing an extensive scene while illuminating that scene simultaneously or partially simultaneously only in a number of discrete and well-defined spots, in particular a predefined spot pattern. By using VCSEL lasers with an outstanding bundle quality and a very narrow output spectrum, it is possible to obtain a detection range with a limited amount of output power, even in the presence of daylight. The actual ranging performed in the system of WO 2015/004213 A1 relies on displacement detection, in particular triangulation, which was understood to be the only method practically available in the context of the long (quasi-stationary) pulse durations that were necessary in view of the power budget. To date, it had not been possible to achieve the same power/performance characteristics with a compact, semiconductor based time-of-flight based system. 
     The present invention overcomes this limitation by radically changing the way the time-of-flight based system operates. The invention increases the total amount of light energy emitted for each time-of-flight measurement (and thus, the number of photons available for detection at the detector for each time-of-flight measurement) by increasing the duration of individual pulses and by producing a virtual “composite pulse”, consisting of a sequence of a large number of individual pulses. This bundling of extended pulses allowed the inventors to obtain the required amount of light energy (photons) for the desired operational range with low-power VCSELs. 
     Where an individual pulse of pre-existing LIDAR systems may have a duration of 1 ns, the systems according to the present invention benefit from a substantially longer pulse duration to partially compensate for the relatively low power level of semiconductor lasers such as VCSELs; in embodiments of the present invention, individual pulses within a sequence may have an exemplary duration of 1 μs (this is one possible value, chosen here to keep the description clear and simple; more generally, in embodiments of the present invention, the pulse duration may for example be 500 ns or more, preferably 750 ns or more, most preferably 900 ns or more). In an exemplary system according to the present invention, a sequence may consist of 1000 pulse cycles, thus adding up to a duration of 1 ms. Given the fact that light would need approximately 0.66 μs to travel to a target at a distance of 100 m and back to the detector, it is possible to use composite pulses of this duration for ranging at distance of this order of magnitude; the skilled person will be able to adjust the required number of pulse cycles in function of the selected pulse width and the desired range. The detection of the sequence preferably comprises detecting the individual pulses in synchronization with the VCSEL-based light source, and accumulating the charges generated in response to the incoming photons at the pixel well level for the entire sequence prior to read-out. The term “exposure value” is used hereinafter to designate the value representative of the charge (and thus of the amount of light received at the pixel) integrated over the sequence. The sequence emission and detection may be repeated periodically. 
     The present invention operates by using range gating. Range gated imagers integrate the detected power of the reflection of the emitted pulse for the duration of the pulse. The amount of temporal overlap between the pulse emission window and the arrival of the reflected pulse depends on the return time of the light pulse, and thus on the distance travelled by the pulse. Thus, the integrated power is correlated to the distance travelled by the pulse. The present invention uses the principle of range gating, as applied to the sequences of pulses described hereinabove. In the following description, the integration of individual pulses of a sequence at the level of a picture element to obtain a measurement of the entire sequence is implicitly understood. 
       FIG. 1  represents a flow chart of an embodiment of the method according to the present invention. Without loss of generality, the ranging method is described with reference to a range gating algorithm. In a first time window  10 , the method comprises projecting  110  a pattern of spots of laser light (e.g. a regular or an irregular spatial pattern of spots) from a light source comprising a solid-state light source  210  onto any objects in the targeted area of the scenery. The spatial pattern is repeatedly projected in a sequence of pulses. 
     As indicated above, the solid-state light source may comprise a VCSEL array. In order for the system to operate optimally, even at long ranges and with high levels of ambient light (e.g., in daylight), a VCSEL for use in embodiments of the present invention is preferably arranged to emit a maximum optical power per spot per unit of area. Thus, lasers with a good beam quality (low M2-factor) are preferred. More preferably, the lasers should have a minimal wavelength spread; a particularly low wavelength spread can be achieved with monomode lasers. Thus, substantially identical pulses can reproducibly be generated, with the necessary spatial and temporal accuracy. In systems were the optics are not arranged to ensure that light rays following different paths all pass through the narrow bandpass filter under the same (perpendicular) angle, the VCSEL array is configured such that different spots have different respective wavelengths, the different respective wavelengths corresponding to the maximum of the passband of the narrow bandpass filter under different angles of incidence. This configuration may be obtained by using a tiled laser array, or by providing means for modulating the wavelength of individual VCSELs in the VCSEL array. 
     During the same time window in which a pulse is emitted, or in a substantially overlapping time window, a first amount of light representing the pattern of spots as reflected by the object of interest is detected  120  at a detector, which is preferably arranged as near as possible to the light source. The synchronicity or near synchronicity between the projection  110  of the spot pattern and the first detection  120  of its reflection, is illustrated in the flow chart by the side-by-side arrangement of these steps. In a subsequent second predetermined time window  20 , a second amount of light representing the reflected light spot is detected  130  at the detector. During this second window  20 , the solid-state light source is inactive. The distance to the object can then be calculated  140  as a function of the first amount of reflected light and the second amount of reflected light. 
     The first predetermined time window  10  and the second predetermined time window  20  are preferably back-to-back windows of substantially equal duration, to facilitate noise and ambient light cancellation by subtracting one of the detected amounts from the other one. An exemplary timing scheme will be described in more detail below in conjunction with  FIG. 3 . 
     The detector comprises a plurality of picture elements, i.e. it consists of a picture element array with adequate optics arranged to project an image of the scenery (including the illuminated spots) onto the picture element. The term “picture element” as used herein may refer to an individual light-sensitive area or well of a pixel, or to an entire pixel (which may comprise multiple wells, see below). For every given projected spot, the detecting  120  of the first amount of light and the detecting  130  of the second amount of light occurs at the same one or the same group of the plurality of picture elements. 
     Without loss of generality, each of the picture elements may be a pixel comprising at least two charge storage wells  221 ,  222 , such that the detecting  120  of the first amount of light and the detecting  130  of the second amount of light can occur at the respective charge storage wells  221 ,  222  of the same pixel or pixel group. 
       FIG. 2  schematically represents an embodiment of the system according to the present invention, in relation to an object  99  in the scenery of interest. The system  200  comprises a solid-state light source  210  for projecting a pattern of a sequence of spots, which may be repeated periodically, onto the object  99 . A detector  220  is arranged near the light source and configured to detect light reflected by the object. 
     The light beam bouncing off the object  99  is illustrated as an arrow in dashed lines, travelling from the light source  210  to the object  99  and back to the detector  220 . It should be noted that this representation is strictly schematic, and not intended to be indicative of any actual relative distances or angles. 
     A synchronization means  230 , which may include a conventional clock circuit or oscillator, is configured to operate the solid-state light source  210  so as to project the pattern of spots onto the object during first predetermined time windows  10  and to operate the detector  220  so as to detect a first amount of light representing the light spot(s) reflected by the object  99  at substantially the same time. It further operates the detector  220  to detect a second amount of light representing the light spots reflected by the object  99 , during respective subsequent second predetermined time windows  20 . Appropriate processing means  240  are configured to calculate the distance to the object as a function of the first amount of reflected light and the second amount of reflected light. 
       FIG. 3  represents a timing diagram for light projection and detection in embodiments of the present invention. For clarity reasons, only a single pulse of the pulse sequence which is repeated periodically of  FIG. 1  is illustrated, which consists of a first time window  10  and a second time window  20 . 
     As can be seen in  FIG. 3 a   , during the first time window  10 , the solid-state light source  210  is in its “ON” state, emitting the pattern of light spots onto the scenery. During the second time window  20 , the solid-state light source  210  is in its “OFF” state. 
     The arrival of the reflected light at the detector  220  is delayed relative to the start of the projection by an amount of time that is proportional to the distance travelled (approximately 3.3 ns/m in free space). Due to this delay, only a part of the reflected light will be detected at the first well  221  of the detector  220 , which is only activated during the first time window  10 . Thus, the charge accumulated in this first well during its period of activation (the first time window  10 ) consists of a part representing only the noise and the ambient light impinging on the pixel prior to the arrival of the reflected pulse, and a part representing the noise, the ambient light, and the leading edge of the reflected pulse. 
     The latter part of the reflected pulse will be detected at the second well  222  of the detector  220 , which is only activated during the second time window  20 , which preferably immediately follows the first time window  10 . Thus, the charge accumulated in this second well during its period of activation (the second time window  20 ) consists of a part representing the noise, the ambient light, and the trailing edge of the reflected pulse, and a part representing only the noise and the ambient light impinging on the pixel after the arrival of the reflected pulse. 
     The greater the distance between the reflecting object  99  and the system  200 , the smaller the proportion of the pulse that will be detected in the first well  221  and the larger the proportion of the pulse that will be detected in the second well  222 . 
     If the leading edge of the reflected pulse arrives after the closing of the first well  221  (i.e., after the end of the first time window  10 ), the proportion of the reflected pulse that can be detected in the second well  222  will decrease again with increasing time of flight delay. 
     The resulting amounts of charge A, B in each of the respective wells  221 ,  222  for varying distances of the object  99  is shown in  FIG. 3 b   . To simplify the representation, the effect of the attenuation of light with distance, according to the inverse square law, has not been taken into account in the diagram. It is clear that for time of flight delays up to the combined duration of the first time window  10  and the second time window  20 , the time of flight delay can in principle unambiguously be derived from the values of A and B:
         For time of flight delays up to the duration of the first time window  10 , B is proportional to the distance of the object  99 . To easily arrive at a determination of the absolute distance, the normalized value B/(B+A) may be used, removing any impact of non-perfect reflectivity of the detected object and of the inverse square law.   For time of flight delays exceeding the duration of the first time window  10 , A consists of daylight and noise contributions only (not illustrated), and C−B is substantially proportional (after correcting for the inverse square law) to the distance of the object  99 , where C is an offset value.       

     While  FIGS. 3 a  and 3 b    illustrate the principle of the invention in relation to a single pulse emitted in the time window  10 , it shall be understood that the illustrated pulse is part of a sequence of pulses as defined above.  FIG. 3 c    schematically illustrates exemplary timing characteristics of such a sequence. As illustrated, the illumination scheme  40  consists of a repeated emission of a sequence  30  of individual pulses  10 . The width of the individual pulses  10  is determined by the maximal operating range. The entire sequence may be repeated at a frequency of, for example, 60 Hz. 
     Reflections of light by objects at a short distance are more likely to cause pixel saturation, because the attenuation of such a reflection will be much less than that of a reflection originating from a more distant object (due to the inverse-square law of light attenuation over distance). As certain applications, such as automotive applications, require accurate system operation up to relatively long distances, a large photon span must be covered between the nearest distances of operation and the farthest distances of operation. With these constraints, pixel saturation at short range is a very real risk, in particular at the first set of wells (which receives the bulk of the reflection at short range). The inventors have found that for given total pixel space, the saturation problem can be mitigated by using an asymmetric well arrangement, in which the photon capacity represented by the first set of wells is increased, and the photon capacity represented by the second set of wells is decreased. If the increase and decrease are balanced, an increase of the dynamic range can be obtained at no additional pixel surface cost. 
     Blooming is a phenomenon that happens when the charge in a pixel exceeds the saturation level of that specific pixel. Consequently, the charge starts to overflow and causes nuisance in adjacent pixels. This creates inaccurate data in the neighboring pixels. Preferably, the pixels of the system according to the present invention are provided with anti-blooming electronics, to bleed off the excess charge before it saturates the relevant set of wells and spills over to the wells of adjacent pixels. In particular when the information from neighboring spots is used for the elimination of background light, it is of great importance to have an accurate estimation of the background light which is obtained independently (and without contamination from) neighboring pixels. 
     Embodiments of the present invention may employ correlated double sampling to correct the samples for the thermal noise related to the capacity of the wells (also designated as “kTC noise”). To this end, the electronics of the pixel may be designed to carry out a differential measurement between the reset voltage (V reset ) and the signal voltage (V signal ), for example by measuring V reset  at the beginning of the frame and measuring V signal  at the end of the frame. As an alternative to an electronic (in-pixel) implementation, correlated double sampling may also be implemented by digitally subtracting the read-out signals (V signal −V reset ) in a processor. 
     To increase the amount of light that reaches the photosensitive elements (in particular diodes) in the pixel structure, embodiments of the present invention may use backside illumination; in that case, the pixel circuitry is behind the photosensitive layer, thus reducing the number of layers that must be traversed by the impinging photons to read the photosensitive elements. 
     The ranging system according to the present invention may be integrated with a triangulation-based system in accordance with WO 2015/004213 A1. If miniaturization is aimed for, the triangulation-based system will end up having a relatively small distance between its projector and its detector, thus leaving it with a reduced operating range. However, it is precisely at short range that the combination presents its benefit, because the triangulation-based system can cover the distances at which the time-of-flight based system cannot operate sufficiently accurately. 
     The entire ranging process may be repeated iteratively, so as to monitor the distance to the detected object or objects over time. Thus, the result of this method can be used in processes that require information about the distance to detected objects on a continuous basis, such as advanced driver assistance systems, vehicles with an active suspension, or autonomous vehicles. 
     In order for all elements of the system as described to operate optimally, the system has to be thermally stable. Thermal stability avoids, among other things, undesired wavelength shifts of the optical elements (thermal drift), which would otherwise impair the proper functioning of the optical filters and other elements of the optical chain. Embodiments of the system according to the present invention achieves thermal stability by their design, or by active regulation by means of a temperature control loop with a PID-type controller. 
     WO 2015/004213 A1 discloses various techniques to minimize the amount of ambient light that reaches the pixels during the detection intervals, thus improving the accuracy of the detection of the patterned laser spots. While these techniques have not been disclosed in the context of a LIDAR system, the inventors of the present invention have found that several such techniques yield excellent results when combined with embodiments of the present invention. This is particularly true for the use of narrow bandpass filters at the detector, and the use of adequate optical arrangements to ensure nearly perpendicular incidence of the reflected light onto the filters. The details of these arrangements as they appear in WO 2015/004213 A1 are hereby incorporated by reference. Further features and details are provided hereinafter. 
     While various techniques known from WO 2015/004213 A1 may be applied to embodiments of the present invention to minimize the amount of ambient light that reaches the pixels during the detection intervals, a certain amount of ambient light cannot be avoided. In a multi-pixel system, only some of the pixels will be illuminated by reflected spots, while others will be illuminated by residual ambient light only. The signal levels of the latter group of pixels can be used to estimate the contribution of the ambient light to the signals in the pixels of interest, and to subtract that contribution accordingly. Additionally or alternatively, background light or ambient light may be subtracted from the detected signal at pixel level. This requires two exposures, one during the arrival of the laser pulse and one in the absence of a pulse. 
     In some embodiments, the detector may be a high dynamic range detector, i.e. a detector having a dynamic range of at least 90 dB, preferably at least 120 dB. The presence of a high dynamic range sensor, i.e. a sensor capable of acquiring a large amount of photons without saturation while maintaining sufficient discrimination of intensity levels in the darkest part of the scene, is an advantage of the use of such a sensor; it allows for a sensor that has a very long range and yet remains capable of detection objects at short distance (where the reflected light is relatively intense) without undergoing saturation. The inventors have found that the use of a true high dynamic range sensor is more advantageous than the use of a sensor that applies tone mapping. In tone mapping, the sensor linear range is compressed towards the higher resolution. In literature, several compression methods are documented, such as logarithmic compression or multilinear compression (see  FIG. 4 ). However, this non-linear compression necessitates relinearisation of the signals before performing logical or arithmetic operations on the captured scene to extract the relief information. The solution according to the invention therefore increases detection accuracy without increasing the computational requirements. It is a further advantage of some embodiments to use a fully linear high dynamic range sensor as presented in  FIG. 5 . A pixel architecture and an optical detector that are capable of providing the desired dynamic range characteristics are disclosed in US patent application publication no. US 2014/353472 A1, in particular paragraphs 65-73 and 88, the content of which is incorporated by reference for the purpose of allowing the skilled person to practice this aspect of the present invention. 
     Embodiments of the present invention use a high dynamic range pixel. This can be obtained by a sizeable full-well capacity of the charge reservoir or by designs limiting the electronic noise per pixel or by usage of CCD gates that do not add noise at charge transfer, or through a design with a large detection quantum efficiency (DQE) (e.g., in the range of 50% for front illumination or 90% in case of back illumination, also known as back thinning), or by a special design such as shown in  FIG. 6  (see below), or by any combination of the listed improvements. Furthermore, the dynamic range can be further enlarged by adding an overflow capacity to the pixel in overlay at its front side (this implementation requires back thinning). Preferably, the pixel design implements an anti-blooming mechanism. 
       FIG. 6  presents a schematic illustration of an advantageous implementation of a pixel with high dynamic range. The example in this figure makes use of two storage gates  7 ,  8 , connected to the floating diffusion. After exposure, the electron generated by the scene AND the laser pulse, is transferred on the floating diffusion using the transfer gate  11 . Both Vgate 1  and Vgate 2  gate voltages are set high. The charges are then spread over both capacitors, realizing a significant Full Well. Once this high full-well data is read via connection to the amplifier, the voltage Vgate 2  is set low. The electrons reflow towards capacitor  7 , increasing the total pixel gain. The data can be read through the amplifier. It is further possible to achieve an even higher gain by applying later a low voltage on Vgate 1 . The electrons reflow towards the floating diffusion  2 . 
       FIG. 7  represents a possible dual-well or dual-bin implementation of an envisaged pixel to be used in CMOS technology. The impinging signal is distributed over two charge storages. Each reservoir has a separate transfer gate controlled by an external pulse which is synchronized with the pulse of the laser sources. 
       FIGS. 8-10  illustrate cameras that may be used in embodiments of the invention, where the light radiation source emits monochromatic light and the at least one detector is equipped with a corresponding narrow bandpass filter and optics arranged so as to modify an angle of incidence onto said narrow bandpass filter, to confine said angle of incidence to a predetermined range around a normal of a main surface of said narrow bandpass filter, said optics comprising an image-space telecentric lens. The term “camera” is used herein as a combination of a sensor and associated optics (lenses, lens arrays, filter). In particular, in  FIG. 9 , the optics further comprise a minilens array arranged between the image-space telecentric lens and the at least one detector, such that individual minilenses of the minilens array focus incident light on respective light-sensitive areas of individual pixels of the at least one detector. It is an advantage of this one-minilens-per-pixel arrangement that the loss due to the fill factor of the underlying sensor can be reduced, by optically guiding all incident light to the light-sensitive portion of the pixels. 
     These examples all result in radiation travelling a substantially equal length through the filter medium or in other words in that the incident radiation is substantially orthogonal to the filter surface, i.e. it is confined to an angle of incidence within a predetermined range around the normal of the filter surface, thus allowing in accurate filtering within a narrow bandwidth to e.g. filter the daylight, the sunlight and in order to for the spots to surpass the daylight. 
     The correction of the angle of incidence is of particular importance in embodiments of the present invention where the entire space around a vehicle is to be monitored with a limited number of sensors, for instance 8 sensors, such that the incident rays may extend over a solid angle of for example 1×1 rad.  FIG. 8  schematically illustrates a first optical arrangement of this type. It comprises a first lens  1030  and a second lens  1040 , with approximately the same focal length f, in an image space telecentric configuration. That means that all chief rays (rays passing through the center of the aperture stop) are normal to the image plane. An exemplary numerical aperture of 0.16 corresponds to a cone angle of 9.3° (half cone angle). The maximum incidence angle on the narrow bandpass filter  1060 , arranged between the lens system  1030 - 1040  and the sensor  102 , would thus be 9.3°. 
     As illustrated in  FIG. 9 , the preferred design consists of a tandem of two lenses  1130 ,  1140  with approximately the same focal length f, in an image-space telecentric configuration (the configuration is optionally also object-space telecentric), a planar stack of mini-lens array  1150 , a spectral filter  1160  and a CMOS detector  102 . Since the center O of the first lens  1130  is in the focus of the second lens  1140 , every ray that crosses O will be refracted by the second lens  1140  in a direction parallel to the optical axis. Consider now a particular laser spot S  1110  located at a very large distance as compared to the focal length of the first lens  1130 . Thus the image of this spot  1110  by the first lens  1130  is a point P located close to the focal plane of this lens, thus exactly in the middle plane of the second lens  1140 . The light rays that are emitted from the spot S  1110  and captured by the first lens  1130  form a light cone that converges towards the point P in the second lens  1140 . The central axis of this light cone crosses the point O and is refracted parallel the optical axis and thus perpendicular to the spectral filter  1160  so as to achieve optimal spectral sensitivity. Hence, the second lens  1140  acts as a correcting lens for the angle of the incident light beam. The other rays of the cone can also be bent in a bundle of rays parallel to the optical axis by using a small convex mini-lens  1150  behind the second lens  1140  in such a way that the point P is located in the focal point of the mini-lens  1150 . In this way all the imaging rays of the spot S  1110  are bent in a direction nearly perpendicular to the spectral filter. This can now be done in front of every pixel of the CMOS detector separately by using an array of mini-lenses positioned in front of every pixel. In this configuration, the minilenses have an image-telecentric function. The main advantage is that the pupil of the first lens  1030  can be enlarged, or the aperture can be eliminated while compensating for the increase in spherical aberration by a local correction optics in the mini-lens  1150 . In this way the sensitivity of the sensor assembly can be improved. A second mini-lens array (not shown in  FIG. 11 ) may be added between the spectral filter  1160  and the CMOS pixels  102 , to focus the parallel rays back to the photodiodes of the pixels so as to maximize the fill factor. 
     For the first and second lenses  1130 ,  1140 , commercially available lenses may be used. The skilled person will appreciate that lenses typically used in other smart phone cameras or webcams of comparable quality can also be used. The aforementioned iSight camera has a 6×3 mm CMOS sensor with 8 megapixels, 1.5 μm pixel size, a very large aperture of f/2.2, an objective focal length of about f=7 mm, and a pupil diameter about 3.2 mm. The viewing angle is of the order of 1 rad×1 rad. If we assume that the resolution of the camera is roughly the pixel size (1.5 micron), we can conclude (from Abbe&#39;s law) that the aberrations of the lens are corrected for all the rays of the viewing angle selected by the aperture. 
       FIG. 10  illustrates a variation of the arrangement of  FIG. 11 , optimized for manufacturing in a single lithographic process. The first lens  1230  is similar to the first lens  1130  of the previous embodiment, but the angle-correcting second lens  1140  is replaced by a Fresnel lens  1240  with the same focal length f and the mini-lens arrays  1150  by Fresnel lens arrays  1250 . The advantage is that they are completely flat and can be produced by nano-electronics technology (with discrete phase zones). A second mini-lens array  1270  may be added between the spectral filter  1260  and the CMOS pixels  102 , to focus the parallel rays back to the photodiodes of the pixels so as to maximize the fill factor. Thus the camera is essentially a standard camera as the iSight but in which the CMOS sensor is replaced by a specially designed multi-layer sensor in which all the components are produced in one integrated block within the same lithographic process. This multilayer sensor is cheap in mass production, compact, robust and it need not be aligned. Each of these five layers  1240 ,  1250 ,  1260 ,  1270 ,  102  has its own function to meet the requirements imposed by the present invention. 
     As the minimal angle of a cone generated by a lens of diameter d is of the order of λ/d, with λ the wavelength of the light, the minimal cone angle is 1/10 radian for a mini-lens diameter d=8.5 μm and λ=850 nm. With a good quality spectral interference filter this corresponds to a spectral window of about 3 nm. 
     In the arrangements of  FIGS. 8-10 , the characteristics of the optics will result in a non-planar focal plane. To compensate this effect, the picture elements of the detector may be arranged on a substrate having a curvature that follows the focal plane of the optics. As a result, the reflected and filtered spots will be in focus, regardless of where they reach the detector. The desired curvature of the substrate of the detector can be obtained by using flex-chip technology, or by composing the substrate by combining differently oriented tiles. This solution is schematically illustrated in  FIG. 11 , which shows telecentric optics  1330 , followed by a narrow band-pass filter  1360 , and a curved pixel layer  102 , the curvature of which is adapted to follow the shape of the focal plane of the telecentric optics  1330 . 
     When it is not possible (or not desirable) to arrange the optics in such a way as to ensure that light rays following different paths all pass through the narrow bandpass filter under the same (perpendicular) angle, the problem of having different filter characteristics with different angles of incidence may be resolved at the source. In particular, the VCSEL array may be configured such that different spots have different respective wavelengths. This configuration may be obtained by using a tiled laser array, or by providing means for modulating the wavelength of individual VCSELs in the VCSEL array. This solution is schematically illustrated in  FIG. 12 , which shows a narrow band-pass filter  1460  arranged before the optics  1430  and the sensor array  102 . For clarity purposes and without loss of generality, two different angles of incidence with different respective wavelengths (λ 1 , λ 2 ) have been indicated on the Figure. The different wavelengths (λ 1 , λ 2 ) of the light sources are chosen to correspond to the maximum of the passband of the narrow bandpass filter under their respective angles of incidence. 
     In some embodiments of the present invention, the intensity of the spots can be kept substantially constant over the full depth range, by applying a stepped or variable attenuation filter at the detector. Alternatively or in addition, also a non-symmetrical lens pupil can be provided for weakening the intensity of spots closer to the detector, while the intensity of the spots further away from the detector are received at full intensity. In this way clipping of the detector is avoided and the average intensity can be made substantially the same for all spots. 
     In some embodiments, the radiation source can be a VCSEL that can be split in different zones, whereby the laser ON time is controlled for the different zones. The images of the spots can thus be controlled to have a constant intensity, e.g. ⅔ rd  of the A/D range. Alternatively the driving voltage can be driven over the array of spots as function of the height, again to obtain a constant intensity. Such controlling can be referred to as a saturation avoidance serving loop. The different VCSELs within the array can be controlled individually for intensity, varying the intensity of the individual VCSELs in the pattern while projected simultaneously. 
     In some other embodiments of the present invention, a micro prism matrix can be used in front of the narrow bandwidth filter, such that the radiation is incident within an angle of incidence between +9° and −9° on the filter. This allows to obtain narrow bandwidth filtering. The prism matrix can for example be made by plastic moulding. 
     In embodiments of the present invention, e.g. where active suspension vehicle applications are envisaged, the projection of the spot pattern is advantageously directed downwards, i.e. towards the road. 
     A system according to the invention may include an implementation of steps of the methods described above in dedicated hardware (e.g., ASIC), configurable hardware (e.g., FPGA), programmable components (e.g., a DSP or general purpose processor with appropriate software), or any combination thereof. The same component(s) may also include other functions. The present invention also pertains to a computer program product comprising code means implementing the steps of the methods described above, which product may be provided on a computer-readable medium such as an optical, magnetic, or solid-state carrier. 
     The present invention also pertains to a vehicle comprising the system described above. 
     Embodiments of the present invention may be used advantageously in a wide variety of applications, including without limitation automotive applications, industrial applications, gaming applications, and the like, and this both indoor and outdoor, at short or long range. In some applications, different sensors according to embodiments of the present invention may be combined (e.g., daisy-chained) to produce panoramic coverage, preferably over a full circle (360° field of view). 
     While the invention has been described hereinabove with reference to separate system and method embodiments, this was done for clarifying purposes only. The skilled person will appreciate that features described in connection with the system or the method alone, can also be applied to the method or the system, respectively, with the same technical effects and advantages. Furthermore, the scope of the invention is not limited to these embodiments, but is defined by the accompanying claims.