CA2950937A1

CA2950937A1 - Apparatus for distance measurement

Info

Publication number: CA2950937A1
Application number: CA2950937A
Authority: CA
Inventors: Andreas Hofbauer; Peter Rieger; Rainer Reichert; Andreas Ullrich
Original assignee: Riegl Laser Measurement Systems GmbH
Current assignee: Riegl Laser Measurement Systems GmbH
Priority date: 2015-12-15
Filing date: 2016-12-08
Publication date: 2017-06-15
Anticipated expiration: 2036-12-08
Also published as: EP3182159A1; EP3182159B1; CA2950937C

Abstract

An apparatus for distance measurement of a target comprises a laser transmitter for transmitting a laser pulse in the form of a transmission laser beam, at a transmission time point, a laser receiver for receiving the laser pulse reflected at the target, in the form of a reception laser beam, at a reception time point, on a reception surface of the laser receiver, a beam deflection device placed after the laser transmitter and ahead of the laser receiver, which device deflects the transmission and reception laser beams at a known angular velocity, an evaluation device connected to the laser transmitter and the laser receiver, for measuring the running time between transmission and reception time point, and from that the distance of the target, and a selection device, which is configured for selecting the reception surface of the laser receiver used for the measurement as a function of the angular velocity and of a predetermined range of measurable distances.

Description

Apparatus for distance measurement The present invention relates to an apparatus for distance measurement of a target by means of running-time measurement of laser pulses reflected by the target, comprising:
a laser transmitter for transmitting a laser pulse, in the form of a transmission laser beam, at a transmission time point;
a laser receiver for receiving the laser pulse reflected at the target, in the form of a reception laser beam, at a reception time point, on a reception surface of the laser receiver;
a beam deflection device placed after the laser transmitter and ahead of the laser receiver, which device deflects the transmission and reception laser beams at a known angular velocity; and an evaluation device connected to the laser transmitter and the laser receiver for measuring the running time between said transmission and reception time points and, from that, the distance of the target.
Apparatuses of this type are used, for example, in air-borne surveying of landscapes. At a high altitude, a laser scanner on board of an aircraft transmits a plurality of laser pulses to many target points on the ground, by means of the beam deflection device, and the distances to the targets are determined from the running-time measurements of the target reflections, and from that - with knowledge of the position and location of the laser scanner and the respective deflection angle - a point model ("3D point cloud") of the landscape is created. Dust, humidity, or clouds can here cause interference reflections, which make it more difficult to create the landscape model or can distort it. Vice versa, it can be desirable, specifically when surveying clouds or dust, for example ash clouds, to record these weak cloud reflections against the background of the strong ground reflections.

- 2 -The invention sets itself the goal of creating a laser scanning apparatus that can cope with these different measurement requirements better than the known solutions.
This goal is achieved with an apparatus of the aforementioned type, which is characterized, according to the invention, by a selection device that is configured for selecting the reception surface of the laser receiver as a function of the angular velocity of the beam deflection device and of a predetermined range of measurable distances.
During the running time of the laser pulse from the laser transmitter to the laser receiver, the beam deflection device continues to move, and therefore the impact point of the reception laser beam on the reception surface of the laser receiver is subject to a transverse offset, which is proportional to the product of the pulse running time and the angular velocity of the beam deflection device. Because the pulse running time in turn is dependent on the distance to the target, the distance measurement range of the apparatus can be set by means of selecting the reception surface used for the measurement. In this manner, undesirable distance ranges, for example, can be masked, for example in order to suppress interference reflections of dust, humidity or clouds, or, vice versa, to isolate weak useful reflections against the background of strong interference reflections, for example for air-supported cloud surveying.
The distance measurement range of the laser scanning apparatus selected by the selection device can be set both before and while the measurement task is carried out; in ongoing measurement operation, it can thereby be adapted to changing atmospheric conditions or to the respective surroundings, for example in the case of changes in altitude or terrain jumps. In some embodiments, which will be explained in greater detail below, the distance measurement range can even be changed afterward, after the measurement task has been

- 3 -performed, for example in order to suppress undesirable interference signals.
The selection device according to the invention also achieves numerous further advantages by means of the possibility of distance-selective masking of interference reflections or distance-selective emphasis on useful reflections:
After distance-related interference reflections have been masked out, remaining interference signals in the measurement signal of the laser receiver can be classified and handled more simply, for example as near reflections from the region of the housing of the laser scanning apparatus or as double reflections of foliage close to the ground or the like.
By means of masking out interference cloud reflections, surveying aircraft having the laser scanning apparatus according to the invention can be used to good effect even under poor weather conditions, and this allows greater capacity utilization of the devices.
By means of suppression of interference reflections that might be great relative to weaker useful reflections, the laser receiver can be operated without over-modulation or saturation, at the maximal sensitivity possible, thereby also making it possible to measure greater target distances at an unchanged interference signal rate.
Vice versa, greater laser outputs of the laser transmitter can also be used, without leading to overly great interference reflections and thereby saturations or over-modulations of the laser receiver, and thereby greater target distances can be measured at an interference signal rate that remains low.
At a high pulse repetition rate (pulse repetition rate, PRR) of the laser transmitter, multiple laser pulses can be in the air at greater target distances ("multiple pulses in the air," MPIA or "multiple time around," MTA), i.e. the next laser pulse (or the one after that, etc.) can already be

- 4 -transmitted even before the first laser pulse is received back in the laser receiver. In this case, assigning a received laser pulse to the correct transmission pulse that caused it is ambiguous or difficult, something that is also known as an MPIA
or MTA problem. In this regard, the value of a clearly assignable distance measurement range, called an "MTA zone," is determined by the pulse distance of the transmission pulses; in this regard, every possible assignment of a reception pulse to a prior transmission pulse corresponds to a specific MTA zone, in each instance. By means of setting the selection device according to the invention appropriately, the MTA zone in which the measurement is to be performed can thereby be selected.
When using multiple laser scanning apparatuses according to the invention on a common measurement platform, for example an aircraft, the distance measurement ranges of the individual apparatuses can be selected differently and, in particular, can be set in such a manner, using the invention, that reciprocal interferences of the apparatuses are suppressed or minimized.
According to a preferred characteristic of the invention, collection optics are provided in the beam path between the beam deflection device and the selection device, which optics can focus the reception laser beam at a point, specifically in the plane of the selection device or of the reception surface.
In this way, the boundaries of the selected distance measurement range can be set very precisely.
The selection device can be implemented in different ways.
In a first embodiment, the selection device comprises a shutter that lies ahead of the laser receiver, which shutter restricts the reception surface of the receiver. In other words, the reception surface of the laser receiver is so great here that it can be restricted, in terms of its geometrical expanse or surface area, by a shutter that lies in front of it.
The shutter can have a single or actually multiple shutter openings, in order to delimit multiple "active" regions on the

- 5 -reception surface of the laser receiver, which are at a distance from one another. Each shutter opening predetermines a distance measurement range of the laser scanning apparatus.
The shutter can be a "fixed" shutter, i.e. having a (or multiple) predetermined shutter opening(s), and can then be moved, relative to the laser receiver, by means of a drive, in order to set the distance measurement range.
Alternatively, the shutter can also be a "movable"
shutter, for example an iris diaphragm as in a camera, and can then have plates controlled by the drive.
In a further variant, the shutter can have a disk that can be rotated by the drive, having shutter openings that are different in size and/or placed differently, in the manner of a "revolver lens," which openings can be brought into position in front of the reception surface of the laser receiver, in each instance.
According to a further characteristic of the invention, an electro-optical shutter can also be used instead of a mechanical shutter. The shutter then has an electro-optical element, the optical transparency or opacity of which can be changed by means of electrical control. In this regard, the change in the transparency or opacity can take place globally for the entire element or locally for small segments (pixels) of it, thereby making it possible to implement even very complex shutter openings, for example having an irregular circumference shape and/or multiple shutter openings spaced apart from one another. A practical embodiment of such an electro-optical element is, for example, an LCD panel, the transparency or opacity of which can be electronically controlled, pixel by pixel.
In each of the aforementioned shutter variants, the shutter can be structured either as a "black shutter" having (at least) one shutter opening, i.e. in the region of a shutter opening it is transmissive at 100 %, and next to the shutter opening it is transmissive at 0 % (100 % opaque, "black"), or,

- 6 -alternatively, the shutter is an attenuation filter having (at least) two regions having different opacity or transparency.
Such a shutter can establish a transmission, opacity or attenuation progression over the reception surface of the laser receiver, which progression thereby attenuates the reception laser pulses in distance-selective manner. In this way, the signal amplitudes of the laser pulses of near and far targets can be equalized with one another, for example, or, alternatively, the amplitude contrast of the laser pulses of near and far targets can be amplified, in order to make them more easily differentiated from one another. All of these embodiments of a "soft" restriction of the reception surface of the laser receiver are covered by the term used here, that of "selection of the reception surface" of the laser receiver "used for measurement."
According to a second embodiment of the selection device according to the invention, the reception surface of the laser receiver used for measurement is selected in that the laser receiver, together with its reception surface, can be moved relative to the reception laser beam. In other words, here the reception surface of the laser receiver is smaller than the offset or angle region that the reception laser beam can pass over after being deflected by the deflection device. Only if the "correct" offset between reception laser beam and laser receiver is set can the reception surface of the latter be "hit" and thereby the desired distance measurement range set in this manner can be measured.
Setting of the reception surface relative to the reception laser beam can be achieved in two different ways, specifically, on the one hand, in that the selection device comprises a further beam deflection device that lies ahead of the reception surface of the laser receiver, which device selectively deflects the reception laser beam in such a manner that the reception surface of the laser receiver is hit only in the selected distance measurement range; or, on the other hand, in

- 7 -that the selection device comprises a drive by means of which the laser receiver can be moved approximately parallel to its reception surface. In this way, too, the reception surface of the laser receiver, in turn, is hit by the reception laser beam in the desired distance measurement range.
In a third embodiment of the selection device according to the invention, the reception surface of the laser receiver used for measurement is selected in that the reception surface of the laser receiver is divided up into a plurality of light-sensitive detector elements, and the selection device is configured for selecting one or more detector elements for the measurement. The selection of the respective detector element of the reception surface can take place here both before, during, and even only after the measurement procedure. From the parallel measurement signals of the detector elements - in the case of corresponding parallel recording in a memory - the detector elements or measurement signals corresponding to the desired distance measurement range can be selected even after the measurement ("offline"), in order to establish the distance measurement range.
Such a selection device built up from many individual detector elements furthermore opens up further possibilities.
For one thing, targets can be measured in two or more different distance measurement ranges that are at a distance from one another, as in the case of a shutter having two or more shutter openings. For another thing, multiple targets hit by a single laser pulse, at different distances, can be better distinguished from one another, in order to measure them simultaneously ("multi-target capacity"). For example, clouds and terrain can be simultaneously surveyed in a single laser scanning procedure in this way: The reflections that come from nearby clouds hit the reception surface of the laser receiver at a slight offset, and the reflections that come from the far-away ground hit at a large offset. If different detector elements are now selected and evaluated for the two offset

- 8 -ranges and thereby distance measurement ranges, both cloud reflections and ground reflections can be measured. The information regarding cloud reflections that lie in between can subsequently also be used to improve or correct measurement of the ground reflections, for example in order to compensate for the dependence on atmosphere of the speed of light, or for cloud-related amplitude attenuation of the ground reflections.
According to a further preferred embodiment of the invention, accordingly the selection device is configured for selecting at least two detector elements, and the evaluation device is configured for measuring a first distance resulting from the transmission time point and a first reception time point measured by the first detector element, and a second distance resulting from the transmission time point and a second reception time point measured by the second detector element.
In the simplest case, the said detector elements of the reception surface of the laser receiver can be distributed only in one direction over the reception surface of the laser receiver, specifically the expected offset direction of the reception laser beam. This results in a one-dimensional array of detector elements on the reception surface.
In a further embodiment, the detector elements can be distributed over the reception surface in two directions, so that a two-dimensional array of detector elements occurs. In this way, in particular, trajectories of the impact point of the reception laser beam on the reception surfaces, even those having any desired curvature, such as those produced by more complex beam deflection devices having two or more axes, can be recorded, without already having to orient the detector elements along a specific trajectory during design of the laser scanning apparatus.
In a preferred embodiment, a Time-of-Flight camera (ToF
camera) is used for this purpose; its pixels form the detector elements of the reception surface of the laser receiver. In

- 9 -this way, the selection device can be structured using industrially pre-fabricated components and corresponding downstream evaluation electronics.
As has been explained, the distance measurement range of the laser scanning apparatus set by means of the selection device can be set not only before but also during and after the measurement procedure. According to a preferred embodiment of the invention, this can take place, in every embodiment, also as a function of at least one previously measured target distance. The distance measurement range used during a measurement can thereby be adaptively adapted to the surroundings of the surveyed targets or made to track them, in such a manner as to automatically mask out interference signals from other distance measurement ranges or to make the distance measurement range follow gradual or sudden changes in the terrain.
According to a further characteristic of the invention, in the case of a laser scanning apparatus with consecutive laser pulses, i.e. at least a "first" and a later "second" pulse, the selection device is configured for selecting the reception surface for the running-time measurement of the second laser pulse also as a function of the running-time measurement of the first laser pulse. The said dependence also includes dependence on more than one previous running-time measurement, for example on an average of multiple earlier measurements.
The beam deflection device of the laser scanning apparatus can be, for example, a rotating mirror pyramid or a rotating mirror facet wheel, in which case its angular velocity is generally essentially constant. In the case of such a constant (e.g. predetermined known or currently measured) angular velocity, the geometric offset of the impact point of the reception laser beam on the reception surface of the laser receiver is also constant for a specific target distance.
If, on the other hand, a beam deflection device having an angular velocity that can change over time is used, as is the

- 10 -case, for example, for a mirror that swings back and forth, the angular velocity of which varies in sine shape, for example, then the offset of the impact point of the reception laser beam on the reception surface of the laser receiver is also dependent on the current angular velocity of the beam deflection device.
A further embodiment of the invention, which is also suitable for beam deflection devices having an angular velocity that can change over time, accordingly comprises a selection device that additionally also receives a measured value of the angular velocity of the beam deflection device and is configured for additionally adapting the selected reception surface also to the change over time of the angular velocity of the beam deflection device. The reception surface of the laser receiver used for the measurement then quasi "resonates" with the change in angular velocity of the beam deflection device, in order to compensate it, and this in turn allows setting a constant distance measurement range.
In the following, the invention will be explained in greater detail using the exemplary embodiments shown in the attached drawings. The drawings show:
Fig. 1 the use of an apparatus according to the invention on board an aircraft for surveying a landscape, with some beam progressions of transmission and reception laser beams drawn in as examples;
Fig. 2 exemplary time diagrams of transmission and reception pulses in the apparatus of Fig. 1;
Fig. 3 the apparatus of Fig. 1 in a block schematic, with beam progressions drawn in schematically;
Fig. 4 a first embodiment of the apparatus of the invention in a block schematic, with beam progressions drawn in schematically;
Fig. 5 the reception surface of the laser receiver of the apparatus of Fig. 4 in a face view, with geometrical

- 11 -relationships of the impact trajectory of the reception laser beam drawn in;
Fig. 6a to 6c three different variants of the shutter of the apparatus of Fig. 4 in a schematic side view (Fig. 6a) and a schematic face view (Fig. 6b, 6c);
Fig. 7 a further variant of the shutter of the apparatus of Fig. 4 in a schematic side view, with some block-schematic components of the apparatus of Fig. 4;
Fig. 8a and 8b an alternative embodiment of the shutter of Fig. 4, 6a - 6c or 7 in the form of an attenuation filter in a face view (Fig. 8a) and in the form of a diagram of the opacity progression Op over the radius r (Fig. 8b);
Fig. 9 a second embodiment of the apparatus of the invention in a block schematic, with beam progressions drawn in schematically;
Fig. 10 a variant of the second embodiment of Fig. 9 in a block schematic, with beam progressions drawn in schematically;
Fig. 11 a third embodiment of the apparatus of the invention in a block schematic, with beam progressions drawn in schematically; and Fig. 12 a variant of the embodiment of Fig. 11 in a face view of the reception surface of the laser receiver.
According to Fig. 1 and 2, a pulsed transmission laser beam 2 is guided to scan a terrain 3, e.g. a landscape, a terrain gradient, the bottom of a strip mine, the ocean floor, the skyline of a city, etc., with individual surroundings targets (scanning points) U1, U2, -, in general Un, using an aircraft-supported distance measurement apparatus ("laser scanner") 1, for example, with scanning taking place line by line, in fan shape, for example. The target distances Dlr D2r ..., in general Dn, from the measurement location of the laser scanner 1 to the individual surroundings targets Un can be determined from measurements of the running times 6.Tn at the individual transmission laser pulses Si, S2r in general Sn, contained in the transmission laser beam 2, which are received

- 12 -back in the laser scanner 1 after target reflection, as reception laser pulses E1, E2, in general En, in the form of a reception laser beam 4, according to the known equation:
Dn = c-ATn/2 = c-(ts, n - (1) with ts,n ................................................................
transmission time point of the transmission laser pulse S"
ts,n ................................................................
reception time point of the reception laser pulse En, and .............. speed of light.
Intermediate targets Un' can lie on the path of the transmission and reception laser beams 2, 4, for example in the form of interference reflections at a cloud 5 that is semitransparent for laser beams. Therefore, if applicable, two or more reception laser pulses En, En' can be received back at different reception time points ts,n, tE,,' for a transmission laser pulse Sn. If surveying of the clouds 5 is specifically of interest, the reflections at the intermediate targets Un' represent the useful signals and the reflections at the ground targets represent the interference signals, and the measurement task is that of determining the target distances Dn' to the intermediate targets Un'.
The laser scanner 1 works at a very high pulse repetition rate (pulse repetition rate, PRR) and thereby determines the target distances Dn to a plurality of surroundings targets Un in rapid sequence, thereby making great time resolution and local resolution possible at a short total measurement period.
If a surroundings target U. is farther removed from the laser scanner 1 than corresponds to the quotient of the speed of light c and twice the pulse repetition rate PRR, then the next transmission pulse S"I is already transmitted even before the reflection En of the last transmission pulse Sn has been received. In this regard, the incoming reception pulses En generally can no longer be clearly assigned to the transmission

- 13 -pulse Sn that caused them (MTA problem). In the example of Fig.
1, five clearly measurable distance ranges ("MTA zones"), Z1, Z2, ..., Z5, in general Zm, form in this manner, the width of which ranges is alma. = c/(2-PRR), in each instance. For example, for surveying a target such as the cloud reflection Un' in the second MTA zone Z2, the running time ATNITA2 between the reception time point tE,2 of the reception laser pulse E2 and the transmission time point ts,i of the pre-preceding transmission laser pulse Si is decisive, as shown in the left half of Fig. 2.
Fig. 3 schematically shows the internal structure of the laser scanner 1 of Fig. 1 for generating the scanning (scanning) transmission and reception laser beams 2, 4, which contain the transmission and reception laser pulses Sn, En. A
laser transmitter 6 transmits the transmission laser beam 2 to the targets Un by way of a mirror 7 and a beam deflection device 8. The beam deflection device 8 is, for example, a rotating mirror pyramid, a rotating mirror facet wheel (here:
with three facets), or a rotating or oscillating mirror. The deflection angle a of the transmission laser beam 2 thereby changes at an angular velocity w.
In spite of its strong bundling as a laser beam, the transmission laser beam 2 always experiences a certain beam , widening a over the great distance Dn to the target Un and back, in practice, so that the reception laser beam 4 received back after reflection at the target Un, after impacting back on the beam deflection device 8, has a certain beam diameter (shown with a dotted line), which diameter, after being redeflected again by way of the beam deflection device 8, by means of collection optics 9, e.g. a collection lens or the like, is focused on an impact point 10 on a reception surface 11 of a laser receiver 12 of the laser scanner 1.
The beam diameter of the reception laser beam 4 when it hits the beam deflection device 8 again, however, is generally widened not only due to what is called the bundling error a,

- 14 -but also due to the reflection at the target Un, which is generally diffuse and leads to a multidirectional or omnidirectional reflection b, from which the entry aperture c of the laser scanner 1 and/or the usable deflection surface of the deflection device 8 and/or the collection optics 9 "cut(s) out" the reception laser beam 4 passed on to the reception surface 11.
The laser receiver 12 measures the reception time point tE,õ of the reception laser pulse En received back in the reception laser beam 4, and passes this measured value on to an evaluation device 13. The evaluation device 13 furthermore has knowledge of the transmission time point ts,õ of the causal transmission laser pulse Sõ, either because it triggered transmission of the transmission pulse S, in the laser transmitter 6, or because it receives a measured value of the transmission time point ts,õ. From the difference ATn = tE,n -ts,n (taking into consideration the correct MTA assignment of the transmission and reception laser pulses, in each instance, in accordance with the correct MTA zone Zm according to Fig.
2), the evaluation device 13 subsequently calculates the distance Dn to the target.
Fig. 4 shows a simplified and enlarged representation of the internal components of the laser scanner 1 of Fig. 1 and 3, wherein for the sake of clarity, here the beam widening of the transmission and reception laser beams 2, 4 and the collection optics 9 for focusing on the impact point 10 of the reception surface 11 of the reception laser receiver 12 are no longer shown, although they are present. To represent them, now only the optical axes of the transmission and reception laser beams 2, 4 are shown.
As illustrated in Fig. 4, the beam deflection device 8 continues to move at the angular velocity w (shown with a dotted line) during the running time ATn, so that the reception laser beam 4, when it impacts the beam deflection device 8, does in fact already experience a different deflection angle a2

- 15 -than the transmission laser beam 2 did previously, which experienced the deflection angle al. As a result, the impact point 10 of the reception laser beam 4 on the reception surface 11 of the laser receiver 12 is subject to an offset Ax, which is proportional to the product of the target distance Dn and the angular velocity co of the beam deflection device 8, i.e.
Ax = f(D0 = co) (2) In the situation shown in Fig. 4, with the transmission and reception laser beams 2, 4 lying in the drawing plane, and an axis of rotation 14 of the beam deflection device 8 that lies normal to the drawing plane, the trajectory on which the impact point 10 moves on the reception surface 11 according to the offset Ax lies on a straight line in the drawing plane. The actual curve shape of the trajectory of the impact point 10 on the reception surface 11, however, depends on the concrete reciprocal geometrical relationships of the transmission and reception laser beams 2, 4, the axis of rotation 14 of the beam deflection device 8, and the position of the mirror surface of the beam deflection device 8 relative to its axis of rotation 14; furthermore, beam deflection devices 8 are also possible, which simultaneously rotate and/or oscillate about more than one axis 14. Fig. 5 therefore shows the general case of a trajectory 15 having any desired curvature, along which the impact point 10 of the reception laser beam 4 moves by the offset Ax, specifically proceeding from an impact point 100 when the deflection device 8 is standing.still (co = 0) to an impact point 101 at a smaller target distance Dn or an impact point 102 at a greater target distance Dn (at a constant angular velocity co).
If the angular velocity co of the beam deflection device 8 is not constant but rather changes over time, then the impact point 10 moves accordingly on the reception surface 11, at the rhythm of the change in the angular velocity co(t). In the case

- 16 -of a two-axis or multi-axis movement of the beam deflection device 8, the impact point 10 or its trajectory 15 can also "draw" loops, Lissajous figures or the like on the reception surface 11.
At a constant angular velocity w (or at a corresponding compensation by the effect of a changing angular velocity w(t)), a conclusion regarding a specific target distance Dn can therefore be drawn from Equation (2), based on a specific offset Ax, i.e. target distances Dn fall on the reception surface 11 from a specific distance measurement range Gl, G2, in general Gi (Fig. 1), also in a specific section ("measurement window") Fl, F2, -, in general Fi, of the trajectory 15. This effect is now used to selectively mask out, select, attenuate or amplify target reflections from a specific distance measurement range Gi.
For this purpose, the laser scanner 1 comprises a selection device 16, which is configured for selecting the reception surface 11 of the laser receiver 12 used for measurement of the reception time point tE,n of a reception pulse En of the reception laser beam 4, specifically as a function of the angular velocity w or w(t) and of the desired range Gi of measurable distances D.
In a first embodiment of the selection device 16, which is explained using Fig. 4 to 7, the laser receiver 12 has such a large reception surface 11 that it can cover the region of the trajectory 15 in which the desired measurement windows Fi for the desired distance measurement ranges Gi lie, and the selection device 16 comprises a shutter 17 that lies ahead of the reception surface 11 and restricts or limits the latter.
Fig. 4 schematically shows a shutter 17 having a single shutter opening 18, which can be moved, by means of a drive 19, relative to the reception surface 11 of the laser receiver 12.
The drive 19 can be controlled appropriately by the evaluation device 13 or another control device, in order to select the position of the shutter opening 18, thereby of the measurement

- 17 -window F, and thereby, in turn, of the distance measurement range G, of the laser scanner 1.
For example, the distance measurement range G, can be established in such a manner, using the selection device 16, that interference reflections of intermediate targets Unt, such as the cloud 5, are completely suppressed (masked out), because they no longer reach the reception surface 11, but rather only hit the opaque regions of the shutter 17 outside of the shutter opening 18. Vice versa, strong ground reflections such as those of the ground targets Un can also be masked out if the distance measurement range G, is set for the distance range of the cloud 5. In the latter case, the sensitivity of the laser receiver 12 can thereby be selected to be maximal, in order to measure the weak intermediate targets Un' of the cloud 5, without having to fear over-modulation resulting from the strong ground reflections.
In this regard, the shutter 17 can also have more than one shutter opening 181, 182, etc., in each instance, see Fig. 5, in order to be able to simultaneously measure targets in more than one distance measurement range Gi, if desired.
Fig. 6a to 6c show different mechanical embodiments of the shutter 17 of the selection device 16. According to Fig. 6a and 6b, the shutter 17 has two or more lamellae 201, 202, 203, 204, which can be moved relative to one another and, between them, delimit the shutter opening(s) 18, 18,, 182, ... and thereby the measurement window(s) F, for the impact point 10 along its trajectory 15. The lamellae 201 - 204 can, once again, be moved by one or more drives 19. Fig. 6c shows a further embodiment of the shutter 17 in the form of a disk 21 that can be rotated about its center point M, having multiple shutter openings 181, 182, 183, which can be optionally rotated by means of rotation of the disk 21 in front of the trajectory 15 (Fig. 5), in order to establish different measurement windows F.
Fig. 7 shows a further embodiment of the shutter 17 in the form of an electro-optical element 22, the optical transparency

- 18 -of which can be controlled electrically for the laser wavelength that is of interest here, for example by the evaluation device 13 or separate electronics (see arrow 23).
The change in the transparency of the electro-optical element 22 can take place globally for the entire element 22 or locally for small segments (pixels) of the same, thereby also making it possible to even set up shutter openings 18, 184, 182, or measurement windows Fi in front of the reception surface 11, which openings have a complex shape or are divided multiple times, if applicable.
Until now, the shutter 17 was considered as what is called a "black shutter," in which the shutter opening(s) 18, 184, 182, are 100 % transparent (0 % opaque) and the remaining regions of the shutter 17 are 100 % opaque (0 % transparent, "black"). However, it is understood that in each of the embodiments described here, the shutter 17 can be not only a 100 % : 0 % shutter (black shutter), but rather can also have any other desired ratio of transparency to opacity, in other words can be an attenuation filter having regions of different transparency or opacity.
Fig. 8a and 8b show such an embodiment of a shutter 17 having ring-shaped regions 244, 242, 243, and 244, disposed concentrically about its center point M, the opacity Op of which regions is plotted above the radius r in Fig. 8b, proceeding from the center point M. With such an attenuation-filter shutter 17, reception pulses En from different distance measurement ranges G1 of the laser scanner 1 can be attenuated differently, for example in order to attenuate ground reflections relative to cloud reflections, thereby making it possible to operate the laser receiver 12 at a uniform, maximal amplification or sensitivity, without the risk of over-modulation.
Until now, embodiments of the selection device 16 were described, in which an upstream shutter 17 restricts a larger reception surface 11 of the laser receiver 12. In alternative

- 19 -embodiments of the selection device 16, which will now be described, however, it is also possible that the selection device 16 selects the reception surface 11 used for the measurement in that a smaller laser receiver 12 having a reception surface 11 corresponding to the size of the desired measurement window F, is used, and adjusted along the trajectory 15 for the purpose of setting the distance measurement ranges Gl; vice versa, the reception laser beam 4 can also be adjusted along the trajectory 15, relative to such a smaller laser receiver 12. Fig. 9 to 12 show embodiments that are based on such a "kinematic reversal" of the principle of Fig. 4 to 7.
Fig. 9 shows a first embodiment having such a "small-area"
laser receiver 12, which is preceded by a selection device 16 in the form of a further beam deflection device 25 that lies in front of the deflection surface 11. The further beam deflection device 25 is, for example, an adjustable lens system or lens, which, if applicable, can also be combined with the collection lens or collection optics 9 (Fig. 3). The beam deflection device 25 deflects the reception laser beam 4 by an adjustable deflection angle p, so that only reception pulses En from the desired distance measurement range G, can get to the reception surface 11 of the laser receiver 12 that corresponds to the measurement window Fi; reception pulses En outside of the distance measurement range G, selected in this manner, in contrast, miss the small reception surface 11. The further beam deflection device 25 can, once again, be adjustable by a drive 19, which can be controlled by the evaluation device 13 or other electronics.
Fig. 10 shows an alternative variant of the principle of Fig. 9, in which the entire laser receiver 12 with its small reception surface 11, corresponding to the selected measurement window Fi along the trajectory 15, is moved relative to the reception laser beam 4, by means of the drive 19, in order to in turn select the desired distance measurement range Gl.

- 20 -Fig. 11 shows yet another embodiment of the selection device 16, in which the reception surface 11 of the laser receiver 12 is divided into a plurality of light-sensitive detector elements 261, 262, , in general 26k, which can be evaluated individually and are connected with a selector 27.
The selector 27 selects the respective detector element(s) 26k for measuring the reception time point t -E,nr in accordance with the desired measurement window(s) Fi, and thereby the distance measurement range(s) G. The selector 27 can, once again, be controlled by the evaluation device 13 or other electronics (arrow 28).
From the face view of the reception surface 11 of the laser receiver 12 in Fig. 12, it can be seen that the detector elements 26k can also be disposed two-dimensionally in the form of a 2D array, so that even complex, irregularly shaped measurement windows F, can be set along the trajectory 15, by means of a corresponding selection of one or more detector elements 26k in one or more ranges. Here, too, two or more measurement windows F, can be defined.
By means of corresponding control of the sensitivity of the individual detector elements 26k, attenuation filter progressions analogous to Fig. 8a and 8b over the reception surface 11 can also be set up, in order to attenuate or amplify reception pulses En from different distance measurement ranges G, with different intensity.
An array of detector elements 26k, as shown in Fig. 11 and 12, can be implemented, for example, using a Time-of-Flight camera (ToF camera).
It is understood that the division of the reception surface 11 of the laser receiver 12 into individual detector elements 26k does not necessarily require a physical division of the reception surface 11, but rather can also be implemented with a laser receiver 12 that delivers a measured value by way of the position of the impact point 10 on its reception surface 11; this measured value then corresponds to the position of the

- 21 -detector element 26k. An embodiment of such a laser receiver 12 is, for example, a photosensitive plate having electrodes connected at the ends or on its circumference, the measurement signals of which electrodes are compared with one another by the evaluation device 13 and/or the selector 27: The relative ratio of the starting amplitudes then provides information regarding the impact point 10 on the reception surface 11.
Accordingly, the said "division" of the reception surface 11 of the laser receiver 12 into the detector elements 26k can also be implemented in this manner, in quasi "analog" or "continuous" manner.
With the embodiments of the selection device 16 shown in Fig. 11 and 12, the selection of the reception surface 11 of the laser receiver 12 used for the measurement, in each instance, can be undertaken not just before or during the measurement process, but actually even afterward, when the measurement signals of the detector elements 26k are recorded.
Thus, for example, a multi-target situation, as shown in Fig.
1, in other words when a transmission pulse S, yields two reception pulses E,, E,', can lead to a response of detector elements 26k, 261,3 in two different measurement windows Fi and thereby distance measurement ranges Gi, which can be evaluated in parallel. In this way, the laser scanner I becomes "multi-target-capable" and can carry out cloud surveying and ground surveying simultaneously, for example.
The multi-target capacity of the laser scanner 1 can subsequently also be utilized to make use of data regarding cloud-related intermediate targets Uõ' for correcting the surveying of the (useful) ground targets U. For example, it is possible to draw conclusions regarding atmospheric ambient conditions such as air contamination, humidity, scattering properties, etc., from intermediate cloud targets Uõ', all the way up to creation of a layer model of the atmosphere. Such ambient conditions have an effect on the running times 6,T, to the ground targets Uõ, on the one hand, and, on the other hand,

- 22 -on the amplitudes of the reception laser pulses En that stem from the ground targets U. With knowledge of the location and/or reflection capacity of the intermediate targets Un' and thereby of the atmospheric conditions, the running times ATn can thereby be compensated in view of the current speed of light in the atmosphere and/or any atmosphere-related attenuation of the amplitudes of the reception pulses E.
In each of the embodiments described above, the distance measurement range Gi can also be set, by means of the selection device 16, in such a manner that in each instance, only reception pulses En from a specific MTA zone Zm are taken into consideration, in order to allow correct MTA-zone assignment.
The laser scanner 1 thereby becomes MTA-zone-selective.
Furthermore, in each of these embodiments, the evaluation device 13 can calculate and set the measurement windows Fi and thereby the distance measurement ranges Gi for determining the distance measurement values Dn (or Dr') to a specific target Un (or Un'), independent of one or more previous distance measurement values Dn_i, Dn-2,- (or The distance measurement ranges Gi can thereby adaptively track the progression of the landscape 3 or the clouds 5. Within the range of movement of an MTA zone width dr,,x, automatic adaptation of the distance measurement range Gi to an MTA zone zm that is of interest can actually be achieved, so that echo pulses from other MTA zones ("ghost echoes") are suppressed or left out of consideration, thereby allowing automatic MTA-zone-selective measurement.
The invention is not restricted to the embodiments presented, but rather comprises all the variants, modifications, and combinations of the same that fall within the scope of the attached claims. Thus, the laser scanner 1 can, in particular, also have two or more transmitters 6, which aim at the beam deflection device 8 or at different mirror facets of the same at different angles, and accordingly, multiple selection devices 16 and laser receivers 12 can also

- 23 -be provided. Also, it is possible to use a common Selection device 16 and a common laser receiver 12, if applicable having multiple detector elements 26k, which select distance measurement ranges for the individual laser transmitters 6 from the corresponding multiple reception laser beam 4, in each instance, in the manner described.

Claims

Claims:

1. An apparatus for distance measurement of a target, by means of running-time measurement of laser pulses reflected by the target, comprising:
a laser transmitter for transmitting a laser pulse in the form of a transmission laser beam, at a transmission time point;
a laser receiver for receiving the laser pulse reflected at the target, in the form of a reception laser beam, at a reception time point, on a reception surface of the laser receiver;
a beam deflection device placed after the laser transmitter and ahead of the laser receiver, which device deflects the transmission and reception laser beams at a known angular velocity; and an evaluation device connected to the laser transmitter and the laser receiver for measuring the running time between said transmission and reception time points and, from that, the distance of the target;
characterized by a selection device that is configured for selecting the reception surface of the laser receiver used for the measurement as a function of the angular velocity and of a predetermined range of measurable distances.

2. The apparatus according to claim 1, characterized by a collection optics that lies between the beam deflection device and the selection device.

3. The apparatus according to claim 1 or 2, characterized in that the selection device comprises a shutter that lies in front of the laser receiver, which shutter restricts the reception surface of the receiver.

4. The apparatus according to claim 3, characterized in that the shutter is moveable relative to the laser receiver by means of a drive.

5. The apparatus according to claim 3, characterized in that the shutter has lamellae controlled by a drive.

6. The apparatus according to claim 3, characterized in that the shutter has a disk that is rotatable by a drive, the disk having shutter openings that are different in size and/or placed differently.

7. The apparatus according to claim 3, characterized in that the shutter has an electro-optical element, the optical transparency of which can be controlled electrically, preferably an electronically controllable LCD panel.

8. The apparatus according to one of claims 3 to 7, characterized in that the shutter is a black shutter having at least one shutter opening.

9. The apparatus according to one of claims 3 to 7, characterized in that the shutter is an attenuation filter having regions of different opacity.

10. The apparatus according to one of claims 1 to 9, characterized in that the selection device comprises a further beam deflection device that lies in front of the reception surface of the laser receiver.

11. The apparatus according to one of claims 1 to 10, characterized in that the selection device comprises a drive, by means of which the laser receiver can be moved approximately parallel to its reception surface.

12. The apparatus according to one of claims 1 to 11, characterized in that the reception surface of the laser receiver is divided into a plurality of light-sensitive detector elements, and that the selection device is configured for selecting one or more detector elements for the measurement.

13. The apparatus according to claim 12, characterized in that the selection device is configured for selecting at least two detector elements, and that the evaluation device is configured for measuring a first distance from the transmission time point and a first reception time point measured by the first detector element, and a second distance from the transmission time point and a second reception time point measured by the second detector element.

14. The apparatus according to claim 12 or 13, characterized in that the detector elements are formed by the pixels of a Time-of-Flight camera.

15. The apparatus according to one of claims 1 to 14, characterized in that the laser transmitter transmits at least a first and a second laser pulse and that the selection device is configured for selecting the reception surface for the running time measurement of the second laser pulse also as a function of the running time measurement of the first laser pulse.

16. The apparatus according to one of claims 1 to 15, characterized in that the beam deflection device has an angular velocity that changes over time, and that the selection device receives a measured value of the angular velocity in order to adapt the selected reception surface to the change over time of the angular velocity.