Method for Determining and Compensating for Stray Light from a 3D Laser Scanner

Abstract

A method is disclosed for determining and compensating a proportion of stray light of a measuring beam of a 3D laser scanner by which a 3D point cloud of an object to be detected can be generated via phase-based distance measurement including a first sequence by which first parameters of a proportion of stray light can be determined independently of the 3D point cloud and/or a second sequence by which second parameters of the proportion of stray light dependent on the generated 3D point cloud and a step can be determined. The proportion of stray light can be compensated as a function of the first parameters and/or the second parameters.

Claims

1. A method for determining and compensating a proportion of stray light (I.sub.S, φ.sub.S) of a measuring beam (18) of a 3D laser scanner (1) by which a 3D point cloud of an object (22) to be detected can be generated through phase-based distance measurement, comprising: a “first step sequence via which first parameters (I.sub.S, φ.sub.S) of the proportion of stray light can be determined independently of the 3D point cloud” and/or a “second step sequence via which second parameters of the proportion of stray light can be determined dependent on the generated 3D point cloud” and a step of “compensating the proportion of stray light as a function of the first parameters (I.sub.S, φ.sub.S) and/or the second parameters”.

2. The method according to claim 1, wherein the first step sequence comprises a step of “emitting the measuring beam (18) via an emitter of the 3D laser scanner (1) into a space portion (24) away from the object which has no reflectivity or a sufficiently low reflectivity”.

3. The method according to claim 2, wherein the space portion (24) is a sky portion sufficiently free from objects, or wherein the space portion is a board, a box or the like having an absorbing coating, in particular made from Vertically Aligned Nano Tube Arrays.

4. The method according to claim 2, wherein the first step sequence moreover comprises steps of “receiving the proportion of stray light of the emitted measuring beam via a receiver of the 3D laser scanner”, and “determining the first parameters (I.sub.S, φ.sub.S) of the proportion of stray light” via a control unit of the 3D laser scanner (1).

5. The method according to claim 4, wherein the first step sequence is carried out in plural space portions (24) and is supplemented by a step of “averaging the first parameters (I.sub.S, φ.sub.S) over the plural space portions (24)” via the control unit.

6. The method according to claim 2, wherein the first parameters are at least an intensity (I.sub.S) of the proportion of stray light and the phase shift (φ.sub.S) thereof to the emitted measuring beam (18), or the mean values thereof.

7. The method according to claim 2, wherein the first step sequence is supplemented by a step of “selecting the space portion or portions (24) at least as a function of their degree of reflection and/or their angle of elevation (β) and of an associated criterion” via the control unit.

8. The method according to claim 7, wherein a subset of the selected space portions (24) is considered in the determination of the first parameters (I.sub.S, φ.sub.S) or the mean values thereof.

9. The method according to claim 1 comprising steps for generating the 3D point cloud of “emitting a measuring beam (18) onto the object (22)” via an emitter of the 3D laser scanner (1), “receiving a proportion (18′) of the measuring beam (18)” via a receiver of the 3D laser scanner (1), “determining parameters of intensity (I.sub.A) of the proportion (18′) and phase shift (φ.sub.A) of the proportion (18′) to the emitted measuring beam (18)” via a control unit of the 3D laser scanner (1), “generating a 3D point” as a function of the parameters (I.sub.A, φ.sub.A) and “generating the 3D point cloud by repeatedly “emitting the measuring beam (18) onto the object (22)”, “receiving the proportion (18′) of the measuring beam (18), “determining the parameters (I.sub.A, φ.sub.A)” and “generating the 3D point” via the control unit.

10. The method according to claim 9, wherein the second step sequence comprises steps of “analyzing the 3D point cloud for waves depending on the proportion of stray light” via a control unit of the M laser scanner (1), and “determining second parameters from the waves” via the control unit.

11. The method according to claim 10, wherein each of the waves depending on the proportion of stray light is distinguished by a characteristic dependence of their wave length (l) and wave height (h) on their pixel intensity (I.sub.P).

12. The method according to claim 10, wherein the second parameters, in particular an intensity of the proportion of stray light and a phase shift of the proportion of stray light to the measuring beam, are estimated or determined from intensities and phases of the waves.

13. The method according to claim 9 comprising a step of “generating the 3D point” as a function of the parameters (I.sub.A, φ.sub.A) and the first parameters (I.sub.S, φ.sub.S) via the control unit.

14. The method according to claim 9 comprising a step of “correcting the 3D point cloud” as a function of the parameters (I.sub.A, φ.sub.A) and the second parameters as a function of the difference thereof via the control unit.

15. The method according to claim 1, wherein the detection is suppressed, interrupted and/or a warning message is output, when the intensity (I.sub.S) of the determined proportion of stray light is above a predetermined limit value.

16. The method according to claim 9, comprising a step of “generating the 3D point” as a function of the difference or differences of the parameters (I.sub.A, φ.sub.A) and the first parameters (I.sub.S, φ.sub.S), via the control unit.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0036] Hereinafter, an example of the method according to the disclosure will be illustrated in detail in the Figures, wherein:

[0037] FIG. 1 shows a 3D laser scanner according to the disclosure in which a method according to the disclosure is stored for implementation;

[0038] FIG. 2 shows the laser scanner according to FIG. 1 shown in cross-section in its measuring environment and with an object to be detected;

[0039] FIG. 3 shows the laser scanner according to FIG. 2 when detecting the object;

[0040] FIG. 4 shows a defective 3D point cloud of the object; and

[0041] FIG. 5 shows a 3D point cloud of the object.

DESCRIPTION

[0042] FIG. 1 illustrates a 3D laser scanner 1 comprising a base 2 and a housing 4 attached thereto. The latter is rotatable about a vertical axis 6 by a motor (not shown) integrated in the base 2 at an angle of rotation a. The housing 4 includes two substantially symmetric portions 8, 10 supporting accumulators, electronics, a control unit and other components of the laser scanner 1. In the center between the portions 8, 10, the housing is formed to have a yoke 12 through which the portions 8, 10 are connected. Above the yoke 12, a measuring head 16 rotatable about a transverse axis 14 at the angle of elevation p is accommodated via which measuring head 16 a measuring beam 18 can be emitted onto an object 22 to be detected. The measuring head 16 includes a transparent cover glass 20 behind which deflecting optics for emitting the measuring beam 18 and finally for receiving a reflected proportion 18′ of the measuring beam 18 are protected. For detecting the object, the laser scanner 1 rotates about its vertical axis 6 at up to 360° and the measuring head 16 rotates about its transverse axis 14 at up to 320°.

[0043] Such laser scanner 1 is sufficiently known, apart from the method described in the following, from prior art, for example as Applicant's Imager® 5016, so that further explanations in this respect can be dispensed with.

[0044] FIG. 2 illustrates the laser scanner 1 according to FIG. 1 in cross-section in its measuring environment and with the object 22 to be detected. The laser scanner 1 is shown in a cut state, with the sectional plane extending from the vertical axis 6 so that it forms a plane of symmetry of the two portions 8, 10 of the housing 4. Cut areas are hatched to facilitate representation, although in reality they are naturally hollow to a certain extent so that they can incorporate the afore-mentioned components.

[0045] In the shown embodiment of the method, the laser scanner 1 is positioned outdoors, though it can also be used indoors. The object (building) 22 is arranged at a comparatively large distance from the laser scanner 1. Those detections over large distances are particularly susceptible to errors of stray light. Light that passes from the emitter of the laser scanner directly to the receiver without having been incident on the object 22 to be detected is referred to as stray light or proportion of stray light. Frequently, soiling of the cover glass 20 of the measuring head 16 in which the exit of the emitter and the entry of the receiver are arranged is responsible for this. Such soiling results in the afore-mentioned scattering.

[0046] Thus, in the case of corresponding soiling of the cover glass 20, an intensity I.sub.S of the proportion of stray light of the measuring beam 18 and a phase shift φ.sub.S of the proportion of stray light based on the emitted measuring beam 18 are associated with each emitted measuring beam 18 having the intensity I.sub.M. As afore illustrated, the intensities I.sub.R and I.sub.S of a reflected proportion 18′ of the measuring beam 18 and of the proportion of stray light of the measuring beam 18 sum up. The same applies mutatis mutandis to the phase shifts φ.sub.R and φ.sub.S thereof. As a consequence, the 3D point cloud is incorrectly detected via the control unit of the laser scanner 1 unless the proportion of stray light is compensated.

[0047] For determining and compensating said proportion of stray light, and thus for increasing the accuracy of the detection, the method according to the disclosure is applied.

[0048] In the shown example, the starting situation for this is an arrangement of the laser scanner 1 relative to the object 22 according to FIG. 3, viz. under outdoor conditions. The method can also be carried out indoors, viz. in interior rooms, of course, in an adapted variant.

[0049] Since, in the example, a sky free from objects or at least a low-reflection sky is visible from the location of the 3D laser scanner 1, irrespective of the 3D point cloud (this has not been detected yet) initially the first step sequence of the method is carried out. According to FIG. 2, at first a space portion 24 referred to as “free from objects” according to the foregoing description is determined. Said space portion 24 meets the criteria of a sufficiently large angle of elevation p and ideally has “zero” or at least sufficiently low reflectivity. Thus, the measuring beam 18 in the subsequent step emitted into said space portion 24 cannot be reflected so that a reflected proportion 18′ of the measuring beam is zero. The parameters I.sub.R and φ.sub.R usually resulting from the reflection therefore are zero. The signal then received by the receiver consequently must be the proportion of stray light of the measuring beam 18 having an associated intensity I.sub.S and a phase shift cps to the emitted measuring beam 18. Of course, it must be pointed out here that also a certain receiving noise caused by the present sunlight and inherent noise of the receiving electronics is given. These first parameters I.sub.S and φ.sub.S originating from the first step sequence usually result from soiling of the cover 20.

[0050] In order to increase the reliability and the accuracy of the determined proportion of stray light and of its first parameters I.sub.S and φ.sub.S, the first step sequence can be carried out for different space portions 24, wherein at the end the first parameters I.sub.S and φ.sub.S of the proportion of stray light are averaged over the space portions 24. In order to obtain first parameters I.sub.S and φ.sub.S which exhibit as little standard deviation as possible, averaging is carried out over an as large area of the space free from objects, viz. the sky.

[0051] The determined first parameters I.sub.S and φ.sub.S and, resp., the mean values thereof are incorporated, during the later step of generating a 3D point cloud 26 or 26′ of the object 22 based on the scanning by measuring beams 18 (cf FIG. 3), in a respective difference with the proportions of the measuring beams 18 received by the receiver.

[0052] The first step sequence of the method can be carried out prior to detection or during detection of the 3D point cloud, i.e., in particular between the detections of individual 3D points.

[0053] For determining a space portion 24 free from objects having zero or sufficiently low reflection, the method in the first step sequence includes a step for selecting those space portions 24. They must meet the criterion of going below a limit value for a received intensity stored in the control unit of the laser scanner 1. In this way, space portions containing a remote building, trees, heavy clouds, dust or the like are not used for determining the first parameters I.sub.S and φ.sub.S.

[0054] The space portions 24 are selected in the first step sequence by initially exactly analyzing individual pixels or 3D measuring points, as described by way of FIG. 2. Apart from the afore-mentioned criterion of intensity, also a stored angle of elevation p has to be exceeded, as only above said angle an open sky can be expected. From the space portions 24 meeting the criteria, in the first step sequence an intelligent or proprietary algorithm of the laser scanner 1 determines those of the space portions 24 which are actually used to determine the first parameters I.sub.S and φ.sub.S of the proportion of stray light.

[0055] Under favorable circumstances, the described compensation based on the first step sequence of the method, viz. based on the described setting of the first parameters I.sub.S and φ.sub.S off against the signals received by the receiver, already results in the 3D point cloud 26 according to FIG. 5 which is sufficiently accurate.

[0056] In case that the first step sequence cannot be carried out due to a lack of space portions 24 free from objects, for example when the detection takes place indoors or outdoors with an overcast or hazy sky, the method flexible according to the disclosure includes a second step sequence to determine and compensate the proportion of stray light which, unlike the first step sequence, is based on the detected 3D point cloud 26′ according to FIG. 4.

[0057] Initially, the starting situation according to FIG. 3 when the first step sequence has been carried out according to the preceding description is assumed. However, the determination and compensation with the first parameters I.sub.S and φ.sub.S has not yet resulted in a sufficiently accurate point cloud so that said point cloud 26′ has a deformed shape as exemplified in FIG. 4. Such shape results from larger stray light values or proportions which, due to a cyclic phase error, generate typical patterns in the detected 3D point cloud 26′. On surfaces they can be identified as waves similar to the waves of a water surface with a breeze. However, the waves usually are too small to be noticed by the user during detection. Therefore, FIG. 4 illustrates those waves in an extremely scaled representation.

[0058] According to the second step sequence of the method, those waves are specifically searched for in the 3D point cloud 26′ and, resp., the latter is analyzed for them. The waves of the 3D point cloud 26′ caused by stray light have a characteristic connection of their wavelength 1 and wave height h depending on their pixel intensity I. By a step for analyzing the 3D point cloud 26′ directed to said characteristic, the waves, even those which would not be noticed by the human viewer, can be identified by the control unit of the laser scanner 1. In a subsequent step of the second step sequence, second parameters of the proportion of stray light can be estimated or determined from the identified waves. The second parameters then are set off against the 3D points of the already detected 3D point cloud. The remaining error of the proportion of stray light that resulted in the waves according to FIG. 4 is thus compensated. After carrying out the second step sequence, now the correct 3D point cloud 26 according to FIG. 5 is provided.

[0059] Alternatively or additionally to the compensation, a warning message can be output as a function of the determined first and/or second parameters of the proportion of stray light. For example, this could be an invitation to clean the cover glass 20 or the like.

[0060] A method is disclosed for determining and compensating a proportion of stray light of a measuring beam of a 3D laser scanner through which a 3D point cloud of an object to be detected can be generated via phase-based distance measurement. The method includes at least two different step sequences, viz. sub-methods, for determining and compensating which are applicable depending on the ambient conditions, the proportion of stray light, the user's option and/or the like either individually or in combination. One of the step sequences enables the determination and compensation independently of the 3D point cloud, the other depending on the detected 3D point cloud.

LIST OF REFERENCE NUMERALS

[0061] 1 3D laser scanner [0062] 2 base [0063] 4 housing [0064] 6 vertical axis [0065] 8, 10 housing portion [0066] 10 housing portion [0067] 12 yoke [0068] 14 transverse axis [0069] 16 measuring head [0070] 18 measuring beam [0071] 18′ reflected proportion of measuring beam [0072] 20 cover glass [0073] 22 object [0074] 24 space portion [0075] 26 compensated 3D point cloud [0076] 26′ insufficiently compensated 3D point cloud [0077] α angle of rotation [0078] β angle of elevation [0079] I intensity [0080] φ phase shift [0081] I.sub.M intensity of measuring beam [0082] I.sub.R intensity of reflected proportion [0083] I.sub.S intensity of proportion of stray light [0084] φ.sub.R phase shift of reflected proportion [0085] φ.sub.S phase shift of proportion of stray light