WO2023285860A1 - System to determine the maximum range of a lidar sensor - Google Patents

System to determine the maximum range of a lidar sensor Download PDF

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Publication number
WO2023285860A1
WO2023285860A1 PCT/IB2021/056370 IB2021056370W WO2023285860A1 WO 2023285860 A1 WO2023285860 A1 WO 2023285860A1 IB 2021056370 W IB2021056370 W IB 2021056370W WO 2023285860 A1 WO2023285860 A1 WO 2023285860A1
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WIPO (PCT)
Prior art keywords
optical
lidar
diffuser
collimator
previous
Prior art date
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PCT/IB2021/056370
Other languages
French (fr)
Inventor
Nelssom FERNANDEZ DA CUNHA
José Carlos VIANA GOMES
Filipe André PEIXOTO OLIVEIRA
Luis Manuel FERNANDES REBOUTA
Manuel José DE LIMA FERREIRA RODRIGUES
Narciso Alexandre GOUVEIA SOARES
Flávio Pedro GONÇALVES FERNANDES FERREIRA
Original Assignee
Bosch Car Multimedia Portugal, S.A.
Universidade Do Minho
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Application filed by Bosch Car Multimedia Portugal, S.A., Universidade Do Minho filed Critical Bosch Car Multimedia Portugal, S.A.
Publication of WO2023285860A1 publication Critical patent/WO2023285860A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/497—Means for monitoring or calibrating
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
    • G01S17/06—Systems determining position data of a target
    • G01S17/08—Systems determining position data of a target for measuring distance only
    • G01S17/10—Systems determining position data of a target for measuring distance only using transmission of interrupted, pulse-modulated waves

Definitions

  • the present application describes a system and method to determine the maximum range of a LiDAR sensor.
  • Disclosed methods for determining the maximum range of LiDAR sensors may be classified into three categories: methods relying on time of flight (ToF) measures; methods relying on signal detection measures; mixed methods relying on both ToF measures and signal detection measures.
  • TOF time of flight
  • the maximum range of a LiDAR is fundamentally limited by the emitted power and the receiver sensitivity which impose an extinction of the LiDAR sensor detection when a target of a given reflectance is at a distance larger than the LiDAR maximum range for that target. For this reason, hereafter disclosures based solely on ToF measures will not be discussed.
  • EP0601872A1 Laser rangefinder testing system incorporating range simulation discloses a range test simulator system that evaluates a laser rangefinder's output energy, with an optical diffuser and a photodetector, and programs the laser transmitter to fire into the rangefinder a delayed pulse at a specific optical power level needed to simulate a return signal from a target for a given range, atmospheric attenuation, and target reflectivity.
  • This invention has the ability of testing in a small area the maximum range of a laser rangefinder. Nevertheless, it has the disadvantages of regenerating the emitted pulse, which loses its temporal shape, and of requiring a system calibration in order to generate an optical signal with a specific power level to imitate the return from a real target.
  • US5281813A Laser rangefinder test system discloses a technique comprising a mechanism for detecting the emission of a pulse from the laser rangefinder under test and to trigger, with a predetermined delay, a radiation source which, after collimation with a convex lens, simulates the return optical signal from a hypothetical target.
  • this invention has the advantage of incorporating a collimation lens which generates a simulated return optical signal with an infinite wavefront radius, close to the one from a distant real target, but still has the disadvantages of not preserving the temporal shape of the rangefinder emitted pulse and of requiring a radiometric calibration.
  • Range calibration instrument of laser rangefinder discloses a range calibration instrument of a laser rangefinder comprising an optical fiber and two convex lenses.
  • the rangefinder emitted beam is injected in one end of the optical fiber by means of the first lens, with the light transmitted by the optical fiber being collimated by the second lens.
  • the fiber length produces the same time delay as with a distant target.
  • this utility model has the advantage of generating a simulated return with the same temporal shape as the return from a real distant target, but unlike previous mentioned documents, is not directly applicable to LiDARs with a single optical axis.
  • US2020363528A1 Determining the maximum range of a lidar sensor discloses a method to estimate the maximum range comprising the steps of sending a LIDAR beam at a predetermined normal emission power and receiving sensitivity onto a set of targets at known distances; decreasing by a certain amount the emitted power and/or receiving sensitivity until some objects are no longer detected; estimating the maximum range.
  • this method is simple and removes the need of ToF measures for determining a LiDAR maximum range. Only a radiometric equivalence is considered based on the extinction of the detection of objects at known distances when the signal is attenuated by a given amount. However, as stated by the inventors, it is necessary an empirical calibration to ascertain the LiDAR maximum range for objects placed at particular distances.
  • the present invention describes a system to determine the maximum range of a LiDAR sensor, comprising: an optical collimator, comprising an optical axis; an optical diffuser, positioned at a focal point which is aligned with the optical axis of the optical collimator; and an optical attenuator; wherein the optical axis of the optical collimator is parallel aligned with the optical path of a laser beam emitted by the LiDAR sensor; the optical attenuator is installed within the optical path of the laser beam and/or within the optical path of a light beam reflected by the optical diffuser (13); and the optical collimator is adapted to focus the laser beam on the optical diffuser and to collimate the reflected light beam received by the LiDAR sensor.
  • the optical attenuator is installed between the optical diffuser and the optical collimator.
  • the output collimated light beam is modulated by adjusting the optical attenuator transmittance.
  • the output collimated light beam is modulated by adjusting the optical diffuser reflectance.
  • the optical collimator comprises one of a convex lens, a concave mirror, or any combination of optical elements with an equivalent focal point.
  • the optical diffuser is adapted to back-scatter light beams with an angular distribution larger than the numerical aperture angle of the optical collimator.
  • the optical attenuator comprises one of an optical density filters, polarizers, or any other conveniently adapted to alone or in combination attenuate the laser beam and/or the output collimated light beam.
  • the optical collimator comprises a parabolic mirror adapted to focus the laser beam parallel to the optic axis onto the optical diffuser positioned at the focal point, said optical diffuser comprising a Lambertian diffusing target.
  • the optical collimator comprises an off-axis parabolic mirror adapted to focus the laser beam parallel to the optic axis onto the optical diffuser positioned at the focal point, said optical diffuser comprising a Lambertian diffusing target with a target surface normal comprising orientation adaptability in order to vary the amount of backscattered light along the optic axis direction.
  • the present invention also describes the method to determine the maximum range of a LiDAR sensor based on the above- described embodiments of the invention, comprising the steps of: decreasing the amplitude of the light signal reflected by the system, by varying the optical attenuator transmittance or the optical diffuser reflectance; monitoring the light signal detected by the LiDAR; checking the light signal extinction; calculating the distance range of the LiDAR sensor based on a radiometric equivalence to a distant free space diffusing target with a specified reflectance.
  • the radiometric equivalence is obtained by the ratio between the optical irradiance received by the LiDAR sensor and the optical power emitted by the LiDAR sensor.
  • the present application describes a system and method thereof for testing the maximum range of LiDAR sensors in a small space.
  • the invention overcomes the need of a radiometric calibration. This feature enables the building of a test system that can be used to determine the maximum range of different LiDARs using the same setup and without the costs of recalibrating the test system, an advantage for both the development and the production of LiDAR sensors
  • the developed system for determining the maximum range of a LiDAR comprises an optical collimator, an optical diffuser, and an optical attenuator, wherein the optical diffuser is placed at the focal point of the optical collimator.
  • a method thereof to determine the maximum range of the LiDAR whereby the optical attenuator transmittance or the optical diffuser reflectance are decreased until no signal detection, and whereby the maximum range is determined by means of a radiometric equivalence to a free space diffusing target.
  • the apparatus in one of the proposed arrangements comprises:
  • An optical diffuser placed at the optical collimator focal position
  • An optical attenuator The laser beam emitted by the LiDAR is focused by the optical collimator on the optical diffuser. The light reflected by the optical diffuser is collimated back to the LiDAR receiver optics. The irradiance of the output collimated light is modulated by adjusting the optical attenuator transmittance or the optical diffuser reflectance.
  • the method to determine the LiDAR maximum comprises the steps of:
  • the present invention disclosure will enable the testing of the maximum range of LiDAR sensors within a small space, which is fundamental for the industrialization of such devices, doing that by overcoming the need of a radiometric calibration.
  • the herein disclosed system allows to determine the maximum range of LiDAR sensors, including, in a nonlimiting way, laser rangefinders, spinning LiDARs, and scanning LiDARs.
  • Fig. 1 - illustrates a preferred embodiment for the system (100) to determine the maximum range of a LiDAR sensor, wherein the reference numbers relate to:
  • Fig. 2 - illustrates a preferred embodiment for the system (100), wherein the reference numbers relate to:
  • Fig. 3 - illustrates a preferred embodiment for the system (100), wherein the reference numbers relate to:
  • FIG. 4 - illustrates a preferred embodiment for the operating method of the system (100), wherein the reference numbers relate to:
  • the present invention refers to a system and method to determine the maximum range of a LiDAR.
  • embodiments of the invention are described in detail; wherein like parts of the invention are designated by like numbers in the accompanying drawings.
  • the invention discloses a system (100) to determine the maximum range of a LiDAR sensor, shown schematically in FIG. 1, comprising an optical collimator (10) with an optical axis (11) and focal point (12), an optical diffuser (13) placed at the focal point (12), an optical attenuator (14) located in front or in the back of the collimator (10) to attenuate the input beam (16) emitted by the LiDAR or the output beam (17) received by the LiDAR.
  • the optical collimator (10) comprises one of a convex lens, a concave mirror, or any combination of optical elements capable of focusing a collimated beam into a focal point (12) and, conversely, to collimate a divergent beam coming from the focal point (12).
  • This feature ensures that the optical return from the system (100), i.e., the output beam (17) received by the LiDAR, has the same direction as the LiDAR beam (16) entering the system (100), as with a real target at a far distance.
  • the optical diffuser (13) comprises an element capable of back-scatter light with a scattering angular distribution substantially larger than the numerical aperture of the focusing and collimation optics (10).
  • the attenuator element i.e., the optical attenuator (14), may comprise one of an optical density filters, polarizers, or any other convenient mean that alone or in combination attenuate the transmitted light beams, in particular input beam emitted by the LiDAR (16) and output beam received by the LiDAR (17).
  • W /X f (a) and forms with the LiDAR receiver optics, with focal length f' and usually focused on infinity, an imaging system with magnification —f'/f, producing a spot size on the focal plane of the LiDAR receiver optics given by
  • the focusing and collimation optics (10) produces a virtual image of the spot focused (12) on the diffusing target (13) which appear to be at infinity, whereas the spot of the LiDAR beam on a far real target is at a finite distance not exceeding its maximum range d.
  • the optical collimator (10) comprises a parabolic mirror which focus the emitted LiDAR beam (16) parallel to the optic axis (11) onto the mirror focal point (12) irrespective of the beam wavelength, and the optical diffuser (13) is a Lambertian diffusing target placed at the optics focal point (12) and having a known reflectance for the LiDAR wavelength.
  • the optical collimator (10) is an off-axis parabolic mirror which focus the LiDAR beam (16) parallel to the optic axis (11) onto the mirror focal point (12), the optical diffuser (13) is a Lambertian diffusing target whose surface normal (35) can be oriented to vary the amount of backscattered light along the optic axis direction (11), and the optical attenuator (14) is an optical density filter with a known transmittance.
  • the disclosed invention includes a method thereof to determine the maximum range of a LiDAR, shown schematically in FIG. 4, and comprising the steps of: decreasing the amplitude of the signal reflected by the system (40); monitoring the signal detected by the LiDAR (41) and checking its extinction (42); calculating the distance range (43) of the LiDAR using a radiometric formula equating the ratio between the irradiance received by the LiDAR and the power emitted by the LiDAR to the corresponding ratio for a free space diffusing target with a specified reflectance.
  • the system (100) response is controlled by rotating a Lambertian diffusing target (13), with reflectance p, producing a reflected intensity I for an incident power P varying with the angle Q between the diffusing target normal (35) and the optical axis direction (11), according to the Lambert's cosine law:
  • the attenuator (14) has a fixed transmittance T, resulting in a ratio between the output irradiance and input power, given by being R the mirror reflectivity.
  • the optical response for a real target with reflectance p', placed at a distance d, with an atmosphere with attenuation coefficient y a tm, is
  • the distance d given by equation (10) is the maximum range of the LiDAR: if the atmospheric attenuation is neglected, it directly gives the maximum range; otherwise, it can be solved numerically to find the maximum range value.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • General Physics & Mathematics (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Electromagnetism (AREA)
  • Optical Radar Systems And Details Thereof (AREA)

Abstract

"System to determine the maximum range of a LiDAR sensor" The present invention describes a system (100) and method to determine the maximum range of a LiDAR sensor. The proposed system comprises: an optical collimator (10); an optical diffuser (13), placed at the optical collimator focal position (12); and an optical attenuator (14). The laser beam (16) emitted by the LiDAR is focused by the optical collimator (10) on the optical diffuser (13), and the light reflected by the optical diffuser (13) is collimated back to the LiDAR receiving optics. The irradiance of the output collimated light (17) is modulated by adjusting the optical attenuator (14) transmittance or the optical diffuser (13) reflectance.

Description

DESCRIPTION
"System to determine the maximum range of a LiDAR sensor"
Technical Field
The present application describes a system and method to determine the maximum range of a LiDAR sensor.
Background art
Over the last decades, some methods have been disclosed how to solve the problem of testing the maximum range of LiDAR sensors within a limited space, instead of the large space required in an outdoor field test. A reliable solution to this problem is of particularly importance in a development laboratory or in a manufacturing floor since it allows to reduce maintenance costs, relieve safety concerns, and avoid uncontrolled atmospheric conditions.
Disclosed methods for determining the maximum range of LiDAR sensors may be classified into three categories: methods relying on time of flight (ToF) measures; methods relying on signal detection measures; mixed methods relying on both ToF measures and signal detection measures.
Whereas the testing of ToF accuracy of a LiDAR is useful to determine the corresponding range accuracy, the maximum range of a LiDAR is fundamentally limited by the emitted power and the receiver sensitivity which impose an extinction of the LiDAR sensor detection when a target of a given reflectance is at a distance larger than the LiDAR maximum range for that target. For this reason, hereafter disclosures based solely on ToF measures will not be discussed. EP0601872A1 Laser rangefinder testing system incorporating range simulation, discloses a range test simulator system that evaluates a laser rangefinder's output energy, with an optical diffuser and a photodetector, and programs the laser transmitter to fire into the rangefinder a delayed pulse at a specific optical power level needed to simulate a return signal from a target for a given range, atmospheric attenuation, and target reflectivity. This invention has the ability of testing in a small area the maximum range of a laser rangefinder. Nevertheless, it has the disadvantages of regenerating the emitted pulse, which loses its temporal shape, and of requiring a system calibration in order to generate an optical signal with a specific power level to imitate the return from a real target.
US5281813A Laser rangefinder test system, discloses a technique comprising a mechanism for detecting the emission of a pulse from the laser rangefinder under test and to trigger, with a predetermined delay, a radiation source which, after collimation with a convex lens, simulates the return optical signal from a hypothetical target. Relative to EP0601872A1, this invention has the advantage of incorporating a collimation lens which generates a simulated return optical signal with an infinite wavefront radius, close to the one from a distant real target, but still has the disadvantages of not preserving the temporal shape of the rangefinder emitted pulse and of requiring a radiometric calibration.
CN203455473U Range calibration instrument of laser rangefinder discloses a range calibration instrument of a laser rangefinder comprising an optical fiber and two convex lenses. The rangefinder emitted beam is injected in one end of the optical fiber by means of the first lens, with the light transmitted by the optical fiber being collimated by the second lens. The fiber length produces the same time delay as with a distant target. Relative to previously mentioned EP0601872A1 and US5281813A, this utility model has the advantage of generating a simulated return with the same temporal shape as the return from a real distant target, but unlike previous mentioned documents, is not directly applicable to LiDARs with a single optical axis.
US2020363528A1 Determining the maximum range of a lidar sensor, discloses a method to estimate the maximum range comprising the steps of sending a LIDAR beam at a predetermined normal emission power and receiving sensitivity onto a set of targets at known distances; decreasing by a certain amount the emitted power and/or receiving sensitivity until some objects are no longer detected; estimating the maximum range. Relative to previously mentioned patent documents, this method is simple and removes the need of ToF measures for determining a LiDAR maximum range. Only a radiometric equivalence is considered based on the extinction of the detection of objects at known distances when the signal is attenuated by a given amount. However, as stated by the inventors, it is necessary an empirical calibration to ascertain the LiDAR maximum range for objects placed at particular distances.
Summary
The present invention describes a system to determine the maximum range of a LiDAR sensor, comprising: an optical collimator, comprising an optical axis; an optical diffuser, positioned at a focal point which is aligned with the optical axis of the optical collimator; and an optical attenuator; wherein the optical axis of the optical collimator is parallel aligned with the optical path of a laser beam emitted by the LiDAR sensor; the optical attenuator is installed within the optical path of the laser beam and/or within the optical path of a light beam reflected by the optical diffuser (13); and the optical collimator is adapted to focus the laser beam on the optical diffuser and to collimate the reflected light beam received by the LiDAR sensor.
Yet in another proposed embodiment of present invention, the optical attenuator is installed between the optical diffuser and the optical collimator.
Yet in another proposed embodiment of present invention, the output collimated light beam is modulated by adjusting the optical attenuator transmittance.
Yet in another proposed embodiment of present invention, the output collimated light beam is modulated by adjusting the optical diffuser reflectance.
Yet in another proposed embodiment of present invention, the optical collimator comprises one of a convex lens, a concave mirror, or any combination of optical elements with an equivalent focal point.
Yet in another proposed embodiment of present invention, the optical diffuser is adapted to back-scatter light beams with an angular distribution larger than the numerical aperture angle of the optical collimator. Yet in another proposed embodiment of present invention, the optical attenuator comprises one of an optical density filters, polarizers, or any other conveniently adapted to alone or in combination attenuate the laser beam and/or the output collimated light beam.
Yet in another proposed embodiment of present invention, the optical collimator comprises a parabolic mirror adapted to focus the laser beam parallel to the optic axis onto the optical diffuser positioned at the focal point, said optical diffuser comprising a Lambertian diffusing target.
Yet in another proposed embodiment of present invention, the optical collimator comprises an off-axis parabolic mirror adapted to focus the laser beam parallel to the optic axis onto the optical diffuser positioned at the focal point, said optical diffuser comprising a Lambertian diffusing target with a target surface normal comprising orientation adaptability in order to vary the amount of backscattered light along the optic axis direction.
The present invention also describes the method to determine the maximum range of a LiDAR sensor based on the above- described embodiments of the invention, comprising the steps of: decreasing the amplitude of the light signal reflected by the system, by varying the optical attenuator transmittance or the optical diffuser reflectance; monitoring the light signal detected by the LiDAR; checking the light signal extinction; calculating the distance range of the LiDAR sensor based on a radiometric equivalence to a distant free space diffusing target with a specified reflectance. In a proposed embodiment of the method, the radiometric equivalence is obtained by the ratio between the optical irradiance received by the LiDAR sensor and the optical power emitted by the LiDAR sensor.
General Description
The present application describes a system and method thereof for testing the maximum range of LiDAR sensors in a small space. Relative to prior art, the invention overcomes the need of a radiometric calibration. This feature enables the building of a test system that can be used to determine the maximum range of different LiDARs using the same setup and without the costs of recalibrating the test system, an advantage for both the development and the production of LiDAR sensors
The developed system for determining the maximum range of a LiDAR comprises an optical collimator, an optical diffuser, and an optical attenuator, wherein the optical diffuser is placed at the focal point of the optical collimator.A method thereof to determine the maximum range of the LiDAR, whereby the optical attenuator transmittance or the optical diffuser reflectance are decreased until no signal detection, and whereby the maximum range is determined by means of a radiometric equivalence to a free space diffusing target.
Herein, the system and method thereof are summarized. The apparatus in one of the proposed arrangements comprises:
An optical collimator;
An optical diffuser, placed at the optical collimator focal position; and
An optical attenuator. The laser beam emitted by the LiDAR is focused by the optical collimator on the optical diffuser. The light reflected by the optical diffuser is collimated back to the LiDAR receiver optics. The irradiance of the output collimated light is modulated by adjusting the optical attenuator transmittance or the optical diffuser reflectance.
The method to determine the LiDAR maximum comprises the steps of:
1. Adjusting the optical attenuator transmittance or the optical diffuser reflectance to decrease the system output irradiance;
2. Verify the extinction of signal detection on the LiDAR receiver;
3. Calculate the maximum range of the LiDAR for a specified reflectance by a radiometric equivalence of the predicted system output irradiance to the predicted irradiance from a free-space diffusing target with that reflectance.
The present invention disclosure will enable the testing of the maximum range of LiDAR sensors within a small space, which is fundamental for the industrialization of such devices, doing that by overcoming the need of a radiometric calibration. The herein disclosed system allows to determine the maximum range of LiDAR sensors, including, in a nonlimiting way, laser rangefinders, spinning LiDARs, and scanning LiDARs.
Brief description of the drawings
For better understanding of the present application, figures representing preferred embodiments are herein attached which, however, are not intended to limit the technique disclosed herein.
Fig. 1 - illustrates a preferred embodiment for the system (100) to determine the maximum range of a LiDAR sensor, wherein the reference numbers relate to:
10 - optical collimator;
11 - optical axis;
12 - focal point;
13 - optical diffuser;
14 - optical attenuator;
16 - input beam emitted by the LiDAR / laser beam;
17 - output beam received by the LiDAR / output collimated light beam;
Fig. 2 - illustrates a preferred embodiment for the system (100), wherein the reference numbers relate to:
10 - optical collimator;
11 - optical axis;
12 - focal point;
13 - optical diffuser;
16 - input beam emitted by the LiDAR.
Fig. 3 - illustrates a preferred embodiment for the system (100), wherein the reference numbers relate to:
10 - optical collimator;
11 - optical axis;
12 - focal point;
13 - optical diffuser;
14 - optical attenuator;
16 - input beam emitted by the LiDAR;
35 - target surface normal. Fig. 4 - illustrates a preferred embodiment for the operating method of the system (100), wherein the reference numbers relate to:
40 - decrease reflected signal amplitude;
41 - monitor signal detection;
42 - signal extinction verification;
43 - calculate distance range.
Description of Embodiments
With reference to the figures, some embodiments are now described in more detail, which are however not intended to limit the scope of the present application.
The present invention refers to a system and method to determine the maximum range of a LiDAR. Herein, embodiments of the invention are described in detail; wherein like parts of the invention are designated by like numbers in the accompanying drawings.
The invention discloses a system (100) to determine the maximum range of a LiDAR sensor, shown schematically in FIG. 1, comprising an optical collimator (10) with an optical axis (11) and focal point (12), an optical diffuser (13) placed at the focal point (12), an optical attenuator (14) located in front or in the back of the collimator (10) to attenuate the input beam (16) emitted by the LiDAR or the output beam (17) received by the LiDAR.
The optical collimator (10) comprises one of a convex lens, a concave mirror, or any combination of optical elements capable of focusing a collimated beam into a focal point (12) and, conversely, to collimate a divergent beam coming from the focal point (12). This feature ensures that the optical return from the system (100), i.e., the output beam (17) received by the LiDAR, has the same direction as the LiDAR beam (16) entering the system (100), as with a real target at a far distance.
The optical diffuser (13) comprises an element capable of back-scatter light with a scattering angular distribution substantially larger than the numerical aperture of the focusing and collimation optics (10).
The attenuator element, i.e., the optical attenuator (14), may comprise one of an optical density filters, polarizers, or any other convenient mean that alone or in combination attenuate the transmitted light beams, in particular input beam emitted by the LiDAR (16) and output beam received by the LiDAR (17).
The optical collimator (10), with focal length f, focus the LiDAR beam (16) with divergence f on the diffusing target, producing a focal spot size,
W = /X f (a) and forms with the LiDAR receiver optics, with focal length f' and usually focused on infinity, an imaging system with magnification —f'/f, producing a spot size on the focal plane of the LiDAR receiver optics given by
Figure imgf000011_0001
On the other hand, the spot size of the LiDAR beam hitting a target placed at a far-field distance d is given by w = dx f (c) and the corresponding spot size on the focal plane of the LiDAR receiver optics, with focal length f' and approximate magnification — f'/d, is given by
Figure imgf000012_0001
which is the same as the spot size of equation (b). Therefore, the virtual target seen through the focusing and collimation optics (10) produces the same spot size on the focal plane of the LiDAR receiver optics as with a real distant target.
The focusing and collimation optics (10) produces a virtual image of the spot focused (12) on the diffusing target (13) which appear to be at infinity, whereas the spot of the LiDAR beam on a far real target is at a finite distance not exceeding its maximum range d. However, the LiDAR receiver optics cannot distinguish the two situations if d > H = f#— (e)
C where H is the hyperfocal distance, f# is the f-number, D the entrance pupil aperture diameter and c the size of the circle of confusion. Typical values for a well corrected lens are c 5 micron; f# 2; D « 2.5 cm, leading to H = 25m, which is substantially smaller than the maximum range of most LiDAR sensors. In a preferred embodiment of the system, depicted in FIG. 2, the optical collimator (10) comprises a parabolic mirror which focus the emitted LiDAR beam (16) parallel to the optic axis (11) onto the mirror focal point (12) irrespective of the beam wavelength, and the optical diffuser (13) is a Lambertian diffusing target placed at the optics focal point (12) and having a known reflectance for the LiDAR wavelength.
In a preferred embodiment of the system (100), disclosed in FIG. 3, the optical collimator (10) is an off-axis parabolic mirror which focus the LiDAR beam (16) parallel to the optic axis (11) onto the mirror focal point (12), the optical diffuser (13) is a Lambertian diffusing target whose surface normal (35) can be oriented to vary the amount of backscattered light along the optic axis direction (11), and the optical attenuator (14) is an optical density filter with a known transmittance.
The disclosed invention includes a method thereof to determine the maximum range of a LiDAR, shown schematically in FIG. 4, and comprising the steps of: decreasing the amplitude of the signal reflected by the system (40); monitoring the signal detected by the LiDAR (41) and checking its extinction (42); calculating the distance range (43) of the LiDAR using a radiometric formula equating the ratio between the irradiance received by the LiDAR and the power emitted by the LiDAR to the corresponding ratio for a free space diffusing target with a specified reflectance.
In a preferred embodiment of the invention, the system (100) response is controlled by rotating a Lambertian diffusing target (13), with reflectance p, producing a reflected intensity I for an incident power P varying with the angle Q between the diffusing target normal (35) and the optical axis direction (11), according to the Lambert's cosine law:
/= P X-X cos(Q) (f) p
The resulting central irradiance E of the output beam (17) collimated by the collimation optics (10) with focal length f, is
Figure imgf000014_0001
In the preferred embodiment of the method, the attenuator (14) has a fixed transmittance T, resulting in a ratio between the output irradiance and input power, given by
Figure imgf000014_0002
being R the mirror reflectivity. The optical response for a real target with reflectance p', placed at a distance d, with an atmosphere with attenuation coefficient yatm, is
£ _ P’exP(~2Tatm P p d
Figure imgf000014_0003
Equating the optical responses of equation (8) and equation (9), one gets an equation to determine the distance of a real target that produces the same LiDAR detected signal as with the test system:
Figure imgf000015_0001
When the angle Q is increased, the signal detected by the LiDAR decreases until there is no detection, in which case the distance d given by equation (10) is the maximum range of the LiDAR: if the atmospheric attenuation is neglected, it directly gives the maximum range; otherwise, it can be solved numerically to find the maximum range value.
While preferred embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes, and modifications may be made without departing from the spirit and scope of the invention.

Claims

1. System (100) to determine the maximum range of a LiDAR sensor, comprising: an optical collimator (10), comprising an optical axis
(ID ; an optical diffuser (13), positioned at a focal point (12) which is aligned with the optical axis (11) of the optical collimator (10); and an optical attenuator (14); wherein the optical axis (11) of the optical collimator (10) is parallel aligned with the optical path of a laser beam (16) emitted by the LiDAR sensor; the optical attenuator (14) is installed within the optical path of the laser beam (16) and/or within the optical path of a light beam (17) reflected by the optical diffuser (13); and the optical collimator (10) is adapted to focus the laser beam (16) on the optical diffuser (13) and to collimate the reflected light beam (17) received by the LiDAR sensor.
2. System (100) according to any of the previous claims, wherein the optical attenuator (14) is installed between the optical diffuser (13) and the optical collimator (10).
3. System (100) according to any of the previous claims, wherein the output collimated light beam (17) is modulated by adjusting the optical attenuator (14) transmittance.
4. System (100) according to any of the previous claims, wherein the output collimated light beam (17) is modulated by adjusting the optical diffuser (13) reflectance.
5. System (100) according to any of the previous claims, wherein the optical collimator (10) comprises one of a convex lens, a concave mirror, or any combination of optical elements with an equivalent focal point.
6. System (100) according to any of the previous claims, wherein the optical diffuser (13) is adapted to back-scatter light beams with an angular distribution larger than the numerical aperture angle of the optical collimator (10).
7. System (100) according to any of the previous claims, wherein the optical attenuator (14) comprises one of an optical density filters, polarizers, or any other conveniently adapted to alone or in combination attenuate the laser beam (16) and/or the output collimated light beam (17).
8. System (100) according to any of the previous claims, wherein the optical collimator (10) comprises a parabolic mirror adapted to focus the laser beam (16) parallel to the optic axis (11) onto the optical diffuser (13) positioned at the focal point (12), said optical diffuser (13) comprising a Lambertian diffusing target.
9. System (100) according to any of the previous claims, wherein the optical collimator (10) comprises an off-axis parabolic mirror adapted to focus the laser beam (16) parallel to the optic axis (11) onto the optical diffuser (13) positioned at the focal point (12), said optical diffuser (13) comprising a Lambertian diffusing target with a target surface normal (35) comprising orientation adaptability in order to vary the amount of backscattered light along the optic axis direction (11).
10. Method to determine the maximum range of the LiDAR sensor, according to the system described in claims 1 through 9, comprising the steps of: decreasing the amplitude of the light signal reflected by the system (40), by varying the optical attenuator transmittance or the optical diffuser reflectance; monitoring the light signal detected by the LiDAR (41); checking the light signal extinction (42); calculating the distance range (43) of the LiDAR sensor based on a radiometric equivalence to a distant free space diffusing target with a specified reflectance.
11. Method to determine the maximum range of the LiDAR sensor, according to claim 10 wherein the radiometric equivalence is obtained by the ratio between the optical irradiance received by the LiDAR and the optical power emitted by the LiDAR.
PCT/IB2021/056370 2021-07-13 2021-07-14 System to determine the maximum range of a lidar sensor WO2023285860A1 (en)

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EP0048014A1 (en) * 1980-09-16 1982-03-24 Siemens Aktiengesellschaft Apparatus for measuring the extinction coefficient of laser range-finders
US5281813A (en) 1992-06-30 1994-01-25 Hughes Aircraft Company Laser rangefinder test system
EP0601872A1 (en) 1992-12-11 1994-06-15 Hughes Aircraft Company Laser rangefinder testing system incorporating range simulation
CN203455473U (en) 2013-08-30 2014-02-26 珠海天峰光电有限公司 Range calibration instrument of laser rangefinder
US20200363528A1 (en) 2017-08-09 2020-11-19 Valeo Schalter Und Sensoren Gmbh Ascertaining the maximum range of a lidar sensor

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4167328A (en) * 1977-09-28 1979-09-11 Westinghouse Electric Corp. Passive optical range simulator device
EP0048014A1 (en) * 1980-09-16 1982-03-24 Siemens Aktiengesellschaft Apparatus for measuring the extinction coefficient of laser range-finders
US5281813A (en) 1992-06-30 1994-01-25 Hughes Aircraft Company Laser rangefinder test system
EP0601872A1 (en) 1992-12-11 1994-06-15 Hughes Aircraft Company Laser rangefinder testing system incorporating range simulation
CN203455473U (en) 2013-08-30 2014-02-26 珠海天峰光电有限公司 Range calibration instrument of laser rangefinder
US20200363528A1 (en) 2017-08-09 2020-11-19 Valeo Schalter Und Sensoren Gmbh Ascertaining the maximum range of a lidar sensor

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