OPTICAL DISTANCE MEASURING SENSOR WITH SWITCHABLE RECEIVING APERTURE

Description

[0001] The invention relates to an optical distance meter for performing distance measurements on both retroreflective and diffusely scattering targets.

[0002] The optical distance meter according to the invention is suitable, for example, for distance and coordinate measuring devices in the fields of geodesy and industrial surveying, where LIDAR, laser trackers, laser scanners, total stations, or theodolites are typically used.

[0003] For example, such measuring devices have an accuracy of 100 µm or better. Laser trackers, for instance, are equipped with distance meters that achieve an accuracy of 3-5 µm when measuring retroreflective targets. With reflectorless targets, such as optically matte surfaces, this level of accuracy is typically no longer guaranteed, especially since the collecting capability of the receiving optics is usually insufficient.

[0004] Such laser trackers are used, for example, in industrial surveying, e.g., for the coordinate position determination of points on a component such as a vehicle body during an inspection, or for the continuous position monitoring of a moving machine part. Typically, these laser trackers are designed for the coordinate position determination of a retroreflective target point and usually for the continuous tracking of this target point. A target point can be represented by a retroreflective unit (e.g., a cube-shaped prism) that is targeted with an optical measuring beam, in particular a laser beam, generated by a beam source of the measuring device or the distance meter of the measuring device. The laser beam is reflected parallel back to the measuring device, and the reflected beam is detected by the measuring device or the distance meter. This process involves the emission or...The direction of beam reception is determined, for example, using angle sensors assigned to a deflection mirror or targeting unit of the system. Additionally, the distance from the measuring device to the target point is determined upon beam detection, e.g., by time-of-flight or phase difference measurement, using an optical interferometer, or the Fizeau principle. Based on the emission or reception direction and the distance, the position coordinates of the target point are determined.

[0005] Furthermore, in modern tracker systems, a fine-tuning sensor determines the offset of the received measuring beam from a zero position. Using this measurable offset, the positional difference between the center of a retroreflector and the point of impact of the laser beam on the reflector can be determined. The laser beam's orientation can then be corrected or adjusted based on this deviation, reducing the offset on the fine-tuning sensor, ideally to "zero," thus aligning the beam towards the reflector's center. By continuously tracking the laser beam's orientation, the target point can be tracked, and its distance and position relative to the measuring device can be continuously determined.Tracking can be achieved by changing the orientation of the motorized deflection mirror, which is designed to deflect the laser beam, and / or by swiveling the aiming unit, which contains the beam-guiding laser optics. With rapid movements of the target object, continuous tracking is often not precise, resulting in an angular deviation from the ideal, calibrated measuring direction. However, this directional difference can be measured and compensated for in real time by the fine-tuning sensor, which typically incorporates a camera. This ensures that the distance and position, or rather the coordinates of the target point, are continuously known.

[0006] The target point or retroreflector can be attached to a measuring instrument, such as a probe, which is positioned with a contact point on a point of the object to be measured. The probe has markings, e.g., light points, and a reflector that represents the target point on the probe and can be targeted with the laser beam of the tracker, whereby the positions of the markings and the reflector relative to the contact point of the probe are precisely known.

[0007] Measuring distances without the use of measuring aids with a retroreflector, i.e., measuring directly to a surface of an object to be measured, is not possible with ordinary state-of-the-art laser trackers.

[0008] In contrast, there are special laser trackers that enable the scanning measurement of surface points, i.e., the determination of a large number of point coordinates on the surface of an object to be measured, which is done in a comparatively short time. However, this method results in a loss of accuracy compared to measuring retroreflective targets.

[0009] US Patent 2014 / 0226145 A1 discloses a laser tracker capable of measuring both a retroreflective target and a natural (i.e., non-retroreflective) surface. The laser tracker comprises a first absolute distance sensor, configured as is known for measuring to a retroreflector. Additionally, the laser tracker includes a second absolute distance sensor, configured for measuring to an object surface. While the respective absolute distance sensors emit their measuring beams through a single exit optic, they are separate, independent units. The need to provide two completely independent, separate absolute distance sensors is complex and therefore expensive to manufacture.

[0010] DE 102014215721 B3 discloses an optical sensor with a photomixing detector with a dimming device for influencing the shape and position of its optically effective surface.

[0011] To ensure the required high sensitivity for detecting the comparatively weak returning signals in reflectorless measurements, the cross-section of the receiving optics must be as large as possible. Retroreflective target objects, however, typically have a small diameter, for example, between 10 mm and 40 mm.

[0012] In the case of so-called misalignment with the retroreflector, a parallel beam offset occurs, and the received beam can therefore arrive laterally at a large receiving objective depending on the aiming, which can lead to distance measurement errors, typically characterized by a so-called misalignment diagram.

[0013] To compensate for the intensity differences of the returning radiation when measuring retroreflective targets compared to measurements on natural, diffusely scattering targets, adjustable attenuation filters are used in the transmit channel, for example, to adapt the transmitted signal amplitude to the electronic receiving unit.

[0014] Furthermore, with reflective, especially retroreflective, targets, multiple reflections can occur between the target object and the instrument, or multiple reflections within the rangefinder's receiving channel. The probability of such multiple reflections is proportional to the size of the receiver's field of view (FoV). These parasitic signal components ("boomerang signals") impair measurement accuracy. Even with a small FoV, these boomerang signals are still strong enough to prevent, for example, a measurement accuracy of better than 100 µm. Therefore, rangefinders that measure on retroreflectors employ special anti-boomerang filters, either as fixed filters or adjustable attenuation filters.However, as fixed filters, these antiboomerang filters would interfere with measurements on natural targets, since the signal is already weak and there are no multiple reflections between the target object and the instrument anyway.

[0015] Furthermore, in known coaxial and biaxial distance measuring arrangements from the prior art, shadowing of the received light typically occurs in the near range (e.g., < 5 m), primarily in receiver-side fixed-focus arrangements. Such shadowing is caused, for example, by elements placed on the optical axis.

[0016] Transmitting beam mirrors which, in the near field, block the view of the receiving diode to the light point on the target object.

[0017] These fixed-focus arrangements result in virtually no signal being detected by the receiver in a very short distance range, from 0 m up to a certain limit. Such unmeasurable distance ranges are disruptive and complicate the workflow of the respective surveying task.

[0018] It is therefore an object of the present invention to provide an improved optical distance meter which enables measurement to both a retroreflective target and a diffusely scattering target, thereby avoiding the disadvantages known from the prior art.

[0019] This problem is solved by realizing the characterizing features of the independent claim. Features that further develop the invention in an alternative or advantageous way are to be found in the dependent claims.

[0020] The invention relates to an optical distance meter, particularly for use in a laser tracker, a laser scanner, a total station, or a theodolite, comprising a transmitter configured to generate a directed, and in particular substantially collimated, transmit beam defining a target axis, a receiver configured to detect at least a portion of the transmit beam returning from the target, hereinafter referred to as the received radiation, and a receiving channel defining a substantially single detection direction for detecting the received radiation. The distance meter is configured to perform distance measurement by means of pulse time-of-flight measurement in a first measurement mode in which the distance meter is set for distance measurement to a retroreflective target, and a second measurement mode in which the distance meter is set for distance measurement to a diffusely scattering (natural) target.

[0021] According to the present invention, the receiving channel is configured such that, for the purpose of capturing the received radiation in the first measurement mode, a first aperture of the receiving channel is set, i.e., a first free opening of the receiving channel through which the part of the received radiation used for distance measurement in the first measurement mode passes, and in the second measurement mode, a second aperture of the receiving channel is set, i.e., a second free opening of the receiving channel through which the part of the received radiation used for distance measurement in the second measurement mode passes, wherein the first aperture is smaller than the second aperture.

[0022] For example, the receiving channel is configured such that the ratio between the area of ​​the first aperture and the area of ​​the second aperture is less than 1 / 10, in particular less than 1 / 20.

[0023] Furthermore, according to one embodiment, the receiving channel has, for example, an attenuation element that can be switched depending on whether the first or second measurement mode is being performed, i.e., that is present to varying degrees in the first and second measurement modes. This attenuation element acts, for example, as a switchable antiboomerang filter, thereby increasing the measurement accuracy. For example, the switchable attenuation element can be designed as a filter that can be swung in and out of the receiving channel with a transmission of less than 50 percent, or as a graduated filter wheel.

[0024] For example, the receiving channel has a switchable aperture diaphragm defining the first aperture, wherein the switchable aperture diaphragm is configured such that in the first measurement mode the switchable aperture diaphragm is swung into the beam path of the receiving channel, or the aperture opening of the switchable aperture diaphragm is adjustable such that by means of a first setting the first aperture is set and by means of a second setting the second aperture is set, in particular wherein the switchable aperture diaphragm is an iris diaphragm.

[0025] One effect of the first aperture is, for example, the support of an optically sufficiently well-defined state, where the lengths of the optical paths in the receiving beam path are all interferometrically the same. This results in an improvement of the receiver's error pattern, since the received beam for measurements on a retroreflector always has the same shape, regardless of how precisely the center of the retroreflector is targeted.

[0026] The switchable attenuator and the switchable aperture diaphragm can, for example, be arranged on a common adjustment element of the receiving channel and thus be swung into the beam path of the receiving channel together in the first measurement mode and swung out of the beam path of the receiving channel together in the second measurement mode.

[0027] According to a further embodiment, the receiving channel has a fixed attenuator element, present identically in both the first and second measurement modes, for attenuating the received radiation passing through the attenuator element. The fixed attenuator element is configured in its lateral geometry and positioned in its lateral position within the receiving channel such that, in the first measurement mode, it attenuates at least the beams of the received radiation passing through the first aperture, and in the second measurement mode, the received radiation passing through the second aperture does not exhibit beams attenuated by the fixed attenuator element.

[0028] For example, the optical system of the distance meter according to the invention can be designed coaxially, wherein the distance meter has a beam splitter arranged on the optical axis of the receiving channel for coupling the transmitting beam onto an axis coaxial to the optical axis of the receiving channel and corresponding to the target axis. The beam splitter is configured and arranged such that the beam splitter has a partially transparent layer in a central zone around the optical axis of the receiving channel, in particular wherein a beam splitting ratio of less than 50 percent of the beam splitter is generated for the receiving channel by means of the partially transparent layer in the central zone, and the partially transparent layer forms at least a part of the fixed attenuator, in particular wherein the partially transparent layer forms the fixed attenuator.

[0029] In contrast to prior art coaxial distance meters, which typically use a radiation-impermeable mirror element to coaxially couple the transmitted radiation onto the optical axis of the receiving channel, the shadowing of the received beam returning from the retroreflector in the near field (e.g., < 5 m) is avoided. Therefore, the fixed attenuation element essentially eliminates the need for special near-field optics for measurements on retroreflectors, as known from the prior art.

[0030] In a further embodiment, the receiving channel has a correction optic defining a correction zone of the receiving channel, in particular configured as a near-range optic, to compensate for a focus error when focusing parts of the received radiation returning from a distance shorter than a defined near-range limit distance onto the receiver, namely wherein parts of the received radiation lying in the correction zone are deflected onto the receiver by the correction optic.

[0031] The correction optics can, for example, be arranged such that at least part of the received radiation passing through the second aperture lies within the correction zone, and received radiation passing through the first aperture lies essentially outside the correction zone, wherein the correction optics are in particular configured such that the time-of-flight contribution caused by the receiver to the time of flight of the beams of the received radiation within the correction zone is identical to the time-of-flight contribution caused by the receiver to the time of flight of the beams of the received radiation outside the correction zone.

[0032] According to a further embodiment, the fixed attenuator, the correction optics, and the switchable aperture diaphragm are arranged relative to each other and configured in their lateral geometry such that, in the first measurement mode, essentially all received radiation reaching the receiver is attenuated by the fixed attenuator and lies outside the correction zone, and in the second measurement mode, the aperture diaphragm is swung out of the beam path of the receiving channel, and received radiation passing through the second aperture exhibits beams not attenuated by the fixed attenuator, which simultaneously lie within the correction zone. In particular, the correction zone does not overlap with an attenuation zone defined by the fixed attenuator.

[0033] Furthermore, the inventive adjustment of the receiver-side aperture can be combined, for example, with a beam-expanding optic in the transmitting channel, such as a negative lens that can be pivoted into place in the first measurement mode, a pinhole aperture, or a diffractive optical element. This widens the beam divergence, for example, by a factor of ten, so that the retroreflected beam illuminates the entire opening of the receiving channel, even if there is an aiming error towards the center of the retroreflector. By increasing the transmitter-side divergence, deviations in the modulation wavefront of the optical pulses, which determines the time of flight, are locally smoothed by a so-called lateral zoom. This, in combination with the inventive adjustment of the receiver-side aperture, reduces systematic spatial errors caused by aiming error towards the center of the reflector.

[0034] For example, according to another embodiment, the transmitter is configured to generate the transmitting beam with a beam divergence in the first measurement mode, such that when performing the distance measurement at a defined minimum measurement distance, the first aperture is completely illuminated by the received radiation, and the beam divergence is less than five times the receiver-side field of view angle.

[0035] According to another embodiment, the transmitter is configured to set the transmitting beam in a defined variable focusing state, wherein in the first measurement mode a first focusing state of the transmitting beam is set, wherein the transmitting beam has a divergence in the first focusing state, and in the second measurement mode a second focusing state of the transmitting beam is set, wherein the transmitting beam has essentially a parallel beam propagation in the second focusing state.

[0036] For example, the transmitter can also be configured such that in the first measurement mode the transmitting beam has a defined adjustable beam divergence, in particular where the beam divergence of the transmitting beam is set based on a distance to the target.

[0037] When using two adjacent retroreflectors, it is essential to ensure that the divergent beam illuminates only one reflector; otherwise, the distance measurement will be inaccurate. Such interference from a nearby target can be avoided by adjusting the beam divergence based on the lateral distance to the target.

[0038] Furthermore, in another embodiment, the transmitting channel of the optical distance meter has a switchable attenuation element, in particular based on at least one adjustable gray wedge filter, a liquid crystal attenuator, and a fiber optic Mach-Zehnder interferometer.

[0039] The optical distance meter according to the invention is described in more detail below with reference to exemplary embodiments schematically depicted in the drawings. Identical elements are marked with the same reference numerals in the figures. The described embodiments are generally not drawn to scale and are not to be understood as limiting the scope of the invention.

[0040] In detail, they show Fig. 1a,b: Schematic representation of an optical distance meter according to the invention configured for measuring diffusely scattering targets (a) and retroreflectors (b), respectively; Fig. 2: Schematic representation of a further embodiment of the optical distance meter according to the invention; Fig. 3: Schematic representation of a further embodiment of the inventive optical distance meter in a configuration for retroreflective target objects; Fig. 4: Schematic representation of a further embodiment of the inventive optical distance meter with a fiber optic setup in both the source and receive channels; Fig. 5: Schematic representation of a correction optic in the receive channel; Fig. 6a,b: Schematic representation of the combined effect of the correction optic together with the inventive, switchable aperture diaphragm in the receive channel; Fig. 7: Schematic representation of a further embodiment of the inventive, switchable optical aperture diaphragm in the receive channel.

[0041] The Figures 1a and 1b Each figure shows a schematic representation of an optical distance meter according to the invention in a configuration for measuring diffusely scattering targets ( Fig. 1a ) or on retroreflectors.

[0042] In the illustrated embodiment, the optical system is coaxial, wherein the transmitting beam 2 generated by the transmitter 1 is coupled by means of a beam splitter 3 onto an axis coaxial to the optical axis of the receiving channel and corresponding to the target axis. According to the invention, the beam splitter 3 has, for example, a partially transparent layer 4 in a central zone around the optical axis of the receiving channel, which partially reflects and transmits.

[0043] The beam splitter 3 is therefore designed to act as a near-field optic for the receiving beam 5 from a retroreflective object 6 by forming a central zone 7 in the receiving channel with attenuated beams of the receiving radiation 5.

[0044] When measuring on a diffusely scattering target object 8, as in the Figure 1a As depicted, the target object itself, unlike a retroreflection mirror 6, acts as the light source. It emits divergent received radiation 5, which strikes the entire aperture of the receiving lens 9. Part of the radiation is attenuated by the partially transparent layer 4 of the beam splitter 3 and lies in the central zone. However, a larger part of the received radiation 5 lies in and passes through the beam splitter 3 in an outer zone with high transmission, e.g., approximately 100%.

[0045] If the receiving lens 9 is designed as a fixed-focus optic, it focuses the radiation completely onto the receiver 10, for example a photodiode or a SPAD array (single-photon avalanche photodiode array, also called SiPM), only at distances greater than the spotlight's limiting distance. At short object distances, the received beam is significantly larger in diameter than the receiver 10 at its axial position, resulting in a large portion of the received radiation 5 being lost. This effect is advantageous for reflective target objects because the signal is sufficiently strong and does not increase further at short distances, so the amplitude dynamics of the received signal remain within an appropriate modulation range. However, for diffusely scattering surfaces, the spotlight's limiting distance is approximately twice as long as for retroreflective targets and, due to optical scattering, also shorter.To increase the received signal for diffusely scattered targets and thus improve the signal-to-noise ratio, an additional near-range optic 11 is placed, for example, near the aperture of the receiving lens 12. Various corrective optics 11 for the near-range are known in the prior art, for example, based on optical wedges in a planar plate or directly integrated with the receiving lens, where, for example, the optical path in the wedge region is identical to the path in the carrier plate.

[0046] When aiming at a retroreflector 6, as in the Figure 1b As depicted, the transmitting beam 2 is optically perfectly reflected and reflected parallel to the optical axis onto the beam splitter 3. The partially transparent layer 4, with a partial transmission of, for example, 30%, directs the received radiation 5 onto the receiving lens 9, which focuses the radiation onto the receiver 10.

[0047] When aiming beside the center of the retroreflector 6, a lateral displacement of the reflected beam 5 occurs relative to the optical axis of aiming. As a result, the receiving beam strikes next to the partially transparent layer of the beam splitter and may, under certain circumstances, collide with the near-range optics 11, which are designed for reflectorless distance measurement. Since the near-range optics 11 are designed for diffuse targets at close distances of a few meters, the light reflected by a retroreflector is deflected uncontrollably within the receiver space, which can lead to uncontrolled stray light and thus inaccurate distance measurements. To prevent this, according to the invention, in measurement mode for measurements on retroreflectors, the receiving aperture is adjusted, for example with a pivoting aperture 13, such that the receiving aperture is reduced to the central zone 7. The radiation outside the central zone 7 is thus blocked for the receiver.This results in a clearly defined optical state, and the lengths of the optical paths in the receiving beam path are all interferometrically the same length, thus achieving very high measurement accuracy, e.g. down to one micrometer.

[0048] To further ensure high measurement accuracy, for measurements on retroreflectors 6 in the receiving channel, for example, an attenuation filter 14 is additionally swung into the beam path. This suppresses multiple reflections between receiver 10 and retroreflector 6 (receiver-side antiboomerang filter) and can, for example, be integrated directly into the swiveling aperture 13 as shown in the figure.

[0049] Figure 2 This shows a further embodiment of the optical distance meter according to the invention. In contrast to the Figures 1a, 1bThe beam splitter 3 with the partially transparent layer 4 is designed such that the transmitting beam 2 passes through the beam splitter 3 in transmission and the receiving beam 5 passes through the beam splitter 3 in reflection. For example, the transmission of the transmitting beam 2 at the beam splitter 3 in the region of the central zone 7 is greater than 50% and the net transmission for the receiving beam 5 is less than 50%.

[0050] Figure 3 Figure 6 shows a schematic representation of a further embodiment of the inventive optical distance meter in a configuration for retroreflective target objects.

[0051] Aiming the center of a retroreflector 6 using a transmitter-side collimated, essentially parallel laser beam 2 requires a highly precise and complex aiming process. If the beam is misaligned with the reflector 6, the back-reflected beam 5 will be offset from the optical axis of the receiving channel, and the receiving optics 9 will only be partially or not at all struck. This leads to undesirable signal fluctuations and unpredictably blocks parts of the measuring beam, resulting in distance measurement inaccuracies and measurement interruptions.

[0052] To prevent this, the transmitter 1' is configured to generate the transmit beam 2 with a defined variable beam divergence. For example, a beam-expanding optic 16, such as a negative lens, a diffracting aperture, or a diffractive optical element, is swung into position in the transmit channel after the laser beam source 15, thereby widening the beam divergence. The beam divergence is set, for example, such that it corresponds at least to the divergence angle of the receiver's field of view. This ensures that the retroreflected beam 5 illuminates the entire opening of the receiving channel from the receiver's illuminator's limit distance onward, even if there is a misalignment towards the center of the retroreflector 6.

[0053] For example, the angles of the receiver's field of view are 1 to 3 mrad. If the transmit divergence is 5 mrad, the cross-section of the retroreflected receive beam is then already 10 mm at a Cornercube distance of 1 m and completely covers the central zone 7 relevant in the first measurement mode.

[0054] By increasing the transmitter-side divergence, deviations of the modulation wavefront of the optical pulses, which determines the transit time, are locally smoothed by a so-called lateral zooming, thereby reducing systematic spatial errors that arise from incorrect aiming at the center of a reflector 6.

[0055] Furthermore, as shown in the figure, another antiboomerang filter 17 in the form of an adjustable attenuation filter, e.g. a graduated filter wheel, can also be installed in the transmitting channel.

[0056] Figure 4schematically shows another embodiment with fiber optic construction in both the sending and receiving channels.

[0057] Examples of suitable radiation sources include fiber-coupled laser diodes, fiber lasers, or seeded fiber amplifiers. The generated radiation is split into a start path 19 and a transmit path 20 (TX path) by means of a fiber coupler 18.

[0058] All fibers in the transmit path are, for example, single-mode fibers or PM fibers (polarization-preserving). The radiation in the start path 19 is coupled to the receive channel via an attenuation filter 21, which can be electrically adjustable, e.g., a gray wedge or a fiber-optic Mach-Zehnder interferometer, and internally via a beam splitter 22. An optical reference signal is generated via the start path 19, which defines the exact start time of the measurement signal, i.e., the measurement pulse for a pulse transit-time measurement.

[0059] Furthermore, the systematic signal-strength-dependent distance measurement error (range walk) can be continuously determined using the profile filter 21. For example, the recorded distance offsets are stored in a lookup table and subsequently used to compensate for raw distances in subsequent distance measurements.

[0060] The fiber-optic transmit channel 20 (TX path) is equipped, for example, with a variable optical attenuator 23 (VOA). Typical fiber-optic attenuators 23 are based, for example, on accusto-optic modulators (AOMs) or electro-optic modulators.

[0061] Furthermore, the systematic signal-strength-dependent distance measurement error (range walk) can be continuously determined using the profile filter 21. For example, the recorded distance offsets are stored in a lookup table and subsequently used to compensate for raw distances in subsequent distance measurements.

[0062] The fiber-optic transmit channel 20 (TX path) is equipped, for example, with a variable optical attenuator 23 (VOA). Typical fiber-optic attenuators 23 are based, for example, on accusto-optic modulators (AOM) or electro-optic modulators (EOM) with adjustment speeds from a few microseconds to 100 nanoseconds. For example, this allows the signal amplitude to be precisely adjusted from measurement to measurement at a measurement rate of up to 10 MHz.

[0063] The transfer time is hardly affected by the attenuation setting, meaning the absolute distance is barely distorted. Of course, a range-walk correction can also be determined here and taken into account in the final distance correction.

[0064] The transmitted radiation in the transmission channel is then converted into a free beam via a delay line 24, e.g., a 10 m delay fiber, and an angle-faceted fiber collimator 25. The delay line has the effect, for example, of allowing the electronic crosstalk to decay before the measurement signal reaches the receiver.

[0065] The use of single-mode fibers, for example, has the advantage of low dispersion, which means the transmitted signals are not broadened. A delay fiber several meters long therefore does not produce pulse distortion and thus no distance errors.

[0066] On the receiver side, however, multimode fibers typically have to be used because of the spatial phase mixing in the beam caused by rough target surfaces. The use of multimode fibers as delay fibers, however, has the disadvantage of high dispersion, which produces distance-dependent errors, especially with fixed-focus optics.

[0067] Furthermore, an electrically adjustable attenuation filter 17, e.g. a gray wedge wheel, optionally acts as a transmitter-side anti-boomerang filter 17.

[0068] As an alternative to Figure 3 As a further example, instead of a mechanically swiveling divergence lens, a switchable diffractive optical element (DOE) or an electro-optically controllable spatial light modulator (SLM) is used for adjustable beam expansion 16.

[0069] The receiving channel is equipped, for example, with a multimode fiber that guides the received light to the receiver (not shown). Furthermore, in the example shown, a receiver-side antiboomerang filter is omitted, since, for example, an obliquely faceted fiber can be used as an additional antiboomerang measure.

[0070] Figure 5 Figure 1 shows an embodiment of a corrective optic for use in an optical distance meter according to the present invention. This optic consists, for example, of a flat plate 26 which has, for example, inserted optically effective areas 27 with a corrective effect, e.g., optical wedges, cylindrical lenses, optics with freeform surfaces, sliding refractive surfaces, Fresnel structures, or diffractive structures. This corrects the portions of the received radiation passing through the optically effective areas 27 onto the receiver 10. Fig. 1a) deflected to form a correction zone in the receiving channel to compensate for a focus error when focusing parts of the received radiation returning from a distance shorter than a defined near-field limit distance. Alternatively, the optically effective areas 27 extend the receiver-side field of view (RX-FOV) so that the measurement light spot on the target object can be seen even at short distances.

[0071] The effect of the swiveling aperture 13 on the beam at the location of the correction optics 11 is shown by the hatched line 130, which represents the first aperture in the first measurement mode. The second aperture (with the aperture 13 swiveled out) essentially comprises the entire free opening of the correction optics 11.

[0072] Figures 6a and 6bFigure 1 shows a schematic representation of the combined effect of the corrective optics 11 together with the inventive, switchable aperture diaphragm in the receiving channel in the so-called footprint representation. In this representation, the beams of light are projected into a common plane perpendicular to the optical axis. This makes visible the effect of several optical components in the system and their mutual arrangement along the optical axis.

[0073] The figures relate to two embodiments of the switchable aperture diaphragm 13,13' and show the two essential cross-sections of the receiving beams when set for retroreflective targets. The attenuation element 4, which is fixed in place ( Fig. 1a The central zone 7 defined is not covered by the optically effective areas 27 of the correction optics 11, and is therefore essentially located within the first aperture 130 defined by the switchable aperture diaphragm.

[0074] As in Figure 6a As shown, the switchable aperture diaphragm 13 creates the first aperture 130 and ensures that the beams of the received radiation lying in this central zone 7, which is relevant for reflector measurements, do not pass through any further optical components (outside the first aperture 130). Furthermore, the fixed attenuator element 4 also acts as a near-field optic for reflector measurements, so that even at short distances in the near field, a sufficient signal is available for accurate measurements.

[0075] For reflectorless targets, the aperture should typically be as large as possible, and the switchable aperture diaphragm 13 is swung out. In addition to the beam from the central zone 7, the light beam now also reaches the receiver via the entire correction optics 11. Furthermore, the switchable attenuator 14 is also swung out, so that the distance measurement signal increases further.

[0076] The optical system is designed such that all beam paths have the same optical length, regardless of whether they pass through the central zone 7, the optically effective areas 27 of the correction optics 11 or outside these zones.

[0077] Furthermore, in order to increase the received signal even more during reflector measurements, for example in measurements over long distances where boomerang effects are automatically reduced by the lowered received signal, the switchable attenuating element 14 can optionally be swung out of the beam path of the receiving channel, thereby increasing the signal-to-noise ratio even further.

[0078] Figure 6bFigure 1 shows cross-sections of the receiving beams according to a further embodiment of the switchable aperture diaphragm 13'. In this example, the switchable aperture diaphragm 13' consists of two segments, each of which only switches on or off the optically effective areas 27 of the correction optics 11. In this case, the retroreflective beam is attenuated less in the receiver than in the Figure 6a The illustrated embodiment, however, has the disadvantage of, for example, potential stray light in the near field at very short measuring distances.

[0079] Figure 7Figure 1 shows an embodiment of the switchable aperture diaphragm 13" combined with a attenuating element 14' integrated into the switchable aperture diaphragm 13", which thus simultaneously forms the inventive switchable attenuating element. The combination of the two inventive components in a single assembly has the advantage, for example, that a single actuator unit is sufficient, thus requiring less space.

[0080] In the first measurement mode for retroreflective targets, this inventive unit is swung into the receiving beam, so that the central zone 7 ( Fig. 1a ) is guided as homogeneously and axially close as possible from the beam splitter 3 to the receiver 10.

[0081] In the second measurement mode for natural, diffusely scattering surfaces, the combined component is swung out of the receiving beam path so that as much radiation as possible is captured and directed towards the receiving unit, both for measurements at short and long object distances.

[0082] It is understood that these figures only schematically represent possible embodiments. The various approaches can also be combined with each other and with prior art methods.

Claims

1. An optical distance meter for use in a laser tracker, a laser scanner, a total station or a theodolite, having • a transmitter (1, 1') which is configured for generating a directed, in particular substantially collimated, transmission beam (2) defining a target axis, • a receiver (10) which is configured for detecting at least a part of the transmission beam coming back from the target, referred to below as received radiation (5), and • a receiving channel essentially defining an individual detection direction for detecting the received radiation, wherein the distance meter is configured for carrying out a distance measurement by means of a pulse time of flight measurement of the transmission beam (2) in • a first measuring mode, in which the distance meter is set up for distance measurement to a retroreflective target (6), and • a second measuring mode, in which the distance meter is set up for distance measurement to a diffusely scattering target (8), characterized in that the receiving channel is configured in such a manner that, for the detection of the received radiation, • in the first measuring mode, a first aperture (130) of the receiving channel is set, namely a first free opening of the receiving channel, through which the part of the received radiation used for the distance measurement in the first measuring mode passes, • in the second measuring mode, a second aperture of the receiving channel is set, namely a second free opening of the receiving channel, through which the part of the received radiation used for the distance measurement in the second measuring mode passes, • wherein the first aperture is smaller than the second aperture.

2. The optical distance meter according to Claim 1, characterized in that the receiving channel has an attenuating element (14, 14') which is switchable depending on the carrying out of the first and second measuring mode, namely which is present in a different way in the first and second measuring modes, in particular wherein • the switchable attenuating element (14, 14') is formed as a filter, which can be pivoted into and out of the receiving channel, with a transmission smaller than 50 per cent, or • the switchable attenuating element (14, 14') is formed as a gradient filter wheel.

3. The optical distance meter according to any one of the preceding claims, characterized in that the receiving channel has a switchable aperture diaphragm (13, 13', 13") defining the first aperture (130), wherein the switchable aperture diaphragm (13, 13', 13") is configured in such a manner that • the switchable aperture diaphragm (13, 13', 13") is pivoted into the ray path of the receiving channel in the first measuring mode, or • the diaphragm opening of the switchable aperture diaphragm (13, 13') can be set up in such a manner that the first aperture (130) is set by means of a first setting and the second aperture is set by means of a second setting, in particular wherein the switchable aperture diaphragm (13, 13') is an iris diaphragm.

4. The optical distance meter according to Claim 2 and 3, characterized in that the switchable attenuating element (14, 14') and the switchable aperture diaphragm (13, 13', 13") are arranged on a common adjustment element of the receiving channel and thus are together pivoted into the ray path of the receiving channel in the first measuring mode or together pivoted out of the ray path of the receiving channel in the second measuring mode.

5. The optical distance meter according to any one of the preceding claims, characterized in that • the receiving channel has an attenuating element (4), which is fixedly arranged, namely which is present in an identical way in the first and second measuring modes, for attenuating the received radiation passing through the attenuating element, and • the fixedly arranged attenuating element (4) is configured in such a manner in terms of its lateral geometry and is arranged in its lateral position in the receiving channel in such a manner that ∘ in the first measuring mode, the fixedly arranged attenuating element (4) attenuates at least the bundle of rays of the received radiation passing through the first aperture (130), and ∘ in the second measuring mode, received radiation passing through the second aperture has bundles of rays that are not attenuated by the fixedly arranged attenuating element (4) .

6. The optical distance meter according to Claim 5, characterized in that • the distance meter has a beam splitter (3) arranged on the optical axis of the receiving channel for coupling the transmission beam onto an axis which is coaxial to the optical axis of the receiving channel and corresponds to the target axis, • the beam splitter (3) is configured and arranged in such a manner that the beam splitter (3) has a partially transparent layer (4) in a central zone (7) around the optical axis of the receiving channel, in particular wherein a beam-splitting ratio of the beam splitter of less than 50 per cent is generated for the receiving channel by means of the partially transparent layer (4) in the central zone (7), and • the partially transparent layer (4) forms at least a part of the fixedly arranged attenuating element (4), in particular wherein the partially transparent layer (4) forms the fixedly arranged attenuating element (4).

7. The optical distance meter according to any one of the preceding claims, characterized in that the receiving channel has a correction optical system (11), which defines a correction zone of the receiving channel, for compensating a focus error during focusing of parts of the received radiation coming back on the receiver (10) from a distance shorter than a defined close-range limit distance, namely wherein parts of the received radiation located in the correction zone are deflected by the correction optical system (11) onto the receiver (10), in particular wherein the correction optical system (11) is arranged in such a manner that • at least part of the received radiation passing through the second aperture is located inside the correction zone, and • received radiation passing through the first aperture is essentially located outside the correction zone.

8. The optical distance meter according to Claims 3, 5 and 7, characterized in that the fixedly arranged attenuating element (4), the correction optical system (11) and the switchable aperture diaphragm (13, 13', 13") are arranged in such a manner with respect to one another and are in each case configured in such a manner in terms of their lateral geometry that • in the first measuring mode, essentially the entirety of the received radiation hitting the receiver (10) is attenuated by the fixedly arranged attenuating element (4) and is located outside the correction zone, and • in the second measuring mode, the aperture diaphragm (13, 13', 13") is pivoted out of the ray path of the receiving channel and received radiation passing through the second aperture has bundles of rays that are not attenuated by the fixedly arranged attenuating element (4), which are at the same time located inside the correction zone, in particular wherein the correction zone has no overlap with an attenuation zone (7) defined by the fixedly arranged attenuating element (4).

9. The optical distance meter according to any one of the preceding claims, characterized in that a transmission channel of the optical distance meter has a switchable attenuating element (21), in particular based on at least one of an adjustable neutral wedge filter, a liquid crystal attenuator, and a fibre-optic Mach-Zehnder interferometer.

10. The optical distance meter according to any one of the preceding claims, characterized in that the ratio between the area of the first aperture (130) and the area of the second aperture is less than 1 / 10, in particular less than 1 / 20.