"Method for material discrimination and respective

implementation system"

Technical field

This application relates to a method for performing material discrimination and respective implementation system.

Background art

Light Detecting and Ranging (LIDAR) systems are used in a wide range of practical applications requiring remote measurements. In general, LIDAR comprises a set of techniques that use laser light to measure the distance to a specific target. By using appropriate optical scanning elements or by illuminating/ flashing a specific area of a target, it is possible to obtain 3D images with range information and backscattering properties of the target. Such systems provide a 3D point cloud frame that can be processed by software in order to obtain additional information of the surrounding scene. Consequently, 3D LIDAR imaging has been widely recognized as an attractive possibility for vehicular applications such as hazard and collision avoidance and autonomous/assisted navigation. However, object identification and recognition based on data provided by 3D LIDAR systems represent a complex multiparameter problem. In fact, given a 3D point cloud frame, the obtained data still needs to be processed in order to recognize, discriminate and classify key elements such as vehicles, pedestrians, buildings or any other obstacle. This kind of classification is of uttermost relevance for autonomous and driven-assisted navigation when hazard avoidance and self-steering decisions need to be made. In order to do so, several object recognition and mapping techniques and methodologies have been proposed, not only for terrestrial, but also for airborne applications. These techniques use the 3D information to determine the geometrical shape and edges of different objects in the illuminated scene to discriminate different types of targets. However, object recognition based solely on pure geometrical and dimensional properties is difficult, in particular when the resolution of the LIDAR cameras is not very high and when different types of targets have similar geometry.

In addition to 3D mapping, LIDAR sensors also provide information on the reflectivity properties of the illuminated targets by measuring the intensity of the reflected/backscattered light, as mentioned in document US9360554 B2. The referred document discloses a LIDAR system comprised by at least one emitter and a detector array, covering a given field of view where each of the emitters emits a single pulse or a multi-pulse packet of light that is sampled by the detector array. On each emitter cycle the detector array will sample the incoming signal intensity at the pre-determined sampling frequency that generates two or more samples per emitted light packet. This allows for time- of-flight measurements from the signal portion of each emitted light packet after reflection by one or more objects in the field of view and its detection at the detector.

For those skilled in the art, it is known that the polarization properties of light may vary when interacting with matter. As an example, the state of polarization of scattered light may be rotated or even become unpolarized at all (random polarization) . Document US7339670 B2 reports the use of polarization properties of backscattered light in airborne LIDAR systems for remote sensing of aerosols.

Summary

The present application describes a method for material discrimination comprising the following steps:

— emission of a polarized light beam;

— detection of the light beam that was reflected from a target; determination of target's range data, by measuring the time-of-flight of said light beam;

— generation of a 3D point cloud based on a 2D image complemented with the target's range data;

— determination of polarization data, by measuring the variation on the polarization properties of the reflected light beam in relation to the polarization properties of the emitted light beam;

— generation of a 3D point cloud with polarization information, by combining the 3D point cloud with the polarization data by means of image processing techniques .

In one embodiment of the method, the variations on the polarization properties of the reflected light beam in relation to the polarization properties of the emitted light beam are determined based on coefficients:

— p, representing the current ratio between the orthogonal polarization components of the reflected light ;

— c, representing the ratio between the current's perpendicular polarization component and total current, given by p/(l+p); — k, representing the degree of polarization of the detected light, given by (p-1 ) / ( 1+p) ;

— r, representing the fluorescence anisotropy parameter, given by (1- p)/(l+2p) .

In another embodiment of the method, the values of coefficients p,x,K,r are calculated for each field of view angle .

In another embodiment of the method, the 3D point cloud with polarization information is complemented with reflectivity data of the light beam.

In one embodiment of the method, the reflectivity data is determined by measuring the current generated by the reflected light beam.

In one embodiment of the method, the 3D point cloud with polarization information is complemented with velocity data.

In one embodiment of the method, the velocity data is determined by calculating the target's range difference between consecutive frames of the 3D point cloud.

The present application also describes a system for implementing the method for material discrimination of described above. Said system comprises:

— an emission unit comprising a light source;

— a detection unit;

a central processing unit provided with processing means adapted to control the operation of the emission unit and the detection unit, by actuating the light source; said central processing unit being configured to measure the optical properties of the reflected light related to polarization and reflectivity data and information regarding target's range and velocity; said processing means programed to execute image processing techniques configured to generate a 3D point cloud with polarization information and with reflectivity and velocity data.

In one embodiment of the system, the light source is a linearly-polarized laser diode or a circularly polarized laser diode with a quarter-wave plate.

In another embodiment of the system, the light source is a unit comprising unpolarized light sources and polarizers.

In one embodiment of the system, the emission unit comprises a scanner.

In another embodiment of the system, the emission unit comprises beam shaping optics.

In another embodiment of the system, the emission unit comprises a polarizer.

In one embodiment of the system, the emission unit is a rotating house.

In one embodiment of the system, the detection unit is comprised by a photodetector array with integrated micropolarizers and an optical unit.

In another embodiment of the system, the photodetector comprises a solid state detector array with a polarizing assembly, wherein each detector is divided into smaller subdetectors each with a micropolarizer with the polarizing axis at a specific orientation.

In another embodiment of the system, each detector is divided into four subdetectors with two micropolarizers oriented along a direction parallel to the polarization state of the emitter, and two micropolarizers oriented along a direction perpendicular to the polarization state of the emitter.

Yet in another embodiment of the system, each detector is divided into four subdetectors with micropolarizers, wherein a first micropolarizer is oriented along a direction parallel to the polarization state of the emitter; a second micropolarizer is oriented 45° with respect to the polarization state of the emitter; a third micropolarizer is oriented -45° with respect to the polarization state of the emitter; and a fourth micropolarizer is oriented along a direction perpendicular to the polarization state of the emitter .

In another embodiment of the system, the detection unit is comprised by a polarization-rotating element, a polarizer, an optical unit and a photodetector array.

In one embodiment of the system, the polarization-rotating element is a rotating half-wave plate.

In another embodiment of the system, the polarization- rotating element is a liquid crystal polarization rotator.

In another embodiment of the system, the polarization- rotating element is a Faraday rotator. Yet in another embodiment of the system, the polarization- rotating element is a Pockels cell.

In one embodiment of the system, the detection unit is comprised by a polarizing beam splitter, an optical unit, and two photodetectors (11) .

Finally, in another embodiment of the system, the optical unit is a lens or an arrangement of lenses, prisms and/or mirrors .

General Description

The present application relates to a method for material discrimination, based on LIDAR techniques and on the properties of the scattered light, in particularly the variations of the state of polarization of light backscattered from a target. In the context of this description, a target is assumed as any fixed or moving body such as for example, buildings, traffic signs, vehicles, animals or humans that are characterized by polarization properties of its respective constituent materials. The method and respective system now developed can be integrated with autonomous or assisted terrestrial vehicle' s driving systems based on LIDAR sensors, but should not be limited to that implementation.

The present application intends to solve the problem of object identification and recognition in 3D images, providing a way to discriminate different types of materials such as fabrics, fur, wood, paints, metal or concrete. In connection to this, the present technology combines information on the backscattered light parameters and image processing techniques for identification of people, vehicles, trees, traffic signs, buildings or animals. This approach allows performing up to a 6D analysis, where a 2D location in an image is combined with information regarding range, reflectivity, velocity and polarization of light, in order to provide material discrimination and consequently target classification.

Disclosed is a method for material discrimination, based on LIDAR techniques and on variations in the state of polarization of light backscattered from a specific target. By measuring the variation on the state of polarization of backscattered light allows gathering additional information on the constituent material of a specific target, which is used as complementary information for target recognition and classification.

In order to accomplish that, the method is implemented by means of a system comprised by an emission unit, a detection unit and a central processing unit. According to one aspect of the present application, the emission unit works as light source, and comprises a linearly-polarized laser diode. The emission unit also includes a scanner or beam shaping optics in order to scan or illuminate targets respectively.

According to another aspect of the present application, the detection unit comprises photodetectors, and is responsible for receiving the backscattered light from the targets.

In another aspect of the present application, the central processing unit controls the operation of both emission and detection unit, by actuating the laser source on the configuration parameters of the emitted light. It is also configured to determine the range information and the optical properties of the reflected light, including reflectivity and polarization data. Additionally, it can also be configured to determine velocity information. Range, reflectivity and polarization can be directly obtained from the electrical signals at the photodetectors. On the other hand, velocity can be computed via signal processing techniques. Range information, is determined by measuring the time-of-flight between emission and detection of the light beam. By means of image processing techniques, and using the range information applied to a 2D image, it is possible to generate a 3D point cloud. Based on that, the velocity information can be determined by measuring the target's range difference between consecutive frames of said 3D point cloud. Note that this provides velocity values with respect to a camera sensor position. The reflectivity is measured according to intensity of the light beam reflected from the target and can be quantified by the photocurrent generated in the photodetectors of the detection unit. The sum of electrical currents generated by said photodetectors, which are oriented along the parallel and perpendicular polarization components of emitter, provides the total intensity of the light backscattered form the target. Depending on the type of emitter used, scanning or flash type, a single photodetector or a photodetector array can be used to detect scattered light. In the former case, the field of view angle of the scattered light reaching the detector is determined by the deflection angle of the scanner. In the second case, each detector of the arrays can be sensitive only to light coming from a specific angle. In order to gather polarization properties of the reflected light, the detection unit may also comprise a set of other optical elements, such as beam splitters, or wave plates, or polarizers, that are arranged in a specific manner depending on the embodiment, as will be described later. Notwithstanding, the access to polarization properties of the reflected light, provides additional information on the constituent material of a specific target. In fact, when polarized light beam hits a specific target, the polarization properties of the reflected light are modified depending on the target's material. Said light beam is defined by a component that is oriented along a direction parallel to the polarization state of the emission unit, and another component that is oriented along a perpendicular direction. Based on that, a set of parameters can be defined in order to quantify the modification on the polarization properties of said reflected light beam. Assuming for simplicity that the orthogonal axis of the reflected light are aligned with directions parallel (II) and perpendicular (1) to the state of polarization of the emission unit, the respective coefficients are defined in Eq. 1, Eq. 2, Eq. 3 and Eq. 4 and are described hereinafter, wherein / _|| and 7 _± are the electrical currents, generated by the photodetectors of the detection unit, which are proportional to the intensity of the parallel and perpendicular polarization components of the reflected light, respectively. Coefficient p represents the current ratio between the perpendicular and parallel polarization components of the reflected light, whereas coefficient c gives the ratio between the perpendicular and total current. Coefficient k is related to the degree of polarization of the detected light and coefficient r is the fluorescence anisotropy parameter. r = / _|| -/ _L _ 1-p

Eq. 4

2/_L+/ _|| l+2p

All the previous parameters can be used to evaluate the variation of the polarization properties of the reflected light, without having to modify the detection architecture. Similarly to the intensity measurements, different values of the previous coefficients can be obtained for each field of view angle, thus enabling to obtain a 3D point cloud with polarization information. The central processing unit then uses the 3D point cloud with polarization information which complemented with reflectivity and velocity data, by means of image processing techniques, allows performing up to a 6D analysis able to provide type material discrimination and therefore target classification.

Brief description of drawings

For easier understanding of this application, figures are attached in the annex that represent the preferred forms of implementation which nevertheless are not intended to limit the technique disclosed herein.

Figure 1 illustrates one embodiment for the system architecture, where the reference signs represent:

1 - light source; 2 - scanner or beam shaping optics;

3.1, 3.2, 3.3 - targets;

4 - optical unit;

5 - photodetector array with integrated micropolarizers .

Figure 2 illustrates two different schemes for the photodetector array with integrated micropolarizers. On the left side, the polarizing grid is oriented along the parallel and perpendicular directions. On the right side, the integrated micropolarizers are oriented at the parallel and perpendicular directions and at 45° and -45°. The reference signs represent:

5 - photodetector array with integrated micropolarizers ;

6 - detector;

6.1 - sub-detector oriented along perpendicular direction;

6.2 - sub-detector oriented along parallel direction;

6.3 - sub-detector oriented at 45°;

6.4 - sub-detector oriented at -45°.

Figure 3 illustrates one embodiment for the system architecture, where the reference signs represent:

1 - light source;

2 - scanner or beam shaping optics;

3.1, 3.2, 3.3 - targets;

4 - optical unit;

7 - polarization-rotating element;

8 - polarizer;

9 - photodetector array. Figure 4 illustrates one embodiment for the system architecture, where the reference signs represent:

1 - light source;

2 - scanner or beam shaping optics;

3.1, 3.2, 3.3 - targets;

4 - optical unit;

10 - polarizing beam splitter;

11 - photodetectors.

Description of embodiments

Now, embodiments of the present application will be described with reference to the annexed drawings. However, they are not intended to limit the scope of this application.

It is described a method for material discrimination, based on LIDAR techniques and on the properties of the reflected light, in particularly the variations of the state of polarization of light backscattered from a target. In one embodiment, a polarized light beam is emitted, scanning or illuminating a target. The reflected light beam from the target is detected, and the respective polarization properties are determined. In one embodiment, the polarization data is quantified by measuring the variation on the polarization properties of the reflected light beam in relation to the polarization properties of the emitted light beam.

In one embodiment, the variations on the polarization properties of the reflected light beam in relation to the polarization properties of the emitted light beam are determined based on coefficients p, representing the current ratio between the orthogonal polarization components of the reflected light; c, representing the ratio between the current's perpendicular polarization component and total current, given by the degree of

p- 1

polarization of the detected light, given by ^an d ^r _>

representing the fluorescence anisotropy parameter, given by -——. The values of the above mentioned coefficients are l+2p

calculated for each field of view angles.

The central processing unit is configured to generate a 3D point cloud from a 2D image, captured from a sensor camera, complemented with range data extracted from the time-of- flight measurement between the emission and respective detection of a light beam. Then, by means of image processing techniques, a 3D point cloud with polarization information is generated.

In another embodiment, reflectivity data of the light beam is also considered, being determined by measuring the current generated by the reflected light beam on the photodetectors.

In another embodiment, velocity parameter is also considered, and for that, the central processing unit is configured to measure the target's range difference between consecutive frames of the 3D point cloud.

Combining 3D point cloud with polarization information with reflectivity data and velocity data, by means of image processing techniques, the central processing unit is able to perform a 6D analysis used to discriminate materials and classify targets. It is also described a system for implementing the method for material discrimination. In one embodiment of the system, it is comprised by an emission unit, a detection unit and a central processing unit. The emission unit is comprised by a light source (1) . In one embodiment, said light source (1) is a linearly-polarized laser diode, which either scans or illuminates targets by using a scanner or beam shaping optics (2) . As alternative, other configurations can be used such as, for instance, circularly polarized lasers with quarter- wave plate, or unpolarized light sources used in combination with polarizer elements. In an alternative embodiment, the emission unit could be a rotating housing that contains the light source (1) and the detectors within. Also, depending on the type of laser polarization properties of the emitted light, an additional polarizer may be required after the light source (1) in order to obtain a highly linearly- polarized beam.

In another embodiment, the detection unit comprises a photodetector array with integrated micropolarizers (5) and an optical unit (4), as illustrated in figure 1. In one embodiment, the optical unit (4) is a lens. The light backscattered from the targets is directly led to the lens, which focuses the reflected light to the photodetector array with integrated micropolarizers (5) . Said photodetector (5) comprises a solid state detector array with a polarizing assembly, e.g. metallic grid, formed directly on top of the detectors, though other alternatives are possible. The photodetector array (5) is composed of several detectors (6), which in turn are divided into smaller subdetectors, each with a micropolarizer with the polarizing axis at a specific orientation as illustrated in figure 2. The micropolarizer is used to select the polarization component arriving at each subdetector, so different orientations of the micropolarizer enable detecting different polarization components in the same detector (6), enabling to improve the sensibility of the detection unit to the polarization properties of the reflected light. According to figure 2, each detector (6) is divided into four subdetectors, with two of them oriented along the direction parallel (6.2) to the state of polarization of the emission unit - and the other two along the perpendicular direction (6.1) . Other configurations of the micropolarizers are possible, for example, each detector (6) is divided into four subdetectors with micropolarizers oriented (i) along a parallel direction (6.2), (ii) 45° with respect to the parallel direction (6.3), (iii) -45° with respect to parallel direction (6.4) and (iv) along a perpendicular direction (6.1) in relation to the polarization state of the emission unit.

In another embodiment, as illustrated in figure 3, the detection unit comprises a photodetector array (9), an optical unit (4), a polarization-rotating element (7) and a polarizer (8) . In one embodiment, the polarization-rotating element (7) is a rotating half-wave plate and the optical unit (4) is a lens. In this case, the reflected light passes through the half-wave plate and the polarizer (8) before the focusing lens and photodetector array (9) . The principle of operation of this embodiment is as follows. Polarizer (8) acts as a polarization filter as it lets only one of the polarization components reaching the photodetector array (9) . If the fast axis of the half-wave plate is aligned with the axis of the polarizer (8), the perpendicular polarization component of the reflected light is blocked by the polarizer (8), and only the parallel polarization component reaches the photodetector array (9) . On the other hand, if the fast axis of the half-wave plate is aligned at 45° with respect to the parallel direction, the perpendicular and parallel polarization components are rotated by 90°. By doing so, only the perpendicular component of the reflected light passes through the polarizer (8) and reaches the photodetector array (9), as the half-wave plate rotates the perpendicular polarization component to the parallel direction. Hence, it is possible to consecutively measure the parallel and perpendicular polarization components by rotating the half-wave plate between 0° and 45°. In one embodiment, the polarization-rotating element (7) is a liquid crystal polarization rotator, a Faraday rotator or a Pockels cell.

Yet in another embodiment, as illustrated in figure 4, the detection unit comprises a polarizing beam splitter (10), an optical unit (4) and two photodetectors (11) . In one embodiment, the optical unit (4) is an arrangement formed by two lenses. The light reflected is led to the polarizing beam splitter (10), which then separates the incoming beams into two orthogonal polarizations components, one parallel to the polarization state of the emission unit and other perpendicular. The two orthogonal polarization components of the incoming light are respectively led to the lenses aligned to each photodetector (11), which focus the received light onto the photodetectors (11) . The two photodetectors (11) are aligned each one with the parallel and perpendicular polarization state of the emitter, generating electrical currents which are proportional to the intensity of the parallel and perpendicular polarization components of the reflected light. In the embodiments described above the lens that formed the optical unit (4) can be a single lens or a complex optical arrangement of lenses, prisms and/or mirrors in order to correct for spherical aberrations, image distortions or to improve the field of view or the angular resolution.

This description is of course not in any way restricted to the forms of implementation presented herein and any person with an average knowledge of the area can provide many possibilities for modification thereof without departing from the general idea as defined by the claims. The forms of implementation described above can obviously be combined with each other. The following claims further define the forms of implementation.