System and method for acquiring a spectral reflectance and a relative position of a point of a scene

Abstract

Acquisition system (10), and related method, for acquiring a spectral reflectance and a relative position of a point (101 ), the acquisition system comprising: - a laser source (8) to emit in time sequence broadband illumination pulses (9), wherein at each illumination pulse (9) illuminating the point (101 ) corresponds an echoed pulse (12) having an echoed spectrum (13); - an optical dispersive system (2) to time stretch the illumination pulses (9) and / or the echoed pulses (12), so that each echoed pulse has a stretched time waveform (15) representative of the echoed spectrum (13); - a detection system (4) to detect the stretched time waveform (15) of each echoed pulse (12) and to generate a first signal (40) comprising a detected time waveform representative of the stretched time waveform (15); - a processing unit (7) configured to: - process the first signal (40) to determine, for each echoed pulse (12), the echoed spectrum (13) based on the detected time waveform and to determine the spectral reflectance of the point (101 ) by comparison between the echoed spectrum (13) and the illumination spectrum (11 ) of the corresponding illumination pulse (9); and - process the first signal (40) to determine a time of flight of each echoed pulse (12) based on the detected time waveform, and to determine the relative position of the point (101) based on the time of flight.

Description

[0001] DESCRIPTION

[0002] Title: SYSTEM AND METHOD FOR ACQUIRING A SPECTRAL REFLECTANCE AND A RELATIVE POSITION OF A POINT OF A SCENE

[0003] Technical field of the invention

[0004] The present invention relates to an acquisition system and a method for acquiring a spectral reflectance and a relative position of a point of a scene.

[0005] State of the art

[0006] Spectral imaging technologies are known which are based on spectroscopy to measure the spectrum of the light coming from a scene of interest in the real world. The measured spectrum can be used to acquire information on the material composition of the objects in the scene, for example by comparison with spectral fingerprints of chemical substances.

[0007] Also known are time-of-flight ranging techniques which sends consecutive singlewavelength pulses hitting different points in a scanned scene and detect the time-of-flight of each pulse to calculate the relative position of the points.

[0008] Summary of the invention

[0009] “Scene” means the ensemble of spatial points, or the 3D point cloud, visible from a single point of view (e.g. the point of view of a detector) within a given solid angle. “Point of a scene” is a single point of a scene which can be fully illuminated by a single illumination pulse.

[0010] “Reflectance” of a point is the effectiveness of the surface of the point in reflecting radiant energy, e.g. the fraction of incident electromagnetic power that is reflected at the boundary surface. Reflectance is generally a component of the response of the electronic structure of the material of the surface of the point to the electromagnetic field of light and is in general a function of the wavelength of the light. Reflectance can be relative (i.e. with respect to other wavelengths of the spectrum) or absolute (i.e. the ratio of reflected energy to the incident energy).

[0011] “Spectral reflectance” refers to the dependence of the reflectance on the wavelength of the light.

[0012] “Relative position” of a point of a scene, or “3D position”, is the spatial position of the point with respect to a reference point, e.g. the point of location of the photodetetector or the launching point of the illumination pulses.

[0013] “3D spectral reconstruction” of a scene means a model (or hyperspectral 3D image)wherein each point of the real scene is associated with a spectral reflectance (“spectral reconstruction”) and with the relative position of the point (“3D reconstruction”) with respect to the reference point.

[0014] “Broadband” refers to a spectrum of the pulse which spans (discontinuously or, more preferably, continuously) at least 100 nm (i.e. the difference between the longest and shortest wavelengths is at least 100 nm), or at least 200 nm. In general, the ratio between the above wavelength difference and the central wavelength may be at least 0,2 or 0,3.

[0015] “Visible band” generally spans from about 400 nm to about 750 nm.

[0016] “Near infrared band” generally spans above about 750 nm, e.g. to about 1400 nm. “Time waveform” of a pulse refers to the temporal profile of the pulse intensity.

[0017] “Downstream” and “upstream” refers to the direction of propagation of light (i.e. the pulses).

[0018] “Time of flight” of an echoed pulse refers to the overall time of flight of the pulse from launch to reception (in other words it is the sum of the time of propagation of the illumination pulse from launch to the illuminated point and the time of propagation of the echoed pulse from the illuminated point to reception).

[0019] The Applicant has faced the problem of contemporaneously acquiring the spectral reflectance of a point of a scene and the relative position of said point, with high velocity (e.g. millions of acquisitions per second), high spatial and / or spectral resolution, high spatial accuracy (e.g. submillimetre-scale) and / or long-range (e.g. tens or hundreds of meters).

[0020] One or more of the above problems are solved by an acquisition system and a method for acquiring a spectral reflectance and a relative position of a point of a scene in accordance with the appended claims and / or having the following characteristics. According to an aspect the invention relates to an acquisition system for acquiring a spectral reflectance and a relative position of a point.

[0021] The acquisition system comprises:

[0022] - a laser source to emit in time sequence illumination pulses having an illumination spectrum, said illumination spectrum being broadband, wherein at each illumination pulse illuminating said point corresponds an echoed pulse echoed by said point having an echoed spectrum;

[0023] - an optical dispersive system to time stretch said illumination pulses and / or saidechoed pulses, so that after time stretching each echoed pulse has a stretched time waveform representative of said echoed spectrum;

[0024] - a detection system downstream to said dispersive system to detect said stretched time waveform of each echoed pulse and to generate a first signal comprising a detected time waveform representative of said stretched time waveform;

[0025] - a processing unit to process said first signal to determine, for each echoed pulse, said echoed spectrum based on said detected time waveform and to determine said spectral reflectance of said point by comparison between said echoed spectrum and said illumination spectrum.

[0026] Preferably said processing unit is configured to process said first signal to determine a time of flight of each echoed pulse based on said detected time waveform, and to determine said relative position of said point based on said time of flight.

[0027] According to an aspect, the invention relates to a method for acquiring a spectral reflectance and a relative position of a point.

[0028] Preferably said method comprises:

[0029] - emitting in time sequence illumination pulses having an illumination spectrum, said illumination spectrum being broadband;

[0030] - illuminating said point with one or more of said illumination pulses, wherein at each illumination pulse illuminating said point corresponds an echoed pulse echoed by said point having an echoed spectrum;

[0031] - time stretching said illumination pulses and / or said corresponding echoed pulses, so that each echoed pulse has a stretched time waveform representative of said echoed spectrum;

[0032] - subsequently to said time stretching, detecting said stretched time waveform of each echoed pulse and generating a first signal comprising a detected time waveform representative of said stretched time waveform;

[0033] - for each echoed pulse, determining said echoed spectrum based on said detected time waveform of said echoed pulse and determining said spectral reflectance of said point by comparing said echoed spectrum and said illumination spectrum of the corresponding illumination pulse.

[0034] Preferably said method comprises determining a time of flight of each echoed pulse based on said detected time waveform, and determining said relative position of said point based on said time of flight.According to an aspect, the invention relates to a 3D spectral reconstruction system of a scene comprising a plurality of points, the 3D spectral reconstruction system comprising the above acquisition system in one or more of the different embodiments. Preferably the 3D spectral reconstruction system comprises a scan system, more preferably downstream to said optical dispersive system, to direct said illumination pulses time sequentially onto each point of said scene, to sequentially illuminate each point of the scene by one or more of said illumination pulses, wherein for any given point of the scene, at each illumination pulse illuminating said point corresponds an echoed pulse echoed by said point.

[0035] Preferably the processing unit is configured to generate said 3D spectral reconstruction of said scene based on said spectral reflectance and said relative position of all the points of the scene.

[0036] According to an embodiment, the invention relates to a method for 3D spectral reconstruction of a scene comprising a plurality of points.

[0037] Preferably the method comprises the above method for acquiring a spectral reflectance and a relative position.

[0038] Preferably the method further comprises:

[0039] - sequentially illuminating each point of the scene by one or more of said illumination pulses, wherein for any given point of the scene, at each illumination pulse illuminating said point corresponds an echoed pulse echoed by said point.

[0040] Preferably the method comprises generating said 3D spectral reconstruction of said scene based on said spectral reflectance and said relative position of all the points of the scene.

[0041] The Applicant has found that by time stretching the pulses (at launch and / or at reception), so that each echoed pulse has a stretched time waveform representative of the illumination spectrum (in other words the intensity temporal profile of the pulse maps the spectral profile), allows to capture in the time domain said stretched time waveform. Thanks to the above technique, each spectral wavelength will have a different arrival time to the detector, thus mapping wavelengths in time. This in turns unlocks the possibility of measuring spectra with very high rate (e.g. tens of millions of spectra per second), much faster (e.g. three orders of magnitude faster) than any standard CCD / C MOS -based spectrometers (limited to few kHz camera speed), or tunable filters, which require building the spectrum by sequentially stacking single-wavelength datapoints.

[0042] The above solution allows not to use gratings for detection, which gratings display limited efficiency and limited spectral coverage due to ghosting by consecutive diffraction orders.

[0043] The time-stretching technique allows a very broadband spectrum coverage, because, at a given dispersion rate, the total spectral band can be extended by increasing the stretching ratio and the detection window, without the need to increase the number of detectors.

[0044] The use of illumination pulses, i.e. a controlled active remote illumination by the laser source (as opposed to common spectral imaging systems which use environmental light or a light source illuminating the whole scene such as a lamp), allows working both indoor and outdoor, also during night or in dark environments, irrespective of the ambient conditions and of the possibly irregular and uncontrolled ambient illumination (direct sunlight, dynamic shadows from clouds, diffused / reflected light from surrounding objects, halogen lamps, LEDs, or fluorescent light sources).

[0045] Laser pulse illumination may also provide sub-millimeter accuracy, and long-range capabilities, e.g. spanning from below 1m up to several km, as well high spatial resolution given by the small spot size of the illumination pulse.

[0046] The Applicant believes that the present invention provides great advantages with respect to a comparative solution, not forming part of the present invention, in which a hyperspectral camera sensor, which captures images of the target scene, is mounted next to a single-wavelength time-of-flight ranging sensor, distinct and separate from the latter. In other words, the spectral reflectance and relative position acquisitions are taken by mere juxtaposition of two separate devices, using fusion algorithms to combine hyperspectral data with 3D information from two different points of view. The Applicant believes that this comparative solution, as opposed to the present invention, is impractical if not impossible given the spatially uncorrelated data, measured at different moments in time from different observation angles and different spot sizes / resolution, hard to be fused, requiring complex calibrations and tracking algorithms.

[0047] Instead, the Applicant has found that the same time stretched broadband pulse once detected is suitable both for spectral reflectance acquisition and for time-of-flight position acquisition. According to the Applicant, determining both the spectralreflectance and the relative position based on the detected time waveform representative of the stretched time waveform of the same echoed pulse (i.e. the same detected pulse is used for both acquisitions contemporaneously) allows highly coherent correlation among the two acquisitions. Among others, the above solution assures that both acquisitions are taken by the same point of view and at the same time.

[0048] In addition, the entire acquisition process is simplified, with advantages in terms of speed and feasibility.

[0049] The high integration of the acquisition system provides advantages in terms of compactness, lightweight and portability of the equipment.

[0050] The present invention can have one or more of the following preferred features.

[0051] In one embodiment, said time stretching said illumination pulses and / or said corresponding echoed pulses comprises time stretching each illumination pulse to generate a stretched illumination pulse having a stretched illumination time waveform representative of said illumination spectrum.

[0052] In one embodiment, said time stretching each illumination pulse comprises:

[0053] - spectrally splitting each illumination pulse into two or more illumination sub-pulses, each illumination sub-pulse having a corresponding sub-spectrum, wherein the subspectra of all the illumination sub-pulses form said illumination spectrum, and

[0054] - individually time stretching each illumination sub-pulse and recombining all stretched illumination sub-pulses to form said stretched illumination pulse. Possibly said recombining comprises time delaying one or more stretched illumination sub-pulses relatively to the other stretched illumination sub-pulses to temporally overlap all recombined stretched illumination sub-pulses.

[0055] In this way the stretching requirements are relaxed, and the time duration of the stretched illumination pulse can be reduced, which is particularly advantageous in case of short distances.

[0056] In one embodiment, the method comprises spectrally splitting each echoed pulse into two or more echoed sub-pulses, each echoed sub-pulse having a corresponding echoed sub-spectrum, wherein the echoed sub-spectra of all the echoed sub-pulses form said echoed spectrum.

[0057] In one embodiment, said time stretching said illumination pulses and / or said corresponding echoed pulses comprises time stretching each echoed pulse to impartsaid stretched time waveform.

[0058] In one embodiment, said time stretching each echoed pulse comprises individually time stretching each echoed sub-pulse.

[0059] In one embodiment said time sequence of illumination pulses has a pulse repetition rate greater than or equal to 1 MHz, more preferably greater than or equal to 10 MHz, and / or smaller than or equal to 100 MHz.

[0060] In one embodiment said illumination pulses upon emission (before time stretching) have a time duration shorter than or equal to 100 ps, more preferably shorter than or equal to 10 ps, even more preferably shorter than or equal to 1 ps.

[0061] Preferably said illumination spectrum spans (discontinuously or, more preferably, continuously) at least 500 nm, more preferably at least 700 nm, even more preferably at least 900 nm, and / or no more than 2000 nm of bandwidth.

[0062] Preferably said illumination spectrum spans over at least part of the visible band and at least part of the near infrared band.

[0063] Preferably the shortest wavelength of said illumination spectrum is smaller than or equal to 600 nm, more preferably smaller than or equal to 500 nm, and / or higher than 200 nm.

[0064] Preferably the longest wavelength of said illumination spectrum is higher than or equal to 1200 nm, more preferably higher than or equal to 1400 nm, and / or smaller than 2000 nm.

[0065] Preferably said illumination pulses upon emission have energy greater than or equal to 10 nJ, more preferably greater than or equal to 50 nJ.

[0066] Preferably said laser source comprises a super-continuum laser.

[0067] The above parameters, made feasible by the present invention, provide high spectral and / or spatial resolution, and broad acquired spectral reflectance.

[0068] Preferably a ratio between a time duration of said stretched time waveform of each echoed pulse and a time duration of said corresponding illumination pulse upon emission has at least two orders of magnitude, preferably has three orders of magnitude or more.

[0069] Preferably a time duration of said stretched time waveform of each echoed pulse is greater than or equal to 1 ns, more preferably greater than or equal to 10 ns.

[0070] In this way, very broad illumination spectrum can be mapped into the stretched waveform.Preferably said optical dispersive system comprises a dispersive medium having group-velocity dispersion (i.e. the propagation speed of light varies as a function of its wavelength).

[0071] Preferably said dispersive medium comprises one or more optical fibers and / or one or more chirped fiber Bragg gratings.

[0072] Preferably said optical dispersive system comprises a diffraction system, comprising one or more diffraction elements (such as diffraction gratings or prisms), and, downstream to the diffraction system, said dispersive medium, e.g. a multimode waveguide or fiber. Preferably a lens is interposed between the diffraction system and the dispersive medium, wherein an input end of the dispersive medium is preferably placed at a focus of said lens.

[0073] Preferably said time stretching comprises subjecting (e.g. by said diffraction system) said illumination pulses (or sub-pulses) and / or said corresponding echoed pulses (or sub-pulses) to angular chromatic dispersion and, subsequently, to modal dispersion (e.g. by propagation through said multimode waveguide), to couple spectral components of said illumination spectrum into different propagation modes (e.g. of said multimode waveguide).

[0074] The combination of angular chromatic dispersion and modal dispersion provides very large group-velocity dispersion in any spectral region of the broadband illumination spectrum. It is also possible to tune the amount and sign of the group-velocity dispersion (e.g. by adjusting the alignment of the lens relative to the spatially dispersed spectrum).

[0075] Preferably said illumination spectrum and / or said echoed spectrum comprises a respective spectral reference feature at a fixed wavelength for all the pulses. The spectral reference feature is any recognizable spectral characteristic at the fixed wavelength, e.g. a spectral peak or a spectral notch, preferably having intensity (or prominence) at least about 5 dB or 10 dB higher or lower than an intensity of a surrounding (or remaining) part of said illumination and / or echoed spectrum, respectively.

[0076] Preferably said detected time waveform comprises a detected reference feature (possibly corresponding to said spectral reference feature), more preferably a detected peak or notch having a value at least about 5 dB or 10 dB higher or lower than a value of a surrounding (e.g. remaining) part of said detected time waveform.Preferably it is envisaged to relate a time stamp of each value of said detected time waveform to a time stamp of said detected reference feature. In this way, it is possible to realize a spectral self-calibration, wherein the spectral axis is adjusted for each pulse to assign each value to the correct spectral wavelength.

[0077] Preferably determining said time of flight is based on a time stamp of said detected reference feature. In this way, it is possible to accurately measure the time of flight, notwithstanding the relatively long duration of the stretched pulse. In fact, knowing the wavelength associated with the reference feature and its position in the waveform, it is possible to spectrally calibrate the signal and determine the arrival time of each individual wavelength with respect to the transmitted one. Consequently, the invention is capable of accurately measuring the time of flight for each spectral component, thereby resolving a possible ambiguity between spatial variations and spectral variations. This allows to maintain high accuracy even when using “long” pulses. Preferably said detection system comprises one or more first photodetectors, more preferably each one being single-pixel photodetector, to detect said stretched time waveform of the echoed pulses. In one embodiment said detection system comprises a plurality of first photodetectors, each first photodetector being configured to detect echoed sub-pulses having one respective echoed sub-spectrum among said echoed sub-spectra. In one embodiment detecting said stretched time waveform of each echoed pulse comprises individually detecting said respective two or more echoed sub-pulses and combining the respective detected stretched time waveforms. In this way it is possible to receive each sub-band with a dedicated optimized photodetector. Preferably the acquisition system comprises an optical (power) splitting system to generate, for each illumination pulse, a corresponding reference pulse having said illumination spectrum. Preferably said splitting system is downstream to said dispersive system, so that each reference pulse has a reference stretched time waveform representative of said illumination spectrum.

[0078] Preferably said detection system is configured to detect said reference stretched time waveform of each reference pulse and to generate a second signal comprising a reference detected time waveform representative of said reference stretched time waveform of said reference pulse.

[0079] Preferably said detection system comprises one or more second photodetectors, more preferably each one being single-pixel, to detect said reference stretched timewaveform of each reference pulse.

[0080] Preferably said processing unit is configured to process said second signal to determine, for each illumination pulse, said illumination spectrum based on said reference detected time waveform.

[0081] In this way, also the illumination spectrum is acquired in real-time and the echoed spectrum is compared with the illumination spectrum of the corresponding illumination pulse, thus obviating possible spectral fluctuations of the laser.

[0082] In an alternative solution, for example when the illumination spectrum is stable along the sequence of illumination pulses, the processing unit receives in input said illumination spectrum. In other words, there is no need to generate and detect the reference pulse for calibrating the spectral response.

[0083] Preferably said detection system comprises, downstream to said one or more first and / or second photodetectors, one or more (preferably real-time) analog-to-digital converter (ADC), more preferably having GSample / s digitizing speed.

[0084] Preferably, determining said spectral reflectance comprises determining an absolute spectral reflectance of each point of the scene based on said relative position of said point. In fact, knowing the distance of the object from the relative position, it is possible to calculate the solid angle of view and, in turn, estimate the absolute spectral reflectance of the point.

[0085] Brief description of the figures

[0086] Figure 1 shows a schematic diagram in terms of functional blocks of a 3D spectral reconstruction system according to the present invention;

[0087] figure 2 schematically shows the principle of operation of an exemplary optical dispersive system;

[0088] figure 3 schematically shows a time-wavelength diagram of a pulse according to an embodiment;

[0089] figure 4 shows a schematic diagram in terms of functional blocks of an acquisition system according to an embodiment of the present invention.

[0090] Detailed description of some embodiments of the invention

[0091] The features and the advantages of the present invention will be further apparent from the following detailed description of some embodiments, presented by way of nonlimiting example of the present invention, with reference to the attached figures.

[0092] In figure 1, the reference number 1 refers to a 3D spectral reconstruction systemaccording to the invention, able to acquire the spectral reflectance and the relative position of each point of a scene 100 comprising a plurality of points.

[0093] The 3D spectral reconstruction system 1 comprises the acquisition system 10 for acquiring a spectral reflectance and a relative position of a (single) point 101 of the scene 100 according to the invention.

[0094] The acquisition system 10 comprises a laser source 8 to emit in time sequence illumination pulses 9 having a broadband illumination spectrum 11 (illustratively depicted in the figures), wherein at each illumination pulse 9 illuminating the point corresponds an echoed pulse 12 echoed by the point and having an echoed spectrum 13 (illustratively depicted in the figures), generally different from the illumination spectrum 11 because of, among others, the spectral reflectance of the reflection / diffusion point 101.

[0095] Exemplarily, the laser source 8 comprises a super-continuum laser with a repetition rate between 1 to 80 MHz (depending on the target range, the shorter the range the higher the pulse rate), a pulse energy greater than 50 nJ and a pulse time duration upon emission in the femtosecond or picosecond range (figure 1 illustratively depicts a time waveform 14 of an illumination pulse 9 upon emission). The illumination spectrum exemplarily spans, preferably continuously, from about 450 nm (blue band) to about 1700 nm (IR band).

[0096] Preferably the illumination spectrum 11 and the echoed spectrum 13 comprises a respective spectral peak 16 (e.g. at about 1040 nm) having the function of a spectral reference feature. In fact, supercontinuum is typically generated via the interaction of nonlinear processes which initiate from short pulses at «1040-nm wavelength, with narrow (5-1 Onm) bandwidth. The conversion efficiency of this fundamental laser line into the other spectral components can be as high as 50%. The residual 1040-nm line generally remains distinguishable in the resulting supercontinuum spectrum as a spectral peak with about 10-dB higher intensity than the other wavelengths. This results in that also the pulse time waveforms 14’ and 15 after stretching typically show a respective intensity peak 17, 17’. The spectral reference feature can be, additionally or alternatively, a notch or any other spectral feature which is detectable and recognizable, to properly allocate the corresponding spectral position.

[0097] The acquisition system 10 further comprises an optical dispersive system 2 to time stretch the illumination pulses 9 and / or the echoed pulses 12, so that after timestretching each echoed pulse 12 has a stretched time waveform 15 (illustratively depicted in figure) representative of the echoed spectrum 13.

[0098] In one embodiment, the optical dispersive system 2 is placed solely at launch (upper dashed box 2 in figure 1) to time stretch solely the illumination pulses 9, so that after time stretching each stretched illumination pulse 9’ has a stretched illumination time waveform 14’ (illustratively depicted in figure) representative of the illumination spectrum 11.

[0099] In one embodiment, the optical dispersive system 2 is placed solely at reception (lower dashed box 2 in figure 1 ) to time stretch solely the echoed pulses 12. In this case, both the illumination pulse 9 and the echoed pulse 12 before time stretching maintain the short duration at emission.

[0100] In one embodiment, the optical dispersive system 2 is placed both at launch (upper dashed box 2 in figure 1 ) and at reception (lower dashed box 2 in figure 1 ): in other words, the total time stretching is the combination of a respective time stretching given both immediately after emission and, after reflection / diffusion, immediately before reception.

[0101] Preferably, as exemplarily shown in figure 2, the optical dispersive system 2 comprises, possibly at each site of deployment, a diffraction system 20, exemplarily comprising a pair of diffraction gratings, a multimode waveguide or fiber 21, downstream to the diffraction system 20 and a lens 22 interposed between the diffraction system 20 and the multimode waveguide 21 , an input end of the latter being placed at a focus of the lens (for further details see, e.g., the article “Giant tunable optical dispersion using chromo-modal excitation of a multimode waveguide” by Eric D. Diebold et al, Optics Express Vol. 19, Issue 24, pp. 23809-23817 (2011)).

[0102] Preferably the acquisition system 10 comprises an optical (power) splitting system 30 to generate, for each illumination pulse 9, a corresponding reference pulse 31 also having the illumination spectrum 11. Preferably the splitting system 30 is downstream to the dispersive system 2 (or to the portion of dispersive system 2 placed at launch). In other words, the illumination pulse 9 is split after the time stretching, so that also the reference pulse 31 is stretched with a stretched time waveform representative of the illumination spectrum 11.

[0103] Preferably the acquisition system 10 comprises a detection system 4 downstream to said dispersive system 2 to detect the echoed pulses 12, in particular to detect anddiscriminate in time the stretched time waveform 15 of each echoed pulse and to generate a first signal 40 comprising a detected time waveform representative of the stretched time waveform 15.

[0104] Preferably the detection system 4 comprises one first single-pixel photodetector 41 , to detect the stretched time waveform of the echoed pulses.

[0105] Preferably the detection system 4 comprises one second single-pixel photodetector 42, distinct from the first photodetector, to detect the reference stretched time waveform of each reference pulse 31. In this way, the detection system 4 is configured to detect the reference stretched time waveform of each reference pulse 31 and to generate a second signal 43 comprising a reference detected time waveform representative of the reference stretched time waveform of the reference pulse 31. Preferably the detection system 4 comprises, downstream to the first and / or second photodetector (e.g. avalanche photodiodes (APD), featuring >1 GHz bandwidth, such as the APD210 and APD310 models from Menlo Systems), one or more real-time analog-to-digital converter s (ADC), having GSample / s digitizing speed.

[0106] Preferably the acquisition system 10 comprises a processing unit 7 to process the first signal 40.

[0107] Preferably the 3D spectral reconstruction system 1 comprises a scan system 3, wherein the scene 100 is depicted as viewed by the point of view (e.g. the reception point). Exemplarily, the scan system 3 comprises a pair of galvanometric mirrors (not shown) rotating respectively about two orthogonal axes to direct the illumination pulses time sequentially onto each point 101 of the scene 100 (exemplarily row by row) to x,y scan the scene, i.e. to illuminate each point of the scene by one or more illumination pulses one point at a time.

[0108] The 3D spectral reconstruction system 1 is able to implement the method for 3D spectral reconstruction of the scene according to the invention.

[0109] The laser source 8 emits in time sequence the illumination pulses 9 which are time stretched in the optical dispersive system 2 as explained above. Exemplarily the time duration of each stretched pulse is about 10 ns.

[0110] The stretched pulses are then directed to the x,y scan system 3 which sequentially directs the illumination pulses to each point of the scene, at each illumination pulse illuminating the point corresponding an echoed pulse echoed by the point.

[0111] In one embodiment each point is illuminated by one and only one illumination pulse,but the invention contemplates any number of pulses for each point, followed by averaging the results for each point (to increase the precision of the result).

[0112] The echoed pulses 12 are time sequentially detected by the first photodetector which time-resolve the stretched time waveform of each echoed pulse and generate a first electric signal comprising a detected time waveform representative of the stretched time waveform. The signal is then sampled by the ADC.

[0113] The processing unit 7 processes the digitalized first signal and, for each echoed pulse, determines:

[0114] - the echoed spectrum based on the detected time waveform and the spectral reflectance of the corresponding point 101 by comparison between the echoed spectrum 11 and the illumination spectrum 11 of the corresponding illumination pulse 9; and

[0115] - an overall time of flight of the echoed pulse 12 based on the detected time waveform, and the relative position of the point 101 based on the overall time of flight.

[0116] Preferably the processing unit 7 processes the second signal to determine, for each illumination pulse 9, the illumination spectrum 11 based on the reference detected time waveform.

[0117] Preferably the detected time waveform comprises a detected peak corresponding to the spectral peak 16, having a value at least about 5dB higher than a value of a remaining part of the detected time waveform. Preferably the processing unit 7 references a time stamp of each value of the detected time waveform to a time stamp of the detected peak to realize a spectral self-calibration.

[0118] Preferably, each echoed pulse is detected before the subsequent illumination pulse is launched.

[0119] The processing unit 7 generates the 3D spectral reconstruction of the whole scene 100 based on the spectral reflectance and relative position of all the points 101 of the scene acquired as above.

[0120] Preferably, the processing unit 7 determines the absolute spectral reflectance of each point 101 based on the relative position of the point.

[0121] In one embodiment, as exemplary shown in figure 3 for two sub-bands, the pulses emitted by the laser are spectrally split (e.g. by way of a dichroic mirror) into two or more illumination sub-pulses spatially separated, each illumination sub-pulse having a corresponding sub-spectrum, wherein the sub-spectra of all the illumination sub-pulsesform said illumination spectrum. In the example shown, a first sub-band 50 span over the visible band (e.g. from about 450nm to about 1100 nm) and a second sub-band 51 spans over part of the near infrared band (e.g. from about 1000 nm to about 1700 nm). The two sub-bands partially overlap around the 1040 nm peak 16 or any other recognizable spectral feature (to facilitate subsequent recombination). After splitting, each illumination sub-pulse is individually time stretched (e.g. as described above) and the stretched illumination sub-pulses 52, 53 thus obtained are spatially recombined (e.g. by means of a further dichroic mirror) to form the stretched illumination pulse 9’. Possibly the sub-pulses before recombination are time delayed one with respect to the other to temporally overlap (partially or totally) the stretched illumination sub-pulses 52, 53 upon recombination (in this way the overall illumination pulse 9’ has a shorter overall duration, wherein at each instant correspond a pair of wavelengths, one for each sub-band).

[0122] After reflection by the point 101, the echoed pulse is spectrally split into two or more echoed sub-pulses before photodetection. In the example, each echoed pulse is split in the same two sub-bands described above, e.g. by way of a further dichroic mirror. Each sequence of sub-pulses having the same sub-band is photodetected by a respective dedicated photodetector. For example a Si-photodetector may be used for the first sub-band (visible) and an InGaAs-photodetector may be used for the second sub-band (NIR). The overall echoed spectrum is then reconstructed by merging the information from the two (or more) photodetectors.

[0123] Similar processing also applies for the reference pulse 31.

[0124] Fig. 4 shows an embodiment of a method for acquiring a spectral reflectance and a relative position of a point 101 by way of an acquisition system 10 of the present invention, wherein the scan system is absent or inactivated so that the illuminated point 101 is fixed in the x,y plane (only the z distance can possibly vary).

[0125] A material 102 in continuous form (such as an extruded compound or a polymeric or metal sheet or ribbon) flows continuously as indicated by the arrow.

[0126] In this way it is possible to simultaneously monitor in real time with very high rate both the thickness (based on the variation of the z distance) and the material composition of the continuous material 102.

Claims

CLAIMS1. Acquisition system (10) for acquiring a spectral reflectance and a relative position of a point (101), the acquisition system comprising:- a laser source (8) to emit in time sequence illumination pulses (9) having an illumination spectrum (11), said illumination spectrum (11) being broadband, wherein at each illumination pulse (9) illuminating said point (101 ) corresponds an echoed pulse (12) echoed by said point having an echoed spectrum (13);- an optical dispersive system (2) to time stretch said illumination pulses (9) and / or said echoed pulses (12), so that after time stretching each echoed pulse has a stretched time waveform (15) representative of said echoed spectrum (13);- a detection system (4) downstream to said dispersive system (2) to detect said stretched time waveform (15) of each echoed pulse (12) and to generate a first signal (40) comprising a detected time waveform representative of said stretched time waveform (15);- a processing unit (7) to process said first signal (40) to determine, for each echoed pulse (12), said echoed spectrum (13) based on said detected time waveform and to determine said spectral reflectance of said point (101) by comparison between said echoed spectrum (13) and said illumination spectrum (11) of the corresponding illumination pulse (9),the processing unit being configured to process said first signal (40) to determine a time of flight of each echoed pulse (12) based on said detected time waveform, and to determine said relative position of said point (101) based on said time of flight.

2. Acquisition system (10) according to claim 1, wherein said laser source comprises a super-continuum laser, and wherein said optical dispersive system (2) comprises a diffraction system (20), a dispersive medium (21) downstream to the diffraction system and a lens (22) interposed between the diffraction system and the dispersive medium and wherein said detection system (4) comprises one or more first photodetectors (41 ), each one being single-pixel photodetector, to detect said stretched time waveform of the echoed pulses.

3. Acquisition system (10) according to any one of the previous claims, further comprising an optical splitting system (30) to generate, for each illumination pulse, a corresponding reference pulse (31) having said illumination spectrum (11), said optical splitting system (30) being downstream to at least part of said dispersive system (2),so that each reference pulse (31) has a reference stretched time waveform representative of said illumination spectrum (11), wherein said detection system (4) comprises one or more second photodetectors (42), each one being single-pixel, to detect said reference stretched time waveform of each reference pulse (31), wherein said detection system (4) is configured to generate a second signal (43) comprising a reference detected time waveform representative of said reference stretched time waveform of said reference pulse and wherein said processing unit (7) is configured to process said second signal (43) to determine, for each illumination pulse (9), said illumination spectrum (11) based on said reference detected time waveform.

4. A 3D spectral reconstruction system (1) of a scene (100) comprising a plurality of points (101), the 3D spectral reconstruction system comprising:- the acquisition system (10) according to one or more of the preceding claims, - a scan system (3) to direct said illumination pulses (9) time sequentially onto each point (101) of said scene, to sequentially illuminate each point of the scene by one or more of said illumination pulses, wherein for any given point of the scene, at each illumination pulse (9) illuminating said point corresponds an echoed pulse echoed by said point,wherein the processing unit (7) is configured to generate said 3D spectral reconstruction of said scene (100) based on said spectral reflectance and said relative position of all the points (101 ) of the scene.

5. Method for acquiring a spectral reflectance and a relative position of a point (101), comprising:- emitting in time sequence illumination pulses (9) having an illumination spectrum (11 ), said illumination spectrum being broadband;- illuminating said point with one or more of said illumination pulses, wherein at each illumination pulse illuminating said point corresponds an echoed pulse (12) echoed by said point having an echoed spectrum (13);- time stretching said illumination pulses and / or said corresponding echoed pulses, so that each echoed pulse has a stretched time waveform (15) representative of said echoed spectrum (13);- subsequently to said time stretching, detecting said stretched time waveform of each echoed pulse and generating a first signal (40) comprising a detected time waveform representative of said stretched time waveform;- for each echoed pulse:- determining said echoed spectrum based on said detected time waveform of said echoed pulse and determining said spectral reflectance of said point by comparing said echoed spectrum and said illumination spectrum of the corresponding illumination pulse; and- determining a time of flight of each echoed pulse based on said detected time waveform, and determining said relative position of said point based on said time of flight.

6. Method according to claim 5, wherein said time stretching said illumination pulses and / or said corresponding echoed pulses comprises time stretching each illumination pulse (9) to generate a stretched illumination pulse (9’) having a stretched illumination time waveform representative of said illumination spectrum (11).

7. Method according to the preceding claim, wherein said time stretching each illumination pulse (9) comprises:- spectrally splitting each illumination pulse (9) into two or more illumination sub-pulses, each illumination sub-pulse having a corresponding sub-spectrum (50, 51), wherein the sub-spectra of all the illumination sub-pulses form said illumination spectrum, and - individually time stretching each illumination sub-pulse and recombining all stretched illumination sub-pulses (52, 53) to form said stretched illumination pulse (9’).

8. Method according to the any one of claims 5 to 7, further comprising spectrally splitting each echoed pulse into two or more echoed sub-pulses, each echoed subpulse having a corresponding echoed sub-spectrum, wherein the echoed sub-spectra of all the echoed sub-pulses form said echoed spectrum, and wherein detecting said stretched time waveform of each echoed pulse comprises individually detecting said respective two or more echoed sub-pulses and combining the respective detected stretched time waveforms.

9. Method according to any one of claims 5 to 8, wherein said illumination spectrum (11) and / or said echoed spectrum (13) comprises a respective spectral reference feature (16) at a fixed wavelength for all the pulses, wherein said detected time waveform comprises a detected reference feature corresponding to said spectral reference feature, the method further comprising relating a time stamp of each value of said detected time waveform to a time stamp of said detected reference feature, and wherein determining said time of flight is based on said time stamp of said detectedreference feature.

10. Method for 3D spectral reconstruction of a scene (100) comprising a plurality of points (101), the method comprising:- executing the method according to any one of claims 5 to 9;- sequentially illuminating each point of the scene by one or more of said illumination pulses, wherein for any given point of the scene, at each illumination pulse illuminating said point corresponds an echoed pulse echoed by said point; and- generating said 3D spectral reconstruction of said scene based on said spectral reflectance and said relative position of all the points of the scene.

Images